4999
J. Phys. Chem. 1991,95,4999-5002
Transient Resonance Coherent Anti-Stokes Raman Scattering Spectra of Ion Radicals of all- frans 1,4-Diphenyi- 1,3-butadiene
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Todor Dudev,? Toshio Kamisuki, Naotoshi Akamatsu, and Chiaki Hirose* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta, Midori- ku, Yokohama 227, Japan (Received: September 19, 1990; In Final Form: January 29, 1991)
Transient CARS (coherent anti-Stokes Raman scattering) spectra of the photoinduced species of all-trans- 1,Cdiphenyl1,3-butadiene (DPB) were observed in exciplex-forming systems: DPB/p-dicyanobenzene (DCNB) and DPB/N,N-dimethylaniline (DMA) in p-dioxane, and DPB alone in such polar solvents as acetone, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). The transient species are ascribed to the DPB cation radical for the DPB/DCNB system and the anion radical for the DPB/DMA system, while either or both of DPB'+ and DPB* were detected for the DPB solo solutions according to the change of electron-donating property of the polar solvents. The lifetimes of the photoproduced species in acetone and in the exciplex systems were much longer (-500 ns for DPB in acetone, 100 ns for DPB/DCNB, and 130 ns for DPB/DMA in p-dioxane) than the reported fluorescence lifetime (less than 1 ns) of SI.The exciplex fluorescence spectra were also measured. A tentative assignment of the observed vibrational frequencies is given and trends concerning spectral characteristics and structures of the transient species for the series of shorter a,w-diphenylpolyenes are briefly discussed.
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Introduction A good deal of attention has been paid to investigations of transient species of different polyenic systems in the past decade. The shorter polyenes serve as model compounds for the chromophores in important biological systems and their transient species are expected to play key roles in mediating the interaction with light. The series of linear all-trans-cr,w-diphenylpolyenes belongs to these systems and many papers have been dedicated to its lowest member, stilbene, and a vast amount of data, both experimental and theoretical, for this molecule, such as knowledge about SI, TI,anion, and cation radicals and conformational and vibrational analyses, is available. Our recent study of diphenylhexatriene (DPH) by using the transient CARS (coherent anti-Stokes Raman scattering) method revealed the formation of DPW+ cation radical in photoirradiated solutions in polar solvents' and the formation of (DPH+-DCNB-)* and (DPH--DMA+)* exciplexes.2 Much more, however, remains to be done in our attempt to unveil the properties of excited-state species of the next homologue (DPB). Recent in the series: all-trans-l,4-diphenyl-l,3-butadiene investigations have shown that the ordering of lowest excited 2'A and I'B, electronic states in DPB depends on ~ o l v e n t sand ~,~ attempts have been made to explain the phenomena by using semiempirical M O calculations.5 Vibrational spectra in the SI and TI states obtained by different techniques such as resonance Raman coherent Stokes Raman scattering (CSRS), coherent anti-Stokes Raman scattering (CARS): laser spectroscopy of supersonic jet expansion, and low-temperature absorptionI0J1 have been reported. Transient absorption spectra of the DPB ion radical^,'^-'^ have been published too. N o vibrational data, however, for ionic species have been reported so far. In this paper we report the results of transient resonance CARS and fluorescence spectra of DPB in exciplex-forming systems such as DPB in p-dioxane together with N,N-dimethylaniline (DMA) or p-dicyanobenzene (DCNB) (as an electron donor or electron acceptor). The spectra of DPB in acetone, DMF, and DMSO are observed and compared with those of the exciplex systems. The lifetimes of the photoproduced species are reported. The observed vibrational frequencies are compared with those of other DPB species and the ion radicals of trans-stilbene (tSB) and all-trans- 1,6-diphenyl- 1,3,5-hexatriene (DPH) to propose a tentative assignment. Some trends concerning the spectroscopic properties and structures of the transient species in the series of *The author to whom correspondence should be sent. 'Present address: Department of Chemistry, University of Sofia, 1126 Sofia, Bulgaria.
0022-3654/91/2095-4999$02.50/0
shorter a,o-diphenylpolyenes are discussed.
Experimental Section Diphenylbutadiene (Kanto Kagaku) was purified by recrystallization from methanol. DMA (Wako Jyunyaku), DMSO (Wako Jyunyaku), and DCNB (Aldrich) were used without further purification. Solvents (acetone and p-dioxane, Wako Jyunyaku) were of spectroscopic grade. The concentration of DPB in solution was in the range 1-3 mM. Solutions with electron donor and electron acceptor were obtained by adding DMA and DCNB, respectively, in concentration of 0.1 M. Degassed solutions prepared by repeated freeze-pumpthaw cycles under vacuum were used. The experimental setup used to measure transient resonance CARS spectra was described in previous publications.'5-'6 In short, a nitrogen laser (Molectron Corp., UV-24,9 mJ/pulse, 337 nm, 10 ns pulse width) was used to pump two dye lasers which generated o1and o2beams. Part of the UV light emitted by the nitrogen laser was used for photoexcitation. The frequency of the pump beam, olrwas set in resonance with the absorption bands of the transient species which lies in the 540-570-nm region.'"'' In the timeresolved CARS measurement, the third harmonic (355 (1) Watanabc, M.; Kamisuki, T.; Akamatsu. N.; Hirose, C. Chem. Phys. Lert. 1990, 170, 451. (2) Kamisuki, T.; Hirasc, C. J . Phys. Chem., in press. (3) Bennet, J.; Birge, R. J . Chem. Phys. 1980, 73, 4234. (4) Swofford, R.;McLain, W. J . Chem. Phys. 1973, 59, 5740. (5) Pierce, B.; Birge, R.J . Phys. Chem. 1982,86, 2651. (6) Wilbrandt, R.;Gmmann, W.; Killough, P.; Bennet, J.; Hester, R.J . Phys. Chem. 1904,88, 5964. (7) Wilbrandt, R.;Jensen, N. H.; Langkilde, F. W. Chem. Phys. Lori. 1984, 1 1 1 , 123. (8) Gustafson, T.; Palmer, J.; Roberts, D. Chem. Phys. Lert. 1986, 127, 505. (9) Kasama, A.; Taya, M.; Kamisuki, T.; Adachi, Y.; Ma&, S.In Time
Resolued Vibrational Spectroscopy; Atkinson, G.; Ed.;Gordon and Breach Sci.: New York, 1987; p 304. (10) Honuitz, J.; Kohler, B.; Spiglanin, T. J . Chem. Phys. 1985,83,2186. (11) Shepanski, J. F.; Keelan, B. W.; Zewail, A. H. Chem. Phys. h i t . 1983. 103. 9. (12) Yamamoto, Y.; Aoyama, T.; Hayashi, K. J . Chem. Soc., Faraday Trans. I 1988.84, 2209. (13) Shida, T.; Hamill. W. J . Chem. Phys. 1966.44.4372. (14) Hotjtink, G.; van der Meij, H. Z . Physik. Chem. (Munich) 1959.20, 1.
(15) Maeda, S.;Kamisuki, T.; Kataoka, H.; Adachi. Y. Appl. Spectrosc. Reo. 1985, 21, 21 1. (16) Kamisuki, T.; Akamatsu, N.; Adachi, Y.; Hirose, C.; Yamamoto. N.; Takuma, K.; Iwamura, M. Bull. Chem. SOC.Jpn. 1989,62, 1415.
0 1991 American Chemical Society
so00 The Journal of Physical Chemistry, Vol. 95, No. 13, 199' I
Dudev et ai.
n
/"\ DPBIDMA
D P D / D C N D in p-dioxane
(4
- - -l
20000
(
I
I
'
I
I
t
l
25000 em-'
Figure 1. Fluorescence spectra of DPB/DMA in p-dioxane (curve l), DPB/DCNB in p-dioxane (curve 2). and DPB in n-hexane (curve 3) obtained by 337-nm-pulse excitation, concentration was 1 mM for DPB and 0.1 M for DMA and DCNB.
nm) of a Nd:YAG laser (Molectron, MY-32, 15 ns pulse width) output was used for photoexcitation and the nitrogen laser which pumped the two dye lasers were operated by taking an appropriate delay time from the 355-nm pulse. Fluorescence spectra were measured by the 337-nm-pulse excitation of the N2laser. All measurements were done at room temperature.
Results and Discussion A. Transient Resonance CARS and Fluorescence Spectra. 1. Exciplex-Forming Systems. Figure 1 shows the fluorescence spectra of the DPB exciplex systems in p-dioxane (curves 1 and 2) and that of DPB in n-heptane solution (curve 3). Unresolved vibrational structure was observed on the SI fluorescence located in the shorter wavelength region in every case. Structureless and broad bands at longer wavelength for the exciplex systems, whose peak positions are slightly different from each other, are identified as exciplex fluorescence. The lifetime of the broad bands were found to be less than the used pulse time profile (10 ns). For the n-heptane solution, a possibility of excimer fluorescence in the studied condition can not be excluded. The observed transient resonance CARS spectra of diphenylbutadiene in p-dioxane containing DCNB as a strong electron acceptor and DMA as a strong electron donor are shown in Figure 2, a and c, respectively. The characteristic features displayed by the present results are summarized as follows. (i) Observed Raman bands show resonance enhancement when olfrequency is set at 546 nm (18 300 cm-I) which correspond to the absorption maximum of DPB'+ 12~13or a t 560 nm (17 800 cm-I) corresponding to the absorption peak of DPB"." When wI was set at different frequencies, the spectral shapes of the transient bands deviated from normal dispersion shape. This confirms that the spectra shown in Figure 2a, are under rigorous one-photon resonance. (ii) The observed Raman frequencies differ significantly from those of other DPB transient species: SI, and TI9(see Table I). (iii) Although the Raman frequencies of both systems are mutually different to some extent, the spectral features observed with resonance enhancement are similar as a whole. Such similarities were also observed between the cation and anion radicals of tSB and DPH; in the case of tSB and DPH, the spectra of photoproduced anion radicals in the presence of aromatic amines showed close correspondence with those of anion radicals obtained chemically by sodium metal reduction. Small deviations in vibrational frequencies were observed between the species prepared by different methods and were attributed to the different environments around tSB'- 17-20 or DPH'-.'J ( I 7) Hamaguchi, H. In Vibrational Specrra and Structure; Durig, J. R.; Ed.; Elsevier: Amsterdam, 1987; Chapter 4. (18) Takahashi, C.; Maeda, S.Chem. Phys. Lett. 1974, 28, 22. (19) Hub,W.; Schneider, S.;Doerr, F.; Simpson, J.; Oxmann, J.; Lewis, F. J. Am. Chem. Soc. 1982, 104,2044. (20) Hub, W.;Schneider, S.;Doerr, F.; Oxmann, J.; Lewis, F. J . Am. Chem. Soc. 1984,106,708.
D P B / D M A in p-dioxane
-11
1
I
lGO0
1200
1300
1
.
7
tl
1000
7
600
(Rainan shift /et.-')
Figure 2. Transient resonance CARS spectra of (a) DPB/DCNB in p-dioxane, (b) DPB in acetone, and (c) DPB/DMA in p-dioxane, observed under the irradiation by 337-nm UV light. For (a) and (b), wI was set at 18 300 cm-I for the observation in the region 1700-900-cm-' and at 18 OOO cm-' for the observation in the 900-500-cm-' region. For (c) wI= 17800 cm-I for the observation in the 17C!04W-"1 region and w I = 18 OOO cm-I for the observation in the 900-500 cm-I region. The
solvent band is denoted by s.
TABLE I: Vibrational Frequencies (in cm-') of Diphenylbutadiene in the Ground State and Transient States SO SI TI cation anion ref 5 ref 9 ref 6 ref 8 ref 9 ref 6 this work this work
1630 1613 1601 1583 I499 1456 1352 1331 1315 1303 1291 1182 1163 1144 1002 992
1625 1595 1577 1495 1452 1331 1311 1299 1287 1180 1160 1141 1000
1585 1575
1570
1480
1478 1390 1335
1230 1164 1135 1070
1225 1161 1131 1071
990
980
607
1582 1585 1556 1560
1598
1574
1298 1252 1 I90
1250 1220 1170
993 863 614 603
982
1338
1217
1220
1146
1145
974
980
596
156
(iv) As will be described in section B, the observed bands for the exciplex-forming solutions of DPB show good correspondence to the reported bands of cation and/or anion radicals of tSB and DPH. (v) From the time-resolved measurements of the most intense signals a t 1598 cm-' for DPB/DCNB and at 1574 cm-' for DPB/DMA, the lifetime of the transient species were estimated as 100 and 130 ns in pdioxane, respectively, which are much longer than the SIfluorescence lifetime of 600 nm). The DPB solutions in the solvents having higher electron-donating properties such as dimethyl sulfoxide (DMSO) and N,Ndimethylformamide (DMF), on the other hand, gave the transient resonance CARS signals as shown in Figure 3. It is seen that the signals consist of the bands which were observed in the DPB/DMA system and of the CARS at 1598 and 993 cm-I which originate from DPB'+. Figure 4 shows the fluorescence spectra of DPB in DMF (curve 1). n-heptane (curve 2), and acetone (curve 3). No apparent exciplex fluorescence was observed. Fluorescence lifetime is found to be not longer than the used laser pulse width (10 ns) even in the longer wavelength region. A study to pursue the role of solvents in forming the ion radicals of DPB and DPH is in progress and will be described in a forthcoming publication. B. Tentative Assignment of Observed Vibrational Frequencies. The observed Raman frequencies of the cation and anion radicals of DPB are given in Table I together with the related data. Table
20600
Z5;OO
em-'
Figure 4. Fluorescence spectra of DPB in DMF (curve I), n-heptane (curve 2), and in acetone (curve 3) as obtained by 337-nm-pulse excitation; concentration was 0.5-1 mM.
I1 lists the Raman frequencies of the ion radicals of tSB, DPB, and DPH to demonstrate the presence of good correspondence among the species. It may be worth trying to assign qualitatively the observed vibrational modes on the basis of qualitative considerations. To this end recently published data about the trans-stilbene (tSB) cation and anion radicalsI7 are useful, where a detailed analyses on both positive and negative ion radicals of tSB and three of its deuterated species were carried out and the assignment of the observed vibrational frequencies in transient Raman spectra was given. The most intense resonance CARS signals at 1598 cm-' for DPB'+ and at 1574 cm-' for DPB' have the corresponding band at slightly shifted frequencies in the corresponding signals for ion radicals of tSB (Table II), and the latter bands were assigned to be mainly associated with phenyl C-C stretch vibrations (8a mode of monosubstituted benzene in Wilson's notation, denoted by Y ~ ) . The bands at 1588 cm-' of DPH'+ and at 1570 cm-' of DPH'can be also assigned to this mode.2 In view of the most intense nature of these bands, however, a considerable mixing with polyenic C = C stretch mode is probably taking place, because the resonantly enhanced mode should be correlated with the corresponding electronic transition. In the absorption spectra of DPB'+ and DPB'-, either or both of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), in the neutral DPB should contribute significantly to the electronic transition according to the results from a simple semiempirical (PPP) M O calculation which shows that both orbitals have more dominant coefficients in the polyenic part of the molecule than in the phenyl moieties. The absorption bands actually show a remarkable red shift by the increase of polyenic chain length. The polyenic part, therefore, must be mainly responsible for the resonance enhancement under study. A weak band at 1585 cm-' was observed as a shoulder of the 1598-cm-' band in DPW+ but the band of the DPB' was too weak to reveal the presence of similar shoulder. The corresponding band in the tSB'+ spectrum occurs at 1562 cm-l and that of tSB'- a t 1553 cm-I, and the band was assigned to the Cb==Cb stretch (denoted by v-).I7 In the case of DPH, the corresponding band appears at the even lower frequency of 1532 cm-' (DPH'+) and 1521 cm-' (DPH').'J We thus ascribe these bands to the Cb+b
5002 The Journal of Physical Chemistry, Vol. 95, No. 13. 1991 stretching mode of the polyenic chain. The low intensity of the band may be due to the coupling with the phenyl 8a mode in such a way that only slight matching exists between the electronic resonance transition and vibrational mode. The 1285-cm-' (tSW+) and the 1251-cm-l (tSB'-) bands observed in the resonance Raman spectrum were assigned to methylene CH in-plane bending vibration with a participation of Ph-C stretch." Then, it is quite likely that the bands at 1298 cm-' in DPB'+ and at 1250 cm-I in DPB' possess similar nature. "Extra" bands were observed in the 1210-1260-cm-' region for DPW+ ( 1252 cm-I), DPH'+ ( 1248 cm-I), DPB' ( 1220 cm-I), and DPH'- (12 15 cm-I) but were not observed for tSB'+ and tSB'-. Most probably they are due to a vibrational mode which includes the stretch of central cb-cb bond@) in the polyenic chain. trans-Stilbene does not have such a bond. The lower frequency bands of the DPB'+ (1 190,993, and 863 cm-I), DPB' (1 170 and 982 cm-I), DPH'+ (1 185,998 cm-l), and DPH'- (1 176, 998 cm-') species may be attributed to benzene ring vibrations, but a mode mixing with polyenic C-C stretching seems to be necessary for them to display resonance enhancement. No reliable sources exist to help the assignment of the bands near 600 cm-'. It is seen from Table I that the frequencies assigned to mainly phenyl modes of the DPB cation radical (1598, 1190, and 993 cm-I) show little shift from the corresponding ground-state frequencies. On the other hand, the bands mostly associated with stretching vibrations in polyenic chain (1 585 and 1252 cm-I) deviate significantly from those in the So state. The observed differences seem to imply that the change of electronic state from So to DPB'+ is localized primarily on its polyenic part. In spite of the similarity in resonance-enhanced features of DPB" and DPB', the observed Raman frequencies of DPB' are down-shifted compared with those of DPB in the ground state (Table I). No significant difference exists between the Raman frequencies of DPB+ and those of the DPB in the Sostate. The Raman bands which we ascribed as being associated with vibrations in the polyenic chain are more strongly affected (shifts of between 50 and 70 cm-l) than those ascribed to phenyl modes (shifts of 10-20 cm-I). One can thus expect that the considerable part of the odd electron is localized on an antibonding MO of the polyenic chain of DPB-. The report on the ESR spectrum of the chemically produced DPB'- supports this deduction.22
Dudev et al. It is worth mentioning that some of the frequencies of DPB'
seems to be close to the respcctive values of DPB in the SIstate, implying, as in the ease of tSB,' similarity in the structure of the transient species. A reservation should be made, however, that the present mode correspondences are rather conventional and quite a difference exists between the spectral (intensity) patterns of the resonance CARS spectra of DPB' and that of DPB in SI.
Summary Resonance CARS spectra of the transient species, identified as the diphenylbutadiene (DPB) cation and anion radicals, were measured in the exciplex-formingsystems of DPB/dicyanobenzene (DCNB) and DPB/dimethylaniline (DMA) in p-dioxane. The assignment was confirmed by the measurement of the lifetime which are longer by 2 orders of magnitude than the fluorescence lifetime. The fluorescence spectra of the DPB/DCNB and DPB/DMA systems were observed to consist of a structureless and broad band in the longer wavelength region and of the SIfluorescence and the former band was assigned to the bands due to the CT exciplexes. The duration time of the exciplex fluorescence was not longer than the pulse width of 10 ns of the laser used. This confirms that the long-lived species detected by CARS are nonfluorescent geminated ion pairs or solvated free ions. The transient resonance CARS spectra of DPB in polar solvents having different electron-donating properties were observed under the similar resonance conditions as the cases of the exciplex systems, giving the cation radical in acetone and DMF solutions and the anion radicals in more electron-donating DMF and DMSO solvents. The lifetime of DPB'+ in acetone of about 500 ns was obtained. The observed Raman bands were compared with those of the ion radicals of tSB and DPH to derive a tentative assignment of the observed Raman bands. A trend was found that all frequencies of the anion radicals shift to lower frequency from those of the cation radicals. The considerable deviation of the ufrequency of DPW+ from those of tSB'- and DPH'- is not explainable at this stage, but this fact and the unexpected intensity behavior of the us, and vM modes suggested the importance of mode mixing. (22) Chippendale, J. C.; Gill, P.S.;Warhurst, E. Trans. Faraday Soc. 1967, 63, 1088.
