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J . Phys. Chem. 1987, 91, 4490-4495
state. One might be tempted to additively correct both states by the error in the calculated l'B, excitation energy, arguing that both states are a a* excitations, and that similar errors should be engendered in calculations on each. This could be an incorrect procedure, since the evidence on the cis isomer suggests that errors in calculated excitation energies are not uniform for all a a* excitations, as do results on the low-lying triplet and other Rydberg state^.^,^-* In comparison with the experimental estimate for the l'B,,, our best results place the 2'A, above this value by from 0.3 to 0.9 eV. The correct interpretation may not be so simple, however, if the picture of the llB, is altered by some of the factors discussed above. W e believe past estimates of the 2lA, state at approximately 7.0 eV6q7are in error. It is apparent that observation of the 2'A, state in the region in which we predict it to lie will be extremely difficult.
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-
V. Conclusions Results from a b initio calculations on the low-lying valencelike states for cis- and trans-1,3-butadiene have been presented. A variety of C I methods were employed which reveal a qualitative difference between the cis and trans isomers. Our results show the cis to have a lowest-lying 'B2 state a t approximately 5.5-5.6 eV, in goad agreement with experimental estimates for this state. The second valencelike singlet state contains significant doubly excited character relative to the ground state and is analogous to the 2'Ag state of the trans isomer, as well as similar states found in longer polyenes. The 21B2 state is predicted to lie approximately 1.4 eV higher in energy than the l'Bz and is found to be a completely Rydberg-like state.
In contrast, the l'B, state for the trans isomer was found to lie 0.3 eV above the experimental intensity maximum for this state, and to have significantly more Rydberg character than the corresponding state in the cis isomer. The oscillator strength for this transition is, nevertheless, in reasonable agreement with the experimental value. On the basis of our results, we estimate that the 2'A, state lies above the experimental intensity maximum for the l'B, state by from 0.3 to 0.9 eV (Le. from 6.2 to 6.8 eV). The results suggest that the qualitative nature of the lowest a a* state of 1,3-butadiene is highly sensitive to the geometry of the molecule. Further work is in progress to assess the sensitivity of the states to other geometric variations. Finally, the results presented here indicate that ab initio C I treatments on the a a* excited states of such polyenes which only treat a d correlation effects will be inadequate for the description of either excitation energies or of the spatial extents of the states. We conclude from this that the relative success of semiempirical treatments in the past must result from the approximate inclusion of m a correlation via the effective Hamiltonian employed.
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Acknowledgment. The authors gratefully acknowledge financial support of this research by the National Institute of Health, Grant No. 2 R01 GM34081-03. The calculations were performed at the Indiana University Chemistry Computer Facility, the establishment of which was in part made possible by grants from the National Science Foundation, Grant No. CHE-83-09446 and CHE-84-0585 1. Registry No. 1,3-Butadiene, 106-99-0.
Picosecond Laser Spectroscopy of 4-( 9-Anthry1)-N,N-dimethylaniline and Related Compounds Tadashi Okada,* Noboru Mataga,* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Wolfram Baumann, Institute of Physical Chemistry, University of Mainz, D 6500 Mainz, West Germany
and Aleksander Siemiarczuk The Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 587, Canada (Received: December 2, 1986; In Final Form: March 2, 1987)
Picosecond time-resolved absorption spectra of 4-(9-anthryl)-N,N-dimethylaniline(ADMA) and related methyl derivatives in the excited singlet state have been measured in various solvents. Analysis of the solvent dependenceof the transient absorption spectra indicates the existence of "multiple excited states" of ADMA with different degrees of charge transfer. Picosecond time-resolved transient absorption spectra in a viscous polar solvent, butanol, show a gradual change of the electronic structure of excited ADMA in the course of the reorientational relaxation of solvent molecules. The mechanisms of the solvent-induced change of the electronic structure of excited ADMA are discussed in relation to the TICT (twisted intramolecular charge transfer) model.
Introduction Mechanisms of intramolecular charge transfer (CT) in the excited state of compounds with electron donor and acceptor groups separated by a single bond have been studied extensively mainly by means of stationary as well as time-resolved fluorescence measurements. Especially, the photoinduced intramolecular C T of 4-(N,N-dimethylamino)benzonitrile(DMABN), which is a typical compound showing the dual fluorescence phenomena in polar solvents, has been investigated intensively.'-I0 (1) Lippert, E.; Liider, W.; Boos, H. In Aduances in Molecular Spectroscopy; Mangini, A,, Ed.; Pergamon: New York, 1962; Vol. 1 , p 443.
0022-3654/87/2091-4490$01.50/0
To explain the origin of the dual fluorescence spectra of DMABN, level inversion between 'Lband 'La states induced by (2) (a) Nakashima, N.; Inoue, H.; Mataga, N.; Yamanaka, C. Bull. Chem. SOC.Jpn. 1973,46, 2288. (b) Nakashima, N.; Mataga, N. Bull. Chem. SOC. Jpn. 1973, 46, 3016. (3) (a) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z . R. Chem. Phys. Lett. 1973, 19, 315. (b) Rotkiewicz, K.; Grabowski, 2.R.; Jasny, J. Chem. Phys. Lett. 1975,34, 55. (c) Rotkiewicz, K.; Grabowski, Z. R.; Krowczynski, A.; Kuhnle, W. J . Lumin. 1976, 12/13, 877. (d) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. NOUL'.J . Chim. 1979, 3, 443. (4) Chandross, E. A. In The Exciplex; Gordon, M., Ware, W R., Eds; Academic: New York, 1975; p 187.
0 1987 American Chemical Society
4-(9-Anthryl)-N,N-dimethylaniline and Related Compounds
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
the solvent reorientation was initially proposed by Lippert et al.' On the other hand, Grabowski et al. proposed that a twisted intramolecular charge-transfer (TICT) state which has a comformation with the phenyl and dimethylamino groups perpendicular to each other is formed in the excited state in polar ~olvents.~ More complex molecules which have two aromatic ?r-systems acting as donor and acceptor such as 4-(9-anthryl)-N,N-dimethylaniline (ADMA) and 9,9'-bianthryl are also reported as the examples showing similar p h e n ~ m e n a . ~ ~ - However, ' ~ J ~ ' ~ it is not quite clear whether or not this TICT interpretation is applicable to these kinds of molecules."J3 Our primary aim in this work is to elucidate this problem and make clear the mechanism of the solvent-induced change of the electronic structure of excited ADMA by means of the measurements of wide-band picosecond time-resolved transient absorption spectra. Picosecond time-resolved absorption spectroscopy provides direct information on the electronic structure in the excited state, of which the studies by means of time-resolved fluorescence spectroscopy are rather d i f f i c ~ l t . ~ ' ' ~For ' ~ example, the time-resolved fluorescence spectra of an ion-pair-like C T state in a polar solvent show a large red shift owing to the strong solvation of the ionpair-like state. Accordingly, the fluorescence rise and decay curves depend strongly upon the wavelength where the C T fluorescence is observed. In contrast to this result of fluorescence measurements, the time-resolved absorption spectra of the CT state, which is the superposition of the absorption bands due to the ions in the pair, remain in the same wavelength region and, moreover, keep their bandshape in the course of the solvation. Furthermore, the transient absorption spectra of the ion-pair-like C T state measured in various polar solvents of different polarity are quite insensitive to the solvent polarity compared with the C T fluorescence spectra. Some examples of such transient absorption spectra are actually demonstrated in the present work. In view of these facts, the time-resolved transient absorption spectral measurements should be the best method for the study of the dynamics of the change of the electronic structures of the photoinduced C T systems. ~
~~
~
( 5 ) (a) Struve, W. S.; Rentzepis, P. M. Chem. Phys. Lett. 1974, 29, 23. (b) Huppert, D.; Rand, S. D.; Rentzepis, P. M.; Barbara, P. F.; Struve, W. S.; Grabowski, Z. R. J . Chem. Phys. 1981, 75, 5714. (6) (a) Wang, Y.; Eisenthal, K. B. J . Chem. Phys. 1982, 77, 6076. (b) Hicks, J.; Vandersall, M.; Babarogic, Z.; Eisenthal, K. B. Chem. Phys. Lett. 1985, 116, 18. (7) (a) Rettig, W.; Bonacic-Koutecky,V. Chem. Phys. Lett. 1979.62, 115. (b) Rettig, W. J . Phys. Chem. 1982, 86, 1970. (c) Rettig, W.; Gleiter, R. J . Phys. Chem. 1985, 89, 4676. (8) (a) Visser, R. J.; Varma, C. A. G. 0. J . Chem. SOC.,Faraday Trans. 2 1980, 76, 453. (b) Visser, R. J.; Varma, C. A. G. 0.;Konijnenberg, J.; Bergwerf, P. J . Chem. Soc., Faraday Trans. 2 1983, 79, 347. (c) Visser, R. J.; Weisenborn, P. C. M.; Varma, C. A. G. 0.;DeHaas, M. P.; Warman, J. M. Chem. Phys. Lett. 1984, 104, 38. (9) (a) Rotkiewicz, K.; Rubaszewska, W. J . Lumin. 1982, 27, 221. (b) Bischof, H.; Baumann, W.; Detzer, N.; Rotkiewicz, K. Chem. Phys. Lett. 1985, 116, 180. (c) Baumann, W. 2.Naturforsch., A . 1981, 36A, 868. (10) (a) Heisel, F.; Miehe, J. A. Chem. Phys. Lett. 1983, 100, 183. (b) Heisel, F.; Miehe, J. A. Chem. Phys. 1985, 98, 233. (c) Heisel, F.; Miehe, J. A.; Martinho, J. M. G. Chem. Phys. 1985, 98, 243. (1 1) (a) Okada, T.; Fujita, T.; Kubota, M.; Masaki, S.;Mataga, N.; Ide, R.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1972,14,543. (b) Okada, T.; Fujita, T.; Mataga, N. 2.Phys. Chem. (Wiesbaden) 1976, 101, 75. (c) Okada, T.; Kawai, M.; Ikemachi, T.; Mataga, N.; Sakata, Y.; Misumi, S.; Shionoya, S. J. Phys. Chem. 1984, 88, 1976. (12) (a) Siemiarczuk, A.; Grabowski, Z. R.; Krowczynski, A,; Asher, M.; Ottolenghi, M. Chem. Phys. Lett. 1977, 51, 315. (b) Siemiarczuk, A,; Koput, J.; Pohorille, A. 2.Naturforsch., A 1982, 35A, 598. (13) (a) Baumann, W.; Petzke, F.;h e n , K. D. Z . Naturforsch., A 1979, 3 4 4 1070. (b) Baumann, W. Presented at the 12th International Conference on Presented at the 12th International Conference on Photochemistry, Tokyo, Japan, Aug 4-9, 1985. (c) Baumann, W. Presented at the Meeting of Photoinduced Electron Transfer and Related Phenomena, Kyoto, Japan, Aug 12-13, 1985. (d) Baumann, W.; Schwager, B.; Detzer, N.; Okada, T.; Mataga, N., to be submitted for publication. (14) Schneider, F.; Lippert, E. Ber. Bunsenges. Phys. Chem. 1968, 72, 1155; 1970, 74, 624. (15) Nakashima N.; Murakawa, M.; Mataga, N. Bull. Chem. SOC.Jpn. 1976, 49, 854. (16) (a) Rettig, W.; Zander, M. Chem. Phys. Lett. 1982, 87, 229. (b) Zander, M.; Rettig, W. Chem. Phys. Lett. 1984, 110, 602. (c) Rettig, W.; Chandross, E. A. J . A m . Chem. SOC.1985, 107, 5617. (17) Mataga, N. Pure Appl. Chem. 1984, 56, 1255 and references cited
therein.
N ''
@
HE%
4491
I EE
I
THF
I
I
DCE
I
ACN AC N
I
I
600 700 800 900 600 700 800 Figure 1. Solvent dependence of transient absorption spectra of ADMA (a) and MA1 (b). Solvents used are indicated in the figure by the
abbreviations given in the Experimental Section. Broken lines denote the reconstructed spectra. See text.
In the following, we demonstrate the dynamic behaviors in the excited state of ADMA and related compounds by means of the picosecond transient absorption measurements in various solvents. The compounds investigated here are ADMA, 4-(9-anthryl)2,6,N,N-tetramethylaniline(2,6-ATMA), 4-(9-anthryl)-3,5,N,Ntetramethylaniline (3,5-ATMA), 4-(9-anthryl)-2,3,5,6,N,Nhexamethylaniline (AHMA), l-methyl-5-(9-anthryI)indoline (MAI), and 9-anthryl-4-(N,N-dimethylamino)phenylmethane (Al), and their structures are given.
ADMA
2,6ATMA
3,5ATMA
'N/
AHMA
MAI
Ai
Experimental Section Time-resolved transient absorption spectra in the picosecond time region were measured w i t h a passively mode-locked Nd3+:YAG laser system." The third harmonic of a single picosecond pulse (355 nm, 25-ps duration) was used for excitation of the sample. The monitoring light generated by focusing the fundamental pulse into D,O is detected by a multichannel photodiode array (MCPD) through a spectrograph. The output of MCPD is connected to a microcomputer system which calculates the transient absorption spectra. The obtained spectra are stored (18) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, 1, 357.
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The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
&
b
I'
'
.,...., .
I
"
b HEX
w I
THF \
'
Okada et al.
1
THF
---_
ACN
l
.
I
600
,
l
700
1
800
,
I
b 600
IJ
900
600
700
800
900
Figure 2. Solvent dependence of transient absorption spectra of 2,6ATMA (a) and 3,5-ATMA (b). Broken lines denote the reconstructed spectra. See text.
in a magnetic disk for the analysis. The preparations of ADMA,I9 MAI,12a3,5-ATMA,12a2,6ATMA,20AHMA,*O and All9 are reported elsewhere, and the ground-state absorption spectra, fluorescence spectra, as well as lifetimes of ADMA,1'a,bJ2aMAI,12 3,5-ATMA,I2 2,6-ATMA,20 and AHMAZ0are given elsewhere. The concentratlons of the compounds were (0.8-5) X lo4 M which give an absorbance of 0.3-3 at the wavelength of the excitation laser pulse. No spectral change of the transient absorption was observed under the concentration change within this range. Spectrograde hexane (HEX), ethyl ether (EE), ethyl acetate (EA), tetrahydrofuran (THF), 1,2-dichloroethane (DCE), acetonitrile (ACN), and 1-butanol were used without further purification. Quartz cells with 1-cm optical path were used for the measurements. All solutions for the measurements were deoxygenated by freeze-pumpthaw cycles or irrigating with a nitrogen gas stream. Measurements were carried out at room temperature (23 "C). Absorption and fluorescence spectra were carefully measured before and after laser excitation, and it was confirmed for all solutions that no detectable photochemical decomposition occurred.
Results and Discussion Transient Absorption Spectra in Various Solutions. Absorption spectra of the excited state of ADMA and related compounds are shown in Figures 1-3. The abbreviated letters in the figures denote the solvents used. No change of the spectral bandshape has been observed in low-viscosity solvents examined here within the time resolution of the present apparatus. The reorientational relaxation of these solvents seems to reach their equilibrium state quickly. The time dependence of the transient spectrum in viscous solvents will be discussed in the next paragraph. The transient absorption spectrum of ADMA in hexane solution is characterized by two bands: one has a maximum at about 570 (19) Migita, M.; Okada, T.; Mataga, N.; Sakata, Y.; Misumi, S.; Nakashima, N.; Yoshihara, K. Bull. Chem. SOC.Jpn. 1981, 54, 3304. (20) Detzer, N.; Baumann, W.; Schwager, B.; Frohling, J.-C.; Brittinger, C., to be submitted for publication.
700
800
900
d
€00 700 800 900 Figure 3. Solvent dependence of transient absorption spectra of AHMA (a) and A, (b). Broken lines denote the reconstructed spectra. See text.
nm, and the other is a broad band with a maximum in the wavelength region longer than 900 nm. The band at 570 nm decreases its intensity and shifts to the red while the broad band shows a blue shift and decreases its intensity with increase of the solvent polarity. The spectra indicated by a broken line in each figure denote those reconstructed by the superposition of the spectra observed in hexane and in acetonitrile. This analysis of the obtained spectra has been made by means of the least-squares method in the region 540-900 nm. In the case of ADMA, it is impossible to reproduce the observed spectra in medium polar solvents in the wide wavelengths range as shown in Figure la, although we have made a similar analysis in the previous paper in a narrower wavelengths range (720-870 nm) where the spectra could be reproduced approximately by the superposition of the absorption spectrum in hexane and that in acetonitrile." In the present case, a large deviation between the observed spectrum and the reproduced one in medium polar solvents arose in the shorter wavelength region (530-700 nm) when the analysis was made so as to reproduce the spectrum in the longer wavelength region 750-900 nm. We have obtained quite similar results also in the case of MA1 and 2,6ATMA as shown in Figures 1b and 2a, respectively. On the other hand, in the case of 3,5-ATMA, AHMA, and A,, the solvent dependences of the transient spectra are more simple compared to those of the other three compounds described above. The conjugate *-systems of N,N-dimethylaniline and anthracene of A, are separated by a methylene chain, and therefore, we have used Al as a reference compound which shows transient absorption spectra for an almost completely charge-separated state. The transient absorption spectrum of A I in hexane can be assigned to the S, SI absorption of the anthracene moiety. The excited state of AHMA in hexane seems to be localized in the anthracene part since the absorption spectrum is the same as that of A, in hexane. In the case of 3,5-ATMA in hexane, however, the absorption band is considerably broadened and the absorption intensities at longer wavelengths are increased compared to those of Al in hexane. This result may suggest that there is some interaction between the N,N-dimethylaniline group and the an-
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4-(9-Anthryl)-N,N-dimethylaniline and Related Compounds
The Journal of Physical Chemistry, Vol. 91, No.17, 1987
4493
A
B
c l
l
W
u z 4
m
/
1500s
Y
L/
20dPh
Y
500ps
a
0 m
Y,
4
1
‘b
1 600
l , L
I
600
700
800
900
Figure 4. (A) Transient absorption spectra of PA in hexane, (B) PA in acetonitrile with added N,N-dimethylaniline (0.05 M), (C) ADMA in acetonitrile,(D) 2,6-ATMA in acetonitrile,and (E) MA1 in acetonitrile. Reconstructed spectra obtained by the superposition of the S, SI absorption spectrum of PA (A) and the absorption spectrum of the PA anion radical (B) are indicated broken lines.
-
thracene part in the excited state of 3,5-ATMA in hexane solution, even if the coplanarity between two groups is rather strongly hindered sterically. The spectra observed in EE, EA, and THF solutions are very similar to that observed in acetonitrile solution, which is essentially the same as that of anthracene anion radical. Consequently, a small difference between the observed spectrum and the reconstructed one in each solvent may be mainly due to the solvent effects upon the absorption spectrum of the ionic state. According to the results described above, it seems that the electronic interaction between a-systems of the phenyl ring and the anthryl part plays an important role in the stabilization of the S1 absorption spectrum of excited state of ADMA. The S, 9-phenylanthracene (PA) in hexane and the absorption spectrum of the anion radical of PA obtained by laser photolysis of a ternary solution of PA and N,N-dimethylaniline (0.05 M) in acetonitrile are shown in Figure 4 together with the transient absorption spectra of ADMA, 2,6-ATMA, and MAX in acetonitrile. Although the N,N-dimethylamino group of these three compounds is in a different conformation with respect to the phenyl ring with each other, the wavelengths of the band maxima of the transient absorption spectrum in acetonitrile solution are very similar to those of the PA anion radical as indicated in Figure 4. Therefore, in the case of the excited states of ADMA, 2,6-ATMA, and MA1 in acetonitrile solutions, an electron seems to be transferred from the amino group to the 9-phenylanthracene part, in contrast to the cases of 3,5-ATMA, AHMA, and A, where electron transfer in the excited state takes place from the (dimethy1amino)phenyl or dimethylamino group to the anthracene part in polar solvents. Of course, the density of the transferred electron in the phenyl-
-
700
800
900
Figure 5. Time-resolved transient absorption spectra of ADMA in 1butanol. Delay times from the exciting picosecond pulse are indicated in the figure.
anthracene group may be larger in the anthracene ring than in the phenyl ring. Nevertheless, there may be a considerable density in the phenyl ring, which is quite different from the case of 3,5-ATMA and AHMA where two rings are twisted by steric hindrance. It should be noted here that, if we regard ADMA, 2,6-ATMA, and MAX as composed of a (dimethy1amino)phenyl group (donor) and an anthryl group (acceptor), the intramolecular electron transfer in the excited state of these systems does not lead to the complete charge separation but only the partial chargetransfer state is realized and the degree of the charge transfer may change, depending upon the interadtion with polar solvents and the twisting between the phenyl and anthracene rings. Also, in medium polar solvents, the transient absorption spectra of ADMA, 2,6-ATMA, and MA1 cannot be reproduced by suSI absorption spectrum of PA and the perposition of the S, absorption spectrum of the PA anion radical; that is, the electronic structures of excited ADMA as well as 2,6-ATMA and MAX seem to change, depending on the nature of the solvent as discussed above. It is not possible to explain these results in terms of the simple two-state TICT mechanism. It seems to be necessary to invoke “multiple states” with different degrees of charge transfer and twisting angle, depending on the interaction with solvent. Time-Resolved Transient Absorption Spectra of ADMA and MAI in Butanol. Figures 5 and 6 give the transient absorption spectra of ADMA and MAX in 1-butanol at several delay times, respectively. The origin of the time scale was determined by measuring the building up at the maximum wavelength (468 nm) of the S, Sl absorption of pyrene in cyclohexane. As the generation of the blue part of the monitoring pulse is delayed relative to the red part,*] the correct delay time at about 650 nm is about 10 ps faster than the values indicated in the figures.
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(21) (a) Varma, C. A. G. 0.;Rentzepis, P. M. J . Chem. Phys. 1973, 58, 5237. (b) Masuhara, H.; Miyasaka, H.; Karen, A.; Uemiya, T.; Mataga, N.; Koishi, M.; Takeshima, A,; Tsuchiya, Y. Opr. Commun. 1983, 44, 426.
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The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
I1
Okada et ai.
1
I 600
I I I I I ] 700 800 900 Figure 6. Time-resolved transient absorption spectra of MA1 in l-butanol. Delay times from the exciting picosecond pulse are indicated in the figure. Nevertheless, the errors in the delay time arising from the wavelength-dependent arrival time of the monitoring pulse in the observed spectral region are estimated to be within a few picoseconds, which are rather short compared to the duration of the monitoring pulse. The transient absorption spectrum of ADMA detected at 3 3 ps after laser excitation is rather similar to that observed in hexane. The absorbance of the band at about 600 nm decreases, and the broad band at the longer wavelength region shifts to blue about 50 nm at first and decreases its absorbance gradually with increasing delay time. The transient spectra of ADMA at delay times of 70-200 ps cannot be reproduced by the superposition of spectra observed at 3 3 and 500 ps, and the difference between the observed and reconstructed spectra is analogous to the case of the solvent dependence of the transient spectra as shown in Figure l a . In the case of MAI in butanol solution, a broad and structureless spectrum was detected at 3 3 ps. The absorption intensities in the wavelength regions at 540-640 and 780-900 nm decrease, within the first several tens of picoseconds. After that, intensities around 700 nm decrease gradually and the peak at 650 nm grows. It is difficult again to reproduce the spectra observed at delay times of 70-200 ps by the superposition of those observed at 3 3 and 500 PS. It should be noted here that, as described in the next section, the transient absorption spectra of AHMA in 1-butanol can be SIabsorption band reproduced by the superposition of the S, of anthracene and the anthracene anion like band due to the intramolecular ion-pair state and that the rise time of the anthracene anion band owing to the formation of the ion-pair state by solvation is about 45 ps. This rise time appears to be considerably shorter than the longitudinal relaxation time of 1-butanol (ca. 70 ps) and the Debye relaxation time (ca. 620 ps). For the ADMA and MA1 in I-butanol, on the other hand, the time-resolved transient absorption spectra change their bandshapes
-
600
700
800
900
Figure 7. Time-resolved transient absorption spectra of AHMA in 1butanol. Delay times from the exciting picosecond pulse are indicated in the figure.
gradually over the delay times longer than 200 ps, indicating the gradual change of electronic structure accompanied by the conformational change between donor and acceptor groups in the course of the solvation process. On the basis of the above results of the measurements of the transient absorption spectra in various solvents as well as the time-dependent absorption spectra in viscous polar solvent, we can conclude that there exist many states with different degrees of charge transfer and, accordingly, with different degrees of solvation and twisting between phenyl and anthryl moieties in the excited state of ADMA, MAI, and 2,6-ATMA. It should be emphasized that the twisting in question here is not that between amino group and phenyl ring but that between the phenyl and anthryl rings. Contrary to this, in the case of 4-(N,N-dimethylamino)benzonitrile (DMABN), the twisting of the amino group seems to be essential for understanding its ion-pair-like electronic structure in the excited state in polar solvents. Actually, we have confirmed that its transient absorption spectrum in acetonitrile is very close to that of the benzonitrile anion radical.22 Level Inversion in the Excited A H M A in Butanol Solution. Figure 7 shows the time-resolved transient absorption spectra of AHMA in 1-butanol. The spectra observed at delay times of 0-100 ps can be reproduced approximately by the superposition of those in hexane and in acetonitrile, and at delay times longer than 100 ps, the spectra remain unchanged in the bandshape. It has been confirmed by the analysis of these spectra that the decay time of the spectral component similar to the absorption band in hexane solution and the rise time of the component similar to the absorption band in acetonitrile solution are the same and approximately 45 ps. According to the above results, the excited state of AHMA in butanol is localized in the anthracene part at first and then in(22) Okada, T.; Mataga, N.; Baumann, W. J . Phys. Chem. 1987,91, 760.
J. Phys. Chem. 1987, 91, 4495-4504 tramolecular electron transfer induced by the solvation process takes place. This result is consistent with the fact that dual fluorescence from the locally excited state of the anthracene part and the intramolecular charge-transfer state has been observed in polar solvents.’3c Recently, dipole moments of the excited equilibrium state (p,) as well as the Franck-Condon state ( p T c ) of the intramolecular charge-transfer systems have been determined by means of the electrooptical emission and absorption m e a ~ r e m e n t s . ’The ~ values of pe of AHMA in dioxane, fluorobenzene, 2-chlorobutane, and benzotrifluoride are obtained to be as large as 60 X C m,13w which seems to be in agreement with the present results obtained by the transient absorption measurements. Dynamic Aspects of Photoinduced Intramolecular ChargeTransfer Interaction in the Excited State of ADMA. The results obtained by the present studies have confirmed clearly the solvent-dependent change of the electronic structures of excited ADMA, Le., the existence of various excited states of different degrees of intramolecular CT depending upon the interaction with the polar solvent, which was suggested by the early experiments of the fluorescence and absorption measurements.llb” In the case of DMABN, however, the TICT mechanism seems to be favorable to explain the behaviors in the excited state. The reason for this difference between ADMA and DMABN with respect to the behaviors in the excited state in polar solvents may be considered as follows. Since an unpaired electron of aromatic ions is delocalized over all the aromatic rings in general, the solvation energy by the reorientation of solvent molecules as well as the Coulombic interaction energy between ions may be smaller for larger aromatic rings compared to the case of smaller aromatic rings or of localized ions of aliphatic groups. This effect may lead to the increase of the role of electronic delocalization interactions between positively and negatively charged parts in the case of ADMA compared to DMABN for the stabilization of the intramolecular C T state.
4495
It is well-known that 9,9’-bianthry1,I4-l6 in which the steric hindrance between two aromatic moieties is larger than in ADMA, shows the dual fluorescence phenomena. We have also measured the transient absorption spectra of 9,9’-bianthryl in various solv e n t ~ .Even ~ ~ in the case of 9,9’-bianthryl in acetonitrile solution, the transient absorption spectrum seems to be slightly different from the superposition of the anthracene anion and cation absorption bands. This result suggests that, in spite of the considerable steric hindrance between two chromophores in 9,9’-bianthryl which prevents taking on the planar configuration, we must invoke some delocalization interactions between them in the excited state, for the interpretation of the observed transient spectra even in strongly polar solvents. According to the above considerations, it is necessary to modify the TICT model to include the electronic delocalization interaction between donor cation and acceptor anion groups, depending on the interaction with polar solvents, in order to explain the observed results of the compounds with two large conjugate a-systems connected by a single bond. It should be noted here that the existence of the “multiple structure” in the intramolecular C T state of ADMA, depending on the interaction with solvent molecules, may be a special example of the original idea24that both the electronic and geometrical structures of an exciplex changes, depending on the solvent polarity. Acknowledgment. N.M. acknowledges the support by Grants-in-Aid from the Japanese Ministry of Education, Science, and Culture and financial support from the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie is gratefully acknowledged by W.B. (23)Okada, T.;Mataga, N., to be submitted for publication. (24)Mataga, N.; Okada, T.; Yamamoto, N. Chem. Phys. Lett. 1967,I, 119.
Role of Mode-Mode Energy Flow in a Model of the 1,5 Hydrogen Shift Isomerization of Malonaldehyde John S. Hutchinson Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77251 (Received: December 31, 1986; In Final Form: March 10, 1987)
The classical dynamics are presented and analyzed for a two-degree-of-freedommodel for the 1,5 hydrogen shift isomerization of P-hydroxyacrolein (the enol form of malonaldehyde) at energies above the reaction barrier. A simple double minimum potential energy surface is developed to describe the motion of the hydrogen in the locale of the fixed frame of the remainder of the molecule. Stable classical trajectories are observed corresponding to zero-order motions for both 0-H stretching and C-0-H bending. In this case, the 0-H stretching is very strongly anharmonic, but a nonlinear resonance analysis can still be successfullyapplied to describe the interaction of the stretching and bending modes. The isomerization reaction is restricted by a critical dependence both on the relative phases of the stretch and bend and on the phase of the mode-mode energy flow. Quantum eigenstates are calculated and analyzed for comparison to the classical analysis, and a strong correspondence is observed.
I. Introduction Intramolecular vibrational energy flow involving light atomheavy atom bonds in polyatomic molecules’ has been the focus of a great deal of experimenta12v3and t h e ~ r e t i c a l research ~,~ in recent years. In large part, interest in the dynamics of local H-X ~~~~
(1) Henry, B. R. Ace. Chem. Res. 1977,10, 207.
(2) Crim, F. F. Annu. Rea Phys. Chem. 1984,35, 651. (3) Reddy, K. V.;Heller, D. F.; Berry, M. J. J . Chem. Phys. 1982,76, 2814. (4)Halonen, L.;Child, M. S. Adu. Chem. Phys. 1984,57,1; Sage, M. L.; Jortner, J. Adu. Chem. Phys. 1981, 47,293. (5) Heller, D. F.; Mukamel, S. J . Chem. Phys. 1979,70,463.
0022-3654/87/2091-4495$01.50/0
bonds (where X is carbon, nitrogen, oxygen, silicon, etc.) is motivated by the observation of single-photon high-energy transitions to overtone states of these local mode^.^^^ It is thus possible to analyze the spectra (and even photo~hemistry*~~-’~) of these ( 6 ) Scherer, G . J.; Lehmann, K. K.; Klemperer, W. J . Chem. Phys. 1983, 78,2817.Fang, H.L.; Swofford, R. L. J . Chem. Phys. 1980,72,6382.Fang, H. L.; Meister, D. M.; Swofford, R. L. J . Phys. Chem. 1984,88, 405,410. Jasinski, J. M. Chem. Phys. Lett. 1984,109,462.Bernheim, R.A,; Lampe, F. W.; OKeefe, J. F.; Qualey, J. R. Chem. Phys. Lett. 1983,100,45. (7) Jasinski, J. M.; Frisoli, J. K.; Moore, C . B. Faraday Discuss. Chem. Soc. 1983,75,289.Chuang, M. C.; Bagott, J. E.; Chandler, D. W.; Farneth, W. E.; Zare, R. N. Faraday Discuss. Chem. Soc. 1983,75,301.
0 1987 American Chemical Society
