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Direct measurements of the electric-field-induced change in fluorescence decay profile of pyrene doped in a polymer film
Chemical Physics Letters 395 (2004) 346–350 www.elsevier.com/locate/cplett
Direct measurements of the electric-field-induced change in fluorescence decay profile of pyrene doped in a polymer film Takakazu Nakabayashi a b
a,b
, Takehiro Morikawa b, Nobuhiro Ohta
a,b,*
Research Institute for Electronic Science (RIES), Hokkaido University, Sapporo 060-0812, Japan Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received 14 May 2004; in final form 28 July 2004 Available online 21 August 2004
Abstract Electric-field-induced change in fluorescence lifetime and pre-exponential factor has been measured for each fluorescence component of pyrene doped in a polymer film at a high concentration. Based on the results, the origin both of the field-induced quenching of structured fluorescence with a peak at 380 nm and broad excimer fluorescence with a peak at 470 nm and of the field-induced enhancement of another excimer fluorescence with a peak at 415 nm is discussed. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Elucidation of the electric field effects on excitation dynamics is very important for through understanding of reaction mechanism in biological systems since electric fields produced by protein membranes seem to play a significant role in photochemical reactions [1–4]. The elucidation of the field effects is also important for design and development of new materials having photoinduced functions such as photoconductivity or electroluminescence properties [5–7]. For example, applied field strength dependence of the field-induced quenching of excimer fluorescence was shown to be strongly related to the generation efficiency of electroluminescence for methylene-linked compounds of pyrene doped in a polymer film [8], based on the measurements of electrofluorescence spectra (plots of the electric-field-induced change in fluorescence intensity as a function of wavelength). The efficiency of electroluminescence of pyrene doped in a polymer film seems to be not so efficient *
Corresponding author. E-mail address: [email protected] (N. Ohta).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.103
as that of linked compounds of pyrene, but fluorescence intensity of bare molecules of pyrene doped in a poly(methyl methacrylate) (PMMA) film is also markedly influenced by an electric field at high concentrations [9]. Electrofluorescence spectra show that different components of fluorescence of pyrene give different field dependence of quantum yield from each other. As far as the experiments are limited to the steady-state measurements, however, it is very difficult to confirm the mechanism of the electric field effects on each fluorescence component. For example, a field-induced change in emission intensity can be ascribed to a change in emission lifetime or a change in the initial population of the emitting states. In order to elucidate the mechanism of field effects on photoexcitation dynamics, therefore, direct measurements of the field-induced change in emission decay profile are essential. In the present study, we have carried out such a measurement for pyrene doped in a polymer film, using a picosecond time-resolved decay measurement system combined with an electromodulation apparatus [10]. The origins of the field-induced changes in fluorescence quantum yield are discussed on the basis of the results obtained.
T. Nakabayashi et al. / Chemical Physics Letters 395 (2004) 346–350
2. Experimental Pyrene and PMMA were purified in the same manner as mentioned previously [11]. A certain amount of benzene solution of pyrene and PMMA was cast on an ITO coated quartz substrate by a spin coating method. Then, a semitransparent aluminum (Al) film was deposited on the dried polymer film. The ITO and Al films were used as electrodes. The thickness of the polymer film was typically 0.6 lm. All the measurements were performed at room temperature under vacuum conditions. Measurements of the field-induced change in fluorescence decay profile were carried out using a singlephoton counting lifetime measurement system combined with a pulse generator supplying a bipolar square wave, which was constructed recently in our laboratory [10,12]. Briefly, the third harmonic of the mode-locked Ti:sapphire laser (Spectra Physics, Tsunami), whose pulse duration was 200 fs, was used for excitation. The repetition rate of the laser pulse was selected to be 1.6 MHz with a pulse picker (Conoptics, model 350– 160). Fluorescence from the sample was detected by a microchannel-plate photomultiplier (Hamamatsu, R3809 U-52). Fluorescence signal was discriminated and then led to a time-to-amplitude converter system. Applied voltage was a repetition of rectangular waves of positive, zero, negative, and zero bias. The time duration of each bias was 30 ms, but the first 3 ms was used as a dead time for emission detection to exclude an overshooting effect of the applied field. Four different decays were collected, corresponding to positive, zero, negative, and zero sample bias, respectively. The instrumental response function had a pulse width of 60 ps (FWHM). Hereafter, applied field is denoted by F. Steady-state electrofluorescence spectra were also measured using electric field modulation spectroscopy with the same apparatus as reported previously [13–15].
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the first excimer with increasing pyrene concentration in a PMMA film. As reported in a previous paper [9], monomer fluorescence as well as the first excimer fluorescence is quenched by an applied electric field, whereas the second excimer fluorescence is enhanced by an applied electric field, indicating that three fluorescence emissions give different electric field effects from each other (see Fig. 1). In order to examine the origin of the field-induced change in fluorescence intensity, decay profiles of pyrene doped in a PMMA film at 10 mol% were measured at zero field and in the presence of electric field of 0.9 MV cm1 by monitoring the emission at 380, 415 and 480 nm, respectively, which corresponds to monomer, second excimer and first excimer fluorescence, respectively. In every case, excitation wavelength was 299.0 nm, where the field-induced change in absorption intensity is negligible. The difference between the decay observed at zero field (I0(t)) and that at 0.9 MV cm1 (IF(t)), i.e., IF(t) I0(t), referred to as DIf(t), was obtained. Time profiles of DIf(t) are shown in Fig. 2, together with the fluorescence decay observed at zero field. As reported previously, all the emissions give a nonexponential decay, probably because of the presence of different interactions between pyrene chromophores, which gives different fluorescence lifetimes from each other [19]. As shown in Fig. 2, DIf(t) observed at 380 and 480 nm is negative during the full decay, while DIf(t) at 415 nm is positive during the decay, indicating that the former emissions are quenched in the presence of F and that the latter emission is enhanced by F. These results agree with the ones obtained from the steady-state measurements of electrofluorescence spectra (see Fig. 1). The time dependence of DIf(t) is different in shape from that of I0(t) as shown in Fig. 2, which clearly indicates that the lifetimes of monomer, first excimer, and second excimer are influenced by F. All the time profiles P are fitted by assuming a triexponential decay, i.e., iAi exp (t/si), where Ai and si de-
3. Results and discussion Fluorescence of pyrene doped in a PMMA film shows at least three components at high concentrations [8,9]: (1) a structured fluorescence in the region from 370– 450 nm, which is assigned as the fluorescence emitted from the locally excited state S1 of pyrene; (2) a broad emission with a peak at 470 nm, which is assigned as the fluorescence emitted from the sandwich type of pyrene excimer [16,17]; (3) a very weak emission with a peak at 415 nm, which is assigned as the fluorescence emitted from a partially overlapping type of pyrene excimer [18]. Here, these three emissions are denoted by monomer fluorescence, first excimer fluorescence and second excimer fluorescence, respectively. Note that monomer fluorescence, which is dominant at low concentrations below 1 mol%, becomes weaker in relative intensity to
Fig. 1. Fluorescence spectrum (solid line) and electrofluorescence spectrum (shaded line) of pyrene doped in a PMMA film at 10 mol% observed in the steady state measurement. The applied field strength was 0.8 MV cm1 in rms.
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Fig. 2. Fluorescence decay observed at zero field (dotted line) and the difference between the decays observed at zero field and at 0.9 MV cm1 (solid line) of pyrene doped in a PMMA film at 10 mol%. Excitation wavelength was 299.0 nm. Monitoring wavelength was 380 nm (a), 415 nm (b) and 480 nm (c). The response function is also shown by a broken line in (a)–(c).
note the pre-exponential factor and lifetime of component i, respectively. Observed and simulated results of the time profiles of I0(t), DIf(t) and the ratio of IF(t)/ I0(t) are shown in Figs. 3–5 for emission at 380, 415 and 480 nm, respectively. It is noted that a rise is observed for the first excimer fluorescence. The lifetime and pre-exponential factor of each component both at zero field and in the presence of 0.9 MV cm1 are shown in Table 1 for monomer fluorescence (380 nm), first excimer fluorescence (480 nm) and second excimer fluorescence (415 P nm). The P average lifetime ðsf Þ, which is given by A s / i i i iAi, is then determined to be 10.96 ± 0.02 and 10.91 ± 0.02 ns for monomer fluorescence, 50.66 ± 0.03 and 50.57 ± 0.03 ns for the first excimer fluorescence, and 34.38 ± 0.02 and 34.32 ± 0.02 ns for the second excimer fluorescence at zero field and at 0.9 MV cm1, respectively. These results show that the lifetime of every fluorescence emission becomes shorter in the presence of F. A field-induced change in lifetime
Fig. 3. (a) Fluorescence decay of the monomer of pyrene doped in a PMMA film at 10 mol% observed at 380 nm at zero field. (b) The difference between the decays observed at zero field and at 0.9 MV cm1. (c) The ratio of the decay observed at 0.9 MV cm1 relative to that at zero field. Simulated time profiles are shown by a dotted line in every case.
affects the time profile of IF(t)/I0(t) more than the decay profile itself, and a change in pre-exponential factor largely affects both the time profile of DIf(t) and the value of IF(t)/I0(t) at t = 0. By simulating not only the decay profile but also the time profile both of DIf(t) and of IF(t)/I0(t), therefore, the field-induced change in lifetime and pre-exponential factor could be determined very precisely, though the error within 0.05% or so is unavoidable in the field-induced change shown in Table 1. The present average lifetime at zero field is essentially the same as the one reported previously in every case, but the lifetime of the fastest decaying component of the monomer fluorescence is a little different from the previous one. This difference may come from an inhomogeneous distribution of pyrene in PMMA. It must be confessed that lifetimes as well as pre-exponential factors of different samples are not identical with each other completely, even when the concentrations are the same. It is worth mentioning, however, that the sample used for the decay measurements shown in Figs. 2–5 as well as other samples with the same concentration gives essentially the same electrofluorescence spectra with the
T. Nakabayashi et al. / Chemical Physics Letters 395 (2004) 346–350
Fig. 4. (a) Fluorescence decay of the second excimer of pyrene doped in a PMMA film at 10 mol% observed at 415 nm at zero field. (b) The difference between the decays observed at zero field and at 0.9 MV cm1. (c) The ratio of the decay observed at 0.9 MV cm1 relative to that at zero field. Simulated time profiles are shown by a dotted line in every case.
one reported previously (see Fig. 1), i.e., the field-induced quenching is observed for the monomer and first excimer fluorescence and the field-induced enhancement is observed for the second excimer fluorescence. As shown in Fig. 3c, the magnitude of IF(t)/I0(t) at the initial stage of time is almost unity, suggesting that the initial population of the locally excited state of S1 of pyrene is not influenced by F, though the lifetime is influenced by F. Therefore, the field-induced quenching of the monomer fluorescence can be ascribed to the lifetime shortening of the monomer fluorescence in the presence of F, indicating that the nonradiative process from the locally excited state of S1 is accelerated by F. The field-induced acceleration of the nonradiative process in the S1 state may be ascribed to the field-induced increase of the formation rate of the first excimer. In fact, the quantum yield of the monomer fluorescence is not influenced by an applied electric field at low concentrations where excimer fluorescence does not appear
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Fig. 5. (a) Fluorescence decay of the first excimer of pyrene doped in a PMMA film at 10 mol% observed at 480 nm at zero field. (b) The difference between the decays observed at zero field and at 0.9 MV cm1. (c) The ratio of the decay observed at 0.9 MV cm1 relative to that at zero field. Simulated time profiles are shown by a dotted line in every case.
[9]. This consideration is also supported by the fact that the formation rate of the first excimer is accelerated by F (see Table 1), as mentioned below. A rise is observed only for the first excimer (see Fig. 2). The simulation of the rise and decay profile of this emission was made by assuming that the rise is given by a single exponential function. The rise becomes a little faster in the presence of F, suggesting that the formation process of the first excimer is slightly accelerated by F (see Table 1). Unless the field-induced change in rise time is considered, the time profile of DIf(t) could not be reproduced; the time profile of DIf(t) simulated without a change in rise time shows a minimum at much earlier time than the observed one. The magnitude of Ai is little affected by F, suggesting that the initial population of the first excimer remains unchanged even in the presence of F. These results lead us to a conclusion that the field-induced quenching of the first excimer fluorescence is attributed to the field-induced decrease in fluorescence lifetime of the first excimer, i.e., the field-induced increase in nonradiative rate of the first excimer.
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Table 1 Fluorescence lifetime and pre-exponential factor of pyrene doped in a PMMA film at 10 mol% observed at zero field and at 0.9 MV cm1a Wavelength
F (MV cm1)
380 380 415 415 480 480
0 0.9 0 0.9 0 0.9
s1 (ns)b
s2 (ns)
s3 (ns)
s4 (ns)
9.55 (0.362) 9.53 (0.362)
1.86 (0.243) 1.85 (0.243) 6.80 (0.280) 6.79 (0.282) 16.80 (0.221) 16.78 (0.221)
5.81 (0.426) 5.77 (0.426) 39.04 (0.604) 38.97 (0.605) 59.45 (0.732) 59.33 (0.732)
24.27 24.17 76.71 76.63 73.04 72.98
(0.331) (0.331) (0.116) (0.117) (0.047) (0.047)
a Pre-exponential factor of each component is given in parenthesis, and the sum of the factors at zero field is normalized to unity. Errors within ±0.2% or so are unavoidable in all the values. b Time constant of the rise.
It is not necessary to consider a rise for the fluorescence decay of the second excimer, suggesting that most of the second excimer are formed with time constants much faster than the present time resolution. One of the most noticeable results in the electric field effects on fluorescence decay is that the pre-exponential factor is increased by F markedly for theP second excimer fluorescence (see Table 1). The sum of iAi at 0.9 MV cm1 is larger than that at zero field by about 0.4%, indicating that the initial population of the second excimer is enhanced by F, which is confirmed from the time profiles of IF(t)/I0(t), where the ratio at the initial stage of time is higher than unity (see Fig. 4c). Thus, there is no doubt that the field-induced enhancement of the second excimer fluorescence is attributable to the field-induced increase in the initial population of the second excimer. The magnitude of the increase in the initial population exceeds the fluorescence quenching induced by a lifetime shortening, resulting in the field-induced enhancement of the second excimer fluorescence. The field-induced increase in the initial population of the second excimer may result from a field-induced change in orbital overlap, which probably increases the stability of the rather unstable second excimer. In conclusion, direct measurements of the field-induced change in fluorescence decay profile are carried out for pyrene doped in a PMMA film at a high concentration of 10 mol% with a picosecond time resolution. A field-induced decrease in lifetime is observed in all the emission components, which results in the field-induced quenching of the monomer and first excimer. A field-induced enhancement of the second excimer, in spite of the field-induced decrease in lifetime, is attributed to a remarkable field-induced increase in the initial population of the second excimer following photoexcitation. Finally, it should be stressed that time-resolved measurements of the field-induced change in emission decay profile provide detailed information on electric field effects on photoexcitation dynamics. Combinations of the present method with theoretical models, e.g., proposed by Tachiya and his co-workers [20] will give a new scope for elucidation of many photochemical processes.
Acknowledgements This work has been supported in part by Grants-inAid for Scientific Research (Grant Nos. 15205001, 15655001, and 15685005) and for Scientific Research on Priority Area (417) from the Ministry of Education, Culture, Sports, Science, and Technology in Japan.
References [1] A. Ogrodnik, U. Eberl, R. Heckmann, M. Kappl, R. Feick, M.E. Michel-Beyerle, J. Phys. Chem. 95 (1991) 2036. [2] D.J. Lockhart, S.G. Boxer, Chem. Phys. Lett. 144 (1988) 243. [3] G.G. Kochendoerfer, S.W. Lin, T.P. Sakmar, R.A. Mathies, Trends Biochem. Sci. 24 (1999) 300. [4] A.S. Klymchenko, A.P. Demchenko, J. Am. Chem. Soc. 124 (2002) 12372. [5] N. Ohta, Bull. Chem. Soc. Jpn. 75 (2002) 1637. [6] Z.D. Popovic, M.I. Khan, A.-M. Hor, J.L. Goodman, J.F. Graham, J. Phys. Chem. B 106 (2002) 8625. [7] V. Gulbinas, R. Kananavicius, L. Valkunas, H. Ba¨ssler, Phys. Rev. B 66 (2002) 233203. [8] N. Ohta, H. Kawabata, S. Umeuchi, I. Yamazaki, Chem. Phys. Lett. 310 (1999) 397. [9] N. Ohta, S. Umeuchi, T. Kanada, Y. Nishimura, I. Yamazaki, Chem. Phys. Lett. 279 (1997) 215. [10] M. Tsushima, T. Ushizaka, N. Ohta, Rev. Sci. Instrum. 75 (2004) 479. [11] S. Umeuchi, Y. Nishimura, I. Yamazaki, H. Murakami, M. Yamashita, N. Ohta,, Thin Solid Films 311 (1997) 239. [12] M. Tsushima, N. Ohta, J. Chem. Phys. 120 (2004) 6238. [13] N. Ohta, M. Koizumi, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Phys. Chem. 100 (1996) 16466. [14] N. Ohta, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Phys. Chem. B 102 (1998) 3784. [15] N. Ohta, S. Mikami, Y. Iwaki, M. Tsushima, H. Imahori, K. Tamaki, Y. Sakata, S. Fukuzumi, Chem. Phys. Lett. 368 (2003) 230. [16] E.A. Chandross, C.J. Demster, J. Am. Chem. Soc. 92 (1970) 3586. [17] Y.C. Wang, H. Morawetz, J. Am. Chem. Soc. 98 (1975) 3611. [18] I. Yamazaki, N. Tamai, T. Yamazaki, J. Phys. Chem. 91 (1987) 3572. [19] M. Ishikawa, J.Y. Ye, Y. Maruyama, H. Nakatsuka, J. Phys. Chem. A 103 (1999) 4319. [20] M. Hilczer, S. Traytak, M. Tachiya, J. Chem. Phys. 115 (2001) 11249.
Direct measurements of the electric-field-induced change in fluorescence decay profile of pyrene doped in a polymer film Takakazu Nakabayashi a b
a,b
, Takehiro Morikawa b, Nobuhiro Ohta
a,b,*
Research Institute for Electronic Science (RIES), Hokkaido University, Sapporo 060-0812, Japan Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received 14 May 2004; in final form 28 July 2004 Available online 21 August 2004
Abstract Electric-field-induced change in fluorescence lifetime and pre-exponential factor has been measured for each fluorescence component of pyrene doped in a polymer film at a high concentration. Based on the results, the origin both of the field-induced quenching of structured fluorescence with a peak at 380 nm and broad excimer fluorescence with a peak at 470 nm and of the field-induced enhancement of another excimer fluorescence with a peak at 415 nm is discussed. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Elucidation of the electric field effects on excitation dynamics is very important for through understanding of reaction mechanism in biological systems since electric fields produced by protein membranes seem to play a significant role in photochemical reactions [1–4]. The elucidation of the field effects is also important for design and development of new materials having photoinduced functions such as photoconductivity or electroluminescence properties [5–7]. For example, applied field strength dependence of the field-induced quenching of excimer fluorescence was shown to be strongly related to the generation efficiency of electroluminescence for methylene-linked compounds of pyrene doped in a polymer film [8], based on the measurements of electrofluorescence spectra (plots of the electric-field-induced change in fluorescence intensity as a function of wavelength). The efficiency of electroluminescence of pyrene doped in a polymer film seems to be not so efficient *
Corresponding author. E-mail address: [email protected] (N. Ohta).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.103
as that of linked compounds of pyrene, but fluorescence intensity of bare molecules of pyrene doped in a poly(methyl methacrylate) (PMMA) film is also markedly influenced by an electric field at high concentrations [9]. Electrofluorescence spectra show that different components of fluorescence of pyrene give different field dependence of quantum yield from each other. As far as the experiments are limited to the steady-state measurements, however, it is very difficult to confirm the mechanism of the electric field effects on each fluorescence component. For example, a field-induced change in emission intensity can be ascribed to a change in emission lifetime or a change in the initial population of the emitting states. In order to elucidate the mechanism of field effects on photoexcitation dynamics, therefore, direct measurements of the field-induced change in emission decay profile are essential. In the present study, we have carried out such a measurement for pyrene doped in a polymer film, using a picosecond time-resolved decay measurement system combined with an electromodulation apparatus [10]. The origins of the field-induced changes in fluorescence quantum yield are discussed on the basis of the results obtained.
T. Nakabayashi et al. / Chemical Physics Letters 395 (2004) 346–350
2. Experimental Pyrene and PMMA were purified in the same manner as mentioned previously [11]. A certain amount of benzene solution of pyrene and PMMA was cast on an ITO coated quartz substrate by a spin coating method. Then, a semitransparent aluminum (Al) film was deposited on the dried polymer film. The ITO and Al films were used as electrodes. The thickness of the polymer film was typically 0.6 lm. All the measurements were performed at room temperature under vacuum conditions. Measurements of the field-induced change in fluorescence decay profile were carried out using a singlephoton counting lifetime measurement system combined with a pulse generator supplying a bipolar square wave, which was constructed recently in our laboratory [10,12]. Briefly, the third harmonic of the mode-locked Ti:sapphire laser (Spectra Physics, Tsunami), whose pulse duration was 200 fs, was used for excitation. The repetition rate of the laser pulse was selected to be 1.6 MHz with a pulse picker (Conoptics, model 350– 160). Fluorescence from the sample was detected by a microchannel-plate photomultiplier (Hamamatsu, R3809 U-52). Fluorescence signal was discriminated and then led to a time-to-amplitude converter system. Applied voltage was a repetition of rectangular waves of positive, zero, negative, and zero bias. The time duration of each bias was 30 ms, but the first 3 ms was used as a dead time for emission detection to exclude an overshooting effect of the applied field. Four different decays were collected, corresponding to positive, zero, negative, and zero sample bias, respectively. The instrumental response function had a pulse width of 60 ps (FWHM). Hereafter, applied field is denoted by F. Steady-state electrofluorescence spectra were also measured using electric field modulation spectroscopy with the same apparatus as reported previously [13–15].
347
the first excimer with increasing pyrene concentration in a PMMA film. As reported in a previous paper [9], monomer fluorescence as well as the first excimer fluorescence is quenched by an applied electric field, whereas the second excimer fluorescence is enhanced by an applied electric field, indicating that three fluorescence emissions give different electric field effects from each other (see Fig. 1). In order to examine the origin of the field-induced change in fluorescence intensity, decay profiles of pyrene doped in a PMMA film at 10 mol% were measured at zero field and in the presence of electric field of 0.9 MV cm1 by monitoring the emission at 380, 415 and 480 nm, respectively, which corresponds to monomer, second excimer and first excimer fluorescence, respectively. In every case, excitation wavelength was 299.0 nm, where the field-induced change in absorption intensity is negligible. The difference between the decay observed at zero field (I0(t)) and that at 0.9 MV cm1 (IF(t)), i.e., IF(t) I0(t), referred to as DIf(t), was obtained. Time profiles of DIf(t) are shown in Fig. 2, together with the fluorescence decay observed at zero field. As reported previously, all the emissions give a nonexponential decay, probably because of the presence of different interactions between pyrene chromophores, which gives different fluorescence lifetimes from each other [19]. As shown in Fig. 2, DIf(t) observed at 380 and 480 nm is negative during the full decay, while DIf(t) at 415 nm is positive during the decay, indicating that the former emissions are quenched in the presence of F and that the latter emission is enhanced by F. These results agree with the ones obtained from the steady-state measurements of electrofluorescence spectra (see Fig. 1). The time dependence of DIf(t) is different in shape from that of I0(t) as shown in Fig. 2, which clearly indicates that the lifetimes of monomer, first excimer, and second excimer are influenced by F. All the time profiles P are fitted by assuming a triexponential decay, i.e., iAi exp (t/si), where Ai and si de-
3. Results and discussion Fluorescence of pyrene doped in a PMMA film shows at least three components at high concentrations [8,9]: (1) a structured fluorescence in the region from 370– 450 nm, which is assigned as the fluorescence emitted from the locally excited state S1 of pyrene; (2) a broad emission with a peak at 470 nm, which is assigned as the fluorescence emitted from the sandwich type of pyrene excimer [16,17]; (3) a very weak emission with a peak at 415 nm, which is assigned as the fluorescence emitted from a partially overlapping type of pyrene excimer [18]. Here, these three emissions are denoted by monomer fluorescence, first excimer fluorescence and second excimer fluorescence, respectively. Note that monomer fluorescence, which is dominant at low concentrations below 1 mol%, becomes weaker in relative intensity to
Fig. 1. Fluorescence spectrum (solid line) and electrofluorescence spectrum (shaded line) of pyrene doped in a PMMA film at 10 mol% observed in the steady state measurement. The applied field strength was 0.8 MV cm1 in rms.
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Fig. 2. Fluorescence decay observed at zero field (dotted line) and the difference between the decays observed at zero field and at 0.9 MV cm1 (solid line) of pyrene doped in a PMMA film at 10 mol%. Excitation wavelength was 299.0 nm. Monitoring wavelength was 380 nm (a), 415 nm (b) and 480 nm (c). The response function is also shown by a broken line in (a)–(c).
note the pre-exponential factor and lifetime of component i, respectively. Observed and simulated results of the time profiles of I0(t), DIf(t) and the ratio of IF(t)/ I0(t) are shown in Figs. 3–5 for emission at 380, 415 and 480 nm, respectively. It is noted that a rise is observed for the first excimer fluorescence. The lifetime and pre-exponential factor of each component both at zero field and in the presence of 0.9 MV cm1 are shown in Table 1 for monomer fluorescence (380 nm), first excimer fluorescence (480 nm) and second excimer fluorescence (415 P nm). The P average lifetime ðsf Þ, which is given by A s / i i i iAi, is then determined to be 10.96 ± 0.02 and 10.91 ± 0.02 ns for monomer fluorescence, 50.66 ± 0.03 and 50.57 ± 0.03 ns for the first excimer fluorescence, and 34.38 ± 0.02 and 34.32 ± 0.02 ns for the second excimer fluorescence at zero field and at 0.9 MV cm1, respectively. These results show that the lifetime of every fluorescence emission becomes shorter in the presence of F. A field-induced change in lifetime
Fig. 3. (a) Fluorescence decay of the monomer of pyrene doped in a PMMA film at 10 mol% observed at 380 nm at zero field. (b) The difference between the decays observed at zero field and at 0.9 MV cm1. (c) The ratio of the decay observed at 0.9 MV cm1 relative to that at zero field. Simulated time profiles are shown by a dotted line in every case.
affects the time profile of IF(t)/I0(t) more than the decay profile itself, and a change in pre-exponential factor largely affects both the time profile of DIf(t) and the value of IF(t)/I0(t) at t = 0. By simulating not only the decay profile but also the time profile both of DIf(t) and of IF(t)/I0(t), therefore, the field-induced change in lifetime and pre-exponential factor could be determined very precisely, though the error within 0.05% or so is unavoidable in the field-induced change shown in Table 1. The present average lifetime at zero field is essentially the same as the one reported previously in every case, but the lifetime of the fastest decaying component of the monomer fluorescence is a little different from the previous one. This difference may come from an inhomogeneous distribution of pyrene in PMMA. It must be confessed that lifetimes as well as pre-exponential factors of different samples are not identical with each other completely, even when the concentrations are the same. It is worth mentioning, however, that the sample used for the decay measurements shown in Figs. 2–5 as well as other samples with the same concentration gives essentially the same electrofluorescence spectra with the
T. Nakabayashi et al. / Chemical Physics Letters 395 (2004) 346–350
Fig. 4. (a) Fluorescence decay of the second excimer of pyrene doped in a PMMA film at 10 mol% observed at 415 nm at zero field. (b) The difference between the decays observed at zero field and at 0.9 MV cm1. (c) The ratio of the decay observed at 0.9 MV cm1 relative to that at zero field. Simulated time profiles are shown by a dotted line in every case.
one reported previously (see Fig. 1), i.e., the field-induced quenching is observed for the monomer and first excimer fluorescence and the field-induced enhancement is observed for the second excimer fluorescence. As shown in Fig. 3c, the magnitude of IF(t)/I0(t) at the initial stage of time is almost unity, suggesting that the initial population of the locally excited state of S1 of pyrene is not influenced by F, though the lifetime is influenced by F. Therefore, the field-induced quenching of the monomer fluorescence can be ascribed to the lifetime shortening of the monomer fluorescence in the presence of F, indicating that the nonradiative process from the locally excited state of S1 is accelerated by F. The field-induced acceleration of the nonradiative process in the S1 state may be ascribed to the field-induced increase of the formation rate of the first excimer. In fact, the quantum yield of the monomer fluorescence is not influenced by an applied electric field at low concentrations where excimer fluorescence does not appear
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Fig. 5. (a) Fluorescence decay of the first excimer of pyrene doped in a PMMA film at 10 mol% observed at 480 nm at zero field. (b) The difference between the decays observed at zero field and at 0.9 MV cm1. (c) The ratio of the decay observed at 0.9 MV cm1 relative to that at zero field. Simulated time profiles are shown by a dotted line in every case.
[9]. This consideration is also supported by the fact that the formation rate of the first excimer is accelerated by F (see Table 1), as mentioned below. A rise is observed only for the first excimer (see Fig. 2). The simulation of the rise and decay profile of this emission was made by assuming that the rise is given by a single exponential function. The rise becomes a little faster in the presence of F, suggesting that the formation process of the first excimer is slightly accelerated by F (see Table 1). Unless the field-induced change in rise time is considered, the time profile of DIf(t) could not be reproduced; the time profile of DIf(t) simulated without a change in rise time shows a minimum at much earlier time than the observed one. The magnitude of Ai is little affected by F, suggesting that the initial population of the first excimer remains unchanged even in the presence of F. These results lead us to a conclusion that the field-induced quenching of the first excimer fluorescence is attributed to the field-induced decrease in fluorescence lifetime of the first excimer, i.e., the field-induced increase in nonradiative rate of the first excimer.
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Table 1 Fluorescence lifetime and pre-exponential factor of pyrene doped in a PMMA film at 10 mol% observed at zero field and at 0.9 MV cm1a Wavelength
F (MV cm1)
380 380 415 415 480 480
0 0.9 0 0.9 0 0.9
s1 (ns)b
s2 (ns)
s3 (ns)
s4 (ns)
9.55 (0.362) 9.53 (0.362)
1.86 (0.243) 1.85 (0.243) 6.80 (0.280) 6.79 (0.282) 16.80 (0.221) 16.78 (0.221)
5.81 (0.426) 5.77 (0.426) 39.04 (0.604) 38.97 (0.605) 59.45 (0.732) 59.33 (0.732)
24.27 24.17 76.71 76.63 73.04 72.98
(0.331) (0.331) (0.116) (0.117) (0.047) (0.047)
a Pre-exponential factor of each component is given in parenthesis, and the sum of the factors at zero field is normalized to unity. Errors within ±0.2% or so are unavoidable in all the values. b Time constant of the rise.
It is not necessary to consider a rise for the fluorescence decay of the second excimer, suggesting that most of the second excimer are formed with time constants much faster than the present time resolution. One of the most noticeable results in the electric field effects on fluorescence decay is that the pre-exponential factor is increased by F markedly for theP second excimer fluorescence (see Table 1). The sum of iAi at 0.9 MV cm1 is larger than that at zero field by about 0.4%, indicating that the initial population of the second excimer is enhanced by F, which is confirmed from the time profiles of IF(t)/I0(t), where the ratio at the initial stage of time is higher than unity (see Fig. 4c). Thus, there is no doubt that the field-induced enhancement of the second excimer fluorescence is attributable to the field-induced increase in the initial population of the second excimer. The magnitude of the increase in the initial population exceeds the fluorescence quenching induced by a lifetime shortening, resulting in the field-induced enhancement of the second excimer fluorescence. The field-induced increase in the initial population of the second excimer may result from a field-induced change in orbital overlap, which probably increases the stability of the rather unstable second excimer. In conclusion, direct measurements of the field-induced change in fluorescence decay profile are carried out for pyrene doped in a PMMA film at a high concentration of 10 mol% with a picosecond time resolution. A field-induced decrease in lifetime is observed in all the emission components, which results in the field-induced quenching of the monomer and first excimer. A field-induced enhancement of the second excimer, in spite of the field-induced decrease in lifetime, is attributed to a remarkable field-induced increase in the initial population of the second excimer following photoexcitation. Finally, it should be stressed that time-resolved measurements of the field-induced change in emission decay profile provide detailed information on electric field effects on photoexcitation dynamics. Combinations of the present method with theoretical models, e.g., proposed by Tachiya and his co-workers [20] will give a new scope for elucidation of many photochemical processes.
Acknowledgements This work has been supported in part by Grants-inAid for Scientific Research (Grant Nos. 15205001, 15655001, and 15685005) and for Scientific Research on Priority Area (417) from the Ministry of Education, Culture, Sports, Science, and Technology in Japan.
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