J. Phys. Chem. 1900, 84, 2363-2368
and in the absence of olefin
2363
phorescence data in the presence of sensitizer to determine
X. References and Notes Research carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences. Hammond, 0. S.; Saltlei, J.; Lamoia, A. A.; Turro, N. J.; Bradshaw, J. S.; Cowan, D. 0.; Counsell, R. C.; Vogt, V.; Dalton, C. J . Am. Chem. SOC. 1964, 86, 3197-217. Cowan, D. 0.; Drisko, R. L. “Elements of Organic Photochemistry”; Plenum: New York, 1976; pp 367-88. Turro, N. J. “Molecular Photochemistty”; Benjamin: New Yolk, 1965 pp 92-123. Wamser, C. C.; Medary, R. T.; Kochevar, I. E.; Turro, N. J.; Chang, P. L. J. Am. Chem. SOC. 1975, 97, 4864-9. Berends, W.; Posthuma, W. J. Phys. Chem. 1962, 66, 2547-50. Posthuma, J.; Berends, W. Biochim. Biophys. Acta 1966, 112, 422-35. E m k e v , V. L.; Grudev, V. P.; Tachin, V. S. Izv. Akad. Nauk SSSR, Ser. Fiz. 1972, 36, 984-7. Bodunov, E. N. IbH. 1972, 36, 996-9. Bodunov, E. N. Opt. Spektrosk. 1973, 34, 490-6. Ruggli, P.; Grun, F. He&. Chim. Acta 1941, 24, 197-212. Tadros, W.; Latlf, A. J. Chem. SOC. 1949, 3337-40. Perkin, A. G. J . Chem. SOC. 1063, 43, 187-94. Merer, A. J.; Muliiken, R. S. Chem. Rev. 1969, 69, 639. Reference 3, p 370. I n the case of the piperylenes, however, the triplet energy of the cis isomer is 1.9 kcai lower than that of the trans isomer.* Porter, G.; Suppan, P. Trans. Faraday SOC. 1965, 61, 1664-73. Nagakura, S. J. Chim. Phys. phys.-Chim. Bioi. 1964, 60, 217-21. Morita, H.; Fuke, K.; Nagakura, S. Bull. Chem. Soc. Jpn. 1977, 50, 645-9. Latajka, 2.; RataJczak, H. Chem. Phys. Lett. 1977, 49, 407-9. Morokuma, K.; Iwata, S.; Lathan, W. A. “The World of Quantum Mechanics”; Daudel, R., Pullman, B., Eds.; Reidel, Dordrecht: Holland, Netherlands, 1974; pp 277-316. Isaacson, A. D., private communication, 1979. Wiikinson, F. J. Phys. Chem. 1962, 66, 2569-74. Lamola, A. A.; Hammond, G. S. J. Chem. phys. 1965, 43,2129-35. Greenzaid, P.; Luz, 2.; Samuel, D. J. Am. Chem. SOC. 1967, 89, 749-56.
where a’ = -- 10-e’(B)l).r$lISc is the intersystem crossing efficiency of biacetyl and E’ is its extinction coefficient. Dividing eq A14 by A15 and equating the ratio of the concentrations of triplet biacetyl molecules to the ratio of phosphorescence intensities yields eq A16. Al-
though generation of 3Bis here through direct irradiation, the result above for itis quenching is, of course, independent of its mode of generation. In actuality the phosphorescence due to direct excitation of biacetyl in the presence of bis(p-(trimethy1ammonio)phenyl)methanonebis(bisulfate) a t the concentrations used for these quenching studies is only about 0.02% of the biacetyl phosphorescence resulting from energy transfer from the sensitizer. Equation A13 can be expressed as
P O .- = [:I + X(OO)][l + Y(OO)] P
where the quantity in the right-hand bracket represents a small correction due to energy transfer from biacetyl to olefin. Y is determinod separately in Stern-Volmer “plots” in the absence of sensitizer. Once determined, eq A17 can be rearranged to eq A18, which is used with the phos-
--P[l
PO
+ Y(OO)]- 1 + X(00)
Laser Photochemistry of Poly(N-vinylcarbarole) in Solution Hiroshl Masuhara, Satoshi Ohwada, Noboru Mataga, Deparfment of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Akira Itaya, Ken-lchl Okamoto, and Shlgekazu Kusabayashl Deparfment of Chemical Engineering, Faculty of Engineering, Yamaguchi University, Ube 755, Japan (Receivd: January 25, 1980)
The primary photoprocesses of N-vinylcarbazole oligomers and polymers excited with a pulsed intense laser were studied in solution. The triplet-triplet absorbance, fluorescencespectra, lifetime, intensity, and quenching efficiency show a distinct dependence upon excitation intensity and degree of polymerization. From these measurements it was concluded that several fluorescent states are produced in one polymer chain and their mutual interaction leads to efficient deactivation. One remaining fluorescent state is converted to the triplet state with a yield characteristicof the particular polymer. Some of these fluorescent chromophores are converted to the cation by a quenching reaction involving the electron acceptor, dimethyl terephthalate. This ionic photodissociation results in formation of a transient polyelectrolyte in organic solvents.
Introduction The majority of our studies of primary photoprocesses of aromatic molecular systems have been in dilute homogeneous solution. However, it is unclear whether these concepts hold for systems of practical importance such as organic synthetic reactions, solar cells, organic photoconductors, and biological photosynthesis. In these systems the local concentration of chromophores is high, and their distribution nonuniform. In order to understand such systems it is necessary to investigate the primary photo0022-3654/80/2084-2363$0 1.OO/O
processes of molecular associate systems in solution. From this viewpoint we have initiated laser photolysis studies in solution on polymers with pendant aromatic hydrocarbon gr0ups.l Using such polymers one can find it possible to observe photoprocesses involving molecular associates under the usual optical condition of laser photolysis. The present paper deals with poly(N-vinylcarbazole) (abbreviated as PVCz-n, where n is the mean degree of polymerization), which was selected for the following reasons: (i) it is a commercially useful polymer: 0 1980 American Chemical Society
2304
The Journal of Physical Chemistry, Vol. 84, No. 19, 1980
TABLE I: Calculated Concentration of Polymer, Carbazole Chromophore, and the Triplet Chromophore of EtCz and PVCz-n in N,N-Dimethvlformamidea [polymer], samples M 1.5 x EtCzb PVCz-4 6.0 X PVCz-17 1.5 X los5
[CZ],M ~ [Tz],M ~ 1.5 X 4.8 x 2.4 X 2.3 X 2.6 X PVCz-17 > PVCz-394. This tendency is opposite to the inner filter effect expected from Figure 3, and the depletion effect (iii) is considered to be the more possible factor. However, the u value of the present compounds decreases as the degree of polymerization increases, predicting that curves of PVCz-n with a high degree of polymerization should approach linearity. Instead, saturation increases in the following order: PVCz-394 > PVCz-17 > PVCz-4 N EtCz. Any other process appears to be operating in addition to the depletion effect. 3. Interactions between Excited Carbazole Chromophores Bonded t o the Same Polymer Chain. It is con-
The Journal of Physical Chemistty, Vol. 84, No. 19, 1980 2367
Laser Photochemistry of Poly(N-vinylcarbazole)
~
sidered that interactions between excited states bonded to the same polymer chain are involved in the present polymer systems. A number of excited states are produced in one polymer chain by intense laser excitation, and these can migrate along the chain and interact with each other, providing an efficient deactivation mechanism. The following results support this scheme. (i) As mentioned above, most of the EtCz in the ground state is excited by the normal power of the present photolysis condition. Since the carbazole chromophores are connected to the same polymer chain, several excited states are produced in one chain. (ii) The polarization degree of fluorescence for N-isopropylcarbazole, 1,3-bis(N-carbazolyl)propane,and PVCz were measured in a rigid glass at 77 Ke4The polarization degree of polymers is smaller than that of monomer and dimer model compounds, indicating that singlet energy migration occurs efficiently along the polymer chain even a t low temperature. (iii) Formation and decay processes of the second and sandwich excimer states6B2may be affected by interaction between these states, which leadsito a change of relative intensity of both excimer emissions. This is clearly demonstrated by the excitation intensity dependence of fluorescence spectra (Figure 2). (iv) From the studies on molecular crystals and concentrated solutions of aromatic hydrocarbons,33interaction between the excited states accelerates fluorescence decay. The measured lifetimes of the long-lifetime component of PVCz-17 and PVCz-394 (Table 11)can be explained well from this viewpoint. The short-lifetime component observed at 375-380 nm (second excimer state) is determined by the decay of an excitation pulse and is independent of excitation intensity. Since this excimer migrates rapidly, its interaction with another second or sandwich excimer may be complete within an excitation pulse width. (v) When an electron acceptor is added to the present polymer systems, the interaction between excited states is replaced to a certain extent by quenching processes. In this case, the main deactivation mechanism is given in eq 1-4. Here Soand SI are the ground and fluorescent states, Si So + hv (1)
-
-+ + - + + s1 + s1 - s, + so - s1 + so SI
SI Q
So
So
heat
Q or
S* Q’
(2)
(3) (4)
respectively, and Q represents a quencher molecule. If one defines the yield of process 1 with 6, the quenching efficiency for weak and intense excitation intensity is given by eq 5 and 6. Here a and a’ represents the yield of
Q, = V‘o Q, =
- PJ’o)/J’o = 1 - P
Fo(1 - a) - Fo(1- a’)P
-Fo(1. - a)
=I-(-)@
(5) 1 - a’ 1-a
(6)
process 4 without and with quencher, respectively. Because of the relation a’ < a,&, is greater than or equal to Q,, as confirmed in the present work. Furthermore the increase of quencher concentration leads P and a’values to zero, resulting in equal values of Q, and QP This is consistent with the results in Table 111. On the basis of the above considerations, it is concluded that there arises interaction between fluorescent states produced in the same polymer. The probability that the excited state degrades through this interaction increases with the degree of polymerization. Therefore the total
a
b
@ ...’ ..
.....I
...’
A
I
I
A-
Flgure 4. Schematic diagram showing photoprlmary processes of polymers wlth pendant carbazolyl groups: carbazole (0), excited carbazole (O), carbazole cation (O’), electron acceptor (A), and acceptor anion (A-); (a) quencher-free system, (b) quencher-containing system.
number of the resultant singlet states decreases with increasing molecular weight. Since the concentration of the triplet state is proportional to that of the excited singlet state, the polymer effect on the triplet yield is due to this interaction. The concentration ratios of triplet to polymer are calculated and listed in the last column of Table I. These values show a rather small scatter. The efficient interaction between excited chromophores leads to one fluorescent state, which is converted to the triplet state with a yield determined by the relative contribution of the two excimer states. 4. Photoinduced Formation of Transient Polyelectrolyte. Under high-intensity excitation, the formation of ionic species is complicated, since interaction between excited chromophores and depletion of the ground state operate. In this section the concentration ratio of the ionized chromophore to the polymer is considered apart from the mechanism. As listed in Table IV, 90% of EtCz is ionized, and PVCz-4 and PVCz-17 have about two and four carbazole cations, respectively. In the case of PVCz-394, ca. 18% of the chromophores are converted into cationic species immediately after excitation. Namely, PVCz-n polymers have several carbazole cations and their numbers increase with the degree of polymerization. Although conversion efficiency to ions is rather low, it is proposed that a kind of polyelectrolyte with a lifetime of the order of microseconds is produced under intense laser excitation. Contrary to the excited states of quencher-free systems, ionized chromophores produced by photochemical electron transfer remain rather stable. Energy migration and interaction between excited states are determined by dipole-dipole effects, while transfer of the positive charge between carbazole chromophores is due t o an exchange interaction. The former interaction has much stronger than the latter. Moreover, ionized chromophores may be localized on one site of the polymer chain because hole transfer requires solvent reorientation and segment motion. Thus transient polyelectrolyte formation is possible under the present experimental conditions. These electronic processes, schematically shown in Figure 4, are characteristic of polymers and laser excitation and reported for the first time here.
2368
J. Pbys. Cbem. 1980, 84, 2368-2374
From the present work and from related studies on ionic p h o t o d i s s o ~ i a t i o nthe , ~ ~conditions ~ ~ ~ ~ favoring formation of a transient polyelectrolyte are as follows: (i) the absorption cross section of the chromophore attached to the polymer chain at the excitation wavelength is large; (ii) energy migration between chromophores is slow; (iii) the solvation process to free ions is very rapid; and (iv) any kind of radiationless transition other than ionic dissociation occurs with a small rate constant. Further studies of these phenomena are in progress.
Acknowledgment. The cost of the present investigation was partly defrayed by Matsunaga Science Foundation and the grant-in-aid from the Ministry of Education. We express our sincere thanks to Mr. Yasuyuki Seki for his help. Thanks are due to Professor Masahide Yamamato (Kyoto University), Dr, Akira Kira (Institute for Physical and Chemical Research), Dr. Sada-aki Yamamoto (Tohoku University), and Dr. Seiichi Tagawa (Tokyo University) for communicating their data to us. Thanks are also due to the reviewers for their advice on English usage and comments. References and Notes (1) H. Masuhara, S. Ohwada, N. Mataga, A. Itaya, K. Okamoto, and S. Kusabayashi, Cbem. Pbys. Lett., 59, 188 (1978). (2) K. Okamoto, S. Kusabayashi, M. Yokoyama, K. Kato, and H. Mikawa, Electropbotogr., Int. Conf., Znd, 7973, 152 (1974). (3) K. Okamoto, S. Kusabayashi, and H. Mikawa, Bull. Cbem. Soc. Jpn., 46, 1953, 2324, and 2613 (1973); M. Yokoyama, Y. Endo, and H. Mikawa, /bid., 49, 1538 (1976); M. Yakoyama, M. Hanabata, T. Tamamura, T. Nakano, and H. Mikawa, J . Cbem. Pbys., 87, 1742 (1977). (4) A. Itaya, K. Okamoto, and S. Kusabayashi, Bull. Cbem. SOC.Jpn., 49, 2082 (1976). (5) A. Itaya, K. Okamoto, and S. Kusabayashi, Bull. Cbem. SOC.Jpn., 50, 22 (1977). (6) G. E. Johnson, J . Cbem. Phys., 81, 3002 (1974); 82, 4697 (1975); 83, 4047 (1975). (7) G. Pflster, D. J. Williams, and G. E. Johnson, J. Pbys. Cbem., 78, 2009 (1974); T. Ishli, Y. Utena, and T. Handa, Rep. Prog. Polym. Pbys. Jpn., 19, 399 (1976). (8) C. E. Hoyle and J. E. Guiliet, Macromolecules, 8, 713 (1975). (9) K. Okamoto, M. Oreki, A. Itaya, S. Kusabayashi, and H. Mikawa, Bull. Cbem. SOC.Jpn., 48, 1362 (1975). (10) (a) H. Masuhara, M. Shimada, N. Tsujino, and N. Mataga, Bull. Chem. SOC.Jpn., 44, 3310 (1971); (b) J. Hinatu, H. Masuhara, N. Mataga,
Y. Sakata, and S. Misumi, ibid., 51, 1032 (1978). (1 1) T. Hayashi, T. Suzuki, N. Mataga, Y. Sakata, and S. Misumi, Chem. Pbys. Lett., 38, 599 (1976). (12) J. S. Brinen in “Molecular Luminescence”, E. C. Lim, Ed., W. A. Benjamin, New York, 1969, p 333. (13) H. Labhart and W. Heinzelmann, Org. Mol. Photophys., 7973- 1975, 1, 297 (1973). (14) S. Yamamoto, private communication, 1976. (15) A. Kira, private communication, 1976. (16) The cationic species produced in polymers is expected to give rather broadened or shifted spectra. See S.Tagawa, M. Washio, Y. Tabata, and M. Imamura, Abstracts for 26th IUPAC Congress Tokyo, Session V, 1977. (17) M. Yamamoto, private communication, 1976. (18) J. R. McDonald, W. E. Echols, T. R. Price, and R. B. Fox, J . Chem. Pbys., 57, 1746 (1972). (19) K. Hayashi, M. Irie, J. Kiwi, and W. Schnabel, Polym. J., 9, 41 (1977); J. Kiwl and W. Schnabel, Macromolecules, 8, 430 (1975). (20) T. Ohno and S.Kato, Chem. Lett., 263 (1976). (21) M. Higuchi, T. Nakayama, and N. Itoh, J. Pbys. SOC.Jpn., 40, 250 (1978). (22) J. M. Morris and K. Yoshihara, Mol. Pbys., 38, 993 (1978). (23) R. B. Cundall, L. C. Pereira, and D. A. Robinson, Cbem. Pbys. Lett., 13, 253 (1972). (24) R. R. Hentz and R. M. Thibauit, J. Pbys. Cbem., 77, 1105 (1973). (25) T. Medinger and F. Wilkinson, Trans. Faraday SOC.,82, 1785 (1966). (26) W. Helnzelmann and H. Labhart, Chem. Pbys. .Left., 4, 20 (1969). (27) C.R. GoldSchmidt in ”Laser in Physical Chemistry and Biophysics”, J. Joussot-Dublen, Ed., Elsevier, Amsterdam, 1975. (28) H. Masuhara and N. Mataga, Cbem. Pbys. Lett., 8, 608 (1970); Bull. Cbem. SOC. Jpn., 45, 43 (1972). (29) For example, M. M. Fisher, 8. Veyret, and K. Weiss, Cbem. Pbys. Lett ., 28, 60 (1974). (30) U. Lachish, A. Shafferman, and G. Stein, J. Chem. Pbys., 84, 4205 (1976). (31) R. N. Griffln, Pbotochem. Pbotobiol., 7, 175 (1968). (32) K. P. Ghiggino, R. D.Wright, and D. Phillips, Eur. Polym. J., 14, 567 (1978); C. E. Hoyle, T. L. Nemzek, A. Mar, and J. E. Guillet, Macromolecules, 11, 429 (1978); S. Tagawa, M. Washio, and Y. Tabata, Cbem. Pbys. Lett., 88 276 (1979). (33) N. A. Toistoi and A. P. Abramov, Sov. Pbys.-Solid Sfate (€ngl. Trans/.), 9, 255 (1967); S. D. Babenko, V. A. Benderski, V. I. Goldanski, A. G. Laurushko, and V. P. Tychinski, Pbys. Status Solidi B, 45,91(1971); A. Bergmann, M. Levine, and J. Jortner, Pbys. Rev. Lett., 18, 593 (1967); S. D. Babenko, V. A. Benderski, V. I. Gddanskl, A. G. Laurushko, and V. P.Tychinski, Cbem. Pbys. Lett., 8, 598 (1971); H. Masuhara and N. Mataga, ibld,, 7, 417 (1970); N. Nakashima, Y. Kume, and N. Mataga, J . Pbys. Cbem., 79, 1788 (1975). (34) H. Masuhara, T. Hino, and N. Mataga, J. phys. Chem., 79, 994 (1975); T. Hino, H. Akazawa. H. Masuhara, and N. Mataga, ibM., 80, 33 (1976); T. Hino, H. Masuhara, and N. Mataga, Bull. Cbem. SOC.Jpn., 49, 394 (1976); H. Masuhara, T. Saito, Y. Ma&, and N. Mataga, J . Mol. Struct., 47, 243 (1978); J. Hinatu, F. Yoshida, H. Masuhara, and N. Mataga, Cbem. Pbys. Lett., 59, 80 (1978).
Formation of Intramolecular Exciplexes in Electrogenerated Chemiluminescence. 2 Mikio Kawai,t Kingo
Itaya,”$ and Shinobu Toshimat
Depattment of Applied Chemistry, Faculty of Engineering, Tohoku Unlversity, Aramaki, Sendai, 980 Japan: Institute of Electrical Communication, Tohoku University, Katahira- 1, Sendai 980, Japan (Received: January 21, 1980)
Electrogenerated chemiluminescence (ecl) of intramolecular donor-acceptor compounds was examined in acetonitrile and acetonitrile-benzene mixtures. Anthracene, 10-phenylanthracene,and pyrene rings directly bonded to N,N-dimethylaniline,N,N-di-p-tolylaniline,and N,N-di-p-anisylaniline. High values of ecl yields (cp,, = 5-1%) were obtained in this series of compounds. The time dependence of ecl emission intensity of N,N-di-p-anisylanilinederivatives revealed that the reaction mechanism was S route (direct population of the singlet states of intramolecular exciplexes).
Introduction There have been numerous investigations in e ~ l l to -~ date since the first reports were published by Chandross et a1.4 and H e r ~ u l e s .Electron-transfer ~ reactions between
radical anions and cations of aromatic hydrocarbons (A) were initially investigated (denoted by A-./ A+. system). The reaction mechanisms of the A-./A+. system will be summarized in the following
t Department of Applied Chemistry, Faculty of Engineering, Tohoku University. Institute of Electrical Communication, Tohoku University.
S route
0022-3654/80/2084-2368$0 1.OO/O
A-* + A+*
IA* 0 1980 American
-+
+
‘A*
A
+ hv
Chemical Society
+A
(1)
