J. Phys. Chem. 1994,98, 7608-7612
7608
Triplet Excimer Formation of Dibromocarbazole Chromophores in Methacrylate Copolymer Films Measured by Time-Resolved Phosphorescence and Transient Absorption Spectroscopy Shinzaburo Ito and Masahide Yamamoto. Division of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Sakyo, Kyoto 606, Japan
William R. Liebe and Richard D. Burkhart Department of Chemistry, University of Nevada, Reno, Nevada 89509
Yoshio Wada Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Sakyo, Kyoto 606, Japan Received: February 18, 1994; In Final Form: May 12, 1994’
Time-resolved phosphorescence and transient absorption spectroscopy have been carried out for studying the triplet states of the 3,bdibromocarbazole (DBCz) chromophore in a solid matrix of methacrylate copolymer, poly[2-(3,6-dibromo-9-carbazoyl)ethylmethacrylate-co-methyl methacrylate]. The sample film containing 10% DBCz units showed critical behavior in the time-resolved phosphorescence spectra which altered the shape from the monomer state 3M* to two kinds of excimer states: El and E2 (460 and 510 nm at the maximum intensity, respectively). These excimeric species have identical profiles with those reported for poly(3,6-dibromo9-vinylcarbazole), although the polymer structures are totally different. This result shows that the DBCz chromophores tend to take some preferential geometry which results in the formation of two distinct excimer sites in the polymer film. The spectral alteration with time was drastically accelerated by thermal activation in the temperature range 25-77 K. Iterative trapping and detrapping processes determine the rate of relaxation to the deeper traps, El and E2.
Introduction Triplet state photophysics in polymer solids has been a subject of discussion not only from the view of application aiming to utilize photofunctional polymers but also from a fundamental standpointarising from photophysical interests.’-3 Polymer solids are useful for understanding triplet behavior in condensed phases where the chromophores are embedded and frozen in a matrix with a concentration high enough to undergo interchromophore interactions, while being kept in a statistically uniform dispersion without microcrystalline or aggregate formation? During the last decade, many works have revealed that an excited triplet state is able to interact with the neighboring chromophores, consequently it forms “triplet traps” or “triplet excimers”,depending on the magnitudeof the interactionenergy.)J Among various chromophoric systems, 3,6-dibromocarbazole (DBCz) is a particular chromphore providing an unambiguous indication of triplet excimer. The substitution of heavy atoms on the aromatic rings causes drastic enhancement of the singlettriplet intersystem crossing which makes phosphorescence spectroscopy much easier than usual. As for poly( 3,6-dibromo-9vinylcarbazole) (PDBVCz), Yokoyama et al. first reported triplet excimer phosphorescence in MTHF rigid glass.6 Recently, Starzyk and Burkhart reinvestigatedthis polymer system in detail using a time-resolving technique,’ and they demonstrated that there are at least two excimeric species besides the monomeric triplet state. In the previous report, we appliedthe copolymerization method to the DBCz chromophore which was introduced into a methacrylate polymer as a side group.* This is a very convenient and useful method capable of observing triplet interactions as a function of chromophore concentration, i.e., the distance of separation,which is an important experimentalvariable in a study of photophysics. Regardless of the existence of spacer atoms between the polymer main chain and the chromophoric unit, this *Abstract published in Advance ACS Abstracts, July 1, 1994.
OO22-3654f 94f 2098-7608$04.5O/O
polymer clearly showed triplet excimers in the solid films at concentrations higher than 10% in mole fraction. The results suggested that the DBCz chromophore has a longer interaction distance and a weaker stabilization energy at the triplet state, compared with the unsubstituted carbazole. To clarify the mechanism and processes of the triplet stabilization in polymer solids, it has been highly desirable to perform kinetic analyses using a time-resolving technique. In this respect, we have performed time-resolving measurements for the copolymers bearing DBCz chromophores. The current paper describes the trapping and relaxation processes of the triplet energies directly observed by transient spectroscopy.
Experimental Section Materials. 2-(3,6-dibromo-9-carbazolyl)ethyl methacrylate (DBCzEMA) was prepared by esterification of 3,6-dibromo-9(2-hydroxyethy1)carbazole and methacryloylchloride (Aldrich). Carbazole was synthesized in this laboratory, because contaminants such as benzocarbazoleoften appear in purchased material. The procedure has been described elsewhere.* Methyl methacrylate (MMA) (Wako Pure Chemical Industries, Ltd.) was purified by distillation under reduced pressure before use. These monomers were polymerized at 60 OC in benzene using azobis(isobutyronitrile) as an initiator. Obtained copolymers (P(DBCzEMA-co-MMA))were purified by repeated reprecipitation
A,, -
PDBVCz
Br P(DBCZEMA-CO-MMA)
from benzene and methanol. The homopolymer (PDBCzEMA) was polymerized in DMF, becauseof the poor solubility in benzene. 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 31. 1994 7609
Triplet Excimer Formation
TABLE 1: Compositions and Molecular Weight (Mw)of the Cowlvmers content Mwa sample content Mw" no. (mol %) (10') sample no. (mol 96) (103) 1
0.20
2 3 4
0.98 4.9 10 a
71 74 78 68
5 6
PDBCzEMA
15 20 100
74 78 98
! A
Determined by GPC with reference to standard polystyrene.
Table 1 shows the compositions of the copolymers and molecular weights determined by GPC. These copolymers are hereinafter referred with the sample number listed in Table 1. The copolymer film was cast on a quartz plate from a solution of dichloromethane (Spectrometric grade, Dojindo Lab.) and then dried in vacuo. Since the homopolymer was insoluble in dichloromethane,distilled T H F was used as the casting solvent. The film thickness was about 10 pm on average. For the measurements of triplet-triplet absorption spectra in solution, thecopolymersweredissolved in distilled THF or MTHF then degassed by repeated freezethaw cycles. The absorbance at 308 nm was adjusted to be ca. 1. Measurements. Steady-state phosphorescence spectra were recorded with a Hitachi 850 spectrophotometer fitted with a phosphorescence attachment, where the sample was immersed in liquid nitrogen in a vacuum bottle made from quartz. To measure time-resolved phosphorescence and transient absorption spectra, an excimer laser system was used; the details have been described in recent publication^.^ The sample was excited with 308-nm pulsed light from a Questek Model 21 10 XeCl excimer laser. The phosphorescence emission was detected by a Princeton Instruments Model IRY-700-SIB diode array located at the exit focal plane of a SPEX Model 1681C spectrograph. The diode array was triggered and gated by a Princeton Instruments pulser, and the signals were accumulated and averaged on a computer more than 200 times. Transient absorption spectra were recorded with the same apparatus using a probe beam from a xenon arc lamp. Temperature control was provided by a closed helium system (APD Cryogenics) which cooled a copper tip to which the copper sample holder was attached. The sample on a quartz disk was placed between the copper holder and another copper ring which was tightly fixed by machine screws and indium gaskets.
Results and Discussion Steady-StateExcitation Spectra. Let us introduce briefly the triplet behavior of P(DBCzEMA-co-MMA) films observed under steady-state spectroscopy.* Figure 1 presents emission spectra of copolymer films, which were recorded with a conventional spectrophotometer. The spectra consist of mostly phosphorescence, due to the so-called heavy atom effect of bromine which markedlyacceleratesintersystemcrossing from the excited singlet state to the triplet state. As a result of this effect, the fluorescence efficiency becomes very small compared with that of the unsubstituted carbazole. The use of copolymers allowed us to observe triplet interactions as a function of chromophore concentration,i.e., as a function of distance of separation between chromophoresin the copolymer film. The spectra clearly become broader with increasing DBCz concentration, and their features alter to that of an excimeric spectrum. A further increase of concentration gives rise to spectra shifted to longer wavelengths. These facts indicate that DBCz chromophores are able to form excimers in the triplet state. The interaction energy, which is observed as a shift of wavelength at the maximum intensity compared with the monomeric phosphorescence band, becomes larger with the decrease of the mean distance between chromophores. It is worth noting that we should considertwo possible factors determining the triplet behavior. One is the formation probabilityof excimer sites where two neighboringchromophores
400
600
500 Wavelength / nm
Figure 1. Emission spectra of P(DBCzEMA-ceMMA) films at 77 K: (a) no. 2 (0.98 mol %), (b) no. 3 (4.9 mol a), (c) no. 4 (10 mol %), (d) no. 5 (15 mol %), and (e) no. 6 (20 mol %). The samples were excited at 330 nm, and the spectra were recorded with a bandwidth of 2 nm.
I
300
\
d ,
400 500 Wavelength / nm
600
Figure 2. Time-resolved phosphorescence spectra of P(DBCzEMA-wMMA) film no. 2: (a) at 25 K using a delay time of 1.5 ps after excitation, (b) at 77 K with the same delay time as a, (c) at 77 K with a 700-ps delay time, and (d) at 77 K with a 7-msdelay. All spectra were recorded with a gate time of 800 ps and accumulated 200 times.
are fixed to a configuration in which the triplet state is stabilized with a mutual interaction. The other is energy migration between chromophores,which is responsible for the efficient flow of triplet energy to specific sites having a lower energy level such as an excimer site. The increase of chromophore concentration results in positive effects on both of these two factors. The main aim of this work is to verify the existence of these triplet species and to show the kinetics of their various modes of relaxation directly by transient spectroscopy. Transient Phosphorescence Spectra. Figure 2 shows transient phosphorescence spectra for the copolymer no. 2 (0.98 mol %) film. Parts a and b of figure 2 were recorded at 25 and at 77 K, respectively,and thedelay time wasset to 1.5 psfrom theexcitation pulse. Emissive species appear around 420-430 nm and are assigned to the monomer triplet state, 3M*. Parts b-d of figure 2 show the spectra at 77 K with various delay times, where only a little broadening was seen in the later time range. As for this copolymer film, the interaction of chromophores is so weak that one can hardly see spectral change with the elapse of time, Le., the chromophore behaves as an isolated one. Sample no. 4 (10 mol 5%) showed critical behavior in the timeresolved spectra, which alter the shape from 3M* to excimeric
7610 The Journal of Physical Chemistry, Vol. 98, No. 31, I994
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I
300
Figure 3. Time-resolved phosphorescence spectra of P(DBCzEMA-coMMA) film no. 4 at 25 K at delay times of (a) 1.5 ps, (b) 700 ps, and (c) 7 ms, using a 800-ps gate.
i
n
Figure 4. Time-resolved phosphorescence spectra of P(DBCzEMA-coMMA) film no. 4 at 77 K at delay times of (a) 1.5 ps, (b) 700 ps, and (c) 7 ms, using a 800-ps gate.
emission, depending on the observed time range and on the temperature. For example, Figure 3 shows the spectra at 25 K, which consist of monomer-like phosphorescence as a major component and excimer-like phosphorescence as a minor component which appeared at 460 nm. Carefully observed, the monomer emission seems to be shifted to the longer wavelengths compared with the spectrum of the no. 2 film; this indicates the formation of a shallow trap ST*by a weak interaction with the neighboring chromophores. The component of excimer-like emission becomes larger with time, and after 7 ms, additional emission could be seen around 510 nm as a shoulder in the longwavelength tail of the dominant 460-nm emission. Let us name these excimeric species as El and E2: 460 and 510 nm at the maximum intensity, respectively. When the temperature was increased to 77 K, the spectral alteration was accelerated by thermal activation. The spectrum in Figure 4a, which is the same sample as Figure 3 but recorded at 77 K,has already changed to that of E l , and later, E2 becomes dominant as shown in Figure 4c. Therefore, we could see three obviously distinct species as a function of time and temperature. We tried to divide these spectra into the components of T, El, and E2. The spectral profiles of T and E2 were taken from Figure 2 and Figure 5 which are predominantly of the monomer and the deeper excimeric emission, respectively. As for the fitting procedure, the spectrum of E l was estimated from Figure 3 parts
400
500
,
Wavelength / nm
,
600
Figure 5. Time-resolved phosphorescence spectra of (a) P(DBCzEMAco-MMA) film no. 5 and (b) a PDBCzEMA film at 25 K. The spectra were recorded with a delay time of 1.5 ps and a 800-ps gate. Spectrum a was accumulated 200 times, and b was accumulated 400 times.
band c, then the spectrumwas constructedfrom these base spectra with an appropriate proportion. The result is shown with the broken lines in Figures 3c and 4c. The good fit with the actual spectra indicates that these three are representativesof the triplet species. The samples with higher concentrations showed only E2 emission in the whole range of time and temperature. The examplesof no. 5 (15 mol %) and the homopolymer PDBCzEMA films are presented in Figure 5 . The flow of excitons to the lowest energy level is too fast to be seen with the present time resolution. Here we will discuss the triplet species observed in this work in comparison with ones previously reported. Yokoyama et al. reported the phosphorescence spectra of PDBVCz in rigid MTHF glasse6 The spectrum clearly showed a broad excimeric band with a maximum around 460 nm. Burkhart et al. carefully reinvestigated this compound using time-resolved spectroscopy and found that there are at least three emissive species: at 448, 473, and 501 nm at the maximum intensity.' They assigned the 448-nm species to a monomeric or weakly interactingchromophore (T), the 473-nm species to an excimeric deep trap (E), and the 501-nm species to an uncertain trap component in the system. These spectra are quite similar to those in the current compound, P(DBCzEMA-co-MMA), although the polymer structures are different. We have to be cautious of contaminants. Recently, we have studied the copolymerssynthesizedindependently at the Yamamotolab and at the Wada labusingcarbazolefromdifferent sources.8 Both gave the same result. Furthermore, the fraction of E l and E2 depended on the casting conditions of copolymer films, and the maximum position of the excimer emission also changed with the conditions. This suggests that the casting procedure affects the manner of chromophore packing in the solid matrix. Juding from these facts, we conclude that these triplet species in Figures 2-5 originate from DBCz chromophores and the deeper traps of E l and E2 are the triplet excimers of DBCz. There is controversy as to the configuration of the triplet excimer.2J0-12 Theoretical calculation predicted that the steric conformation of the triplet excimer is significantlydifferent from a sandwich-parallelgeometry of aromatic rings, which is favored by the singlet excimer.13.14 Due to the loose potential curve for the triplet state with respect to the geometrical arrangement of two chromophores,it is said that the triplet excimer need not take a strict parallel arrangement. This characteristic probably yields the broad excimeric spectra observed in the triplet state; the spectra are much broader compared with the relevant singlet excimer.5 Especiallyin an amorphous polymer film, the chromophores are
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7611
Triplet Excimer Formation 0.03
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0*061 0.02
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Figure 6. Transient absorption spectra of P(DBCzEMA-co-MMA)film no. 4 at 25 K at delay times of (a) 270 ns, (b) 8 p, and (c) 650 p, using a 140-11s gate.
Figure 7. Transient absorption spcctra of (a) P(DBCzEMA-co-MMA) no, 2 in MTHF rigid glass at 25 K, (b) P(DBCzEMA-co-MMA) no. 2 in THF solution at 170 K, and (c) PDBCzEMA homopolymer in THF solution at 170 K. All spectra were recorded with a delay time of 270
able to take various spatial arrangements with respect to the neighboring ones and are stably fixed in the solid matrix. The variety of the spatial arrangements also seems to be responsible for the broader excimer emission. However, the current chromophore, DBCz, showed two distinct excimers, E l and E2, and the profileswereverysimilar toPDBVCzinspiteof thecompletely different polymer structure. This result suggests that there is some preferential geometry when the chromophores approach each other. One possibleexplanation for this is thesteric hindrance of bromine atoms attached to the carbazole ring. The van der Waals radius of the bromine atom is known to be 0.2 nm, which is comparable with that of a methyl group.*S The bulkiness may forcechromophoresintosome preferential configurationin which the triplet state interaction can be exerted. Triplet-Triplet (T-T) Absorption Spectra. T-T absorption spectra also give us important information on the triplet state. But, it is usually difficult to measure the transient absorption of film samples because of some experimental reasons, e.g., the thinness of films results in a short path length of the probe beam and the intense laser light often damages film samples. In the current study, fortunately, we could observe the T-T absorption spectra of the copolymer films using the same cryogenics and diode array systems. Although the spectra were noisy compared with those of solution samples, clear absorption bands appeared in the wavelengths corresponding to the carbazole triplet. Figure 6 presents the spectra of the copolymer no. 4 film at 25 K with various delay times after the excitation. As mentioned above, this sample showed phosphorescence spectra intermediate between T and E. In the early time range, 270-410 ns from the excitationpulse, the absorption band of the triplet monomer state could be seen around 420 nm. This band decreases rapidly, and after 8 ps, the rest of the spectra provides broad and structureless absorption in a wide wavelength range. In order to make sure the assignments of excited species in Figure 6 were correct, we measured T-T absorption spectra of no. 2 in MTHF rigid glass (at 25 K), where the chromophores are not able to interact with each other. The spectrum is shown in Figure l a . Another spectrum in Figure 7b was measured in THF solution at a temperature of 170 K which is slightly higher than the melting point of the solvent THF. These two figures are quite similar to Figure 6a, therefore, the species in Figure 6 parts a and b can be safely assigned to the triplet monomer state in the copolymer film, which is also consistent with the result of phosphorescence spectra. Figure 7c shows the spectrum of the homopolymer, PDBCzEMA, in THF at 170 K. Due to the high local concentration of chromophores, the absorption is very weak and broadeven at theinitialstageafter the excitation. Theinteraction
ns using a 140-11sgate.
1
t 500 'do' Wavelength ' / nm '
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' 600 '
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Figure 8. Transient absorption spectra in THF solution at room temperature: (a) P(DBCzEMA-co-MMA) no. 2 and (b) PDBCzEMA. The spectra were recorded with a delay time of 1 p using a 140-11sgate.
of the triplet state can be observed in Figure 8, where the spectra were taken at room temperature. The homopolymer PDBCzEMA again provided a broad T-T absorption spectrum. These experimental results reasonably assign the remaining absorption features in Figure 6c to the absorption of a stabilized triplet, which is probably due to the formation of triplet excimer. The T-T absorption bands observed in the copolymer films are mainly of the monomer state, so we could not obtain details about excimer states by the transient absorption technique. This means that most of excited states produced by the laser excitation dissipate to the ground state by the annihilation processes of triplet excitons, therefore, the triplet population on the trap sites and excimer sites is too small to be detected with enough of a signal to noise ratio of the absorbance, even though the phosphorescence of these species could be observed.
Conclusion Transient spectroscopy provided clear features of individual triplet species, which had been observed in the steady-state excitation spectra as a superimposedform of each component. A striking result is that these species have profiles nearly identical with those of the PDBVCz polymer measured in MTHF glass.
7612 The Journal of Physical Chemistry, Vol. 98, No. 31, I994
We thought that the DBCz chromophores attached to the methacrylate copolymer would behave more intermolecularly, because they are linked to the main chain with a few spacer atoms. But, the result indicated that DBCz chromophores in both polymers (PDBVCz and DBCzEMA copolymers) have a quite similar configuration in the excimer state. This probably arises from the conformational restriction of bromine atoms on the way of mutual approach of aromatic rings. Consequently, this chromophore showed two distinct excimer states in the copolymer films. It was difficult to determine precise time constants for the energy flow between these triplet species, because the intense laser excitation caused rapid T-T annihilation of mobile triplet excitons. As seen in the T-T absorption measurement, most of the triplet energy dissipates through annihilation processes. However, one could see the alteration of triplet species with time from the triplet monomer to excimer states, and the process was accelerated by thermal activation. These results are consistent with the kinetic scheme that iterative trapping and detrapping processes determine the triplet state kinetics in polymer s01id.~
Acknowledgment. This work was financed by the Japan-US Cooperative Science Program of Japan Society for the Promotion of Science (JSPS). The authors at Nevada acknowledge support by the U S . Department of Energy (Grant No. DE-FG0392ER45476) and by the NSF US-Japan Cooperative Research Program.
Ito et al.
References and Notes (1) (a) Kldpffer, W.; Fisher, D. J. Polym. Sci. Part C 1973,40,43. (b) Rippen, G.; Kaufmann, G.; KIbpffer, W. Chem. Phys. 1980, 52, 165. (c) Kldpffer, W. Chem. Phys. 1981,57, 75. (d) Klbpffer, W. Ann. N . Y.Acad. Sci. 1981, 366, 373. (2) Kldpffer, W. EPA " V I . 1987, 29, 15. (3) (a) Burkhart, R. D.; Aviles, R. G. J . Phys. Chem. 1979, 83, 1897. (b) Burkhart, R. D. Macromolecules 1983, 16, 820. (c) Burkhart, R. D.; Dawood, I. Macromolecules 1986,19,447. (d) Chakraborty, D. K.; Burkhart, R. D .J. Phys. Chem. 1989,93,4791. (e) Burkhart, R. D.; Chakraborty, D. K. J. Phys. Chem. 1990,94,4143. ( f ) Chakraborty, D. K.; Burkhart, R. D. Macromolecules 1990, 23, 121. (4) (a) Ito, S.; Yamashita, K.; Yamamoto, M.; Nishijima, Y. Chem. Phys. Lerr. 1985, 117, 171. (b) Ohmori, S.; Ito, S.;Yamamoto, M. Ber. Bunsen-Ges. Phys. Chem. 1989,93, 815. ( 5 ) (a) Ito, S.; Katayama, H.; Yamamoto, M. Macromolecules 1988, 21, 2456. (b) Ito, S.; Numata, N.; Katayama, H.; Yamamoto, M. Macromolecules 1989,22, 2207. (c) Katayama, H.; Ito, S.; Yamamoto, M. J. Photopolym. Sci. Technol. 1991,4217. (d) Katayama, H.; Tawa, T.; Ito, S.; Yamamoto, M. J. Chem. SOC.,Faraday Trans. 1992, 88, 2743. (e) Katayama, H.; Tawa, T.; Haggquist, G. W.; Ito, S.; Yamamoto, M. Macromolecules 1993, 26, 1265. (6) Yokoyama, M.; Funaki, M.; Mikawa, H. J. Chem. SOC.,Chem. Commun. 1974, 372. (7) Starzyk, F. C.; Burkhart, R. D. Macromolecules 1989, 22, 782. (81 Wada. Y.: Ito. S.: Yamamoto. M. J. Phvs. Chem. 1993. 97. 11164. (9j Haggquist, G.'W:; Burkhart, R.D. J. Phys. Chem. 1993,97,2576. (10) Lim, E. C. Acc. Chem. Res. 1987, 20, 8 . (11) Waldmann, J.; Von Schiitz, J. U.; Wolf, H. C. Chem. Phys. 1985,
92, 1. (12) (a) Locke, R. J.; Lim, E. C. Chem. Phys. Leu. 1987,134, 107. (b) Locke, R. J.; Lim, E. C. J. Phys. Chem. 1989, 93,6017. (13) Schweitzer,D.;Colpa, J.P.;Behnke, J.;Hausser,K. H.;Haenel, M.; Staab, H. A. Chem. Phys. 1975, 1 1 , 373. (14) Chandra, A. K.; Lim, E. C. Chem. Phys. Lerr. 1977,45,79. (15 ) Pauling, L. The Narure of Chemical Bond; Cornell University Press: Ithaca, NY, 1960.
