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Exterplex intermediate in the photodimerization of N-vinylcarbazole
17
J Photochem. Photobiol. A: Chem., 77 (1994) 17-21
Exterplex
intermediate
in the photodimerization
Akira Tsuchida and Masahide Yamamoto Lkpwtmentof Polymer Chemists, Graduate School of Engineering,
of IV-vinylcarbazole
Kyoto Universiry,Yoshida, Sakyo-lot, Kyoto 606 (Jopan)
(Received May 17, 1993; accepted July 8, 1993)
Abstract The
photocyclodimerization mechanism of N-vinylcarbazole (VCZ) via an exterplex intermediate is presented. In polar solvents, such as acetonitrile, VCZ efficiently photcdimerizes by a chain reaction, with VCZ radical cation (VCz’+) as a chain carrier. However, in the non-polar solvent benzene, photoirradiation of VCZ gives only a negligible amount of the cyclobutane photodimer due to the lack of VCz’+. Nevertheless, even in the non-polar solvent benzene, the addition of aromatic electron acceptors (A) to the system increases the photodimer yield. The effects of VCZ concentration on the photodimer yield and emission spectra were intetpreted by the participation of the (VCZ. . .VCZ.. .A)* exterplex as an intermediate of photodimer formation.
1. Introduction Since the first report of an exterplex (this termolecular excited complex is also called a triplex) formed between two naphthalenes and one 1,4dicyanobenzene by Beens and Weller [l], much spectroscopic evidence has been accumulated on exterplex formation [2]. This exterplex is reported to play an important role in exciplex quenching [3, 41 and successive charge separation [5]. The participation of exterplex intermediates in photochemical reactions is also of current interest, especially in photosensitized cycloadditions [6] and cycloreversions [7]. In this paper, exterplex formation between two N-vinylcarbazoles (VCZ) and an aromatic acceptor (A) molecule was studied as an intermediate of VCZ photocyclodimerization. Exterplex formation between two carbazole (CZ) chromophores and one dimethylterephthalate (DMTP) molecule has been reported by Hoyle and Guillet [g]. In polar solvents, VCZ efficiently forms a cyclobutane dimer of trans-1,2-di(N-carbazolyl)cyclobutane (CB) on photoirradiation [9]. This dimerization proceeds by a chain reaction with VCZ radical cation (VCZ+) as a chain carrier species [lo]. However, in non-polar solvents, it is not possible to produce free VCz’+ by photoexcitation and hence the photodimerization of VCZ in the non-polar solvent benzene is suppressed. Nevertheless, even in benzene, the addition of an electron acceptor to a concentrated VCZ solution increases the CB photodimer yield. This VCZ photodimer is interpreted
lOlO-6030/94/$07.00
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Elsevier
Sequoia.
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to be formed via a (VCZ. . .VCZ* . .A)* exterplex intermediate. In most exterplex reaction systems reported thus far, the yield is enhanced by the longer exterplex lifetime compared with the shorter monomer excited state, whereas in the present system the polarity of the exterplex structure plays an important role in the reactivity.
2. Experimental
details
2.1. Materials VCZ (Nakarai Tesque) was purified by recrystallization from methanol and hexane three times. The electron acceptors 1,3-dicyanobenzene (DCNB, Wako Pure Chemical Industries) and dimethylterephthalate (DMTP, Wako) were purified by recrystallization from ethanol several times. Benzene (Nakarai) was washed with concentrated HzS04, dried by CaCi, and purified by distillation several times. Care was taken to eliminate the remaining water (azeotrope). 2.2. Photoreactions and product analysis A benzene solution of VCZ containing an acceptor was degassed by repeated freezepumpthaw cycles (less than 10 -5 mmHg) in a Pyrex ampoule. The degassed sample was photoinadiated by a 300 W high-pressure merctiry lamp (Toshiba) for 6 h at 298 K [ll, 121: VCZ was selectively photoexcited in this system at the 313 and 334 nm Hg lines. After photoirradiation, the benzene was evaporated and the product yields
18
A. Tsuchida, M. Yamamoto
I Photodimetiation
were determined by high performance liquid chromatography (HPLC) (JASCO, TRI-ROTAR-V, W detector, fitted with a silica gel column (Finepak SIL) with hexaue-cthyl acetate (4:l v/v) as the eluent). Control experiments in the dark gave minor products and the fractions of these products were corrected. 2.3, Spectroscopic measurements All samples for spectroscopic measurements were degassed in a Pyrex ampoule fitted with a 1 cm quartz cell [13, 141. Absorption spectra were measured with a W-200s (Shimadzu) spectrophotometer. The emission spectra were obtained on a Hitachi 850 spectrofluorophotometer, whose spectral response was calibrated using a standard tungsten lamp. The emission lifetime was measured by the single-photon-counting method (PR4 Inc., model 510B). The time-resolved emission spectra were obtained using a Unisoku USP-500 optical multichannel analyser (OMA) with a 5 ns gate width of the image intensifier. In these measurements, a Lambda Physik EMGlOlMSC excimer laser (351 nm; full width at half-maximum (FWHM), approximately 14 ns) was used as the excitation light pulse source. All measurements were performed at 298 K. 3. Results and discussion The photodimerization of VCZ has been studied in detail by Ledwith [lo]. As mentioned above, photoirradiation of VCZ in polar solvents efficiently produces the cyclobutane dimer CB; the quantum yield of CB formation is greater than unity under a variety of experimental conditions and therefore the existence of a chain reaction is evident. The addition of oxidative metal ions to this reaction system [lo] reveals the participation of VCz’+ as an intermediate. The existence of VCz’+ was also confirmed by laser photolysis measurements [15]. However, in non-polar solvents, the photodimerization of VCZ is scarcely observed due to the lack of VCz’+ formation. In non-polar solvents, such as benzene, free VCz’+ is not produced due to poor solvation; no transient absorption of VCZ+ was detected in benzene by nanosecond laser photolysis. However, in non-polar solvents, it is possible to produce CB using an exterplex as an intermediate. The cation radical shared by two neighbouring VCZ molecules and the close approach of two VCZ molecules in the exterplex state favour CB formation in non-polar solvents. On addition of
of N-vinylcarbazole
an electron acceptor (A) to a benzene solution of VCZ, a (VCZ. * *A)* exciplex is formed initially. With au increase in VCZ concentration, the (VCZ * . *A)* exciplex is quenched by a second VCZ molecule and a (VCZ - * - VCZ * . -A)* exterplex is formed. In this exterplex, the radical cationic nature of the two adjacent CZ chromophores induces CB formation. The fact that two CZ chromophores form an exterplex with one DMTP molecule has been reported by Hoyle and Guillet [8] and Masuhara et al. [16]. Figure 1 shows the results of the photoreaction. A degassed benzene solution of VCZ with (or without) an electron acceptor was photoirradiated for 6 h and the CB dimer yield was analysed by HPLC. In the non-polar solvent benzene, photoirradiation of VCZ without an acceptor gave only a negligible amount of photodimer CB in the range of VCZ concentration studied in this experiment. However, the addition of an electron acceptor (DMTP or DCNB) to the solution appreciably increased the CB yield; the yield of CB increased with increasing VCZ concentration. This concentration effect strongly suggests the participation of an exterplex in CB formation. The acceptor concentration of Fig. 1 is sufficient to quench VCZ* fluorescence (~~8.7 ns). The quenching rate constant of VCZ* by DMTP was determined in benzene from the Stern-Vohner plots: the value was almost diffusion controlled (1.8~10”’ mold1 1 s-l). Due to this quenching, a new emission band appeared at 465 nm as shown
VCZ I mol dm” Fig. 1. Effect of VCZ concentration and electron acceptor additives on the photodimer CB yield in non-polar benzene at 298 K. [DMTP] =0.13 mot I-‘, [DCNB] =0.20 mot I-‘. Irradiation time, 6 h.
A. Tsuchida. M. Yamanoto I Photodimerimtion of N-vinylcatiawIe
in Fig. 2 (curve (1)). This emission band is ascribed to the (VCZ. - - DMTP)* exciplex whose emission decay, measured by the single-photon-counting method, is shown in Fig. 3 (curve (1)). The quenching of VCZ* by the high DMTP concentration and the slight overlap of the VCZ* emission tail with the exciplex band resulted in almost no rise of the exciplex emission. The OMA measurements at a longer delay time (more than 25 ns) gave only the (VCZ. - - DMTF’)* exciplex emission and no VCZ* emission. This means that, in these conditions, the exciplex dissociation is negligible.
500
4cn
WAVELENGTH
600
I nm
Fig. 2. Emission spectra of VCZ in the presence of DMTP in benzene at 298 K. Spectra are normalized at the 351.0 nm peak of the VCZ monomer band. (1) [VCZ]=2.6X lo-’ mol 1-l with [DMTP]=0.13 mol 1-l; (2) [vCZ]=5.2XlO-* mol I-’ with [DMTF’]=0.13 mol 1-l; (3) [VCZ]=O.13’ mol 1-l with [DMTF]=0.13 mol 1-l; (4) [VCZ]=O.26 mol l-’ with [DMTP]=0.13 mol l-‘; (5) [VC2]=2.6~ lo-’ mol l-‘.
CHANNEL Fig. 3. Decay curves of the VCZDMTP exciplex emission obtained by the single-photon-counting method (0.61 ns per channel) in benzene at 298 K with the filters transparent at 424L460 nm. (1) [VCZ]=2.6~10-’ mol I-’ with [DMTP]=0.13 mol 1-l; (2) [VCZ]=5.2~10-’ mol 1-l with [Dh4lT]-0.13 mol I-‘; (3) [VC’2]=0.13mo11-‘with[DMTP]=0.13mo11-’;(4)[VCZ]=0.26 mol I-’ with [DMTP]=0.13 mol 1-l.
19
Therefore the slope in Fig. 3 (curve (1)) gave an exciplex lifetime of 46 ns. With an increase in VCZ concentration, this exciplex emission was quenched and the band peak was red shifted as shown in Fig. 2 (curves (2)-(4)). Figure 3 shows the decrease in the (VCZ . . .DMTP)* exciplex emission lifetime on addition of VCZ. The exciplex quenching rate constant obtained from the Stern-Volmer plots of exciplex intensity or lifetime was 5.9 X 10’ mol-1 1 s-l. The exciplex quenching and emission peak shift also support (VCZ. . .VCZ. . .DMTP)* exterplex formation. The initially formed exciplex is quenched by another VCZ molecule to form an exterplex whose emission peak is located at about 480 nm. This exterplex emission overlaps with the exciplex emission and the peak is red shifted. Although the pure emission spectrum of the (VCZ.. .vcz- . . DMTP)* exterplex was difficult to measure due to the overlap of the remaining strong exciplex emission, the lifetime of the exterplex was determined to be 35 ns by decay analysis. The lifetime of the exciplex (VCZ * * *DMTP)* decreased with increasing VCZ concentration according to the Stern-Volmer relationship, whereas the 35 ns component observed at longer wavelengths {greater than 500 nm) was unchanged by biexponential curve fitting in the 20-120 ns region. This means that exterplex dissociation is almost negligible under these experimental conditions. Therefore we assigned this 35 ns component to the (VCZ- - . VCZ- . *DMTP)* exterplex emission Lifetime. Figure 4 shows the time-resolved emission spectra from exciplex to exterplex. These time-resolved emission spectra were obtained by OMA measurements at different delay times with a 5 ns gate width. The spectral response of the OMA is not corrected. The peak shifts to longer wavelengths can be seen at longer delay times. These spectral changes show the existence of exterplex emission. The possibility that the red shift of the peak may be caused by an increase in polarity of the solution with an increase in VCZ concentration was excluded, i.e. the measurement of I&(30) (polarity index) gave an increase of only 0.1 in the dielectric constant of the benzene solution at a concentration of VCZ of 0.26 mol I-’ [17]. The exterplex formed by poly(N-vinylcarbazole) with DMTP has an emission maximum at 520 nm [S]. The bichromophoric compounds of meso- and racemic-2,4-di-(N-carbazolyl)pentanes show exterplex emission by DMTP quenching at 560 and 510 nm respectively [23. The former corresponds to the fulLoverlaptype and the latter corresponds to the partial-
A. Tsuchida, h4. Yamamoto I Photodimetiation
20
of N-vinyicarbazole nonpolar solvent
T
E
G
E
A;
radical ions
c
iTi f
vcz+ +
polar solvent 11) (2) (3) (4) (5) (6)
Scheme
‘“fi A cz cz CB
1.
4. Conclusions WAVELENGTH
/ nm
Fig. 4. Time-resolved emission spectra of a system containing VCZ (0.26 mol 1-l) and DMTP (0.13 mol I-‘), obtained by OMA measurements at a gate width of 5 ns in benzene at 298 K. Delay times after the exciting laser pulse are: (I) IO ns; (2) 1.5 ns; (3) 20 ns; (4) 25 ns; (5) 30 ns; (6) 3.5 ns.
overlap-type exterplex [16]. In these cases, the high local concentration of intramolecular CZ stable (CZ. * sCZ* * * chromophores allows DMTP)* exterplex formation with a definite emission band. In contrast, Fig. 2 shows that the emission quantum yield of intermolecular is not high and the (VCZ. * *vcza * - DMTP)” emission band peak is located in the blue region. This can be explained by a loose unstable structure of the intermolecular exterplex. The emission properties of the exterplex of VCZ with DCNB are similar to those of the exterplex (VCZ- . *vcz* * *DMTP)*. In the case of DCNB as quencher, the (VCZ. * -DCNB)* exciplex band appeared at 419 nm. On addition of 0.26 mol 1-l VCZ, this emission peak was red shifted to 429 nm. This short-wavelength peak is consistent with that reported by Masuhara et al. [ 161. The reduction potentials of DMTP and DCNB (a measure of electron acceptability) are similar: -2.11 and -1.97 V UUS.Ag/O.l N Ag+, respectively [18]. Therefore the shorter exterplex emission band and the higher CB yield (see Fig. 1) for DCNB seem to be caused by the different exterplex conformation. The stereochemistry of the VCZ dimer produced via the exterplex mechanism was determined by nuclear magnetic resonance (NMR) and found to be a trans-type CB. This CB structure supports the loose intermolecular (VCZ. . .VCZ- . .A)* exterplex structure indicated by the emission spectra [ll, 121, and may indicate the existence of an open-ring-type CB intermediate after exterplex deactivation.
As shown in Scheme 1, the photoreactivity of VCZ to form a CB dimer is enhanced by the participation of the exterplex intermediate. This exterplex intermediate allows CB formation even in non-polar solvents where the free radical cation of VCZ cannot exist. In this system, the reactivity of VCZ in non-polar solvents is ascribed to the polar exterplex structure, whereas for the other systems reported thus far the enhanced reactivity was assigned to the longer lifetime of the exterplex.
Acknowledgment This work was supported by a Grant-in-Aid for Developmental Scientific Research (No. 04555218) from the Ministry of Education, Science and Culture of Japan.
References 1 H. Beens and A. Weller, Chem Phys. Let& 2 (1968) 140. 2 Y. Tsujii, K. Takami, A. Tsuchida, S. Ito and M. Yamamoto, Mucromokcules, 26 (1993) 1411. 3 RA. CakhveIl, D. Creed, D.C. DeMarco, L.A. Melton, H. Ohta and P.H. Wine, J. Am Chem. Sot., 102 (1980) 2369. 4 W. Hub, S. Schneider, F. Diirr, J.D. Oxman and F.D. Lewis, J. Am. Chem. Sot.. IO6 (1984) 101. 5 N.C. Yang, R. Gerald II and hk. Wasielewski, J Am. Chem Sec., 107 (1985) 5531. 6 K. Mizuno, T. Hashizume and Y. Otsuji,J. Chem. Sec., Chem. Commun., (1983) 772. 7 C. Pat, Pure Appl. Chem, 58 (1986) 1ZA9. 8 C.E. Hoyle and J.E. Guillet, Macromolecules, II (1978) 221. 9 Y. Shirota and H. Mikawa, J. Macromol. Sci., Rev. Macromol. Chem., C16 (1977) 129. 10 A. Ledwith, Act. Chem. Res., 5 (1972) 133. 11 A. Tsuchida, M. Yamamoto and Y. Nishijima, L Chem. SW., Per&in Trans. 2, (1986) 239. 12 A Tsuchida, M. Yamamoto and Y. Nishijima, Bull. Chem. SK Jpn., 60 (1987) 2899.
A. Tsuchida, M. Yamamolo I Photodimeritation 13 A. Tsuchida, M. Yamamolo and Y. Nishijima, I. Chem. Sac., Per!& Trans. 2, (1987) 507. 14 A. Tsuchida, M. Yamamoto and Y. Nishijima, Bull. Chem. SOL .&WI., 64 (1991j 3402. 15 M. Yamamoto, M. Ohoka, K. Kitagawa, S. Nishimoto and Y. Nishijima, Chem. Len., (1973) 745.
of N-vittylcarbazole
21
16 H. Masuhara, J. Vandendriessche, K. Demeyer, N. Boens and F.C. De Schryver, Macromolecules, 15 (1982) 1471. 17 K. Dimroth and C. Reichardt, Fresenius’Z Anal. Chem., 215 (1966) 344. 18 N.L. Weinberg (ed.), Technique of Elecfroorganic Synthesis, Wiley, New York, 1975.
J Photochem. Photobiol. A: Chem., 77 (1994) 17-21
Exterplex
intermediate
in the photodimerization
Akira Tsuchida and Masahide Yamamoto Lkpwtmentof Polymer Chemists, Graduate School of Engineering,
of IV-vinylcarbazole
Kyoto Universiry,Yoshida, Sakyo-lot, Kyoto 606 (Jopan)
(Received May 17, 1993; accepted July 8, 1993)
Abstract The
photocyclodimerization mechanism of N-vinylcarbazole (VCZ) via an exterplex intermediate is presented. In polar solvents, such as acetonitrile, VCZ efficiently photcdimerizes by a chain reaction, with VCZ radical cation (VCz’+) as a chain carrier. However, in the non-polar solvent benzene, photoirradiation of VCZ gives only a negligible amount of the cyclobutane photodimer due to the lack of VCz’+. Nevertheless, even in the non-polar solvent benzene, the addition of aromatic electron acceptors (A) to the system increases the photodimer yield. The effects of VCZ concentration on the photodimer yield and emission spectra were intetpreted by the participation of the (VCZ. . .VCZ.. .A)* exterplex as an intermediate of photodimer formation.
1. Introduction Since the first report of an exterplex (this termolecular excited complex is also called a triplex) formed between two naphthalenes and one 1,4dicyanobenzene by Beens and Weller [l], much spectroscopic evidence has been accumulated on exterplex formation [2]. This exterplex is reported to play an important role in exciplex quenching [3, 41 and successive charge separation [5]. The participation of exterplex intermediates in photochemical reactions is also of current interest, especially in photosensitized cycloadditions [6] and cycloreversions [7]. In this paper, exterplex formation between two N-vinylcarbazoles (VCZ) and an aromatic acceptor (A) molecule was studied as an intermediate of VCZ photocyclodimerization. Exterplex formation between two carbazole (CZ) chromophores and one dimethylterephthalate (DMTP) molecule has been reported by Hoyle and Guillet [g]. In polar solvents, VCZ efficiently forms a cyclobutane dimer of trans-1,2-di(N-carbazolyl)cyclobutane (CB) on photoirradiation [9]. This dimerization proceeds by a chain reaction with VCZ radical cation (VCZ+) as a chain carrier species [lo]. However, in non-polar solvents, it is not possible to produce free VCz’+ by photoexcitation and hence the photodimerization of VCZ in the non-polar solvent benzene is suppressed. Nevertheless, even in benzene, the addition of an electron acceptor to a concentrated VCZ solution increases the CB photodimer yield. This VCZ photodimer is interpreted
lOlO-6030/94/$07.00
Q, 1994
Elsevier
Sequoia.
Al1 rights reserved
to be formed via a (VCZ. . .VCZ* . .A)* exterplex intermediate. In most exterplex reaction systems reported thus far, the yield is enhanced by the longer exterplex lifetime compared with the shorter monomer excited state, whereas in the present system the polarity of the exterplex structure plays an important role in the reactivity.
2. Experimental
details
2.1. Materials VCZ (Nakarai Tesque) was purified by recrystallization from methanol and hexane three times. The electron acceptors 1,3-dicyanobenzene (DCNB, Wako Pure Chemical Industries) and dimethylterephthalate (DMTP, Wako) were purified by recrystallization from ethanol several times. Benzene (Nakarai) was washed with concentrated HzS04, dried by CaCi, and purified by distillation several times. Care was taken to eliminate the remaining water (azeotrope). 2.2. Photoreactions and product analysis A benzene solution of VCZ containing an acceptor was degassed by repeated freezepumpthaw cycles (less than 10 -5 mmHg) in a Pyrex ampoule. The degassed sample was photoinadiated by a 300 W high-pressure merctiry lamp (Toshiba) for 6 h at 298 K [ll, 121: VCZ was selectively photoexcited in this system at the 313 and 334 nm Hg lines. After photoirradiation, the benzene was evaporated and the product yields
18
A. Tsuchida, M. Yamamoto
I Photodimetiation
were determined by high performance liquid chromatography (HPLC) (JASCO, TRI-ROTAR-V, W detector, fitted with a silica gel column (Finepak SIL) with hexaue-cthyl acetate (4:l v/v) as the eluent). Control experiments in the dark gave minor products and the fractions of these products were corrected. 2.3, Spectroscopic measurements All samples for spectroscopic measurements were degassed in a Pyrex ampoule fitted with a 1 cm quartz cell [13, 141. Absorption spectra were measured with a W-200s (Shimadzu) spectrophotometer. The emission spectra were obtained on a Hitachi 850 spectrofluorophotometer, whose spectral response was calibrated using a standard tungsten lamp. The emission lifetime was measured by the single-photon-counting method (PR4 Inc., model 510B). The time-resolved emission spectra were obtained using a Unisoku USP-500 optical multichannel analyser (OMA) with a 5 ns gate width of the image intensifier. In these measurements, a Lambda Physik EMGlOlMSC excimer laser (351 nm; full width at half-maximum (FWHM), approximately 14 ns) was used as the excitation light pulse source. All measurements were performed at 298 K. 3. Results and discussion The photodimerization of VCZ has been studied in detail by Ledwith [lo]. As mentioned above, photoirradiation of VCZ in polar solvents efficiently produces the cyclobutane dimer CB; the quantum yield of CB formation is greater than unity under a variety of experimental conditions and therefore the existence of a chain reaction is evident. The addition of oxidative metal ions to this reaction system [lo] reveals the participation of VCz’+ as an intermediate. The existence of VCz’+ was also confirmed by laser photolysis measurements [15]. However, in non-polar solvents, the photodimerization of VCZ is scarcely observed due to the lack of VCz’+ formation. In non-polar solvents, such as benzene, free VCz’+ is not produced due to poor solvation; no transient absorption of VCZ+ was detected in benzene by nanosecond laser photolysis. However, in non-polar solvents, it is possible to produce CB using an exterplex as an intermediate. The cation radical shared by two neighbouring VCZ molecules and the close approach of two VCZ molecules in the exterplex state favour CB formation in non-polar solvents. On addition of
of N-vinylcarbazole
an electron acceptor (A) to a benzene solution of VCZ, a (VCZ. * *A)* exciplex is formed initially. With au increase in VCZ concentration, the (VCZ * . *A)* exciplex is quenched by a second VCZ molecule and a (VCZ - * - VCZ * . -A)* exterplex is formed. In this exterplex, the radical cationic nature of the two adjacent CZ chromophores induces CB formation. The fact that two CZ chromophores form an exterplex with one DMTP molecule has been reported by Hoyle and Guillet [8] and Masuhara et al. [16]. Figure 1 shows the results of the photoreaction. A degassed benzene solution of VCZ with (or without) an electron acceptor was photoirradiated for 6 h and the CB dimer yield was analysed by HPLC. In the non-polar solvent benzene, photoirradiation of VCZ without an acceptor gave only a negligible amount of photodimer CB in the range of VCZ concentration studied in this experiment. However, the addition of an electron acceptor (DMTP or DCNB) to the solution appreciably increased the CB yield; the yield of CB increased with increasing VCZ concentration. This concentration effect strongly suggests the participation of an exterplex in CB formation. The acceptor concentration of Fig. 1 is sufficient to quench VCZ* fluorescence (~~8.7 ns). The quenching rate constant of VCZ* by DMTP was determined in benzene from the Stern-Vohner plots: the value was almost diffusion controlled (1.8~10”’ mold1 1 s-l). Due to this quenching, a new emission band appeared at 465 nm as shown
VCZ I mol dm” Fig. 1. Effect of VCZ concentration and electron acceptor additives on the photodimer CB yield in non-polar benzene at 298 K. [DMTP] =0.13 mot I-‘, [DCNB] =0.20 mot I-‘. Irradiation time, 6 h.
A. Tsuchida. M. Yamanoto I Photodimerimtion of N-vinylcatiawIe
in Fig. 2 (curve (1)). This emission band is ascribed to the (VCZ. - - DMTP)* exciplex whose emission decay, measured by the single-photon-counting method, is shown in Fig. 3 (curve (1)). The quenching of VCZ* by the high DMTP concentration and the slight overlap of the VCZ* emission tail with the exciplex band resulted in almost no rise of the exciplex emission. The OMA measurements at a longer delay time (more than 25 ns) gave only the (VCZ. - - DMTF’)* exciplex emission and no VCZ* emission. This means that, in these conditions, the exciplex dissociation is negligible.
500
4cn
WAVELENGTH
600
I nm
Fig. 2. Emission spectra of VCZ in the presence of DMTP in benzene at 298 K. Spectra are normalized at the 351.0 nm peak of the VCZ monomer band. (1) [VCZ]=2.6X lo-’ mol 1-l with [DMTP]=0.13 mol 1-l; (2) [vCZ]=5.2XlO-* mol I-’ with [DMTF’]=0.13 mol 1-l; (3) [VCZ]=O.13’ mol 1-l with [DMTF]=0.13 mol 1-l; (4) [VCZ]=O.26 mol l-’ with [DMTP]=0.13 mol l-‘; (5) [VC2]=2.6~ lo-’ mol l-‘.
CHANNEL Fig. 3. Decay curves of the VCZDMTP exciplex emission obtained by the single-photon-counting method (0.61 ns per channel) in benzene at 298 K with the filters transparent at 424L460 nm. (1) [VCZ]=2.6~10-’ mol I-’ with [DMTP]=0.13 mol 1-l; (2) [VCZ]=5.2~10-’ mol 1-l with [Dh4lT]-0.13 mol I-‘; (3) [VC’2]=0.13mo11-‘with[DMTP]=0.13mo11-’;(4)[VCZ]=0.26 mol I-’ with [DMTP]=0.13 mol 1-l.
19
Therefore the slope in Fig. 3 (curve (1)) gave an exciplex lifetime of 46 ns. With an increase in VCZ concentration, this exciplex emission was quenched and the band peak was red shifted as shown in Fig. 2 (curves (2)-(4)). Figure 3 shows the decrease in the (VCZ . . .DMTP)* exciplex emission lifetime on addition of VCZ. The exciplex quenching rate constant obtained from the Stern-Volmer plots of exciplex intensity or lifetime was 5.9 X 10’ mol-1 1 s-l. The exciplex quenching and emission peak shift also support (VCZ. . .VCZ. . .DMTP)* exterplex formation. The initially formed exciplex is quenched by another VCZ molecule to form an exterplex whose emission peak is located at about 480 nm. This exterplex emission overlaps with the exciplex emission and the peak is red shifted. Although the pure emission spectrum of the (VCZ.. .vcz- . . DMTP)* exterplex was difficult to measure due to the overlap of the remaining strong exciplex emission, the lifetime of the exterplex was determined to be 35 ns by decay analysis. The lifetime of the exciplex (VCZ * * *DMTP)* decreased with increasing VCZ concentration according to the Stern-Volmer relationship, whereas the 35 ns component observed at longer wavelengths {greater than 500 nm) was unchanged by biexponential curve fitting in the 20-120 ns region. This means that exterplex dissociation is almost negligible under these experimental conditions. Therefore we assigned this 35 ns component to the (VCZ- - . VCZ- . *DMTP)* exterplex emission Lifetime. Figure 4 shows the time-resolved emission spectra from exciplex to exterplex. These time-resolved emission spectra were obtained by OMA measurements at different delay times with a 5 ns gate width. The spectral response of the OMA is not corrected. The peak shifts to longer wavelengths can be seen at longer delay times. These spectral changes show the existence of exterplex emission. The possibility that the red shift of the peak may be caused by an increase in polarity of the solution with an increase in VCZ concentration was excluded, i.e. the measurement of I&(30) (polarity index) gave an increase of only 0.1 in the dielectric constant of the benzene solution at a concentration of VCZ of 0.26 mol I-’ [17]. The exterplex formed by poly(N-vinylcarbazole) with DMTP has an emission maximum at 520 nm [S]. The bichromophoric compounds of meso- and racemic-2,4-di-(N-carbazolyl)pentanes show exterplex emission by DMTP quenching at 560 and 510 nm respectively [23. The former corresponds to the fulLoverlaptype and the latter corresponds to the partial-
A. Tsuchida, h4. Yamamoto I Photodimetiation
20
of N-vinyicarbazole nonpolar solvent
T
E
G
E
A;
radical ions
c
iTi f
vcz+ +
polar solvent 11) (2) (3) (4) (5) (6)
Scheme
‘“fi A cz cz CB
1.
4. Conclusions WAVELENGTH
/ nm
Fig. 4. Time-resolved emission spectra of a system containing VCZ (0.26 mol 1-l) and DMTP (0.13 mol I-‘), obtained by OMA measurements at a gate width of 5 ns in benzene at 298 K. Delay times after the exciting laser pulse are: (I) IO ns; (2) 1.5 ns; (3) 20 ns; (4) 25 ns; (5) 30 ns; (6) 3.5 ns.
overlap-type exterplex [16]. In these cases, the high local concentration of intramolecular CZ stable (CZ. * sCZ* * * chromophores allows DMTP)* exterplex formation with a definite emission band. In contrast, Fig. 2 shows that the emission quantum yield of intermolecular is not high and the (VCZ. * *vcza * - DMTP)” emission band peak is located in the blue region. This can be explained by a loose unstable structure of the intermolecular exterplex. The emission properties of the exterplex of VCZ with DCNB are similar to those of the exterplex (VCZ- . *vcz* * *DMTP)*. In the case of DCNB as quencher, the (VCZ. * -DCNB)* exciplex band appeared at 419 nm. On addition of 0.26 mol 1-l VCZ, this emission peak was red shifted to 429 nm. This short-wavelength peak is consistent with that reported by Masuhara et al. [ 161. The reduction potentials of DMTP and DCNB (a measure of electron acceptability) are similar: -2.11 and -1.97 V UUS.Ag/O.l N Ag+, respectively [18]. Therefore the shorter exterplex emission band and the higher CB yield (see Fig. 1) for DCNB seem to be caused by the different exterplex conformation. The stereochemistry of the VCZ dimer produced via the exterplex mechanism was determined by nuclear magnetic resonance (NMR) and found to be a trans-type CB. This CB structure supports the loose intermolecular (VCZ. . .VCZ- . .A)* exterplex structure indicated by the emission spectra [ll, 121, and may indicate the existence of an open-ring-type CB intermediate after exterplex deactivation.
As shown in Scheme 1, the photoreactivity of VCZ to form a CB dimer is enhanced by the participation of the exterplex intermediate. This exterplex intermediate allows CB formation even in non-polar solvents where the free radical cation of VCZ cannot exist. In this system, the reactivity of VCZ in non-polar solvents is ascribed to the polar exterplex structure, whereas for the other systems reported thus far the enhanced reactivity was assigned to the longer lifetime of the exterplex.
Acknowledgment This work was supported by a Grant-in-Aid for Developmental Scientific Research (No. 04555218) from the Ministry of Education, Science and Culture of Japan.
References 1 H. Beens and A. Weller, Chem Phys. Let& 2 (1968) 140. 2 Y. Tsujii, K. Takami, A. Tsuchida, S. Ito and M. Yamamoto, Mucromokcules, 26 (1993) 1411. 3 RA. CakhveIl, D. Creed, D.C. DeMarco, L.A. Melton, H. Ohta and P.H. Wine, J. Am Chem. Sot., 102 (1980) 2369. 4 W. Hub, S. Schneider, F. Diirr, J.D. Oxman and F.D. Lewis, J. Am. Chem. Sot.. IO6 (1984) 101. 5 N.C. Yang, R. Gerald II and hk. Wasielewski, J Am. Chem Sec., 107 (1985) 5531. 6 K. Mizuno, T. Hashizume and Y. Otsuji,J. Chem. Sec., Chem. Commun., (1983) 772. 7 C. Pat, Pure Appl. Chem, 58 (1986) 1ZA9. 8 C.E. Hoyle and J.E. Guillet, Macromolecules, II (1978) 221. 9 Y. Shirota and H. Mikawa, J. Macromol. Sci., Rev. Macromol. Chem., C16 (1977) 129. 10 A. Ledwith, Act. Chem. Res., 5 (1972) 133. 11 A. Tsuchida, M. Yamamoto and Y. Nishijima, L Chem. SW., Per&in Trans. 2, (1986) 239. 12 A Tsuchida, M. Yamamoto and Y. Nishijima, Bull. Chem. SK Jpn., 60 (1987) 2899.
A. Tsuchida, M. Yamamolo I Photodimeritation 13 A. Tsuchida, M. Yamamolo and Y. Nishijima, I. Chem. Sac., Per!& Trans. 2, (1987) 507. 14 A. Tsuchida, M. Yamamoto and Y. Nishijima, Bull. Chem. SOL .&WI., 64 (1991j 3402. 15 M. Yamamoto, M. Ohoka, K. Kitagawa, S. Nishimoto and Y. Nishijima, Chem. Len., (1973) 745.
of N-vittylcarbazole
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16 H. Masuhara, J. Vandendriessche, K. Demeyer, N. Boens and F.C. De Schryver, Macromolecules, 15 (1982) 1471. 17 K. Dimroth and C. Reichardt, Fresenius’Z Anal. Chem., 215 (1966) 344. 18 N.L. Weinberg (ed.), Technique of Elecfroorganic Synthesis, Wiley, New York, 1975.