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Azonia aromatic heterocycles as a new acceptor unit in D-π-A+ vs D-A+ nonlinear optical chromophores
Accepted Manuscript + + Azonia aromatic heterocycles as a new acceptor unit in D-π-A vs D-A nonlinear optical chromophores Tatiana Cañeque, Ana M. Cuadro, Raúl Custodio, Julio Alvarez-Builla, Belén Batanero, Pilar Gómez-Sal, Javier Pérez-Moreno, Koen Clays, Obis Castaño, José L. Andrés, Thais Carmona, Francisco Mendicuti, Juan J. Vaquero PII:
S0143-7208(17)30422-9
DOI:
10.1016/j.dyepig.2017.05.005
Reference:
DYPI 5967
To appear in:
Dyes and Pigments
Received Date: 28 February 2017 Revised Date:
1 May 2017
Accepted Date: 2 May 2017
Please cite this article as: Cañeque T, Cuadro AM, Custodio Raú, Alvarez-Builla J, Batanero Belé, Gómez-Sal P, Pérez-Moreno J, Clays K, Castaño O, Andrés JoséL, Carmona T, Mendicuti F, + + Vaquero JJ, Azonia aromatic heterocycles as a new acceptor unit in D-π-A vs D-A nonlinear optical chromophores, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.05.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Azonia aromatic heterocycles as a new acceptor unit in D-π-A+ vs D-A+ nonlinear optical chromophores Tatiana Cañeque,1 Ana M. Cuadro,1,* Raúl Custodio,1 Julio Alvarez-Builla,1 Belén Batanero,1 Pilar
Francisco Mendicuti4 and Juan J. Vaquero1,* 1
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Gómez-Sal,1 Javier Pérez-Moreno,2 Koen Clays,3 Obis Castaño,4 José L. Andrés,4 Thais Carmona,4
Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá, 28871-Alcalá de
Henares, Madrid, Spain. 2Department of Physics. Skidmore College, Saratoga Springs, NY. 3Department
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of Chemistry, University of Leuven, Celestijnenlaan 200 D,3001 Leuven, Belgium. 4Departamento de Química Analítica, Química Física e Ingeniería Química, Universidad de Alcalá, 28871 Alcalá de
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Henares, Madrid, Spain
[email protected], [email protected]
ABSTRACT: A comparison of D-π-A+ and D-A+ cationic chromophores based on the quinolizinium system as new acceptor units is reported along with the results of studies into their linear and non-linear
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optical properties and electrochemical data. Experimental and theoretical data show that quinoliziniumbased chromophores may provide a new generation of second-order non-linear materials with enhanced performance. The first hyperpolarizabilities were measured by Hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis. In some chromophores the absence of a
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bridge (D-A+) between the donor and acceptor fragments enhances the NLO properties and the single
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crystal structure of such a material has been determined by X-ray diffraction
KEYWORDS: NLO, quinolizinium acceptor unit, charged chromophores, azonia aromatic heterocycles, synthesis D-A, theoretical calculations
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GRAPHICAL ABSTRACT:
Research highlight
Design and synthesis of new D-π-A+ vs D-A+ chromophores based on quinolizinium cation Unbridged D-A+ chromophores with remarkable large beta values
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Cyclic voltammetry measurements compared with DFT calculations
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The work provides valuable hints for rational design of novel D-A+ materials
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Appendix A. Supplementary information (SI)
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ACCEPTED MANUSCRIPT 1. Introduction There is considerable interest in organic non-linear optical (NLO) materials[1] with large second-order optical non-linearities in various fields (chemistry, physics, materials) because these materials have found diverse applications in optoelectronics[2], all-optical
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data processing technology[3], bioimaging[4], sensitizers for solar cells[5], and photodynamic therapy[6] amongst others. Generally, most effort has been focused on the design of new chromophores with large first hyperpolarizability values (β)[7], which are related to an electronic intramolecular charge transfer (ICT) due to the ease with which
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the molecular structures, and hence the properties, can be tuned for various applications. Second-order NLO organic materials are most often based on π-conjugated molecules
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(chromophores) with strong electron donor and acceptor groups at the ends of the π-conjugated structure[8]. Although π-electron delocalization is the most widely studied and exploited form of ICT, electrons are also known to delocalize over σ bonds[9] and this topic has barely been studied. Conjugated compounds have been the focus of most research within this field, but
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today it is recognized and accepted that ICT occurs through a synergistic combination of different phenomena[10]. Although a wide range of materials has been studied over the past few decades, the most recent approaches to the design of highly NLO-active systems is based on
Me2 N
Me2 N
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Me2N
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charged molecules with the presence of cationic acceptors.
Me2 N
NMe2
N Me + PF6 S +N
+ N
PF6
NMe2
PF Me
N +
N + PF
PF
N + PF
+ PF
N
N+ PF
Figure 1. Representative examples of nonlinear optical chromophores based on azolium and pyridiniun cations as acceptor units.
Cationic chromophores are of interest because they possess an easy tunability by counter ion exchange to crystallize into non-centrosymmetric structures[11]. Furthermore, by tuning their 3
ACCEPTED MANUSCRIPT solubility, potential applications in imaging can also be achieved. Other potential advantages of charged chromophores include their greater stability and higher chromophore number densities when compared with other NLO materials. In this context, charged acceptors units are referred to benzothiazolium[12] and azinium[13] (pyridinium, quinolinium) cations. These latter having
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interest in the generation of terahertz (THz) radiation[14] as 1D and 2D chromophores represented by DAST(4-(??,N-dimethylamino)-4´-N′-methyl-stilbazoliumtosylate)/DSTMS(4(??,N-dimethyl
amino-4′-??′-methyl-stilbazolium-2,4,6-trimethylbenzenesulfonate)[15]
and
diquats/helquats [16] (Figure 1). However, azonia aromatic heterocycles (AZAH)[17] have not
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been explored to date as an acceptor unit, except in our work, as a marker in nonlinear optical bioimaging, exhibiting a large two-photon absorption (2PA) and as push-pull cationic
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chromophores (D-π-A+-π-D)[18] (Figure 2). These kinds of heteroaromatic cations containing a quaternary bridgehead nitrogen are well represented in nature, forming part of a wide range of alkaloids[19]. Furthermore, some derivatives of these azonia salts have proven useful in important applications such as cyanine dyes and highly fluorescent compounds used as probes
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for the detection of biomolecules[20].
R = OMe, NMe 2
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Figure 2. Quinolizinium as acceptor unit in 1D (D-π-A+) and 2D (D-π-A+-π-D) chromophores.
As a continuation of our research on the applications of heteroaromatic cations in the NLO field[21], we explored the potential of new chromophores based on the quinolizinium cation, which may exhibit better non-linear optical properties (NLO) than other heteroaromatic cations tested to date, i.e., azinium[22]and azolium[23] salts. In addition, comparative studies on the NLO properties were carried out in order to investigate the role of quinolizinium as an acceptor and to establish the relevant structure activity relationships in simple dipolar molecules. This
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ACCEPTED MANUSCRIPT was achieved by changing the nature of the π-bridge that connects the donor units and comparing these chromophores with unbridged ones (Figure 3). EDG
EDG
+
D
D
D-A+ EDG: electron-donating group
+
X
D-π-A+
+
+
D D-π-A+
N +
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D
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Figure 3. General structure of push-pull systems based on the quinolizinium cation.
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We report here our results on the use of the quinolizinium cation as an acceptor unit in push-pull chromophores in which the quinolizinium moiety is connected to an electron donor (D) through a π-conjugated bridge (D-π-A+) or directly (D-A+) by Pd-catalysed cross-coupling reactions (Figure 4). The aim was to gain a better understanding of these systems and to contribute to the recent interest in β-enhancement strategies[21a,24].
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The second-order NLO properties of the selected chromophores were evaluated and compared to experimental results and theoretical calculations previously carried out on azinium cations. The resulting D-A+ and D-π-A+ chromophores with the quinolizinium acceptor were studied by
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hyper-Rayleigh scattering experiments and by theoretical analysis, including Density Functional Theory (DFT) and correlated Hartree–Fock-based methods (RCIS(D)). In addition, the redox
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activity was evaluated by cyclic voltammetry and the linear optical properties related to the ICT characteristics of the D-A+ and D-π-A+ are included along with the X-ray structures of selected D-A+ chromophores.
2. Results and discussion 2.1 Synthesis The quinolizinium cation is the simplest model that offers four possible positions of substitution by palladium-mediated cross-coupling processes and these products exhibit markedly different
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ACCEPTED MANUSCRIPT behaviour from an electronic point of view at the C1/C3 and C2/C4 positions. Moreover, one would also expect a different steric effect on C1/C4 and C2/C3. Our strategy for the construction of push–pull molecules was to combine quinolizinium cations – employed as acceptor units (A+) – with donor units (D) through a π-bridge using the Sonogashira[25] and
reactions
using
either
the
appropriate
arylboronic
-
-
PF6
or
potassium
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R = OMe, NMe2
acids
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organotrifluoroborates[27,28](Figure4) .
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Heck reactions or to connect the units directly by Stille or Suzuki−Miyaura[26] coupling
Figure 4. Synthesis of cationic (D-π-A+) and D-A+) chromophores by Pd catalysed cross-coupling reactions.
The first series of quinolizinium-based chromophores was previously obtained by us from the four isomeric bromoquinolizinium cations[29] (1a–d) (Scheme 1), which were
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coupled with different arylalkynes under Sonogashira conditions[25]. This method allowed easy access for the first time to representative D-π-A+ quinolizinium-based chromophores and allowed the efficient incorporation of arylethynyl substituents into the quinolizinium system,
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particularly at the C2 and C3 positions (Scheme 1).
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10% CuI 5% Pd2Cl2(PPh3)2
Et 3N, DMF, 60 ºC or rt
1 1a = 1-Br 1b = 2-Br 1c = 3-Br 1d = 4-Br
2a (1-) 47% 2b (2-) 78% 2c (3-) 71% 2d (4-) 61%
3b (2-) 55%
2-5 X = Br, PF6 R = OMe, NMe2, Me, Br
-
-
4b (2-) 74% 4c (3-) 76%
5b (2-) 71% 5c (3-) 70%
Scheme 1. Synthesis of D-π-A+ chromophores 2-5 by Sonogashira reaction 6
ACCEPTED MANUSCRIPT In order to study and compare the NLO properties of the quinolizinium cation as an acceptor in D-π-A+ to D-A+, we discuss here previously published results on quinolizinium[18b] linked to C1-C4 with dimethylamino- and methoxystyryl derivatives. These compounds were synthesized by Heck reactions using the corresponding vinyl quinolizinium hexafluorophosphate and aryl
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iodides as reaction partners or, alternatively, by a more classical approach involving a Knoevenagel reaction between 2-methylquinolizinium hexafluorophosphate[30] and the corresponding aryl aldehyde in higher yields as indicated in Scheme 2. I N +_
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R N _+
6,7
R
PF6
PF6
N _+
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R
N _+
N _+
PF6
N _+
PF6
6b: R = NMe2(42%) 7b: R = OMe (81%)
6a: R = NMe2 (37%)
R
PF6
37-81%
PF6
6c: R = NMe2(42%) 7c: R = OMe (37%)
R R 6d: R = NMe2(42%)
CHO
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Me
N _+
PF6
R
MeCN Piperidin
6b: R = NMe2(92%) 7b: R = OMe (81%)
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Scheme 2. Synthesis of chromophores 6 and 7 by Heck and Knoevenagel reaction
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Although initial studies in our laboratory showed that the Stille[26] coupling reaction was the most efficient procedure in comparison to other palladium-catalysed reactions to produce high yields of some aryl- and heteroaryl-quinolizinium cations, we prepared the new quinolizinium derivatives 9 and 10 by the Suzuki–Miyaura reaction[31], which is a greener procedure according to conditions employed by us to synthesize heteroaryl quinolizinium derivatives[27] (Scheme 3). Optimization of the reaction conditions was carried out first with the 2-bromoquinolizium salt, with different bases, catalytic systems and temperatures evaluated. It was found that the best
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ACCEPTED MANUSCRIPT catalytic system was Pd2(dba)3 as this allowed the reaction to be carried out at room temperature in conjunction with bulky phosphines: P(o-Tol)3 or (2-biphenyl)-di-tert-butylphosphine.
DMF, r.t. Method A or B
52-80% , -
1 H2O, 65º C Method C Method C
8%mol Pd(o-tol)3
8b: R = 3-OMe; 71% (C) 9b: R = 4-OMe; 52% (A); 73% (B); 76% (C) 10b: R = 4-NMe2: 73% (b); 80% (C)
1%mol, Pd(OAc)2;
8c: R = 3-OMe; 77% (C) 9c: R = 4-OMe; 60% (B); 80% (C) 10c: R = 4-NMe2: 60% (B)
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8a: R= 3-MeO; 93% (C)
Method B 4%mol, Pd2(dba)3;
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Method A 2%mol, Pd2(dba)3; 4%mol Pd(o-tol)3
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9-10
1b= 2-Br 1c= 3-Br
8d: R= 3-MeO; 63% (C)
Scheme 3. Suzuki–Miyaura coupling reaction with boronic acids and potassium organotrifluoroborates
Thus, the coupling between 1b and phenylboronic acid with electron-donating groups (OMe and
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NMe2) afforded the product 9b in moderate yield (52%) when Pd2(dba)3/P(o-Tol)3 in a ratio of 2%/4% (Method A) was used or in good yield for 9 and 10 with a 4%/8% ratio (Method B) in DMF in the presence of K2CO3 at room temperature for 16 hours (Scheme 3). The 3-
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bromoquinolizinium bromide (1c) was also subjected to Suzuki coupling in order to provide an overview of the reactivity of the C2 and C3 positions, which are electronically very different
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and similar to C4 and C1, respectively. Alternatively, coupling with potassium organotrifluoroborates, (Method C) an efficient reagent that allows functionalization of quinolizinium under mild conditions by reaction with 1b, afforded chromophores 9b–10b in higher yield (Scheme 3) and similar to 8a–d, which were previously synthesized by us[27].
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ACCEPTED MANUSCRIPT 2.2 Linear optical properties The UV-Vis spectra which are shown in the wavelength range where the main absorption bands appeared (Figures 5-8), were analyzed to evaluate the effectiveness of different electrondonating substituents (D) with different π-linkers in D-π-A+ vs D-A+ chromophores.
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The absorption spectra of a group of representative chromophores (2b–5b) bearing different electron-donating substituents (D) in position C2 of the quinolizinium (A+) with an acetylenic πlinker are represented in Figure 5. As expected, a good correlation was found between the electron-donating capabilities of the substituents and the relative position of the maximum of
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the ICT band (λabs,max). The electron-donating capability increased in the order Br- < Me- < MeO- < Me2N- and a corresponding bathochromic displacement of λabs,max was observed. In
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general, the λabs,max for compounds containing dimethylamino-substituents (Me2N–) shifted towards longer wavelengths (430 nm for 3b) with respect to those containing Br-substituents (341 nm for 5b).
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2b λmax=378 nm 3b λmax=430 nm
0.8
4b λmax=367 nm 5b λmax=341 nm
0.6 0.4
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Absorbance (a.u.)
1.0
AC C
0.2
0.0 300
350
400
450
500
550
λ (nm)
Figure 5. Absorption spectra (normalized at the maximum of intensity) in the 300–580 nm range for 2– 5b in MeOH as a solvent at 25 °C.
The normalized absorption spectra of chromophores 6a–d with styryl moieties, which are collected in Figure 6, also showed the intense ICT absorption band attributed to the Me2NC6H4→QZ charge transfer. This band was significantly shifted to the red in compounds 6b (473 nm) and 6d (445 nm) when compared to chromophores 6a (425 nm) and 6c (433 nm). For the
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ACCEPTED MANUSCRIPT former derivatives the Me2NC6H4– group was linked through a styryl moiety to the quinolizinium acceptor at the C2 and C4 positions, for the latter ones, however, the donors were located in the less activated C1 and C3 positions. This fact could be explained by a more efficient conjugation in positions 2 and 4 compared to those in 1 and 3, due to a combination of
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electronic and steric effects. A similar situation was observed for 2a–d and 8a–d, where the greatest stabilization took place for compounds b and/or d (see SI Figures S1 and S2).
The absorption spectra of chromophores 6b,c and 7b,c are also depicted in Figure 7. The ICT absorption bands were shifted to significantly longer wavelengths for compounds 6b,c
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substituted at C2- and C3-, when compared to 7b,c, by about 91 and 81 nm respectively. For unbridged chromophore D-A+ pairs 9b, 10b and 9c, 10c the displacements were approximately
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83 nm and 60 nm, respectively (see Figure S3 in the SI). For compounds 2b-3b, which contain acetylenic π-linkers, the maxima of the bands were displaced by around 52 nm. The stronger electron-donating character of Me2N– favors the stabilization of compounds that contain this substituent relative to those bearing MeO– regardless of the linker. The stabilization of
members of the series.
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compounds b with respect to c due to the position of the substituent was also evident for all
1.0
6a λmax=425 nm
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Absorbance (a.u.)
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6b λmax=473 nm 6c λmax=433 nm
0.8
6d λmax=445 nm
0.6 0.4 0.2
0.0 300
350
400
450
500
550
600
650
λ (nm) Figure 6. Absorption spectra (normalized at the maximum of intensity) in the 300–680 nm range for chromophores 6a–d in MeOH at 25 °C.
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1.0
7b λmax=382 nm
Absorbance (a.u.)
7c λmax=352 nm 0.8
6b λmax=473 nm 6c λmax=433 nm
0.6
0.2 0.0
300
350
400
450
500
550
600
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0.4
650
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λ (nm) Figure 7. Absorption spectra (normalized at the maximum of intensity) in the 300–650 nm range of pushpull 7b-c and 6b-c chromophores in MeOH at 25ºC.
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A good correlation was also found between λabs,max for D-A+ unbridged chromophores 8b,c and 9b,c (see Figure S4 of the SI). The λabs,max for the methoxyphenyl groups in para-positions (9b,c) showed a bathochromic displacement of the ICT band relative to 8b,c by about 13 and 5 nm, respectively, because meta-substitution leads to less effective conjugation.
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Certainly, the para-substituted electron-rich MeOC6H4– and Me2NC6H4– groups at the C2 quinolizinium position (compounds 2b, 7b, 9b and 3b, 6b, 10b respectively) strongly favor DA+ conjugation stabilizing the ICT complex when compared to other series of studied
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compounds. For this reason, a more extensive photophysical study was performed on these compounds in order to gain a deeper insight into the linear properties and their relationship with
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the non-linear ones.
The normalized absorption and emission spectra for the quinolizinium derivatives 10b, 6b and 3b, which have Me2NC6H4– as donors, are shown in Figures 8(a) and 8(b), respectively. The absorption spectrum of the olefinic chromophore 6b showed a more marked displacement of the ICT band to the red (472 nm) when compared to the acetylenic derivative 3b (447 nm) and the unbridged Me2NC6H4– chromophore 10b (431 nm). This particular trend was previously reported by Moylan et al.,[32] for a series of D-A chromophores connected by simple acetylenic, olefinic and aza linkers. Absorption spectra for the MeOC6H4- substituted 2b, 7b and
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ACCEPTED MANUSCRIPT 9b derivatives showed a similar trend (see Figure S5 SI) although bands are less shifted to the red than their Me2NC6H4– substituted counterparts. The fluorescence spectra showed analogous features for the band displacements in both series of chromophores. The most red-shifted emission bands correspond to Me2NC6H4– donors
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containing compounds when compared to their counterparts, which have MeOC6H4- donors. However, the extent of such bathochromic changes depend on donor-linker structure. The absorption and emission maxima, fluorescence quantum yields and molar absorptivities for para-substituted MeOC6H4- and Me2NC6H4– quinolizinium derivatives are listed in Table 1.
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The absorption maxima λabs,max and λem,max were similar to those previously reported by us [18a,c]. The emission quantum yield and molar absorptivity values for some chromophores,
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however, showed some discrepancies but they were of the same order of magnitude. Thus, Stokes shifts of approximately 160, 123 and 171 nm for 10b, 3b and 6b and of 76, 147 and 123 nm for 9b, 2b and 7b, respectively, were found. Different behaviors may arise from the formation of TICT (Twisted intramolecular charge transfer) states in dimethylamino derivatives
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whose stability in polar media depends strongly on the type of donor to the quinolizinium acceptor linker and their internal rotation number. For this reason, as previously reported, the interpretation of some of the photophysical behaviors in these systems is complex[33]. The
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fluorescence quantum yields were lower for the Me2NC6H4-containing derivatives than for the MeOC6H4- ones. It is well known that the fluorescence quantum yields usually decrease with
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the electron substituent donating ability. The additional non-radiative deactivation excited state path arising from the dimethylamino twisting in the TICT may also contribute to this. Although with a less efficient electron donating ability, a surprisingly high fluorescence quantum yield was obtained for the unbridged MeOC6H4-containing chromophore 9b which, as expected, was decreased significantly for compounds 7b and 2b, which have olefinic and acetylenic π-linkers.
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ACCEPTED MANUSCRIPT Table 1. Photophysical properties of the quinolizinium derivatives (see Figure 9) in methanol at 25 °C λexc (nm)
λem,max (nm)
φ
εmax (× 10-3 cm-1Mol-1L)
2b
378
360
525
0.01 ± 0.00a
31.8 ± 0.0
7b
386
360
509
0.02± 0.00a
14.9 ± 1.2
9b
360
360
436
0.35 ± 0.06a
42.1 ± 0.2
3b
447
430
570
< 0.001b
6b
472
460
643
0.006± 0000b
10b
431
420
591
0.08 ± 0.01b
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λabs,max (nm)
Comp.
37.0 ± 0.3 16.6 ± 0.2 36.4 ± 0.5
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Maximum absorption wavelength (λabs,max), excitation wavelength (λexc), emission maxima wavelength (λem,max), emission quantum yield (ϕ) and molar absorptivity at λabs,max (εmax). (a) Using Quinine Sulphate
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0.4
b)
0.8
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Absorbance (a.u.)
0.8
0.6
1.0
10b 6b 3b
a)
Fluorescence (a.u.)
1.0
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(φ=0.546) in H2SO4 0.1N or (b) coumarine 153 in ethanol (φ=0.38) as standards[34].
0.6
0.4
0.2
0.0
0.0
AC C
0.2
300 350 400 450 500 550 600
λ (nm)
500 550 600 650 700 750 800
λ (nm)
Figure 8. Absorption (a) and emission spectra (b), both normalized to 1 at the maximum of intensity, for 10b (black), 6b (red) and 3b (blue) in methanol at 25 °C.
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OMe ACCEPTED MANUSCRIPT
OMe
OMe
N +_ PF6
N _ + PF6
N PF6 9b
7b
2b
N(Me)2 NMe2
N(Me)2
PF6
PF6 10b
6b
3b
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N
N +_ PF6
N _ +
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Figure 9. Selected chromophores for the study of linear and non-linear optical properties (Tables 1 and 2).
2.3 Electrochemical properties
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The electrochemical properties of selected quinolizinium salts (Figure 9) were studied by cyclic voltammetry (CV) in acetonitrile/LiClO4 as SSE (solvent-supporting electrolyte system). The cathodic and the anodic peak potentials, measured at a scan rate of 100mV/s and summarized in Table 2, are quoted with respect to the Ag/Ag+ (sat) reference electrode. These cationic chromophores are electroreducible to the corresponding stabilized radicals in the
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same potential range (Epc ∼ –1.2 V) in all cases due to the positive charge on the nitrogen atom of the quinolizinium ring. The reduction of the electrogenerated radical to the corresponding anion requires a more negative potential value and this was only observed in some cases.
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A relationship between the energy of the highest occupied molecular orbital (HOMO) and the
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oxidation potential of a molecular organic semiconductor was described by Forrest[35]. The data
for
the
estimation
of
such
energy (EHOMO,
eV)
applied
to quinolizinium
hexafluorophosphates represented in Figure 9 are summarized in Table 2. These experimental results are consistent with theoretical computational calculations, with a very similar trend in both series of chromophores (9b>2b>7b>10b>3b>6b). The phenyl-substituted linker in these chromophores represents a conjugated system that extends the electron delocalization of the heterocycle. The HOMO is extended over the entire πsystem and mainly involves the heteroatom electron pairs of the substituent joined to the aromatic ring. For this reason, the oxidation is strongly influenced by the electron-donor 14
ACCEPTED MANUSCRIPT character of the substituents in the para-position of the phenyl group attached to the quinolizinium cation. Strongly donating substituents, such as dimethylamino (3b, 6b, 10b), lead to more negative reduction potentials (and less positive oxidation potentials) than those obtained when weaker donor substituents (2a, 7b, 9b) are present in the same position. The LUMO is
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located on the nitrogen atom in the quinolizinium cation core. This unit is only weakly influenced by the electron-donor character of substituents located at the end of the π-system and the reduction potentials for 3b, 6b, 10b, 2a, 7b and 9b are therefore very similar.
The oxidation and reduction of a functional group in a given compound proceeds more easily
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when it has a ‘lower’ (in absolute value) oxidation or reduction potential, respectively. Maps of the HOMO and LUMO orbitals of the chromophores under investigation show that reduction at
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the nitrogen atom in the quinolizinium cation involves the entire π-system when the reduction occurs. However, the more limited delocalization of the π-electron system in 4-methoxyderivative 9b results in a more negative reduction potential (and more positive oxidation potential) than those in 2b and 7b (see Figure 10). In turn this gives rise to a larger gap between
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the Eox and Ered values for 9b in comparison with 2a and 7b. The order of the HOMO-LUMO gap should be: 9b > 2a > 7b.
Compound 3b, in which the quinolizinium salt is substituted at the 2-position with an acetylenic
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chain joined to a 2-(4-dimethylamino phenyl) ring, provides a cyclic voltammetry Epa value of +0.94 V, which correlates with the expected reluctance of this compound to undergo oxidation
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compared to the homologous ethylenic compound 6b (Epa = +0.7 V). Stabilization of the electrogenerated radical cation in 3b is more difficult than in 6b because it cannot be highly delocalized. The oxidation of 3b should take place directly at the nitrogen atom rather than the acetylenic moiety, as indicated in Scheme 4. Cyclic voltammetry results for 9b can be compared with those for 7b. The latter alkenylbridged chromophore is much easier to oxidize. The absence of a connecting chain between the quinolizinium ring and the aryl group in 9b (Epa = +1.85 V) produces a poorly stabilized radical cation (Scheme 5) where the contribution of the resonance
15
ACCEPTED MANUSCRIPT forms with an oxygen atom supporting the positive charge is tiny. In this case the aromatic ring is oxidized but is also slightly stabilized by the alkoxy group.
7b
+1.42
9b 3b
+1.85 +0.94
6b 10b
+0.71 +0.98
+1.57
+0.87 +1.32
-1.24
-6.56
-5.97
-1.35 -1.25
-7.19 -5.92
-6.25 -5.40
-1.28 -1.47
-5.58 -5.97
-5.15 -5.47
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Table 2. Experimental values of the peak potentials E (V, vs Ag/Ag+) (± 0.03V) of selected D-π-A+ and D-A+ chromophores (Figure 9). Scan rate: 100 mV/s. Energy Energy Comp. Epa1 Epa2 Epc ∆Eb/eV HOMOa/eV HOMOb /eV +1.67 -1.27 -6.94 -5.98 -2.52 2b -2.27
-2.28 -2.33
-2.21 -2.06
SC
a:Estimated from CV data (Epa1 values) by applying the relationship by Forrest[33].
Me
Me
N
N
Me
Me N
Me
-e N
N
N
PF6-
PF6-
3b
Me
Me
N
N Me
-e
-
N
N PF6-
Me
6b
PF6-
Me
Me
Me
N
N
Me
N
N
PF6-
PF6-
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PF6-
M AN U
b: B3LYP/6-31g(d) calculations including the solvent effect.
Me
OMe
-e N
9b
OMe
OMe
N
N
PF6-
PF6-
AC C
PF6-
EP
Scheme 4. Comparative delocalization D-π-A+ chromophores 3b and 6b
Scheme 5. Delocalization for chromophore 9b
The redox abilities of the aforementioned quinolizinium salts were investigated with the main focus on the first oxidation and first reduction potentials, since these values are related to the HOMO and LUMO energies, respectively. The electronic influence of individual substituents was followed in an effort to understand the relationship between redox properties and structure. The reduction of these chromophores occurs on the quinolizinium heteroatom and is
16
ACCEPTED MANUSCRIPT consequently not strongly influenced by the donor/acceptor character of the substituents. However, this process is rather dependent on the extended π-delocalized electron system involving stabilization of the electrogenerated radical intermediate. Whereas in the presence of strong donating groups these compounds are reduced more
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negatively (Er(10b) = –1.47V), the weaker donor substituents produce a shift to more positive reduction potentials (Er(9b) = –1.35V). The reduction of 9b therefore proceeds more easily than that of 10b.
The oxidation of these molecules occurs on the methoxy/amino group located at the para-
SC
position of the benzene ring, in opposite side. The oxidation potential strongly reflects the donor/acceptor character of the substituents, although the entire π-system chain is also
M AN U
considerable. On increasing the electron-donating character (Me2N related to MeO) and the extent of the conjugation in the molecule (cf. 6b and 10b), less positive oxidation potentials are
AC C
EP
TE D
registered and this is consistent with the easier oxidation of these chromophores.
Figure 10. Cyclic voltammograms of 2b, 7b and 9b in acetonitrile/LiClO4 (0.1M) as SSE, at Pt as working and auxiliary electrodes. Ag/Ag+ (sat) reference electrode. Scan rate: 100 mV/s. (see SI voltammograms of 3b, 6b and 10b).
2.4 Non-linear Optical Properties: Hyper-Rayleigh Scattering (HRS) Measurements and Computational Details.
17
ACCEPTED MANUSCRIPT The results of Femtosecond hyper-Rayleigh scattering measurements performed at 800 nm on selected compounds are compiled in Table 3 and these confirm the potential of these small ionic compounds for second-order non-linear optics. Congruent with the trends already observed in the linear optical (absorption) experiments, the first hyperpolarizability of the compounds is
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higher when the methoxyphenyl group is present as the aryl donor and the most marked effect is seen when the quinolizinium is substituted in the C-2 position with a π-linker (2b) and in the DA+ unbridged chromophore 9b, which was the chromophore with the highest first
SC
hyperpolarizability value when compared with D-π-A+ compounds.
Table 3 Experimental non-linear Optical Properties of some Chromophores 2–10 λmax
βHRS
βzzz
350
122 ± 10
300 ± 20
60 ± 4
2b
378
284 ± 14
690 ± 40
152 ± 5
2c
350
245 ± 20
600 ± 40
112 ± 6
2d
380
202 ± 18
490 ± 40
37 ± 3
4b
367
380 ± 60
918 ± 140
110 ± 20
4c
334
74 ± 3
178 ± 7
45 ± 2
5b
350
186 ± 28
350 ± 70
86 ± 12
5c
341
34 ± 8
80 ± 12
18 ± 2
6a
425
142 ± 7
343 ± 17
31 ± 2
6b
472
199 ± 6
482 ± 51
49 ± 5
6c
433
191 ± 28
462 ± 49
56 ± 6
6d
445
83 ± 6
202 ± 42
38 ± 8
7b
382
118 ± 17
285 ± 41
19 ± 2
7c
353
162 ± 16
391 ± 39
70 ± 7
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EP
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M AN U
Comp. . 2a
βzzz, 0
8b
347
30 ± 7
72 ± 17
15 ± 3
9b
360
500 ± 50
1190 ±120
180± 20
9c
337
61 ± 7
147 ± 17
35 ± 4
10b
431
286 ± 30
450 ± 70
50 ± 7
Maximum absorption wavelength λmax (nm) predicted by CIS(D), resonance enhanced HRS first hyperpolarizability βHRS (10–30 esu), resonance enhanced diagonal component of the molecular first hyperpolarizability βzzz (10–30 esu), off-resonance diagonal component of the molecular first hyperpolarizability βzzz,0 (10–30 esu).
18
ACCEPTED MANUSCRIPT The first hyperpolarizability value in D-π-A+ chromophores is higher in compound 2b (β = 284.10–30 esu) than in 2c and 2d, which have β values of 245.10–30 esu and 202.10–30 esu, respectively. The replacement of the donor unit (N,N-dimethylaminostyryl) at the C2 position with a double bond resulted in 6b (β = 199.10–30 esu), the chromophore with the highest first
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hyperpolarizability value when compared with compounds in which quinolizinium was substituted at the C1, C3 and C4 positions with the same substituent. In agreement with the greater influence of the position of the donor substituent at the quinolizinium acceptor, which is more marked than the relative electron-donating strength characteristics, and the better
SC
conjugation and stabilization, the highest β values correspond to the compounds with substituents at the C2 positions, thus confirming that the best conjugation and charge transfer
M AN U
from donor to acceptor is achieved in these positions.
The experimental results reported here for cationic chromophores (D-A+), when compared to the (D-π-A+) systems reported previously by us, reveal that the hyperpolarizability of (D-A+) molecules seems to be determined by the strength of the charge-transfer. Thus, the highest first
TE D
hyperpolarizability values were found for the dimethoxystyryl donor unit at the C2 (9b: β = 500·10–30 esu) and the N,N-dimethylaminostyryl donor unit at the C2 (10b: 286·10–30 esu). However, in the case of 9b, the common assumption that β can be maximized by using a
EP
conjugated bridge[36] to link D and A acceptor units is not clear because, surprisingly, the highest hyperpolarizability was found for 9b with dimethoxystyril donor unit connected directly to
AC C
quinolizinium acceptor when compared with 2b and 7b. Theoretical calculations present a similar
correlation for the resonant and off-resonant molecular hyperpolarizabilities βzzz and βzzz,0, respectively.
Theoretical calculations were performed using the Gaussian suite of quantum chemical programs[37]. For all of the studied cationic chemical systems, the molecular structure used to compute optical properties corresponded to an energy minimum calculated at the HF/6-31G(d) level of theory. These structures were confirmed as minima by evaluation of the harmonic vibrational frequencies, which were all real.
19
ACCEPTED MANUSCRIPT Table 4. Theoretical linear and nonlinear optical properties selected compounds λmax exp.
λmax Cal. CIS(D)
βHRS
βzzz,800
βzzz, 0
2a
350
312
83
197
79
2b
378
364
255
617
187
2c
350
323
131
-318
-126
2d
380
336
104
-276
-96
3b
447
449
233
429
297
9b
360
334
63
154
10b
431
414
209
506
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Comp.
64
152
Maximum absorption wavelength λmax (nm) predicted by CIS(D), resonance enhanced HRS first
SC
hyperpolarizability βHRS (10–30 esu), resonance enhanced diagonal component of the molecular first
hyperpolarizability βzzz (10–30 esu), off-resonance diagonal component of the molecular first
M AN U
hyperpolarizability βzzz,0 (10–30 esu).
Calculated wavelengths (λmax, in nm) were estimated by the CIS(D)[38] method, which generally provides accurate predictions of excited states that are mainly one-electron transitions from a single reference ground state. The λmax values were obtained as vertical excitations from the ground state to the first excited singlet.
TE D
As CIS(D) is one of the simplest approaches amongst the correlated excited state theories, our calculations provided good results with wavelength values that differed by less than 10% with respect to the experimental data, as predicted in other
EP
investigations[39]. A more pronounced overestimation of the excitation energies by the quantum chemical methods was observed for TDDFT calculated values.
AC C
Theoretically predicted values of the second harmonic generation (SHG) of hyperpolarizability [40] β(-2ω;ω,ω) were obtained, including the electron correlation effect at the MP2 level. Molecular components of the first hyperpolarizability were computed as dipole moment second derivatives and these values were then spatially averaged to obtain the βHRS values[41]. Details of the mathematical procedure to obtain the hyper-Rayleigh hyperpolarizability from molecular first hyperpolarizabilities are reported elsewhere[21a,b]. Both the resonance βzzz and off-resonance βzzz,0 diagonal components of the molecular first hyperpolarizability were directly calculated as dipole moment derivatives rather than being estimated from the simple two-state model. As a consequence, comparisons could not be made between these two sets of data.
20
ACCEPTED MANUSCRIPT These MOs (Figure 11) clearly show the charge transfer from the donor to the acceptor fragments of these systems. The π-π* transition bands are due to the vertical excitation between these MOs (see SI Figure 11). The discrepancies between the theoretical energy gap (∆E) values and vertical excitation (λmax) for cation 9b shows deficiencies for this HF wavefunction that are not
M AN U
SC
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completely corrected in a third order property like hyperpolarizability.
Figure 11. HOMO and LUMO of cation 3b
2.5 Crystallographic studies
The crystal and molecular-ion structures of compounds 9b and 9c were obtained by X-
TE D
ray diffraction studies (Figure 12). The PF6– counter ion in both cases is found in the crystal but omitted in tables and figures for the sake of clarity. Compound 9b crystallizes in the monoclinic P21/c space group and 9c in the triclinic P-1 space group, both of which are centrosymmetric.
EP
Selected bond distances and angles are shown in Table 5 (see SI) for both molecules. It is
AC C
interesting to note that the distance C(2)–C (11) in 9b and C (3)–C(11) in 9c are 1.485(5) and 1.481(4) Å, respectively. These are slightly shorter than the normal[42] C–C single bond (1.54(3) Å), which indicates π-conjugation between the donor and quinolizinium system. In compounds 9b and 9c the aryl group is nearly coplanar with the quinolizinium system, with angles between the planes of the donor and acceptor units of 11.87(15)° and 13.91(11)°, respectively. The practically coplanar arrangement in the chromophore corroborates the delocalization and charge transfer between the aryl and quinolizinium, which in turn explains the high hyperpolarizability values of D-A+ unbridged chromophores 9b – as predicted by the two-levels model[43]. 21
ACCEPTED MANUSCRIPT Observation of the dihedral angles between the phenyl and methoxy group for both molecules shows that for 9b the dihedral angle C(15)–C(14)–O(2)–C(17) is 1.5° and for 9c C(13)–C(14)–O(1)–C(17) is 6°. This difference probably indicates that the planarity of 9b is somewhat higher and this promotes better charge transfer (βHRS = 500±50 10–30
SC
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esu for 9b and βHRS = 61±7 10–30 esu for 9c).
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Figure 12. ORTEP drawings of compounds 9b and 9c. The PF6– counter ion has been omitted for clarity
EP
Figure 13. Weak intermolecular interactions compounds 9b and 9c
3. Experimental section
AC C
3.1 Materials and Methods
3.1.1 Chemistry. Reactions were monitored by thin-layer chromatography (TLC) using TLC silica gel-coated aluminium plates 60F-254 (Merck). Column chromatography was performed using Merck silica gel 60, 0.040–0.063 mm (230–400 mesh). NMR spectroscopy was performed on Varian UNITY-Plus 300 (1H and 75 MHz 13C) or VNMRS-500 (1H and 125 MHz 13
C) spectrometers equipped with a cryoprobe. Chemical shifts were reported as δ values (ppm)
and coupling constants (J) in Hz. The mass spectra (MS) as ESI+ and HRMS masses were recorded on a Thermo Scientific TSQ Quantum LC/MS equipped with an ESI ionization source and a TOF detector. All starting materials, namely Pd(OAc)2, CuI, PdCl2(PPh3)2, 4-methoxy22
ACCEPTED MANUSCRIPT and 4-N,N-dimethylamino- benzaldehyde, 1-ethynyl-4-methoxybenzene, and the corresponding potassium organotrifluoroborates, were purchased from Aldrich and were used without further purification. The synthesis of 1-bromo-, 2-bromo-. 3-bromo- and 4-bromoquinolizinium [28] and 2-Methylquinolizinium salts [29] were obtained by previously reported methods. For
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chromophores 2a–d, 4b,c and 5b,c see reference 24 and for the preparation of the (E)-2(arylvinyl)quinolizinium hexafluorophosphates by Heck reaction and Knoevenagel reaction see references 18b and 25a. The procedure with potassium organotrifluoroborates is described in reference 27.
SC
3.1.2 General optical properties. Absorption Spectra were recorded on a Perkin-Elmer L35 UVVis spectrophotometer in the 200–1000 nm range. Steady-state fluorescence measurements
M AN U
were performed by using an SLM 8100 AMINCO spectrofluorimeter equipped with polarizers and a double (single) concave grating monochromator in the excitation (emission) path and a cooled photomultiplier. Slit widths were set at 8 nm for excitation and emission and polarizers at the magic angle. Fluorescence decay measurements were performed on an Edinburgh
TE D
Instruments FL900 time-correlated single-photon-counting system.
The fluorescence quantum yield was calculated using equation 1 (E1): grad i ni2 2 grad s t ns t
φi = φs t
E1
EP
where φst is the fluorescence quantum yield of the standard (st) in each case, gradi/st are the slope
AC C
of the surface plot under the full emission spectrum versus absorption at the excitation wavelength of derivative i or standard, respectively, and ni/st are the refractive indexes of methanol and water (or ethanol), respectively. 3.1.3 General voltammetric measurements. Voltammetric curves with IR compensation were obtained with a Tracelab Radiometer POL150 polarographic analyser in a Voltalab MDE150 Polarographic Stand in a three-electrode cell. Two Pt wires were used as working and auxiliary electrodes. The electrochemical properties of the chromophores indicated in Figure 9 were studied by cyclic voltammetry (CV) in acetonitrile (ACN)/LiClO4 as SSE (solvent-supporting electrolyte 23
ACCEPTED MANUSCRIPT systems). Before the voltammetric experiments, the solutions were deaerated by bubbling N2 (99.98% purity, purchased from Air Liquid) for 5 minutes. The cathodic and anodic peak potentials measured at a scan rate of 100 mV/s (summarized in Table 2) are quoted with respect to the Ag/Ag+ (sat) reference electrode.
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3.1.4 Hyper-Rayleigh scattering. General details of the hyper-Rayleigh scattering (HRS) experiments were discussed previously[44]. The HRS measurements were performed at room temperature in methanol, with crystal violet as the reference molecule and with high-frequency demodulation of the multiphoton fluorescence contribution[45].
SC
The HRS signal was analyzed towards a single major dipolar hyperpolarizability tensor element βzzz along the molecular z-axis. The dynamic on resonantly enhanced βzzz,800 value obtained at
M AN U
800 nm was reduced to the static, or off-resonance βzzz.0, value by applying the classical twolevel model[39]. From the fitting of the apparent βzzz,800 as a function of modulation frequency, a fluorescence lifetime could be obtained along with the accurate fluorescence-free hyperpolarizability value[46]. The reported β values are the averages taken from measurements
TE D
at different amplitude modulation frequencies.
Theoretical calculations: The theoretical resonant or frequency-dependent hyperpolarizabilities were obtained at the MP2/6-31G(d) level and include the electron correlation effect in this NLO
EP
property βMP2(-2ω;ω,ω). This property was estimated from the resonant or dynamic β(-2ω;ω,ω) at the HF/6-31G(d) level, and the static or non-resonant β(0;0,0) were calculated at both the
AC C
HF/6-31G(d) and MP2/6-31G(d) levels following the multiplicative approximation scheme, where the frequency dispersion is estimated to be similar at these two levels of theory[39a,47]. β MP 2 (− 2ω;ω , ω ) ≈ β HF (− 2ω;ω , ω )
β MP 2 (0;0,0) β HF (0;0,0)
The first hyperpolarizability β was calculated as a second dipole moment derivative with respect to electric fields. At the Hartree–Fock (HF) level these dipole derivatives were calculated analytically by solving the corresponding CPHF equations. When the electron correlation effect was included by means the perturbative MP2 approach, these dipole derivatives were obtained numerically. Calculated hyperpolarizabilities were obtained on 24
ACCEPTED MANUSCRIPT the molecular axes basis βijk and then transformed to the laboratory axes βXYZ by applying the expressions of Civin et al[48]. 3.1.5 Single-crystal X-ray structure determinations of 9b and 9c. Selected bond distances and angles are shown in Table 5 (see ESI) for both molecules. Details of the X-ray
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experiments, data reduction and final structure refinement calculations are summarized in Table 6 (see ESI). Single crystals suitable for X-ray diffraction were selected for data collection. The crystals were mounted on a glass fibre using an inert perfluorinated ether oil and placed in a low temperature N2 stream (200(2) K) in a Bruker-Nonius Kappa
SC
CCD single crystal diffractometer equipped with a graphite-monochromated Mo-Kα radiation source (λ = 0.71073 Å) and an Oxford Cryostream 700 unit. The structures
M AN U
were solved by direct methods (SHELXS-97) using the WINGX package[49] and completed by subsequent difference Fourier techniques and refined by using full-matrix least-squares against F2(SHELXL-97)[50]. All non-hydrogen atoms were anisotropically refined. Most of the hydrogen atoms were geometrically placed and left riding on their
TE D
parent atoms, and others were found in the Fourier difference maps. In the case of 9b the PF6– ion was found to be within the crystal with evident structural disorder, which was modelled.
EP
Crystallographic data (excluding structure factors Å) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary
AC C
publication nos. CCDC 1523732-1523733. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-033; email: [email protected]).
3.2 Synthesis 3.2.1
General
procedure
for
the
preparation
of
arylethynyl-quinolizinium
hexafluorophosphate. A flame-dried vial was charged under argon with 50 mg (0.173 mmol) of the corresponding bromoquinolizinium salt, 10 mol % CuI (3.3 mg, 0.0173 mmol) and 5 mol % PdCl2(PPh3)2 (6.9 mg, 0.0087 mmol) in dry DMF (2.5 mL). 125
ACCEPTED MANUSCRIPT Ethynyl-4-methoxybenzene (0.208 mmol, 27 µL) and Et3N (0.259 mmol, 45 µL) were added. After heating at 60 °C for 14 h, the solution was filtered through a small pad of Celite® and washed with methanol (5 mL). The solution was concentrated, treated with saturated solution NaHCO3 (20 mL) and extracted with EtOAc (15 mL). The organic
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phase was extracted with H2O (2 × 10 mL), the aqueous phase was treated with saturated solution NH4PF6 the resulting phase was extracted with CH2Cl2 (3 × 15 mL). The organic phase was dried over Na2SO4, the solvent evaporated under reduced pressure and the
(9.6:0.4) as eluent.
hexafluoro
M AN U
2-(4-N,N-Dimethylamino-phenylethynyl)quinolizinium
SC
product was purified by column chromatography on silica gel using CH2Cl2/MeOH
phosphate
(3b).
Following the general procedure, from 4-ethynyl-N,N-dimethylaniline (0.2077 mmol, 30 mg) stirring at room temperature for 5 h and using CH2Cl2/MeOH (9:1) afforded 39.7 mg (55%) of 3b as a brown-red solid: mp 273–277 °C; IR (KBr) νmax (cm–1) 3345, 2190, 1644, 1600, 1375,
TE D
837. 1H-NMR (500 MHz, Acetone-d6) δ (ppm) 9.30 (d, J = 7.0 Hz, 1H), 8.56 (d, J = 1.5 Hz, 1H), 8.52 (d, J = 8.6 Hz, 1H), 8.43 (ddd, J = 8.6, 7.2, 1.0 Hz, 1H), 8.13–8.06 (m, 1H), 7.51 (d, J = 9.0 Hz, 1H), 6.80 (d, J = 9.0 Hz, 1H), 3.07 (s, 3H). 13C-NMR (125 MHz, acetone) δ 154.61,
EP
139.99, 139.58, 139.21, 136.67, 135.63, 129.52, 127.65, 126.03, 114.46, 108.82, 107.31, 88.16,
AC C
41.81. HRMS (ESI-TOF, MeOH) Calcd. for C19H17N2 [M+]: 273,1386 [M+] Found: 273.1384.
3.2.2 General procedure for the preparation of the quinolizinium salts (9–10). Suzuki reaction
Method A. A flame-dried 10 mL vial was charged under argon with the corresponding quinolizinium bromide (50 mg, 0.173 mmol), 2 mol % of Pd2(dba)3 (3.2 mg, 0.0035 mmol), 4 mol % P(o-Tol)3 (2.1 mg, 0.0069 mmol), 1.4 equiv. of K2CO3 (36.8 mg, 0.242 mmol), and 1.3 equiv. of the boronic acid (0.225 mmol) in dry DMF (2.5 mL). After stirring at room temperature as indicated, the reaction mixture was filtered through a small pad of Celite® and washed with MeOH (5 mL). The solution was concentrated, treated with H2O (15 mL) and 26
ACCEPTED MANUSCRIPT extracted with AcOEt (10 mL). The organic phase was extracted with H2O (2 × 10 mL), the aqueous phase was treated with saturated solution NH4PF6 and the resulting phase was extracted with CH2Cl2 (3 × 15 mL). The organic phase was dried over Na2SO4, the solvent was evaporated under reduced pressure and the product was purified by column chromatography on silica gel
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using CH2Cl2/MeOH (9.6:0.4) as eluent. Method B. 4 mol % of Pd2(dba)3 (6.3 mg, 0.0069 mmol), 8 mol % of P(o-Tol)3 (4.2 mg, 0.0138 mmol).
SC
2-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9b). From 4-methoxyphenyl boronic (34.2 mg, 0.225 mmol) and stirring the reaction mixture for 23 h afforded 34.5 mg
M AN U
(52%, method A) and 47.8 mg (73%, method B) of 9b as a white solid. mp. 230–232 °C. IR (KBr): υmax (cm–1) 3122, 2933, 1645, 1606, 1405, 1178, 836, 558. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.38 (d, 1H, J = 7.3 Hz), 9.31 (d, 1H, J = 6.8 Hz), 8.90 (d, 1H, J = 2.0 Hz), 8.59–8.52 (m, 2H), 8.40 (td, 1H, J = 8.5, 1.1 Hz), 8.14–8.04 (m, 3H), 7.21 (d, 2H, J = 9.0 Hz),
TE D
3.93 (s, 3H). 13C NMR (75 MHz, Acetone-d6) δ (ppm) 163.3, 148.3, 144.2, 137.6, 137.4, 136.9, 130.1, 127.9, 127.1, 123.7, 122.2, 122.1, 115.9, 56.0. MS (ESI+) m/z (relative intensity) 236 (M+, 100). Anal. Calcd. For C16H14F6NOP (381.26 g/mol) C (50.41), H (3.70), N (3.67) Found:
EP
C (50.49), H (3.85), N (3.54).
3-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9c). Following Method B, from 4-
AC C
methoxyphenylboronic acid (34.2 mg, 0.225 mmol) and stirring the reaction mixture for 22 h afforded 39.6 mg (60%) of 9c as a pale yellow solid: 217–218 °C. IR (KBr): υmax (cm–1) 3113, 2942, 1609, 1509, 1401, 1292, 1257, 1203, 835, 558. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.70 (s, 1H), 9.42 (d, 1H, J = 6.8 Hz), 8.80 (dd, 1H, J = 9.0, 1.8 Hz), 8.69 (d, 1H, J = 9.5 Hz), 8.65 (d, 1H, J = 10.1 Hz), 8.45 (ddd, 1H, J = 9.7, 7.3, 1.1 Hz), 8.21 (t, 1H, J = 6.0 Hz), 7.95 (d, 2H, J = 8.8 Hz), 7.19 (d, 2H, J = 8.8 Hz), 3.91 (s, 3H). 13C NMR (75 MHz, Acetone-d6) δ (ppm) 162.6, 142.5, 137.6, 137.5, 137.3, 136.7, 133.9, 129.9, 128.2, 128.0, 126.6, 125.3, 116.0, 55.9.
27
ACCEPTED MANUSCRIPT MS (ES+) m/z (relative intensity) 236 (M+, 100). Anal. Cal. For: C16H14F6NOP (381.26 g/mol) C (50.41), H (3.70), N (3.67) Found : C (50.64), H (3.53), N (3.78). 2-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10b). Procedure B, from 4-N,N-dimethylaminophenylboronic (37.1 mg, 0.225 mmol), and stirring the reaction
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mixture for 20 h, gave 49.7 mg (73%) of 10b as a red solid. mp: 242–244 °C (Acetone:Et2O). IR (KBr): υmax (cm–1) 2927, 1649, 1598, 1406, 1214, 842, 559. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.24 (d, 1H, J = 7.3 Hz), 9.17 (d, 1H, J = 6.6 Hz), 8.77 (d, 1H, J = 2.2 Hz), 8.47 (dd,
1H, J = 7.1, 2.2 Hz), 8.44 (d, 1H, J = 10.4 Hz), 8.27 (td, 1H, J = 9.9, 1.1 Hz), 8.03 (d, 2H, J =
SC
9.2 Hz), 7.91 (td, 1H, J = 6.8, 1.0 Hz), 6.94 (d, 2H, J = 9.2 Hz), 3.12 (s, 6H). 13C NMR (75
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MHz, Acetone-d6) δ (ppm) 153.8, 149.0, 144.4, 137.3, 136.7, 136.5, 129.7, 127.7, 122.6, 121.3, 120.8, 119.5, 113.1, 40.0. MS (ESI+) m/z (relative intensity) 249 (M+, 100). Anal. Calcd. for C17H17F6N2P (394.30 g/mol) C (51.78), H (4.35), N (7.10), found C (51.64), H (4.50), N (7.22). 3-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10c). Method B, from 4-N,N-dimethyl aminophenylboronic acid (37.1 mg, 0.225 mmol), stirring the reaction mixture
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for 30 h, gave 41.0 mg (60%) of 10c as an orange solid: mp 229–230 °C (Acetone:Et2O).IR (KBr): υmax (cm–1) 2930, 1605, 1511, 1401, 1216, 842, 559. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.62 (s, 1H), 9.37 (d, 1H, J = 6.6 Hz), 8.77 (dd, 1H, J = 9.2, 1.8 Hz), 8.61 (d, 1H, J = 8.8
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Hz), 8.58 (d, 1H, J = 6.4 Hz), 8.36 (td, 1H, J = 9.7, 1.1 Hz), 8.15 (t, 1H, J = 7.1 Hz), 7.85 (d, 2H, J = 9.0 Hz), 6.94 (d, 2H, J = 9.0 Hz), 3.07 (s, 6H).
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C NMR (75 MHz, Acetone-d6) δ
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(ppm). 152.7, 141.8, 137.9, 137.1, 136.3, 135.9, 132.1, 129.0, 127.9, 127.8, 125.1, 120.4, 113.4, 40.1. MS (ES+) m/z (relative intensity) 249 (M+, 100). Anal. Calcd. for C17H17F6N2P (394.30 g/mol) C (51.78), H (4.35), N (7.10). Found: C (51.90), H (4.55), N (7.02).
3.2.3 General procedure for the preparation of the quinolizinium with organotrifluoroborates (9b and 10b) Method C. The corresponding potassium organotrifluoroborate (0.2076 mmol, 1.2 equiv) was added to a mixture of the corresponding bromoquinolizinium bromide (0.1730 mmol), K2CO3
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ACCEPTED MANUSCRIPT (0.0857 g, 0.5190 mmol, 3 equiv), 1 mol % of Pd(OAc)2 (0.0004 g, 0.00017 mmol) in H2O (2.5 mL). The reaction mixture was heated at 60–65 °C with stirring for the time indicated (2–4.5 h). The mixture was then cooled to room temperature and treated with saturated solution NH4PF6. The resulting solid was filtered off and washed with water and diethyl ether. For a further
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purification, the solid was chromatographed on silica gel using CH2Cl2/MeOH) (9:1) as eluent. 2-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9b). Following the general procedure and using potassium 4-methoxyphenyltrifluoroborate (44.4 mg) gave 49.8 mg (76%) of 9b as a white solid.
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2-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10b). Following the general procedure and using potassium 4-(N,N-dimethylamino)phenyltrifluoro borate (47.1 mg)
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gave 54.5 mg (80%) of 10b as a red solid.
2-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9b). Following the general procedure and using potassium 4-methoxyphenyltrifluoroborate (44.4 mg), were obtained 49.8 mg (76%) of 9b as a white solid.
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2-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10b). Following the general procedure and using potassium 4-(N,N-dimethylamino )phenyltrifluoro borate (47.1
4. Conclusions
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mg), were obtained 54.5 mg (80%) of 10b as a red solid.
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In summary, we have prepared a series of D-A+ and D-π-A+ chromophores based on quinolizinium using diffferent palladium-promoted C–C bond formation reactions. The optical properties of these chromophores were investigated by absorption and emission spectroscopy. The spectra of compounds with electron-rich substituents at the C2 position of the quinolizinium show a strong effect on the stabilization energy of the chromophores and a bathochromic effect is observed in the absorption spectrum, which is consistent with a better conjugation or a lower energy gap. The fluorescence spectra also presented displacements with a similar trend.
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ACCEPTED MANUSCRIPT The electrochemical behaviour of the chromophores is mainly affected by the electronic nature of the donor/acceptor character of the substituents, although the entire π-system chain is also considerable. On increasing the electron-donating character and the extent of conjugation of the molecule, less positive oxidation potentials were registered. This finding is consistent with the
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easier oxidation of these compounds. The calculated HOMO/LUMO energies and their differences correlate with the electrochemical measurements obtained by cyclic voltammetry. The experimental first hyperpolarizabilities (βHRS) of the series of charged chromophores show that compounds 2b, 3b, 9b and 10b have significantly higher values than the rest of the
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synthesized cationic systems. The low transition energy and high CT level are the decisive factors that provide a higher first hyperpolarizability in compounds substituted at C2.
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Theoretical calculations support the experimental results and also predict a similar trend. However, chromophore 9b showed some discrepancies between the theoretical calculation and experimental results. Further investigation of 9b by X-ray analysis revealed significant planarization.
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In comparison to previously reported D-π-A+ molecules[21a], the lack of a conjugated bridge in these D-A+ chromophores does not seem to be significant in terms of obtaining higher βHRS values, with the magnitude of β in this kind of chromophore being dominated by additive
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contributions from the donors and the acceptors. The results highlight an attractive strategy in guiding the design of new NLO materials, with unbridged D-A+ chromophores allowing a
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remarkable degree of control of the linear and non-linear optical response by changing the position and electronic nature of the substituents and, more importantly, the nature of the linker.
Acknowledgements
Financial support from the Spanish Ministerio de Ciencia y Competitividad (projects MICINN/CTQ2011-24715 and MINECO/ CTQ2014-52488-R and CTQ2015-64425-C2-1-R and CTQ2016-80600-P). Instituto de Salud Carlos III (FEDER funds, ISCIII RETIC REDINREN RD012/20021/0014) are gratefully acknowledged
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Highly Dipolar, Optically Nonlinear Adducts of Tetracyano-p-quinodimethane: Synthesis, Physical Characterization, and Theoretical Aspects. J Am Chem Soc 1997;119:3144-54. [25] Garcia D, Cuadro AM, Alvarez-Builla J, Vaquero JJ. Sonogashira Reaction on Quinolizium Cations. Org Lett. 2004;6:4175-8. [26] (a) Garcia-Cuadrado D, Cuadro AM, Barchin BM, Nunez A, Caneque T, Alvarez-Builla J, et al. PalladiumMediated Functionalization of Heteroaromatic Cations: Comparative Study on Quinolizinium Cations. J Org Chem. 2006;71:7989-95. (b) Garcia-Cuadrado D, Cuadro AM, Alvarez-Builla J, Sancho U, Castano O, Vaquero JJ. First Synthesis of Biquinoliziniun Salts: Novel Example of a Chiral Azonia Dication. Org Lett. 2006;8:5955-8. (c) Garcia D, Cuadro AM, Alvarez-Builla J, Vaquero JJ. Sonogashira Reaction on Quinolizium Cations. Org Lett. 2004;6:4175-8. (d) Barchin BM, Valenciano J, Cuadro AM, Alvarez-Builla J, Vaquero JJ. Use of the Stille Coupling Reaction on Heteroaromatic Cations: Synthesis of Substituted Quinolizinium Salts. Org Lett. 1999;1:545-7. [27] Caneque T, Cuadro AM, Alvarez-Builla J, Vaquero JJ. Efficient functionalization of quinolizinium cations with organotrifluoroborates in water. Tetrahedron Lett. 2009;50:1419-22. [28] For leading references, see: (a) Molander GA, Ellis N. Organotrifluoroborates: Protected Boronic Acids That Expand the Versatility of the Suzuki Coupling Reaction. Acc Chem Res. 2007;40:275-86. (b)Molander GA, Figueroa R. Organotrifluoroborates: Expanding organoboron chemistry. Aldrichimica Acta. 2005;38:49-56. (c) Darses S, Genet J. Potassium Organotrifluoroborates: New Perspectives in Organic Synthesis. Chem Rev 2008;108:288-325. [29] For the synthesis of 1-bromo-, 3-bromo- and 4-bromoquinolizinium, see: (a) Sanders GM, Van Dijk M, Van der Plas HC. Synthesis and deuteration of some halogenoquinolizinium bromides. Heterocycles 1981;15(1):213-223. (b) For the synthesis of 2-bromoquinolizinium see: (b) Fozard A, Jones G. Quinolizines. V. The synthesis and properties of some hydroxyquinolizinium salts. J Chem Soc 1963:2203-2209. [30] (a) Miyadera T, Iwai I. The studies on quinolizinium salts. I. Synthesis of quinolizinium and 1-, 2-, 3-, and 4methylquinolizinium bromides. Chem Pharm Bull 1964;12:1338-43. (b) Sato K, Okazaki S, Yamagishi T, Arai S. The synthesis of azoniadithia[6]helicenes. J Heterocycl Chem. 2004;41:443-7. [31] Ishiyama T, Murata M, Miyaura N. Palladium(0)-Catalyzed Cross-Coupling Reaction of Alkoxydiboron with Haloarenes: A Direct Procedure for Arylboronic Esters. J Org Chem. 1995;60:7508-10. [32] Moylan CR, Twieg RJ, Lee VY, Swanson SA, Betterton KM, Miller RD. Nonlinear optical chromophores with large hyperpolarizabilities and enhanced thermal stabilities. J Am Chem Soc. 1993;115:12599-600. [33] (a) Jedrzejewska B, Pietrzak M, Paczkowski J. Solvent effect on the spectroscopic properties of styryl quinolinium dyes series. Dyes Pigm. 2013;97:278-85; (b) Li D, Yu D, Zhang Q, Li S, Zhou H, Wu J, TianY. Synthesis, crystal structure and third-order nonlinear optical properties in the near-IR range of a novel stilbazolium dye substituted with flexible polyether chains. Dyes Pigm. 2013;99:673-85; (c) Jędrzejewska B, Pietrzak M, Pączkowski J. Solvent Effects on the Spectroscopic Properties of Styrylquinolinium. Dyes Series. J. Fluoresc. 2010; 20:73-86. (d)Molecular Fluorescence: Principles and Applications; Valeur B. Wiley-VCH: Weinheim, 2002, Chapter 3, p.62 [34] Brouwer AM. Standards for photoluminescence quantum yield measurements in solution Pure Appl Chem. 2011;83:2213-28.
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[35] D'Andrade BW, Datta S, Forrest SR, Djurovich P, Polikarpov E, Thompson ME. Relationship between the ionization and oxidation potentials of molecular organic semiconductors. Org Electron. 2005;6:11-20. [36] (a) Alain V, Thouin L, Blanchard-Desce M, Gubler U, Bosshard C, Gunter P, et al. Molecular engineering of push-pull phenylpolyenes for nonlinear optics. Improved solubility, stability, and nonlinearities. Adv Mater. 1999;11(14):1210-4. (b) Rao VP, Jen AKY, Wong KY, Drost KJ. Dramatically enhanced second-order nonlinear optical susceptibilities in (tricyanovinyl)thiophene derivatives. J Chem Soc , Chem Commun. 1993:1118-20. (c) Cheng LT, Tam W, Stevenson SH, Meredith GR, Rikken G, Marder SR. Experimental investigations of organic molecular nonlinear optical polarizabilities. 1. Methods and results on benzene and stilbene derivatives. J Phys Chem. 1991;95:10631-43. [37] Gaussian 09, Revision B.1. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J,.Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Montgomery JA, Peralta JE Jr, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD , Farkas O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ. Gaussian, Inc., Wallingford CT, 2014 For details of these calculations, see reference 20a,b. [38] Head-Gordon M, Rico RJ, Oumi M, Lee TJ. 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Frequency-dependent hyperpolarizabilities in the coupled-cluster method: the Kerr effect for molecules. Chem Phys Lett. 1995;234:87-93. (c)Rice JE, Handy NC. The calculation of frequency-dependent hyperpolarizabilities including electron correlation effects. Int J Quantum Chem. 1992;43:91-118. [42] Wilberg N, Holeman AF, Wiberg E, Holleman-Wiberg E. Inorganic Chemistry. Eds. Walter de Gruyter GmbH & Co. Berlin, 2001;1757-59. [43] (a) Isborn CM, Leclercq A, Vila FD, Dalton LR, Bredas JL, Eichinger BE, Robinson, B. H. Comparison of Static First Hyperpolarizabilities Calculated with Various Quantum Mechanical Methods. J Phys Chem A. 2007;111:1319-27. (b) Carella A, Centore R, Fort A, Peluso A, Sirigu A, Tuzi A. Tuning second-order optical nonlinearities in push-pull benzimidazoles. Eur J Org Chem. 2004;12:2620-6. (c) Oudar JL, Chemla DS. Hyperpolarizabilities of the nitroanilines and their relations to the excited state dipole moment. J Chem Phys. 1977;66:2664-8. 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S0143-7208(17)30422-9
DOI:
10.1016/j.dyepig.2017.05.005
Reference:
DYPI 5967
To appear in:
Dyes and Pigments
Received Date: 28 February 2017 Revised Date:
1 May 2017
Accepted Date: 2 May 2017
Please cite this article as: Cañeque T, Cuadro AM, Custodio Raú, Alvarez-Builla J, Batanero Belé, Gómez-Sal P, Pérez-Moreno J, Clays K, Castaño O, Andrés JoséL, Carmona T, Mendicuti F, + + Vaquero JJ, Azonia aromatic heterocycles as a new acceptor unit in D-π-A vs D-A nonlinear optical chromophores, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.05.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Azonia aromatic heterocycles as a new acceptor unit in D-π-A+ vs D-A+ nonlinear optical chromophores Tatiana Cañeque,1 Ana M. Cuadro,1,* Raúl Custodio,1 Julio Alvarez-Builla,1 Belén Batanero,1 Pilar
Francisco Mendicuti4 and Juan J. Vaquero1,* 1
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Gómez-Sal,1 Javier Pérez-Moreno,2 Koen Clays,3 Obis Castaño,4 José L. Andrés,4 Thais Carmona,4
Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá, 28871-Alcalá de
Henares, Madrid, Spain. 2Department of Physics. Skidmore College, Saratoga Springs, NY. 3Department
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of Chemistry, University of Leuven, Celestijnenlaan 200 D,3001 Leuven, Belgium. 4Departamento de Química Analítica, Química Física e Ingeniería Química, Universidad de Alcalá, 28871 Alcalá de
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Henares, Madrid, Spain
[email protected], [email protected]
ABSTRACT: A comparison of D-π-A+ and D-A+ cationic chromophores based on the quinolizinium system as new acceptor units is reported along with the results of studies into their linear and non-linear
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optical properties and electrochemical data. Experimental and theoretical data show that quinoliziniumbased chromophores may provide a new generation of second-order non-linear materials with enhanced performance. The first hyperpolarizabilities were measured by Hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis. In some chromophores the absence of a
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bridge (D-A+) between the donor and acceptor fragments enhances the NLO properties and the single
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crystal structure of such a material has been determined by X-ray diffraction
KEYWORDS: NLO, quinolizinium acceptor unit, charged chromophores, azonia aromatic heterocycles, synthesis D-A, theoretical calculations
1
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GRAPHICAL ABSTRACT:
Research highlight
Design and synthesis of new D-π-A+ vs D-A+ chromophores based on quinolizinium cation Unbridged D-A+ chromophores with remarkable large beta values
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Cyclic voltammetry measurements compared with DFT calculations
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The work provides valuable hints for rational design of novel D-A+ materials
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Appendix A. Supplementary information (SI)
2
ACCEPTED MANUSCRIPT 1. Introduction There is considerable interest in organic non-linear optical (NLO) materials[1] with large second-order optical non-linearities in various fields (chemistry, physics, materials) because these materials have found diverse applications in optoelectronics[2], all-optical
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data processing technology[3], bioimaging[4], sensitizers for solar cells[5], and photodynamic therapy[6] amongst others. Generally, most effort has been focused on the design of new chromophores with large first hyperpolarizability values (β)[7], which are related to an electronic intramolecular charge transfer (ICT) due to the ease with which
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the molecular structures, and hence the properties, can be tuned for various applications. Second-order NLO organic materials are most often based on π-conjugated molecules
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(chromophores) with strong electron donor and acceptor groups at the ends of the π-conjugated structure[8]. Although π-electron delocalization is the most widely studied and exploited form of ICT, electrons are also known to delocalize over σ bonds[9] and this topic has barely been studied. Conjugated compounds have been the focus of most research within this field, but
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today it is recognized and accepted that ICT occurs through a synergistic combination of different phenomena[10]. Although a wide range of materials has been studied over the past few decades, the most recent approaches to the design of highly NLO-active systems is based on
Me2 N
Me2 N
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Me2N
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charged molecules with the presence of cationic acceptors.
Me2 N
NMe2
N Me + PF6 S +N
+ N
PF6
NMe2
PF Me
N +
N + PF
PF
N + PF
+ PF
N
N+ PF
Figure 1. Representative examples of nonlinear optical chromophores based on azolium and pyridiniun cations as acceptor units.
Cationic chromophores are of interest because they possess an easy tunability by counter ion exchange to crystallize into non-centrosymmetric structures[11]. Furthermore, by tuning their 3
ACCEPTED MANUSCRIPT solubility, potential applications in imaging can also be achieved. Other potential advantages of charged chromophores include their greater stability and higher chromophore number densities when compared with other NLO materials. In this context, charged acceptors units are referred to benzothiazolium[12] and azinium[13] (pyridinium, quinolinium) cations. These latter having
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interest in the generation of terahertz (THz) radiation[14] as 1D and 2D chromophores represented by DAST(4-(??,N-dimethylamino)-4´-N′-methyl-stilbazoliumtosylate)/DSTMS(4(??,N-dimethyl
amino-4′-??′-methyl-stilbazolium-2,4,6-trimethylbenzenesulfonate)[15]
and
diquats/helquats [16] (Figure 1). However, azonia aromatic heterocycles (AZAH)[17] have not
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been explored to date as an acceptor unit, except in our work, as a marker in nonlinear optical bioimaging, exhibiting a large two-photon absorption (2PA) and as push-pull cationic
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chromophores (D-π-A+-π-D)[18] (Figure 2). These kinds of heteroaromatic cations containing a quaternary bridgehead nitrogen are well represented in nature, forming part of a wide range of alkaloids[19]. Furthermore, some derivatives of these azonia salts have proven useful in important applications such as cyanine dyes and highly fluorescent compounds used as probes
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for the detection of biomolecules[20].
R = OMe, NMe 2
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Figure 2. Quinolizinium as acceptor unit in 1D (D-π-A+) and 2D (D-π-A+-π-D) chromophores.
As a continuation of our research on the applications of heteroaromatic cations in the NLO field[21], we explored the potential of new chromophores based on the quinolizinium cation, which may exhibit better non-linear optical properties (NLO) than other heteroaromatic cations tested to date, i.e., azinium[22]and azolium[23] salts. In addition, comparative studies on the NLO properties were carried out in order to investigate the role of quinolizinium as an acceptor and to establish the relevant structure activity relationships in simple dipolar molecules. This
4
ACCEPTED MANUSCRIPT was achieved by changing the nature of the π-bridge that connects the donor units and comparing these chromophores with unbridged ones (Figure 3). EDG
EDG
+
D
D
D-A+ EDG: electron-donating group
+
X
D-π-A+
+
+
D D-π-A+
N +
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D
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Figure 3. General structure of push-pull systems based on the quinolizinium cation.
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We report here our results on the use of the quinolizinium cation as an acceptor unit in push-pull chromophores in which the quinolizinium moiety is connected to an electron donor (D) through a π-conjugated bridge (D-π-A+) or directly (D-A+) by Pd-catalysed cross-coupling reactions (Figure 4). The aim was to gain a better understanding of these systems and to contribute to the recent interest in β-enhancement strategies[21a,24].
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The second-order NLO properties of the selected chromophores were evaluated and compared to experimental results and theoretical calculations previously carried out on azinium cations. The resulting D-A+ and D-π-A+ chromophores with the quinolizinium acceptor were studied by
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hyper-Rayleigh scattering experiments and by theoretical analysis, including Density Functional Theory (DFT) and correlated Hartree–Fock-based methods (RCIS(D)). In addition, the redox
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activity was evaluated by cyclic voltammetry and the linear optical properties related to the ICT characteristics of the D-A+ and D-π-A+ are included along with the X-ray structures of selected D-A+ chromophores.
2. Results and discussion 2.1 Synthesis The quinolizinium cation is the simplest model that offers four possible positions of substitution by palladium-mediated cross-coupling processes and these products exhibit markedly different
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ACCEPTED MANUSCRIPT behaviour from an electronic point of view at the C1/C3 and C2/C4 positions. Moreover, one would also expect a different steric effect on C1/C4 and C2/C3. Our strategy for the construction of push–pull molecules was to combine quinolizinium cations – employed as acceptor units (A+) – with donor units (D) through a π-bridge using the Sonogashira[25] and
reactions
using
either
the
appropriate
arylboronic
-
-
PF6
or
potassium
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R = OMe, NMe2
acids
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organotrifluoroborates[27,28](Figure4) .
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Heck reactions or to connect the units directly by Stille or Suzuki−Miyaura[26] coupling
Figure 4. Synthesis of cationic (D-π-A+) and D-A+) chromophores by Pd catalysed cross-coupling reactions.
The first series of quinolizinium-based chromophores was previously obtained by us from the four isomeric bromoquinolizinium cations[29] (1a–d) (Scheme 1), which were
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coupled with different arylalkynes under Sonogashira conditions[25]. This method allowed easy access for the first time to representative D-π-A+ quinolizinium-based chromophores and allowed the efficient incorporation of arylethynyl substituents into the quinolizinium system,
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particularly at the C2 and C3 positions (Scheme 1).
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10% CuI 5% Pd2Cl2(PPh3)2
Et 3N, DMF, 60 ºC or rt
1 1a = 1-Br 1b = 2-Br 1c = 3-Br 1d = 4-Br
2a (1-) 47% 2b (2-) 78% 2c (3-) 71% 2d (4-) 61%
3b (2-) 55%
2-5 X = Br, PF6 R = OMe, NMe2, Me, Br
-
-
4b (2-) 74% 4c (3-) 76%
5b (2-) 71% 5c (3-) 70%
Scheme 1. Synthesis of D-π-A+ chromophores 2-5 by Sonogashira reaction 6
ACCEPTED MANUSCRIPT In order to study and compare the NLO properties of the quinolizinium cation as an acceptor in D-π-A+ to D-A+, we discuss here previously published results on quinolizinium[18b] linked to C1-C4 with dimethylamino- and methoxystyryl derivatives. These compounds were synthesized by Heck reactions using the corresponding vinyl quinolizinium hexafluorophosphate and aryl
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iodides as reaction partners or, alternatively, by a more classical approach involving a Knoevenagel reaction between 2-methylquinolizinium hexafluorophosphate[30] and the corresponding aryl aldehyde in higher yields as indicated in Scheme 2. I N +_
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R N _+
6,7
R
PF6
PF6
N _+
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R
N _+
N _+
PF6
N _+
PF6
6b: R = NMe2(42%) 7b: R = OMe (81%)
6a: R = NMe2 (37%)
R
PF6
37-81%
PF6
6c: R = NMe2(42%) 7c: R = OMe (37%)
R R 6d: R = NMe2(42%)
CHO
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Me
N _+
PF6
R
MeCN Piperidin
6b: R = NMe2(92%) 7b: R = OMe (81%)
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Scheme 2. Synthesis of chromophores 6 and 7 by Heck and Knoevenagel reaction
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Although initial studies in our laboratory showed that the Stille[26] coupling reaction was the most efficient procedure in comparison to other palladium-catalysed reactions to produce high yields of some aryl- and heteroaryl-quinolizinium cations, we prepared the new quinolizinium derivatives 9 and 10 by the Suzuki–Miyaura reaction[31], which is a greener procedure according to conditions employed by us to synthesize heteroaryl quinolizinium derivatives[27] (Scheme 3). Optimization of the reaction conditions was carried out first with the 2-bromoquinolizium salt, with different bases, catalytic systems and temperatures evaluated. It was found that the best
7
ACCEPTED MANUSCRIPT catalytic system was Pd2(dba)3 as this allowed the reaction to be carried out at room temperature in conjunction with bulky phosphines: P(o-Tol)3 or (2-biphenyl)-di-tert-butylphosphine.
DMF, r.t. Method A or B
52-80% , -
1 H2O, 65º C Method C Method C
8%mol Pd(o-tol)3
8b: R = 3-OMe; 71% (C) 9b: R = 4-OMe; 52% (A); 73% (B); 76% (C) 10b: R = 4-NMe2: 73% (b); 80% (C)
1%mol, Pd(OAc)2;
8c: R = 3-OMe; 77% (C) 9c: R = 4-OMe; 60% (B); 80% (C) 10c: R = 4-NMe2: 60% (B)
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8a: R= 3-MeO; 93% (C)
Method B 4%mol, Pd2(dba)3;
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Method A 2%mol, Pd2(dba)3; 4%mol Pd(o-tol)3
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9-10
1b= 2-Br 1c= 3-Br
8d: R= 3-MeO; 63% (C)
Scheme 3. Suzuki–Miyaura coupling reaction with boronic acids and potassium organotrifluoroborates
Thus, the coupling between 1b and phenylboronic acid with electron-donating groups (OMe and
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NMe2) afforded the product 9b in moderate yield (52%) when Pd2(dba)3/P(o-Tol)3 in a ratio of 2%/4% (Method A) was used or in good yield for 9 and 10 with a 4%/8% ratio (Method B) in DMF in the presence of K2CO3 at room temperature for 16 hours (Scheme 3). The 3-
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bromoquinolizinium bromide (1c) was also subjected to Suzuki coupling in order to provide an overview of the reactivity of the C2 and C3 positions, which are electronically very different
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and similar to C4 and C1, respectively. Alternatively, coupling with potassium organotrifluoroborates, (Method C) an efficient reagent that allows functionalization of quinolizinium under mild conditions by reaction with 1b, afforded chromophores 9b–10b in higher yield (Scheme 3) and similar to 8a–d, which were previously synthesized by us[27].
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ACCEPTED MANUSCRIPT 2.2 Linear optical properties The UV-Vis spectra which are shown in the wavelength range where the main absorption bands appeared (Figures 5-8), were analyzed to evaluate the effectiveness of different electrondonating substituents (D) with different π-linkers in D-π-A+ vs D-A+ chromophores.
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The absorption spectra of a group of representative chromophores (2b–5b) bearing different electron-donating substituents (D) in position C2 of the quinolizinium (A+) with an acetylenic πlinker are represented in Figure 5. As expected, a good correlation was found between the electron-donating capabilities of the substituents and the relative position of the maximum of
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the ICT band (λabs,max). The electron-donating capability increased in the order Br- < Me- < MeO- < Me2N- and a corresponding bathochromic displacement of λabs,max was observed. In
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general, the λabs,max for compounds containing dimethylamino-substituents (Me2N–) shifted towards longer wavelengths (430 nm for 3b) with respect to those containing Br-substituents (341 nm for 5b).
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2b λmax=378 nm 3b λmax=430 nm
0.8
4b λmax=367 nm 5b λmax=341 nm
0.6 0.4
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Absorbance (a.u.)
1.0
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0.2
0.0 300
350
400
450
500
550
λ (nm)
Figure 5. Absorption spectra (normalized at the maximum of intensity) in the 300–580 nm range for 2– 5b in MeOH as a solvent at 25 °C.
The normalized absorption spectra of chromophores 6a–d with styryl moieties, which are collected in Figure 6, also showed the intense ICT absorption band attributed to the Me2NC6H4→QZ charge transfer. This band was significantly shifted to the red in compounds 6b (473 nm) and 6d (445 nm) when compared to chromophores 6a (425 nm) and 6c (433 nm). For the
9
ACCEPTED MANUSCRIPT former derivatives the Me2NC6H4– group was linked through a styryl moiety to the quinolizinium acceptor at the C2 and C4 positions, for the latter ones, however, the donors were located in the less activated C1 and C3 positions. This fact could be explained by a more efficient conjugation in positions 2 and 4 compared to those in 1 and 3, due to a combination of
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electronic and steric effects. A similar situation was observed for 2a–d and 8a–d, where the greatest stabilization took place for compounds b and/or d (see SI Figures S1 and S2).
The absorption spectra of chromophores 6b,c and 7b,c are also depicted in Figure 7. The ICT absorption bands were shifted to significantly longer wavelengths for compounds 6b,c
SC
substituted at C2- and C3-, when compared to 7b,c, by about 91 and 81 nm respectively. For unbridged chromophore D-A+ pairs 9b, 10b and 9c, 10c the displacements were approximately
M AN U
83 nm and 60 nm, respectively (see Figure S3 in the SI). For compounds 2b-3b, which contain acetylenic π-linkers, the maxima of the bands were displaced by around 52 nm. The stronger electron-donating character of Me2N– favors the stabilization of compounds that contain this substituent relative to those bearing MeO– regardless of the linker. The stabilization of
members of the series.
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compounds b with respect to c due to the position of the substituent was also evident for all
1.0
6a λmax=425 nm
AC C
Absorbance (a.u.)
EP
6b λmax=473 nm 6c λmax=433 nm
0.8
6d λmax=445 nm
0.6 0.4 0.2
0.0 300
350
400
450
500
550
600
650
λ (nm) Figure 6. Absorption spectra (normalized at the maximum of intensity) in the 300–680 nm range for chromophores 6a–d in MeOH at 25 °C.
10
ACCEPTED MANUSCRIPT
1.0
7b λmax=382 nm
Absorbance (a.u.)
7c λmax=352 nm 0.8
6b λmax=473 nm 6c λmax=433 nm
0.6
0.2 0.0
300
350
400
450
500
550
600
RI PT
0.4
650
SC
λ (nm) Figure 7. Absorption spectra (normalized at the maximum of intensity) in the 300–650 nm range of pushpull 7b-c and 6b-c chromophores in MeOH at 25ºC.
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A good correlation was also found between λabs,max for D-A+ unbridged chromophores 8b,c and 9b,c (see Figure S4 of the SI). The λabs,max for the methoxyphenyl groups in para-positions (9b,c) showed a bathochromic displacement of the ICT band relative to 8b,c by about 13 and 5 nm, respectively, because meta-substitution leads to less effective conjugation.
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Certainly, the para-substituted electron-rich MeOC6H4– and Me2NC6H4– groups at the C2 quinolizinium position (compounds 2b, 7b, 9b and 3b, 6b, 10b respectively) strongly favor DA+ conjugation stabilizing the ICT complex when compared to other series of studied
EP
compounds. For this reason, a more extensive photophysical study was performed on these compounds in order to gain a deeper insight into the linear properties and their relationship with
AC C
the non-linear ones.
The normalized absorption and emission spectra for the quinolizinium derivatives 10b, 6b and 3b, which have Me2NC6H4– as donors, are shown in Figures 8(a) and 8(b), respectively. The absorption spectrum of the olefinic chromophore 6b showed a more marked displacement of the ICT band to the red (472 nm) when compared to the acetylenic derivative 3b (447 nm) and the unbridged Me2NC6H4– chromophore 10b (431 nm). This particular trend was previously reported by Moylan et al.,[32] for a series of D-A chromophores connected by simple acetylenic, olefinic and aza linkers. Absorption spectra for the MeOC6H4- substituted 2b, 7b and
11
ACCEPTED MANUSCRIPT 9b derivatives showed a similar trend (see Figure S5 SI) although bands are less shifted to the red than their Me2NC6H4– substituted counterparts. The fluorescence spectra showed analogous features for the band displacements in both series of chromophores. The most red-shifted emission bands correspond to Me2NC6H4– donors
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containing compounds when compared to their counterparts, which have MeOC6H4- donors. However, the extent of such bathochromic changes depend on donor-linker structure. The absorption and emission maxima, fluorescence quantum yields and molar absorptivities for para-substituted MeOC6H4- and Me2NC6H4– quinolizinium derivatives are listed in Table 1.
SC
The absorption maxima λabs,max and λem,max were similar to those previously reported by us [18a,c]. The emission quantum yield and molar absorptivity values for some chromophores,
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however, showed some discrepancies but they were of the same order of magnitude. Thus, Stokes shifts of approximately 160, 123 and 171 nm for 10b, 3b and 6b and of 76, 147 and 123 nm for 9b, 2b and 7b, respectively, were found. Different behaviors may arise from the formation of TICT (Twisted intramolecular charge transfer) states in dimethylamino derivatives
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whose stability in polar media depends strongly on the type of donor to the quinolizinium acceptor linker and their internal rotation number. For this reason, as previously reported, the interpretation of some of the photophysical behaviors in these systems is complex[33]. The
EP
fluorescence quantum yields were lower for the Me2NC6H4-containing derivatives than for the MeOC6H4- ones. It is well known that the fluorescence quantum yields usually decrease with
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the electron substituent donating ability. The additional non-radiative deactivation excited state path arising from the dimethylamino twisting in the TICT may also contribute to this. Although with a less efficient electron donating ability, a surprisingly high fluorescence quantum yield was obtained for the unbridged MeOC6H4-containing chromophore 9b which, as expected, was decreased significantly for compounds 7b and 2b, which have olefinic and acetylenic π-linkers.
12
ACCEPTED MANUSCRIPT Table 1. Photophysical properties of the quinolizinium derivatives (see Figure 9) in methanol at 25 °C λexc (nm)
λem,max (nm)
φ
εmax (× 10-3 cm-1Mol-1L)
2b
378
360
525
0.01 ± 0.00a
31.8 ± 0.0
7b
386
360
509
0.02± 0.00a
14.9 ± 1.2
9b
360
360
436
0.35 ± 0.06a
42.1 ± 0.2
3b
447
430
570
< 0.001b
6b
472
460
643
0.006± 0000b
10b
431
420
591
0.08 ± 0.01b
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λabs,max (nm)
Comp.
37.0 ± 0.3 16.6 ± 0.2 36.4 ± 0.5
SC
Maximum absorption wavelength (λabs,max), excitation wavelength (λexc), emission maxima wavelength (λem,max), emission quantum yield (ϕ) and molar absorptivity at λabs,max (εmax). (a) Using Quinine Sulphate
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0.4
b)
0.8
EP
Absorbance (a.u.)
0.8
0.6
1.0
10b 6b 3b
a)
Fluorescence (a.u.)
1.0
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(φ=0.546) in H2SO4 0.1N or (b) coumarine 153 in ethanol (φ=0.38) as standards[34].
0.6
0.4
0.2
0.0
0.0
AC C
0.2
300 350 400 450 500 550 600
λ (nm)
500 550 600 650 700 750 800
λ (nm)
Figure 8. Absorption (a) and emission spectra (b), both normalized to 1 at the maximum of intensity, for 10b (black), 6b (red) and 3b (blue) in methanol at 25 °C.
13
OMe ACCEPTED MANUSCRIPT
OMe
OMe
N +_ PF6
N _ + PF6
N PF6 9b
7b
2b
N(Me)2 NMe2
N(Me)2
PF6
PF6 10b
6b
3b
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N
N +_ PF6
N _ +
SC
Figure 9. Selected chromophores for the study of linear and non-linear optical properties (Tables 1 and 2).
2.3 Electrochemical properties
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The electrochemical properties of selected quinolizinium salts (Figure 9) were studied by cyclic voltammetry (CV) in acetonitrile/LiClO4 as SSE (solvent-supporting electrolyte system). The cathodic and the anodic peak potentials, measured at a scan rate of 100mV/s and summarized in Table 2, are quoted with respect to the Ag/Ag+ (sat) reference electrode. These cationic chromophores are electroreducible to the corresponding stabilized radicals in the
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same potential range (Epc ∼ –1.2 V) in all cases due to the positive charge on the nitrogen atom of the quinolizinium ring. The reduction of the electrogenerated radical to the corresponding anion requires a more negative potential value and this was only observed in some cases.
EP
A relationship between the energy of the highest occupied molecular orbital (HOMO) and the
AC C
oxidation potential of a molecular organic semiconductor was described by Forrest[35]. The data
for
the
estimation
of
such
energy (EHOMO,
eV)
applied
to quinolizinium
hexafluorophosphates represented in Figure 9 are summarized in Table 2. These experimental results are consistent with theoretical computational calculations, with a very similar trend in both series of chromophores (9b>2b>7b>10b>3b>6b). The phenyl-substituted linker in these chromophores represents a conjugated system that extends the electron delocalization of the heterocycle. The HOMO is extended over the entire πsystem and mainly involves the heteroatom electron pairs of the substituent joined to the aromatic ring. For this reason, the oxidation is strongly influenced by the electron-donor 14
ACCEPTED MANUSCRIPT character of the substituents in the para-position of the phenyl group attached to the quinolizinium cation. Strongly donating substituents, such as dimethylamino (3b, 6b, 10b), lead to more negative reduction potentials (and less positive oxidation potentials) than those obtained when weaker donor substituents (2a, 7b, 9b) are present in the same position. The LUMO is
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located on the nitrogen atom in the quinolizinium cation core. This unit is only weakly influenced by the electron-donor character of substituents located at the end of the π-system and the reduction potentials for 3b, 6b, 10b, 2a, 7b and 9b are therefore very similar.
The oxidation and reduction of a functional group in a given compound proceeds more easily
SC
when it has a ‘lower’ (in absolute value) oxidation or reduction potential, respectively. Maps of the HOMO and LUMO orbitals of the chromophores under investigation show that reduction at
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the nitrogen atom in the quinolizinium cation involves the entire π-system when the reduction occurs. However, the more limited delocalization of the π-electron system in 4-methoxyderivative 9b results in a more negative reduction potential (and more positive oxidation potential) than those in 2b and 7b (see Figure 10). In turn this gives rise to a larger gap between
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the Eox and Ered values for 9b in comparison with 2a and 7b. The order of the HOMO-LUMO gap should be: 9b > 2a > 7b.
Compound 3b, in which the quinolizinium salt is substituted at the 2-position with an acetylenic
EP
chain joined to a 2-(4-dimethylamino phenyl) ring, provides a cyclic voltammetry Epa value of +0.94 V, which correlates with the expected reluctance of this compound to undergo oxidation
AC C
compared to the homologous ethylenic compound 6b (Epa = +0.7 V). Stabilization of the electrogenerated radical cation in 3b is more difficult than in 6b because it cannot be highly delocalized. The oxidation of 3b should take place directly at the nitrogen atom rather than the acetylenic moiety, as indicated in Scheme 4. Cyclic voltammetry results for 9b can be compared with those for 7b. The latter alkenylbridged chromophore is much easier to oxidize. The absence of a connecting chain between the quinolizinium ring and the aryl group in 9b (Epa = +1.85 V) produces a poorly stabilized radical cation (Scheme 5) where the contribution of the resonance
15
ACCEPTED MANUSCRIPT forms with an oxygen atom supporting the positive charge is tiny. In this case the aromatic ring is oxidized but is also slightly stabilized by the alkoxy group.
7b
+1.42
9b 3b
+1.85 +0.94
6b 10b
+0.71 +0.98
+1.57
+0.87 +1.32
-1.24
-6.56
-5.97
-1.35 -1.25
-7.19 -5.92
-6.25 -5.40
-1.28 -1.47
-5.58 -5.97
-5.15 -5.47
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Table 2. Experimental values of the peak potentials E (V, vs Ag/Ag+) (± 0.03V) of selected D-π-A+ and D-A+ chromophores (Figure 9). Scan rate: 100 mV/s. Energy Energy Comp. Epa1 Epa2 Epc ∆Eb/eV HOMOa/eV HOMOb /eV +1.67 -1.27 -6.94 -5.98 -2.52 2b -2.27
-2.28 -2.33
-2.21 -2.06
SC
a:Estimated from CV data (Epa1 values) by applying the relationship by Forrest[33].
Me
Me
N
N
Me
Me N
Me
-e N
N
N
PF6-
PF6-
3b
Me
Me
N
N Me
-e
-
N
N PF6-
Me
6b
PF6-
Me
Me
Me
N
N
Me
N
N
PF6-
PF6-
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PF6-
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b: B3LYP/6-31g(d) calculations including the solvent effect.
Me
OMe
-e N
9b
OMe
OMe
N
N
PF6-
PF6-
AC C
PF6-
EP
Scheme 4. Comparative delocalization D-π-A+ chromophores 3b and 6b
Scheme 5. Delocalization for chromophore 9b
The redox abilities of the aforementioned quinolizinium salts were investigated with the main focus on the first oxidation and first reduction potentials, since these values are related to the HOMO and LUMO energies, respectively. The electronic influence of individual substituents was followed in an effort to understand the relationship between redox properties and structure. The reduction of these chromophores occurs on the quinolizinium heteroatom and is
16
ACCEPTED MANUSCRIPT consequently not strongly influenced by the donor/acceptor character of the substituents. However, this process is rather dependent on the extended π-delocalized electron system involving stabilization of the electrogenerated radical intermediate. Whereas in the presence of strong donating groups these compounds are reduced more
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negatively (Er(10b) = –1.47V), the weaker donor substituents produce a shift to more positive reduction potentials (Er(9b) = –1.35V). The reduction of 9b therefore proceeds more easily than that of 10b.
The oxidation of these molecules occurs on the methoxy/amino group located at the para-
SC
position of the benzene ring, in opposite side. The oxidation potential strongly reflects the donor/acceptor character of the substituents, although the entire π-system chain is also
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considerable. On increasing the electron-donating character (Me2N related to MeO) and the extent of the conjugation in the molecule (cf. 6b and 10b), less positive oxidation potentials are
AC C
EP
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registered and this is consistent with the easier oxidation of these chromophores.
Figure 10. Cyclic voltammograms of 2b, 7b and 9b in acetonitrile/LiClO4 (0.1M) as SSE, at Pt as working and auxiliary electrodes. Ag/Ag+ (sat) reference electrode. Scan rate: 100 mV/s. (see SI voltammograms of 3b, 6b and 10b).
2.4 Non-linear Optical Properties: Hyper-Rayleigh Scattering (HRS) Measurements and Computational Details.
17
ACCEPTED MANUSCRIPT The results of Femtosecond hyper-Rayleigh scattering measurements performed at 800 nm on selected compounds are compiled in Table 3 and these confirm the potential of these small ionic compounds for second-order non-linear optics. Congruent with the trends already observed in the linear optical (absorption) experiments, the first hyperpolarizability of the compounds is
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higher when the methoxyphenyl group is present as the aryl donor and the most marked effect is seen when the quinolizinium is substituted in the C-2 position with a π-linker (2b) and in the DA+ unbridged chromophore 9b, which was the chromophore with the highest first
SC
hyperpolarizability value when compared with D-π-A+ compounds.
Table 3 Experimental non-linear Optical Properties of some Chromophores 2–10 λmax
βHRS
βzzz
350
122 ± 10
300 ± 20
60 ± 4
2b
378
284 ± 14
690 ± 40
152 ± 5
2c
350
245 ± 20
600 ± 40
112 ± 6
2d
380
202 ± 18
490 ± 40
37 ± 3
4b
367
380 ± 60
918 ± 140
110 ± 20
4c
334
74 ± 3
178 ± 7
45 ± 2
5b
350
186 ± 28
350 ± 70
86 ± 12
5c
341
34 ± 8
80 ± 12
18 ± 2
6a
425
142 ± 7
343 ± 17
31 ± 2
6b
472
199 ± 6
482 ± 51
49 ± 5
6c
433
191 ± 28
462 ± 49
56 ± 6
6d
445
83 ± 6
202 ± 42
38 ± 8
7b
382
118 ± 17
285 ± 41
19 ± 2
7c
353
162 ± 16
391 ± 39
70 ± 7
AC C
EP
TE D
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Comp. . 2a
βzzz, 0
8b
347
30 ± 7
72 ± 17
15 ± 3
9b
360
500 ± 50
1190 ±120
180± 20
9c
337
61 ± 7
147 ± 17
35 ± 4
10b
431
286 ± 30
450 ± 70
50 ± 7
Maximum absorption wavelength λmax (nm) predicted by CIS(D), resonance enhanced HRS first hyperpolarizability βHRS (10–30 esu), resonance enhanced diagonal component of the molecular first hyperpolarizability βzzz (10–30 esu), off-resonance diagonal component of the molecular first hyperpolarizability βzzz,0 (10–30 esu).
18
ACCEPTED MANUSCRIPT The first hyperpolarizability value in D-π-A+ chromophores is higher in compound 2b (β = 284.10–30 esu) than in 2c and 2d, which have β values of 245.10–30 esu and 202.10–30 esu, respectively. The replacement of the donor unit (N,N-dimethylaminostyryl) at the C2 position with a double bond resulted in 6b (β = 199.10–30 esu), the chromophore with the highest first
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hyperpolarizability value when compared with compounds in which quinolizinium was substituted at the C1, C3 and C4 positions with the same substituent. In agreement with the greater influence of the position of the donor substituent at the quinolizinium acceptor, which is more marked than the relative electron-donating strength characteristics, and the better
SC
conjugation and stabilization, the highest β values correspond to the compounds with substituents at the C2 positions, thus confirming that the best conjugation and charge transfer
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from donor to acceptor is achieved in these positions.
The experimental results reported here for cationic chromophores (D-A+), when compared to the (D-π-A+) systems reported previously by us, reveal that the hyperpolarizability of (D-A+) molecules seems to be determined by the strength of the charge-transfer. Thus, the highest first
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hyperpolarizability values were found for the dimethoxystyryl donor unit at the C2 (9b: β = 500·10–30 esu) and the N,N-dimethylaminostyryl donor unit at the C2 (10b: 286·10–30 esu). However, in the case of 9b, the common assumption that β can be maximized by using a
EP
conjugated bridge[36] to link D and A acceptor units is not clear because, surprisingly, the highest hyperpolarizability was found for 9b with dimethoxystyril donor unit connected directly to
AC C
quinolizinium acceptor when compared with 2b and 7b. Theoretical calculations present a similar
correlation for the resonant and off-resonant molecular hyperpolarizabilities βzzz and βzzz,0, respectively.
Theoretical calculations were performed using the Gaussian suite of quantum chemical programs[37]. For all of the studied cationic chemical systems, the molecular structure used to compute optical properties corresponded to an energy minimum calculated at the HF/6-31G(d) level of theory. These structures were confirmed as minima by evaluation of the harmonic vibrational frequencies, which were all real.
19
ACCEPTED MANUSCRIPT Table 4. Theoretical linear and nonlinear optical properties selected compounds λmax exp.
λmax Cal. CIS(D)
βHRS
βzzz,800
βzzz, 0
2a
350
312
83
197
79
2b
378
364
255
617
187
2c
350
323
131
-318
-126
2d
380
336
104
-276
-96
3b
447
449
233
429
297
9b
360
334
63
154
10b
431
414
209
506
RI PT
Comp.
64
152
Maximum absorption wavelength λmax (nm) predicted by CIS(D), resonance enhanced HRS first
SC
hyperpolarizability βHRS (10–30 esu), resonance enhanced diagonal component of the molecular first
hyperpolarizability βzzz (10–30 esu), off-resonance diagonal component of the molecular first
M AN U
hyperpolarizability βzzz,0 (10–30 esu).
Calculated wavelengths (λmax, in nm) were estimated by the CIS(D)[38] method, which generally provides accurate predictions of excited states that are mainly one-electron transitions from a single reference ground state. The λmax values were obtained as vertical excitations from the ground state to the first excited singlet.
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As CIS(D) is one of the simplest approaches amongst the correlated excited state theories, our calculations provided good results with wavelength values that differed by less than 10% with respect to the experimental data, as predicted in other
EP
investigations[39]. A more pronounced overestimation of the excitation energies by the quantum chemical methods was observed for TDDFT calculated values.
AC C
Theoretically predicted values of the second harmonic generation (SHG) of hyperpolarizability [40] β(-2ω;ω,ω) were obtained, including the electron correlation effect at the MP2 level. Molecular components of the first hyperpolarizability were computed as dipole moment second derivatives and these values were then spatially averaged to obtain the βHRS values[41]. Details of the mathematical procedure to obtain the hyper-Rayleigh hyperpolarizability from molecular first hyperpolarizabilities are reported elsewhere[21a,b]. Both the resonance βzzz and off-resonance βzzz,0 diagonal components of the molecular first hyperpolarizability were directly calculated as dipole moment derivatives rather than being estimated from the simple two-state model. As a consequence, comparisons could not be made between these two sets of data.
20
ACCEPTED MANUSCRIPT These MOs (Figure 11) clearly show the charge transfer from the donor to the acceptor fragments of these systems. The π-π* transition bands are due to the vertical excitation between these MOs (see SI Figure 11). The discrepancies between the theoretical energy gap (∆E) values and vertical excitation (λmax) for cation 9b shows deficiencies for this HF wavefunction that are not
M AN U
SC
RI PT
completely corrected in a third order property like hyperpolarizability.
Figure 11. HOMO and LUMO of cation 3b
2.5 Crystallographic studies
The crystal and molecular-ion structures of compounds 9b and 9c were obtained by X-
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ray diffraction studies (Figure 12). The PF6– counter ion in both cases is found in the crystal but omitted in tables and figures for the sake of clarity. Compound 9b crystallizes in the monoclinic P21/c space group and 9c in the triclinic P-1 space group, both of which are centrosymmetric.
EP
Selected bond distances and angles are shown in Table 5 (see SI) for both molecules. It is
AC C
interesting to note that the distance C(2)–C (11) in 9b and C (3)–C(11) in 9c are 1.485(5) and 1.481(4) Å, respectively. These are slightly shorter than the normal[42] C–C single bond (1.54(3) Å), which indicates π-conjugation between the donor and quinolizinium system. In compounds 9b and 9c the aryl group is nearly coplanar with the quinolizinium system, with angles between the planes of the donor and acceptor units of 11.87(15)° and 13.91(11)°, respectively. The practically coplanar arrangement in the chromophore corroborates the delocalization and charge transfer between the aryl and quinolizinium, which in turn explains the high hyperpolarizability values of D-A+ unbridged chromophores 9b – as predicted by the two-levels model[43]. 21
ACCEPTED MANUSCRIPT Observation of the dihedral angles between the phenyl and methoxy group for both molecules shows that for 9b the dihedral angle C(15)–C(14)–O(2)–C(17) is 1.5° and for 9c C(13)–C(14)–O(1)–C(17) is 6°. This difference probably indicates that the planarity of 9b is somewhat higher and this promotes better charge transfer (βHRS = 500±50 10–30
SC
RI PT
esu for 9b and βHRS = 61±7 10–30 esu for 9c).
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Figure 12. ORTEP drawings of compounds 9b and 9c. The PF6– counter ion has been omitted for clarity
EP
Figure 13. Weak intermolecular interactions compounds 9b and 9c
3. Experimental section
AC C
3.1 Materials and Methods
3.1.1 Chemistry. Reactions were monitored by thin-layer chromatography (TLC) using TLC silica gel-coated aluminium plates 60F-254 (Merck). Column chromatography was performed using Merck silica gel 60, 0.040–0.063 mm (230–400 mesh). NMR spectroscopy was performed on Varian UNITY-Plus 300 (1H and 75 MHz 13C) or VNMRS-500 (1H and 125 MHz 13
C) spectrometers equipped with a cryoprobe. Chemical shifts were reported as δ values (ppm)
and coupling constants (J) in Hz. The mass spectra (MS) as ESI+ and HRMS masses were recorded on a Thermo Scientific TSQ Quantum LC/MS equipped with an ESI ionization source and a TOF detector. All starting materials, namely Pd(OAc)2, CuI, PdCl2(PPh3)2, 4-methoxy22
ACCEPTED MANUSCRIPT and 4-N,N-dimethylamino- benzaldehyde, 1-ethynyl-4-methoxybenzene, and the corresponding potassium organotrifluoroborates, were purchased from Aldrich and were used without further purification. The synthesis of 1-bromo-, 2-bromo-. 3-bromo- and 4-bromoquinolizinium [28] and 2-Methylquinolizinium salts [29] were obtained by previously reported methods. For
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chromophores 2a–d, 4b,c and 5b,c see reference 24 and for the preparation of the (E)-2(arylvinyl)quinolizinium hexafluorophosphates by Heck reaction and Knoevenagel reaction see references 18b and 25a. The procedure with potassium organotrifluoroborates is described in reference 27.
SC
3.1.2 General optical properties. Absorption Spectra were recorded on a Perkin-Elmer L35 UVVis spectrophotometer in the 200–1000 nm range. Steady-state fluorescence measurements
M AN U
were performed by using an SLM 8100 AMINCO spectrofluorimeter equipped with polarizers and a double (single) concave grating monochromator in the excitation (emission) path and a cooled photomultiplier. Slit widths were set at 8 nm for excitation and emission and polarizers at the magic angle. Fluorescence decay measurements were performed on an Edinburgh
TE D
Instruments FL900 time-correlated single-photon-counting system.
The fluorescence quantum yield was calculated using equation 1 (E1): grad i ni2 2 grad s t ns t
φi = φs t
E1
EP
where φst is the fluorescence quantum yield of the standard (st) in each case, gradi/st are the slope
AC C
of the surface plot under the full emission spectrum versus absorption at the excitation wavelength of derivative i or standard, respectively, and ni/st are the refractive indexes of methanol and water (or ethanol), respectively. 3.1.3 General voltammetric measurements. Voltammetric curves with IR compensation were obtained with a Tracelab Radiometer POL150 polarographic analyser in a Voltalab MDE150 Polarographic Stand in a three-electrode cell. Two Pt wires were used as working and auxiliary electrodes. The electrochemical properties of the chromophores indicated in Figure 9 were studied by cyclic voltammetry (CV) in acetonitrile (ACN)/LiClO4 as SSE (solvent-supporting electrolyte 23
ACCEPTED MANUSCRIPT systems). Before the voltammetric experiments, the solutions were deaerated by bubbling N2 (99.98% purity, purchased from Air Liquid) for 5 minutes. The cathodic and anodic peak potentials measured at a scan rate of 100 mV/s (summarized in Table 2) are quoted with respect to the Ag/Ag+ (sat) reference electrode.
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3.1.4 Hyper-Rayleigh scattering. General details of the hyper-Rayleigh scattering (HRS) experiments were discussed previously[44]. The HRS measurements were performed at room temperature in methanol, with crystal violet as the reference molecule and with high-frequency demodulation of the multiphoton fluorescence contribution[45].
SC
The HRS signal was analyzed towards a single major dipolar hyperpolarizability tensor element βzzz along the molecular z-axis. The dynamic on resonantly enhanced βzzz,800 value obtained at
M AN U
800 nm was reduced to the static, or off-resonance βzzz.0, value by applying the classical twolevel model[39]. From the fitting of the apparent βzzz,800 as a function of modulation frequency, a fluorescence lifetime could be obtained along with the accurate fluorescence-free hyperpolarizability value[46]. The reported β values are the averages taken from measurements
TE D
at different amplitude modulation frequencies.
Theoretical calculations: The theoretical resonant or frequency-dependent hyperpolarizabilities were obtained at the MP2/6-31G(d) level and include the electron correlation effect in this NLO
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property βMP2(-2ω;ω,ω). This property was estimated from the resonant or dynamic β(-2ω;ω,ω) at the HF/6-31G(d) level, and the static or non-resonant β(0;0,0) were calculated at both the
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HF/6-31G(d) and MP2/6-31G(d) levels following the multiplicative approximation scheme, where the frequency dispersion is estimated to be similar at these two levels of theory[39a,47]. β MP 2 (− 2ω;ω , ω ) ≈ β HF (− 2ω;ω , ω )
β MP 2 (0;0,0) β HF (0;0,0)
The first hyperpolarizability β was calculated as a second dipole moment derivative with respect to electric fields. At the Hartree–Fock (HF) level these dipole derivatives were calculated analytically by solving the corresponding CPHF equations. When the electron correlation effect was included by means the perturbative MP2 approach, these dipole derivatives were obtained numerically. Calculated hyperpolarizabilities were obtained on 24
ACCEPTED MANUSCRIPT the molecular axes basis βijk and then transformed to the laboratory axes βXYZ by applying the expressions of Civin et al[48]. 3.1.5 Single-crystal X-ray structure determinations of 9b and 9c. Selected bond distances and angles are shown in Table 5 (see ESI) for both molecules. Details of the X-ray
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experiments, data reduction and final structure refinement calculations are summarized in Table 6 (see ESI). Single crystals suitable for X-ray diffraction were selected for data collection. The crystals were mounted on a glass fibre using an inert perfluorinated ether oil and placed in a low temperature N2 stream (200(2) K) in a Bruker-Nonius Kappa
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CCD single crystal diffractometer equipped with a graphite-monochromated Mo-Kα radiation source (λ = 0.71073 Å) and an Oxford Cryostream 700 unit. The structures
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were solved by direct methods (SHELXS-97) using the WINGX package[49] and completed by subsequent difference Fourier techniques and refined by using full-matrix least-squares against F2(SHELXL-97)[50]. All non-hydrogen atoms were anisotropically refined. Most of the hydrogen atoms were geometrically placed and left riding on their
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parent atoms, and others were found in the Fourier difference maps. In the case of 9b the PF6– ion was found to be within the crystal with evident structural disorder, which was modelled.
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Crystallographic data (excluding structure factors Å) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary
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publication nos. CCDC 1523732-1523733. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-033; email: [email protected]).
3.2 Synthesis 3.2.1
General
procedure
for
the
preparation
of
arylethynyl-quinolizinium
hexafluorophosphate. A flame-dried vial was charged under argon with 50 mg (0.173 mmol) of the corresponding bromoquinolizinium salt, 10 mol % CuI (3.3 mg, 0.0173 mmol) and 5 mol % PdCl2(PPh3)2 (6.9 mg, 0.0087 mmol) in dry DMF (2.5 mL). 125
ACCEPTED MANUSCRIPT Ethynyl-4-methoxybenzene (0.208 mmol, 27 µL) and Et3N (0.259 mmol, 45 µL) were added. After heating at 60 °C for 14 h, the solution was filtered through a small pad of Celite® and washed with methanol (5 mL). The solution was concentrated, treated with saturated solution NaHCO3 (20 mL) and extracted with EtOAc (15 mL). The organic
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phase was extracted with H2O (2 × 10 mL), the aqueous phase was treated with saturated solution NH4PF6 the resulting phase was extracted with CH2Cl2 (3 × 15 mL). The organic phase was dried over Na2SO4, the solvent evaporated under reduced pressure and the
(9.6:0.4) as eluent.
hexafluoro
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2-(4-N,N-Dimethylamino-phenylethynyl)quinolizinium
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product was purified by column chromatography on silica gel using CH2Cl2/MeOH
phosphate
(3b).
Following the general procedure, from 4-ethynyl-N,N-dimethylaniline (0.2077 mmol, 30 mg) stirring at room temperature for 5 h and using CH2Cl2/MeOH (9:1) afforded 39.7 mg (55%) of 3b as a brown-red solid: mp 273–277 °C; IR (KBr) νmax (cm–1) 3345, 2190, 1644, 1600, 1375,
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837. 1H-NMR (500 MHz, Acetone-d6) δ (ppm) 9.30 (d, J = 7.0 Hz, 1H), 8.56 (d, J = 1.5 Hz, 1H), 8.52 (d, J = 8.6 Hz, 1H), 8.43 (ddd, J = 8.6, 7.2, 1.0 Hz, 1H), 8.13–8.06 (m, 1H), 7.51 (d, J = 9.0 Hz, 1H), 6.80 (d, J = 9.0 Hz, 1H), 3.07 (s, 3H). 13C-NMR (125 MHz, acetone) δ 154.61,
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139.99, 139.58, 139.21, 136.67, 135.63, 129.52, 127.65, 126.03, 114.46, 108.82, 107.31, 88.16,
AC C
41.81. HRMS (ESI-TOF, MeOH) Calcd. for C19H17N2 [M+]: 273,1386 [M+] Found: 273.1384.
3.2.2 General procedure for the preparation of the quinolizinium salts (9–10). Suzuki reaction
Method A. A flame-dried 10 mL vial was charged under argon with the corresponding quinolizinium bromide (50 mg, 0.173 mmol), 2 mol % of Pd2(dba)3 (3.2 mg, 0.0035 mmol), 4 mol % P(o-Tol)3 (2.1 mg, 0.0069 mmol), 1.4 equiv. of K2CO3 (36.8 mg, 0.242 mmol), and 1.3 equiv. of the boronic acid (0.225 mmol) in dry DMF (2.5 mL). After stirring at room temperature as indicated, the reaction mixture was filtered through a small pad of Celite® and washed with MeOH (5 mL). The solution was concentrated, treated with H2O (15 mL) and 26
ACCEPTED MANUSCRIPT extracted with AcOEt (10 mL). The organic phase was extracted with H2O (2 × 10 mL), the aqueous phase was treated with saturated solution NH4PF6 and the resulting phase was extracted with CH2Cl2 (3 × 15 mL). The organic phase was dried over Na2SO4, the solvent was evaporated under reduced pressure and the product was purified by column chromatography on silica gel
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using CH2Cl2/MeOH (9.6:0.4) as eluent. Method B. 4 mol % of Pd2(dba)3 (6.3 mg, 0.0069 mmol), 8 mol % of P(o-Tol)3 (4.2 mg, 0.0138 mmol).
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2-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9b). From 4-methoxyphenyl boronic (34.2 mg, 0.225 mmol) and stirring the reaction mixture for 23 h afforded 34.5 mg
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(52%, method A) and 47.8 mg (73%, method B) of 9b as a white solid. mp. 230–232 °C. IR (KBr): υmax (cm–1) 3122, 2933, 1645, 1606, 1405, 1178, 836, 558. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.38 (d, 1H, J = 7.3 Hz), 9.31 (d, 1H, J = 6.8 Hz), 8.90 (d, 1H, J = 2.0 Hz), 8.59–8.52 (m, 2H), 8.40 (td, 1H, J = 8.5, 1.1 Hz), 8.14–8.04 (m, 3H), 7.21 (d, 2H, J = 9.0 Hz),
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3.93 (s, 3H). 13C NMR (75 MHz, Acetone-d6) δ (ppm) 163.3, 148.3, 144.2, 137.6, 137.4, 136.9, 130.1, 127.9, 127.1, 123.7, 122.2, 122.1, 115.9, 56.0. MS (ESI+) m/z (relative intensity) 236 (M+, 100). Anal. Calcd. For C16H14F6NOP (381.26 g/mol) C (50.41), H (3.70), N (3.67) Found:
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C (50.49), H (3.85), N (3.54).
3-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9c). Following Method B, from 4-
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methoxyphenylboronic acid (34.2 mg, 0.225 mmol) and stirring the reaction mixture for 22 h afforded 39.6 mg (60%) of 9c as a pale yellow solid: 217–218 °C. IR (KBr): υmax (cm–1) 3113, 2942, 1609, 1509, 1401, 1292, 1257, 1203, 835, 558. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.70 (s, 1H), 9.42 (d, 1H, J = 6.8 Hz), 8.80 (dd, 1H, J = 9.0, 1.8 Hz), 8.69 (d, 1H, J = 9.5 Hz), 8.65 (d, 1H, J = 10.1 Hz), 8.45 (ddd, 1H, J = 9.7, 7.3, 1.1 Hz), 8.21 (t, 1H, J = 6.0 Hz), 7.95 (d, 2H, J = 8.8 Hz), 7.19 (d, 2H, J = 8.8 Hz), 3.91 (s, 3H). 13C NMR (75 MHz, Acetone-d6) δ (ppm) 162.6, 142.5, 137.6, 137.5, 137.3, 136.7, 133.9, 129.9, 128.2, 128.0, 126.6, 125.3, 116.0, 55.9.
27
ACCEPTED MANUSCRIPT MS (ES+) m/z (relative intensity) 236 (M+, 100). Anal. Cal. For: C16H14F6NOP (381.26 g/mol) C (50.41), H (3.70), N (3.67) Found : C (50.64), H (3.53), N (3.78). 2-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10b). Procedure B, from 4-N,N-dimethylaminophenylboronic (37.1 mg, 0.225 mmol), and stirring the reaction
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mixture for 20 h, gave 49.7 mg (73%) of 10b as a red solid. mp: 242–244 °C (Acetone:Et2O). IR (KBr): υmax (cm–1) 2927, 1649, 1598, 1406, 1214, 842, 559. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.24 (d, 1H, J = 7.3 Hz), 9.17 (d, 1H, J = 6.6 Hz), 8.77 (d, 1H, J = 2.2 Hz), 8.47 (dd,
1H, J = 7.1, 2.2 Hz), 8.44 (d, 1H, J = 10.4 Hz), 8.27 (td, 1H, J = 9.9, 1.1 Hz), 8.03 (d, 2H, J =
SC
9.2 Hz), 7.91 (td, 1H, J = 6.8, 1.0 Hz), 6.94 (d, 2H, J = 9.2 Hz), 3.12 (s, 6H). 13C NMR (75
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MHz, Acetone-d6) δ (ppm) 153.8, 149.0, 144.4, 137.3, 136.7, 136.5, 129.7, 127.7, 122.6, 121.3, 120.8, 119.5, 113.1, 40.0. MS (ESI+) m/z (relative intensity) 249 (M+, 100). Anal. Calcd. for C17H17F6N2P (394.30 g/mol) C (51.78), H (4.35), N (7.10), found C (51.64), H (4.50), N (7.22). 3-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10c). Method B, from 4-N,N-dimethyl aminophenylboronic acid (37.1 mg, 0.225 mmol), stirring the reaction mixture
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for 30 h, gave 41.0 mg (60%) of 10c as an orange solid: mp 229–230 °C (Acetone:Et2O).IR (KBr): υmax (cm–1) 2930, 1605, 1511, 1401, 1216, 842, 559. 1H NMR (300 MHz, Acetone-d6) δ (ppm) 9.62 (s, 1H), 9.37 (d, 1H, J = 6.6 Hz), 8.77 (dd, 1H, J = 9.2, 1.8 Hz), 8.61 (d, 1H, J = 8.8
EP
Hz), 8.58 (d, 1H, J = 6.4 Hz), 8.36 (td, 1H, J = 9.7, 1.1 Hz), 8.15 (t, 1H, J = 7.1 Hz), 7.85 (d, 2H, J = 9.0 Hz), 6.94 (d, 2H, J = 9.0 Hz), 3.07 (s, 6H).
13
C NMR (75 MHz, Acetone-d6) δ
AC C
(ppm). 152.7, 141.8, 137.9, 137.1, 136.3, 135.9, 132.1, 129.0, 127.9, 127.8, 125.1, 120.4, 113.4, 40.1. MS (ES+) m/z (relative intensity) 249 (M+, 100). Anal. Calcd. for C17H17F6N2P (394.30 g/mol) C (51.78), H (4.35), N (7.10). Found: C (51.90), H (4.55), N (7.02).
3.2.3 General procedure for the preparation of the quinolizinium with organotrifluoroborates (9b and 10b) Method C. The corresponding potassium organotrifluoroborate (0.2076 mmol, 1.2 equiv) was added to a mixture of the corresponding bromoquinolizinium bromide (0.1730 mmol), K2CO3
28
ACCEPTED MANUSCRIPT (0.0857 g, 0.5190 mmol, 3 equiv), 1 mol % of Pd(OAc)2 (0.0004 g, 0.00017 mmol) in H2O (2.5 mL). The reaction mixture was heated at 60–65 °C with stirring for the time indicated (2–4.5 h). The mixture was then cooled to room temperature and treated with saturated solution NH4PF6. The resulting solid was filtered off and washed with water and diethyl ether. For a further
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purification, the solid was chromatographed on silica gel using CH2Cl2/MeOH) (9:1) as eluent. 2-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9b). Following the general procedure and using potassium 4-methoxyphenyltrifluoroborate (44.4 mg) gave 49.8 mg (76%) of 9b as a white solid.
SC
2-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10b). Following the general procedure and using potassium 4-(N,N-dimethylamino)phenyltrifluoro borate (47.1 mg)
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gave 54.5 mg (80%) of 10b as a red solid.
2-(4-Methoxyphenyl)quinolizinium hexafluorophosphate (9b). Following the general procedure and using potassium 4-methoxyphenyltrifluoroborate (44.4 mg), were obtained 49.8 mg (76%) of 9b as a white solid.
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2-(4-N,N-Dimethylaminophenyl)quinolizinium hexafluorophosphate (10b). Following the general procedure and using potassium 4-(N,N-dimethylamino )phenyltrifluoro borate (47.1
4. Conclusions
EP
mg), were obtained 54.5 mg (80%) of 10b as a red solid.
AC C
In summary, we have prepared a series of D-A+ and D-π-A+ chromophores based on quinolizinium using diffferent palladium-promoted C–C bond formation reactions. The optical properties of these chromophores were investigated by absorption and emission spectroscopy. The spectra of compounds with electron-rich substituents at the C2 position of the quinolizinium show a strong effect on the stabilization energy of the chromophores and a bathochromic effect is observed in the absorption spectrum, which is consistent with a better conjugation or a lower energy gap. The fluorescence spectra also presented displacements with a similar trend.
29
ACCEPTED MANUSCRIPT The electrochemical behaviour of the chromophores is mainly affected by the electronic nature of the donor/acceptor character of the substituents, although the entire π-system chain is also considerable. On increasing the electron-donating character and the extent of conjugation of the molecule, less positive oxidation potentials were registered. This finding is consistent with the
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easier oxidation of these compounds. The calculated HOMO/LUMO energies and their differences correlate with the electrochemical measurements obtained by cyclic voltammetry. The experimental first hyperpolarizabilities (βHRS) of the series of charged chromophores show that compounds 2b, 3b, 9b and 10b have significantly higher values than the rest of the
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synthesized cationic systems. The low transition energy and high CT level are the decisive factors that provide a higher first hyperpolarizability in compounds substituted at C2.
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Theoretical calculations support the experimental results and also predict a similar trend. However, chromophore 9b showed some discrepancies between the theoretical calculation and experimental results. Further investigation of 9b by X-ray analysis revealed significant planarization.
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In comparison to previously reported D-π-A+ molecules[21a], the lack of a conjugated bridge in these D-A+ chromophores does not seem to be significant in terms of obtaining higher βHRS values, with the magnitude of β in this kind of chromophore being dominated by additive
EP
contributions from the donors and the acceptors. The results highlight an attractive strategy in guiding the design of new NLO materials, with unbridged D-A+ chromophores allowing a
AC C
remarkable degree of control of the linear and non-linear optical response by changing the position and electronic nature of the substituents and, more importantly, the nature of the linker.
Acknowledgements
Financial support from the Spanish Ministerio de Ciencia y Competitividad (projects MICINN/CTQ2011-24715 and MINECO/ CTQ2014-52488-R and CTQ2015-64425-C2-1-R and CTQ2016-80600-P). Instituto de Salud Carlos III (FEDER funds, ISCIII RETIC REDINREN RD012/20021/0014) are gratefully acknowledged
30
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