J. Phys. Chem. 1989,93, 5351-5353 the Soret band excited Raman intensity pattern for modes of monomeric chlorophyll a and Ni(pheo a ) are similar. This suggests a larger contribution of Condon-type scattering than is the case for scattering induced with red light in resonance with the Q band. On the other hand, Condon-type scattering dominates the R E P for Soret band excited modes of an approximately Tshaped dimeric chlorophyll a complex in hexane.* We conclude that different vibronic coupling mechanisms between electronic
5351
surfaces govern the intensity patterns of modes of these large molecules. Acknowledgment. We thank Dr. P. Wong of the National Research Council of Canada for his cooperation by allowing T.M. to use instrumentation enabling us to construct the REPS. This research was supported by the Natural Sciences and Engineering Research Council of Canada.
Time-Dependent Fluorescence Spectral Shift and Unusual Slow Decay of Exciplex in Poly(N-vinylcarbatole) Films Hisashi Sakai, Akira Itaya,* and Hiroshi Masuhara* Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan (Received: March 9, 1989)
Exciplex fluorescence of poly(N4nylcarbazole) films doped with dimethyl terephthalate and pdicyanobenzene was investigated by using the nanosecond time-correlated single-photon-countingmethod. The fluorescence peak in the time-resolved spectra shifted continuously to the long wavelength with time. The exciplex fluorescence showed nonexponential decays with slow tails up to the microsecond time region, and the decay process was enhanced by applying an electric field. New mechanisms underlying these results were considered.
Introduction
Considerable data on the fluorescence properties of poly(Nvinylcarbazole) (abbreviated hereafter as PVCz) have been accumulated because of an interest in its photoconductive properties, two spectrally distinct excimer fluorescences, and a clear relation between photophysical properties and tacticity.' It is well-known that doping of weak electron acceptors such as dimethyl terephthalate (DMTP) in PVCz films causes the chemical sensitization of the photoconductivity2 and gives exciplex fluorescence.f$ In order to make clear the carrier-photogeneration mechanism, electric and magnetic field effects on steady-state exciplex fluorescence have been investigated in detail." However, very little is known about dynamics of exciplex fluorescence, except for our reports concerning its formation process.',9 On the other hand, there exist some reports on exciplex fluorescence dynamics in solution.'*'2 It was reported that the sandwich excimer fluorescence was quenched more effectively than the second excimer fluorescence by doping with DMTP.'O Ex(1) Itaya, A,; Sakai, H.; Masuhara, H. Chem. Phys. Lett. 1987, 138,231 and references cited therein. (2) Okamoto, K.; Kusabayashi, S.;Mikawa, H. Bull. Chem. SOC.Jpn. 1973, 46, 2613. (31 Okamoto, K.; Yano, A.; Kusabayashi, S.; Mikawa, H. Bull. Chem. Soc. Jpn. 1974, 47, 749. (4) Itaya, A,; Okamoto, K.; Kusabayashi, S. Bull. Chem. SOC.Jpn. 1977, 50, 22. (5) Yokoyama, N.; Endo, Y.; Mikawa, H. Bull. Chem. Soc. Jpn. 1976.49, 1538. (6) Yokoyama, M.; Endo, Y.; Matsubara, A.; Mikawa, H. J . Chem. Phys. 1981, 75, 3006. (7) Okamoto, K.; Itaya, A.; Kusabayashi, S. Chem. Lett. 1976, 99. (8) Kim, N.; Webber, S. E. Macromolecules 1985, 18, 741. (9) Masuhara, H.; Tamai, N.; Mataga, N. Chem. Phys. Lett. 1982, 91, 209. (IO) Itaya, A.; Okamoto, K.; Kusabayashi, S. Bull. Chem. Soc. Jpn. 1976. 49, 2082. (1 I ) Hoyle, C. E.; Guillet, J. E. Macromolecules 1978, 11, 221. (12) Hoyle, C. E.; Guillet, J. E. Macromolecules 1979, 12, 956.
amining the PVCz-DMTP system, Hoyle and Guillet revealed the presence of both an exciplex and an exterplex. Both are deemed to be complexes composed of donor cation and acceptor anion. The exterplex consists of two carbazolyl chromophores and one molecule of DMTP, and the hole is delocalized over two carbazolyl chromophores.",12 As model compounds for exciplexes from isotactic and syndiotactic sequences of PVCz, respectively, meso- and rac-2,4-di(N-carbazolyl)pentanes,giving sandwich ahd partial overlap excimer fluorescences, respectively, are very interesting.13 Studies on the time-resolved fluorescence spectra of these compounds in the presence of m-dicyanobenzene (m-DCNB) indicated that the special geometrical structure of the two carbazolyl groups leads to two kinds of exterplex where the hole is delocalized in the corresponding sandwich and partial overlap structures.14 As summarized above, studies on the dynamics of exciplex fluorescence in film are much less than those in solution and the mechanism of exciplex formation in films is still beyond our knowledge, although it has been discussed on the basis of the results on the electric field effect on the steady-state exciplex fluorescence. We applied here for the first time a nanosecond time-correlated single-photon-counting technique to PVCz films doped with DMTP or p-dicyanobenzene (p-DCNB) without and with the applied electric field. Experimental Section PVCz was prepared by radical polymerization described previ~usly.'~DMTP and p-DCNB were recrystallized from benzene and ethanol, respectively, and subsequently sublimed in vacuo. Polymer films were cast on quartz or nesa-coated quartz plate from 1,2-dichloroethane or benzene solution containing PVCz and (1 3) Vandendriessche, J.; Palmans, P.; Toppet, S.; Boens,N.; De Schryver, F. C.; Masuhara, H. J . Am. Chem. SOC.1984, 106, 8057. (14) Masuhara, H.; Vandendriessche, J.; Demeyer, K.; Boens, N.; De Schryver, F. C. Macromolecules 1982, 15, 1471. (15) Itaya, A,; Okamoto, K.; Kusabayashi, S. Polym. J . 1985, 17, 557.
0022-3654/89/2093-5351$01.50/00 1989 American Chemical Society
5352 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
Letters
1 2227-2422
Y
u
WAVELENGTH / nm
Figure 1. Normalized fluorescence spectra of PVCz films (1) undoped and doped with (2) DMTP (3.0 mol %) and (3) p-DCNB (1.8 mol %). The spectra were obtained by integrating the fluorescence intensity at each wavelength up to 168,4688,and 4668 ns after excitation and not corrected for the detector sensitivity. a known amount of the dopant and dried under vacuum for several hours. If necessary, a semitransparent electrode was formed by evaporating gold onto the film on nesa-coated quartz plates. By use of this sandwich-type cell, electric field effects upon exciplex dynamics were examined. All the measurements were performed under vacuum. Time-resolved fluorescence spectra and fluorescence rise and decay curves were measured with a nanosecond time-correlated single-photon-counting system with a hydrogen-filled flash lamp. Fluorescence decay curves were measured for some wavelengths. The wavelength of the monochromator was driven in a 1- or 2-nm step, and the accumulation time for one decay curve was usually 180 s. From a series of the decay curves thus obtained, the time-resolved spectra were obtained by plotting the fluorescence intensity at the delay time concerned versus wavelength. All the measurements and data processing were fully controlled by the personal computer. Spectra were not corrected for the detector sensitivity.
Results Fluorescence spectra of PVCz films undoped and doped with DMTP and p-DCNB were obtained by integrating the fluorescence intensity at each wavelength up to 168,4688, and 4668 ns and are shown in Figure 1 . The spectrum of the undoped PVCz = 420 film consists of the sandwich excimer fluorescence (A, nm) and the second excimer one (A, = 370 nm).4916 By the doping of DMTP (3.0 mol %) or p-DCNB (1.8 mol %), the host fluorescence is quenched and is replaced by the exciplex fluorescence with the peak of 478 and 473 nm, respectively. Time-resolved fluorescence spectra of PVCz films doped with DMTP and p-DCNB are shown in Figure 2, where each spectrum is normalized at the maximum intensity. The wavelength dependence of the instrument response function was not corrected. In the early-gated spectrum, the host fluorescence was mainly observed with a peak at ca. 430 nm and the exciplex fluorescence in the long wavelength region was weak. The peak of spectra continuously shifted to the long wavelength with time. At the latest gated time, the peak is located at 492 and 497 nm for DMTP and p-DCNB systems, respectively. Their peak wavelength is longer than that of fluorescence spectra (a sum of time-resolved spectra) for both systems shown in Figure 1. Since the decay time of the sandwich excimer fluorescence of PVCz film is 35 ns,' it is difficult to explain the continuous red-shift of the spectra in the time region after 35 ns only by assuming a superposition of spectra of excimer and exciplex fluorescence. Figure 3 shows a series of difference spectra for the p-DCNB system which were obtained by subtracting the spectrum in the latest gated time (16) Rippen, G . ; Klopffer, W. Ber. Bunsen-Ges. Phys. Chem. 1979, 83,
431.
0-19.5
400
500
600
0-19.5
500 600 WAVELENGTH / nm
400
WAVELENGTH / nm
Figure 2. Normalized time-resolved fluorescence spectra of PVCz films doped with (a) DMTP (3.0 mol %) and (b) p-DCNB (1.8 mol 95). Excitation wavelength is 295 nm. The time window is given in the figure. All the spectra are not corrected for detector sensitivity.
19.5-39.1
t/i 400
500
600
WAVELENGTH / nm
Figure 3. Normalized difference spectra of PVCz films doped with p-DCNB. See text. region from the each time-resolved fluorescence spectrum after smoothing the spectra. The peak shifts to the long wavelength with time even in the time region after 35 ns. This result indiqtes clearly that the exciplex fluorescence of doped PVCz films shifted gradually to the long wavelength with time. Yokoyama et al. reported that the steady-state excimer fluorescence was not quenched by applying an electric field, but the exciplex fluorescence of PVCz films doped with weak electron acceptors such as DMTP and p-DCNB was quenched. The decrease of the fluorescence intensity depends upon the applied
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5353
Letters
lobs.
=540 n m
1000 150 T I M E /ns Figure 4. Exciplex fluorescence decay curves of PVCz film doped with p-DCNB (1.4 mol 5%) with and without applied electric field. 0
500
relaxed fluorescent exc@Iexstate
electric field strength, which was interpreted well in terms of the Onsager theory? The decay curves of fluorescence of the p D C N B system observed at 540 nm with and without the applied electric field are shown in Figure 4. The decay curve has a long tail to the microsecond time region and does not obey a multiexponential function. This behavior is quite different from that observed for PVCz s o l ~ t i o n . ~The ~ J present ~ decay became fast by applying the electric field, while an appreciable change of the rise curve was not observed. The total fluorescence intensity at this wavelength was decreased by applying the electric field, which is in agreement with the result reported by Yokoyama et al. for the steady-state fluorescence.6 The decay curves of the excimer fluorescence of the undoped PVCz film monitored at 370 and 465 nm were not affected by the applied electric field.
Discussion On the basis of results on the electric field induced exciplex fluorescence quenching and photoconductivity measurements, Scheme I for the formation of the fluorescent exciplex state has been considered as the most probable me~hanism.~,"A singlet excited state D* in the PVCz film migrates effectively through the carbazolyl chromophores and encounters an acceptor (A) during its lifetime and forms an encounter complex (D*.-A). The complex goes through a rapid electron transfer and changes to a nonrelaxed exciplex state (D+-A-)**. This state has an excess energy and undergoes a thermalization of the excess energy, giving an ion pair state (D+--A-). Relaxation from the ion pair to the relaxed fluorescent exciplex state (D+-A-)* occurs in competition with the electric field assisted thermal dissociation into free carrier, D+ + A-. The interionic distance r, in the ion pair is larger than that of the contact fluorescent exciplex. In Scheme I, only one (17) Okamoto,K.; Oda, A.; Itaya, A.; Kusabayashi, S. Chem. Phys. f e r ? . 1985, 35, 483.
kind of exciplex has been assumed. It is also worth noting that the electric field may affect the formation rate of the fluorescent exciplex state. One might expect that the rise curve of the exciplex fluorescence changes by applying the electric field and that the decay curve does not change. Therefore, our results cannot be interpreted by Scheme I. So, we propose to modify it. In solution, two kinds of exterplex, of which the precursors are sandwich and partial overlap excimers, have been reported by examining bichromophoric model compound and m-DCNB system.I4 An exterplex formation has been proposed also in PVCz-DMTP solution,"-'* which is consistent with the fact that PVCz shows both partial overlap and sandwich excimer fluorescences in solution.I0 The present results are in disagreement with these results considered until now. We could not separate an exciplex with distinct spectrum and temporal behavior, as the exciplex fluorescence peak shifts to the long wavelength with time. Taking into account the facts that PVCz film shows also both excimer fluorescencesand that the structure of the exciplex is more loose than that of excimer, we consider that multiple exciplex and exterplex species with various relative configurations between donors and acceptor are responsible for the dynamic behavior in the doped film. It is difficult to assume a gradual relaxation of exciplex species such as reorientation of chromophores, because molecular motion in solid film is very slow. Thus, the continuous shift of the peak of the time-resolved fluorescence spectra seems to be attributed to a superposition of various exciplex and exterplex species with temporal characteristics. One more thing which we have to point out here is the nonexponential decay curve with a long tail up to several microsecond time region. It is impossible to interpret this behavior by considering the presence of multiple species, since the lifetime of the singlet exciplex and exterplex is shorter than 100 ns. Introducing a variety of distribution of ion pairs and relaxed exciplexes with different distance in Scheme I, we can explain the slow tail of the decay curve and the electric field induced decay time decrement as follows. The relaxation from the ion pair state (D+-A-) to the relaxed fluorescent exciplex state (D+-A-)* as well as free carriers determines the decay of the fluorescent exciplex states. This relaxation time constant may be around the microsecond order, while the exciplex lifetime is about a few tens of nanoseconds. That is, the geminate recombination of the ion pair with an interionic separation ro and the dissociation to free carriers are rate-determining steps for the decay process of fluorescent exciplex states. Thus, the formation of fluorescent exciplex states continues even in the microsecond time region. Since this geminate recombination competes with the electric field assisted thermal dissociation into free carriers, the electric field can affect the decay of the exciplex fluorescence. Actually we have confirmed here for the first time that the electric field caused the increment of the fluorescence decay rate. Recently, Kim and Webber have reported that the PVCz film doped with DMTP showed the exciplex phosphorescence with a peak at ca. 550 nm at room temperature.* As the peak of the exciplex fluorescence shifts continuously from the nanosecond to the microsecond time regions, the present emissions are considered to be due to the singlet state. Furthermore, the electric field effect on the decay curve could not be interpreted in terms of the phosphorescent exciplex. However, we could not exclude a possibility that the slow tail of the decay curve up to the microsecond time region includes the phosphorescence to a certain extent. The present result is the first report on the exciplex dynamics in organic solid films. More detailed analysis and experimental work are now in progress. Acknowledgment. We are indebted to T. Okamoto for help with the experiments. The present work is partly supported by the Grant-in-Aid for Scientific Research on Priority Area for Macromolecular Complexes (63612510) and on Special Project Research for Photochemical Processes (63104007) and the Grant-in-Aid for Scientific Research (63430003)from the Japanese Ministry of Education, Science, and Culture.
