Photoinduced intramolecular charge transfer process of betaine pyridinium: A theoretical spectroscopic study

Chemical Physics Letters 515 (2011) 42–48

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Photoinduced intramolecular charge transfer process of betaine pyridinium: A theoretical spectroscopic study Aurélie Perrier a,⇑, Stéphane Aloïse a,b,⇑, Zuzanna Pawlowska b, Michel Sliwa b, François Maurel a, Jiro Abe c a

Univ Paris Diderot, Sorbonne Paris Cité, ITODYS, CNRS UMR 7086, 15 rue Jean Antoine de Baïf, 75205 Paris Cedex 13, France Université Lille Nord de France, Lille 1, LASIR, CNRS UMR 8516, F-59655 Villeneuve d’Ascq, France c Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan b

a r t i c l e

i n f o

Article history: Received 10 July 2011 In final form 2 September 2011 Available online 7 September 2011

a b s t r a c t Using Time-Dependent Density Functional Theory and taking into account bulk solvent effects, we investigate the absorption and emission spectra of a betaine pyridinium molecule, the 2-(1-pyridinio) benzimidazolate (SBPa). This molecule exhibits strong photoinduced intramolecular charge transfer (ICT). We have identified two different electronic states involved, respectively, in the strong bathochromic ICT absorption band (S2) and in the moderate emission band (S1). The ICT process is analyzed in terms of charge distribution and dipole moment evolutions upon photoexcitation. These results are compared with steady-state spectroscopic measurements. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Ophthalmic glasses with variable transition [1], optical memories [2], optoelectronic devices [3] are concrete illustrations of a very simple principle: the modulation of a macroscopic property upon light irradiation. Such photofunctional properties are based on photoinduced modifications at the molecular scale such as a change of configuration [4], a creation of a covalent bond [2] or a proton or an hydrogen transfer [5]. Another class of elementary processes concerns the charge transfer (CT) reaction and more especially the intramolecular CT (ICT): after irradiation, a charge is transferred from one part of the molecule to another. Consequently, the dipole moment of the excited state differs drastically from that of the ground state and this phenomenon may induce strong solvatochromism [6] and enhanced nonlinear optical (NLO) properties [7]. Among all the molecules exhibiting photoinduced ICT properties, 2-(1-pyridinio) benzimidazolate molecule (SBPa in Figure 1) has received much attention [8–15]. This molecule is made of a negatively charged aromatic electron-donating group, the benzimidazole ring, and a positively charged aromatic electron-withdrawing group, the pyridinium moiety. SBPa presents an unusual large dipole moment, +10.33 D for the ground state in dioxane [9], and shows a large hypsochromic shift of the predominant absorption band in polar solvents [8–10,15] (negative solvatochromism). This behavior contrasts with the positive solvatochromism (a bathochromic shift is observed upon increas⇑ Corresponding authors at: Univ Paris Diderot, Sorbonne Paris Cité, ITODYS, CNRS UMR 7086, 15 rue Jean Antoine de Baïf, 75205 Paris Cedex 13, France. E-mail addresses: [email protected] (A. Perrier), [email protected] (S. Aloïse). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.09.013

ing the solvent polarity) and the reversed solvatochromism (a change from the positive to negative behavior is observed at a given solvent polarity). Interestingly, the three types of solvatochromism are observed in the family of phenolate betaine dyes and Dominguez et al. recently proposed a model to rationalize these behaviors in terms of the chemical hardness of donor and acceptor fragments [16]. For SBPa, the large negative solvatochromism suggests that an important decrease of the dipole moment arises upon photoexcitation [8]. Consequently, SBPa has an extremely large molecular hyperpolarizability while compared to other push–pull systems of similar size, jbj = (115 ± 25)  1030 cm5 esu1 [11], and thus constitutes a good candidate for the use in NLO devices [13,14]. More recently, we have reinvestigated the solvatochromism of SBPa with an innovative solvatochromic analysis in order to deduce the ground and excited dipole moments as well as the related hyperpolarizability [15]. In this work, it has also been shown that SBPa is fluorescent and that the emission band shows only moderate positive solvatochromism while compared to absorption features. This result appears unusual because the reverse situation, solvatochromic emission versus nonsolvatochromic absorbance, is frequently reported in the literature [17]. This behavior suggests that the emissive transition does not involve the same excited state as the CT absorption band. A challenging task is therefore the thoughtful identification of the excited state involved, respectively, in the absorption and emission processes. In that framework, theoretical studies are well-recognized as valuable complements to experimental investigations. Previously, a series of ab initio (HF, CIS, MP2) and semi-empirical calculations (INDO/S) have been performed by Abe and co-workers [11,12]. An ICT character has been identified for the first excited state, with a displacement of the p charge density from the betaine ring to the

A. Perrier et al. / Chemical Physics Letters 515 (2011) 42–48

N N N Figure 1. Representation of the 2-(1-pyridinio) benzimidazolate (SBPa) molecule.

pyridinium moiety. MP2 calculations have successfully reproduced the experimental dipole moment value for the ground state [9] and for the first excited state, CIS calculations have shown that the overall quinoid character of the molecule is enhanced and is accompanied by a decrease and a flip of the dipole moment [13,14]. More recently, Hartree–Fock calculations [18,19] and high-level electron correlation studies (CASSCF, MP2, coupledcluster) [20] have investigated the photoinduced electron transfer in large betaine molecules by analyzing the variations of the atomic charges and dipole moment from the ground state to the first excited state. In the course of proposing a new solvatochromic analysis, we have recently given some preliminary and promising results based upon (Time-Dependent) Density Functional Theory (TD-DFT) concerning the absorption and fluorescence properties [15]. So far, only semi-empirical calculations (INDO/S) were able to reproduce qualitatively the absorption spectrum [13]. Therefore, for SBPa, an exhaustive theoretical investigation of the absorption/ emission phenomena as well as their solvatochromic behavior has never been reported. This contribution thus aims at filling this gap. In a first step, we test a computational procedure, within the TDDFT [21,22] and the Polarisable Continuum Model (PCM) [23] frameworks, which should be able to accurately predict both the absorption and fluorescence spectra of SBPa as well as the influence of the solvent polarity. Recently, TD-DFT has been successfully used to study the absorption and emission properties of ICT organic molecules [24,25] and should thus be an accurate tool. In a second step, we aim at identifying the electronic states involved in the absorption and emission phenomena. In order to lay the foundation for the study of the photoinduced ICT mechanism, we present a detailed description of the charge distribution and dipole moment of the ground and excited states.

2. Computational details All DFT and TD-DFT calculations have been performed with the GAUSSIAN09 program [26] using default thresholds and parameters. The bulk solvent effects have been included at all computational stages by means of the PCM [23]. Within this model, the solvent (or reaction field) is considered as a polarizable medium characterized by a uniform dielectric constant and the solute is immersed in the medium. We perform comparisons with the experiments carried out in three different solvents [15]: toluene, tetrahydrofurane (THF) and acetonitrile (ACN). With these aprotic solvents, the PCM approach, that neglects specific solute–solvent interactions, can be straightforwardly applied. All the calculations (geometry optimization, absorption and emission) have been carried out with the PBE0/6-31G+(d) level of theory [27,28]. Among a group of hybrid functionals (B3LYP [29], PBE0, BMK [30] and BHandHLYP [31]), the choice of the PBE0 has been made by comparing the computed transition energies and the experimental absorption spectrum of SBPa [15]. In the case of vertical absorption transitions, our calculations rely on a three-step protocol [32]. In a first step, the geometrical parameters of the ground state minimum (hereafter S0(opt)) have been optimized without symmetry constraints. In a second stage, vibrational spectra were computed to check that the optimized structures correspond to true minima. In the last step, we have calculated the first 10 singlet low-lying electronic excited-states

43

Sn(FC) (with n = 1–10 and FC standing for Franck–Condon) using the vertical TD-DFT approximation. In the case of vertical emission transitions, a complete optimization procedure of the two lowest excited states S1 and S2 was accomplished following the TD-DFT analytical gradients available in GAUSSIAN09 software [33]. During the exploration of the excited state potential energy surfaces, we used the equilibrium, linear response (LR) solvatation scheme [34]. True minima are validated by ensuring that no imaginary frequencies stand in the computed vibrational spectra. Starting from S1(FC) and S2(FC), we obtained two optimized structures, respectively, S1(opt) and S2(opt). For each optimized state, the emission to the final ground state was obtained by exploiting the state-specific (SS) model [35]. The emission wavelength is then simply deduced from the energy gap between the optimized excited state S1(opt) (S2(opt)) and the corresponding lower vertical state S00 (S000 ). At this stage, the ground state energy is computed with non-equilibrium solvatation, at the excited state geometry, with the static solvatation reaction field from the excited state. To analyze the charge transfer within the molecule, we use the Merz–Kollman (MK) charges [36]. This method of charge analysis has been previously applied to investigate the charge transfer mechanism in Nile Red dye [25].

3. Results and discussion 3.1. Absorption spectrum and description of the first excited states in the Franck–Condon region For the ground state, the PBE0/6-31+G(d) geometry optimization leads to a planar geometry with a symmetry close to C2v. This result is in agreement with previous HF and MP2 calculations [12] and with our recent DFT calculations relying on the comparison of experimental and theoretical NMR spectra [15]. The calculated bond distances are given in Figure 2a. The comparison of the optimized structure with X-ray diffraction analysis [9] shows that the PBE0/6-31+G(d) scheme yields an average absolute deviation of 0.9% when comparing the theoretical and experimental bond lengths (see Table SI1). Given the fact that the theoretical and experimental data are obtained in different media (solid state for X-ray, toluene for DFT), the agreement is very satisfactory. In Figure 2a, the C–C bond length alternation shows that the conjugation is more pronounced in the pyridine than in the benzene cycle. Besides, the N1–C7 bond has a simple bond character whereas the C–N bonds in the betaine (N8–C7) and the pyridine (N1–C2) present intermediate single/double bond character. These features are in agreement with the chemically-intuitive resonant structure displayed in Figure 1. As shown on Figure 3a, the UV/Vis absorption spectra of SBPa measured in the 300–600 nm range present a strong band in the 350–550 nm region. Additionally, important variations of the absorption band positions and intensities are observed while increasing the solvent polarity. In Table 1, the TD-DFT wavelengths of the three lowest electronic transitions are compared with experimental measurements and the calculated absorption spectra are displayed in Figure 3b. Among the first computed transitions, the S0(opt) ? S2(FC) transition is the only excitation with a significant oscillator strength. It can thus be reasonably ascribed to the observed absorption band. For this transition, the TD-PBE0/631+G(d)//PBE0/6-31+G(d) computation scheme leads to absolute deviations with experimental data of 0.12, 0.01 and 0.12 eV in toluene, THF and ACN, respectively, which is in the line of the expected accuracy for TD-DFT [37]. Furthermore, the observed negative solvatochromism is nicely reproduced by our calculations. The hypsochromic displacements of the kmax measured while going

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A. Perrier et al. / Chemical Physics Letters 515 (2011) 42–48

(a)

1.385

11 1.404

12 1.416

1.369

10

0.05

2 1.332

3 1.394

1

1.428

15

1.380

1.354

7

4

N

N

14

0.06

N

1.431

13

(b)

8

6

9

5

S1(opt)-S0(opt) S2(opt)-S0(opt)

Toluene

0.04 0.03

Δr (Å)

0.02 0.01 0.00 -0.01

C12C13

C11C12

C11C10

C10C15

C10-N8 N8-C7

C7-N1

N1-C2

C2-C3

C3-C4

-0.02 -0.03 -0.04 -0.05

(c)

0.06 0.05

S1(opt)-S0(opt) S2(opt)-S0(opt)

ACN

0.04 0.03

Δr (Å)

0.02 0.01 0.00 -0.01

C12C13

C11C12

C11C10

C10C15

C10-N8 N8-C7

C7-N1

N1-C2

C2-C3

C3-C4

-0.02 -0.03 -0.04 -0.05

Bond distances Figure 2. (a): Representation of SBPa with the atom numbering, the bond distances of the ground state minimum S0(opt) in toluene (in Å) are also indicated in italics. (b) and (c): Variations of the C–C and C–N bond distances (in Å) on going from S0(opt) to S1(opt) (light blue) and from S0(opt) to S2(opt) (dark purple) in toluene and ACN. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this Letter.)

from toluene to THF and from THF to ACN are, respectively, 19 nm (0.13 eV) and 30 nm (0.23 eV) while theory yields 36 nm (0.23 eV) and 17 nm (0.12 eV). Nevertheless, upon increasing the solvent polarity, TD-DFT calculations predict a small decrease of the intensity of the absorption band whereas a strong hyperchromic effect is observed. This result is unsurprising since TD-DFT’s oscillator strengths are known to be less accurate than the corresponding excitation energies [38–40]. In the following, we discuss the nature of the two first excited states since both S1 and S2 play a part in the absorption (S2) or emission spectra (S1, next section). The S0(opt) ? S1(FC) and S0(opt) ? S2(FC) transitions arise, respectively, from a HOMO ? LUMO and HOMO1 ? LUMO electronic excitations. The representation of these orbitals as well as their energies in the different solvents are given in Figure 4. The occupied orbitals are mainly localized on the betaine part whereas the LUMO is centred on the pyridine ring. Therefore, it appears clearly that both transitions have CT character from the betaine donor group to the pyridine acceptor. The evolution of the frontier orbital energies while

increasing the solvent polarity helps us to rationalize the negative solvatochromism of the absorption band. From toluene to ACN, there is a quasi parallel energy stabilization of the HOMO1 and HOMO and a concomitant small destabilization of the LUMO. These evolutions result in the increase of the HOMO–LUMO and (HOMO1)–LUMO gap and thus in a hypsochromic shift of the corresponding absorption wavelengths. In order to obtain additional information on the nature of the excited states, we analyze the variation of the charge distribution and dipole moment from ground to excited states. In Figure 5 and 6, all the computed dipole moments for the S0(opt), S1(opt) and S2(opt) geometries are reported. The variation of the MK charge distribution in the FC region is represented in the left-hand side of Figure 5 for toluene (Figure 6 for ACN). In toluene, for the ground state S0(opt), the dipole moment is orientated from the betaine to the pyridinium moiety. The computed dipole moment is +10.3 D, this value being in good agreement with the experimental value of +10.33 D reported by Alcalde et al. in dioxane [9] and the +9.1 D value (in vacuum) that we obtained in a previous study

A. Perrier et al. / Chemical Physics Letters 515 (2011) 42–48

Figure 3. (A) Experimental and (B) simulated UV/Vis absorption and emission spectra in toluene, THF and ACN (black solid, green dashed and red dotted-dashed lines, respectively). The theoretical curves are obtained using a convolution Gaussian function with a FWHM of 0.4 eV. More details concerning the experimental data acquisition are given in Ref. [15]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this Letter.) Table 1 PCM-TD-PBE0/6-31+G(d)//PCM-PBE0/6-31+G(d) absorption wavelengths (k, in nm), oscillator strengths (f) and orbital composition for the three first S0(opt) ? Sn(FC) vertical transitions of SBPa. Experimental values (wavelengths in nm and molar absorption coefficients in 103 M1 cm1) are taken from Ref. [15]. Solvent

Toluene

THF

ACN

State

Theory

45

benzene and pyridine moieties and is mostly localized on the pyridine ring. This charge distribution is thus in accordance with the ‘‘conventional’’ picture of SBPa (Figure 1). If we focus on the state responsible for absorption, the S0(opt) ? S2(FC) transition corresponds to a CT of 0.3jej from the benzene to the pyridinium group, the imidazole charge keeping a constant value. Accordingly, the value of the dipole moment decreases and is now +0.5 D for the excited state. This result is thus in agreement with experimental assumptions [8] and with the 1.5 D obtained through the solvatochromic study [15]. The CT character of the S0(opt) ? S1(FC) transition is more pronounced with a charge transfer from both the benzene (0.27jej = 0.69 + 0.42) and the imidazole group (0.24jej = 1.11 + 0.87) to the pyridinium ring. Consequently, this electronic excitation leads to an inversion of the dipole moment, the barycentre of the negative charges becoming closer to the pyridinium unit (see Figure 5). The value of the dipole moment is now 3.7 D and can be compared with the +0.3 D value obtained previously at the CIS/631G(d,p) level in the gas phase [18]. We are now analyzing the effect of the solvent on the charge distribution and dipole moment for the ground and excited states. For the ground state, the comparison of toluene (Figure 5) and ACN (Figure 6) shows that the charge difference between the (benzene + imidazole) donor groups and the pyridinium acceptor becomes larger when the polarity of the solvent increases (+0.69jej in toluene, +0.74jej in ACN). The variation of the ground state charge distribution can be understood considering that the charge delocalization is enhanced when the solvent polarity increases, in order to benefit from the solvent electrostatic potential. Accordingly, the dipole moment l of the ground state increases. For the first excited state S1(FC), the CT character is getting more important with the solvent polarity: in ACN (respectively toluene), the S0(opt) ? S1(FC) transition corresponds to a global 0.61jej (resp. 0.27 to 0.24 = 0.51jej) transfer to the pyridinium ring. Consequently, the absolute value of l for S1 at the S0(opt) geometry also increases gradually with the solvent polarity. The evolution of the dipole moment of S2 with respect to the solvent change is more subtle: the benzene to pyridinium CT also increases with the solvent polarity (0.32jej in toluene, 0.46jej in ACN). The barycentres of the positive and negative charges thus become closer and finally change their relative positions which leads to an inversion of the dipole moment in polar solvents (THF, ACN). 3.2. Emission spectrum and description of the first excited states after relaxation

Experiment



k

f

Orbital composition

k

S1(FC) S2(FC) S3(FC)

504 457 343

0.01 0.52 0.001

HOMO ? LUMO HOMO1 ? LUMO HOMO1 ? LUMO+1

– 438 –

6.7

S1(FC) S2(FC) S3(FC)

450 421 315

0.01 0.48 0.001

HOMO ? LUMO HOMO1 ? LUMO HOMO1 ? LUMO+1

– 419 –

10.7

S1(FC) S2(FC) S3(FC)

428 404 303

0.01 0.46 0.001

HOMO ? LUMO HOMO1 ? LUMO HOMO1 ? LUMO+1

– 389 –

13.0

through a solvatochromic analysis [15]. To analyze the charge distribution, one has to consider not two parts for SBPa (betaine and pyridinium) but three as illustrated on Figure 5: (i) the benzene ring and (ii) the imidazole bridge N–C–N, behaving as two distinct donor groups, and (iii) the pyridinium group which is the only acceptor unit. According to this partition, for the ground state S0(opt), the imidazole bridge bears a negative charge (1.11jej) while the compensative positive charge is shared between the

Experimentally, Figure 3a shows that SBPa presents an emission spectrum with one large band centred, respectively, at 669, 675 and 687 nm in toluene, THF and ACN. As stated in the introduction, the fluorescence band thus shows moderate positive solvatochromism suggesting that the emissive transition does not involve the same excited state as the CT absorption transition. The significant Stokes shift is also an indication of an electronic and/or structural change in the excited state [25]. We thus investigate the nature of the excited state involved in the emission spectrum. Starting, respectively, from S1(FC) and S2(FC), we obtained two optimized structures, resp. S1(opt) and S2(opt), which are both planar and close to C2v symmetry. As a fundamental result, one can note that geometry optimizations starting with a twisting angle between the betaine and the pyridinium parts of the molecule systematically led to planar structures. This thus discredits the twisted intramolecular charge transfer (TICT) mechanism for SBPa. This finding is supported by the work of Ishida [41,42] who has studied the coupling between solvation process and electronic structure for the pyridinium N-phenoxide betaine. For the ground state, in the gas phase, steric conflicts favor a twisted geometry

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A. Perrier et al. / Chemical Physics Letters 515 (2011) 42–48

0 5 .

TOL

THF

ACN

LUMO

S0(opt) →S1(FC)

-2.6

-2.8

-3

-3.2

S1(opt) →S0’ -3.4 -5.4

S0(opt) →S2(FC)

-2.4

LUMO

LUMO

0 5 .

-5.6

HOMO

HOMO

-5.8

-6

-6.2

HOMO-1 HOMO HOMO-1

HOMO-1

-6.4 Figure 4. Energy of the frontier orbitals (in eV) for the ground state geometry S0(opt) (red) and for the S1(opt) geometry (blue). The topology of these orbitals calculated at the PCM (toluene)-PBE0/6-31+G(d) level for the S0(opt) geometry is also given (contour threshold: 0.03 a.u.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this Letter.)

Figure 5. MK charge distribution and dipole moment in toluene of the ground state and the two first excited states for the S0(opt), S1(opt) and S2(opt) geometries. The MK charges are summed over the benzene ring, the N–C–N imidazole bridge and the pyridine ring, respectively. The position of the barycentric positions of the positive and negative charges is given for the S0(opt) geometry.

while the presence of polar solvent weakens the torsional dependence of the charge separation [41]. Moreover, a dynamic study [42] revealed that solvation in the short-time region may be faster in the charge-transfer process than the twisting motion, which weakens the TICT hypothesis. Based on this work, Pinheiro et al. [19] have recently shown that the consideration of torsional effect

is not relevant to study the solvatochromic behavior of large betaines. Figure 2 shows the variations of the interatomic distances on going from S0(opt) to S1(opt) and from S0(opt) to S2(opt) in toluene and ACN. While relaxing on the excited state potential energy surfaces, the bond length variation does not exceed 0.05 Å, indicating

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A. Perrier et al. / Chemical Physics Letters 515 (2011) 42–48

Figure 6. MK charge distribution and dipole moment in ACN of the ground state and the two first excited states for the S0(opt), S1(opt) and S2(opt) geometries. See Figure 5 for more details.

only small geometrical changes. A common point between S1(opt) and S2(opt) is the loss of aromaticity for the pyridinium ring with N1–C2 and C3–C4 elongations versus a C2–C3 bond length diminution. This effect slightly depends on the solvent whereas the solvent influence is clearly seen on the shortening of the C7–N1 bond. Indeed, the decrease of the central CN bond length is more pronounced in polar solvents and is accompanied by a stretching of the C–N bond in the imidazole bridge for both S1(opt) and S2(opt). A distinction between the two first excited state minima appears in the geometry of the benzene ring: there is an increase of aromaticity for S1(opt) constrasting with a loss of aromaticity for S2(opt). For the former, C12–C13 and C11–C10 bond shortenings indicate a larger ‘‘double bond’’ character while the C11–C12 lengthening gives evidence of a more ‘‘single bond’’ character. The trends are opposite for S2(opt). The vertical S1(opt) ? S00 and S2(opt) ? S000 transition wavelengths computed for the three different solvents are reported in Table 2. Unlike the S2(opt) case, the predicted vertical transition from S1(opt) is in very good agreement with the position of the emission band with signed errors of 0.09 eV in toluene and THF and 0.07 eV in ACN. Therefore, the assignment of the fluorescence spectrum shows that the emissive state corresponds to the first excited state while the absorption band involves the S2 state. It is also worth noting that the moderate negative solvatochromism of the emission band is reproduced by TD-DFT calculations with a calculated red-shift of +6 nm while going from toluene to THF (experimentally +6 nm) and of +5 nm from THF to ACN (exp. +12 nm). The S1(opt) ? S00 transition corresponding to a LUMO ? HOMO electronic relaxation, we have plotted the energy of the frontier orbitals as a function of the solvent polarity in Figure 4. Interestingly, the topology of the frontier orbitals does not change while relaxing from the Franck–Condon region to the excited state minimum. When the solvent polarity increases, the HOMO and the LUMO of S1(opt) are stabilized in the same way. Therefore, the HOMO–LUMO gap is nearly constant and this can account for the moderate solvatochromism of the emission band. Furthermore, TD-DFT calculations predict a Stokes shift of 246 nm (0.95 eV) in toluene, 288 nm (1.20 eV) in THF and 310 nm (1.33 eV) in ACN

Table 2 Computed emission wavelength for the two first excited states in toluene and ACN. Solvent

State

Theory

Experiment [15]

Toluene

S1(opt) ? S00 S2(opt) ? S000

703 460

669 –

THF

S1(opt) ? S00 S2(opt) ? S000 S1(opt) ? S00 S2(opt) ? S000

709 443 714 457

675 – 687 –

ACN

and are again on the spot with respect to experimental data: 231 nm (0.98 eV) in toluene, 256 nm (1.12 eV) in THF and 298 nm (1.38 eV) in ACN. The variation of the charge distribution while relaxing on the excited state potential energy surface is then analyzed. If we focus on the toluene in Figure 5, we can see that there is no major changes of the charge distribution of the S1 state on going from S0(opt) to S1(opt). Indeed, the relaxation on the S1 excited state potential energy surface is characterized by a small reverse charge transfer from the pyridinium ring to the imidazole bridge and thus an increase of the dipole moment from 3.7 D for the S0(opt) geometry to 1.4 D for the S1(opt) geometry. At the same time, for the ground state, the charge distribution calculated for the S1(opt) geometry is very similar to the S0(opt) geometry. To pave the way towards a complete mechanistic study and help to understand ultrafast data measured on SBPa [43], we also analyze the variation of the charge distribution while relaxing on the non-emissive S2 state. This relaxation process is also accompanied with a reverse charge transfer of ca. 0.10jej, from the pyridinium unit to the benzene ring. Therefore, the dipole moment increases from +0.5 D for the S0(opt) geometry to +3.8 D for the S2(opt) geometry. Consequently, more globally, the previously described geometry modifications are in accordance with the variation of the charge distribution. From the ground state of S0(opt) to the first excited state of S1(opt), there is a strong charge transfer from the betaine to the pyridinium (0.45jej) which results in the loss of aromaticity of the latter moiety. From the ground state of S0(opt) to the

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A. Perrier et al. / Chemical Physics Letters 515 (2011) 42–48

second excited state of S2(opt), the charge transfer is more limited: the benzene moiety gives 0.24 jej to the pyridinium ring. The loss of aromaticity of the pyridinium is thus less pronounced for S2(opt) than for S1(opt) and some geometrical differences appear in the betaine moiety between S1(opt) and S2(opt) because of the differences in charge transfer intensity. Increasing the solvent polarity yields the same qualitative conclusions for both the S1 and S2 states. However, quantitatively, the reverse charge transfer observed while relaxing on the excited state potential energy surfaces is more important and the increase of the dipole moment is thus more pronounced. For instance, for S1, from the FC region to the S1(opt) geometry, the dipole moment variation is +2.3 D in toluene (1.4 + 3.7) and +3.8 D in ACN. Finally, it is worth noting that the S2 dipole moment for the S2(opt) geometry is still sensitive to the solvent polarity with a value of 3.8 D in toluene and 4.9 D in ACN. This result is in accordance with the solvatochromic behavior of the absorption band. Reversely, it is found that S1 dipole moment at the S1(opt) geometry is nearly constant with respect to the solvent polarity (1.5 D in toluene, 1.4 D in ACN) which is fully consistent with the non solvatochromic behavior of the emission band described above.

4. Conclusions The structure and the nature of the excited states involved in the absorption and emission spectra of SBPa has been investigated using (TD)-DFT calculations. Three different solvents (toluene, THF and ACN) have been studied within the framework of the PCM approach. The agreement between our TD-DFT calculations performed at the PCM-PBE0/6-31G+(d) level and the measured data is very satisfying with an absolute average error of 0.08 eV for both the position of the absorption and emission bands (average over the three solvents). Moreover, our calculations are able to reproduce the strong hypsochromic shift of the absorption band and the moderate solvatochromism of the fluorescence band. A simple analysis of the energy gap between the frontier orbitals involved in the electronic transition is efficient enough to rationalize the observed solvatochromic behavior. TD-DFT calculations also show that S2 is the absorption state while S1 is the emissive state. Both states have charge transfer character. The analysis of MK charges have shown that S2 present a charge transfer from the benzene ring to the pyridinium moiety while the CT of S1 is more pronounced with a charge migration from the entire betaine to the pyridinium part. The nature of the excited states is preserved while relaxing on the excited state potential energy surfaces. To go further in the comprehension of the photoinduced ICT reaction, the interesting photophysics of SBPa, which involves the two different singlet states fully characterized in this Letter, will be unveiled thanks to a combination of steady-state and femtosecond spectroscopies supported by the results of our TD-DFT calculations [43]. Acknowledgments This work was performed using HPC resources from GENCI-CINES (Grant 2011-c2011086680). The authors thank the CNRS for fundings through the GDRI 93 PHENICS. The authors

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