Single site electronic spectra of free base porphin in an n-octane crystal at 5 K

SPECTROCHIMICA ACTA PART A

ELSEVIER

Spectrochimica Acta Part A 53 (1997) 589 595

Single site electronic spectra of free base porphin in an n-octane crystal at 5 K Wen-Ying Huang, Erik Van Riper, Lawrence

W. Johnson *

Department o[ Chemisto', York College, The City University o[New York, Jamaica, N Y 11451, USA

Received 30 March 1996: accepted 2 August 1996

Abstract

The single site fluorescence and excitation spectra of free base porphin in an n-octane crystal at 5 K are reported. The vibrational frequencies of the ground and excited states (Q,., Q, and B bands) have been determined from the spectra. The Q,. region is very similar to the previously reported results from supersonic jet expansion. However, the Q~. region is quite different in the jet and mixed crystal; the crystal spectrum exhibits extensive vibronic coupling. The Sorer band shows little vibronic structure. © 1997 Elsevier Science B.V. Kevwords: Absorption: Fluorescence: Low temperature; Porphin; Spectroscopy

1. Introduction

The electronic spectroscopy of free base porphin (H2P) has been and continues to be the subject of numerous theoretical [1-5] and experimental [6-14] investigations. (The structural formula of H2P is illustrated in Fig. 1.) The interest in this molecule arises for many reasons, the most prominent one being the fact that it is the parent molecule of the biologically important porphyrin family of chromophores; more recently, studies have also been prompted by the ability of H2P to undergo photochemical hole-burning [15] and by the possible use of porphyrins in opto-electric devices [16]. Most of the previous investigations have focused on the lowest energy singlet JrTr* * Corresponding author.

transition, Q,. (S~). Only a little work has been done on the higher excited states, Q~ and B. Even and Jortner [7] looked at the Q region spectrum of H2P using supersonic expansion; Kim and Bohandy [12] examined the same region with a low temperature mixed crystal; Radziszewski et al. [11] employed Fourier-transform spectroscopy to study H2P in low temperature noble gas matrices; also, van Dorp [17] previously reported the fluorescence excitation spectrum of H2P in an n-octane crystal over the Q,. and Q~ regions, but it was not a single site spectrum and no attempt was made to identify the peak positions. In this work, we set out to investigate the electronic transitions of H2P in an n-octane single crystal at 5 K. Our data for Q, are very similar to those found from the supersonic jet experiment [7]. However, our results in the Q, region are considerably different to those of the cold isolated

1386-1425/97517.00 C 1997 Elsevier Science B.V. All rights reserved. PII S1386-1425(96)01801-6

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Spectrochimica Acre Part A 53 (1997) 589 595

molecule; there is a substantial shift to lower energy, no clear origin, a very complex spectrum and a very sharp band which could be the origin of a low lying nzr* state. We have also examined the Soret band, but there is little clear structure.

2. Experimental Free base porphin was synthesized and purified using the methods of Longo et al. [19]. The H2P was dissolved in spectral grade n-octane (n-Cs) which had been distilled over sodium chips. The H2P/n-C ~ solution ( ~ 10 6 M) was put into a crystal growing tube, degassed, sealed and slowly ( - 48 h) lowered into liquid nitrogen. The resulting mixed crystal was cut under liquid nitrogen and a piece was mounted in the sample holder. The low temperatures were reached using an Oxford Instruments CF1204 continuous flow cryostat. The absorption spectra were obtained as fluorescence excitation spectra. Excitation was accomplished with a Lambda Physik FL2001 dye laser pumped with their Lextra 50 excimer (XeCI) laser. Detection of the fluorescence was at 90 ° to the laser beam using a Jobin Yvon 1500 m o n o c h r o m a t o r and a cooled EMI 9203QA phototube. Signal detection was done with a Stanford Research Systems SR400 photon counter (excitation spectra) and a Stanford Research Systems SR510 lock-in amplifier (fluorescence spectra). The excitation spectra were corrected for dye output. The room temperature solution spectrum was measured using a Perkin Elmer L a m b d a 2 UV visible spectrometer; the fourth derivative of the spectrum was obtained with the Perkin-Elmer PECSS software. (A fourth order derivative spectrum was used to more precisely determine the peak positions, and to isolate shoulders and weak signals from the background: while the maximum of a band in an odd order derivative spectrum corresponds to a pass through zero, and a second order derivative gives a minimum, a fourth order derivative produces a sharp m a x i m u m [20].)

3. Results and discussion In Fig. 1, we show the high resolution fluorescence spectrum of H2P in an n-octane crystal at 5 K. The vibrational frequencies were reported earlier by Voelker and Macfarlane [6], but the spectrum was not published; Arabei et al. [13] have examined this system at 77 K. Our emission spectrum is doubled since we used an excitation energy that was absorbed by both of the H2P tautomers, so as to eliminate the errors induced by photochemical hole-burning of the sample during excitation. This doubling of the spectrum arises because when an H2P molecule is in a particular crystal site, in this case an A-type site [21], there are two stable positions for the inner pair of protons; each resulting tautomer produces an individual spectrum. For this (H2P/n-Cs) mixed crystal the origins are separated by 65 cm ~; these tautomers can be interchanged by laser excitation [15]. The positions of the features associated with the 16330 cm ~ origin are listed in Table 1. These are identified in c m - I units from the origin band. We have also included the relative intensities of the vibronic bands; the origin was arbitrarily assigned an intensity o f 1000. (They were not corrected for the phototube response.) Finally, using the results of Radziszewski et al. [11] and Arabei et al. [13] as guides, we have assigned the symmetries of these

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591

W.-Y. Huang et al./ Spectrochimica Acta Part A 53 (1997) 589 595

Table 1 Vibrational frequencies from the emission spectrum of H2P in an n-octane crystal at 5 K. Only the data for the origin at 16 330 cm ~ are listed. The column headed ~a (cm- ~) gives the distance of the spectral feature from the origin. The origin band intensity (I) was independently assigned a value of 1000; all subsequent peaks were compared to it. 'F' signifies a fundamental vibration. The symmetries and numerical values in brackets come from Refs. [11] and [13] i;g d

(cm ~)

0 36 116 305 719 952 974 1054 1129 1174 1223 1316 1385 1497 1603 1615

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1000 145 106 216 221 290 160 242 157 300 206 248 513 283 669 530

Origin ( S l ~ S o ) Phonon F (big, 109) F (ag, 309) F (ag, 723) F (ag, 952) F (big, 976) F (ag, 1063) F (big. 1138) F (ag, 1182) F (big. 1226) F (big, 1318) F (big. 1388) F (ag, 1493) F (big. 1600) F (ag, 1610)

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vibrations; the numbers in brackets are the frequencies from their data. Fig. 2 shows the absorption spectrum of the Q~ and Q, regions. In part (a) we show the 298 K,

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Fig. 2. (a) Absorption spectrum of the Q, and Q,. regions of HzP in n-octane at room temperature; (b) fourth derivative of the spectrum shown in (a); (c) single site (16330 cm -~) excitation spectrum of H2P in an n-octane crystal at 5 K. The peak positions in (b) and (c) are with respect to the Q, origins.

n-octane solution spectrum of H2P. Just below it in part (b) is the fourth derivative of the spectrum; this was included to help extract the positions of the features from the room temperature spectrum [20]. Part (c) of this figure displays the overall single site (16330 cm 1) excitation spectrum of H2P in an n-octane crystal at 5 K. In Fig. 2(c), the Q, region, from the origin (16 330 cm l) to about 19 000 cm ~, is shifted to higher energy with respect to the room temperature solution spectrum by about 117 cm 1. The uniformity of the peak blue shifts supports the belief that this region (Q,.) is due to a single electronic transition. Further, the fact that the shift is to a higher energy may indicate that there has been a crystal induced change in the geometry of the HzP molecule [7,18]. There is also some support for this idea from the Stark effect experiments on the Q, origin band which measured a change in dipole moment [19]; this was not expected from a molecule with D2h symmetry. The low temperature bands in the Q, region (19000-21500 cm ~), however, all seem to be shifted to lower energy, but by a wider range of values ( ~ 100 200 cm 1). This red shift presumably arises from the dispersive interactions induced by the moderately large oscillator strength of this transition [7,22]. Fig. 3 illustrates the Q, region associated with the origin at 16 330 cm ~. (Voelker and Macfarlane [6] and Arabei et al. [13] previously published

w.-Y. Huang et al. Spectrochimh'a Acta Part A 53 (1997) 589 595

592

Table 2 Vibrational frequencies from the excitation spectrum of H=P in an n-octane crystal at 5 K. Only the data for the Q, origin at 16 330 cm ~ are listed. The column headed /;~ (cm -~) gives the distance of the spectral feature from the origin. The origin band intensity (1) was independently assigned a value of 1000: all subsequent peaks were compared to it. 'F' signifies a fundamental vibration. The symmetries and numerical values in brackets come from Refs. [11] and [13]. Numbers with stars (*) indicate the Q,. origin position %~ (cm ~) 0 118 154 307 385 414 460 613 708 780 941 967 983 991 1015 1049 1106 1160 1175 1214 1224 1235 1247 1258 1273 1292 1304 1312 1317 1324 1329 1330 1334 1349 1353 1362 1367 1390 1412 1467 1480 1489 1508 1518 1524 1532 1544

1

Assignment

1000 30

Origin (SI~S~) F (big. 109) F (a~, 157) F (a~, 309) F (big. 389) F (hl~, 418) (154+ 307)- 1 (307 × 2 ) - 1 F (ag, 723) F (bl~, 786} F (a~, 952) F (big, 976) F 1as, 988) F (bit, 1005) 307 + 708 F {a¢, 1063) F (big, 1138) F (a~, 1182) F (big, 1182) F (bl~, 1226) 118+ 1106 (154+ 3 0 7 + 7 8 0 ) - 6 (307+941)- 1 (154+ 1106)-2 (307+967)-1 F (b~g, 1318) (307 + 991t + 6 (154 + 1160)- 2 (154+ 1160)-3 F (ag. 1353) 154+ 1175 (385 + 9 4 1 ) + 4 (118+ 1214)+2 F (bls, 13741 (307 + 1049)- 3 F (big, 1388) (154+ 1214)-- 1 F (ag. 1425) (708 × 2 ) - 4 F (big, 1493) F (ag, 1493) {708 ~ 780)+1 (460 + 1049)- 1 (307+ 1214)-3 F (at, 15461 (118 +708 x 2 ) - 2 (385+ 1160) 1

221 392 59 52 66 123 993 517 443 291 328 162 169 724 521 1359 212 807 196 I10 170 158 191 781 312 570 574 707 568 494 518 664 523 447 205 271 146 457 713 244 382 912 969 514 446

i~.,,. (cm ~) 1550 1570 1575 1581 1585 1591 1656 1697 1705 1732 1739 1766 1773 1787 1825 1831 1859 1893 1933 2011 2065 2101 2168 2199

2269 2303 2839* 2852* 2915" 2944* 2969* 3048* 3097 3146 3365 3386 3463 3544 3560 3628 3656 3683 3732 3769 3805 3832 3856 3897 3946 3992 4013 4070 4119 4194 8778 9358 9821 10 461

1

Assignment

1367 403 475 588 926 460 226 133 146 88 93 87 88 170 230 254 431 313 115 94 85 76 94 92 144 94 143 140 141 205 240 161 166 124 288 166 167 291 294 350 353 353 522 498 490 543 513 569 700 623 683 551 51)5 482 540 600 543 325

F tbl~, 1600) 154+708 x 2 (414+ 1160)+ 1 (118+ 1467)--4 F (ae, 1610) (414+1175)+2 307+ 1349 (154+385+1160)-2 (154+1550)+1 (154+1581)-3 154-1585 (414+ 1353) 1 154+307+1312 307+ 1480 (307 x 2 + 1214) 3 307+ 1524 (307+ 1550)-2 (307+1585)+1 (967 x 2 i - 1 154+307+1550 (307+708+ 1049)4- 1 (1049×2t+3 (941+1224)+3 307×2+1585 (941 + 1324)+4 (780+ 1524)- 1 (1362+ 1480)-3 (1390+ 1467) 5 1385+941 + 1585)+4 (1467+1480)-3 (708+780+ 1480)+ 1 {1467+ 1585)-4 1154+2944") I (154+2969")+23 or 307+2839* n~* (414+2969")+3 (414+3468")+ 1 (708+2839*)-3 708+2852* (708+2915)+5 (708+2944*)+4 (708+2969*)+6 (154+708+2852")+18 (154+708+2915")-8 ( 154 + 708 + 2944') - 1 (154±708+2969')+1 941+2915" (1049+2852")-4 (1106+2839")+1 (1049 + 2944*) 1 (1049~- 2 9 6 9 " ) - 5 (1106+2969") 5 1175 + 2944* (118+1106+2969")+1 Origin (S~ "---S~)

W.-Y. Huang et al. / Spectrochimica Acta Part ,4 53 (1997) 589 595

the first ~ 1000 cm l of this spectrum and the first 1600 cm I of the vibrational analysis.) The peak positions and intensities relative to the origin band are listed in Table 2; the origin was assigned an intensity value of 1000. (Both of the crystal origins give essentially the same Q,. spectra.) Since free base porphin has D2h symmetry and the lowest allowed singlet gg* transition (Q,) has B~u symmetry, the active vibrations in the B3u ,-- Ag excitation should then have ag and big symmetries. Consequently, we have made symmetry assignments of the observed bands using the works of Arabei et al. [13] and Radziszewski et al. [11] as guides. There is a close correspondence in this region between our low temperature, mixed crystal data and the jet experimental data produced by Even and Jortner [7]; the origin and subsequent vibrational bands look similar and have essentially identical values. Our experiment gives values of 16 330 cm-~ and 16265 cm ~ for the respective Q,. (rc~r*) tautomer origins of H2P in n-octane at 5 K; the corresponding number from Even and Jortner in a supersonic expansion experiment is 16 320 cm ~. This similarity, however, does not extend to the higher energy portion of the spectrum. Fig. 4 shows only the Q,. region of the spectrum. This allowed singlet ~g* transition has B2u symmetry. Since there is a marked difference between the Q,. region spectra of the two tautomers, we have included both of them; Fig. 4(a) shows the one from the 16265 cm l origin and Fig. 4(b) shows that from the 16 330 cm ~ Qx origin. Table 2 lists the positions of the peaks and intensities associated with the Q,. origin at 16 330 cm 1. Table 3 contains the values connected with the 16 265 cm 1 Q, origin. These spectra are considerably different from that obtained from the jet by Even and Jortner [7]. Their spectrum has a relatively intense and sharp origin at 19 884 cm - ~ (full width at half maximum (FWHM): 12 cm -1) followed by an orderly set of vibronic bands. In our mixed crystal spectra there are no clear origins for the Q,. regions; earlier room temperature solution [1] and low temperature [11,12] studies have identified it as the weak band observed in the n-octane solution at 19425 cm 1. This corresponds to the structured, broad bands in Fig. 4(a)Fig. 4(b) centered at about 3046 cm i and 2969 cm - 1, respectively, in the low

593

temperature crystal spectra. This gives a red shift of about 580 cm 1 on going from the cold isolated molecule in the jet to the cold molecule imbedded in the alkane crystal. The Q~. region spectrum of the higher energy (16330 cm i) origin is very congested and the vibrational bands are extensively overlapped. However, the spectrum associated with the lower energy (16 265 c m - 1) origin is more resolved and shows a clear pattern of repeating pairs of peaks; our contention is that the group of about four peaks clustered at about 19311 cm ~ (Fig. 4(a)), 3046 c m - 1 from the Q, origin, is the Qy origin which is coupled to the higher energy components of the Q,. vibrational manifold. The F W H M of this cluster is about 200 cm 1, which we assume is the magnitude of the vibronic interaction between the B2u and B3u states. Once the Q,. origin has been identified as the cluster with the two prominent peaks at 19271 cm ~ and 19351 c m i the higher energy part of the Q~ region can be analyzed in terms of these peaks spaced by normal vibrational modes from the origin. This is illustrated in Fig. 5 for the region corresponding to the 19 311 cm 1 Q,. origin; in this figure we show the major components of the origin group of bands spaced at 385 cm ~, 780 cm 1, 956 cm 1, 1224 cm - l , 1534 cm l, and 1645 cm i from the origin. These values were determined from the best fit of the spectral data: the fact that they differ a little from the Q,. numbers is not surprising since they correspond to another electronic state. Table 3 contains the peak locations for Fig. 4(a) and Fig. 5. The numbers marked with a star (*) indicate the origin band cluster produced by vibronic coupling of the B2u state (Q~.) with background vibronic manifold of the underlying B3u state (Q,). The coupling arises through the participation of a big vibrational mode; the starred vibrational numbers have the symmetry form big + nag. The remainder of the Q,. spectrum is generated by the repetition of the origin by ag and big vibrations. In Table 2 we have also identified the Q~. origin peaks in Fig. 4(b) with stars and then assigned some of the subsequent spectral features; however, the pattern is not as clear as that in Fig. 4(a). Consequently, without the vibronic coupling between the B3u and B2u states,

594

W.-Y. Huang et al.

,,

Spectrochirnica Acta Part A 53 (1997) 589 595

Acknowledgements This research was supported by grant GM08153 from the National Institutes of Health. =

References

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Fig. 5. Single site spectrum of the Q,. region associated with the 16265 c m - ~ origin band of H2P in an n - octane crystal at 5 K. The initial cluster of four peaks centered at about 19311 cm ~ has been moved across the region to show how' its pattern is repeated at vibrational intervals of 385, 780, 952. 1224, 1534 and 1645 c m - i

and low intensity features of the excitation spectrum of the Soret band imply that there is probably an interacting nrr* state energetically just below it.

(a) / XI/8 /

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22500

23500

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Fig. 6. (a) Absorption spectrum of the Soret region of H,P in n-octane at room temperature: (b) single site (16330 cm ~) excitation spectrum of H2P in an n-octane crystal at 5 K. The intensity scales are the same as those in Fig. 2.

[1] M. Gouterman, in D. Dolphin (Eds.), The Porphyrins, Vol. II1, Academic Press, New York, 1978, Chapter I. [2] U. Nagashima, T. Takada and K. Ohno, J. Chem. Phys., 85 (1986) 4525. [3] J.D. Petke, G.M. Maggiora. L i . Shipman and R.E. Christoffersen, J. Mol. Spectrosc., 71 (1978) 64; 73 (1978) 311. [4] D.C. Rawlings, E.R. Davidson and M. Gouterman, Int. J. Quantum Chem., 26 (1984) 237, 251D.C. Rawlings, M. Gouterman, E.R. Davidson and D. Feller, Int. J. Quantum Chem., 28 (1985) 773, 797, 823. [5] D. Lamoen and M. Parriello, Chem. Phys. Lett., 248 (1996) 309. [6] S. Voelker and R.M. MacFarlane, J. Chem. Phys., 73 (1980) 4476. [7] U. Even and J. Jortner. J. Chem. Phys., 77 (1982) 4391. [8] M. Gouterman, P.M. Rentzepis and K.D. Starb, Porphyrins: Excited States and Dynamics, ACS Syrup. Ser. 32l, Washington. DC, 1986. [9] K.N. Solovyov, A.T. Gradyushko, M.P. Tsvinko and V.N. Knyukshto, J. Lumin., 41 (1976) 365. [10] A.I.M. Dicker. M. Noort, S. Voelker and J.H. van der Waals, Chem. Phys. Lett., 73 (1980) 1. [11] J.G. Radziszewski, J. Waluk and J. Michl, J. Mol. Spectrosc., 140 (1990) 373. [12] B.F. Kim and J. Bohandy, J. Mol. Spectrosc., 73 (1978) 332. [13] S.M. Arabei, S.K. Shkirman, K.N. Solov'ev and G.D. Yegorova, Spectrosc. Lett.. 10 (1977) 677. [14] A.T. Gradyuishko, K.N. Solov'ev and A.S. Starukhin, Opt. Spectrosc., 40 (1976) 469. [15] S. Voelker, Ann. Rev. Phys. Chem., 40 (1989) 499. [16] J. Simon and J.-J. Andre, Molecular Semiconductors, Springer, Berlin, 1985. [17] W.G. van Dorp, Thesis. University of Leyden, 1975, p. 20. [18] L. Edwards, D.H. Dolphin, M. Gouterman and A.D. Adler, J. Mol. Spectrosc.. 38 11971) 16. [19] F.R. Longo, E.J. Thorne, A.D. Adler and S. Dym, Heterocycl. Chem., 12 (1975) 1305. [20] H.H. Madden, Anal. Chem., 50 (1978) 1383. [21] G. Jansen, M. Noort, N. van Dijk and J.H. van der Waals, Mol. Phys., 39 (1980) 865. [22] J.A. Pople and H.C. Longuet-Higgins, J. Chem. Phys., 27 (1957) 192. [23] W.-Y. Huang, E. Van Riper and L.W. Johnson, Spectrochim Acta Part A. 52 (1996) 761. [24] R.M. Hochstrasser, Acc. Chem. Res., 1 (1968) 266.

14% Y. Huang et al. Spectrochimica Acta Part A 53 (1997) 589 595

595

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This research was supported by grant GM08153 from the National Institutes of Health.

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References

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Fig. 5. Single site spectrum of the Q~, region associated with the 16265 cm I origin band of H~P in an n octane crystal at 5 K. The initial cluster of four peaks centered at about 19311 cm I has been moved across the region to show how its pattern is repeated at vibrational intervals of 385, 780, 952, 1224, 1534 and 1645 cm ~. and low intensity features of the excitation spect r u m o f t h e S o r e r b a n d i m p l y t h a t t h e r e is p r o b a bly a n i n t e r a c t i n g roT* s t a t e e n e r g e t i c a l l y j u s t b e l o w it.

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Fig. 6. Ca) Absorption spectrum of the Soret region of HeP in n-octane at room temperature; (b) single site (16330 cm-~) excitation spectrum of HeP in an n-octane crystal at 5 K. The intensity scales are the same as those in Fig. 2.

[1] M. Gouterman, in D. Dolphin (Eds.), The Porphyrins, Vol. III, Academic Press, New York, 1978, Chapter 1. [2] U. Nagashima, T. Takada and K. Ohno, J. Chem. Phys., 85 (1986) 4525. [3] J.D. Petke, G.M. Maggiora, L.L. Shipman and R.E. Christoffersen, J. Mol. Spectrosc., 71 (1978) 64; 73 (1978) 311. [4] D.C. Rawlings, E.R. Davidson and M. Gouterman, Int. J. Quantum Chem.. 26 (1984) 237, 251.D.C. Rawlings, M. Gouterman, E.R. Davidson and D. Feller, Int. J. Quantum Chem., 28 (1985) 773, 797, 823. [5] D. Lamoen and M. Parriello, Chem. Phys. Lett., 248 (1996) 309. [6] S. Voelker and R.M. MacFarlane, J. Chem. Phys., 73 (1980) 4476. [7] U, Even and J. Jortner, J. Chem. Phys., 77 (1982) 4391. [8] M. Gouterman, P.M. Rentzepis and K.D. Starb, Porphyrins: Excited States and Dynamics, ACS Symp. Ser. 321, Washington, DC. 1986. [9] K.N. Solovyov, A.T. Gradyushko, M.P. Tsvinko and V.N. Knyukshto, J. Lumin., 41 (1976) 365. [10] A.I.M. Dicker, M. Noort, S. Voelker and J.H. van der Waals, Chem. Phys. Lett., 73 (1980) 1. [1 I] J.G. Radziszewski, J. Waluk and J. Michl, J. Mol. Spectrosc., 140 (1990) 373. [12] B,F. Kim and J. Bohandy, J. Mol. Spectrosc., 73 (1978) 332. [13] S.M. Arab& S.K, Shkirman, K.N. Solov'ev and G.D. Yegorova, Spectrosc. Lett., 10 (1977) 677. [14] A.T. Gradyuishko, K.N. Solov'ev and A.S. Starukhin, Opt. Spectrosc., 40 (1976} 469. [15] S. Voelker, Ann. Rev. Phys. Chem., 40 (1989) 499. [16] J. Simon and J.-J. Andre, Molecular Semiconductors, Springer. Berlin. 1985. [17] W.G. van Dorp, Thesis, University of Leyden, 1975, p. 2t1. [18] L. Ed,sards, D.H. Dolphin, M. Gouterman and A.D. Adler. J. Mol. Spectrosc., 38 (1971) 16. [[9] F.R. Longo, E.J. Thorne, A.D. Adler and S. Dym, Heterocycl. Chem., 12 (1975) 1305. [20] H.H. Madden, Anal. Chem.. 50 (1978) 1383. [21] G. Jansen, M. Noort, N. van Dijk and J.H. van der Waals, Mol. Phys., 39 (1980) 865. [22] J.A. Pople and H.C. Longuet-Higgins, J. Chem. Phys., 27 (1957) 192. [23] W.-Y. Huang, E. Van Riper and L.W. Johnson, Spectrochim Acta Part A, 52 (1996) 761. [24] R.M. Hochstrasser. Acc. Chem. Res., 1 (1968) 266.