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Asymmetry in ground and excited states in styryls and methoxystyryls detected by NMR (13C), absorption, fluorescence and fluorescence excitation spectroscopy
Journal of Molecular Structure 988 (2011) 102–110
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Asymmetry in ground and excited states in styryls and methoxystyryls detected by NMR (13C), absorption, fluorescence and fluorescence excitation spectroscopy A.V. Stanova a,⇑, A.B. Ryabitsky b, V.M. Yashchuk a, O.D. Kachkovsky b, A.O. Gerasov b, Ya.O. Prostota b, O.V. Kropachev b a b
Taras Shevchenko National University of Kyiv, Physics Department, 2 Glushkova Prosp., 03022 Kyiv, Ukraine Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str., 03660 Kyiv, Ukraine
a r t i c l e
i n f o
Article history: Received 31 August 2010 Received in revised form 18 December 2010 Accepted 19 December 2010 Available online 25 December 2010 Keywords: Cyanine dyes Electron transitions Styryls Methoxystyryls Quantum-chemical calculations Spectroscopy
a b s t r a c t Combined quantum-chemical and spectral study of electron structure features of styryls and their oxyanalogues containing benzothiazolium, benzooxazolium, indoleninium, pyridium, quinolinium residues has been fulfilled. It showed that asymmetry degree of molecular geometry and charge distribution in the chromophore of styryls and methoxystyryls considerably differ in the ground and excited states. It was established that two the lowest transitions in styryls are splitting and involve both donor levels, similarly to symmetrical cyanines. If compare with methoxystyryls the long-wave high intensive absorption band is shifted bathochromically due to considerable interaction between the donor quasi-local chromophores. In contrary, because of the low position of a lone electron pair of oxygen in methoxystyryls, only one donor quasi-local chromophore is effective, hence such unsymmetrical dyes absorb appreciably higher. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Unsymmetrical cyanine dyes, including styryls and their methoxyanalogues, continue to be used widely, in parallel with corresponding symmetrical dyes, in numerous applications; they are widely used objects due to their unique electronic and spectral properties which form a basis for the design of new effective materials connected with the light conversion, i.e. spectral sensitization, molecular probing in biology, active and passive components for tuneable lasers, non-linear media exploring excited state absorption, etc. [1–4]. Also, symmetrical and unsymmetrical cyanine molecules remain convenient objects for developing of new theoretical concepts and quantum-chemical models [5–9]. Going from symmetrical dyes to unsymmetrical ones causes significant changes in distribution of the total positive charge in the polymethine chromophore, molecular geometry and hence in spectral properties [1,4,10–12]. The styryls were usually considered as unsymmetrical cyanines with an extremely high degree of asymmetry because of the low basicity of the p-dimethylaminophenylene residue as a terminal group. Nevertheless, the nature of the lowest electron transition is similar to the nature of the one in symmetrical cyanines. Inser⇑ Corresponding author. Tel.: +38 067 19 49 208. E-mail address: [email protected] (A.V. Stanova). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.12.038
tion of an alkyloxy group with a high electronegative oxygen atom instead of amino group into styryls is likely to lead to more considerable change in the molecule electron structure of the dyes and in the nature of the electron transitions. It is expected that the spectral effects should depend on the difference in donor strengths of other terminal groups. So, we use absorption and fluorescence spectra or, more correctly, electron transitions, for studying the asymmetry of the excited state, whereas the degree of the asymmetry in the ground state can be obtained from NMR spectra. Also the quantum-chemical calculations give the information about the asymmetry in the charge distribution and molecular geometry in both ground and excited states. This paper presents the results of the simultaneous quantumchemical simulations and spectral measurements of the absorption spectra of unsymmetrical styryls and methoxystyrys. 2. Experimental 2.1. Objects and methodology The general structures of investigated styryls 1 and corresponding methoxystyryls 2 containing indoleninium (In), benzothiazolium (BT), benzooxazolium (Ox), pyridium (Py) and quinolinium (Qu) residues are presented in Fig. 1. Also, as reference dyes, the symmetrical cyanines 3–5 will be considered. Methods of the syn-
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103
Fig. 1. Formulae of the dyes.
thesis of dyes 1–3 were described earlier [10,13,14]; the spectral characteristics of the known compounds correspond to data in these papers. The spectral data of new dyes change regularly upon the change of the chemical constitution. 2.2. NMR spectra, absorption spectra, fluorescence spectra, fluorescence excitation spectra, fluorescence excitationanizotropy spectra NMR (nuclear magnetic resonance) spectra. All NMR measurements were carried out on Varian GEMINI 2000 spectrometer with 1 H and 13C frequencies of 400.07 and 100.61 MHz, respectively at 293 K. Tetramethylsilane was used as a standard for d (NMR chemical shifts) scale calibrating. 1H NMR spectra were recorded with spectral width 8000 Hz and numbers of points 32,000; 13C NMR spectra were recorded with spectral width 30,000 Hz and numbers of points 128,000. 1H–1H COSY [15] spectra were acquired into 2048 (F2) and 512 (F1) time-domain data matrix and 2048 (F2) 2048 (F1) frequency-domain matrix after zero-filling. NOESY [16] spectra were acquired, if necessary, with parameters similar to COSY spectra. Mixing times were determined preliminary from T1-measurement experiment for each sample by a conventional inversion-recovery method. Heteronuclear chemical shift correlation (HETCOR) [17] was used to determine 1H–13C attachment with 2048 (F2) 256 (F1) time-domain matrix and 2048 (F2) 1024 (F1) frequency-domain matrix after zero-filling. The average value of one bond constant JCH was set to 140 Hz. HETCOR for determination long range correlation had very similar parameters and average value of multibond C–H coupling constant was set to 8 Hz. Absorption spectra were recorded on spectrophotometer Specord UV VIS in ethanol. Fluorescence spectra and Fluorescence excitation spectra were recorded on Varian Cary Eclipse fluorescence spectrophotometer.
Fluorescence excitation spectra were measured upon various concentrations. They showed that an increasing of concentration leads to splitting of the band, which is the result of reabsorption. So we used for measurements such concentrations that fluorescence excitation spectra reproduced absorption spectra. Fluorescence excitation anisotropy spectra were recorded on fluorescence spectrophotometer CM2203 in glycerin. They were recorded to find out the transition S0 ? S2 when it is orthogonally polarized to the first electron transition S0 ? S1. 2.3. Quantum-chemical calculations To study the dependence of the electron structure and electron transitions on molecular constitution quantum-chemical calculations were performed. The equilibrium geometry and charge distribution in the ground state were calculated by DFT B3LYP (6-31G) method (packages GAUSSIAN03) [18]. The electron transition characteristics were calculated by ZINDO/S method using, as a rule, all p-electron single excited configurations. There are some problems in the correlation between the calculated and the experimentally observed energies both for the first and for higher electron transitions, which have been discussed in detail earlier [19]; here we have used the same methodology.
3. Results and discussion 3.1. Ground state 3.1.1. Optimized molecular geometry The performed calculations of the equilibrium molecular geometry give the conjugated part of all unsymmetrical molecules, 1 and 2, and symmetrical cyanines 3, to be planar that is typical for pelectron systems [20]. Only methyl groups in the indolenine resi-
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Fig. 2. Optimized molecular geometry of indostyryl 1-In (a) and indomethoxystyryl 2-In (b).
due in dyes 1-In and 2-In are out of plane of the main chromophore, as one can see from Fig. 2, where the optimized molecular geometries of these compounds are presented. The carbon–carbon bond lengths along the open chain as well as in the benzene ring in both dyes are appreciably alternated, in contrast to the corresponding symmetrical cyanines; for example, the calculation by ab initio method gives the following bond lengths: 1.3289, 1.3959, 1.3969, 1.3969, 1.3959, 1.3289 Å for the open chain of symmetrical indodicarbocyanine. We also depicted atoms’ numeration in Fig. 2a to clarify the part of the molecule we consider in Figs. 3-6. The bond length alternation degree we use further is quantitatively estimated by an amplitude Dlm, according to the following equation [21]:
Dlm ¼ ð1Þm ðlm lmþ1 Þ;
ð1Þ
where lm is the length of the mth bond. For convenience, parameter |Dlm| (scalars) is preferentially used. One can see the difference in the lengths of the adjacent bonds (an alternation degree) in the chain of indomethoxystyryl (0.036 and 0.054 Å) exceed alternation of the bond lengths in indostyryl (0.021 and 0.036 Å) that points to the increase of the alternation degree upon going from styryl to methoxystyryl. At the same time, the alternation of lengths of the adjacent bonds in the styryl benzene ring and in the methoxystyryl benzene ring is shown in Fig. 3 and is practically the same, in contrast to the appreciable difference in the bond length alternation in the open chain of these dyes.
Fig. 3. Bond length in ground (S0) and excited (S1) states in the chromophore of dyes 2-In (a) and 1-In (b), here m is bond number.
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(a)
0.5 0.4
0.324 -0.347 -0.14 -0.292 -0.14 -0.347 0.324
q, a.u.
0.345 -0.342 -0.084 -0.14 -0.16 -0.28 0.24
(a)
0.358 -0.335 -0.074 -0.134 -0.165 -0.29 0.391
1.2
3-In
0.3 0.2
0 -0.1
0.8 0.6
1-In
0.1
|Δq|, a.u.
1
2-In
0.4 1
2
3
4
5
6
7
μ 0.2
-0.2 -0.3
0
-0.4 172.24 102.76 153.28 125.21 153.28 102.76 172.24
(b)
190
179.4 104.93 153.82 122.08 133.73 112.03 154.3
Ox
1.2 1
δ, ppm 3-In 2-In
150
1-In
0.8
0.455 0.259 0.044 0.019 0.126 0.522
1.012 0.307 0.06 0.012 0.129 0.525
0.303 0.185 0.031 0.027 0.124 0.519
0.328 0.179 0.033 0.027 0.125 0.518
In BT Qu Py μ 1
2
(b)
182.6 110.82 153.9 127.85 133.58 115.75 164.5
170
3 0.693 0.261 0.06 0.031 0.125 0.681
|Δq|, a.u. Ox
4 0.448 0.262 0.049 0.028 0.129 0.68
1.012 0.306 0.064 0.021 0.131 0.681
5 0.303 0.186 0.032 0.037 0.126 0.674
6 0.331 0.182 0.035 0.038 0.127 0.674
In
0.6
130
0.4
110
0.2
BT Qu Py μ
90 70
0.687 0.258 0.056 0.02 0.12 0.52
μ 1
2
3
4
5
6
0
1
2
3
4
5
6
7 Fig. 5. Alternation of atomic charges in the chromophore of dyes 1 (a) and 2 (b). l – atom’s number.
Fig. 4. Atomic charges (a) and chemical shifts (b) in the chromophore of symmetrical indocyanines 3-In (n = 1, 2) and unsymmetrical dyes 1-In, 2-In. l – atom’s number.
The plot of values |Dlm| versus number of the bond along the chromophore for both dye series, 1 and 2, are pictured in Fig. 3. One can see that the bond alternation degree is changed considerably along the chromophore in both styryls 1 and methoxystyryls 2. However, the calculated magnitudes of the parameter |Dlm| for the bond in the open methoxystyryl chain are regularly higher than those in styryls with the same terminal group R. In contrast, the alternation of the bond lengths in the branched part of the chromophore (benzene ring), as shown in Fig. 3 is higher in styryls. Also, it should be especially noted that parameter |Dlm| for the bond lengthes in the open chain is highly sensitive to the donor strength of the variable residue R in the following order:
Ox > In > BT > Qu > Py; independently of the second end group nature (styryl or methoxystyryl). This order coincides with the order of increasing basicity of the heterocycles used as terminal groups in cyanine dyes, found by Brooker [11] based on their spectral data (and will be considered below). 3.1.2. Charge distribution and NMR 13C signals Here, we will analyse only the charges and chemical shifts for the atoms in the chromophore, or so-called Kuhn’ chain between two nitrogen atoms [22]. As an example, the calculated charges
and measured chemical shifts for indostyryl 1-In and indomethoxystyryl 2-In, as well as, symmetrical indodicarbocyanine 3 are presented in Fig. 4. One can see that the signals for the carbon atoms in the chromophore are considerably shifted, in comparison with carbon aromatic atoms: to the downfield for the atoms in the odd position; and opposite to the up field for the atoms in the even position, what is typical for the chromophore of cationic polymethine dyes [23]. This effect was found to be connected with the considerable alternation of the electron density within a polymethine chain generated by the injection of the charge (hole) in the collective system of p-electrons in the linear conjugated molecules. The corresponding alternation of NMR chemical shifts, dl(13C), is the experimental confirmation of such character of charge distribution in the chromophore, so as there is an appreciable correlation between values dl(13C) and calculated electron densities ql in carbon atoms [23]:
dl ð13 CÞ ¼ aql þ b;
ð2Þ
where a and b are constants. One can see from Fig. 4a that calculated charges for the atoms in positions 1 and 2 of the open chain coincide practically for symmetrical indocyanine and both unsymmetrical dyes. On the contrary, the maximum difference in the charges produced by asymmetry is found for the carbon atom in positions 4 and 5, whereas the charges in positions 6 and 7 are relatively close, i.e.
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these atoms in the benzene ring of unsymmetrical styryl and methoxystyryl become of ‘‘cyanine’’ type, similar to the corresponding atoms in symmetrical dye 3-In (n = 2) with the open chain. The similar trend is experimentally observed in spectra NMR (13C), as it is shown in Fig. 4b. The magnitudes of chemical shifts for the neighbouring carbon atoms are considerably alternated along the chromophore in all dyes. Values dl(13C): for the atoms in position 1 and 2 in both unsymmetrical cyanine are close to the corresponding chemical shifts in symmetrical indodicarbocyanine 3-In (n = 2). At the same time, the signals at both chromophore ends in both symmetrical and unsymmetrical dyes are maximally shifted in the weak field that is connected with the influence of heteroatoms of terminal groups. It was proposed [24] to use the alternation amplitude calculated by the:
Dql ¼ ð1Þl ðql qlþ1 Þ;
ð3Þ
Taking into consideration the relationship between the calculated electron densities and NMR signals (2), we can also write the similar function for the alternation of the experimental characteristics dl(13C):
Ddl ¼ ð1Þl ðdl dlþ1 Þ;
ð4Þ
It is obvious that both parameters, Dql and Ddl, can be considered as quantitative characteristics of the bond ionicity: quantum-chemical and experimental, correspondingly. Fig. 5 presents a graphical dependence of the charge alternation in the position of carbon atoms along the chromophore of styryls and methoxystyryls. The calculation gives practically the same magnitudes of parameter Dql, for the pairs of carbon atoms in the benzene ring of both series of dyes, 1 and 2. Although, it is noticed that the degree of alternation of electron densities for the last pair of atom (nearest to the heteroatom) is higher for methoxystyryls than for styryls, that is unquestionably connected with the higher electronegativity of the metoxy group oxygen atom, in comparison with the amino group nitrogen atom. The maximum difference in the alternation degree, Dql, for the same pair atoms in both series of dyes is obtained for the open chain, especially nearly to the variable terminal residues R. One can see from Fig. 5. that the increasing of the donor properties (or basicity) of the terminal group in series Ox > In > BT > Qu > Py, (independently of the nature of the second end group: styryl or methoxystyryl), is accompanied by regular decreasing of values Dql, for the first pair of carbon atoms, in opposite to the increasing of the alternation of the bond lengths in the dye series, considered above. It is reasonable that the similar trend should be observed for the alternation of chemical shifts, Ddl. However, one can see from Fig. 6 that there is an appreciable difference in magnitudes Ddl for the pair atoms not only for the open chain, but for some atoms in the benzene ring, nearly to the open chain. Also, the dyes with the benzoxyazole residue (1-Ox, 2-Ox) with their values Ddl for the first pair atoms disrupt the order in the series in some way that is likely to be caused by the influence of a high electronegative oxygen atom of the oxazole cycle. However, for the second pair of carbon atoms parameter Ddl decreases regularly in series:
Ox > In > BT > Qu > Py; for both styryls and methoxystyryls. Thus, the increasing of asymmetry degree upon going to dyes with more basic variable terminal group leads to the increasing of bond length alternation degree and to decreasing of charge alternation, mainly, in the open chain. On the other hand, the difference in electron densities and molecular geometry between styryls and corresponding methoxystyryls is far lower.
90 80 70
|Δδ |, ppm
a
In
Ox
60 BT
50 40 30
Qu Py
20 10
μ
0 1 90
2
60
6
Ox
50
20
5
b
In
70
30
4
|Δδ |, ppm
80
40
3
BT
Qu Py
10
μ
0 1
2
3
4
5
6
Fig. 6. Alternation of chemical shifts in the chromophore of dyes 1 (a) and 2 (b). l – atom’s number.
3.1.3. Delocalized and donor levels In the unsubstituted linear conjugated systems, corresponding MOs are uniformly delocalized along the chromophore. Insertion of donor terminal groups with own p-center or even branched psystem and hence going to a,x-disubstituded cationic polymethine dyes presented by a general formula 5 should be accompanied by the appearance of new MOs
D1 —½pþ —D2
ð5Þ
which can be delocalized partially in the chromophore or localized mainly in the terminal residues [25–27]. The simplest end groups contain dimethylamino- or methoxy-residues: N(CH3)2, OCH3. In cationic cyanine dyes with a short chain, local MOs of nitrogen atoms (or lone electron pairs – LEPs) are positioned higher than the highest delocalized orbital of the main chromophore. In principle, we can consider the two highest occupied MOs as orbitals generated mainly from the LEPs which are conjugated with the delocalized p-electron system of the polymethine chain; these MO could be treated as donor orbitals, as it was proposed earlier [27]. The donor MOs, /(D1) and /(D2), interact between each other to give two new splitting MOs: symmetrical (6) and unsymmetrical (7) linear combinations.
u1 ¼ ð2Þ1=2 f/ðD1 Þ þ /ðD2 Þg; u2 ¼ ð2Þ1=2 f/ðD1 Þ /ðD2 Þg
ð6Þ ð7Þ
Fig. 7a shows schematically the splitting of the donor levels in a symmetrical dye. Here the lowest energy level depicted on Fig. 7a and b and present on Fig. 7c is formed by a conjugated chain. In unsymmetrical cyanines (D1 – D2), as one can see from Fig. 7b and c the energies of both donor levels are different. If taking into consideration that an interaction of the levels is in inverse
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107
Fig. 7. Schematic disposition of delocalized and donor (D1 and D2) levels as well as the first 2 electron transitions in symmetrical cyanine (a), styryl (b) and methoxystyryl (c); DEspl is the splitting energy.
proportionally to the distance between them, then the splitting energy should decrease in asymmetrical polymethine dyes. In styryl 1, both terminal residues contain a nitrogen atom with its high donor level; and hence we expect that the distance between both donor levels is not too large, so that their interaction retains comparatively high, similar to symmetrically cyanines. The situation reverses principally in methoxystyryls 2 with an oxygen atom instead of nitrogen in the low basic terminal p-oxymethylphenylene group; the corresponding quasi-donor level should be disposed rather lower. Fig. 7c demonstrates that the splitting energy in methoxystyryls 2 should be practically negligible, in comparison with symmetrical cyanines or even with unsymmetrical dyes 1.
Of course, the scheme above is strongly simplified. The initial donor MOs interact with the delocalized MOs of the main chromophore. As a result, the direct quantum-chemical calculation of unsymmetrical styryl 1-In and methoxystyryl 2-In (Fig. 8) shows that both highest occupied MO (HOMO) and next orbital (HOMO-1) are considerably delocalized. Nevertheless, the energy gap in dye 1-In with both nitrogenic residues is shifted up in comparison with methoxystyryl 2-In, containing only one nitrogenic terminal group. Thus, we can conclude that the metoxyphenylene residue does not produce the high positioned highest occupied level, in contrast to the nitrogenic heterocycle. At the same time, the position of the lowest vacant level is less sensitive for replacing
Fig. 8. Molecular orbitals of unsymmetrical dyes: 1-In (a) and 2-In (b); and two first transitions S0 ? S1, S0 ? S2.
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dimethylamino group (styryl 1) by metoxy group (methoxystyryl 2). 3.2. Excited state 3.2.1. A general scheme of electron transitions in cyanines The bands observed in visible and near IR spectra of symmetrical and unsymmetrical cyanine dyes are known to correspond to the electron transitions that involve some highest occupied MOs and some vacant orbitals [4,12]. The lowest electron transition is practically connected only with the highest occupied MO (HOMO, further abbreviation H) and the lowest unoccupied MO (LUMO, further abbreviation L): |S1 P |HOMO ? LUMO>. In regard to the next states, their nature depends on the type of the conjugated system. So, |S2> and |S3> electron transitions in polyenes, with symmetrically disposed occupied and vacant electron levels (in respect to the midgap [20]), are degenerated and described by four MOs: |S2 P (2)1(|H-1 ? L> + |H ? L + 1>); |S3 P (2)1(|H1 ? L> |H ? L + 1>). In contrast to it, in cationic cyanine dyes with their shifted down energy gap and considerable distance between the lowest vacant and next levels, the transitions proved to be predominantly one-electron excited states [27,28]. Considerable interaction of donor levels and corresponding interaction of transitions from them to |LUMO> lead to splitting between the excited states S1 and S2. One can see from Fig. 7a that the magnitude of the splitting DEspl in symmetrical dyes should be maximal because of the degeneration of donor levels. Increasing of distance between the donor levels in unsymmetrical dyes should be accompanied by decreasing of the splitting energy. Then, one could expect that the two lowest electron transition, S0 ? S1 and S0 ? S2, in styryls 1 as dyes with both nitrogenic terminal groups (Fig. 7b) should be similar to the corresponding transitions in symmetrical cyanine, whereas these transitions in methoxystyryls 2 (Fig. 7c) should have considerably higher energies. 3.2.2. Absorption and fluorescence excitation anisotropy spectra Fig. 9 presents absorption spectra and spectra of fluorescence excitation, correspondingly, measured in the UV/visible region for indostyryl 1-In (Fig. 9a) and indomethoxystyryls 2-In (Fig. 9b). The shape of the spectral bands for other unsymmetrical dyes 1 and 2 is similar. A comparatively intensive and selective spectral band with its maximum at 560 nm is observed in spectra dye 1In. At the same time, the fluorescence excitation anisotropy spectra points to the existence of the electron transition at nearly 380 nm, which is polarized perpendicularly to the first transition S0 ? S1. The performed quantum-chemical calculations which showed in Table 1 give two electron transitionsß S0 ? S1 and S0 ? S2, for this spectral region, so that the calculated distance between them is 141 nm that is comparatively near to the experimental value. S0 ? S2 transition involves |HOMO-1>. This transition is practically forbidden in one-photon absorption spectra, but it is found for dye 1-BT [28] to be active in two-photon absorption spectra (at 335 nm in the ethanol solvent); a similar electron transition of the same nature is also observed in two-photon spectra of symmetrical thiacarbocyanines 3-BT (n = 1). One can see from Fig. 9 that the going from styryl to its oxyanalogue is accompanied by a considerable hypsochromic effect. As far as the interaction of donor levels in methoxystyryls should be significantly smaller, in comparison with corresponding styryls, the energy of the first electron transition is higher, and hence the absorption band maximum is necessarily shifted in the short wavelength spectral region. The maximum effect is observed for pair 1-In ? 2-In: 127 nm (5238 cm1), the minimum shift of the spectral band is found for dyes 1-Ox ? 2-Ox: 96 nm (4795 cm1). The calculation for 2-In gives the second transition that appeared at approximately 310 nm. The fluorescence excitation
Fig. 9. Absorption, fluorescence and fluorescence excitation spectra of indostyryls 1-In (a) and indomethoxystyryl 2-In (b).
anisotropy spectrum does not point to the existence of the electron transitions; only considerable decreasing of the anisotropy parameter is observed. The fluorescence excitation anisotropy spectra also point to a short wavelength position of the second electron transition, higher than 300 nm. At the same time, the calculated k2 = 375 nm for styryl 1-Qu agrees with the minimum at approximately 377 nm in its fluorescence excitation anisotropy spectrum (see Table 2). Also, one can see that an appreciable large spectral effect upon the annelation of the pyridine cycle is observed for the pairs of pyridinim and quinolinium dye derivatives: Dkabs = 66 nm for the dye pair: 1-Py ? 1-Qu; and Dkabs = 42 nm for the dyes: 2-Py ? 2-Qu. A particular attention should be paid to the fact that quinostyryl 1-Qu absorbs practically at the same spectral region as unsymmetrical indocyanine 1-In does, in spite of symmetrical quinocyanines 3-Qu that absorbs deeper on 150 nm than indocyanines 3-In with the same chain length, because of the larger effective length of the quinolinum residue as a terminal group [4,10,11]. This spectral phenomenon is explained by the deviation of the absorption band maxima in spectra of unsymmetrical cyanine dyes. Deviation is proportional to the difference in the donor strength of both
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A.V. Stanova et al. / Journal of Molecular Structure 988 (2011) 102–110 Table 1 Calculated data of styryls 1 and methoxystyryls 2. R
Transition
Styryl 1
Methoxystyryl 2
k, nm
f, oscillator strength
Main configuration
k, nm
f, oscillator strength
Main configuration
BT
S0 ? S1 S0 ? S2
519 357
1.212 0.073
0.98|H ? L> 0.94|H-1 ? L>
407 280
1.471 0.078
0.98H ? L> 0.75|H-1 ? L > -0.63|H-2 ? L>
Ox
S0 ? S1 S0 ? S2
490 339
1.342 0.083
0.97|H ? L> 0.96|H-1 ? L>
391 273
1.545 0.037
0.98|H ? L> 0.96|H-1 ? L>
In
S0 ? S1 S0 ? S2
545 404
1.024 0.094
0.97|H ? L> 0.96|H-1 ? L>
419 310
1.319 0.082
0.98|H ? L> 0.96|H-1 ? L>
Py
S0 ? S1 S0 ? S2
488 306
1.446 0.061
0.96|H ? L> 0.93|H-1 ? L>
395 268
1.500 0.068
0.97|H ? L> 0.95|H-1 ? L>
Qu
S0 ? S1 S0 ? S2
541 375
1.294 0.036
0.96|H ? L> 0.82|H-1 ? L>
435 316
1.402 0.049
0.97|H ? L> 0.86|H-1 ? L>
Table 2 Spectral data of dyes 1, 2. R
BT Py In Ox Qu
1
2
kanis, nm
kabs, nm
kfl, nm
kabs, nm
kfl, nm
365 336 376 356 377
531 486 560 498 552
591 602 588 554 662
419 388 433 402 430
502 489 523 476 545
terminal groups, i.e. to the asymmetry degree [11]. In the spectra of methoxystyryls 2-Qu and 2-In, the positions of the band maxima (Table 2) coincide practically. 3.2.3. Deviation of absorption band maxima and asymmetry of the electron transition To estimate quantitatively the degree of asymmetry, a deviation D, proposed by Brooker [11] is traditionally used. This parameter is calculated in the following way:
D ¼ ðk1 þ k2 Þ=2 kas ;
ð8Þ
where kas is the absorption maximum of the unsymmetrical dye while k1 and k2 are maxima of the corresponding symmetrical parent molecules. It was found that for typical unsymmetrical cyanines, D > 0, i.e. the position of the maximum for unsymmetrical dyes kas is shifted towards short wavelengths as compared to the arithmetic mean value from the maxima of the parent dyes. This spectral fact was treated to be connected with the appearance of alternation of the CC-bond length along the polymethine chain, if terminal groups are non-equivalent [11]. The corresponding symmetrical carbocyanines 3 (n = 1) and dyes 4 or 5 should be considered as parent dyes for styryls or metoxystyryls. The calculated deviations for the investigated unsymmetrical cyanines 1 and 2 are collected in Table 3. One can see that values D are significantly larger in the spectra of styryls 1 than in the spectra of corresponding metoxystyryls 2. This is in a good agreement with the assumption about connection between the
deviation and non-equivalence of the bond lengths in the polymethine chain. In fact, the degree alternation of the bond lengths (parameter Dlm) in the open chain of metoxystyryls is higher than in the chromophore of methoxystyryl. However, it is noticed that the charge and bond order (and hence bond length) in the polymethine chain of linear conjugated systems depend directly on donor (or basic) properties of terminal groups that can be connected with the position of the highest occupied levels. In turn, the position of this level depends on the splitting of donor MOs that is the maximum in symmetrical cyanines (because of the degeneration of donor levels) and decreases regularly in unsymmetrical dyes. Then, we state that the appearance of deviations in unsymmetrical cyanine is eventually caused by decreasing of interaction of non-degenerated donor levels of non-equivalent terminal groups. The performed analysis shows that spectral characteristic, deviation D, of styryls is proportional to the distance between the initial donor levels DeD of the end groups. These levels were found as a midposition between eHOMO and eHOMO-1 for both symmetrical maternal dyes 4 and corresponding carbocyanine 3 (n = 1) Table 3, using consideration addressed to Fig. 7a. Practically the same order as Brooker’s is obtained for the deviation in spectra of both dye series 1 and 2, excepting the styryl with indolenium residue, 1-In, and somewhat close the value D for the pair dyes 2-Ox and 2-In as well as 2-Qu and 2-Py.
D ðnmÞ ¼ ½51:5ð1-OxÞ; 19:5ð1-InÞ < 54:5ð1-BTÞ < 107 ð1-QuÞ < 122:5ð1-PyÞ D ðnmÞ ¼ ½94ð2-OxÞ; 93ð2-InÞ < 113ð2-BTÞ < 175:5 ð2-QuÞ; ½167ð2-PyÞ The generall conclusion is following: increasing of donor strength of the variable terminal group R is accompanied by regular increasing of deviation. The difference in the observed value D for styryl and corresponding metoxystyryl is considerable, in contrast to the difference in the equilibrium molecular geometry and charge distribution in the ground state. I.e. the excited state is more sensitive to the
Table 3 Deviation (D), Stock’s shift (DmS), DeD and parameter d(pm) for dyes 1, 2. R
BT Py In Ox Qu
1
2
De D
D, nm deviation
Stock’s shift Dm, cm1
d(pm) chain
DeD
D, nm deviation
Stock’s shift Dm, cm1
d(pm) chain
0.2 0.24 0.04 0.08 0.66
54.5 122.5 19.5 51.5 107
1800 3955 753 1860 2890
0.227 0.2132 0.1584 0.1852 0.1961
1.54 1.58 1.38 1.26 2
113 167 93 94 175.5
3840 5027 4018 3832 4670
0.3948 0.3752 0.3346 0.3255 0.3559
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A.V. Stanova et al. / Journal of Molecular Structure 988 (2011) 102–110
change in chemical constitution upon going from styryls 1 to their metoxyanalogues 2. 3.2.4. Relaxation of molecular geometry and fluorescence spectra The florescence bands of indostyryl 1-In and indometoxystyryl 2-In are presented in Fig. 9a and b. One can see that the fluorescence band maximum is appreciably shifted in the longwavelength region that points to the substantial change of the equilibrium energy upon the relaxation in the excited state. It was established [29] that the Stock’s shift, DmS, calculated by the following formulas (9), depends directly on the change of lengths of the bonds in the
DmS ¼ mmax ðabsÞ mmax ðfluorÞ
ð9Þ
chromophore, that could be quantitatively estimated by:
dðlm Þ ¼
ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X 2 lm lm
ð10Þ
where lm and lm are the bond lengths in the excited and ground states; index m runs all bonds. On the other hand, there is a correlation between the bond length, lm, and the bond order, pm, [20]: lm (Å) = 1.54–0.14pm. I.e., parameter pm can be also considered as a characteristic of the bonds. Then, Eq. (9) could be replaced by an equivalent equation: [30]
dðpm Þ ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qX 2ffi pm pm ;
ð11Þ
Thus, value d(pm) can be considered as an integral characteristic of possible geometrical changes upon the relaxation in the excited state. One could assume that parameter d(pm) should correlate with the Stack’s shift. The calculated values d(pm) and the measured magnitudes DmS for unsymmetrical dyes 1 and 2 are collected in Table 3. One can see that value d(pm) obtained for methoxystyryls 2 exceeds practically twice this parameter for corresponding styryls 1 that agrees with their larger Stock’s shifts. Analysis of value DmS (Table 3) has shown that it depends on donor strength of the variable terminal residue R. Similarly to the deviation, dyes 1 and 2 can be arranged into the following order:
DmS ðcm1 Þ ¼ ½1860ð1-OxÞ; 753ð1-InÞ < 1800ð1-BTÞ < 2890ð1-QuÞ < 3955ð1-PyÞ
DmS ðcm1 Þ ¼ 3832ð2-OxÞ < 4018ð2-InÞ½3840ð2-BT < 4660ð2-QuÞ < 5027ð2-PyÞ Consequently, we can state that increasing of difference in donor strengths of both terminal groups regularly leads to increasing of degree of geometrical changes upon excitation and hence to increasing of Stock’s shifts. The spectral effects from Table 3 are larger for methoxystyryls 2.
4. Conclusion Thus, the combined quantum-chemical and spectral investigations showed that asymmetry degree of the chromophore electron structure in styryls and methoxystyryls differs considerably in the ground and excited states. In the ground state with the closed electron shell, the difference in charge distribution and molecular geometry between styryls and corresponding methoxystyryls is comparatively insignificant, so that electron structure is more sensitive to the basicity of the variable terminal group. In contrary, the electron transitions which involve only the individual MOs differ considerably in both types of unsymmetrical cyanine dyes. The appreciable interaction between two donor levels in styryls provides an additional decreasing of the first transition energy and hence leads to a comparative deep color, whereas the similar interaction in methoxystyryls is negligible, and hence these dyes absorb at shorter wavelengths. References [1] A. Mishra, Chem. Rev. 100 (2000) 1973. [2] G. Bach, S. Daehne, Cyanine Dyes and Related Compounds, ROOD’S Chemistry of Carbon Compounds, vol. IVb, Elsevier Science, Amsterdam, 1997 (Chapter 15). [3] F. Meyers, S.R. Marder, J.W. Perry, Introducing to the Nonlinear Optical Properties of Organic Materials, Chemistry of Advanced Materials, Wiley-VCH, New York–Chichester–Weinheim–Brisbane–Singapore–Toronto, 1998 (Chapter 6). [4] N. Tyutyulkov, J. Fabian, A. Mehlhorn, F. Dietz, A. Tadjer, Polymethine Dyes. Structure and Properties, St. Kliment Ohridski University Press, Sofia, 1991. [5] S. Daehne, Color and Constitution. One hundred years of research, vol. 199, Science, 1978. [6] H. Kuhn, J. Chem. Phys. 17 (1949) 1098. [7] N.S. Bayliss, J. Chem. Phys. 16 (1948) 287. [8] W.T. Simpson, J. Am. Chem. Soc. 73 (1951) 5359. [9] L.M. Tolbert, Acc. Chem. Res. 25 (1992) 561. [10] F.M. Hamer, Cyanine Dyes and Related Compounds, Interscience, New York, 1964. [11] L. Brooker, Rev. Mod. Phys. 14 (1942) 275. [12] A.D. Kachkovsky, Uspekhi Khim. (Rus) 66 (1997) 715. [13] R. Wizinger, P. Ulrich, Helv. Chim. 39 (1956) 207. [14] V. Dryanska, C. Ivanov, Synthesis 1 (1976) 37. [15] A.L. Davis, J. Keeler, E.D. Laue, D. Moskau, J. Magn. Reson. 98 (1992) 207. [16] D.J. States, R.A. Haberkorn, D.J. Ruben, J. Magn. Reson. 48 (1982) 286. [17] A. Bax, G.A. Morris, J. Magn. Reson. 42 (1981) 501. [18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., GAUSSIAN03; Revision B.05, Gaussian, Inc., Pittsburgh PA, 2003. [19] Ju. Bricks, A. Ryabitskii, A. Kachkovskii, Eur. J. Org. Chem. (2009) 3439. [20] M.J.S. Dewar, The molecular orbital theory of organic chemistry, McGrow-Hill, New York, 1969. [21] J.S. Craw, J.R. Reimers, G.B. Bacskay, A.T. Wong, N.S. Hush, Chem. Phys. 167 (1992) 77. [22] H. Kuhn, J. Chem. Phys. 16 (1948) 840. [23] R. Radeglia, E. Gey, K.-D. Nolte, S. Daehne, J. Prakt. Chem. 315 (1973) 586. [24] J.S. Craw, J.R. Reimers, G.B. Bacskay, A.T. Wong, Chem. Phys. 167 (1992) 101. [25] A.D. Kachkovski, D.A. Yushchenko, G.A. Kachkovski, Dyes Pigments 66 (2005) 223. [26] A.D. Kachkovski, Dyes Pigments 24 (1994) 171. [27] S. Webster, J. Fu, L.A. Padilha, H. Hu, O.V. Przhonska, D.J. Hagan, E. Van Stryland, M.V. Bondar, Yu.L. Slominsky, A.D. Kachkovski, J. Luminescence 128 (2008) 927. [28] J. Fu, O.V. Przhonska, L.A. Padilha, D.J. Hagan, E.W. Van Stryland, M.V. Bondar, Yu.L. Slominskiy, A.D. Kachkovsky, J. Am. Opt. Soc. (JOSA B) 24 (2007) 56. [29] E.F. McCoy, I.J. Ross, Aus. J. Chem. 15 (1962) 573. [30] A.A. Ishchenko, Quantum Electron. 24 (1984) 471.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Asymmetry in ground and excited states in styryls and methoxystyryls detected by NMR (13C), absorption, fluorescence and fluorescence excitation spectroscopy A.V. Stanova a,⇑, A.B. Ryabitsky b, V.M. Yashchuk a, O.D. Kachkovsky b, A.O. Gerasov b, Ya.O. Prostota b, O.V. Kropachev b a b
Taras Shevchenko National University of Kyiv, Physics Department, 2 Glushkova Prosp., 03022 Kyiv, Ukraine Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str., 03660 Kyiv, Ukraine
a r t i c l e
i n f o
Article history: Received 31 August 2010 Received in revised form 18 December 2010 Accepted 19 December 2010 Available online 25 December 2010 Keywords: Cyanine dyes Electron transitions Styryls Methoxystyryls Quantum-chemical calculations Spectroscopy
a b s t r a c t Combined quantum-chemical and spectral study of electron structure features of styryls and their oxyanalogues containing benzothiazolium, benzooxazolium, indoleninium, pyridium, quinolinium residues has been fulfilled. It showed that asymmetry degree of molecular geometry and charge distribution in the chromophore of styryls and methoxystyryls considerably differ in the ground and excited states. It was established that two the lowest transitions in styryls are splitting and involve both donor levels, similarly to symmetrical cyanines. If compare with methoxystyryls the long-wave high intensive absorption band is shifted bathochromically due to considerable interaction between the donor quasi-local chromophores. In contrary, because of the low position of a lone electron pair of oxygen in methoxystyryls, only one donor quasi-local chromophore is effective, hence such unsymmetrical dyes absorb appreciably higher. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Unsymmetrical cyanine dyes, including styryls and their methoxyanalogues, continue to be used widely, in parallel with corresponding symmetrical dyes, in numerous applications; they are widely used objects due to their unique electronic and spectral properties which form a basis for the design of new effective materials connected with the light conversion, i.e. spectral sensitization, molecular probing in biology, active and passive components for tuneable lasers, non-linear media exploring excited state absorption, etc. [1–4]. Also, symmetrical and unsymmetrical cyanine molecules remain convenient objects for developing of new theoretical concepts and quantum-chemical models [5–9]. Going from symmetrical dyes to unsymmetrical ones causes significant changes in distribution of the total positive charge in the polymethine chromophore, molecular geometry and hence in spectral properties [1,4,10–12]. The styryls were usually considered as unsymmetrical cyanines with an extremely high degree of asymmetry because of the low basicity of the p-dimethylaminophenylene residue as a terminal group. Nevertheless, the nature of the lowest electron transition is similar to the nature of the one in symmetrical cyanines. Inser⇑ Corresponding author. Tel.: +38 067 19 49 208. E-mail address: [email protected] (A.V. Stanova). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.12.038
tion of an alkyloxy group with a high electronegative oxygen atom instead of amino group into styryls is likely to lead to more considerable change in the molecule electron structure of the dyes and in the nature of the electron transitions. It is expected that the spectral effects should depend on the difference in donor strengths of other terminal groups. So, we use absorption and fluorescence spectra or, more correctly, electron transitions, for studying the asymmetry of the excited state, whereas the degree of the asymmetry in the ground state can be obtained from NMR spectra. Also the quantum-chemical calculations give the information about the asymmetry in the charge distribution and molecular geometry in both ground and excited states. This paper presents the results of the simultaneous quantumchemical simulations and spectral measurements of the absorption spectra of unsymmetrical styryls and methoxystyrys. 2. Experimental 2.1. Objects and methodology The general structures of investigated styryls 1 and corresponding methoxystyryls 2 containing indoleninium (In), benzothiazolium (BT), benzooxazolium (Ox), pyridium (Py) and quinolinium (Qu) residues are presented in Fig. 1. Also, as reference dyes, the symmetrical cyanines 3–5 will be considered. Methods of the syn-
A.V. Stanova et al. / Journal of Molecular Structure 988 (2011) 102–110
103
Fig. 1. Formulae of the dyes.
thesis of dyes 1–3 were described earlier [10,13,14]; the spectral characteristics of the known compounds correspond to data in these papers. The spectral data of new dyes change regularly upon the change of the chemical constitution. 2.2. NMR spectra, absorption spectra, fluorescence spectra, fluorescence excitation spectra, fluorescence excitationanizotropy spectra NMR (nuclear magnetic resonance) spectra. All NMR measurements were carried out on Varian GEMINI 2000 spectrometer with 1 H and 13C frequencies of 400.07 and 100.61 MHz, respectively at 293 K. Tetramethylsilane was used as a standard for d (NMR chemical shifts) scale calibrating. 1H NMR spectra were recorded with spectral width 8000 Hz and numbers of points 32,000; 13C NMR spectra were recorded with spectral width 30,000 Hz and numbers of points 128,000. 1H–1H COSY [15] spectra were acquired into 2048 (F2) and 512 (F1) time-domain data matrix and 2048 (F2) 2048 (F1) frequency-domain matrix after zero-filling. NOESY [16] spectra were acquired, if necessary, with parameters similar to COSY spectra. Mixing times were determined preliminary from T1-measurement experiment for each sample by a conventional inversion-recovery method. Heteronuclear chemical shift correlation (HETCOR) [17] was used to determine 1H–13C attachment with 2048 (F2) 256 (F1) time-domain matrix and 2048 (F2) 1024 (F1) frequency-domain matrix after zero-filling. The average value of one bond constant JCH was set to 140 Hz. HETCOR for determination long range correlation had very similar parameters and average value of multibond C–H coupling constant was set to 8 Hz. Absorption spectra were recorded on spectrophotometer Specord UV VIS in ethanol. Fluorescence spectra and Fluorescence excitation spectra were recorded on Varian Cary Eclipse fluorescence spectrophotometer.
Fluorescence excitation spectra were measured upon various concentrations. They showed that an increasing of concentration leads to splitting of the band, which is the result of reabsorption. So we used for measurements such concentrations that fluorescence excitation spectra reproduced absorption spectra. Fluorescence excitation anisotropy spectra were recorded on fluorescence spectrophotometer CM2203 in glycerin. They were recorded to find out the transition S0 ? S2 when it is orthogonally polarized to the first electron transition S0 ? S1. 2.3. Quantum-chemical calculations To study the dependence of the electron structure and electron transitions on molecular constitution quantum-chemical calculations were performed. The equilibrium geometry and charge distribution in the ground state were calculated by DFT B3LYP (6-31G) method (packages GAUSSIAN03) [18]. The electron transition characteristics were calculated by ZINDO/S method using, as a rule, all p-electron single excited configurations. There are some problems in the correlation between the calculated and the experimentally observed energies both for the first and for higher electron transitions, which have been discussed in detail earlier [19]; here we have used the same methodology.
3. Results and discussion 3.1. Ground state 3.1.1. Optimized molecular geometry The performed calculations of the equilibrium molecular geometry give the conjugated part of all unsymmetrical molecules, 1 and 2, and symmetrical cyanines 3, to be planar that is typical for pelectron systems [20]. Only methyl groups in the indolenine resi-
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Fig. 2. Optimized molecular geometry of indostyryl 1-In (a) and indomethoxystyryl 2-In (b).
due in dyes 1-In and 2-In are out of plane of the main chromophore, as one can see from Fig. 2, where the optimized molecular geometries of these compounds are presented. The carbon–carbon bond lengths along the open chain as well as in the benzene ring in both dyes are appreciably alternated, in contrast to the corresponding symmetrical cyanines; for example, the calculation by ab initio method gives the following bond lengths: 1.3289, 1.3959, 1.3969, 1.3969, 1.3959, 1.3289 Å for the open chain of symmetrical indodicarbocyanine. We also depicted atoms’ numeration in Fig. 2a to clarify the part of the molecule we consider in Figs. 3-6. The bond length alternation degree we use further is quantitatively estimated by an amplitude Dlm, according to the following equation [21]:
Dlm ¼ ð1Þm ðlm lmþ1 Þ;
ð1Þ
where lm is the length of the mth bond. For convenience, parameter |Dlm| (scalars) is preferentially used. One can see the difference in the lengths of the adjacent bonds (an alternation degree) in the chain of indomethoxystyryl (0.036 and 0.054 Å) exceed alternation of the bond lengths in indostyryl (0.021 and 0.036 Å) that points to the increase of the alternation degree upon going from styryl to methoxystyryl. At the same time, the alternation of lengths of the adjacent bonds in the styryl benzene ring and in the methoxystyryl benzene ring is shown in Fig. 3 and is practically the same, in contrast to the appreciable difference in the bond length alternation in the open chain of these dyes.
Fig. 3. Bond length in ground (S0) and excited (S1) states in the chromophore of dyes 2-In (a) and 1-In (b), here m is bond number.
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(a)
0.5 0.4
0.324 -0.347 -0.14 -0.292 -0.14 -0.347 0.324
q, a.u.
0.345 -0.342 -0.084 -0.14 -0.16 -0.28 0.24
(a)
0.358 -0.335 -0.074 -0.134 -0.165 -0.29 0.391
1.2
3-In
0.3 0.2
0 -0.1
0.8 0.6
1-In
0.1
|Δq|, a.u.
1
2-In
0.4 1
2
3
4
5
6
7
μ 0.2
-0.2 -0.3
0
-0.4 172.24 102.76 153.28 125.21 153.28 102.76 172.24
(b)
190
179.4 104.93 153.82 122.08 133.73 112.03 154.3
Ox
1.2 1
δ, ppm 3-In 2-In
150
1-In
0.8
0.455 0.259 0.044 0.019 0.126 0.522
1.012 0.307 0.06 0.012 0.129 0.525
0.303 0.185 0.031 0.027 0.124 0.519
0.328 0.179 0.033 0.027 0.125 0.518
In BT Qu Py μ 1
2
(b)
182.6 110.82 153.9 127.85 133.58 115.75 164.5
170
3 0.693 0.261 0.06 0.031 0.125 0.681
|Δq|, a.u. Ox
4 0.448 0.262 0.049 0.028 0.129 0.68
1.012 0.306 0.064 0.021 0.131 0.681
5 0.303 0.186 0.032 0.037 0.126 0.674
6 0.331 0.182 0.035 0.038 0.127 0.674
In
0.6
130
0.4
110
0.2
BT Qu Py μ
90 70
0.687 0.258 0.056 0.02 0.12 0.52
μ 1
2
3
4
5
6
0
1
2
3
4
5
6
7 Fig. 5. Alternation of atomic charges in the chromophore of dyes 1 (a) and 2 (b). l – atom’s number.
Fig. 4. Atomic charges (a) and chemical shifts (b) in the chromophore of symmetrical indocyanines 3-In (n = 1, 2) and unsymmetrical dyes 1-In, 2-In. l – atom’s number.
The plot of values |Dlm| versus number of the bond along the chromophore for both dye series, 1 and 2, are pictured in Fig. 3. One can see that the bond alternation degree is changed considerably along the chromophore in both styryls 1 and methoxystyryls 2. However, the calculated magnitudes of the parameter |Dlm| for the bond in the open methoxystyryl chain are regularly higher than those in styryls with the same terminal group R. In contrast, the alternation of the bond lengths in the branched part of the chromophore (benzene ring), as shown in Fig. 3 is higher in styryls. Also, it should be especially noted that parameter |Dlm| for the bond lengthes in the open chain is highly sensitive to the donor strength of the variable residue R in the following order:
Ox > In > BT > Qu > Py; independently of the second end group nature (styryl or methoxystyryl). This order coincides with the order of increasing basicity of the heterocycles used as terminal groups in cyanine dyes, found by Brooker [11] based on their spectral data (and will be considered below). 3.1.2. Charge distribution and NMR 13C signals Here, we will analyse only the charges and chemical shifts for the atoms in the chromophore, or so-called Kuhn’ chain between two nitrogen atoms [22]. As an example, the calculated charges
and measured chemical shifts for indostyryl 1-In and indomethoxystyryl 2-In, as well as, symmetrical indodicarbocyanine 3 are presented in Fig. 4. One can see that the signals for the carbon atoms in the chromophore are considerably shifted, in comparison with carbon aromatic atoms: to the downfield for the atoms in the odd position; and opposite to the up field for the atoms in the even position, what is typical for the chromophore of cationic polymethine dyes [23]. This effect was found to be connected with the considerable alternation of the electron density within a polymethine chain generated by the injection of the charge (hole) in the collective system of p-electrons in the linear conjugated molecules. The corresponding alternation of NMR chemical shifts, dl(13C), is the experimental confirmation of such character of charge distribution in the chromophore, so as there is an appreciable correlation between values dl(13C) and calculated electron densities ql in carbon atoms [23]:
dl ð13 CÞ ¼ aql þ b;
ð2Þ
where a and b are constants. One can see from Fig. 4a that calculated charges for the atoms in positions 1 and 2 of the open chain coincide practically for symmetrical indocyanine and both unsymmetrical dyes. On the contrary, the maximum difference in the charges produced by asymmetry is found for the carbon atom in positions 4 and 5, whereas the charges in positions 6 and 7 are relatively close, i.e.
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these atoms in the benzene ring of unsymmetrical styryl and methoxystyryl become of ‘‘cyanine’’ type, similar to the corresponding atoms in symmetrical dye 3-In (n = 2) with the open chain. The similar trend is experimentally observed in spectra NMR (13C), as it is shown in Fig. 4b. The magnitudes of chemical shifts for the neighbouring carbon atoms are considerably alternated along the chromophore in all dyes. Values dl(13C): for the atoms in position 1 and 2 in both unsymmetrical cyanine are close to the corresponding chemical shifts in symmetrical indodicarbocyanine 3-In (n = 2). At the same time, the signals at both chromophore ends in both symmetrical and unsymmetrical dyes are maximally shifted in the weak field that is connected with the influence of heteroatoms of terminal groups. It was proposed [24] to use the alternation amplitude calculated by the:
Dql ¼ ð1Þl ðql qlþ1 Þ;
ð3Þ
Taking into consideration the relationship between the calculated electron densities and NMR signals (2), we can also write the similar function for the alternation of the experimental characteristics dl(13C):
Ddl ¼ ð1Þl ðdl dlþ1 Þ;
ð4Þ
It is obvious that both parameters, Dql and Ddl, can be considered as quantitative characteristics of the bond ionicity: quantum-chemical and experimental, correspondingly. Fig. 5 presents a graphical dependence of the charge alternation in the position of carbon atoms along the chromophore of styryls and methoxystyryls. The calculation gives practically the same magnitudes of parameter Dql, for the pairs of carbon atoms in the benzene ring of both series of dyes, 1 and 2. Although, it is noticed that the degree of alternation of electron densities for the last pair of atom (nearest to the heteroatom) is higher for methoxystyryls than for styryls, that is unquestionably connected with the higher electronegativity of the metoxy group oxygen atom, in comparison with the amino group nitrogen atom. The maximum difference in the alternation degree, Dql, for the same pair atoms in both series of dyes is obtained for the open chain, especially nearly to the variable terminal residues R. One can see from Fig. 5. that the increasing of the donor properties (or basicity) of the terminal group in series Ox > In > BT > Qu > Py, (independently of the nature of the second end group: styryl or methoxystyryl), is accompanied by regular decreasing of values Dql, for the first pair of carbon atoms, in opposite to the increasing of the alternation of the bond lengths in the dye series, considered above. It is reasonable that the similar trend should be observed for the alternation of chemical shifts, Ddl. However, one can see from Fig. 6 that there is an appreciable difference in magnitudes Ddl for the pair atoms not only for the open chain, but for some atoms in the benzene ring, nearly to the open chain. Also, the dyes with the benzoxyazole residue (1-Ox, 2-Ox) with their values Ddl for the first pair atoms disrupt the order in the series in some way that is likely to be caused by the influence of a high electronegative oxygen atom of the oxazole cycle. However, for the second pair of carbon atoms parameter Ddl decreases regularly in series:
Ox > In > BT > Qu > Py; for both styryls and methoxystyryls. Thus, the increasing of asymmetry degree upon going to dyes with more basic variable terminal group leads to the increasing of bond length alternation degree and to decreasing of charge alternation, mainly, in the open chain. On the other hand, the difference in electron densities and molecular geometry between styryls and corresponding methoxystyryls is far lower.
90 80 70
|Δδ |, ppm
a
In
Ox
60 BT
50 40 30
Qu Py
20 10
μ
0 1 90
2
60
6
Ox
50
20
5
b
In
70
30
4
|Δδ |, ppm
80
40
3
BT
Qu Py
10
μ
0 1
2
3
4
5
6
Fig. 6. Alternation of chemical shifts in the chromophore of dyes 1 (a) and 2 (b). l – atom’s number.
3.1.3. Delocalized and donor levels In the unsubstituted linear conjugated systems, corresponding MOs are uniformly delocalized along the chromophore. Insertion of donor terminal groups with own p-center or even branched psystem and hence going to a,x-disubstituded cationic polymethine dyes presented by a general formula 5 should be accompanied by the appearance of new MOs
D1 —½pþ —D2
ð5Þ
which can be delocalized partially in the chromophore or localized mainly in the terminal residues [25–27]. The simplest end groups contain dimethylamino- or methoxy-residues: N(CH3)2, OCH3. In cationic cyanine dyes with a short chain, local MOs of nitrogen atoms (or lone electron pairs – LEPs) are positioned higher than the highest delocalized orbital of the main chromophore. In principle, we can consider the two highest occupied MOs as orbitals generated mainly from the LEPs which are conjugated with the delocalized p-electron system of the polymethine chain; these MO could be treated as donor orbitals, as it was proposed earlier [27]. The donor MOs, /(D1) and /(D2), interact between each other to give two new splitting MOs: symmetrical (6) and unsymmetrical (7) linear combinations.
u1 ¼ ð2Þ1=2 f/ðD1 Þ þ /ðD2 Þg; u2 ¼ ð2Þ1=2 f/ðD1 Þ /ðD2 Þg
ð6Þ ð7Þ
Fig. 7a shows schematically the splitting of the donor levels in a symmetrical dye. Here the lowest energy level depicted on Fig. 7a and b and present on Fig. 7c is formed by a conjugated chain. In unsymmetrical cyanines (D1 – D2), as one can see from Fig. 7b and c the energies of both donor levels are different. If taking into consideration that an interaction of the levels is in inverse
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107
Fig. 7. Schematic disposition of delocalized and donor (D1 and D2) levels as well as the first 2 electron transitions in symmetrical cyanine (a), styryl (b) and methoxystyryl (c); DEspl is the splitting energy.
proportionally to the distance between them, then the splitting energy should decrease in asymmetrical polymethine dyes. In styryl 1, both terminal residues contain a nitrogen atom with its high donor level; and hence we expect that the distance between both donor levels is not too large, so that their interaction retains comparatively high, similar to symmetrically cyanines. The situation reverses principally in methoxystyryls 2 with an oxygen atom instead of nitrogen in the low basic terminal p-oxymethylphenylene group; the corresponding quasi-donor level should be disposed rather lower. Fig. 7c demonstrates that the splitting energy in methoxystyryls 2 should be practically negligible, in comparison with symmetrical cyanines or even with unsymmetrical dyes 1.
Of course, the scheme above is strongly simplified. The initial donor MOs interact with the delocalized MOs of the main chromophore. As a result, the direct quantum-chemical calculation of unsymmetrical styryl 1-In and methoxystyryl 2-In (Fig. 8) shows that both highest occupied MO (HOMO) and next orbital (HOMO-1) are considerably delocalized. Nevertheless, the energy gap in dye 1-In with both nitrogenic residues is shifted up in comparison with methoxystyryl 2-In, containing only one nitrogenic terminal group. Thus, we can conclude that the metoxyphenylene residue does not produce the high positioned highest occupied level, in contrast to the nitrogenic heterocycle. At the same time, the position of the lowest vacant level is less sensitive for replacing
Fig. 8. Molecular orbitals of unsymmetrical dyes: 1-In (a) and 2-In (b); and two first transitions S0 ? S1, S0 ? S2.
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dimethylamino group (styryl 1) by metoxy group (methoxystyryl 2). 3.2. Excited state 3.2.1. A general scheme of electron transitions in cyanines The bands observed in visible and near IR spectra of symmetrical and unsymmetrical cyanine dyes are known to correspond to the electron transitions that involve some highest occupied MOs and some vacant orbitals [4,12]. The lowest electron transition is practically connected only with the highest occupied MO (HOMO, further abbreviation H) and the lowest unoccupied MO (LUMO, further abbreviation L): |S1 P |HOMO ? LUMO>. In regard to the next states, their nature depends on the type of the conjugated system. So, |S2> and |S3> electron transitions in polyenes, with symmetrically disposed occupied and vacant electron levels (in respect to the midgap [20]), are degenerated and described by four MOs: |S2 P (2)1(|H-1 ? L> + |H ? L + 1>); |S3 P (2)1(|H1 ? L> |H ? L + 1>). In contrast to it, in cationic cyanine dyes with their shifted down energy gap and considerable distance between the lowest vacant and next levels, the transitions proved to be predominantly one-electron excited states [27,28]. Considerable interaction of donor levels and corresponding interaction of transitions from them to |LUMO> lead to splitting between the excited states S1 and S2. One can see from Fig. 7a that the magnitude of the splitting DEspl in symmetrical dyes should be maximal because of the degeneration of donor levels. Increasing of distance between the donor levels in unsymmetrical dyes should be accompanied by decreasing of the splitting energy. Then, one could expect that the two lowest electron transition, S0 ? S1 and S0 ? S2, in styryls 1 as dyes with both nitrogenic terminal groups (Fig. 7b) should be similar to the corresponding transitions in symmetrical cyanine, whereas these transitions in methoxystyryls 2 (Fig. 7c) should have considerably higher energies. 3.2.2. Absorption and fluorescence excitation anisotropy spectra Fig. 9 presents absorption spectra and spectra of fluorescence excitation, correspondingly, measured in the UV/visible region for indostyryl 1-In (Fig. 9a) and indomethoxystyryls 2-In (Fig. 9b). The shape of the spectral bands for other unsymmetrical dyes 1 and 2 is similar. A comparatively intensive and selective spectral band with its maximum at 560 nm is observed in spectra dye 1In. At the same time, the fluorescence excitation anisotropy spectra points to the existence of the electron transition at nearly 380 nm, which is polarized perpendicularly to the first transition S0 ? S1. The performed quantum-chemical calculations which showed in Table 1 give two electron transitionsß S0 ? S1 and S0 ? S2, for this spectral region, so that the calculated distance between them is 141 nm that is comparatively near to the experimental value. S0 ? S2 transition involves |HOMO-1>. This transition is practically forbidden in one-photon absorption spectra, but it is found for dye 1-BT [28] to be active in two-photon absorption spectra (at 335 nm in the ethanol solvent); a similar electron transition of the same nature is also observed in two-photon spectra of symmetrical thiacarbocyanines 3-BT (n = 1). One can see from Fig. 9 that the going from styryl to its oxyanalogue is accompanied by a considerable hypsochromic effect. As far as the interaction of donor levels in methoxystyryls should be significantly smaller, in comparison with corresponding styryls, the energy of the first electron transition is higher, and hence the absorption band maximum is necessarily shifted in the short wavelength spectral region. The maximum effect is observed for pair 1-In ? 2-In: 127 nm (5238 cm1), the minimum shift of the spectral band is found for dyes 1-Ox ? 2-Ox: 96 nm (4795 cm1). The calculation for 2-In gives the second transition that appeared at approximately 310 nm. The fluorescence excitation
Fig. 9. Absorption, fluorescence and fluorescence excitation spectra of indostyryls 1-In (a) and indomethoxystyryl 2-In (b).
anisotropy spectrum does not point to the existence of the electron transitions; only considerable decreasing of the anisotropy parameter is observed. The fluorescence excitation anisotropy spectra also point to a short wavelength position of the second electron transition, higher than 300 nm. At the same time, the calculated k2 = 375 nm for styryl 1-Qu agrees with the minimum at approximately 377 nm in its fluorescence excitation anisotropy spectrum (see Table 2). Also, one can see that an appreciable large spectral effect upon the annelation of the pyridine cycle is observed for the pairs of pyridinim and quinolinium dye derivatives: Dkabs = 66 nm for the dye pair: 1-Py ? 1-Qu; and Dkabs = 42 nm for the dyes: 2-Py ? 2-Qu. A particular attention should be paid to the fact that quinostyryl 1-Qu absorbs practically at the same spectral region as unsymmetrical indocyanine 1-In does, in spite of symmetrical quinocyanines 3-Qu that absorbs deeper on 150 nm than indocyanines 3-In with the same chain length, because of the larger effective length of the quinolinum residue as a terminal group [4,10,11]. This spectral phenomenon is explained by the deviation of the absorption band maxima in spectra of unsymmetrical cyanine dyes. Deviation is proportional to the difference in the donor strength of both
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A.V. Stanova et al. / Journal of Molecular Structure 988 (2011) 102–110 Table 1 Calculated data of styryls 1 and methoxystyryls 2. R
Transition
Styryl 1
Methoxystyryl 2
k, nm
f, oscillator strength
Main configuration
k, nm
f, oscillator strength
Main configuration
BT
S0 ? S1 S0 ? S2
519 357
1.212 0.073
0.98|H ? L> 0.94|H-1 ? L>
407 280
1.471 0.078
0.98H ? L> 0.75|H-1 ? L > -0.63|H-2 ? L>
Ox
S0 ? S1 S0 ? S2
490 339
1.342 0.083
0.97|H ? L> 0.96|H-1 ? L>
391 273
1.545 0.037
0.98|H ? L> 0.96|H-1 ? L>
In
S0 ? S1 S0 ? S2
545 404
1.024 0.094
0.97|H ? L> 0.96|H-1 ? L>
419 310
1.319 0.082
0.98|H ? L> 0.96|H-1 ? L>
Py
S0 ? S1 S0 ? S2
488 306
1.446 0.061
0.96|H ? L> 0.93|H-1 ? L>
395 268
1.500 0.068
0.97|H ? L> 0.95|H-1 ? L>
Qu
S0 ? S1 S0 ? S2
541 375
1.294 0.036
0.96|H ? L> 0.82|H-1 ? L>
435 316
1.402 0.049
0.97|H ? L> 0.86|H-1 ? L>
Table 2 Spectral data of dyes 1, 2. R
BT Py In Ox Qu
1
2
kanis, nm
kabs, nm
kfl, nm
kabs, nm
kfl, nm
365 336 376 356 377
531 486 560 498 552
591 602 588 554 662
419 388 433 402 430
502 489 523 476 545
terminal groups, i.e. to the asymmetry degree [11]. In the spectra of methoxystyryls 2-Qu and 2-In, the positions of the band maxima (Table 2) coincide practically. 3.2.3. Deviation of absorption band maxima and asymmetry of the electron transition To estimate quantitatively the degree of asymmetry, a deviation D, proposed by Brooker [11] is traditionally used. This parameter is calculated in the following way:
D ¼ ðk1 þ k2 Þ=2 kas ;
ð8Þ
where kas is the absorption maximum of the unsymmetrical dye while k1 and k2 are maxima of the corresponding symmetrical parent molecules. It was found that for typical unsymmetrical cyanines, D > 0, i.e. the position of the maximum for unsymmetrical dyes kas is shifted towards short wavelengths as compared to the arithmetic mean value from the maxima of the parent dyes. This spectral fact was treated to be connected with the appearance of alternation of the CC-bond length along the polymethine chain, if terminal groups are non-equivalent [11]. The corresponding symmetrical carbocyanines 3 (n = 1) and dyes 4 or 5 should be considered as parent dyes for styryls or metoxystyryls. The calculated deviations for the investigated unsymmetrical cyanines 1 and 2 are collected in Table 3. One can see that values D are significantly larger in the spectra of styryls 1 than in the spectra of corresponding metoxystyryls 2. This is in a good agreement with the assumption about connection between the
deviation and non-equivalence of the bond lengths in the polymethine chain. In fact, the degree alternation of the bond lengths (parameter Dlm) in the open chain of metoxystyryls is higher than in the chromophore of methoxystyryl. However, it is noticed that the charge and bond order (and hence bond length) in the polymethine chain of linear conjugated systems depend directly on donor (or basic) properties of terminal groups that can be connected with the position of the highest occupied levels. In turn, the position of this level depends on the splitting of donor MOs that is the maximum in symmetrical cyanines (because of the degeneration of donor levels) and decreases regularly in unsymmetrical dyes. Then, we state that the appearance of deviations in unsymmetrical cyanine is eventually caused by decreasing of interaction of non-degenerated donor levels of non-equivalent terminal groups. The performed analysis shows that spectral characteristic, deviation D, of styryls is proportional to the distance between the initial donor levels DeD of the end groups. These levels were found as a midposition between eHOMO and eHOMO-1 for both symmetrical maternal dyes 4 and corresponding carbocyanine 3 (n = 1) Table 3, using consideration addressed to Fig. 7a. Practically the same order as Brooker’s is obtained for the deviation in spectra of both dye series 1 and 2, excepting the styryl with indolenium residue, 1-In, and somewhat close the value D for the pair dyes 2-Ox and 2-In as well as 2-Qu and 2-Py.
D ðnmÞ ¼ ½51:5ð1-OxÞ; 19:5ð1-InÞ < 54:5ð1-BTÞ < 107 ð1-QuÞ < 122:5ð1-PyÞ D ðnmÞ ¼ ½94ð2-OxÞ; 93ð2-InÞ < 113ð2-BTÞ < 175:5 ð2-QuÞ; ½167ð2-PyÞ The generall conclusion is following: increasing of donor strength of the variable terminal group R is accompanied by regular increasing of deviation. The difference in the observed value D for styryl and corresponding metoxystyryl is considerable, in contrast to the difference in the equilibrium molecular geometry and charge distribution in the ground state. I.e. the excited state is more sensitive to the
Table 3 Deviation (D), Stock’s shift (DmS), DeD and parameter d(pm) for dyes 1, 2. R
BT Py In Ox Qu
1
2
De D
D, nm deviation
Stock’s shift Dm, cm1
d(pm) chain
DeD
D, nm deviation
Stock’s shift Dm, cm1
d(pm) chain
0.2 0.24 0.04 0.08 0.66
54.5 122.5 19.5 51.5 107
1800 3955 753 1860 2890
0.227 0.2132 0.1584 0.1852 0.1961
1.54 1.58 1.38 1.26 2
113 167 93 94 175.5
3840 5027 4018 3832 4670
0.3948 0.3752 0.3346 0.3255 0.3559
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change in chemical constitution upon going from styryls 1 to their metoxyanalogues 2. 3.2.4. Relaxation of molecular geometry and fluorescence spectra The florescence bands of indostyryl 1-In and indometoxystyryl 2-In are presented in Fig. 9a and b. One can see that the fluorescence band maximum is appreciably shifted in the longwavelength region that points to the substantial change of the equilibrium energy upon the relaxation in the excited state. It was established [29] that the Stock’s shift, DmS, calculated by the following formulas (9), depends directly on the change of lengths of the bonds in the
DmS ¼ mmax ðabsÞ mmax ðfluorÞ
ð9Þ
chromophore, that could be quantitatively estimated by:
dðlm Þ ¼
ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X 2 lm lm
ð10Þ
where lm and lm are the bond lengths in the excited and ground states; index m runs all bonds. On the other hand, there is a correlation between the bond length, lm, and the bond order, pm, [20]: lm (Å) = 1.54–0.14pm. I.e., parameter pm can be also considered as a characteristic of the bonds. Then, Eq. (9) could be replaced by an equivalent equation: [30]
dðpm Þ ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qX 2ffi pm pm ;
ð11Þ
Thus, value d(pm) can be considered as an integral characteristic of possible geometrical changes upon the relaxation in the excited state. One could assume that parameter d(pm) should correlate with the Stack’s shift. The calculated values d(pm) and the measured magnitudes DmS for unsymmetrical dyes 1 and 2 are collected in Table 3. One can see that value d(pm) obtained for methoxystyryls 2 exceeds practically twice this parameter for corresponding styryls 1 that agrees with their larger Stock’s shifts. Analysis of value DmS (Table 3) has shown that it depends on donor strength of the variable terminal residue R. Similarly to the deviation, dyes 1 and 2 can be arranged into the following order:
DmS ðcm1 Þ ¼ ½1860ð1-OxÞ; 753ð1-InÞ < 1800ð1-BTÞ < 2890ð1-QuÞ < 3955ð1-PyÞ
DmS ðcm1 Þ ¼ 3832ð2-OxÞ < 4018ð2-InÞ½3840ð2-BT < 4660ð2-QuÞ < 5027ð2-PyÞ Consequently, we can state that increasing of difference in donor strengths of both terminal groups regularly leads to increasing of degree of geometrical changes upon excitation and hence to increasing of Stock’s shifts. The spectral effects from Table 3 are larger for methoxystyryls 2.
4. Conclusion Thus, the combined quantum-chemical and spectral investigations showed that asymmetry degree of the chromophore electron structure in styryls and methoxystyryls differs considerably in the ground and excited states. In the ground state with the closed electron shell, the difference in charge distribution and molecular geometry between styryls and corresponding methoxystyryls is comparatively insignificant, so that electron structure is more sensitive to the basicity of the variable terminal group. In contrary, the electron transitions which involve only the individual MOs differ considerably in both types of unsymmetrical cyanine dyes. The appreciable interaction between two donor levels in styryls provides an additional decreasing of the first transition energy and hence leads to a comparative deep color, whereas the similar interaction in methoxystyryls is negligible, and hence these dyes absorb at shorter wavelengths. References [1] A. Mishra, Chem. Rev. 100 (2000) 1973. [2] G. Bach, S. Daehne, Cyanine Dyes and Related Compounds, ROOD’S Chemistry of Carbon Compounds, vol. IVb, Elsevier Science, Amsterdam, 1997 (Chapter 15). [3] F. Meyers, S.R. Marder, J.W. Perry, Introducing to the Nonlinear Optical Properties of Organic Materials, Chemistry of Advanced Materials, Wiley-VCH, New York–Chichester–Weinheim–Brisbane–Singapore–Toronto, 1998 (Chapter 6). [4] N. Tyutyulkov, J. Fabian, A. Mehlhorn, F. Dietz, A. Tadjer, Polymethine Dyes. Structure and Properties, St. Kliment Ohridski University Press, Sofia, 1991. [5] S. Daehne, Color and Constitution. One hundred years of research, vol. 199, Science, 1978. [6] H. Kuhn, J. Chem. Phys. 17 (1949) 1098. [7] N.S. Bayliss, J. Chem. Phys. 16 (1948) 287. [8] W.T. Simpson, J. Am. Chem. Soc. 73 (1951) 5359. [9] L.M. Tolbert, Acc. Chem. Res. 25 (1992) 561. [10] F.M. Hamer, Cyanine Dyes and Related Compounds, Interscience, New York, 1964. [11] L. Brooker, Rev. Mod. Phys. 14 (1942) 275. [12] A.D. Kachkovsky, Uspekhi Khim. (Rus) 66 (1997) 715. [13] R. Wizinger, P. Ulrich, Helv. Chim. 39 (1956) 207. [14] V. Dryanska, C. Ivanov, Synthesis 1 (1976) 37. [15] A.L. Davis, J. Keeler, E.D. Laue, D. Moskau, J. Magn. Reson. 98 (1992) 207. [16] D.J. States, R.A. Haberkorn, D.J. Ruben, J. Magn. Reson. 48 (1982) 286. [17] A. Bax, G.A. Morris, J. Magn. Reson. 42 (1981) 501. [18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., GAUSSIAN03; Revision B.05, Gaussian, Inc., Pittsburgh PA, 2003. [19] Ju. Bricks, A. Ryabitskii, A. Kachkovskii, Eur. J. Org. Chem. (2009) 3439. [20] M.J.S. Dewar, The molecular orbital theory of organic chemistry, McGrow-Hill, New York, 1969. [21] J.S. Craw, J.R. Reimers, G.B. Bacskay, A.T. Wong, N.S. Hush, Chem. Phys. 167 (1992) 77. [22] H. Kuhn, J. Chem. Phys. 16 (1948) 840. [23] R. Radeglia, E. Gey, K.-D. Nolte, S. Daehne, J. Prakt. Chem. 315 (1973) 586. [24] J.S. Craw, J.R. Reimers, G.B. Bacskay, A.T. Wong, Chem. Phys. 167 (1992) 101. [25] A.D. Kachkovski, D.A. Yushchenko, G.A. Kachkovski, Dyes Pigments 66 (2005) 223. [26] A.D. Kachkovski, Dyes Pigments 24 (1994) 171. [27] S. Webster, J. Fu, L.A. Padilha, H. Hu, O.V. Przhonska, D.J. Hagan, E. Van Stryland, M.V. Bondar, Yu.L. Slominsky, A.D. Kachkovski, J. Luminescence 128 (2008) 927. [28] J. Fu, O.V. Przhonska, L.A. Padilha, D.J. Hagan, E.W. Van Stryland, M.V. Bondar, Yu.L. Slominskiy, A.D. Kachkovsky, J. Am. Opt. Soc. (JOSA B) 24 (2007) 56. [29] E.F. McCoy, I.J. Ross, Aus. J. Chem. 15 (1962) 573. [30] A.A. Ishchenko, Quantum Electron. 24 (1984) 471.