Single configuration interaction study on conjugated betainic chromophores based on DFT optimized geometries

Journal of Molecular Structure (Theochem) 469 (1999) 163–176

Single configuration interaction study on conjugated betainic chromophores based on DFT optimized geometries J. Fabian a,*, G.A. Rosquete a,b, L.A. Montero-Cabrera b a

Technische Universita¨t Dresden, Institut fu¨r Organische Chemie, Mommsenstrasse 13, 01062 Dresden, Germany b Facultad de Quı´mica, Universidad de la Habana, Havana 10400, Cuba Received 10 August 1998; received in revised form 13 November 1998; accepted 16 November 1998

Abstract The structure of a series of heterocyclic betaines was calculated by methods of density functional theory (DFT). The charge distribution and bond characteristics of these compounds were analyzed by Weinhold’s natural bond orbital analysis (NBO) and by natural resonance theory (NRT). In order to probe the aromatic character of the ring fragments, Schleyer’s nucleusindependent chemical shifts (NICSs) were calculated by GIAO-RHF. Ab initio single configuration interaction calculations (SCI) correctly predict intense p ! p * transitions at low energies, but the transition energies of the color bands are overestimated. Torsion around the interfragmental bond increases the charge separation between the molecular fragments and the dipole moment. The molecular fragments become increasingly aromatic. The absorption wavelengths increase on torsion while the oscillator strengths decrease. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Pyridinium betaines; Density functional theory; Single configuration interaction; Molecular geometry

1. Introduction There is a current interest in conjugated organic compounds that have a high dipole moment in the molecular ground state and display a large change of the dipole moment upon excitation. This interest is mainly because of the strong solvatochromic effect of these compounds dependent on the enviromental effects [1,2], and to potential applications in nonlinear optics (NLO) [3–6]. One of the series more extensively investigated experimentally embraces compounds with the pyridinium residue that is linked to different types of donor groups. The donors may be directly attached to the pyridinium nitrogen or to one of the carbon atoms of * Corresponding author. Fax: 149-351-463-7030. E-mail address: [email protected] (J. Fabian)

the ring. If linkage occurs to nitrogen or to carbon in meta-position, the resulting compounds are betainic. These structures are indicated by the formulas A and B. The compounds may lose their betainic character when charge transfer occurs upon electron excitation. When substitution occurs in para-position, the betainic character may completely vanish and the quinoid formulas appear more appropriate. This type of compound is characterized by the formulas C with a dominant contribution of the quinoid resonance structure. Nevertheless, the latter mentioned compounds may also be polar. In these cases electron excitation leads to an excited state of predominantly betainic character. The series A to C have one point in common: the spectral absorption are sensitive to solvent effects with mostly negatively solvatochromic shifts in series A and B and potential positively solvatochromic shifts in series C. In the first case, the

0166-1280/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0166-128 0(98)00585-5

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dipole moment in the ground is larger than in the excited state (m g . m e) whereas in second case the opposite holds (m g , m e). Twisting of C-type compounds around the interfragmental bond should increase the weight of the dipolar resonance structure and change the compound from the predominantly quinoid to a betainic one. Reichhardt et al. have much contributed to the present knowledge about solvatochromic betainic and polar compounds. The subject has been reviewed in detail [1,2]. The zwitterionic dye D known as Reichhardt betaine dye belongs to the series A. These dyes are the best studied solvatochromic dyes of the type A so far.

valence electron treatments used for calculation of spectral data employ the singles-only configuration interaction method (CNDO/S, ZINDO/S) using empirical scaling factors for the different type of overlap [10]. AM1/CI and related methods (including singly-excited and, optionally, pairwise doublyexcited configurations) without scaling have been systematically tested for hydrocarbons only [11]. Both kinds of semiempirical approaches were applied in studies of solvatochromism [12,13]. Less approximate calculations of electron excitation by first-principle methods are expensive or for larger compounds not feasible at all (e.g. by MRCI [14], CIPSI [15],

Structure and spectral properties of compounds of the series A to C, including the dye D, were mostly calculated by semiempirical methods. Although the Pariser–Parr–Pople method [7] was very useful in calculating electron transitions of conjugated systems, the results were less satisfactory with zwitterionic compounds [8]. As a result of the partial charge there is some arbitrariness and uncertainty in selecting or defining heteroatom parameters of atoms in that case. Problems were also reported for compounds known as mesoionic heterocycles [9]. The most all-

CASPT2 [16] or EOM-CCSD [17]). There is less experience in calculating excitation energies and excited state properties of dye molecules with the computationally less demanding singles-only ab initio configuration interaction (SCI) method [18,19]. As shown for various small compounds, insufficient consideration of dynamic correlation generally results in absorption wavelengths that are too low. The aim of this article is to derive the molecular and electronic structure of various compounds of the series A, B and C. Solvatochromic effects of some

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˚ ) calculated at the B3LYP/6-311G** level of theory, GIAO-SCF/6-311G** NICS values (ppm) and Fig. 1. Selected bond lengths (A aromaticity indices I.

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dyes will be reported in a forthcoming article. Density functional theory (DFT) optimum geometries were calculated using the B3LYP functional using extended basis set. The electron distribution was derived from natural bond orbitals (NBO) and natural resonance theory (NRT). Cyclic electron delocalization (aromaticity) was evaluated by the nucleus-independent chemical shifts (NICS). Based on the DFT geometry, the lowest-energy vertical transitions were calculated by the ab initio configuration interaction limited to singly excited states (SCI) and, for the sake of comparison, by semiempirical methods. Finally, the solvatochromic pyridinium phenolates were taken as examples for analyzing the effect of torsion around the interfragmental bond.

2. Computational DFT in the Kohn–Sham (KS) formalism proved to be a successful alternative to traditional methods using configuration expansions [20]. The introduction of gradient-corrected density functionals (‘‘generalized gradient approximation’’, GGA) has considerably improved the accuracy of numerical predictions [21,22]. The functional used throughout this study consists of a non-local hybrid DFT/HF exchange functional as defined by Becke’s three-parameterequation [23] in conjunction with the non-local Lee–Yang–Parr correlation functional [24] (B3LYP for short). The hybrid methods are clearly superior to non-hybrid ones. The high performance of the nonlocal functionals such as B3LYP has not only been documented with relative energies, but also with calculated molecular geometries [25]. In addition, DFT electron densities calculated from the SCF converged KS orbitals are found to be sufficiently accurate [26,27]. In order to treat the zwitterionic structures properly the basis set 6-311G** was used throughout this study. The molecular structure was fully optimized by analytical gradient techniques and the minimum structures were checked by frequency calculations. The electronic structure of the molecules is characterized by atomic charges and bond orders. As the extended basis is employed in this study, including a set of diffuse functions atomic charges are calculated by Weinhold’s natural population analysis (NPA) [28]

rather than by Mulliken’s population analysis that is commonly used. Making use of the first-order reduced density matrix, the molecular electron dristibution was analyzed in terms of localized electron pair bonding units corresponding to one center lone pairs and two-center bonds (NBO). These are the elements of the chemist’s Lewis structure picture. The NRT analysis [29–32] of the one electron density in terms of resonance structures (Lewis-type structures) provided the description of the electronic structure in terms of Lewis structures and their weights. The calculations in this study were performed with the program NBO 4.0 implemented into GAUSSIAN 94 in place of the former NBO 3.0 version. The NRT calculations were done by the default NRT option but in some cases the number of reference structures was extended by means of the NRTSTR key list to estimate the contributions of additional Lewis structures of low weight. The benzoid and quinoid nature of the chromophoric systems was evaluated by the atomic charges obtained by NPA of the fragments. Another valuable source of information may be magnetic properties. The criterion of the NICS was introduced by Schleyer et al. [33]. This quantity is defined as the negative magnetic shielding at the center of the ring. These values were calculated by GIAO-RHF using the 6-311G** basis set. According to Schleyer et al., NICS values less than about 23 ppm are observed for aromatic ring systems and more than about 13 ppm for antiaromatic rings. Cyclic compounds with values in the range between 23 and 13 ppm are non-aromatic, e.g. cycloolefinic or quinoid [34]. A traditional parameter in terms of bond orders is the so-called aromaticity index I [35,36]. This index is calculated from the bond orders. By definition, the aromaticity index I is 100% for benzene. The aromaticity indices in this article are calculated from Wiberg’s bond indices within the NBO program [37]. The SCI method is used as detailed in Ref. [18]. The method enables the vertical transition energies (or absorption wavelengths) and oscillator strengths to be calculated. Atomic charges and bond characteristics and their changes upon excitation from the ground to the lowest energy excited state were calculated by natural population analysis. Non-empirical calculations were performed with the GAUSSIAN 94 series of programs [38] and

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Table 1 ˚ and angles of torsion in degrees around this bond, dihedral angles in degrees of the Bond lengths R of the interfragmental bond C3N2 (C3C2) in A methyl group at the pyridinium nitrogen

3 4 5 6 7 8 9 10 11

1.389 1.542 1.406 1.425 — — — 1.411 1.477 a b

15.2 32.0 a 30.4 0.0 b — — — 2.5 16.9

— — — — 88.9 92.8 2 86.8 2 86.8 2 95.6

Exp. value: u ˆ 43.38 [46]. Exp. value: u ˆ 2 1.98 [50].

semiempirical calculations of spectral data by PM3/CI and ZINDO/S calculations by the program packages VAMP [39] and HyperChem [40], respectively.

3. Results and discussion 3.1. Molecular structure and electronic delocalization The compounds studied, the calculated geometric parameters and some characteristics of the electronic structure are assembled in Fig. 1. The CC bond lengths of the pyridinium ions 1 and 7 are nearly equal in lengths and close to the bond length of ˚ . The CN bonds are shorter benzene about 1.40 A ˚ . The aromaticity indices and amount to about 1.35 A I of these compounds of about 81% which indicate aromatic bond delocalization. The same conclusion is reached from NICSs. The NICS values of 1 and 7 (about 2 9 ppm) are in the same order of magnitude as that calculated for benzene ( 2 9.7 ppm [33]). This feature is retained or altered with substitution at the pyridinium by another residue. Replacement of

methyl or hydrogen in 7 by the negatively charged oxygen leads to pyridine N-oxide (2) and N-methyl4-pyridone (9), respectively. As shown in Fig. 1, the cyclic system remains fully delocalized in 2 (NICS: 29.3 ppm) while localization and quinoid character is indicated in 9 (NICS: 22.6 ppm). Substitution of hydrogen of 7 by oxygen in metaposition to nitrogen results in N-methyl-pyridinium-3olate (8). This heterocycle can no longer be written by a single resonance formula without invoking octet expansion at nitrogen. In accordance with the betainic formula, the CO bond is longer than in quinoid structures. The CyO group of 8 is linked by two essential single bonds in the ring structure. Both the aromaticity index and the magnetic criterion indicate a reduced aromatic bond delocalization. This is not reflected in the formula. N-methyl-pyridinium-3-phenolate (8) belongs to the mesoionic heterocycles [41,42]. The related pyridinium-3-phenolate has been characterized spectroscopically [43]. It is a reactive transient compound that evaded any structural investigation hitherto. A strong variation of the properties of pyridine with

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Fig. 2. Lewis-type resonance structures and their weights calculated by natural resonance theory.

substitution is encountered in the molecular and electronic structure of composite molecules with this fragment, such as 3 to 6 and 10 and 11. The two molecular moieties are bound coplanarly in 6 (Table 1) but are more or less twisted in other cases. The angle of twist is surprisingly low for 4-[1-methyl-4(1H)-pyridinyliden]-2,5-cyclohexadien-1-one (or 4-(N-methyl)pyridinum-4-phenolate) (10) with an interfragmental ˚ that is longer than the CC double bond of 1.41 A

bond. In general, twisting is large when the pyridinium ring is linked to six-membered rings with hydrogen atoms in the ortho-positions (about 308). As a result of the underestimation of the torsional angle by DFT [44,45], the dihedrals of the compounds may be actually larger. In fact, according to the Xray analysis of N-pyridinium borinate (4), the interfragmental angle of twist is 43.38 [46,47] compared with 32.08 calculated by DFT. The MBPT(2)/6-31G*

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Table 2 NPA charges of the pyridinium fragment Qpyr, dipole moments m in Debye in the ground and the lowest energy excited singlet state (vertical excitation) and change in these quantities (DQpyr and Dm , respectively) upon excitation. Total energies of the DFT ground state and of the lowest energy excited state (SCI) are given in hartrees a

2 3 4 5 6 8 9 10 11 a b

Qpyr DFT

Qpyr RHF

Qpyr* SCI

DQpyr

m DFT

m RHF

m * CIS

D mb

Etot (DFT)

Etot (SCI)

0.54 0.26 0.24 0.28 0.47 0.67 0.62 0.37 0.61

0.66 0.38 0.33 0.43 0.58 0.79 0.72 0.42 0.74

0.44 0.08 2 0.39 0.09 0.02 0.57 0.24 0.31 0.19

2 0.22 2 0.30 2 0.72 2 0.34 2 0.56 2 0.22 2 0.48 2 0.11 2 0.55

4.46 5.15 7.29 11.46 8.00 7.61 7.74 14.14 17.33

5.52 7.32 9.35 14.92 10.81 8.42 8.22 15.20 20.13

4.05 3.71 2.41 9.50 1.09 6.48 3.85 14.18 9.60

2 1.47 2 3.61 2 6.94 2 5.42 2 9.72 2 1.94 2 4.37 2 1.02 2 10.53

2 323.47188 2 441.19355 2 466.72713 2 554.54658 2 626.96700 2 362.82139 2 362.83998 2 593.88050 2 593.85014

2 321.29893 2 438.18765 2 463.53197 2 550.94238 2 622.87095 2 360.41297 2 360.39755 2 589.97906 2 589.99677

1 hartree ˆ 627.52 kcal/mol. Differences between the dipole moment of the excited state obtained by SCI and the RHF ground state. Both are directed in the long axis.

calculation predicts the molecular distortion more close to the experiment (40.48). However, any more general conclusion about geometrical features and electron delocalization are not affected by this numerical error. N-pyridinium benzimidazolates were extensively studied by Alcalde et al. [48–50] and the parent structure 6 has been more recently calculated by ab initio quantum chemical methods by Abe et al. [51]. The X-ray structure of 6 with a very small angle of ˚ (intratwist (1.98) and CN bond lengths of 1.360 A ˚ (inter-bond) fits quite well the bond) and 1.450 A geometry of the planar structure predicted by DFT (Fig. 1). Reichhardt betaine dye (D) is distinguished from 5 by five additional phenyl groups. This substitution determines the actual molecular distortion and the interfragmental CN bond length. The X-ray analysis of a brom-substituted D resulted in a distortion of ˚ [52]. The experi658 and CN bond length of 1.48 A ˚) mental CO bond length found in this analysis (1.29 A exceeds considerably the CO bond length calculated ˚ ). CO bond lengths of for parent compound (1.25 A ˚ about 1.25 A are rather predicted for mesoionic heterocycles such as 8 and 11. According to the aromaticity indices I and NICSs, the pyridinium betaines 3 to 6 consist of fragments exhibiting aromatic bond delocalization. NICSs of the pyridinium fragments are in the range between 25.8 and 27.7 ppm. The lowest aromaticity (largest NICS value) is calculated for N-pyridinium cyclopentadienide (3). The interfragmental CN bond in 3 is considerably shorter than the corresponding bonds in

5 and 6, thus indicating a stronger interfragmental interaction. NICSs of the donor fragments of the composite molecules 4 to 6 scatter over a large range. While NICS of the cyclopentadienide ring is relatively low (29.0 ppm), the chemical shift of the phenolate fragment of 5 is relatively high (23.3 ppm). This outcome is rationalized by the fact that the NICS value of the deprotonated phenol is likewise high (26.2 ppm [53]) compared to benzene (29.7 ppm [54]). The strong bond fixation in the phenolate donor moiety of 5 contrasts with the bond length equalization in the pyridinium acceptor fragment. The information obtained about the structure of 10 is inconsistent. Apart from the relatively long interfragmental bond, the CC-bond shows the expected quinoid bond alternation. The aromaticity criteria confirm this classification. However, the bond distances of 10 differ not greatly from those of 5, which is the parent chromophore of the betaine dye D. As shown by the consideration of dipole moment and the change in the dipole moment on excitation the particular position of the dye 10 is confirmed. 3.2. Charge distribution The natural resonance theory was employed to describe zwitterionic or polar structures in terms of theoretically derived Lewis-type structure. The results of three representative examples are shown in Fig. 2 (NRT default option). As expected, the quinoid Lewis–Kekule´-type structure dominates in the case

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Fig. 3. Change in energy of N-pyridinium-4-phenolate (5) upon torsion around the interfragmental bond and concomitant changes of the charge at pyridinium fragment (Qpyr) and nucleus-independent chemical shifts (NICSs) of both fragments. The molecular geometries are fully optimized except the angle of rotation.

of 9 (45%). Additional polar resonance structures contribute to the actual charge delocalization. A balanced description of Lewis-type structures obtained for pyridine N-oxide (2) in the NRT non-

default option resulting in some changes in the weights of the main contributors. In this case, the two Lewis–Kekule´-type structures with complete N–O charge separation predominate (51%, in total).

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Additional resonance structures indicate the transfer of charge from oxygen to the ortho- and para-positions of the ring. While the result of the NRT analysis of 2 and 9 obviously reflect the expectation, the outcome of the analysis of 8 is less obvious. In accordance with the large carbonyl character of CO in 8 mentioned earlier the two Lewis–Kekule´-type structures (total contribution of 41%) appear to be composed of the enone and azomethine ylide fragments. This result essentially confirms an earlier conclusion drawn from p -MOs after transformation of canonical MOs to the localized MOs as localization transformation, [55]. As is well known, compounds like 8 actually behave as ylides in chemical reactions (1,3-dipoles [56]). Lewis-type structures equivalent to the common and convenient pyridinium olate formula, however, have a minor weight only (less than 10%). Lewis structures contain formal charges that correspond to p -charges rather than total atomic charges. As expected, the p -charges at the nitrogen atoms of nitrogen-containing rings are positive for all compounds considered. A more general probe of the betainic character is the total charge of the heterocyclic ring moiety indicating the charge transferred from the donor and acceptor fragment of the molecules. In all cases, the heterocyclic moieties bear a positive charge lower than the hypothetical charge 11 corresponding to the pyridinium representation. As a result of the RHF based excited state SCI calculation discussed vide infra RHF fragment charges are listed in addition to DFT charges. Changes in the electron distribution on excitation refer to SCI (excited state) and RHF charges (ground state). The pyridinium fragments of all compounds considered gain electrons on excitation (negative DQpyr values in Table 2). The planar betaine dyes 4 and 6 display the largest changes in the charge and, concomitantly, of the dipole moment. Apart from the mesoionic compound 11 large ground state dipole moments are calculated for the iso-p -electronic compounds 5 and 10. The parent compound of D is the only one which exhibits both a high ground state dipole moment and a relatively strong change on excitation. This is in principal agreement with the available experimental data. The experimental dipole moment of the strongly negative solvatochromic Reichhardt betaine dye (D) amounts to 14.6 Debye [57,58] and decreases on excitation by

171

5.9 Debye. However, the outcome of the related compound 10 is unexpected. The large ground state dipole moment of 10 likewise decreases, though to a lesser extent. Positive solvatochromism is rather suggested by the quinoid formula. Another comparison between theory and experiment is afforded by N-pyridinium benzimidazolate (6). The calculated dipole moment of 8.00 Debye is lower than the experimental value (10.33 Debye [48– 50]). The calculated dipole moment of N-pyridinium cyclopentadienide (5.15 Debye) is clearly at variance with the experimental one (13.2 Debye quoted in Ref. [1]). Torsion around the bond interconnecting the donor and acceptor fragment will affect the interfragmental charge transfer and the change the dipole moment. Npyridinum-4-phenolate (5) and 4-(N-methyl)pyridinum-4-phenolate (10) was selected to show the effect of structural change on the charge-transfer in the betainic structure. As shown in Fig. 3, the charge separation increases when the torsional angle is increased beyond the optimum torsional angle (30.48). The transition structure at 908 defines the barrier to torsion. The barrier height is about 10 kcal/mol. Thus, the rotamers should quickly interconvert at room temperature. When torsion occurs, or is forced by steric hindrance, the charge separation between the pyridinium- and phenol-like moiety is enhanced. The positive charge at the pyridinium increases by about 0.25 to 0.60 (see Fig. 3). As the interaction between the moieties is not removed with nonplanarity in the orthogonal arrangement, the charge separation is far from 100% (70%). The increase in charge separation results in an increased aromatic character of the fragments. This is supported by the NICS values (Fig. 3). As mentioned earlier, the relatively low NICS values of the phenolate-like fragment is because of the O 2 substituent effect. Unexpectedly, the dipole moment of 10 is not only very large but also reduced by excitation. This contradicts its quinoid structural formula and suggests negative rather than a positive solvatochromic dye. 3.3. Electronic excitation Electronic excitation energies calculated by SCI are listed in Table 3 and are compared with the available experimental data. The single-only ab initio approach

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Table 3 Calculated vertical SCI transition energies a in wave numbers˜ (10 23 cm 21) of the lowest-energy transition and oscillator strengths (B3LYP/6311G** optimum geometries). Experimental spectral data, results of semiempirical PM3/CI b, ZINDO/S c and CNDOL/22 d calculations and energy differences of the Kohn–Sham frontier orbitals (cm 21) e are given for comparison

2 3 4 5 6 8 9 10 11

Trans.

SCI/6-311G** nÄ f

PM3/CI nÄ f

ZINDO/S nÄ f

CNDOL/22 nÄ f

DFT nÄ

Exp. nÄ

p p p p p p p p p p

42.3 24.6 27.8 23.6 29.6 33.5 45.6 50.3 30.9 35.4

33.9 21.2 16.1 17.2 21.9 26.9 44.1 34.3 23.0 30.0

33.4 19.5 — 16.4 16.9 27.2 46.4 38.7 29.0 26.9

41.5 16.1 17.5 14.5 19.8 29.7 41.4 44.1 27.3 38.4

37.3 22.0 19.2 18.1 20.3 30.0

36.3 f 19.6 g 21.2 h — 22.5 i 31.0 j 40.8 j 35.9 k — —

! p *, A1 ← A1 ! p *, A ← A ! p *, A ← A ! p *, A ← A ! p *, A1 ← A1 ! p* ! p* ! p *, A 0 ← A 0 ! p *, A 0 ← A 0 ! p*

0.22 0.69 0.23 0.81 0.58 0.23 0.23 0.52 1.70 0.62

0.12 0.34 0.20 0.32 0.47 0.13 0.38 0.15 0.57 0.47

0.44 0.88 — 0.89 0.45 0.26 0.65 0.55 1.03 0.99

0.04 0.17 0.01 0.13 0.02 0.06 0.07 0.08 0.38 0.24

42.6 23.1 9.53

a

Full configuration interaction of singly-excited configurations. 8 MOs were used in PECI. c Four highest occupied orbitals and four lowest unoccupied orbitals (singly-excited configurations). d Up to 75 basis orbitals [61,62]. e Cf. Ref. [59,60]. f In CH3CN, log 1 ˆ 4.14 [63]. g In CHCl3 [64,65]. h In THF, log1 ˆ 3.45 [46]. i In C6H6 [66]. j In 0.1 N NaOH, log 1 ˆ 3.95 [67]. k In CH3CN, log 1 ˆ 4.16 [63]. b

correctly predicts an intense electronic transition mainly of p ! p * character in all cases. However, SCI absorption wave numbers as well as oscillator strengths are greatly overestimated. Semiempirical transition energies are more close to the experiment. Interestingly, most of the experimentally known absorption wave number listed in Table 3 are well reproduced by the energy difference of the frontier KS orbitals (DFT HOMO-LUMO energy gap). Although the interpretation of KS orbitals is more complex than that of RHF ab initio orbitals [59], recent studies actually show a clear relationship between the experimental lowest energy electron transition and the calculated energy gap [60]. To reveal the general features, the results of the SCI calculation are considered. As exemplified are 5 and 10 in Fig. 4, the wavelength of the lowest-energy transition increases upon torsion while the oscillator strength decreases. This relationship is observed both for ‘‘benzoid’’ 5 and ‘‘quinoid’’ 10. In both cases the charge separation is most strong in the completely twisted (908) arrangement. In this arrangement, the

dipole moment are also largest. Thus, rotation out of the plane will result in a change in the charge separation. If this is caused by solvent effects the solvatochromic effect will be affected The MOs of the fragments dominant in the lower energy transitions essentially retain their p -type character (see Fig. 5). Although the definition strictly holds only for planar molecules, the first electronic transition of the slightly distorted 10 is essentially p ! p * with dominant CT-character such as in the orthogonal rotamer.

4. Conclusions and prospective This study included a series of polar compounds that differ greatly in structure and, in consequence, in molecular properties. Iso-p -electronic compounds appear as quinoid, betainic or mesoionic depending on the particular linkage of the molecular subunits. Characteristic structural and electronic features are revealed by the optimized molecular geometries,

Fig. 4. Change of absorption wavelengths (l ), oscillator strengths of color bands, charge at pyridinium (Qpyr) and dipole moment (m *) of N-pyridinium-4-phenolate (5) and the isop -electronic 4-(N-methyl)pyridinum-4-phenolate (10) upon torsion around the interfragmental bond. Single point calculation with unrelaxed DFT optimized ground state geometries.

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Fig. 5. Nature of the frontier orbitals of the tilted ground state N-pyridinium-4-phenolate (5) (C2-symmetry) and the compound with the fragments in orthogonal arrangements (Cs-symmetry).

natural atomic charges of the NBO analyses and Lewis–Kekule´-type resonance structures derived by the NRT calculations. When the molecule is composed of two fragments one of the donor and the other of the acceptor fragment, the charge distribution and the electronic excitation energy strongly depends on the nature of the fragments and the positions of the interfragmental linkage. As shown by inspection of the calculated dipole moments of the iso-p -electronic compounds 5 and 10 the outcome

of this interaction is not obvious. In contrast to the formula representation dye 10 is betainic rather than quinoid, and therefore more close to 5 than expected. The decrease of the dipole moment of 10 may result in negative rather than positive solvatochromism. This needs future verification. More generally, distortion around the interfragmental bond of 5 and 10 reinforces the zwitterionic character of the dyes. Thus, molecular distortion may be favored in polar solvents. The study of solvatochromic effect has

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likewise to consider molecular relaxation of the solute in solution.

[21] [22]

Acknowledgements The study was supported by ‘‘Deutschen Akademischen Austauschdienst’’ (DAAD). We are also grateful to ‘‘Deutsche Forschungsgemeinschaft’’ and to the ‘‘Fonds der Chemischen Industrie’’ for financial support of this research. The authors would like to thank Prof. Reichhardt (Marburg) for information and support.

[23] [24] [25] [26]

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