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Large nonlinear optical activity of chromophores with divinylquinoxaline conjugated π-bridge
Accepted Manuscript Title: Large nonlinear optical activity of chromophores with divinylquinoxaline conjugated -bridge Authors: Alexey A. Kalinin, Sirina M. Sharipova, Timur I. Burganov, Alina I. Levitskaya, Olga D. Fominykh, Tatyana A. Vakhonina, Nataliya V. Ivanova, Ayrat R. Khamatgalimov, Sergey A. Katsyuba, Marina Yu. Balakina PII: DOI: Reference:
S1010-6030(18)31164-X https://doi.org/10.1016/j.jphotochem.2018.10.034 JPC 11550
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
13-8-2018 15-10-2018 18-10-2018
Please cite this article as: Kalinin AA, Sharipova SM, Burganov TI, Levitskaya AI, Fominykh OD, Vakhonina TA, Ivanova NV, Khamatgalimov AR, Katsyuba SA, Balakina MY, Large nonlinear optical activity of chromophores with divinylquinoxaline conjugated -bridge, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.10.034 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.
Large nonlinear optical activity of chromophores with divinylquinoxaline conjugated π-bridge
Alexey A. Kalinin,* Sirina M. Sharipova, Timur I. Burganov, Alina I. Levitskaya, Olga D. Fominykh, Tatyana A. Vakhonina, Nataliya V. Ivanova, Ayrat R. Khamatgalimov, Sergey A.
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Katsyuba, Marina Yu. Balakina
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* Corresponding author
E-mail addresses: [email protected] (A.A. Kalinin); [email protected] (S.M.
(O.D.
[email protected]
Fominykh); (N.V.
Burganov);
[email protected]
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[email protected]
(T.I.
[email protected]
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[email protected]
Ivanova);
[email protected]
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Sharipova);
(A.I.
Levitskaya),
(T.A.
Vakhonina);
(A.R.
Khamatgalimov);
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[email protected] (S.A. Katsyuba); [email protected] (M.Yu. Balakina)
Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS,
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Arbuzov Str. 8, 420088 Kazan, Russia
Graphical abstract
d33, pm/V
120 100 80
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60 40 20
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0 0
10
20
30
40
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Chromophore load (wt%) in PMMA
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Synthesis of D-π-A chromophores with quinoxaline core in the π-bridge High thermal stability and first hyperpolarizability High values of NLO coefficients
ABSTRACT
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Highlights
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Two chromophores with π-deficient quinoxaline (Q) core in the π-electron bridge together with
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dibutylaniline (DBA) donor moiety and tricyanofuranyl (TCF) or dicyanovinyl (DCV) acceptor moieties have been synthesized to investigate their macroscopic nonlinear optical (NLO) activity in guest-host materials. A distinctive feature of the 7-DBA-VQPhV-TCF chromophore is
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predominance of negative solvatochromism over the positive one, which is quite rarely met for the chromophores manifesting inverse solvatochromic behavior as well as the blue shift of the absorption maximum compared to the chromophore FTC with divinylthiophene π-bridge. Introduction of quinoxaline moiety in the conjugated bridge promotes the increase of thermal stability: Td of both compounds is higher than 295 oC. Density functional theory was used to calculate the chromophores first hyperpolarizability (β). Quadratic NLO response characterized
by d33 coefficient was measured by SHG technique for thin guest-host polymer films. The influence of chromophore content on the NLO coefficient of 7-DBA-VQPhV-TCF/PMMA film has been investigated and the d33 value of this film reaches 108 pm/V at 25 wt% load. The poled film 7-DBA-VQPh-DCV/PMMA shows twice smaller d33 values (30 pm/V) at the same
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chromophore load (20 wt%).
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Keywords: NLO Chromophore, divinylquinoxaline π-bridge, solvatochromic behavior, TD-DFT
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calculations, NLO coefficient
1. Introduction Nonlinear optical (NLO) materials have attracted much attention due to their potential at the construction of photonic devices; progress in these investigations contributes to the development of technologies, such as optical communication and data storage, as well as optical
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computing [1-4]. At the creation of organic NLO materials push-pull (D-π-A) chromophores [5,6], which are the source of the NLO response at the molecular level, represent important
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structural elements. Up to now a variety of chromophores were designed and their NLO characteristics were studied by HRS technique or EFISH techniques [7-17]. Modification of πbridge of D-π-A chromophores to increase the NLO activity on the one hand, and to obtain a set
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of useful characteristics on the other hand, represents promising areas in the design and
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development of such compounds and guest-host materials on their basis [18]. Significant
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progress in the creation of NLO chromophores and chromophore-based materials occurred at the
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turn of the century when studying the properties of chromophores with extended
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divinylthiophene and octatetraene π-bridges, in particular, the FTC [19] and CLD [20] ones. The introduction of aromatic heterocycles as an important design element into the π-bridge of D-π-A
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compounds results in maintaining or even increase of the first hyperpolarizability value [14], thermal [16,21] and photostability [22], and also affects the absorption maximum, which in turn
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may influence the optical transparency of the NLO material. In addition to chromophores with thiophene moiety in the π-bridge [15-19,21-26], NLO activity of polymer materials doped with chromophores having divinylpyrrole π-bridge [18,27] has been investigated. More complex
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structure of the chromophores π-bridge included several [16,17] or fused [28,29] thiophene moieties. Composite materials doped with the above-mentioned chromophores showed a good electro-optic (EO) activity (from 30 to 150 pm/V). Excellent transformation of molecular NLO activity into the macroscopic one resulting in further increasing of the NLO (EO) response (up to 300 pm/V and above) has been achieved mainly due to combination of various factors, which
include the introduction of well-chosen isolation groups in the chromophores [30,31] preventing their aggregation, supramolecular organization [32], rational selection of donor and acceptor strength of chromophore end-groups [33,34], as well as the design of more complex chromophore-containing structures (for example, bichromophores [35]), for which the abovementioned factors are valid. Here we investigate nonlinear properties of another type of
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chromophores which do not contain π-excessive heterocyclic moiety in the π-bridge (such as thiophene or any other), but are based on π-deficient quinoxaline core, which acts as an auxiliary acceptor and consequently the whole chromophore bridge has a negative π-charge (Fig. 1). For
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chromophores with such π-bridge in combination with strong donor and acceptor end units high values of the first hyperpolarizabilty with inessential conformational dependence have been
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predicted [36-38].
Fig. 1. Chromophores under study and total π-populations on quinoxaline moiety and on the π-
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bridge as a whole.
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2. Experimental section
2.1. Materials and instrumentation
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1-Phenylpropane-1,2-dione, 4-bromo-1,2-benzenediamine, N,N-dibutylaniline, 3-hydroxy-3methyl-2-butanone and malononitrile, were purchased from Acros or Alfa Aesar. Organic solvents used were purified and dried according to standard methods. 2-Dicyanomethylene-3cyano-4,5,5-trimethyl-2,5-dihydrofuran (Me-TCF) was obtained by condensation of 3-hydroxy3-methyl-2-butanone
with
malononitrile
in
pyridine
[39].
7-Bromo-3-methyl-2-
phenylquinoxaline was obtained according to the literature [40]. The melting points (mp) of
chromophores were determined by the differential scanning calorimetry (DSC); the values are presented in Table 1. The mp of compounds 1 and 2 were determined on a Boetius hot-stage apparatus. Infrared (IR) spectra were recorded on the Bruker Vector-22 FT-IR spectrometer. All NMR experiments were performed with Bruker AVANCE-600 and AVANCE-400 (600 and 400 MHz for 1H NMR, 150 and 100 MHz for 13C NMR) spectrometers. Chemical shifts (δ, in ppm)
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are referenced to the solvent CDCl3. The mass spectra were obtained on Bruker UltraFlex III MALDI TOF/TOF instrument with p-nitroaniline as a matrix. Electronic absorption (UV-Vis)
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spectra were registered at room temperature on a Perkin-Elmer Lambda 35 spectrometer with a scan speed of 480 nm/min using a spectral width of 1 nm. All samples were prepared as solutions in four solvents: dioxane (E = 2.25), chloroform (E = 4.81), 1,2-dichloromethane (E =
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8.93), and acetonitrile (E = 37.5), with the concentrations ranging from ~10–6 to ~10−4 molL-1,
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and placed in 10 mm quartz cells. The IR, NMR, UV-Vis, MALDI spectra were registered on the
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equipment of Assigned Spectral-Analytical Center of FRC Kazan Scientific Center of RAS. The
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thermal stabilities of chromophores were investigated by simultaneous thermal analysis
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(thermogravimetry/differential scanning calorimetry - TG/DSC) using NETZSCH (Selb, Germany) STA449 F3 instrument. Approximately 2.2-3.5 mg of samples were placed in an Al
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crucible with a pre-hole on the lid and heated from 30 to 500 C. The same empty crucible was used as the reference one. High-purity argon was used with a gas flow rate of 50 mL/min.
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TG/DSC measurements were performed at the heating rates of 10 K/min.
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2.2. Computational methods As in our earlier studies devoted to the estimation of molecular NLO characteristics of
newly synthesized chromophores, we have performed DFT calculations of dipole moments and molecular
polarizabilities
of
7-DBA-VQPh-DCV
and
7-DBA-VQPh-TCF
by
DFT.
Chromophores structure was obtained in the course of full optimization with both B3LYP [41,42] and M06-2X [43,44] density functionals using 6-31G(d) basis set; the calculations of the
electric characteristics are performed by TD-DFT [45] at the M06-2X/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ computational levels, thus giving the following three approaches: M062X/6-31G(d)//M06-2X/aug-cc-pVDZ (A), B3LYP/6-31G(d) //M06-2X/aug-cc-pVDZ (B), B3LYP/6-31G(d) //B3LYP/aug-cc-pVDZ (C) (the designation left to the double slash standing for the level used for geometry optimization, that to the right – for electric properties); the
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detailed justification for choosing these computational schemes is presented elsewhere [36]. In spite of the fact that B3LYP density functional is known to overestimate the values of first
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hyperpolarazability [46,47], we have used it for the calculations of β for the sake of comparison with available data for recognized and novel promising chromophores synthesized by other research groups.
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The hyperpolarizability value 𝛽𝑡𝑜𝑡 is calculated as follows
1 ( ikk kik kki ),i x, y , z. 3 ik
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tot x 2 y 2 z 2 ; i iii
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For the data analysis we use “theoretical” convention here, assuming that the chromophore’s
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dipole moment is expanded in Taylor series with respect to local electric field. The coordinate frame was chosen to align z-axis along the dipole moment vector of the chromophore.
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The values of π-electron charges for both chromophores are calculated at B3LYP/6-31G(d)
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computational level. Natural population analysis (NPA) was carried out to compute atomic charges [48]. All calculations were performed with the Jaguar program package [49,50].
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2.3. Syntheses of dyes 2.3.1. E-7-(4-(Dibutylamino)styryl)-3-methyl-2-phenylquinoxaline (1) A mixture of 2-phenyl-3-methyl-7-bromoquinoxaline (400 mg, 1.34 mmol), N,N-dibytyl4-vinylaniline (310 mg, 1.34 mmol), tri(о-tolyl)phosphine (4.0 mg, 0.013 mmol), Pd(OAc)2 (1.5 mg, 0.007 mmol), Et3N (0.33 g, 0.33 mmol), and anhydrous DMF (1 mL) was stirred for 4 h at
120°С. The reaction mixture was cooled, poured into water, and extracted with CH2Cl2. The organic layer was separated, washed with water, dried over anhydrous MgSO4, filtered. The solvent was removed at reduced pressure, and the residue was purified by silica gel column chromatography (eluent petroleum ether – EtOAc, gradient from 100:1 to 20:1). Yield 430 mg (72%), yellow-orange powder. Rf = 0.43 (hexane : ethyl acetate = 3:1); mp 82-84 °C. 1H NMR
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(600 MHz, CDCl3) δ: 8.06 (s, 1H, H-8 quinoxaline), 7.96 (d, 1H, J = 8.7 Hz, H-5 quinoxaline), 7.94 (dd, 1H, J = 8.7, 2.0 Hz, H-6 quinoxaline), 7.67 (d, J = 7.7 Hz, 2H, o-Ph), 7.58-7.40 (m, 3H,
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m,p-Ph), 7.44 (d, J = 8.6 Hz, 2H, 3,5H aniline), 7.23 (d, J = 16.2 Hz, 1H, H ethene), 7.05 (d, J = 16.2 Hz, 1H, H ethene), 6.65 (d, J = 8.6 Hz, 2H, 2,6-H aniline), 3.31 (t, J = 7.6 Hz, 4H, NCH2), 2.76 (s, 3H, CH3), 1.65-1.55 (m, 4H, NCH2CH2), 1.45-1.32 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 13
C NMR (150 MHz, CDCl3) δ: 155.0, 151.1, 148.3, 141.6, 140.6, 139.6,
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7.4 Hz, 6H, CCH3).
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139.3, 131.8, 128.9, 128.8, 128.5, 128.18, 128.16, 127.8, 125.3, 124.0, 122.4, 111.6, 50.8, 29.5,
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24.2, 20.3, 14.0. MALDI-TOF: 450 [M+H]+. IR (max, cm-1, KBr): 3921 (С-Н), 2955 (С-Н),
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2927 (С-Н), 2871 (С-Н), 1599 (С=N, С=С), 1552, 1521, 1489, 1469, 1448, 1424, 1403, 1371,
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1347, 1288, 1250, 1219, 1183, 1149, 1138, 1110, 1031, 1004, 959, 927, 895, 848, 832, 805, 764.
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2.3.3. (E)-7-(4-(Dibutylamino)styryl)-2-phenylquinoxaline-3-carbaldehyde (2)
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A mixture of quinoxaline 1 (200 mg, 0.45 mmol), selenium dioxide (57 mg, 0.51 mmol) and dioxane (3 mL) was stirred at 100 oC for 1 h under argon and then cooled to room temperature. After removal of the solvent by rotary-evaporation, the residue was purified by
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silica-gel column chromatography (eluent: methylene chloride) to give a product as dark red powder (180 mg, 87%). Rf = 0.47 (hexane/ethyl acetate 3:1); mp 111-113 °С. 1H NMR (600 MHz, CDCl3): δ 10.28 (s, 1H, СН=О), 8.20 (d, J = 8.8 Hz, 1Н, H-5 quinoxaline), 8.09 (s, 1H, H8 quinoxaline), 8.07 (dd, J = 8.9, 2.0 Hz, 1H, H-6 quinoxaline), 7.74-7.67 (m, 2Н, о-Ph), 7.597.54 (m, 3Н, m,p-Ph), 7.46 (d, J = 8.5 Hz, 2Н, 3,5-H aniline), 7.35 (d, J = 16.0 Hz, 1H, -
CH=CH-), 7.07 (d, J = 16.0 Hz, 1H, -CH=CH-), 6.66 (d, J = 8.5 Hz, 2Н, 2,6-H aniline), 3.32 (t, J = 7.6 Hz, 4H, NCH2), 1.66-1.57 (m, 4H, NCH2CH2), 1.43-1.34 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.4 Hz, 6H, CCH3).
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C NMR (150 MHz, CDCl3) δ: 191.1, 155.2, 148.8, 143.83, 143.81,
143.5, 140.7, 136.9, 134.0, 130.2, 129.8, 129.7, 129.2, 128.7, 128.6, 124.8, 123.4, 121.6, 111.7, 50.8, 29.5, 20.35, 14.0. MALDI-TOF: 464 [M+H]+. IR (max, cm-1, KBr): 3029 (С-Н), 2955 (С-
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Н), 2927 (С-Н), 2858 (С-Н), 1701 (С=О), 1596 (С=N, С=С), 1553, 1517, 1488, 1465, 1427, 1398, 1350, 1317, 1288, 1277, 1254, 1220, 1187, 1148, 1111, 1037, 1013, 968, 923, 889, 833,
(E)-2-((7-(4-(Dibutylamino)styryl)-2-phenylquinoxalin-3-yl)methylene)
malononitrile
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2.3.2.
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808.
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(7-DBA-VQPh-DCV)
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A solution of aldehyde 2 (77 mg, 0.17 mmol) and malononitrile (11 mg, 0.17 mmol) in
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anhydrous methylene chloride (1 mL) was stirred at room temperature for 48 h. After removal of the solvent by rotary-evaporation, the residue was purified by silica-gel column chromatography
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(eluent: petroleum ether /methylene chloride = 1:11:9) to give a product as black-violet
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powder (70 mg, 82%). Rf = 0.51 (hexane/ethyl acetate 3:1). 1H NMR (400 MHz, CDCl3) δ: 8.13 (d, J = 8.9 Hz, 1Н, H-5 quinoxaline), 8.02 (dd, J = 8.9, 1.7 Hz, 1Н, H-6 quinoxaline), 7.97 (d, J
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= 1.7 Hz, 1H, H-8 quinoxaline), 7.89 (s, 1H, CH=C(CN)2), 7.65-7.57 (m, 5Н, Ph), 7.43 (d, J = 8.8 Hz, 2Н, 3,5-H aniline), 7.30 (d, J = 16.1 Hz, 1H, -CH=CH-), 6.99 (d, J = 16.1 Hz, 1H, CH=CH-), 6.64 (d, J = 8.8 Hz, 2H, 2,6-H aniline), 3.33 (t, J = 7.7 Hz, 4H, NCH2), 1.66-1.55 (m,
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4H, NCH2CH2), 1.45-1.32 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.4 Hz, 6H, CCH3). 13C NMR (100 MHz, CDCl3) δ: 155.1, 153.7, 148.9, 144.4, 144.0, 140.5, 140.1, 136.5, 134.4, 130.2, 129.9, 129.7, 129.6, 129.1, 128.9, 124.6, 123.2, 121.4, 114.1, 112.4, 111.6, 87.2, 50.9; 29.5, 20.3, 13.4. MALDI-TOF: 512 [M+H]+. IR (max, cm-1, KBr): 3043 (С-Н), 2953 (С-Н), 2865 (С-Н), 2228
(CN), 1593 (C=C, C=N), 1515, 1482, 1429, 1395, 1363, 1332, 1282, 1253, 1228, 1185, 1155, 1111, 1014, 968, 957, 916, 893, 827, 813, 766. 2.3.4.
(E,E)-2-(3-Cyano-4-(2-(7-(4-(dibutylamino)styryl)-2-phenylquinoxalin-3-yl)vinyl)-5,5-
dimethyl-2,5-dihydrofuran-2-ylidene)malononitrile (7-DBA-VQPhV-TCF) A mixture of aldehyde 2 (33 mg, 0.07 mmol), Me-TCF (14 mg, 0.07 mmol) and
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anhydrous ethanol (2 mL) was refluxed for 14 h, then cooled to room temperature. After removal of the solvent by rotary-evaporation, the residue was purified by silica-gel column
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chromatography (eluent: petroleum ether /methylene chloride = 1:1methylene chloride) to give a product as black powder (27 mg, 59%). Rf = 0.16 (hexane/ethyl acetate 3:1). 1H NMR
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(400 MHz, CDCl3) δ: 8.10-8.04 (m, 2Н, H-5,6 quinoxaline), 8.02 (s, 1H, H-8 quinoxaline), 7.92
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(d, J = 15.8 Hz, 1Н, -CH=CH-TCF), 7.84 (d, J = 15.8 Hz, 1Н, -CH=CH-TCF), 7.69-7.63 (m, 2Н,
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о-Ph), 7.62-7.56 (m, 3Н, m,p-Ph), 7.46 (d, J = 8.6 Hz, 2Н, 3,5-H aniline), 7.34 (d, J = 16.2 Hz,
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1Н, -CH=CH-DBA), 7.06 (d, J = 16.2 Hz, 1Н, -CH=CH-DBA), 6.67 (d, J = 8.6 Hz, 2Н, 2,6-H aniline), 3.33 (t, J = 7.7 Hz, 4H, NCH2), 1.67 (s, 6H, CH3), 1.66-1.56 (m, 4H, NCH2CH2), 1.44-
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1.33 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.4 Hz, 6H, CCH3).
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C NMR (150 MHz, CDCl3) δ:
174.9, 173.0, 155.7, 148.9, 143.3, 143.0, 141.8, 141.4, 137.2, 133.8, 130.0, 129.8, 129.7, 129.4,
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129.0, 128.8, 128.3, 124.8, 123.3, 121.6, 119.7, 111.7, 111.4, 110.6, 109.8, 102.2, 97.8, 58.7,
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50.8, 29.5, 26.2, 20.3, 14.0. MALDI-TOF: 645 [M+H]+. IR (max, cm-1, KBr): 3071 (С-Н), 2956 (С-Н), 2930 (С-Н), 2871 (С-Н), 2228 (CN), 1582 (C=C, C=N), 1553, 1511, 1478, 1429, 1394,
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1375, 1322, 1303, 1222, 1208, 1181, 1140, 1105, 1040, 1020, 958, 925, 828.
3. Results and discussion 3.1. Synthesis and thermal stability Scheme 1 shows the synthesis of chromophores 7-DBA-VQPh-DCV and 7-DBA-VQPhVTCF with vinylquinoxaline and divinylquinoxaline π-bridge, respectively. The Heck reaction
between N,N-dibutyl-4-vinylaniline and 7-bromo-3-methyl-2-phenylquinoxaline gave the compound 1 as trans-isomer. The time (4 h) of this coupling procedure with π-deficient bromoquinoxaline derivative is significantly less compared to that for similar reaction (72 h) of π-excessive phenothiazine or carbazole derivatives [51]. There are various oxidative procedures for the alkyl acyl transformation in alkylquinoxaline derivatives [52-54]. The oxidation of
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quinoxaline 1 by selenium dioxide leads to aldehyde 2. The Knoevenagel сondensation of compound 2 with malononitrile or Me-TCF without using the base results in the chromophores
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7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF.
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Scheme 1. Synthesis of chromophores 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF
The studied chromophores are identified as highly crystalline compounds with a mp at 229 °C
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and 168 °C for DBA-VQPhV-TCF and DBA-VQPh-DCV, respectively (Table 1). Figure 2a shows the thermogravimetric curves for the studied chromophores according to which they have high thermal stabilities: the decomposition temperatures (Td), at which 5% mass loss occurs at heating, are above 295 °C for both chromophores. However, according to DSC data there is an exothermic peak (the second one) on the DSC curve that can correspond to processes either not related to decomposition (e.g. polymorphism) or related to decomposition but proceeding without mass loss (e.g.
oligomerization). To clarify this, the following experiment was carried out: each chromophore sample was heated on a TG/DSC device in an aluminum crucible to 270 °C (what is higher than that corresponding to the peak on the DSC curve), and then it was cooled. The physicochemical characteristics of the sample after heating did not match those of the original chromophore. Thus, DSC data show that the decomposition temperature of chromophores 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF is 225 °C
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and 234 °C, respectively (Table 1).
Table 1 Thermal properties of the studied chromophores.
Td, °Cb
234
mp, °C
229
295 (369)
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225 168
TGA (at which 5% (10%) mass loss occurs at heating). DSC.
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b
295 (354)
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a
Td, °Ca
7-DBA-VQPh-DCV
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Chromophore 7-DBA-VQPhV-TCF
a
b
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Fig. 2. TGA curves (a) and DSC curves (b) for 7-DBA-VQPhV-TCF and 7-DBA-VQPh-DCV.
The decomposition temperature of 7-DBA-VQPhV-TCF (234 °C) is close to that for FTC chromophore having DBA (220 °C) [24] or diethyl aniline (244 °C) [23] donor moieties and to that for julolidine-based chromophores [27,55].
3.2. Photophysical properties UV-Vis electronic absorption spectra of the chromophores 7-DBA-VQPh-DCV and 7DBA-VQPhV-TCF exhibit broad and highly intensive the lowest-energy absorption bands in the visible region of spectra (λmax of ca. 550-640 nm, see Figure 3, Table 2. To shed light on nature of the main electronic absorption bands quantum-chemical calculations of first 15 electronic
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transitions have been performed. Calculations demonstrated that the longest-wavelength absorption maxima could be ascribed to the transition with at least partial intramolecular charge-
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transfer (ICT) from DBA to DCV/TCF moieties, (see Figure S1 and Table S1 in ESI). The positions of these bands (λmax) in the spectra of chromophore 7-DBA-VQPhV-TCF are redshifted relative to the spectra of chromophore 7-DBA-VQPh-DCV due to the increased acceptor
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strength of TCF compared to DCV. Depending on the solvent polarity the compounds
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demonstrate reversible solvatochromic shifts up to ca. 0.2 eV (Table 2). Increase of the solvent
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polarity in the series from 1,4-dioxane to chloroform causes a clear red shift of λmax by ca. 0.15-
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0.2 eV while in more polar solvents, such as dichloromethane and acetonitrile, all chromophores
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demonstrate reversed blue shift of λmax (Table 2). Similar behavior has been previously described for a number of D-π-A systems with various divinylhetaryl π-bridge [23,27,28,38,56,57].
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A distinctive feature of the chromophore 7-DBA-VQPhV-TCF is the predominance of
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negative solvatochromism in passing from chloroform to acetonitrile solutions over positive one found in passing from dioxane to chloroform solutions, this being a rare phenomenon. In contrast, stronger positive solvatochromic effect relative to the negative one is typical for TCF-
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based chromophores with divinylthiophene [15,18,23,31,34], divinylpyrrole [18,27] and divinylthienothiophene [28] π-bridges combined with dialkyl aniline [23], diaryl pyrrole [15], tetrahydroquinoline [34] and julolidine [27,28,31,34] donor end groups. Comparison of the data of the spectra of 7-DBA-VQPhV-TCF, 7-DBA-VQPh-DCV (Table 1) with the spectra of the related systems bearing quinoxalinone moiety instead of quinoxaline one [57] reveals moderate blue shift of λmax by ca. 0.1 eV (26 nm) and ca. 50% decrease of ε values for 7-DBA-VQPhV-
TCF, 7-DBA-VQPh-DCV in the every solvent used, relative to their quinoxalinone analogs. Slightly greater hypsochromic shift of λmax by ca. 0.13 eV (47 nm) is found for the chromophore 7-DBA-VQPhV-TCF in comparison with FTC chromophore having DBA donor moiety and divinylthiophene π-bridge [24]. Blue-shifted absorption of chromophores without the loss of their molecular NLO properties can improve optical transparency of NLO materials based
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on such chromophores [28].
Table 2 Summarized experimental and calculated data on photophysical and thermal properties of the chromophores.
7-DBAVQPhV-TCF
CH3CN
N
CH2Cl2
A
546/2.27 594/2.09 582/2.13 552/2.25 (30500) (28500) (28800) (27300) [125/0.54] [149/0.54] [147/0.55] [146/0.60] 589/2.11 638/1.94 619/2.00 578/2.15 (29900) (27000) (28100) (30300) [162/0.60] [186/0.58] [188/0.62] [191/0.74]
M
7-DBAVQPh-DCV
1,4-dioxane CHCl3
ED
compound
U
λmaxa, nm/eV (ε, M-1 cm-1), [FWHM]b
a
Δλmaxc, nm/eV
48/0.18
60/0.19
DFT
computational leveld
A B C A B C
µ, D
βtot, 10-30 esu
11.4 12.4 14.0 18.2 19.5 21.6
360 421 832 688 875 1983
position of the maximum of the longest-wave electronic absorption band presented in nm/eV. FWHM – full-width at half maximum is presented in nm/eV c difference between the highest and lowest λmax in nm/eV. d
PT
b
A
CC E
M06-2X/6-31G(d)//M06-2X/aug-cc-pVDZ (A), B3LYP/6-31G(d) //M06-2X/aug-cc-pVDZ (B), B3LYP/6-31G(d) //B3LYP/aug-cc-pVDZ (C).
7-DBA-VQPh-DCV
7-DBA-VQPhV-TCF
1,2
1,4-dioxane CH2Cl2
1,0
1,4-dioxane CH2Cl2
1,2
CHCl3
CHCl3
1,0
CH3CN
CH3CN
0,8
a.u.
0,6
0,4
0,6
0,4
0,2
0,2
0,0
0,0
300
400
500
600
700
800
900
300
400
, nm
500
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a.u.
0,8
600
700
800
900
1000
, nm
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Fig. 3. UV-Vis experimental spectra of 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF in different solvents.
U
3.3. DFT calculations: molecular NLO activity and π-populations on the π-bridges
N
Earlier the TCF acceptor was proved to be an efficient one, thus the replacement of
A
tricyanovinylene moiety by TCF could lead to a substantial increase of optical nonlinearity [19].
M
Our research performed for the related chromophores with 3,7-(di)vinylquinoxaline-2-one
ED
(VQonV) bridge also demonstrated that the incorporation of TCF acceptor resulted in the essential increase of dipole moment and first hyperpolarizability values in comparison with those
PT
for chromophores with DCV acceptor moieties [36]. Table 2 demonstrates that the same observation is valid for 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF chromophores. According
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to the presented data, first hyperpolarizability for 7-DBA-VQPhV-TCF is approximately twice higher than the corresponding values for the 7-DBA-VQPh-DCV chromophore, this tendency persisting regardless of the used calculation scheme. Comparison of the estimations for 7-DBA-
A
VQPh-DCV and 7-DBA-VQPhV-TCF chromophores with those obtained earlier for related compounds gives evidence to the effect of various structural components on the value of first hyperpolarizability. In particular, the replacement of VQonV bridge by the VQPhV one results in the close values of first hyperpolarizability for the chromophores with TCF acceptor, while for those with DCV acceptor and VQPhV bridge βtot values are notably smaller: by ca 30% and 16%
for B3LYP/M06-2X and M06-2X/M06-2X, respectively [58] than βtot values for chromophores with VQonV bridge). We have compared the first hyperpolarizability values obtained for the novel chromophores under study with those published before for the chromophores with thiophene moieties in the π-bridge and TCF acceptors, which got a reputation of effective chromophores.
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For this purpose we have used the results obtained with B3LYP density functional which is often exploited by researchers to perform estimations of μ and β of newly synthesized chromophores.
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The calculation of βtot performed at the B3LYP/aug-cc-pVDZ´ level gave very high value for 7DBA-VQPhV-TCF, 1983⋅10−30 esu, which is more than twice as high as βtot for FTC [58], CLD [58] and julolidine-based chromophores [31,34], at the same time it is ca 17% higher than
U
for the chromophore with VQonV π-bridge [57].
N
Net π-populations, calculated for heterocyclic moiety and the chromophore π-bridge as a
A
whole for quinoxaline-based chromophores and some other chromophores with various
M
divinylhetaryl π-bridges, are presented in Figure 1 and Table 3. π-Excessive thiophene moiety makes π-bridge an auxiliary donor. Thiazole moiety as a part of the bridge in D-π-A
ED
chromophores is conventionally assumed to be auxiliary acceptor; however, as it can be seen
PT
from Table 3, thiazole moiety is rather an auxiliary donor than an acceptor. Even π-deficient quinoxalinone moiety, containing one pyridine-type nitrogen atom and oxygene atom of
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carbonyl group as acceptor sites and one pyrrole-type nitrogen atom as donor site, can’t be treated as auxiliary acceptor, since net π-charge on it is close to zero, what gives grounds to treat Qon as promoting the electron density transfer from donor to acceptor, not withdrawing any
A
density. However, net π-charge on chromophores bridge with π-deficient quinoxaline moiety is found to be negative, thus quinoxaline moiety acts as an auxiliary acceptor opposite to the case of thiazole and even quinoxalinone. It is worth stressing, that only the presence of two pyridinetype nitrogen atoms in this moiety provides this feature.
Table 3 Chromophores with π-populations on the π-bridge. Chromophores
b
a
+0.129
+0.125
b
+0.058
+0.081
c
+0.056
+0.077
d
-0.010 [57]
+0.035 [57]
e
-0.092
-0.097
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-Chargeb
on heterocyclic moiety of the π-bridge. on the π-bridge as a whole.
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a
-Chargea
N
U
3.5. NLO performance
To study the macroscopic NLO activity the guest-host polymer materials were prepared
M
A
with PMMA as polymer matrix (Tg = 105 C) and 7-DBA-VQPhV-TCF and 7-DBA-VQPh-DCV chromophores as guests. Thin polymer films containing 20 wt % of the 7-DBA-VQPh-DCV
ED
chromophore together with those doped by 7-DBA-VQPhV-TCF chromophore with various loads (10, 15, 20, 25 and 30 wt%) were spin cast from 7% cyclohexanone solution. Films were
PT
poled at the corona-triode setup in the corona discharge field, voltage 6.5 kV, poling time ~20
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min, the distance from the tungsten needle electrode to the surface of the film being 1 cm; the field was applied to the films heated to temperatures close to glass transition temperature, Tg. The quality of orientation was controlled by the absorption change in UV-Vis spectra (Fig. 4)
A
detected before and after poling, and characterized by order parameter, η, calculated by the following formula: 1 A A , 0
where A and A0 are the absorptions of the polymer films after and before poling [59].
0,35
15% 15%
before poling after poling
15wt % 0,30 0,25
A
0,20 0,15 0,10 0,05 0,00 600
500
700
800
1000
900
, nm
a 0,8
30wt %
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0,6 0,5
A
30% 30%
before poling after poling
0,7
IP T
400
0,4 0,3
U
0,2
0,0 400
500
600
N
0,1
700
800
900
1000
0,45 0,40
b
M
0,50
A
, nm
20wt %
before poling after poling
20% 20%
ED
0,35
A
0,30 0,25
A
CC E
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0,20 0,15 0,10 0,05 0,00
400
500
600
700
800
900
1000
, nm
c
Fig. 4. UV-Vis electronic absorption spectra: registered before and after poling (a-c) for 7DBA-VQPhV-TCF/PMMA films with chromophore load 15 (a) and 30 wt% (b), for 7DBA-VQPhV-DCV/PMMA film with chromophore load 20 wt% (c).
Polymer films surfaces were studied by AFM technique before and after poling, the surface images and films roughness are presented in Figure 5. As it can be seen, after poling the film roughness somewhat increases (Fig. 5 c, d).
c
U
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a
d
N
b
M
A
Fig. 5. AFM surface images and film roughness for polymer film 7-DBA-VQPhV-TCF/PMMA (25wt%) before (a,c) and after (b,d) poling.
Polymer NLO coefficients were measured by SHG technique; pulse Nd3+:YAG laser radiation
ED
(λ=1064 nm, pulse duration 15 ns, power density at the sample 10 kW/cm2; α-quartz (х-cut plate served as a standard) was used. In the course of measurements it was assumed that d33/d13 ≈ 3.
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The NLO coefficients, order parameters, film thicknesses of obtained composite
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materials are summarized in Table 4. The d33 values for the 7-DBA-VQPh-DCV/PMMA film is 30 pm/V. At the same chromophore load (20 wt%) NLO coefficient for 7-DBA-VQPhVTCF/PMMA film increases twice reaching 61 pm/V. So, higher first hyperpolarizability for
A
chromophore 7-DBA-VQPhV-TCF as compared with that for chromophore 7-DBA-VQPh-DCV (Table 1) provides the better NLO activity at material level.
Table 4 NLO coefficients (d33, pm/V),*order parameters (), and film thickness (h, nm). h, nm
d33, pm/V
7-DBA-VQPh-DCV/PMMA (20)
400
0.24
30
7-DBA-VQPhV-TCF/PMMA (10)
325
0.40
19
7-DBA-VQPhV-TCF/PMMA (15)
366
0.35
57
7-DBA-VQPhV-TCF/PMMA (20)
360
0.30
61
7-DBA-VQPhV-TCF/PMMA (25)
350
0.35
108
7-DBA-VQPhV-TCF/PMMA (30)
450
0.20
76
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*d33 measurements accuracy is about 15%
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Sample (chromophore load, wt%)
Table 4 demonstrates the influence of chromophore content on the NLO coefficient of the
U
7-DBA-VQPhV-TCF/PMMA film. One can see that guest–host polymer with 25 wt% of
N
chromophore displays maximum NLO activity, d33 being equal to 108 pm/V. Further increase of
A
the load up to 30 wt% leads to the decrease of d33 value. Similar trend was obtained for FTC
M
guest-chromophore – maximum EO activity was demonstrated at 25 wt% load in APC, the value
material [60].
ED
of 39 pm/V being achieved [23], the same conclusion was obtained for EZ-FTC/PMMA
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The obtained d33 value of 108 pm/V may be considered rather high, in particular, for the material with a representative of a novel class of NLO chromophores with quinoxaline moiety in
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π-bridge. We believe that optimization of the chromophore structure of 7-DBA-VQPhV-TCF by introducing the isolation groups or making other modifications will result in further increase of
A
nonlinear activity.
4. Conclusion Two chromophores with π-deficient quinoxaline core in the π-electron conjugated bridge connecting dibutylaniline donor and tricyanofuranyl/dicyanovinyl acceptor moieties have been synthesized and their photophysical, thermal and NLO properties were systematically investigated. Both chromophores exhibit positive solvatochromic effect when passing from 1,4-
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dioxane to chloroform and also demonstrate negative solvatochromism when passing from acetonitrile to chloroform, herewith the rare case - domination of the latter over the former has
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been demonstrated for the chromophore 7-DBA-VQPhV-TCF; the latter also shows the strong blue shift of the absorption maximum relative to the FTC chromophore with divinylthiophene πbridge. Chromophores showed excellent thermal stability with the decomposition temperatures
U
above 295 C. For the first time the NLO activity of qinoxaline-based chromophores as guests in
N
the composite materials is studied. The d33 values of poled films containing 20 wt% of these new
A
chromophores doped in PMMA were 30 and 61 pm/V at 1064 nm for chromophores 7-DBA-
M
VQPh-DCV and 7-DBA-VQPhV-TCF, respectively. The influence of chromophore content on
ED
the NLO coefficient of the 7-DBA-VQPhV-TCF/PMMA film was also investigated: the d33 values were shown to be equal to 19, 57, 61, 108 and 76 pm/V at 10, 15, 20, 25 and 30 wt% load,
PT
correspondingly.
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Acknowledgements
Financial support of Russian Science Foundation (grant no. 16-13-10215) for the study of
A
NLO activity of quinoxaline-based chromophores is gratefully acknowledged. The authors are thankful to Prof. V.I. Kovalenko for helpful discussion of the chromophores thermal properties.
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[47] K. Garrett, X. A. Sosa Vazquez, S. B. Egri, J. Wilmer, L.E. Johnson, B.H. Robinson, C.M. Isborn, Optimum exchange for calculation of excitation energies and hyperpolarizabilities of organic electro-optic chromophores, J. Chem. Theor. Comput. 10 (2014) 3821–3831.
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[48] A.E. Reed, R.B. Weinstock, F. Weinhold, Natural-population analysis, J. Chem. Phys. 83 (1985) 735–746.
[49] Schrödinger release 2017-3. LLC, New York, NY: Jaguar, Schrödinger; 2017. [50] A.D. Bochevarov, E. Harder, T. F. Hughes, J.R. Greenwood, D.A. Braden, D.M. Philipp, D. Rinaldo, M.D. Halls, J. Zhang, R.A. Friesner, Jaguar: a high performance quantum
chemistry software program with strengths in life and materials sciences, Int. J. Quant. Chem. 113 (2013) 2110–2142. [51] M.J. Cho, J.Y. Kim, J.H. Kim, S.H. Lee, L.R. Dalton, D. H. Choi, Heterocyclic nonlinear optical
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cyanomethylene-3-cyano-4,5,5- trimethyl-2,5-dihydrofuran acceptor, Bull. Korean Chem.
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Soc. 26 (2005) 77–84. [52] H. Aissaoui, M. Clozel, T. Weller, R. Koberstein, T. Sifferlen, W. Fischli, Quinoxalinone-3-
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[53] V.A. Mamedov, A.A. Kalinin, A.T. Gubaidullin, O.G. Isaikina, I.A. Litvinov, Synthesis and functionalization of 3-ethylquinoxalin-2(1H)-one, Russ. J. Org. Chem. 41 (2005) 599–606.
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[54] V.A. Mamedov, A.A. Kalinin, A.T. Gubaidullin, I.A. Litvinov, Ya.A. Levin, The Kornblum
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reaction of α-substituted 3-benzyl-1,2-dihydro-2-oxoquinoxalines. Synthesis and structure of
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3-benzoyl-2-oxo-1,2-dihydroquinoxaline, Chem. Heterocycl. Compd., 2002, 38, 1504–1510
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[Khim. Geterotsikl. Soedin. 38 (2002) 1704–1710]. [55] H. Wang, F. Liu, Yu. Yang, Huajun Xu, C. Peng, S. Bo, Z. Zhen, X. Liu, L. Qiu, Synthesis
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[56] A.A. Kalinin, S.M. Sharipova, T.I. Burganov, Yu.B. Dudkina, A.R. Khamatgalimov, S.A. Katsyuba, Yu.H. Budnikova, M.Yu. Balakina, Push-pull isomeric chromophores with vinyl-
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[57] O.D. Fominykh, A.A. Kalinin, S.M. Sharipova, A.V. Sharipova, T.I. Burganov, M.A. Smirnov, T.A. Vakhonina, A.I. Levitskaya, A.A. Kadyrova, N.V. Ivanova, A.R. Khamatgalimov, I.R. Nizameev, S.A. Katsyuba, M.Yu. Balakina, Composite materials containing chromophores with 3,7(di)vinylquinoxalinone π-electron bridge doped into
PMMA: Atomistic modeling and measurements of quadratic nonlinear optical activity, Dyes Pigm. 158 (2018) 131–141. [58] C.M. Isborn, A. Leclercq, F.D. Vila, L.R. Dalton, J.L. Bredas, B.E. Eichinger, B.H. Robinson, Comparison of static first hyperpolarizabilities calculated with various quantum mechanical methods, J. Phys. Chem. A 111 (2007) 1319–27.
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S1010-6030(18)31164-X https://doi.org/10.1016/j.jphotochem.2018.10.034 JPC 11550
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
13-8-2018 15-10-2018 18-10-2018
Please cite this article as: Kalinin AA, Sharipova SM, Burganov TI, Levitskaya AI, Fominykh OD, Vakhonina TA, Ivanova NV, Khamatgalimov AR, Katsyuba SA, Balakina MY, Large nonlinear optical activity of chromophores with divinylquinoxaline conjugated -bridge, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.10.034 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.
Large nonlinear optical activity of chromophores with divinylquinoxaline conjugated π-bridge
Alexey A. Kalinin,* Sirina M. Sharipova, Timur I. Burganov, Alina I. Levitskaya, Olga D. Fominykh, Tatyana A. Vakhonina, Nataliya V. Ivanova, Ayrat R. Khamatgalimov, Sergey A.
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Katsyuba, Marina Yu. Balakina
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* Corresponding author
E-mail addresses: [email protected] (A.A. Kalinin); [email protected] (S.M.
(O.D.
[email protected]
Fominykh); (N.V.
Burganov);
[email protected]
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[email protected]
(T.I.
[email protected]
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[email protected]
Ivanova);
[email protected]
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Sharipova);
(A.I.
Levitskaya),
(T.A.
Vakhonina);
(A.R.
Khamatgalimov);
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[email protected] (S.A. Katsyuba); [email protected] (M.Yu. Balakina)
Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS,
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Arbuzov Str. 8, 420088 Kazan, Russia
Graphical abstract
d33, pm/V
120 100 80
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60 40 20
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0 0
10
20
30
40
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Chromophore load (wt%) in PMMA
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Synthesis of D-π-A chromophores with quinoxaline core in the π-bridge High thermal stability and first hyperpolarizability High values of NLO coefficients
ABSTRACT
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Highlights
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Two chromophores with π-deficient quinoxaline (Q) core in the π-electron bridge together with
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dibutylaniline (DBA) donor moiety and tricyanofuranyl (TCF) or dicyanovinyl (DCV) acceptor moieties have been synthesized to investigate their macroscopic nonlinear optical (NLO) activity in guest-host materials. A distinctive feature of the 7-DBA-VQPhV-TCF chromophore is
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predominance of negative solvatochromism over the positive one, which is quite rarely met for the chromophores manifesting inverse solvatochromic behavior as well as the blue shift of the absorption maximum compared to the chromophore FTC with divinylthiophene π-bridge. Introduction of quinoxaline moiety in the conjugated bridge promotes the increase of thermal stability: Td of both compounds is higher than 295 oC. Density functional theory was used to calculate the chromophores first hyperpolarizability (β). Quadratic NLO response characterized
by d33 coefficient was measured by SHG technique for thin guest-host polymer films. The influence of chromophore content on the NLO coefficient of 7-DBA-VQPhV-TCF/PMMA film has been investigated and the d33 value of this film reaches 108 pm/V at 25 wt% load. The poled film 7-DBA-VQPh-DCV/PMMA shows twice smaller d33 values (30 pm/V) at the same
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chromophore load (20 wt%).
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Keywords: NLO Chromophore, divinylquinoxaline π-bridge, solvatochromic behavior, TD-DFT
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calculations, NLO coefficient
1. Introduction Nonlinear optical (NLO) materials have attracted much attention due to their potential at the construction of photonic devices; progress in these investigations contributes to the development of technologies, such as optical communication and data storage, as well as optical
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computing [1-4]. At the creation of organic NLO materials push-pull (D-π-A) chromophores [5,6], which are the source of the NLO response at the molecular level, represent important
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structural elements. Up to now a variety of chromophores were designed and their NLO characteristics were studied by HRS technique or EFISH techniques [7-17]. Modification of πbridge of D-π-A chromophores to increase the NLO activity on the one hand, and to obtain a set
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of useful characteristics on the other hand, represents promising areas in the design and
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development of such compounds and guest-host materials on their basis [18]. Significant
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progress in the creation of NLO chromophores and chromophore-based materials occurred at the
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turn of the century when studying the properties of chromophores with extended
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divinylthiophene and octatetraene π-bridges, in particular, the FTC [19] and CLD [20] ones. The introduction of aromatic heterocycles as an important design element into the π-bridge of D-π-A
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compounds results in maintaining or even increase of the first hyperpolarizability value [14], thermal [16,21] and photostability [22], and also affects the absorption maximum, which in turn
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may influence the optical transparency of the NLO material. In addition to chromophores with thiophene moiety in the π-bridge [15-19,21-26], NLO activity of polymer materials doped with chromophores having divinylpyrrole π-bridge [18,27] has been investigated. More complex
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structure of the chromophores π-bridge included several [16,17] or fused [28,29] thiophene moieties. Composite materials doped with the above-mentioned chromophores showed a good electro-optic (EO) activity (from 30 to 150 pm/V). Excellent transformation of molecular NLO activity into the macroscopic one resulting in further increasing of the NLO (EO) response (up to 300 pm/V and above) has been achieved mainly due to combination of various factors, which
include the introduction of well-chosen isolation groups in the chromophores [30,31] preventing their aggregation, supramolecular organization [32], rational selection of donor and acceptor strength of chromophore end-groups [33,34], as well as the design of more complex chromophore-containing structures (for example, bichromophores [35]), for which the abovementioned factors are valid. Here we investigate nonlinear properties of another type of
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chromophores which do not contain π-excessive heterocyclic moiety in the π-bridge (such as thiophene or any other), but are based on π-deficient quinoxaline core, which acts as an auxiliary acceptor and consequently the whole chromophore bridge has a negative π-charge (Fig. 1). For
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chromophores with such π-bridge in combination with strong donor and acceptor end units high values of the first hyperpolarizabilty with inessential conformational dependence have been
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predicted [36-38].
Fig. 1. Chromophores under study and total π-populations on quinoxaline moiety and on the π-
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bridge as a whole.
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2. Experimental section
2.1. Materials and instrumentation
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1-Phenylpropane-1,2-dione, 4-bromo-1,2-benzenediamine, N,N-dibutylaniline, 3-hydroxy-3methyl-2-butanone and malononitrile, were purchased from Acros or Alfa Aesar. Organic solvents used were purified and dried according to standard methods. 2-Dicyanomethylene-3cyano-4,5,5-trimethyl-2,5-dihydrofuran (Me-TCF) was obtained by condensation of 3-hydroxy3-methyl-2-butanone
with
malononitrile
in
pyridine
[39].
7-Bromo-3-methyl-2-
phenylquinoxaline was obtained according to the literature [40]. The melting points (mp) of
chromophores were determined by the differential scanning calorimetry (DSC); the values are presented in Table 1. The mp of compounds 1 and 2 were determined on a Boetius hot-stage apparatus. Infrared (IR) spectra were recorded on the Bruker Vector-22 FT-IR spectrometer. All NMR experiments were performed with Bruker AVANCE-600 and AVANCE-400 (600 and 400 MHz for 1H NMR, 150 and 100 MHz for 13C NMR) spectrometers. Chemical shifts (δ, in ppm)
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are referenced to the solvent CDCl3. The mass spectra were obtained on Bruker UltraFlex III MALDI TOF/TOF instrument with p-nitroaniline as a matrix. Electronic absorption (UV-Vis)
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spectra were registered at room temperature on a Perkin-Elmer Lambda 35 spectrometer with a scan speed of 480 nm/min using a spectral width of 1 nm. All samples were prepared as solutions in four solvents: dioxane (E = 2.25), chloroform (E = 4.81), 1,2-dichloromethane (E =
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8.93), and acetonitrile (E = 37.5), with the concentrations ranging from ~10–6 to ~10−4 molL-1,
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and placed in 10 mm quartz cells. The IR, NMR, UV-Vis, MALDI spectra were registered on the
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equipment of Assigned Spectral-Analytical Center of FRC Kazan Scientific Center of RAS. The
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thermal stabilities of chromophores were investigated by simultaneous thermal analysis
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(thermogravimetry/differential scanning calorimetry - TG/DSC) using NETZSCH (Selb, Germany) STA449 F3 instrument. Approximately 2.2-3.5 mg of samples were placed in an Al
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crucible with a pre-hole on the lid and heated from 30 to 500 C. The same empty crucible was used as the reference one. High-purity argon was used with a gas flow rate of 50 mL/min.
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TG/DSC measurements were performed at the heating rates of 10 K/min.
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2.2. Computational methods As in our earlier studies devoted to the estimation of molecular NLO characteristics of
newly synthesized chromophores, we have performed DFT calculations of dipole moments and molecular
polarizabilities
of
7-DBA-VQPh-DCV
and
7-DBA-VQPh-TCF
by
DFT.
Chromophores structure was obtained in the course of full optimization with both B3LYP [41,42] and M06-2X [43,44] density functionals using 6-31G(d) basis set; the calculations of the
electric characteristics are performed by TD-DFT [45] at the M06-2X/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ computational levels, thus giving the following three approaches: M062X/6-31G(d)//M06-2X/aug-cc-pVDZ (A), B3LYP/6-31G(d) //M06-2X/aug-cc-pVDZ (B), B3LYP/6-31G(d) //B3LYP/aug-cc-pVDZ (C) (the designation left to the double slash standing for the level used for geometry optimization, that to the right – for electric properties); the
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detailed justification for choosing these computational schemes is presented elsewhere [36]. In spite of the fact that B3LYP density functional is known to overestimate the values of first
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hyperpolarazability [46,47], we have used it for the calculations of β for the sake of comparison with available data for recognized and novel promising chromophores synthesized by other research groups.
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The hyperpolarizability value 𝛽𝑡𝑜𝑡 is calculated as follows
1 ( ikk kik kki ),i x, y , z. 3 ik
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tot x 2 y 2 z 2 ; i iii
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For the data analysis we use “theoretical” convention here, assuming that the chromophore’s
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dipole moment is expanded in Taylor series with respect to local electric field. The coordinate frame was chosen to align z-axis along the dipole moment vector of the chromophore.
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The values of π-electron charges for both chromophores are calculated at B3LYP/6-31G(d)
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computational level. Natural population analysis (NPA) was carried out to compute atomic charges [48]. All calculations were performed with the Jaguar program package [49,50].
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2.3. Syntheses of dyes 2.3.1. E-7-(4-(Dibutylamino)styryl)-3-methyl-2-phenylquinoxaline (1) A mixture of 2-phenyl-3-methyl-7-bromoquinoxaline (400 mg, 1.34 mmol), N,N-dibytyl4-vinylaniline (310 mg, 1.34 mmol), tri(о-tolyl)phosphine (4.0 mg, 0.013 mmol), Pd(OAc)2 (1.5 mg, 0.007 mmol), Et3N (0.33 g, 0.33 mmol), and anhydrous DMF (1 mL) was stirred for 4 h at
120°С. The reaction mixture was cooled, poured into water, and extracted with CH2Cl2. The organic layer was separated, washed with water, dried over anhydrous MgSO4, filtered. The solvent was removed at reduced pressure, and the residue was purified by silica gel column chromatography (eluent petroleum ether – EtOAc, gradient from 100:1 to 20:1). Yield 430 mg (72%), yellow-orange powder. Rf = 0.43 (hexane : ethyl acetate = 3:1); mp 82-84 °C. 1H NMR
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(600 MHz, CDCl3) δ: 8.06 (s, 1H, H-8 quinoxaline), 7.96 (d, 1H, J = 8.7 Hz, H-5 quinoxaline), 7.94 (dd, 1H, J = 8.7, 2.0 Hz, H-6 quinoxaline), 7.67 (d, J = 7.7 Hz, 2H, o-Ph), 7.58-7.40 (m, 3H,
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m,p-Ph), 7.44 (d, J = 8.6 Hz, 2H, 3,5H aniline), 7.23 (d, J = 16.2 Hz, 1H, H ethene), 7.05 (d, J = 16.2 Hz, 1H, H ethene), 6.65 (d, J = 8.6 Hz, 2H, 2,6-H aniline), 3.31 (t, J = 7.6 Hz, 4H, NCH2), 2.76 (s, 3H, CH3), 1.65-1.55 (m, 4H, NCH2CH2), 1.45-1.32 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 13
C NMR (150 MHz, CDCl3) δ: 155.0, 151.1, 148.3, 141.6, 140.6, 139.6,
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7.4 Hz, 6H, CCH3).
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139.3, 131.8, 128.9, 128.8, 128.5, 128.18, 128.16, 127.8, 125.3, 124.0, 122.4, 111.6, 50.8, 29.5,
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24.2, 20.3, 14.0. MALDI-TOF: 450 [M+H]+. IR (max, cm-1, KBr): 3921 (С-Н), 2955 (С-Н),
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2927 (С-Н), 2871 (С-Н), 1599 (С=N, С=С), 1552, 1521, 1489, 1469, 1448, 1424, 1403, 1371,
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1347, 1288, 1250, 1219, 1183, 1149, 1138, 1110, 1031, 1004, 959, 927, 895, 848, 832, 805, 764.
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2.3.3. (E)-7-(4-(Dibutylamino)styryl)-2-phenylquinoxaline-3-carbaldehyde (2)
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A mixture of quinoxaline 1 (200 mg, 0.45 mmol), selenium dioxide (57 mg, 0.51 mmol) and dioxane (3 mL) was stirred at 100 oC for 1 h under argon and then cooled to room temperature. After removal of the solvent by rotary-evaporation, the residue was purified by
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silica-gel column chromatography (eluent: methylene chloride) to give a product as dark red powder (180 mg, 87%). Rf = 0.47 (hexane/ethyl acetate 3:1); mp 111-113 °С. 1H NMR (600 MHz, CDCl3): δ 10.28 (s, 1H, СН=О), 8.20 (d, J = 8.8 Hz, 1Н, H-5 quinoxaline), 8.09 (s, 1H, H8 quinoxaline), 8.07 (dd, J = 8.9, 2.0 Hz, 1H, H-6 quinoxaline), 7.74-7.67 (m, 2Н, о-Ph), 7.597.54 (m, 3Н, m,p-Ph), 7.46 (d, J = 8.5 Hz, 2Н, 3,5-H aniline), 7.35 (d, J = 16.0 Hz, 1H, -
CH=CH-), 7.07 (d, J = 16.0 Hz, 1H, -CH=CH-), 6.66 (d, J = 8.5 Hz, 2Н, 2,6-H aniline), 3.32 (t, J = 7.6 Hz, 4H, NCH2), 1.66-1.57 (m, 4H, NCH2CH2), 1.43-1.34 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.4 Hz, 6H, CCH3).
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C NMR (150 MHz, CDCl3) δ: 191.1, 155.2, 148.8, 143.83, 143.81,
143.5, 140.7, 136.9, 134.0, 130.2, 129.8, 129.7, 129.2, 128.7, 128.6, 124.8, 123.4, 121.6, 111.7, 50.8, 29.5, 20.35, 14.0. MALDI-TOF: 464 [M+H]+. IR (max, cm-1, KBr): 3029 (С-Н), 2955 (С-
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Н), 2927 (С-Н), 2858 (С-Н), 1701 (С=О), 1596 (С=N, С=С), 1553, 1517, 1488, 1465, 1427, 1398, 1350, 1317, 1288, 1277, 1254, 1220, 1187, 1148, 1111, 1037, 1013, 968, 923, 889, 833,
(E)-2-((7-(4-(Dibutylamino)styryl)-2-phenylquinoxalin-3-yl)methylene)
malononitrile
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2.3.2.
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808.
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(7-DBA-VQPh-DCV)
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A solution of aldehyde 2 (77 mg, 0.17 mmol) and malononitrile (11 mg, 0.17 mmol) in
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anhydrous methylene chloride (1 mL) was stirred at room temperature for 48 h. After removal of the solvent by rotary-evaporation, the residue was purified by silica-gel column chromatography
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(eluent: petroleum ether /methylene chloride = 1:11:9) to give a product as black-violet
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powder (70 mg, 82%). Rf = 0.51 (hexane/ethyl acetate 3:1). 1H NMR (400 MHz, CDCl3) δ: 8.13 (d, J = 8.9 Hz, 1Н, H-5 quinoxaline), 8.02 (dd, J = 8.9, 1.7 Hz, 1Н, H-6 quinoxaline), 7.97 (d, J
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= 1.7 Hz, 1H, H-8 quinoxaline), 7.89 (s, 1H, CH=C(CN)2), 7.65-7.57 (m, 5Н, Ph), 7.43 (d, J = 8.8 Hz, 2Н, 3,5-H aniline), 7.30 (d, J = 16.1 Hz, 1H, -CH=CH-), 6.99 (d, J = 16.1 Hz, 1H, CH=CH-), 6.64 (d, J = 8.8 Hz, 2H, 2,6-H aniline), 3.33 (t, J = 7.7 Hz, 4H, NCH2), 1.66-1.55 (m,
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4H, NCH2CH2), 1.45-1.32 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.4 Hz, 6H, CCH3). 13C NMR (100 MHz, CDCl3) δ: 155.1, 153.7, 148.9, 144.4, 144.0, 140.5, 140.1, 136.5, 134.4, 130.2, 129.9, 129.7, 129.6, 129.1, 128.9, 124.6, 123.2, 121.4, 114.1, 112.4, 111.6, 87.2, 50.9; 29.5, 20.3, 13.4. MALDI-TOF: 512 [M+H]+. IR (max, cm-1, KBr): 3043 (С-Н), 2953 (С-Н), 2865 (С-Н), 2228
(CN), 1593 (C=C, C=N), 1515, 1482, 1429, 1395, 1363, 1332, 1282, 1253, 1228, 1185, 1155, 1111, 1014, 968, 957, 916, 893, 827, 813, 766. 2.3.4.
(E,E)-2-(3-Cyano-4-(2-(7-(4-(dibutylamino)styryl)-2-phenylquinoxalin-3-yl)vinyl)-5,5-
dimethyl-2,5-dihydrofuran-2-ylidene)malononitrile (7-DBA-VQPhV-TCF) A mixture of aldehyde 2 (33 mg, 0.07 mmol), Me-TCF (14 mg, 0.07 mmol) and
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anhydrous ethanol (2 mL) was refluxed for 14 h, then cooled to room temperature. After removal of the solvent by rotary-evaporation, the residue was purified by silica-gel column
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chromatography (eluent: petroleum ether /methylene chloride = 1:1methylene chloride) to give a product as black powder (27 mg, 59%). Rf = 0.16 (hexane/ethyl acetate 3:1). 1H NMR
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(400 MHz, CDCl3) δ: 8.10-8.04 (m, 2Н, H-5,6 quinoxaline), 8.02 (s, 1H, H-8 quinoxaline), 7.92
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(d, J = 15.8 Hz, 1Н, -CH=CH-TCF), 7.84 (d, J = 15.8 Hz, 1Н, -CH=CH-TCF), 7.69-7.63 (m, 2Н,
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о-Ph), 7.62-7.56 (m, 3Н, m,p-Ph), 7.46 (d, J = 8.6 Hz, 2Н, 3,5-H aniline), 7.34 (d, J = 16.2 Hz,
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1Н, -CH=CH-DBA), 7.06 (d, J = 16.2 Hz, 1Н, -CH=CH-DBA), 6.67 (d, J = 8.6 Hz, 2Н, 2,6-H aniline), 3.33 (t, J = 7.7 Hz, 4H, NCH2), 1.67 (s, 6H, CH3), 1.66-1.56 (m, 4H, NCH2CH2), 1.44-
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1.33 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.4 Hz, 6H, CCH3).
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C NMR (150 MHz, CDCl3) δ:
174.9, 173.0, 155.7, 148.9, 143.3, 143.0, 141.8, 141.4, 137.2, 133.8, 130.0, 129.8, 129.7, 129.4,
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129.0, 128.8, 128.3, 124.8, 123.3, 121.6, 119.7, 111.7, 111.4, 110.6, 109.8, 102.2, 97.8, 58.7,
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50.8, 29.5, 26.2, 20.3, 14.0. MALDI-TOF: 645 [M+H]+. IR (max, cm-1, KBr): 3071 (С-Н), 2956 (С-Н), 2930 (С-Н), 2871 (С-Н), 2228 (CN), 1582 (C=C, C=N), 1553, 1511, 1478, 1429, 1394,
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1375, 1322, 1303, 1222, 1208, 1181, 1140, 1105, 1040, 1020, 958, 925, 828.
3. Results and discussion 3.1. Synthesis and thermal stability Scheme 1 shows the synthesis of chromophores 7-DBA-VQPh-DCV and 7-DBA-VQPhVTCF with vinylquinoxaline and divinylquinoxaline π-bridge, respectively. The Heck reaction
between N,N-dibutyl-4-vinylaniline and 7-bromo-3-methyl-2-phenylquinoxaline gave the compound 1 as trans-isomer. The time (4 h) of this coupling procedure with π-deficient bromoquinoxaline derivative is significantly less compared to that for similar reaction (72 h) of π-excessive phenothiazine or carbazole derivatives [51]. There are various oxidative procedures for the alkyl acyl transformation in alkylquinoxaline derivatives [52-54]. The oxidation of
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quinoxaline 1 by selenium dioxide leads to aldehyde 2. The Knoevenagel сondensation of compound 2 with malononitrile or Me-TCF without using the base results in the chromophores
PT
ED
M
A
N
U
SC R
7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF.
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Scheme 1. Synthesis of chromophores 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF
The studied chromophores are identified as highly crystalline compounds with a mp at 229 °C
A
and 168 °C for DBA-VQPhV-TCF and DBA-VQPh-DCV, respectively (Table 1). Figure 2a shows the thermogravimetric curves for the studied chromophores according to which they have high thermal stabilities: the decomposition temperatures (Td), at which 5% mass loss occurs at heating, are above 295 °C for both chromophores. However, according to DSC data there is an exothermic peak (the second one) on the DSC curve that can correspond to processes either not related to decomposition (e.g. polymorphism) or related to decomposition but proceeding without mass loss (e.g.
oligomerization). To clarify this, the following experiment was carried out: each chromophore sample was heated on a TG/DSC device in an aluminum crucible to 270 °C (what is higher than that corresponding to the peak on the DSC curve), and then it was cooled. The physicochemical characteristics of the sample after heating did not match those of the original chromophore. Thus, DSC data show that the decomposition temperature of chromophores 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF is 225 °C
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and 234 °C, respectively (Table 1).
Table 1 Thermal properties of the studied chromophores.
Td, °Cb
234
mp, °C
229
295 (369)
U
225 168
TGA (at which 5% (10%) mass loss occurs at heating). DSC.
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PT
ED
M
A
b
295 (354)
N
a
Td, °Ca
7-DBA-VQPh-DCV
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Chromophore 7-DBA-VQPhV-TCF
a
b
A
Fig. 2. TGA curves (a) and DSC curves (b) for 7-DBA-VQPhV-TCF and 7-DBA-VQPh-DCV.
The decomposition temperature of 7-DBA-VQPhV-TCF (234 °C) is close to that for FTC chromophore having DBA (220 °C) [24] or diethyl aniline (244 °C) [23] donor moieties and to that for julolidine-based chromophores [27,55].
3.2. Photophysical properties UV-Vis electronic absorption spectra of the chromophores 7-DBA-VQPh-DCV and 7DBA-VQPhV-TCF exhibit broad and highly intensive the lowest-energy absorption bands in the visible region of spectra (λmax of ca. 550-640 nm, see Figure 3, Table 2. To shed light on nature of the main electronic absorption bands quantum-chemical calculations of first 15 electronic
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transitions have been performed. Calculations demonstrated that the longest-wavelength absorption maxima could be ascribed to the transition with at least partial intramolecular charge-
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transfer (ICT) from DBA to DCV/TCF moieties, (see Figure S1 and Table S1 in ESI). The positions of these bands (λmax) in the spectra of chromophore 7-DBA-VQPhV-TCF are redshifted relative to the spectra of chromophore 7-DBA-VQPh-DCV due to the increased acceptor
U
strength of TCF compared to DCV. Depending on the solvent polarity the compounds
N
demonstrate reversible solvatochromic shifts up to ca. 0.2 eV (Table 2). Increase of the solvent
A
polarity in the series from 1,4-dioxane to chloroform causes a clear red shift of λmax by ca. 0.15-
M
0.2 eV while in more polar solvents, such as dichloromethane and acetonitrile, all chromophores
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demonstrate reversed blue shift of λmax (Table 2). Similar behavior has been previously described for a number of D-π-A systems with various divinylhetaryl π-bridge [23,27,28,38,56,57].
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A distinctive feature of the chromophore 7-DBA-VQPhV-TCF is the predominance of
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negative solvatochromism in passing from chloroform to acetonitrile solutions over positive one found in passing from dioxane to chloroform solutions, this being a rare phenomenon. In contrast, stronger positive solvatochromic effect relative to the negative one is typical for TCF-
A
based chromophores with divinylthiophene [15,18,23,31,34], divinylpyrrole [18,27] and divinylthienothiophene [28] π-bridges combined with dialkyl aniline [23], diaryl pyrrole [15], tetrahydroquinoline [34] and julolidine [27,28,31,34] donor end groups. Comparison of the data of the spectra of 7-DBA-VQPhV-TCF, 7-DBA-VQPh-DCV (Table 1) with the spectra of the related systems bearing quinoxalinone moiety instead of quinoxaline one [57] reveals moderate blue shift of λmax by ca. 0.1 eV (26 nm) and ca. 50% decrease of ε values for 7-DBA-VQPhV-
TCF, 7-DBA-VQPh-DCV in the every solvent used, relative to their quinoxalinone analogs. Slightly greater hypsochromic shift of λmax by ca. 0.13 eV (47 nm) is found for the chromophore 7-DBA-VQPhV-TCF in comparison with FTC chromophore having DBA donor moiety and divinylthiophene π-bridge [24]. Blue-shifted absorption of chromophores without the loss of their molecular NLO properties can improve optical transparency of NLO materials based
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on such chromophores [28].
Table 2 Summarized experimental and calculated data on photophysical and thermal properties of the chromophores.
7-DBAVQPhV-TCF
CH3CN
N
CH2Cl2
A
546/2.27 594/2.09 582/2.13 552/2.25 (30500) (28500) (28800) (27300) [125/0.54] [149/0.54] [147/0.55] [146/0.60] 589/2.11 638/1.94 619/2.00 578/2.15 (29900) (27000) (28100) (30300) [162/0.60] [186/0.58] [188/0.62] [191/0.74]
M
7-DBAVQPh-DCV
1,4-dioxane CHCl3
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compound
U
λmaxa, nm/eV (ε, M-1 cm-1), [FWHM]b
a
Δλmaxc, nm/eV
48/0.18
60/0.19
DFT
computational leveld
A B C A B C
µ, D
βtot, 10-30 esu
11.4 12.4 14.0 18.2 19.5 21.6
360 421 832 688 875 1983
position of the maximum of the longest-wave electronic absorption band presented in nm/eV. FWHM – full-width at half maximum is presented in nm/eV c difference between the highest and lowest λmax in nm/eV. d
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b
A
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M06-2X/6-31G(d)//M06-2X/aug-cc-pVDZ (A), B3LYP/6-31G(d) //M06-2X/aug-cc-pVDZ (B), B3LYP/6-31G(d) //B3LYP/aug-cc-pVDZ (C).
7-DBA-VQPh-DCV
7-DBA-VQPhV-TCF
1,2
1,4-dioxane CH2Cl2
1,0
1,4-dioxane CH2Cl2
1,2
CHCl3
CHCl3
1,0
CH3CN
CH3CN
0,8
a.u.
0,6
0,4
0,6
0,4
0,2
0,2
0,0
0,0
300
400
500
600
700
800
900
300
400
, nm
500
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a.u.
0,8
600
700
800
900
1000
, nm
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Fig. 3. UV-Vis experimental spectra of 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF in different solvents.
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3.3. DFT calculations: molecular NLO activity and π-populations on the π-bridges
N
Earlier the TCF acceptor was proved to be an efficient one, thus the replacement of
A
tricyanovinylene moiety by TCF could lead to a substantial increase of optical nonlinearity [19].
M
Our research performed for the related chromophores with 3,7-(di)vinylquinoxaline-2-one
ED
(VQonV) bridge also demonstrated that the incorporation of TCF acceptor resulted in the essential increase of dipole moment and first hyperpolarizability values in comparison with those
PT
for chromophores with DCV acceptor moieties [36]. Table 2 demonstrates that the same observation is valid for 7-DBA-VQPh-DCV and 7-DBA-VQPhV-TCF chromophores. According
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to the presented data, first hyperpolarizability for 7-DBA-VQPhV-TCF is approximately twice higher than the corresponding values for the 7-DBA-VQPh-DCV chromophore, this tendency persisting regardless of the used calculation scheme. Comparison of the estimations for 7-DBA-
A
VQPh-DCV and 7-DBA-VQPhV-TCF chromophores with those obtained earlier for related compounds gives evidence to the effect of various structural components on the value of first hyperpolarizability. In particular, the replacement of VQonV bridge by the VQPhV one results in the close values of first hyperpolarizability for the chromophores with TCF acceptor, while for those with DCV acceptor and VQPhV bridge βtot values are notably smaller: by ca 30% and 16%
for B3LYP/M06-2X and M06-2X/M06-2X, respectively [58] than βtot values for chromophores with VQonV bridge). We have compared the first hyperpolarizability values obtained for the novel chromophores under study with those published before for the chromophores with thiophene moieties in the π-bridge and TCF acceptors, which got a reputation of effective chromophores.
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For this purpose we have used the results obtained with B3LYP density functional which is often exploited by researchers to perform estimations of μ and β of newly synthesized chromophores.
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The calculation of βtot performed at the B3LYP/aug-cc-pVDZ´ level gave very high value for 7DBA-VQPhV-TCF, 1983⋅10−30 esu, which is more than twice as high as βtot for FTC [58], CLD [58] and julolidine-based chromophores [31,34], at the same time it is ca 17% higher than
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for the chromophore with VQonV π-bridge [57].
N
Net π-populations, calculated for heterocyclic moiety and the chromophore π-bridge as a
A
whole for quinoxaline-based chromophores and some other chromophores with various
M
divinylhetaryl π-bridges, are presented in Figure 1 and Table 3. π-Excessive thiophene moiety makes π-bridge an auxiliary donor. Thiazole moiety as a part of the bridge in D-π-A
ED
chromophores is conventionally assumed to be auxiliary acceptor; however, as it can be seen
PT
from Table 3, thiazole moiety is rather an auxiliary donor than an acceptor. Even π-deficient quinoxalinone moiety, containing one pyridine-type nitrogen atom and oxygene atom of
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carbonyl group as acceptor sites and one pyrrole-type nitrogen atom as donor site, can’t be treated as auxiliary acceptor, since net π-charge on it is close to zero, what gives grounds to treat Qon as promoting the electron density transfer from donor to acceptor, not withdrawing any
A
density. However, net π-charge on chromophores bridge with π-deficient quinoxaline moiety is found to be negative, thus quinoxaline moiety acts as an auxiliary acceptor opposite to the case of thiazole and even quinoxalinone. It is worth stressing, that only the presence of two pyridinetype nitrogen atoms in this moiety provides this feature.
Table 3 Chromophores with π-populations on the π-bridge. Chromophores
b
a
+0.129
+0.125
b
+0.058
+0.081
c
+0.056
+0.077
d
-0.010 [57]
+0.035 [57]
e
-0.092
-0.097
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-Chargeb
on heterocyclic moiety of the π-bridge. on the π-bridge as a whole.
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a
-Chargea
N
U
3.5. NLO performance
To study the macroscopic NLO activity the guest-host polymer materials were prepared
M
A
with PMMA as polymer matrix (Tg = 105 C) and 7-DBA-VQPhV-TCF and 7-DBA-VQPh-DCV chromophores as guests. Thin polymer films containing 20 wt % of the 7-DBA-VQPh-DCV
ED
chromophore together with those doped by 7-DBA-VQPhV-TCF chromophore with various loads (10, 15, 20, 25 and 30 wt%) were spin cast from 7% cyclohexanone solution. Films were
PT
poled at the corona-triode setup in the corona discharge field, voltage 6.5 kV, poling time ~20
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min, the distance from the tungsten needle electrode to the surface of the film being 1 cm; the field was applied to the films heated to temperatures close to glass transition temperature, Tg. The quality of orientation was controlled by the absorption change in UV-Vis spectra (Fig. 4)
A
detected before and after poling, and characterized by order parameter, η, calculated by the following formula: 1 A A , 0
where A and A0 are the absorptions of the polymer films after and before poling [59].
0,35
15% 15%
before poling after poling
15wt % 0,30 0,25
A
0,20 0,15 0,10 0,05 0,00 600
500
700
800
1000
900
, nm
a 0,8
30wt %
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0,6 0,5
A
30% 30%
before poling after poling
0,7
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400
0,4 0,3
U
0,2
0,0 400
500
600
N
0,1
700
800
900
1000
0,45 0,40
b
M
0,50
A
, nm
20wt %
before poling after poling
20% 20%
ED
0,35
A
0,30 0,25
A
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0,20 0,15 0,10 0,05 0,00
400
500
600
700
800
900
1000
, nm
c
Fig. 4. UV-Vis electronic absorption spectra: registered before and after poling (a-c) for 7DBA-VQPhV-TCF/PMMA films with chromophore load 15 (a) and 30 wt% (b), for 7DBA-VQPhV-DCV/PMMA film with chromophore load 20 wt% (c).
Polymer films surfaces were studied by AFM technique before and after poling, the surface images and films roughness are presented in Figure 5. As it can be seen, after poling the film roughness somewhat increases (Fig. 5 c, d).
c
U
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a
d
N
b
M
A
Fig. 5. AFM surface images and film roughness for polymer film 7-DBA-VQPhV-TCF/PMMA (25wt%) before (a,c) and after (b,d) poling.
Polymer NLO coefficients were measured by SHG technique; pulse Nd3+:YAG laser radiation
ED
(λ=1064 nm, pulse duration 15 ns, power density at the sample 10 kW/cm2; α-quartz (х-cut plate served as a standard) was used. In the course of measurements it was assumed that d33/d13 ≈ 3.
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The NLO coefficients, order parameters, film thicknesses of obtained composite
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materials are summarized in Table 4. The d33 values for the 7-DBA-VQPh-DCV/PMMA film is 30 pm/V. At the same chromophore load (20 wt%) NLO coefficient for 7-DBA-VQPhVTCF/PMMA film increases twice reaching 61 pm/V. So, higher first hyperpolarizability for
A
chromophore 7-DBA-VQPhV-TCF as compared with that for chromophore 7-DBA-VQPh-DCV (Table 1) provides the better NLO activity at material level.
Table 4 NLO coefficients (d33, pm/V),*order parameters (), and film thickness (h, nm). h, nm
d33, pm/V
7-DBA-VQPh-DCV/PMMA (20)
400
0.24
30
7-DBA-VQPhV-TCF/PMMA (10)
325
0.40
19
7-DBA-VQPhV-TCF/PMMA (15)
366
0.35
57
7-DBA-VQPhV-TCF/PMMA (20)
360
0.30
61
7-DBA-VQPhV-TCF/PMMA (25)
350
0.35
108
7-DBA-VQPhV-TCF/PMMA (30)
450
0.20
76
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*d33 measurements accuracy is about 15%
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Sample (chromophore load, wt%)
Table 4 demonstrates the influence of chromophore content on the NLO coefficient of the
U
7-DBA-VQPhV-TCF/PMMA film. One can see that guest–host polymer with 25 wt% of
N
chromophore displays maximum NLO activity, d33 being equal to 108 pm/V. Further increase of
A
the load up to 30 wt% leads to the decrease of d33 value. Similar trend was obtained for FTC
M
guest-chromophore – maximum EO activity was demonstrated at 25 wt% load in APC, the value
material [60].
ED
of 39 pm/V being achieved [23], the same conclusion was obtained for EZ-FTC/PMMA
PT
The obtained d33 value of 108 pm/V may be considered rather high, in particular, for the material with a representative of a novel class of NLO chromophores with quinoxaline moiety in
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π-bridge. We believe that optimization of the chromophore structure of 7-DBA-VQPhV-TCF by introducing the isolation groups or making other modifications will result in further increase of
A
nonlinear activity.
4. Conclusion Two chromophores with π-deficient quinoxaline core in the π-electron conjugated bridge connecting dibutylaniline donor and tricyanofuranyl/dicyanovinyl acceptor moieties have been synthesized and their photophysical, thermal and NLO properties were systematically investigated. Both chromophores exhibit positive solvatochromic effect when passing from 1,4-
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dioxane to chloroform and also demonstrate negative solvatochromism when passing from acetonitrile to chloroform, herewith the rare case - domination of the latter over the former has
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been demonstrated for the chromophore 7-DBA-VQPhV-TCF; the latter also shows the strong blue shift of the absorption maximum relative to the FTC chromophore with divinylthiophene πbridge. Chromophores showed excellent thermal stability with the decomposition temperatures
U
above 295 C. For the first time the NLO activity of qinoxaline-based chromophores as guests in
N
the composite materials is studied. The d33 values of poled films containing 20 wt% of these new
A
chromophores doped in PMMA were 30 and 61 pm/V at 1064 nm for chromophores 7-DBA-
M
VQPh-DCV and 7-DBA-VQPhV-TCF, respectively. The influence of chromophore content on
ED
the NLO coefficient of the 7-DBA-VQPhV-TCF/PMMA film was also investigated: the d33 values were shown to be equal to 19, 57, 61, 108 and 76 pm/V at 10, 15, 20, 25 and 30 wt% load,
PT
correspondingly.
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Acknowledgements
Financial support of Russian Science Foundation (grant no. 16-13-10215) for the study of
A
NLO activity of quinoxaline-based chromophores is gratefully acknowledged. The authors are thankful to Prof. V.I. Kovalenko for helpful discussion of the chromophores thermal properties.
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