11164
J . Phys. Chem. 1993,97, 11 164-1 1167
Phosphorescence Spectroscopy for Poly[2-(3,6-dibromo-9-carbazolyl)ethylmethacrylate] and Its Copolymers with Methyl Methacrylate Yoshio Wada Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Sakyo, Kyoto 606, Japan Shinzaburo Ito and Masahide Yamamoto' Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Sakyo, Kyoto 606, Japan Received: August 30, 1993"
Our research project concerned to triplet spectroscopy of 3,6-dibromocarbazole chromophores in a methacrylate copolymer, poly[2-(3,6-dibromo-9-carbazolyl)ethyl methacrylate-co-methyl methacrylate]. Owing to the "heavy atom effect" on the rate of singlet-triplet intersystem crossing, the phosphorescence spectra were clearly observed in a wide range of copolymer compositions. Although the triplet interaction between chromophores was weak in 2-methyltetrahydrofuran rigid glass, the copolymer films showed an alteration in spectra from the monomer to the triplet excimer emission with increasing chromophore concentration. The substituted bromine atoms provided a longer interaction distance between chromophores, and a weaker stabilization of energy at the excimer state, compared with unsubstituted carbazole. These effects were interpreted in terms of the extended electron cloud and the bulkiness of bromine atoms. Introduction Photophysical processes of poly(9-vinylcarbazole) (PVCz) have been the subject of many investigations. In the triplet state as well as in the singlet state, carbazole chromophore is said to form an excimer which is an excited dimer between an excited chromophore and a ground-state one.' Burkhart et al. showed the existence of several triplet species in PVCz film and have studied their behavior in a wide range of temperatures.2 On performing triplet spectroscopy,one often meets with difficulties owing to the weak phosphorescence, which sometimes gives rise to controversy about the existenceof triplet excimer in solution.394 The substitutionof heavy atoms to aromatic rings causes drastic enhancementof the singlet-triplet intersystemcrossing and makes the observation much easier as fluorescence spectroscopy. As for poly(3,6-dibromo-9-vinylcarbazole)(PDBVCz), Yokoyama et al. reported triplet excimer phosphorescence in the polymer system,s and recently, Starsyk and Burkhart reinvestigated the same polymer in detail using time-resolving technique.6 From a view of polymer photophysics, dibromocarbazole (DBCz) is a particular chromophoreproviding an unambiguous indication of triplet excimer. We have been studying the triplet-state behavior in polymer systems, using copolymers of a chromophoric monomer with a spectroscopicallyinert monomer? The use of copolymers allowed us to make the chromophore concentration high enough to see interchromophore interactions, but keeping chromophores in a statistically uniform distribution. This method enabled us to observe triplet interactions as a function of chromophore concentration, which is an important experimental variable in the study of polymer photophysics. An example is poly(carbazoly1ethyl methacrylate) (PCzEMA) and its copolymers with methyl methacrylate. In previous work,8 we reported that this polymer is free from steric restriction intramolecularly imposed by the so-called n = 3 rule,9 which usually results in efficient excimer formationin vinyl aromatic polymers. The pendant chromophore of the methacrylate polymer is linked to the main chain with a few spacer atoms. The triplet exciton behaves more intermolecularly, and the study in this system gave a lot of insight into the triplet-state processes involving excimer formation.
* Abstract published in Aduance ACS Abstracts, November 1, 1993.
In the current study, the copolymerization method is again applied to the 3,6-dibromocarbazole chromophore, which is introduced into a methacrylate polymer as an ethyl ester form. The triplet behavior in a frozen glass solution and in the film state has been investigated. Comparison with the result of PCzEMA is expected to clarify the effect of heavy atoms on the triplet photophysics. Experimental Method Materials. 3,6-Dibromo-9-(2-hydroxyethyl)carbazole(4. 9-(2-Hydroxyethyl)carbazole (II)was synthesized from carbazole and ethylene carbonate by refluxing in dimethylformamide (DMF) for 4 h with trace amounts of NaOH. The reaction
I
UI
mixture was extracted with benzene and washed with water several times. After removing the solvent, recrystallization from a mixed solvent of hexane and benzene gave needles of 11: yield 64%. Bromination of I1 was carried out by the method in ref 10. The product was recrystallized from ethanol: yield 34%. 2-(3,6-Dibromo-9-carbazolyl)ethyl Methacrylate (DBCzEMA) (m.The monomer, III, was synthesized by esterification of I with methacryloyl chloride (Aldrich)in dry 1,Zdichloroethane and pyridine (1O:l) with trace amounts of hydroquinone. The solution was extracted with benzene and washed with dilute hydrochloric acid and water several times. After removing the solvent, the crude product was purified by recrystallization from benzene: yield 63%; mp 170-172 OC. 1H-NMR (CDC13): d 1.79 (s, 3 H), 4.51 (s, 4 H), 5.48 (m, 1 H), 5.89 (s, 1 H), 7.238.11 (m, 6 H). Anal. Calcd for C18H15Br~N02(437.13): C, 49.45; H, 3.46; Br, 36.56; N, 3.21; 0, 7.32. Found: C, 49.62; H, 3.52; Br, 36.40; N, 3.19; 0, 7.38.
0022-365419312097-11164$04.00/0 0 1993 American Chemical Society
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The Journal of Physical Chemistry, Vo1. 97, No. 43, 1993 11165
TABLE I: Composition, Concentration (0of the Copolymer
Film, Average Distance (0)between Chromophores in Film, and Relative Molecular Weight (M,) of the Copolymer sample no. content (mol 5%) C (unit mol L-1) D (nm) Mwa (103) 1 2 3 4 5 6 7
P( DBCZEMA-CO-MMA)~ 0.1 0.012 5.18 4.2 0.44 1.56 5.9 0.59 1.41 7.2 0.69 1.34 9.4 0.86 1.25 1.21 1.11 15.2 2.75 0.85 100
52 40 28 28 31 31 9.6
P(CzEMA-c&MMA) 0.14 2.28 87 2 0.49 1.50 100 3 0.95 1.20 loo 4 1.44 1.05 104 5 2.42 0.88 113 0 Determined by GPC with reference to standard polystyrene. * The concentration and average distance were calculated assuming the film density is 1.20. 1
1.2 4.4 9.2 15.3 31.1
Copolymers. Methyl methacrylate (MMA) (Wako Pure Chemical Industries, Ltd.) was purified by distillation under a reduced pressure before use. These monomers, DBCzEMA and MMA, were dissolved in benzene with small amounts of azobis(isobutyronitrile) and polymerized at 60 OC. Obtained copolymers, P(DBCzEMA-co-MMA), were purified by repeated reprecipitation from benzene to methanol and dried in vacuo. Table I shows the compositions of the copolymers and molecular weights determined by GPC. 2-Methyltetrahydrofuran (MTHF).The purchased MTHF (Aldrich) was purified by an alumina column and vacuum distillation after preliminary distillation over sodium metal. SamplePreparation. Spectroscopicmeasurementswere carried out for the degassed solutions and films. To prepare the film sample, the copolymer was dissolved in dichloromethane (Spectrophotometric grade, Dojindo Laboratory) in a quartz cell or a Pyrex tube, and the solution was deaerated by freeze-pump thaw cycles. The copolymer film was cast onto the inside wall of the cell by gradual evaporation of the solvent under a reduced pressure. After keeping in vacuo for a day, the cell was sealed. Since the homopolymer, PDBCzEMA, was insoluble to dichloromethane, distilled THF was used as a cast solvent. MTHF solution of the copolymer was prepared in a Pyrex tube at a concentration of 1 X 10-4 DBCz unit mol L-' and then degassed by several freeze-pumpthaw cycles under a pressure less than 1 X le5Torr. Measurements. Absorption spectra were measured by a Hitachi Model 200-20 spectrophotometer. Fluorescence and phosphorescence spectra were recorded with a Hitachi MPF-4 spectrophotometer fitted with a phosphorescence attachment. The sample cell was set in a quartz Dewar and cooled by liquid nitrogen. Phosphorescencedecay curves were measured by a nitrogen laser (NDC Co., JH-500) and a gated photomultiplier (Hamamatsu, R1333). The details were described else~here.'~
Results and Mscussion A series of previous works for copolymer films containing chromophoricside groups indicated that the triplet-state behavior of chromophoresin the solid films could be presented well by the interchromophoredistance? The fourth column in Table I shows the mean distance, D,between chromophores evaluated from the density of the film. We have already reported triplet behavior of carbazolylethyl methacrylate copolymers (P(CzEMA-coMMA)) which has the same molecular structure except for the bromine atoms.70 In order to discuss the effect of the heavy atoms on the triplet properties, the compositions and relevant values for P(CzEMA-co-MMA) films are also listed in Table I.
Wavelength I nm
Figure 1. Total emission spectra of sample 2of P(DBCzEMA-co-MMA) in MTHF at 77 K. The sample was excited at 310 nm. The spectra were recorded with a bandwidth of 2 nm.
The chromophore concentration, C, and the average distance, D, are able to be good measures of the triplet interaction in these films. Figure 1 shows total emission spectra of a copolymer sample, sample 2 of P(DBCzEMA-co-MMA), in MTHF at 77 K, which consist of weak fluorescence and intense phosphorescence. The fluorescence intensity is very weak as expected, because the bromine atoms cause efficient singlet-triplet (SI-TI) intersystem crossing. The S l S o band appears at 365 nm, which is shifted ca. 15 nm longer than the band of carbazole chromophore. This shift is probably attributed to the effect of the substitution at the 3,dpositions. Similar effects have been observed for alkyl substituted carbazole" and for PDBVCZ.~The 0-0 band of phosphorescence spectra ( T I S O )appears at 416 nm. The peak positions can be regarded as the intrinsic character of an isolated DBCz chromophore and are employed hereafter as a criterion of the magnitude of intermolecular interaction. Figure 2 shows phosphorescence spectra of the copolymers in MTHF rigid glass. The 0-0 band for all the copolymers locates at the same wavelength, 416-417 nm, and there is no change with the increase of DBCz content. However, the homopolymer, PDBCzEMA, showed a slightly red-shifted spectrum, indicating the existence of weak interaction between chromophores. Yokoyama et al.5 and, later, Starzyk and Burkhart6 reported phosphorescence spectra of PDBVCz, showing a single broad band due to triplet excimer formation. In the case of PVCz, the chromophores are connected directly to the polymer chain, and therefore, they are sterically forced to interact with each other. The strong interaction provides at least two kinds of excimers in the singlet ~ t a t e , ' ~and J ~ several emissive species from triplet excimer have been reported by KliSpfferl and Burkhart et al.2 This is also the case for PDBVCZ.~Compared with the spectra of PDBVCz, the interaction in the current polymer, PDBCzEMA, is rather weak and causes only a slight shift of the spectra to a longer wavelength of about 3 nm. We call this species 'shallow trap" and distinguish it from triplet e ~ c i m e r . ' ~Our previous work demonstrated that the methacrylate polymers, where carbazole moieties are separated from the main chain by a few spacer atoms, did not form singlet excimer.* This structural modification also makes the chromophore free from strong interaction in the triplet state and just keeps it at a shallow trap even in the homopolymer. We could see intramolecular processes in a MTHF rigid glass. On the other hand, in condensed phases such as polymer films, the photophysicsis governed by mainly intermolecular processes, including energy migration and excimer formation. Figure 3 shows phosphorescence spectra of the copolymer films. The spectra clearly become broader with increasing DBCz concentration, and the feature alters to excimeric spectra for the
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11166 The Journal of Physical Chemistry, Vol. 97,No. 43, 1993
TABLE Ik Wavelength of the 0-0 Band of Phosphorescence sample concn (unit mol L-') X(MTHF) (nm) A(film)(nm) P(DBCZEMA-W-MMA)" 416 417 0.012 (5.18)b 417 423 0.44( 1.56) 416 424 0.59 (1.41) 416 425 0.69 (1.34) 417 433 0.86 (1.25) 417 462b 1.21 (1.11) 419 475-485b 2.75 (0.85) P(CzEMA-co-MMA) 412 412 0.14 (2.28) 0.49 (1.50) 414 0.95 (1.20) 422 480-4Ub 1.44 (1.05) 2.42 (0.88) 505-515b a Wavelength at the maximum of excimeric emission. The values in parenthcses are the average distance, D, between chromophores in nanometers.
400
500
600
Wavelength I nm
Figure 2. Phosphorescence spectra of P(DBCzEMA-co-MMA) and PBDCzEMA in MTHFat 77 K. For numbers, see Table I. The samples
were excited at 358 nm.
L-A
1 Ol0
20 t I ms
40
Figure 4. Phosphorescencedecay curves of P(CzEMA-co-MMA)films observed at 420 nm at 77 K. For numbers, see Table I. The samples were excited at 337 nm.
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400
I
500
6
I
Wavelength I nm
Phosphorescencespectra of P(DBCzEMA-co-MMA)films at 77 K. For numbers, see Table I. The samples were excited at 358 nm.
Figure 3.
concentration higher than 0.86 unit mol L-1 (sample 5 ) . Here, two particular characteristics should be noted. One is the smaller shift of excimer band from the monomer phosphorescence band
in comparison to the unsubstituted carbazole chromophore. Table I1 shows the peak wavelength of the triplet excimer. The value for the homopolymer, PDBCzEMA, was found around 475-485 nm, which is much shorter than that of P(CzEMA-co-MMA) films (see Table 11). This means that the stabilization energy of the triplet excimer is smaller than the carbazole chromophore. Although the exact reason for this phenomemon is not clear a t the present stage, it is reasonable to suppose that the bulkiness of bromine atoms prevents the carbazole chromophores from taking an appropriateconfiguration for the stable triplet excimer. The van der Waals radius of the bromine atom is known to be ca. 0.2 nm, which is comparable with that of the methyl group.14 The second important characteristic is that the broadening of phosphorescencespectra can be seen from fairly low concentration samples. Table I1shows the 0-0band of the monomer and shallow trap emission observed for various contents of copolymer films. Even for the 0.44 unit mol L-1 (D = 1.56 nm) film, the spectra show broadening in Figure 3. Figure 4 presents the phosphorescence decay curves a t a wavelength of the monomer emission. A faster decay was observed from this concentration, indicating the existence of interchromophore interaction and quenching of the monomer triplet state. Taking into account the fact that CzEMA copolymers begin to interact at a higher concentration (0.95 unit mol L-1, D = 1.20 nm),'* it is safely said that the interaction distance of DBCz is longer than the carbazole chromophore. In the triplet state, predominant is a short-range
Letters interaction involving a coupling of the molecular orbital of each chromophore. The extended electron cloud due to the attached bromine atoms seems to be responsible for the longer interaction distance of the DBCz triplet. In summary, the substitution with the heavy atoms drastically enhancedboth theSI-TI and TlSointersystem crossings, yielding the strong phosphorescence intensity from the carbazole moieties. Besides this, as for the interchromophore interaction, bromine atoms provided a longer interaction distance and a weaker stabilization energy at the excimer state. These results were interpreted in terms of the bulkiness and the extended electron cloud of the bromine atoms. Kinetic analysisusing time-resolved technique is needed for discussing the mechanism in detail.
References and Notes (1) (a) Klbpffer, W.; Fischer, D. J . Polym. Sci., Part C 1973,40,43. (b) Rippen, G.; Kaufmann, G.; Klbpffer, W. Chem. Phys. 1980, 52, 165. (c) Klbpffer, W.Chem. Phys. 1981,57,75. (d) Klbpffer, W. Ann. N.Y. Acad. Sci. 1981, 366, 373. (2) (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. 1989,93,4797. (e) Burkhart, R. D.; Chakraborty, D.
The Journal of Physical Chemistry, Vol. 97, No. 43, 1993 11167 K. J . Phys. Chem. 1990,94,4143. (f) Chakraborty, D. K.; Burkhart, R. D. Macromolecules 1990, 23, 121. (3) Lim, E. C. Acc. Chem. Res. 1987, 20, 8. (4) Nickel, B.; Prieto, M. F. R. 2.Phys. Chem. (Munich) 1986,150,31. (5) Yokoyama, M.; Funaki, M.; Mikawa, H. J. Chem. Soc., Chem. Commun. 1974, 372. ( 6 ) Starzyk, F. C.; Burkhart, R. D. Macromolecules 1989, 22, 782. (7) (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. 1991,4, 217. (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. (8) (a) Ito, S.;Yamashita, K.; Yamamoto, M.; Nishijima, Y. Chem. Phys. Lett. 1985, 117, 171. (b) Ohmori, S.;Ito, S.;Yamamoto, M. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 815. (9) Hirayama, F. J . Chem. Phys. 1965, 42, 3163. (10) Inami, A.; Morimoto, K.; Murakami, Y. Nihon Kagaku Zasshi 1964, 85, 880. (1 1) (a) Ito, S.;Takami, K.; Yamamoto, M.Makromol. Chem., Rapid Commun. 1989, I O , 79. (b) Ito, S.; Takami, K.; Tsujii, Y.;Yamamoto, M. Macromolecules 1990, 23, 2666. (12) Johnson, G. E. J. Chem. Phys. 1975,62,4697. (13) Itaya, A.; Okamoto, K.; Kusabayashi, S.Bull. Chem.SOC.Jpn. 1976, 49, 2082. (14) Pauling, L. The Nature ofrhe Chemical Bond; Cornell University: Ithaca, NY, 1960.
