J . Phys. Chem. 1990, 94, 4147-4152 it appears that the configuration of chromophores leading to singlet excimers is a more relaxed structure than that leading to triplet excimers. This result is in agreement with earlier predictions based upon spectral shifts of prompt excimer fluorescence compared with delayed excimer fluorescence arising from triplet-triplet annihilation. The latter are red-shifted somewhat compared with the former, suggesting that the excimer configuration for triplets involves relatively larger orbital overlap than that for singlets. This trend is seen in both solid films of PVCA5,13and poly(2-vinylnaphthalene).'4,'5
Conclusions Below 40 K the phosphorescence spectra from solid films of PVCA are primarily nonexcimeric in nature. At temperatures (12) Klopffer, W. EPA Newsletter No. 29, March 1987, p 15. (13) Klopffer, W. Chem. Phys. 1981, 57, 75. (14) Kim, M.; Webber, S. E. Macromolecules 1980, 13, 1233. ( 1 5 ) Kim, N.; Webber, S. E. Macromolecules 1985, 18, 741.
4147
of 55 K and above, only excimeric phosphorescence can be observed. The temperature dependence of the phosphorescence lifetime in the interval from 15 to 50 K indicates that the activation energy for trapping of nonexcimeric triplets is 2.0 kJ/mol and that the shallow and deep excimer states lie at 0.5 and 10.1 kJ/mol, respectively, below the energy of the nonexcimeric species. The ground-state configuration corresponding to that of the shallow trap lies at an energy of 30.0 kJ/mol above the configuration of the relaxed polymer chain. The configuration of the deep trap lies at an even higher energy of 35.2 kJ/mol.
Acknowledgment. We are grateful to Dr. Jack Morgan for loaning the cryogenic equipment and to Dr. Walter Klopffer for helpful comments. Support of this work by the U S . Department of Energy under Grant DE-FG08-84ER45 107 is gratefully acknowledged. Registry No. PVCA, 25067-59-8
Femtosecond-Picosecond Laser Photolysis Studies,on the Dynamics of Excited Charge-Transfer Complexes in Solution. 1. Charge Separation Processes in the Course of the Relaxation from the Excited Franck-Condon State of 1,2,4,5-Tetracyanobenzene in Benzene and Methyl-Substituted Benzene Solutions Seishi Ojima, Hiroshi Miyasaka, and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: October 6, 1989)
Femtosecond and picosecond laser photolysis and time-resolved transient absorption spectral studies have been made to directly observe the charge separation (CS) process in the excited state of charge-transfer complexes in the case of 1,2,4,5-tetracyanobenzene (TCNB) in benzene and methyl-substituted benzene solutions. It has been demonstrated that, immediately after excitation, a slight change of absorption intensity accompanied by a slight sharpening of the band shape takes place with time constants of 2 ps, 1.5 ps, and 550 fs for benzene, toluene, and mesitylene solutions, respectively. This spectral change has been ascribed to the configurational rearrangement within the 1:l donor (D)-acceptor (A) complex from the Franck-Condon excited state with asymmetrical configuration toward a more symmetrical overlapped one, which slightly increases the extent of CS. It has been shown that the slight sharpening of the band shape is not due to the mere vibrational relaxation (cooling), since the observed result in the TCNB-toluene system has not been affected by the change of the wavelength of excitation pulse from 355 to 295 nm. This structural change within the 1:l complex, however, does not lead to the complete CS, but further interaction with donor and structural rearrangement including the formation of the 1:2 complex (A-.DZe)* are of crucial importance for it.
Introduction The mechanisms of the photoinduced charge separation (CS) leading to the formation of charge transfer (CT) and/or ion pair (IP) state and charge recombination (CR) of the produced C T or IP state as well as their dissociation into free ions are the subjects of lively investigations in the photochemical primary pr0cesses.I Those C S and CR processes have been examined mainly in the following cases. (a) The C S a t the encounter between the fluorescer and the electron-donating or -accepting quencher molecule leading to the formation of geminate IP, (As--.Ds+), where C R to the ground state, A+D, or triplet state, 3A+D or A+3D, and the dissociation into free ions are competing with each other.
(b) The intramolecular photoinduced C S and the C R of the produced intramolecular C T or IP state in the electron donor (D) and acceptor (A) system combined by the spacer or directly by the single bond. These systems are useful for the studies on the effects of the magnitude of the D-A electronic interaction and the solvent orientation dynamics on the electron-transfer rate. (c) The excitation of the ground-state C T complexes to the Franck-Condon (FC) state and its relaxation leading to the formation of the geminate IP which undergoes C R and dissociation. This is an extreme case of strong D-A interaction causing the photoinduced CS. The properties and absorption spectra in the ground state of C T complexes were investigated thoroughly for many kinds of systems,2 but properties in the excited singlet state were studied
( I ) See for example: (a) Mataga, N.; Ottolenghi, M. In Molecular Associafion; Foster, R., Ed.; Academic: New York, 1979; Vol. 2, p I . (b) Mataga, N . Pure Appl. Chem. 1984, 56, 1255. (c) Mataga, N. In Photochemical Energy Conuersion; Norris, J., Meisel, D., Eds.; Elsevier: Amsterdam, 1989; p 32.
(2) (a) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New York, 1969. (b) Foster, R. Organic Charge Transfer Complexes; Academic Press: London, 1969. ( c ) Mataga, N.; Kubota, T. Molecular Interaction and Electronic Spectra; Marcel Dekker: New York, 1970.
0022-3654/90/2094-4147$02.50/00 1990 American Chemical Society
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by means of C T fluorescence measurements only for a very limited number of s y s t e m ~because ~ ~ , ~ most of the C T complexes are very weakly fluorescent or nonfluorescent. Behaviors of a very limited number of C T complexes such as the T C N B (1,2,4,5-tetracyanobenzene)-toluene system in the excited state were investigated by means of fluorescence and phosphorescence spectroscopy as well as nanosecond laser photolysis and transient absorption spectral measurement^.^^^^^^ However, it is very important to obtain more detailed and direct information on the mechanisms of the photoinduced C S and the C R of the produced C T or I P state in such systems with strong D-A electronic interactions in addition to those systems with relatively weak D-A interactions as in the case of (a) and some in (b), for a more satisfactory understanding of the nature of photochemical electron-transfer phenomena. The previous luminescence measurements as well as nanosecond laser photolysis studies on the TCNB-toluene and also some MO theoretical studies on its electronic structures in the ground and excited ~ t a t e s indicated ~ ~ . ~ ~a~large structural change within the complex and also including the surrounding solvents in the course of the relaxation from the excited FC to the equilibrium charge-separated state. Although some preliminary picosecond laser photolysis studies were also made on the C S processes of this complex,'b the exact sequence of electronic and geometrical changes involving the donor-acceptor geometry and environmental configurations is still not very clear. For its elucidation. more detailed and direct time-resolved observations of the relaxation processes of various C T complexes in the excited state in various solvents of different polarities are necessary, which have become possible now with the quantitative and accurate femtosecond and picosecond laser photolysis and time-resolved absorption and emission spectra measurements. I n this and subsequent articles, we show the results of our femtosecond and picosecond laser photolysis and time-resolved spectral studies on the photoinduced CS processes of T C N B complexes with aromatic hydrocarbons in various solvents and discuss the mechanisms underlying those ultrafast C S processes. Experimental Section A microcomputer-controlled femtosecond laser photolysis system was used for the measurements of time-resolved absorption spectra in regions from 100 fs to several tens of picoseconds. In the present study, we have used mainly the pyridine-1 dye laser (710 nm) for generating the 100-fs pulse but also employed the rhodamine 6G dye laser (590 nm) in some cases. The output of a C W mode-locked Nd3+:YAG laser operated at 82 M H z was compressed by a fiber-grating system, and the second harmonic generation (SHG) of the compressed pulse synchronously pumped a pyridine-] dye laser (710 nm). The output pulse of the pyridine-I dye laser (300-fs fwhm, 2 nJ) was amplified to 0.4 mJ/pulse by a three-stage pyridine-1 dye amplifier pumped by the S H G of Q-switched Nd3+:YAG laser operating at I O Hz. The amplified pulse with typically 500-fs fwhm was frequency-doubled (355 nm), which was used for exciting the sample solution exclusively at the C T absorption band. The rest of the fundamental pulse was used for generating the white light probe pulse by focusing into DzO. Two sets of multichannel photodiode detectors were used to observe the transient absorption spectra. When it was necessary, the transient absorption band shapes were corrected for the wavelength-dependent arrival times of the white light probe pulse. The pulse generation procedures and the method of spectral measurements in the case of rhodamine 6G dye laser were essentially the same as those employed in the case of the pyridine-I dye laser. The amplified pulse width of (3) (a) Czekalla, J . ; Meyer, K. 0.Z.Phys. Chem. (Munich) 1961,27, 185. (b) Mataga, N.; Murata, Y . J . Am. Chem. SOC.1969, 91, 3144. (c) Kobayashi, T.; Yoshihara, K.; Nagakura, S. Bull. Chem. SOC.Jpn. 1971.44, 2603. (d) Egawa. K . ; Nakashima. N.; Mataga, N.; Yamanaka. C. Ibid. 1971. 44, 3287. (4) Masuhara. H.: Mataga, N. 2. Phys. Chem. (Munich) 1972,80, 113. ( 5 ) Nagakura, S. In Excited Stare; Lim. E. C . , Ed.; Academic: New York, 1 q - 5 : V o l 2 . r' > ? Z
Ojima et al. t
(a)
T
( b )
Figure 1. Possible geometrical structures of the 1:1 TCNB complex: (a) FC excited state; (b) related excited state with symmetrical overlapped configuration
the rhodamine dye laser was, however, typically 950 fs, and the S H G (295 nm) pulse was used for exciting the C T complex. For the measurement of the time-resolved transient absorption spectra in the IO-ps to nanosecond region, a microcomputercontrolled picosecond Nd3+:YAG laser photolysis system was used. The third harmonic generation (THG) pulse (355 nm) with 22-ps fwhm and 0.5-2-mJ output power was used for exciting the sample solution at the C T absorption band. The dynamics of the C T fluorescence in the picosecond to nanosecond time regions was examined by exciting the solution with the T H G pulse (355 nm) of a picosecond Nd3+:YAG laser and detecting the fluorescence by the picosecond streak camera system (Hamamatsu Photonics C2909) with CCD camera. For the fluorescence measurements in the time regions longer than nanoseconds, a microchannel plate (MCP) photomultiplier (Hamamatsu Photonics R1194UX)storage oscilloscope combination was used. Wako G R grade T C N B was recrystallized from ethanol, and its purity was checked by measuring the absorption spectra, C T fluorescence spectra, and decay time. Spectrograde benzene (Bz) and toluene (Tol) and m-xylene (m-Xyl) for the liquid chromatography were passed through a column of silica gel before use. Tokyo Kasei G R grade mesitylene (Mes) was passed through a column of silica gel. Tokyo Kasei G R grade hexamethylbenzene (HMB) was recrystallized twice from ethanol. Durene (Du) was chromatographed on alumina and silica gel and recrystallized from ethanol. Sample solutions for the measurements were deoxygenated by freeze-pump-thaw cycles. In the preparation of the solutions of TCNB in benzene, toluene, m-xylene, and mesitylene, solvents were carefully dried by contact with molecular sieves. The samples of T C N B complexes contained in a poly(methy1 methacrylate) (PMMA) matrix were prepared by polymerization of methyl methacrylate (MMA) solutions containing the T C N B complexes. Results and Discussion A . TCNB in Toluene Solution. As described in the Introduction, previous fluorescence measurements and nanosecond laser photolysis studies on TCNB in toluene solution and also some M O theoretical studies on its electronic structures in the ground and excited states indicated a large change in the geometrical and electronic structures of the C T complex and the rearrangement of the surrounding solvent in the course of the relaxation from the excited FC to the equilibrium charge-separated ~ t a t e . ~ ~ . ~ . ~ Early semiempirical M O calculations on the FC excited state of a complex such as TCNB-benzene and TCNB-toluene predicted an almost equal contribution of the electron-transfer (ET) ('(A-D')) and locally excited (LE) ('A*.D) structures to the wave function of the excited complex. It is assumed that the center of one benzene ring is considerably shifted against the center of another ring in the complex in the FC excited as well as ground state, since the same MO calculation indicated that it was the stable configuration in the ground state and also such a configuration was proved for TCNB-HMB by X-ray analysis in the crystalline state. The MO calculations also predicted a much larger degree of CT character in the configurations where two benzene rings are overlapped symmetrically or their mutual distance is large.3c.4 Our recent femtosecond and picosecond laser photolysis and time-rewlved absorption spectra studies on T C N B in toluene
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4149
Dynamics of Excited Charge-Transfer Complexes
,
m 0
4 00
500
Inm
20
10
TiMElps
,
I n s i div. Figure 3. Fluorescence decay curves of TCNB in toluene solution observed by a streak camera at several different wavelength regions, by exciting with the picosecond 355-nm pulse. [TCNB] = 6 X lo-’ M.
Figure 2. (a) Time-resolved transient absorption spectra of TCNBtoluene solution measured with the femtosecond laser photolysis method by exciting at 295 nm. Spectra are corrected for the chirping of the monitoring white light pulse. [TCNB] = 6 X IO-’ M. (b, c) Time profiles of absorbance at 465 nm.
solution have demonstrated the spectral change which seems to correspond to the intracomplex structural change toward the symmetrical overlapped one in the course of the relaxation from the FC excited state as indicated in Figure 1.6,7 That is, the broad absorption band of the S, state of the complex observed immediately after excitation with a 355-nm, 100-fs pulse shows a slight decrease of absorbance with decay time of ca. 1.5 ps accompanied by a slight sharpening of the band shape, indicating an increase of the C T degree due to the configurational rearrangement from the asymmetrical one toward a more symmetrical overlapped one accompanied by a slight nonradiative deactivation in the course of the structural change. This structural change, however, does not lead to the complete CS, but further interaction with toluene and structural rearrangement involving the formation of the 1:2 complex ‘(TCNB-.To12+)* is of crucial importance for it ~(TcNB-*.T~I+*) -!L+ ~(TcNB-*’.T~I+~’)* 78
= 1.5 ps
structural change within 1:1 complex
r,
= 30 ps
7
‘(TCNB-6”*Tol+6”)*
‘(TCNB-*Tol,+)* (1) where the time dependences of the absorbance change accompanied by the structural rearrangements of the complex can be reproduced approximately by single-exponential curves and, accordingly, the inverse of the decay time ( T ~ and ) rise time ( T ~ ) represents the rate constant of the transformation. The fact that the configurational rearrangement of the complex immediately after the excitation (with relaxation time of 1.5 ps) does not lead to the complete C S indicates that this structural change is not the formation of the completely overlapped symmetrical configuration as predicted by the previous MO calculations but only an incomplete shift from asymmetric toward a more symmetric configuration or that the previous MO predictions are not exact. It might also be possible that the slight change of the band shape and the absorbance immediately after excitation is due to the vibrational relaxation (cooling). However, if it is a mere vibrational relaxation, the absorbance observed a t the peak position should increase, contrary to the actual observation. We have examined the time-resolved absorption spectra of the same system by means of the rhodamine 6G femtosecond laser photolysis system exciting the sample at 295 nm by the pulse with 950-fs fwhm. As indicated in Figure 2, the time-dependent spectral change from the broad absorption band immediately after excitation to the one ( 6 ) Mataga, N.; Miyasaka, H.; Asahi, T.; Ojima, S.;Okada, T. UItrufust Phenomena VI; Springer-Verlag: Berlin, 1988; p 51 1. (7) Miyasaka, H.; Ojima, S.; Mataga, N. J . Phys. Chem. 1989, 93, 3380.
I
-200
0
200 Time I ps
400
Figure 4. Fluorescence decay curve of TCNB in toluene solution measured with a streak camera by detecting the fluorescence emission including all wavelength regions indicated in Figure 3.
with slightly more sharp band shape accompanied by a slight decrease of the absorbance at the peak position (465 nm) has been observed, and the time constant of this change has been estimated to be 1.4 f 0.2 ps, which is practically the same as that obtained by excitation with the 355-nm pulse. Therefore, this slight change of the band shape and absorbance immediately after excitation is not affected by the larger excess vibrational energy in the case of 295-nm excitation compared with 355-nm excitation. This result supports the above argument that the observed spectral change is not due to the mere vibrational relaxation (cooling). Moreover, this result indicates that the excess vibrational energy is not effectively used for the change of the intracomplex configuration from the asymmetric one to the more symmetrical overlapping one. On the other hand, the band shape seems to depend rather strongly upon the degree of the CS. The TCNB- band of the 1:2 complex in the SI state is very similar to that of the free T C N B anion radical. The large change of the electronic structure of the complex in the S, state, as suggested by the previous that the fluorescence lifetime of the TCNB in toluene solution at room temperature was more than 100 ns which was much longer than the radiative lifetime (-50 ns) calculated from the intensity of the C T absorption band, should be ascribed to the last step in eq 1, the almost completely charge-separated 1:2 complex formation from the precursor state ‘(TCNB-6”.T~l+6”)*.This can be demonstrated more directly by the measurements of the fluorescence decay curves by the picosecond streak camera and also by the MCP photomultiplierstorage oscilloscope combination as follows. The results of measurement by streak camera at several wavelength regions are shown in Figure 3. We can recognize the existence of rapid and slow decay components and that the rapid decay component is more dominant in the shorter wavelength region. The decay time of the slow component has been determined to be 120 ns by detection with the M C P photomultiplier, which agrees with the result of the previous mea~urement.~”In Figure 4, we show the fluorescence decay curve measured under
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
c
Ojima et al.
I
w ,-g
h
.->
? v)
o
10
20
0
0 Y
w
9a
m
Qa
m
Figure 5 . ( a ) Time-resolved transient absorption spectra of TCNB in benzene solution measured with the femtosecond laser photolysis method by exciting at 295 nm. Spectra are corrected for the chirping of the
monitoring white light pulse [TCNB] of absorbance a t 46.5 nm
ii:
M. (b, c) Time profiles
higher time resolution by detecting the fluorescence emissions including all wavelength regions indicated in Figure 3. The decay time of the rapid component has been estimated by subtracting the slow component from the observed decay curve to be 30 ps, which agrees with the rise time of the 1:2 complex formation obtained by the transient absorption spectral measurement. The result in Figure 4 shows clearly that the radiative rate constant of the complex decreases remarkably by this structural change, reflecting the change of electronic structure from the precursor state of the 1:l complex to the more extensively charge-separated state of the 1 :2 complex. B. TCh‘B in Benzene Solution. Since the CT absorption band of the TCNB-benzene complex is blue-shifted compared with that of the TCNB-toluene complex, it was difficult to obtain a sufficient concentration of the S I state to measure its absorption spectra by exciting the complex with the 355-nm pulse of a femtosecond laser photolysis system. Therefore, we have employed the 295-nm pulse of a rhodamine 6G dye laser with 950-fs fwhm for exciting the complex. The time-resolved transient absorption spectra are indicated in Figure 5a. At 0.4 ps, a broad band with a maximum around 440 nm can be observed. With increase of the delay time, the absorption band with peak at 465 nm arises. We can observe such a band already at 4 ps, although it is still much broader compared with the absorption band of the free TCNB anion. In Figure 5b,c, time profiles of the absorbance at 465 nm are indicated. A slight decrease of absorbance with decay time of ca. 2.0 ps, accompanied by a slight sharpening of the band shape, has been observed immediately after excitation. After this absorbance decay, the sharp T C N B anion band grows with a rise time of ca. 20 ps. We have also performed transient absorption spectral measurements in the IO-ps to nanosecond region and have observed a broad absorption due to the benzene dimer cation in the wavelength region longer than 600 nm. The rise time of this broad absorption has been confirmed to be the same as that of the sharp T C N B anion band. These results are very similar to those observed in the case of T C N B in toluene solution. Therefore, the mechanism responsible for the observed absorption spectral change may be given as follows. which is very similar to eq 1. ‘(TCNB-*.Benz+*) id
= 2 0 ps
hv
i(TCNB-6’-Benz+6’)*
structural change within 1 . 1 complex
Tl
= 20 ps
1(TCNB-6”.Benz+6”)*------+ Benz
I(TCNB--Benz,+) * ( 2 ) The T~ value of T C N B in benzene solution, which seems to represent the relaxation time of the intracomplex configurational rearrangement from the asymmetrical one toward a more symmetrical overlapped one, is slightly longer than that of T C N B in
LOO
500
600 I n m
TIME~PS
Figure 6 . (a) Time-resolved transient absorption spectra of TCNB in mesitylene solution measured with the femtosecond laser photolysis method by exciting at 3 5 5 nm. Spectra are corrected for the chirping of the monitoring white light pulse. [TCNB] = M. (b-d) Time profiles of absorbance at several wavelengths.
toluene solution. The C T character of the FC excited state of the TCNB-benzene complex should be a little smaller than that of the TCNB-toluene complex due to the higher ionization potential of benzene (by 0.52 eV), and the extent of the structural change necessary to increase the C T character in the excited state may be larger in the former complex than the latter. We have also examined TCNB in m-xylene solution. Although we have not performed femtosecond laser photolysis studies on this system, general features of the results of picosecond laser photolysis and time-resolved transient absorption spectral measurements are quite similar to those of T C N B in benzene and in toluene solutions. Therefore, the mechanism of photoinduced C S in the course of the relaxation from FC excited state to the formation of the 1 :2 complex of T C N B in m-xylene is very similar to those given in eqs I and 2, and the time constant of the 1.2 complex formation has been obtained to be 23 ps. C. TCA’B in Mesitylene Solution. In order to examine the relaxation process to the equilibrium excited state of stronger C T complex, we have performed femtosecond laser photolysis studies on the T C K B in mesitylene solution. Figure 6a shows time-resolved transient absorption spectra of the T C N B in mesitylene solution. The broad absorption band observed immediately after excitation changes to a more sharp one with a peak at 465 nm with increase of the delay time. Immediately after excitation, we have observed a slight decrease of absorbance in the case of the T C N B in benzene and T C N B in toluene solutions (with decay times of 2.0 and 1.5 ps, respectively) at 465 nm accompanied by a slight sharpening of the band shape. In the case of the T C N B in mesitylene solution, such absorbance decay cannot be observed at 465 nm. Nevertheless, we can observe rapid absorbance decay at shorter wavelength regions as indicated in Figure 6b-d, which means that a slight sharpening of the band shape immediately after excitation occurs mainly in the wavelength region shorter than 465 nm. The decay time obtained in the wavelength region of 410-435 nm was 550 f 50 fs, which is shorter than those of T C N B in benzene and T C N B in toluene solutions. The C T characters of the TCNBmesitylene complex in the FC excited as well as ground state should be larger than those of the complexes with benzene and toluene, which seems to shorten this decay time due to the intracomplex configurational rearrangement, although the details of its mechanism are not very clear. This spectral change immediately after excitation is followed by the increase of the sharp TCNB anion band as shown in Figure 7. The rise of the T C N B anion band is accompanied by the rise of the broad absorption in the wavelength region longer than 650 nm which can be ascribed to the formation of the almost completely charge-separated ‘(TCNB-.Mes2+)* state. Therefore, the
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4151
Dynamics of Excited Charge-Transfer Complexes
[E -4
OPS
200 ps
t
3ns
I u
t
1
400
500
400
600
700
5ns
500 600 700 Wavelength /nm
Figure 8. Time-resolved transient absorption spectra of the TCNBtoluene complex in PMMA matrix.
- 26ps -
1-
.
-
200ps
3ns
600 700 800 900 Figure 7. Time-resolved transient absorption spectra of TCNB in mesitylene solution measured with the picosecond laser photolysis method by exciting a t 355 nm: (a) 400-750-nm region; (b) 550-950-11111 region.
reaction scheme of the C S processes from the excited FC state to the equilibrium IP state of the TCNB-mesitylene system seems to be similar to other T C N B in liquid donor systems. The rise time of the TCNB anion band at the long delay time was estimated to be ca. 40 ps, which is longer than the corresponding values of TCNB-benzene and -toluene systems. This result might be partly due to the slower rearrangement motions for the 1:2 IP state formation, owing to the higher viscosity of mesitylene liquid compared to that of other liquid donors. From the above results, the IP state formation process of T C N B in mesitylene solution may be summarized as follows. ‘(TCN B-*.Mes+*) rd
ps observed at room temperature immediately after the structural rearrangement within the 1:l complex. This result means that the large structural change of the 1:2 complex formation which is necessary for the almost completely charge-separated IP state formation is difficult or considerably slowed down at low temperatures. Similar results may be expected also in the case of the T C N B complex in polymer matrix, not only at low temperature but also a t room temperature. According to the result of the previous nanosecond laser photolysis studies of the TCNB-toluene complex in P M M A matrix a t room temperature as well as a t low temperatures,8 the Sn SI absorption band with a peak a t the same wavelength as that of the T C N B anion was observed, although the absorption due to the toluene dimer cation was not clear in the case of those measurements. This result was interpreted as due to the IP state formation due to the geometrical rearrangement from asymmetrical to the overlapped symmetrical structure within the 1:l complex in the course of the relaxation from the FC ~ t a t e . ~ , ~ However, in view of the results of picosecond-femtosecond laser photolysis studies on T C N B in toluene solution at room temperature as well as at low temperatures,’ the above conclusion based on the previous nanosecond laser photolysis studies does not seem to be correct. Therefore, we have conducted the picosecond laser photolysis studies on the TCNB-toluene complex in PMMA matrix. The observed spectra are much broader compared with that of the ‘(TCNB-.ToI,+)* state. The time-resolved transient absorption spectra are indicated in Figure 8. Although the spectra have a peak at 465 nm, they also show broad shoulders and long tails in the shorter and longer wavelength sides of the peak position, respectively, and the spectral band shape does not change with increase of the delay time to more than 5 ns. The long tail does not show any such increase of intensity in the wavelength region longer than 700 nm as observed for the toluene dimer cation. The above results clearly show that the excited TCNB-toluene complex in P M M A matrix shows only incomplete charge separation, which can be ascribed to the fact that the 1:2 complex formation is difficult in P M M A matrix. However, it should be noted here that the shape of the transient absorption band of the TCNB-toluene complex in P M M A matrix at room temperature
-
= 550 fs
hu
1(TCNB-6‘-Mes+6‘) * + l(TCNB-d”.Mes+6”)*
structural change within 1:1 complex
T
-4Ops Mes
‘(TCNB-.Mes2+)* (3)
D. Behaoiors of TCNB Complexes at Low Temperatures and in Polymer Matrix. We have already reported’ that the transient absorption spectrum of TCNB in toluene solution observed at 170 K and at 100-ps delay time is very similar to the spectrum a t 4.5
(8) (a) Masuhara, H.; Tsujino, N.; Mataga, N. Chem. Phys. Letr. 1972, 1 2 , 4 8 1 . (b) Masuhara, H.; Tsujino, N.; Mataga, N. Bull. Chem. SOC.Jpn. 1973, 46, 1088. (c) Masuhara, H.; Mataga, N. Chem. Phys. Leu. 1973, 22, 305.
J . Phys. Chem. 1990, 94, 4152-4155
4152
is slightly sharper than that of the T C N B in toluene solution immediately after the intracomplex geometrical rearrangement (at 4-ps delay time in Figure la). This might be ascribed to the fact that, in addition to the geometrical rearrangement within the 1 : 1 complex, the polar groups of P M M A contribute slightly to the increase of the charge separation. Since an accurate measurement of the transient absorption band shape was difficult in the case of the previous nanosecond laser photolysis the somewhat sharp band shape at 465 nm in P M M A matrix leads to the above conclusion of almost complete charge separation within the 1 : l complex caused by the intracomplex geometrical rearrangement. We have also examined TCNB-benzene, -mesitylene, Aurene, and -hexamethylbenzene complexes in P M M A matrix. In all cases, we have observed a broad transient absorption band with peak at 465 nm and with shoulders and tails. The contribution of the broad tail to the observed transient absorption spectrum decreases with decrease of the ionization potential of the donor. This result suggests that the absorption band shapes with broad tails originate from the incomplete charge separation in the S I state of the I : I complex in the polymer matrix and that the extent
of the charge separation within the 1:l complex increases with decrease of the donor ionization potential. E . Concluding Remarks. It is well-known that the photoinduced CS is greatly enhanced by the interaction of the donoracceptor system with polar solvent molecules.' In the above sections, we have demonstrated by means of femtosecond and picosecond laser photolysis and time-resolved transient spectral measurements on the T C N B in benzene and methyl-substituted benzene solutions that the configurational rearrangements including the geometrical change within the 1:l complex as well as the 1 :2 complex formation are necessary for the IP state formation in the course of the relaxation from the excited FC state. Whether such configurational changes necessary for the C S of these strongly interacting D-A systems in the nonpolar or only slightly polar environment are still necessary for the CS in considerably or strongly polar solvent, is an important and interesting problem and will be examined in subsequent papers.
Acknowledgment. N.M. acknowledges the support by a Grant-in-Aid (No. 62065006) from the Japanese Ministry of Education, Science and Culture.
Theory of Electron-Transfer Rates across Liquid-Liquid Interfaces R. A . Marcus Noyes Laboratory of Chemical Physics,+ California Institute of Technology, Pasadena, California 91 I25 (Receiued: October 6, 1989; In Final Form: December 21, 1989)
The theory developed in a previous paper for the geometry of the encounter complex, the reorganization energy, and the electron-transfer rate constant at a liquid-liquid interface is applied to existing data on the rate constant. To treat cyclic voltammetric (CV) studies of electron transfer across the interface, the nature of the encounters is examined and a bimolecular-type rate treatment is used. When one redox pair is in large excess, it has been pointed out that a single-phase CV analysis for diffusion/reaction can be utilized. In the present paper we avoid in this analysis the assumption that the second ("concentrated") phase is metallike. The experimental result deduced in this way for the true exchange current electron-transfer rate constant at the interface is compared with that estimated from the present theory of the rate constant, using metal-liquid electrochemical exchange rate constants. The type of agreement found is encouraging, considering the various approximations involved, and further experimental studies and tests would be of interest.
1. Introduction
Recently, we described a theory of electron transfer from one reactant to another, across an interface between two immiscible liquids.' The treatment focused on two aspects: the reorganization of the two solvent media prior to and after the electron transfer, and the geometrical aspects of a "precursor state" formed by the two reactants, each confined to its own liquid phase. T o obtain some idea of the effect of partial reactant penetration of the other phase on the geometrical probability of forming a precursor state, an estimate of the latter was made also for the case where such penetration occurs. The results of ref 1 are extended and applied here to recent data on an electron-transfer reaction rate constant.2 2. Theory
We consider first the expression obtained' for the reorganization energy A,, for electron transfer across a liquid-liquid interface. From dielectric continuum theory A, was found to be given by
I n eq 2.1 Di and DPP refer to the static and optical dielectric constants of phase i (i = 1, 2 ) , Ae is the charge transferred, di is the perpendicular distance from the center of reactant i to the interfacial boundary, R is the center-to-center separation distance between the two reactants, and the aiare the radii of the two reactants. Equation 2.1 differs in some respects from the one R. A. J . Phys. Chem. 1990, 94, 1050. (2) Geblewicz, G.;Schiffrin, D. J. J Elecfroanal. Chem. 1988, 244, 27. ( 1 ) Marcus,
'Contribution Yo 8037.
0022-3654/90/2094-4152$02.50/0
0 1990 American Chemical Society
