J. Phys. Chem. 1990, 94, 4143-4147
4143
Binding Energies of Triplet Excimers in Poly(N-vinylcarbazole) Solid Films from Laser-Based Kinetic Spectroscopy between 15 and 55 K Richard D. Burkhart* and Dilip K. Chakrabortyt Department of Chemistry, University of Nevada-Reno,
Reno, Nevada 89557 (Received: September 29, 1989)
Time-resolved delayed luminescence spectra from solid films of poly(Nviny1carbazole) have been recorded in the temperature interval between 15 and 55 K. Below 40 K the phosphorescence spectra are primarily nonexcimeric in character but become totally excimeric at 55 K. Rate constants for the phosphorescence decay in this same temperature interval yield linear Arrhenius plots but with a discontinuous change in slope at 40 K. The activation energy above 40 K is 2.0 kJ/mol and is associated with the trapping of nonexcimeric triplets. When combined with previous determinations of the activation energies for detrapping of triplet excimers, binding energies of 2.5 and 12.1 kJ/mol are found for the shallow and deep excimers relative to the energy of the limited free rotor, the energy of which corresponds to the top of the activation barrier. The molecular configuration corresponding to these triplet excimer states is dissociative on the ground-state potential energy surface. The dissociation energies are 30.2 kJ/mol for the shallow trap and 35.4 kJ/mol for the deep trap. These energies are larger than the corresponding ones for singlet excimers in agreement with earlier predictions.
Introduction It has been well-documented in many laboratories over the past decade that solid films of poly(N-vinylcarbazole) (PVCA) at 77 K emit a phosphorescence signal which is nonstructured and red-shifted compared with frozen glassy solution of this polymer also at 77 K.'v2 Many workers in this field had attributed these phosphorescence spectra to triplet excimers, but reservations were often expressed because impurity emission is difficult to remove from polymeric materia1s.j A variety of different experiments were carried out in a number of different laboratories in an attempt to provide a positive identification of the source of this red-shifted phosphore~cence.~-~ I n many cases, it was concluded that the emission was a real manifestation of the photophysical activity of the polymer as opposed to impurity luminescence. Recent elegant experiments have been conducted by Ito et al.' on copolymers of methyl methacrylate and carbazolylethyl methacrylate showing that, in these copolymers, the relative intensity of the observed excimer-like phosphrescence is related to the density of chromophore units in the polymer coil. Furthermore, in those copolymers from which both excimeric and nonexcimeric phosphorescence could be observed, the intensity of the excimeric component increased with an increase in temperature. In addition, the relative intensity of excimeric phosphorescence from methyl methacrylate films doped with N-phenylcarbazole increased as the concentration of the dopant was increased. Every photophysical test that was applied to this system yielded delayed emission signals whose behavior indicated that the source of the emission was excimeric in nature rather than due to the presence of impurities. Since the existence of triplet excimeric species in these solid polymer films now seems to be well-established, it is important to undertake a characterization of their physical and chemical properties. Two properties of special interest are their structure and their binding energy. In the present work we address the second of these problems in an examination of the triplet photophysics of PVCA solid films at low temperatures. The focus of attention is primarily upon phosphorescence properties associated with triplet excimers. Unlike earlier low-temperature studies of this polymer which were conducted at 77 K or higher,s,9in the present work a lower temperature regime is being investigated between I O and 77 K. There are several motivations for extending the temperature range of these investigations to lower values, and principal among these was a concern about the nature of temperature-dependent shifts of the phosphorescenceemission. Earlier s t u d i e ~ had ~ ? ~shown that the center of gravity of the phosphorescence band shifts to longer wavelengths as the temperature is raised from 77 K to ambient temperature. This trend is evidently 'Present address: Institute of Polymer Science, University of Akron, Akron. OH 44325.
0022-3654/90/2094-4143$02.50/0
due to selective detrapping of higher energy excimeric species, thus producing a higher population of lower energy species as the temperature is raised. It was therefore of interest to determine whether or not a shift to shorter wavelengths might occur as the temperature is decreased below 77 K, possibly leading to a totally nonexcimeric phosphorescence at sufficiently low temperatures. It has frequently been observed that detailed structural features become more evident when emission spectra are recorded at lower temperatures, and this question is particularly important in the case of PVCA films where the potential influence of excimer emission has to be considered. In addition, it was recently noted that the temperature dependence of the delayed fluorescence (df) intensity of solid films of PVCA includes a maximum in the df intensity near 40 K.Io This is in addition to the one discovered earlier9 near 200 K. From this result it was concluded that the rate of exciton trapping at excimer-forming sites increases with an increase in temperature but then declines at higher temperatures due to a reversibility in the trapping-detrapping process which increasingly favors the detrapped triplet exciton as the temperature rises. It may be expected that if the rate of exciton trapping does, in fact, increase in the temperature range from 10 to 40 K, then one should find additional evidence for the existence of these processes by an examination of the kinetics of phosphorescence decay as well as temperature-dependent spectral shifts. In earlier work at 77 K a kinetic analysis of the phosphorescence decay of PVCA yielded two prominent components having lifetimes of 7.7 and 1.7 s.',* Computer-assisted resolution of the phosphorescence band]' also yields two prominent components plus a third much weaker species. An Arrhenius plot of the short-lived lifetime indicated a marked insensitivityto temperature changes below 150 K followed by a change to a significant temperature dependence above this temperature. The activation energy above 160 K was determined to be 12.1 f 0.4 kJ/mol. The activation energy for the longer lived component was evaluated in the temperature range from 77 to 90 K and was found to be 2.5 f 0.8 kJ/mol. For the short-lived species it was concluded (1) Klopffer, W.; Fischer, D. J . Polymn. Sci., Part C 1973, 40, 43. (2) Itaya, A,; Okamoto, K.; Kusabayashi, S . Bull. Chem. SOC.Jpn. 1976, 49, 2037. (3) A particularly well documented example involves the emission from aromatic carbonyls in films of polystyrene. See: Burkhart, R. D.; Caldwell, N. J.; Haggquist, G. W . J . Photochem. Photobiol., A 1988, 45, 369. (4) Burkhart, R. D.; Aviles, R . G. Macromolecules 1979, 12, 1078. (5) Rippen, G.; Kaufmann, G.; Klopffer, W. Chem. Phys. 1980, 52, 165. (6) Webber, S. E.; Avots-Avotins, P.E. J . Chem. Phys. 1980, 7 2 , 3713. (7) [to, S.; Katayama, H.; Yamamoto, M . Macromolecules 1988, 21, 2456. (8) Burkhart, R. D.; Aviles, R. G. J . Phys. Chem. 1979, 83, 1897. (9) Burkhart, R. D.;Aviles, R . G. Macromolecules 1979, 12, 1073. ( I O ) Chakraborty, D. K.; Burkhart, R. D. J . Phys. Chem. 1989, 93, 4797. ( I I ) Burkhart, R. D.; Dawood, I. Macromolecules 1986, 19, 447.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Burkhart and Chakraborty
that the discontinuity in the Arrhenius plot indicated the onset of competition between detrapping and relaxation of the excimeric species to the ground state. That is, above 160 K the rate of detrapping was comparable to that of relaxation. Unfortunately, it is not possible to establish the depth of an excimeric trap simply from the activation energy for detrapping since this activation energy is a sum of the trap depth plus the activation energy for trapping. One might expect the activation energy for trapping to be small but perhaps not negligible. For these reasons an important motivation for the present work involved the hope that, a t sufficiently low temperatures, a structured nonexcimeric phosphorescence signal could be obtained. In this event the determination of an activation energy for trapping might be possible, and an important new physical property, the trap depth or binding energy, would then be available to characterize triplet excimers. As will be seen in the following work, this hope has, in fact, been realized.
Experimental Section Samples of PVCA were prepared in the laboratory by freeradical polymerization in connection with earlier studies on this polymer.11 Solid films were formed by dissolving the polymer in a small quantity of purified benzene and then placing drops of this solution on an optically flat quartz plate. The plates had been placed in the bottom half of a Petri dish which was covered after depositing the solution. When the solvent had evaporated, the films were placed in a vacuum oven a t 100 "C for 12-24 h. Temperature control was provided by a closed cycle liquid helium system (R. G. Hansen Associates) which cooled a copper tip to which the copper sample holder was attached by a threaded connection. A 25-W tip heater was used to counter the cooling action and provide temperature control to within f 0 . 2 K. The sample holder consisted of a copper ring with a recessed ledge into which the quartz sample disk and its cover plate would fit. Another copper ring was placed on top of the first one and was solidly fixed into place by machine screws so that the quartz disks were sandwiched between the copper rings. Indium gaskets were used between the quartz plates and the copper to provide for efficient thermal conductivity. The gaskets had small holes about 3 mm in diameter cut in their center to provide a light path for excitation and emission. A gold-chrome1 thermocouple was also sandwiched between the indium gaskets to provide temperature readings as close to the region of sample excitation as possible. Sample excitation was provided by a Tachisto Model 401 XR excimer laser which was operated either as a XeCl laser (308 nm) or as a nitrogen laser (337 nm). The pulse frequencies were always kept sufficiently small so that no overlapping of individual decay events could take place. Time-resolved spectra were recorded by using a predetermined delay between excitation and monitoring the emission. The wavelength drive speed of the monochromator was correlated with the laser pulse frequency so that each pulse corresponded to 1-nm change in wavelength. The emission signals were detected, after passage through the monochromator, by a photomultiplier which was connected to a Nicolet Model 12/70 signal averager. In some cases the signals were amplified and inverted before reaching the signal averager, but this could only be done if time delays were greater than 200 ks. A fast, noninverting amplifier was used for time delays to about 10 p s , and in these cases a 50-ohm terminating resistor was used at the connection to the signal averager to avoid distortion in arrival time of the signal. For delay times of several hundred nanoseconds to several microseconds, no amplification was used and the 50-ohm terminator was in place. Data stored on the signal averager were transferred to a laboratory computer (Epson Equity II+) and were analyzed by using locally developed software. Results and Discussion Figure I is a series of time-resolved delayed luminescence spectra for solid films of PVCA recorded at 15 K. Photoexcitation at 308 nm was provided by the XeCl excimer laser. As expected for an annihilative mechanism, the intensity of the delayed
Wovelenglh (nm)
>DY
r J F ? 4%
L M
5%
Figure 1. Delayed luminescence spectra of a PVCA solid films at 15 K. Note the slightly different wavelength scale used at 100 ms.
fluorescence component between 350 and 410 nm decays away more rapidly than the phosphorescence band so that, at the longest delay time of 500 ms, there remains only the phosphorescence spectrum of the carbazole chromophore. The remarkable characteristic of this spectrum is that there is no indication of the typical PVCA excimeric phosphorescence. This, to the best of our knowledge, is the first example of a phosphorescence spectrum of a PVCA solid film which lacks the typical broad and red-shifted components characteristic of excimeric emission. W e will provisionally refer to these spectra as nonexcimeric. Whether or not they originate from mobile triplet excitons or from chromophores that are very weakly interacting with neighbors will be more thoroughly discussed below. At the earliest delay time of 5 ms, the delayed fluorescence band strongly overlaps the phosphorescence band and so an unobstructed view of the phosphorescence spectrum is not possible. It appears, however, that the vibronic components near 425 and 450 nm of the nonexcimeric phosphorescence are definitely present at this delay time. Thus, the phosphorescence emission appears to be primarily nonexcimeric even at the shortest delay times used to record these spectra. An interesting contrast to the nonexcimeric phosphorescence emission seen in these spectra is the definitely excimeric delayed fluorescence band. Delayed excimer fluorescence is thought to arise from a heteroannihilation between a mobile triplet exciton and a trapped Thus, the triplet species responsible for these phosphorescence spectra may, in fact, reside in traps which are sufficiently shallow to permit the vibronic components of the emission spectrum to be observed but sufficiently deep to render them immobile. These possibilities will be discussed in more detail below. Using a constant delay time of 100 ms, we recorded a second series of phosphorescence spectra a t various temperatures from 15 to 55 K. These spectra are presented in Figure 2. Here one finds that an increase in temperature results in the disappearance of the vibronic components of the phosphorescence band and that a general shift of the spectrum to longer wavelengths occurs with a concurrent loss of structure. That is, at 55 K the spectrum has become primarily excimeric in character. It is now clear that the phosphorescence spectra so often observed from solid films of PVCA at 77 K are excimeric because this particularly convenient temperature happens to be above the transition region between
ik
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4145
Binding Energies of Triplet Excimers in PVCA
--
2.
c
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Q
3
Wovelength (nm)
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Wavelength (nm)
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1
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0.00
2.00
4.00
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x 100 Figure 3. An Arrhenius graph of the logarithm of the rate constant for phosphorescence decay versus 1/T for a solid film of PVCA. The asterisk denotes experiments utilizing a monitor wavelength of 430 nm and circles, 420 nm. The lower inset with triangle legend is a graph of delayed fluorescence intensity versus 1/T.
1/T
Figure 2. Phosphorescence spectra of a PVCA solid film at 100 ms after the excitation pulse recorded at different temperatures.
excimeric and monomeric emission. This provides important additional evidence that these emission signals are a result of photophysical processes occurring in the polymer sample and probably not a result of accidentally introduced impurities. In fact, other carbazole-containing polymers currently under investigation in these laboratories have similar transition regions between monomeric and excimeric phosphorescence but in different temperature ranges. I n earlier work on the temperature dependence of triplet emission from PVCA? it was established that the delayed fluorescence intensity plotted as a function of temperature goes through a minimum near 140 K and, as stated above, an Arrhenius plot of the rate constant for phosphorescence decay exhibits a discontinuity in slope near 160 K, indicating the emergence of a competing path for the relaxation to the ground state. The fact that both of these properties exhibit transitional behavior in the same temperature range is strong circumstantial evidence that they are associated with the same fundamental physical processes. It has been proposed that the process responsible for this transitional behavior is detrapping of the triplet excimers. Since at least two types of triplet excimers are known to exist in irradiated solid films of PVCA,5 it is expected that a similar type of transitional behavior might be encountered but in a different temperature regime. Such behavior can now be confirmed since the delayed fluorescence intensity of PVCA exhibits a low-temperature maximum near 40 K, which is in the same region over which an alteration in the character of the phosphorescence spectra from monomeric to excimeric has now been observed. In Figure 3 is an Arrhenius graph summarizing experiments on the temperature dependence of the rate constant for phosphorescence decay in the temperature range which includes 40 K near its midpoint. Actually, two independent sets of experiments were carried out indicated by different legends on the graph. Once again, it is seen that a break in the Arrhenius graph occurs. The exact temperature corresponding to the break is sightly different for the two sets of experiments but occurs in the range from 37 to 40 K. The activation energy extracted from the steeper portion of this graph is 2.0 f 0.3 kJ/mol. Also indicated on this graph is an inset showing how the delayed fluorescence intensity varies with 1 / T in that same temperature range. The coincidence in temperature corresponding to the maximum in the delayed fluorescence intensity and the break in the Arrhenius graph is clearly seen. Now, through the collective observations made on this system, we find that there are three different temperature-
dependent phenomena which demonstrate transitions in photophysical behavior in the temperature range near 40 K. The next task is clearly to seek a mechanistic framework for triplet excimer formation which is capable of tying together these various observations. The Arrhenius graph of Figure 3 clearly points to the opening of a new channel for depletion of excited-triplet-state species in this temperature range. It is worth noting that the discontinuity observed here is quite different from the one found in previous work at higher temperatures since in the present case it is the disappearance of nonexcimeric phosphorescence which is being monitored. In fact, the wavelengths chosen to monitor the rate of phosphorescence decay in these experiments (420 and 430 nm) were selected in order to exclude the possibility of contamination by any excimeric phosphorescence. Furthermore, the corresponding energy barrier is very small in the present case at 2.0 kJ. Since the temperature corresponding to the discontinuity of the Arrhenius graph coincides with the onset of excimeric phosphorescence, it is natural to conclude that the increased rate of decay of nonexcimeric triplets is due to trapping at excimerforming sites. Of the various processes involving polymers which might be responsible for trapping, low-amplitude intramolecular rotations about carbonxarbon bonds in the chain backbone appear to be reasonable, possibly combined with small-angle torsional oscillations of the pendant chromophore. Presumably, during the residence time of a triplet exciton at a particular chromophore, one of these rotations or oscillations can lead to the formation of an excimer which then becomes trapped at that location. At temperatures on the order of 40 K the rate of this trapping process begins to compete with the rate of relaxation of the nonexcimeric triplet to the ground state. Thus, the competing processes responsible for the change in slope of the Arrhenius graph may be represented by the equations
where T, represents the triplet state which has been called nonexcimeric and T, is a triplet state at an excimer site. The additional subscripts 1 and 2 take into account the fact that at least two types of triplet excimers can exist in these films. The ground electronic state is represented by IMo. Let us now consider the third of these temperature-dependent phenomena, namely, the passage of the delayed fluorescence intensity through a maximum near 40 K . An important point to recall is that activation barriers for detrapping of triplet excitons
4146
The Journal of Physical Chemistry, Voi.94, No. 10, 1990
Burkhart and Chakraborty liOirilXCrtrlC
TABLE I: Wavelengths and Energies of the Highest Energy Vibronic Band in the Phosphorescence Spectra of Various Carbazole-Containine Molecules compound and state PVCA solid film at 1 5 K PVCA in rigid solution at 77 K (carbazolylethyl methacrylate)-co-(methyl methacrylate) solid film a t 77 K ( I .2% carbazole) N-isopropylcarbazole in rigid solution at I1 K
,,A, nm
energy.
425 420 414
281.3
kJ/mol
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,
284.7 1 288.8 2 290.2
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1
in PVCA have the values 12.1 and 2.5 kJ/mol for the two prominent excimeric species which have been identified. Thus, assuming that the major process for delayed fluorescence (df) production in this temperature range is the heteroannihilation between a mobile triplet exciton and a trapped triplet exciton, T,, or Te2?the increase in df intensity would correpond to an increase in the concentration of trapped excitons as the temperature approaches 40 K from below and RT approaches the value of the activation barrier for trapping. As the temperature increases further, RT approaches the activation barrier for detrapping of the higher energy excimer and this process will begin to compete in rate with trapping. At this point the concentration of T, species begins to decline and a corresponding decline in the df intensity occurs. I n an earlier discussion of the mechanism responsible for this low-temperature maximum in the df-temperature profile,I0 it was suggested that the increasing df intensity between 15 and 40 K may be due to an increasing rate of triplet exciton migration. This proposal was based on the assumption that excimer sites were populated even at the lowest temperatures (near 15 K) examined. The present results clearly indicate that the population of excimer sites is insufficient to produce significant excimeric phosphorescence even at temperatures as large as 36 K. Thus, although the rate of triplet exciton migration may, in fact, be increasing between 15 and 40 K, it seems likely on the basis of present results that the observed increase in the df intensity in this temperature interval is due primarily to an increase in the population of triplet excimers. Although the mechanism to this point has the advantages of self-consistency, there remains the question of just what type of triplet excimer is being formed as the temperature approaches 40 K or whether both types are being formed. When one compares the phosphorescence spectrum of the film at 55 K with, for example, spectra recorded at 77 K, they are seen to be very similar. That is, in both cases the broad band extends from about 435 to 570 nm and a maximum intensity occurs at 485 nm. One minor difference in the spectra at these two temperatures is that at 77 K there is often a longer wavelength tail extending beyond 600 nm. Previous work on computer-assisted resolution of these phosphorescence spectra" shows that the long-wavelength tail is due primarily to a third excimeric species present in minor amount, usually on the order of 15% depending upon the mode of polymerization. We conclude. therefore, that the two major excimeric components are formed in the low-temperature regime and that the activation energy of 2.0 kJ/mol represents an energy barrier for formation of either of these components. A summary of the energy parameters deduced for this system is gathered together in Figure 4 where we are using T,, to represent the shallow trap and Te2 to represent the deeper trap. There is some question about the choice of an appropriate model for a reference system with which to compare the binding energy of the two different triplet excimers in these solid films. Several possibilities are listed in Table I along with the energies corresponding to the highest energy vibronic component in the respective phosphorescence spectra. Another possibility for the reference system would be the state corresponding to the energy of the carbazolyl chromophore at the top of the activation barrier depicted in Figure 4. Here, presumably, sufficient energy is present in internal rotational modes at a localized site to allow a chromophore pair to probe various conformational arrangements, some of which will lead to excimer
7.c iii
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Figure 4. A proposed energy level diagram for the triplet states of solid films of poly(N-vinylcarbazole).T,, and T,, symbolize excimeric species, and DTIand DT2represent dissociation energies of these two excimers, respectively, on the ground-state surface. El and E2are energy differences between the lowest vibrational level of the respective excimeric species and the lowest vibrational level of the nonexcimeric triplet.
formation. We shall refer to this state as the limited free rotor (LFR) for which the energy is 283.3 kJ/mol. The most important criterion for the choice of a reference system is that it should make quantitative comparisons possible of binding energies from one chromophore to another. It will eventually prove important to be able to relate triplet excimer binding energies to the chemical nature of the chromophore. Some portions of the spectral shifts noted in Table I are due to environmental effects; for example, the effective dielectric constant of the solid film at 15 K may be somewhat different than that of the rigid solution. For this reason it is suggested that the reference system should be in the same physical state as that in which the excimer is formed. This eliminates all possibilities except that of the so-called nonexcimeric phosphorescence(28 1.3 kJ/mol) or the LFR state at the top of the activation barrier (283.3 kJ/mol). Of these two choices it seems that the LFR state is preferable since the nonexcimeric state is clearly residing in a shallow trap, the depth of which will vary from one chromophore to another. Using this choice then the binding energies are 2.5 and 12.1 kJ/mol for T,, and Te2, respectively. Although phosphorescence emission from these two excimeric components cannot be observed independently, they have been artificially resolved by using computer methods yielding a peak wavelength of 477 nm (250.6 kJ/mol) corresponding to Tel and 507 nm (235.8 kJ/mol) for Te2. Let us symbolize these photon energies as PE, and PE,, respectively. It will be seen that PE, + E, - E , = 245.4 kJ/mol. This suggests that transitions from T,, terminate on the ground-state potential energy surface at an energy level 5.2 kJ/mol higher than that of T,,. Furthermore, the origin of the phosphorescence band for nonexcimeric triplets, PE,,, is 425 nm (281.3 kJ/mol). Since PE,, = PE, DTl 0.5, we find that DTI = 30.2 kJ/mol and therefore h2= 35.4 kJ/mol. These results are interesting from several points of view. First of all, it is found that the most stable excimeric species has a configuration which is more strongly dissociative on the ground-state surface. This is an intuitively satisfying result since it is expected that the species having the stronger bonding in the excited state would be the least stable on the ground-state surface. I n addition, it is known that the dissociation energy for singlet excimers of PVCA in the ground state is 25.1 kJ/mol.I2 Thus,
+
+
,
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 at 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
