- Email: [email protected]
Measurements of the electric-field-induced change in fluorescence decay profile of methylene-linked carbazole and terephthalic acid methyl ester in a PMMA polymer film
25 June 1999
Chemical Physics Letters 307 Ž1999. 8–14
Measurements of the electric-field-induced change in fluorescence decay profile of methylene-linked carbazole and terephthalic acid methyl ester in a PMMA polymer film Yoshinobu Nishimura a , Iwao Yamazaki a , Masahide Yamamoto b, Nobuhiro Ohta
c,)
a b
Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060-8628, Japan Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniÕersity, Sakyou-ku Kyoto 606-8501, Japan c Research Institute for Electronic Science, Hokkaido UniÕersity, Sapporo 060-0812, Japan Received 29 January 1999; in final form 15 April 1999
Abstract The electric-field-induced change in the fluorescence decay profile has been measured for methylene-linked carbazole ŽCZ. and terephthalic acid methyl ester ŽTAME. doped in a PMMA polymer film, using a picosecond time-resolved decay measurement system combined with a bipolar sample bias. The average lifetime of the fluorescence emitted from the locally excited state of CZ becomes shorter in the presence of an external electric field Ž F ., confirming that the rate of photoinduced electron transfer from CZ to TAME is enhanced by F. Different decaying portions of the multi-exponential decay were also found to give different efficiencies in the electric field effect from each other, depending on the methylene chain length. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The electric field effect on excitation dynamics is one of the current topics in relation to photochemical processes in biological systems. In the photosynthetic reaction center, for example, the electric field produced by the oriented membrane protein in which the chromophores of bacteriochrollophils are encased seems to play an important role in the photoinduced charge separation process, producing a high efficiency and high selectivity. The electric field effects
)
Corresponding author. E-mail: [email protected]
on the initial step of the photoinduced electron transfer process have been extensively examined w1–4x. In our previous papers w5,6x, the external electric field effects Žbased on the measurements of the effects on the fluorescence spectra. on the photoinduced electron transfer processes in a mixture of ethyl carbazole and dimethyl terephthalate and in their methylene-linked compounds were reported. In both systems, it was found that fluorescence emitted from the locally excited state of carbazole was quenched by an electric field. The quenching was attributed to the field-induced enhancement of the rate of the electron transfer from the excited state of carbazole to terephthalic acid, since the electron transfer rate and its field-induced change become larger with increasing sample concentration, i.e. with
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 6 0 - 1
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
decreasing donor–acceptor distance. Even when the electron transfer rate is not affected by an electric field, however, fluorescence quenching occurs if the formation yield of the emissive state of the carbazole chromophore generated by photoexcitation is reduced by an electric field. In the latter case, the fluorescence decay profile is not affected by an
9
electric field. In the former case, on the other hand, the fluorescence lifetime becomes shorter in the presence of an electric field since the electron transfer is considered to compete with the fluorescing process. In order to confirm the mechanism of the field-induced quenching of fluorescence, therefore, the direct measurements of the field effects on the
Fig. 1. Broken line shows fluorescence spectra and solid line shows electrofluorescence spectra of D-Ž4.-A Župper. and D-Ž12.-A Žlower. doped in a PMMA film at a concentration of 1.5 mol%. The electrofluorescence spectra were obtained with a field strength of 0.5 MV cmy1 , and slant lines are inserted to highlight the difference between the positive and the negative parts. Maximum intensity of fluorescence is normalized to unity. The molecular structure of the sample is also shown.
10
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
fluorescence decay profile are necessary, as reported by Popovic and his co-workers in some organic photoconductors w7,8x. In the present study, the direct measurements of the field-induced change in the decay profile have been carried out with picosecond time-resolution, and preliminary results on methylene-linked compounds of CZ– ŽCH 2 . n –TAME with n s 4 and n s 12, hereafter denoted by D-Ž4.-A and D-Ž12.-A, respectively, are reported. Here, CZ Žmethylene-linked carbazole. and TAME represent the carbazole chromophore and terephthalic acid methyl ester, respectively, and the molecular structure of the sample is shown in Fig. 1.
ŽLaboratory Equipment MCArPC98.. The time duration of each bias was 25 ms, but the first 10 ms was a deadtime and the decay measurements were carried out only for the time range 10–25 ms in order to exclude an overshooting effect of the applied field just after the change in applied voltage. The steady-state electroabsorption and electrofluorescence spectra were measured using modulation spectroscopy with the same apparatus as those reported in previous papers w5,6x. The excitation wavelength was 294 nm, where the absorption spectrum shows a maximum intensity of the S 0 ™ S 2 band. The bandwidth of the instrument response function was 39 ps ŽFWHM. at 294 nm when the decay profile was measured with a time intervalrchannel of 5.0 psrch.
2. Experimental The samples were prepared using the methods reported elsewhere w5,6x. A PMMA polymer film which contains D-Ž4.-A or D-Ž12.-A was cast on the ITO-coated quartz substrate by a spin coating method. Then, a semitransparent aluminum ŽAl. film was deposited on the polymer film. ITO and Al films were used as electrodes. The thickness of the polymer film was determined with a nanospecrAFT system ŽM3000, nanometrics.. Fluorescence decay measurements were carried out with a single-photon counting system combined with a custom-made pulse generator supplying the bipolar square wave up to 100 V in amplitude. The emission and scattered light were detected with a microchannel plate ŽHamamatsu R3809. w9x. The excitation source was a mode-locked Ti:Sa laser ŽCoherent, Mira 900. pumped by an Ar ion laser ŽCoherent, Innova 300.. The repetition rate of the laser pulse was 2.9 MHz with a pulse picker ŽCoherent, Model 9200.. The third harmonic generated by an ultrafast harmonic system ŽInrad, Model 5-050. was used for excitation. The applied field strength was evaluated from the applied voltage divided by the film thickness. The applied voltage was a repetition of rectangular waves of negative, zero, positive and zero bias in turn. Three different decay curves were collected, corresponding to positive, negative and zero sample bias. These decays were stored in each of the different memory-channels of a multichannel analyzer
3. Results and discussion Fluorescence spectra of D-Ž4.-A and D-Ž12.-A doped in a PMMA polymer film are regarded as a mixture of sharp-structured fluorescence emitted from the locally excited S 1 state of CZ and a broad exciplex fluorescence with a peak at ; 450 nm w10x. Hereafter, the former fluorescence is referred to as monomer fluorescence. The fluorescence spectra of both compounds at a concentration of 1.5 mol% are shown in Fig. 1, together with the electrofluorescence spectra observed with a field strength of 0.5 MV cmy1 . Note that the concentration is given by the molar fraction of the sample relative to the monomer unit of PMMA. The exciplex fluorescence is attributed to the photoinduced electron transfer from the excited state of CZ to TAME. The presence of a photoinduced electron transfer process was supported by the photocurrent measurements w11x. The exciplex fluorescence is very weak at low sample concentrations. With increasing concentration, however, the exciplex fluorescence relative to the monomer fluorescence becomes stronger, indicating that the observed exciplex fluorescence comes mainly from intermolecular electron transfer, rather than the intramolecular electron transfer. The monomer fluorescence both of D-Ž4.-A and of D-Ž12.-A is quenched by an electric field Ž F ., as is shown in Fig. 1. These results are the same as the ones of D-Ž3.-A or D-Ž20.-A, which were presented
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
previously w6x. The field-induced quenching of the monomer fluorescence is attributed to the field-induced enhancement of the rate of the electron transfer, which competes with the fluorescence process at the locally exited state of CZ. It was suggested in a previous paper w5x that the field-induced change in fluorescence intensity relative to the total intensity, Ž D I FrIF d ., is related to the field-induced change in the electron transfer rate Ž D k et . by the following equation: D k et s yŽ D I FrIF .rt f , when D I FrIF < 1. Here t f is the average fluorescence lifetime. Note that D I FrIF corresponds to the field-induced change in quantum yield relative to the total yield of the monomer fluorescence. Fluorescence decays of D-Ž4.-A and D-Ž12.-A doped in a PMMA polymer film at 1.5 mol% were obtained both in the absence and in the presence of
11
F by monitoring the monomer fluorescence at 370 nm over the time range of 0–40 ns. One example of the results is shown in Fig. 2 both for D-Ž4.-A and for D-Ž12.-A. Hereafter, the zero-field decay and the decay in the presence of F are denoted by I Fs 0 Ž t . and I F Ž t ., respectively. The difference between these two decays is denoted by D I F Ž t .; D I F Ž t . s I Fs0 Ž t . y I F Ž t .. Apparently, both I Fs0 Ž t . and I F Ž t . give the same shape of decay, even when the field strength was as large as 1.0 MV cmy1 , as is shown in Fig. 2. However, D I F Ž t . gives a positive intensity over the whole time region Žsee Fig. 2., indicating that the monomer fluorescence is quenched by F. Further, D I F Ž t . gives a shape that is different from I Fs0 Ž t . or I F Ž t ., as is shown in Fig. 2. This result indicates that the rate of the electron transfer which competes with the fluorescing process is affected by F.
Fig. 2. Fluorescence decays of D-Ž4.-A and D-Ž12.-A doped in a PMMA film at 1.5 mol% observed in the absence Žsolid line. and in the presence of an electric field of 1.0 MV cmy1 Ždotted line. are shown in Ža. and Žb., respectively. The difference between the decay at zero field and the decay at 1.0 MV cmy1 , i.e., D I F Ž t ., is shown in Žc. and Žd. by a thin solid line, together with the decay observed at zero field Žthick solid line., where the maximum intensity is normalized to the intensity of D I F Ž t . at the initial stage of time.
12
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
The monomer fluorescence both of D-Ž4.-A and of D-Ž12.-A shows a multi-exponential decay at a concentration of 1.5 mol% Žsee Fig. 2., and the observed decays were analyzed by assuming that both I F Ž t . and I Fs 0 Ž t . are given by Ý i A i expŽytrt i ., where A i and t i are the pre-exponential factor and lifetime of component i, respectively. Actually, the observed decays at 1.5 mol% could be simulated by assuming a superposition of four single exponential decays in D-Ž4.-A and by assuming a superposition of three single exponential decays in D-Ž12.-A. The average lifetime defined as HI F Ž t . d trIF Ž0., i.e., t f , is given by Ý i A it irÝ i A i . The t f values were determined for both D-Ž4.-A and D-Ž12.-A at different field strengths, and D k et was evaluated at each field strength of F by a subtraction of 1rt f at zero field Ž1rt f0 . from 1rt f at F Ž1rt fF .; D k et s 1rt fF y 1rt f0 . Plots of D k et as a function of F are shown in Fig. 3, together with the plots of D I FrIF as a function of F. As is shown in the figure, D k et increases nearly quadratically with increasing F in both compounds, indicating that the electron transfer is enhanced by F. Note that the plots of D I FrIF against F also suggest that D k et is
increased quadratically with increasing F in agreement with the results of D-Ž3.-A and D-Ž20.-A w6x. It is also found that D k et of D-Ž4.-A is larger than that of D-Ž12.-A at each field strength. The present direct measurements of the electric field dependence of the fluorescence decay profile confirm our previous conclusion that the field-induced quenching of the monomer fluorescence comes from the field-induced enhancement of the electron transfer process, which was derived from the measurements of the electrofluorescence spectra. Note that the field-induced enhancement of the electron transfer process gives a shorter fluorescence lifetime, since the electron transfer from the excited state of CZ to TAME competes with the fluorescing process. It is also confirmed that the magnitude of the fieldinduced enhancement of k et of D-Ž4.-A is larger than that of D-Ž12.-A, in agreement with the methylene chain length dependence of D I FrIF Žsee Fig. 3.. It was also found that different decaying portions of the multi-exponential decays of the monomer fluorescence show different efficiencies of the field dependence from each other, as will be described below.
Fig. 3. Solid circles show plots of D k et as a function of applied field strength in D-Ž4.-A Žleft. and in D-Ž12.-A Žright.. Plots of the field-induced change in intensity relative to the total intensity at 370 nm, which corresponds to the monomer fluorescence, as a function of the field strength are also shown by solid triangle.
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
Multi-exponential fluorescence decays are attributed to the presence of different donor–acceptor pairs which have different distances and orientations w12x. The lifetime and pre-exponential factor of each decaying portion of the multi-exponential decay of D-Ž12.-A observed at 1.5 mol% in the absence of F were determined to be as follows: t 1 s 0.87, t 2 s 3.39, t 3 s 11.40 ns and A1 s 0.455, A 2 s 0.498, A 3 s 0.047. The corresponding values in the presence of 1.0 MV cmy1 were determined to be as follows: t 1 s 0.77, t 2 s 3.17 and t 3 s 10.64 ns and A1 s 0.439, A 2 s 0.507, A 3 s 0.054. The average lifetime, i.e., t f , was determined to be 2.62 ns at zero field and 2.52 ns at 1.0 MV cmy1 . Thus, not only the average lifetime but also the lifetime of every decaying portion becomes shorter in the presence of F. However, the fastest decaying portion gives the largest field effect; the lifetime of the fastest component becomes shorter by a factor of more than 10% in the presence of 1.0 MV cmy1 , while the lifetime of the other decaying portions becomes shorter by a factor of several percentage. These results imply that D k et as well as k et increases with decreasing donor–acceptor distance, in agreement with the conclusion derived from the concentration dependence of the field-induced quenching of the monomer fluorescence w5x. The lifetime and the pre-exponential factor of each decaying portion of the multi-exponential decays of D-Ž4.-A observed at 1.5 mol% at zero field were determined to be as follows: t 1 s 0.32, t 2 s 1.35, t 3 s 3.81, t4 s 13.0 ns and A1 s 0.29, A 2 s 0.38, A 3 s 0.29, A 4 s 0.04. The corresponding values with a field strength of 1.0 MV cmy1 were determined to be as follows: t 1 s 0.27, t 2 s 1.44, t 3 s 4.15, t4 s 14.59 ns and A1 s 0.302, A 2 s 0.418, A 3 s 0.253, A 4 s 0.027. The average lifetime was determined to be 2.23 ns at zero field and 2.13 ns at 1.0 MV cmy1 Žsee Fig. 3., indicating that the electron transfer rate becomes larger in the presence of F. The lifetime of the fastest decaying component becomes shorter in the presence of F by a factor of ; 15%. The decrease of the lifetime of the fastest decaying portion is attributed to the field-induced enhancement of the electron transfer rate, as in the case of D-Ž12.-A. In contrast with D-Ž12.-A, however, the lifetime of the other slowly decaying portions becomes longer in the presence of F. The
13
opposite field dependence of the second and other slowly decaying portions may be attributed to the field effect on the back electron transfer process which produces the excited state of CZ. In order to understand the opposite field dependence in D-Ž4.-A, however, further studies of the chain length dependence are necessary. Here, a rising decay profile at the initial stage of time in D I F Ž t . may be worth mentioning. When the number of the photoexcited molecules is not affected by F, the value of D I F Ž t . at t s 0 must be zero, as long as fluorescence decays are measured with a sufficiently short pulse excitation and with a sufficient time resolution for detection. In such a case, a rising profile must be observed at the initial stage of time in the decay of D I F Ž t ., if field-induced quenching occurs following photoexcitation. In fact, a rising profile was observed in D I F Ž t . of D-Ž12.-A Žsee Fig. 2d., and the peak in D I F Ž t . was found to be shifted from the peak in I F Ž t . by about 1.2 ns, which is essentially the same as the value estimated from the analyzed decay profiles under the present experimental conditions, i.e. 1.1 ns. In contrast with D-Ž12.-A, however, a rising profile was not confirmed in D-Ž4.A Žsee Fig. 2c.. As mentioned above, the lifetime of the shortest component of fluorescence of D-Ž4.-A is much shorter than that of D-Ž12.-A. The time interval between peaks of I F Ž t . and D I F Ž t . in D-Ž4.-A is estimated from the analyzed decay profiles to be as small as 0.28 ns, implying that the time interval between these two peaks expected for D-Ž4.-A is much smaller than that for D-Ž12.-A. Therefore, an apparent lack of the rising profile at the initial stage of time in D I F Ž t . of D-Ž4.-A may be attributed to the small time interval between peaks of I F Ž t . and D I F Ž t ., besides the low signal-to-noise ratio in D I F Ž t .. The above-mentioned values of the lifetime and the pre-exponential factor are the averaged ones of the several decays measured for different samples. The experimental error of the determined lifetime is not so small, but the following can be concluded safely: Ž1. the averaged lifetime becomes shorter in the presence of F; Ž2. the fastest decaying portion shows the most efficient decrease of lifetime by F; Ž3. the decrease both of the average lifetime and of the lifetime of the fastest decaying portion of D-Ž4.-A is larger than that of D-Ž12.-A; Ž4. different decaying
14
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
portions of the multi-exponential decay show different efficiencies of the field dependence from each other; Ž5. the relation of the field dependence among different portions of the multi-exponential decays depends on the methylene chain length. These results indicate that not only the intermolecular photoinduced electron transfer process but also the intramolecular process are concerned in the excitation dynamics of D-Ž4.-A and D-Ž12.-A and its field effects, though the concentration dependence of the fluorescence spectra seems to show that only the intermolecular process occurs efficiently. In the above discussion, we focused our attention to the field effect on the monomer fluorescence. Finally, the field effects on the exciplex fluorescence is briefly discussed. In contrast with the monomer fluorescence, the exciplex fluorescence is enhanced by F at 1.5 mol% in both compounds. A charge recombination of the radical ion pair produced by photoinduced electron transfer, which leads to a quenching of the exciplex fluorescence, is considered to occur at zero field. In the presence of F, the recombination is inhibited and the exciplex fluorescence is recovered. Then, the field-induced enhancement of the exciplex fluorescence is attributed to an inhibition of the charge recombination by F, as mentioned previously w6x. The magnitude of the enhancement of D-Ž4.-A is much larger than that of D-Ž12.-A, indicating that the charge recombination at zero field occurs more efficiently in D-Ž4.-A than in D-Ž12.-A. It should also be noted that the electrofluorescence spectra of the exciplex fluorescence are given by a linear combination of the exciplex fluorescence spectrum and its first derivative spectrum. The latter contribution comes from the Stark shift w13x, as in the case of n s 3 or n s 20 w6x, and the
molecular polarizability of the exciplex is evaluated ˚ 3 by to be as large as ; 2000 in units of 4 p´ 0 A assuming that the local field is the same as the applied field. Acknowledgements This work was supported by Grant-in-Aids for the Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan ŽNo. 10440163 and No. 09740507.. References w1x Z.D. Popovic, G.J. Kovacs, P.S. Vincett, G. Alegria, P.L. Dutton, Biochem. Biophys. Acta 851 Ž1986. 38. w2x D.J. Lockhart, S.G. Boxer, Chem. Phys. Lett. 144 Ž1988. 243. w3x M. Bixon, J. Jortner, J. Phys. Chem. 92 Ž1988. 7148. w4x K. Lao, S. Franzen, R.J. Stanley, D.G. Lambright, S.G. Boxer, J. Phys. Chem. 97 Ž1993. 13165. w5x N. Ohta, M. Koizumi, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Phys. Chem. 100 Ž1996. 16466. w6x N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, Y. Tanimoto, Y. Hatano, M. Yamamoto, H. Kono, J. Phys. Chem. 100 Ž1996. 19295. w7x Z.D. Popovic, R.O. Loutfy, A. Hor, W.B. Jackson, R.A. Street, SPIE 743 Ž1987. 29. w8x Z.D. Popovic, M.I. Khan, S.J. Atherton, A. Hor, J.L. Goodman, J. Phys. Chem. B 102 Ž1998. 657. w9x N. Ohta, T. Tamai, T. Kuroda, T. Yamazaki, Y. Nishimura, I. Yamazaki, Chem. Phys. 177 Ž1993. 591. w10x Y. Hatano, M. Yamamoto, Y. Nishijima, J. Phys. Chem. 82 Ž1978. 367. w11x Y. Nishimura, N. Ohta, M. Yamamoto, I. Yamazaki, Mol. Crys. Liq. Cryst. 315 Ž1998. 181. w12x S. Murata, M. Tachiya, J. Phys. Chem. 100 Ž1996. 4064. w13x W. Liptay, in: E.C. Lim ŽEd.., Excited States, Academic Press, New York, 1974, p. 129.
Chemical Physics Letters 307 Ž1999. 8–14
Measurements of the electric-field-induced change in fluorescence decay profile of methylene-linked carbazole and terephthalic acid methyl ester in a PMMA polymer film Yoshinobu Nishimura a , Iwao Yamazaki a , Masahide Yamamoto b, Nobuhiro Ohta
c,)
a b
Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060-8628, Japan Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniÕersity, Sakyou-ku Kyoto 606-8501, Japan c Research Institute for Electronic Science, Hokkaido UniÕersity, Sapporo 060-0812, Japan Received 29 January 1999; in final form 15 April 1999
Abstract The electric-field-induced change in the fluorescence decay profile has been measured for methylene-linked carbazole ŽCZ. and terephthalic acid methyl ester ŽTAME. doped in a PMMA polymer film, using a picosecond time-resolved decay measurement system combined with a bipolar sample bias. The average lifetime of the fluorescence emitted from the locally excited state of CZ becomes shorter in the presence of an external electric field Ž F ., confirming that the rate of photoinduced electron transfer from CZ to TAME is enhanced by F. Different decaying portions of the multi-exponential decay were also found to give different efficiencies in the electric field effect from each other, depending on the methylene chain length. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The electric field effect on excitation dynamics is one of the current topics in relation to photochemical processes in biological systems. In the photosynthetic reaction center, for example, the electric field produced by the oriented membrane protein in which the chromophores of bacteriochrollophils are encased seems to play an important role in the photoinduced charge separation process, producing a high efficiency and high selectivity. The electric field effects
)
Corresponding author. E-mail: [email protected]
on the initial step of the photoinduced electron transfer process have been extensively examined w1–4x. In our previous papers w5,6x, the external electric field effects Žbased on the measurements of the effects on the fluorescence spectra. on the photoinduced electron transfer processes in a mixture of ethyl carbazole and dimethyl terephthalate and in their methylene-linked compounds were reported. In both systems, it was found that fluorescence emitted from the locally excited state of carbazole was quenched by an electric field. The quenching was attributed to the field-induced enhancement of the rate of the electron transfer from the excited state of carbazole to terephthalic acid, since the electron transfer rate and its field-induced change become larger with increasing sample concentration, i.e. with
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 6 0 - 1
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
decreasing donor–acceptor distance. Even when the electron transfer rate is not affected by an electric field, however, fluorescence quenching occurs if the formation yield of the emissive state of the carbazole chromophore generated by photoexcitation is reduced by an electric field. In the latter case, the fluorescence decay profile is not affected by an
9
electric field. In the former case, on the other hand, the fluorescence lifetime becomes shorter in the presence of an electric field since the electron transfer is considered to compete with the fluorescing process. In order to confirm the mechanism of the field-induced quenching of fluorescence, therefore, the direct measurements of the field effects on the
Fig. 1. Broken line shows fluorescence spectra and solid line shows electrofluorescence spectra of D-Ž4.-A Župper. and D-Ž12.-A Žlower. doped in a PMMA film at a concentration of 1.5 mol%. The electrofluorescence spectra were obtained with a field strength of 0.5 MV cmy1 , and slant lines are inserted to highlight the difference between the positive and the negative parts. Maximum intensity of fluorescence is normalized to unity. The molecular structure of the sample is also shown.
10
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
fluorescence decay profile are necessary, as reported by Popovic and his co-workers in some organic photoconductors w7,8x. In the present study, the direct measurements of the field-induced change in the decay profile have been carried out with picosecond time-resolution, and preliminary results on methylene-linked compounds of CZ– ŽCH 2 . n –TAME with n s 4 and n s 12, hereafter denoted by D-Ž4.-A and D-Ž12.-A, respectively, are reported. Here, CZ Žmethylene-linked carbazole. and TAME represent the carbazole chromophore and terephthalic acid methyl ester, respectively, and the molecular structure of the sample is shown in Fig. 1.
ŽLaboratory Equipment MCArPC98.. The time duration of each bias was 25 ms, but the first 10 ms was a deadtime and the decay measurements were carried out only for the time range 10–25 ms in order to exclude an overshooting effect of the applied field just after the change in applied voltage. The steady-state electroabsorption and electrofluorescence spectra were measured using modulation spectroscopy with the same apparatus as those reported in previous papers w5,6x. The excitation wavelength was 294 nm, where the absorption spectrum shows a maximum intensity of the S 0 ™ S 2 band. The bandwidth of the instrument response function was 39 ps ŽFWHM. at 294 nm when the decay profile was measured with a time intervalrchannel of 5.0 psrch.
2. Experimental The samples were prepared using the methods reported elsewhere w5,6x. A PMMA polymer film which contains D-Ž4.-A or D-Ž12.-A was cast on the ITO-coated quartz substrate by a spin coating method. Then, a semitransparent aluminum ŽAl. film was deposited on the polymer film. ITO and Al films were used as electrodes. The thickness of the polymer film was determined with a nanospecrAFT system ŽM3000, nanometrics.. Fluorescence decay measurements were carried out with a single-photon counting system combined with a custom-made pulse generator supplying the bipolar square wave up to 100 V in amplitude. The emission and scattered light were detected with a microchannel plate ŽHamamatsu R3809. w9x. The excitation source was a mode-locked Ti:Sa laser ŽCoherent, Mira 900. pumped by an Ar ion laser ŽCoherent, Innova 300.. The repetition rate of the laser pulse was 2.9 MHz with a pulse picker ŽCoherent, Model 9200.. The third harmonic generated by an ultrafast harmonic system ŽInrad, Model 5-050. was used for excitation. The applied field strength was evaluated from the applied voltage divided by the film thickness. The applied voltage was a repetition of rectangular waves of negative, zero, positive and zero bias in turn. Three different decay curves were collected, corresponding to positive, negative and zero sample bias. These decays were stored in each of the different memory-channels of a multichannel analyzer
3. Results and discussion Fluorescence spectra of D-Ž4.-A and D-Ž12.-A doped in a PMMA polymer film are regarded as a mixture of sharp-structured fluorescence emitted from the locally excited S 1 state of CZ and a broad exciplex fluorescence with a peak at ; 450 nm w10x. Hereafter, the former fluorescence is referred to as monomer fluorescence. The fluorescence spectra of both compounds at a concentration of 1.5 mol% are shown in Fig. 1, together with the electrofluorescence spectra observed with a field strength of 0.5 MV cmy1 . Note that the concentration is given by the molar fraction of the sample relative to the monomer unit of PMMA. The exciplex fluorescence is attributed to the photoinduced electron transfer from the excited state of CZ to TAME. The presence of a photoinduced electron transfer process was supported by the photocurrent measurements w11x. The exciplex fluorescence is very weak at low sample concentrations. With increasing concentration, however, the exciplex fluorescence relative to the monomer fluorescence becomes stronger, indicating that the observed exciplex fluorescence comes mainly from intermolecular electron transfer, rather than the intramolecular electron transfer. The monomer fluorescence both of D-Ž4.-A and of D-Ž12.-A is quenched by an electric field Ž F ., as is shown in Fig. 1. These results are the same as the ones of D-Ž3.-A or D-Ž20.-A, which were presented
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
previously w6x. The field-induced quenching of the monomer fluorescence is attributed to the field-induced enhancement of the rate of the electron transfer, which competes with the fluorescence process at the locally exited state of CZ. It was suggested in a previous paper w5x that the field-induced change in fluorescence intensity relative to the total intensity, Ž D I FrIF d ., is related to the field-induced change in the electron transfer rate Ž D k et . by the following equation: D k et s yŽ D I FrIF .rt f , when D I FrIF < 1. Here t f is the average fluorescence lifetime. Note that D I FrIF corresponds to the field-induced change in quantum yield relative to the total yield of the monomer fluorescence. Fluorescence decays of D-Ž4.-A and D-Ž12.-A doped in a PMMA polymer film at 1.5 mol% were obtained both in the absence and in the presence of
11
F by monitoring the monomer fluorescence at 370 nm over the time range of 0–40 ns. One example of the results is shown in Fig. 2 both for D-Ž4.-A and for D-Ž12.-A. Hereafter, the zero-field decay and the decay in the presence of F are denoted by I Fs 0 Ž t . and I F Ž t ., respectively. The difference between these two decays is denoted by D I F Ž t .; D I F Ž t . s I Fs0 Ž t . y I F Ž t .. Apparently, both I Fs0 Ž t . and I F Ž t . give the same shape of decay, even when the field strength was as large as 1.0 MV cmy1 , as is shown in Fig. 2. However, D I F Ž t . gives a positive intensity over the whole time region Žsee Fig. 2., indicating that the monomer fluorescence is quenched by F. Further, D I F Ž t . gives a shape that is different from I Fs0 Ž t . or I F Ž t ., as is shown in Fig. 2. This result indicates that the rate of the electron transfer which competes with the fluorescing process is affected by F.
Fig. 2. Fluorescence decays of D-Ž4.-A and D-Ž12.-A doped in a PMMA film at 1.5 mol% observed in the absence Žsolid line. and in the presence of an electric field of 1.0 MV cmy1 Ždotted line. are shown in Ža. and Žb., respectively. The difference between the decay at zero field and the decay at 1.0 MV cmy1 , i.e., D I F Ž t ., is shown in Žc. and Žd. by a thin solid line, together with the decay observed at zero field Žthick solid line., where the maximum intensity is normalized to the intensity of D I F Ž t . at the initial stage of time.
12
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
The monomer fluorescence both of D-Ž4.-A and of D-Ž12.-A shows a multi-exponential decay at a concentration of 1.5 mol% Žsee Fig. 2., and the observed decays were analyzed by assuming that both I F Ž t . and I Fs 0 Ž t . are given by Ý i A i expŽytrt i ., where A i and t i are the pre-exponential factor and lifetime of component i, respectively. Actually, the observed decays at 1.5 mol% could be simulated by assuming a superposition of four single exponential decays in D-Ž4.-A and by assuming a superposition of three single exponential decays in D-Ž12.-A. The average lifetime defined as HI F Ž t . d trIF Ž0., i.e., t f , is given by Ý i A it irÝ i A i . The t f values were determined for both D-Ž4.-A and D-Ž12.-A at different field strengths, and D k et was evaluated at each field strength of F by a subtraction of 1rt f at zero field Ž1rt f0 . from 1rt f at F Ž1rt fF .; D k et s 1rt fF y 1rt f0 . Plots of D k et as a function of F are shown in Fig. 3, together with the plots of D I FrIF as a function of F. As is shown in the figure, D k et increases nearly quadratically with increasing F in both compounds, indicating that the electron transfer is enhanced by F. Note that the plots of D I FrIF against F also suggest that D k et is
increased quadratically with increasing F in agreement with the results of D-Ž3.-A and D-Ž20.-A w6x. It is also found that D k et of D-Ž4.-A is larger than that of D-Ž12.-A at each field strength. The present direct measurements of the electric field dependence of the fluorescence decay profile confirm our previous conclusion that the field-induced quenching of the monomer fluorescence comes from the field-induced enhancement of the electron transfer process, which was derived from the measurements of the electrofluorescence spectra. Note that the field-induced enhancement of the electron transfer process gives a shorter fluorescence lifetime, since the electron transfer from the excited state of CZ to TAME competes with the fluorescing process. It is also confirmed that the magnitude of the fieldinduced enhancement of k et of D-Ž4.-A is larger than that of D-Ž12.-A, in agreement with the methylene chain length dependence of D I FrIF Žsee Fig. 3.. It was also found that different decaying portions of the multi-exponential decays of the monomer fluorescence show different efficiencies of the field dependence from each other, as will be described below.
Fig. 3. Solid circles show plots of D k et as a function of applied field strength in D-Ž4.-A Žleft. and in D-Ž12.-A Žright.. Plots of the field-induced change in intensity relative to the total intensity at 370 nm, which corresponds to the monomer fluorescence, as a function of the field strength are also shown by solid triangle.
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
Multi-exponential fluorescence decays are attributed to the presence of different donor–acceptor pairs which have different distances and orientations w12x. The lifetime and pre-exponential factor of each decaying portion of the multi-exponential decay of D-Ž12.-A observed at 1.5 mol% in the absence of F were determined to be as follows: t 1 s 0.87, t 2 s 3.39, t 3 s 11.40 ns and A1 s 0.455, A 2 s 0.498, A 3 s 0.047. The corresponding values in the presence of 1.0 MV cmy1 were determined to be as follows: t 1 s 0.77, t 2 s 3.17 and t 3 s 10.64 ns and A1 s 0.439, A 2 s 0.507, A 3 s 0.054. The average lifetime, i.e., t f , was determined to be 2.62 ns at zero field and 2.52 ns at 1.0 MV cmy1 . Thus, not only the average lifetime but also the lifetime of every decaying portion becomes shorter in the presence of F. However, the fastest decaying portion gives the largest field effect; the lifetime of the fastest component becomes shorter by a factor of more than 10% in the presence of 1.0 MV cmy1 , while the lifetime of the other decaying portions becomes shorter by a factor of several percentage. These results imply that D k et as well as k et increases with decreasing donor–acceptor distance, in agreement with the conclusion derived from the concentration dependence of the field-induced quenching of the monomer fluorescence w5x. The lifetime and the pre-exponential factor of each decaying portion of the multi-exponential decays of D-Ž4.-A observed at 1.5 mol% at zero field were determined to be as follows: t 1 s 0.32, t 2 s 1.35, t 3 s 3.81, t4 s 13.0 ns and A1 s 0.29, A 2 s 0.38, A 3 s 0.29, A 4 s 0.04. The corresponding values with a field strength of 1.0 MV cmy1 were determined to be as follows: t 1 s 0.27, t 2 s 1.44, t 3 s 4.15, t4 s 14.59 ns and A1 s 0.302, A 2 s 0.418, A 3 s 0.253, A 4 s 0.027. The average lifetime was determined to be 2.23 ns at zero field and 2.13 ns at 1.0 MV cmy1 Žsee Fig. 3., indicating that the electron transfer rate becomes larger in the presence of F. The lifetime of the fastest decaying component becomes shorter in the presence of F by a factor of ; 15%. The decrease of the lifetime of the fastest decaying portion is attributed to the field-induced enhancement of the electron transfer rate, as in the case of D-Ž12.-A. In contrast with D-Ž12.-A, however, the lifetime of the other slowly decaying portions becomes longer in the presence of F. The
13
opposite field dependence of the second and other slowly decaying portions may be attributed to the field effect on the back electron transfer process which produces the excited state of CZ. In order to understand the opposite field dependence in D-Ž4.-A, however, further studies of the chain length dependence are necessary. Here, a rising decay profile at the initial stage of time in D I F Ž t . may be worth mentioning. When the number of the photoexcited molecules is not affected by F, the value of D I F Ž t . at t s 0 must be zero, as long as fluorescence decays are measured with a sufficiently short pulse excitation and with a sufficient time resolution for detection. In such a case, a rising profile must be observed at the initial stage of time in the decay of D I F Ž t ., if field-induced quenching occurs following photoexcitation. In fact, a rising profile was observed in D I F Ž t . of D-Ž12.-A Žsee Fig. 2d., and the peak in D I F Ž t . was found to be shifted from the peak in I F Ž t . by about 1.2 ns, which is essentially the same as the value estimated from the analyzed decay profiles under the present experimental conditions, i.e. 1.1 ns. In contrast with D-Ž12.-A, however, a rising profile was not confirmed in D-Ž4.A Žsee Fig. 2c.. As mentioned above, the lifetime of the shortest component of fluorescence of D-Ž4.-A is much shorter than that of D-Ž12.-A. The time interval between peaks of I F Ž t . and D I F Ž t . in D-Ž4.-A is estimated from the analyzed decay profiles to be as small as 0.28 ns, implying that the time interval between these two peaks expected for D-Ž4.-A is much smaller than that for D-Ž12.-A. Therefore, an apparent lack of the rising profile at the initial stage of time in D I F Ž t . of D-Ž4.-A may be attributed to the small time interval between peaks of I F Ž t . and D I F Ž t ., besides the low signal-to-noise ratio in D I F Ž t .. The above-mentioned values of the lifetime and the pre-exponential factor are the averaged ones of the several decays measured for different samples. The experimental error of the determined lifetime is not so small, but the following can be concluded safely: Ž1. the averaged lifetime becomes shorter in the presence of F; Ž2. the fastest decaying portion shows the most efficient decrease of lifetime by F; Ž3. the decrease both of the average lifetime and of the lifetime of the fastest decaying portion of D-Ž4.-A is larger than that of D-Ž12.-A; Ž4. different decaying
14
Y. Nishimura et al.r Chemical Physics Letters 307 (1999) 8–14
portions of the multi-exponential decay show different efficiencies of the field dependence from each other; Ž5. the relation of the field dependence among different portions of the multi-exponential decays depends on the methylene chain length. These results indicate that not only the intermolecular photoinduced electron transfer process but also the intramolecular process are concerned in the excitation dynamics of D-Ž4.-A and D-Ž12.-A and its field effects, though the concentration dependence of the fluorescence spectra seems to show that only the intermolecular process occurs efficiently. In the above discussion, we focused our attention to the field effect on the monomer fluorescence. Finally, the field effects on the exciplex fluorescence is briefly discussed. In contrast with the monomer fluorescence, the exciplex fluorescence is enhanced by F at 1.5 mol% in both compounds. A charge recombination of the radical ion pair produced by photoinduced electron transfer, which leads to a quenching of the exciplex fluorescence, is considered to occur at zero field. In the presence of F, the recombination is inhibited and the exciplex fluorescence is recovered. Then, the field-induced enhancement of the exciplex fluorescence is attributed to an inhibition of the charge recombination by F, as mentioned previously w6x. The magnitude of the enhancement of D-Ž4.-A is much larger than that of D-Ž12.-A, indicating that the charge recombination at zero field occurs more efficiently in D-Ž4.-A than in D-Ž12.-A. It should also be noted that the electrofluorescence spectra of the exciplex fluorescence are given by a linear combination of the exciplex fluorescence spectrum and its first derivative spectrum. The latter contribution comes from the Stark shift w13x, as in the case of n s 3 or n s 20 w6x, and the
molecular polarizability of the exciplex is evaluated ˚ 3 by to be as large as ; 2000 in units of 4 p´ 0 A assuming that the local field is the same as the applied field. Acknowledgements This work was supported by Grant-in-Aids for the Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan ŽNo. 10440163 and No. 09740507.. References w1x Z.D. Popovic, G.J. Kovacs, P.S. Vincett, G. Alegria, P.L. Dutton, Biochem. Biophys. Acta 851 Ž1986. 38. w2x D.J. Lockhart, S.G. Boxer, Chem. Phys. Lett. 144 Ž1988. 243. w3x M. Bixon, J. Jortner, J. Phys. Chem. 92 Ž1988. 7148. w4x K. Lao, S. Franzen, R.J. Stanley, D.G. Lambright, S.G. Boxer, J. Phys. Chem. 97 Ž1993. 13165. w5x N. Ohta, M. Koizumi, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Phys. Chem. 100 Ž1996. 16466. w6x N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, Y. Tanimoto, Y. Hatano, M. Yamamoto, H. Kono, J. Phys. Chem. 100 Ž1996. 19295. w7x Z.D. Popovic, R.O. Loutfy, A. Hor, W.B. Jackson, R.A. Street, SPIE 743 Ž1987. 29. w8x Z.D. Popovic, M.I. Khan, S.J. Atherton, A. Hor, J.L. Goodman, J. Phys. Chem. B 102 Ž1998. 657. w9x N. Ohta, T. Tamai, T. Kuroda, T. Yamazaki, Y. Nishimura, I. Yamazaki, Chem. Phys. 177 Ž1993. 591. w10x Y. Hatano, M. Yamamoto, Y. Nishijima, J. Phys. Chem. 82 Ž1978. 367. w11x Y. Nishimura, N. Ohta, M. Yamamoto, I. Yamazaki, Mol. Crys. Liq. Cryst. 315 Ž1998. 181. w12x S. Murata, M. Tachiya, J. Phys. Chem. 100 Ž1996. 4064. w13x W. Liptay, in: E.C. Lim ŽEd.., Excited States, Academic Press, New York, 1974, p. 129.