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Measurements of the electric-field-induced change in fluorescence decay profile of a mixture of ethylcarbazole and dimethylterephthalate in a PMMA polymer film
Journal of Luminescence 87}89 (2000) 791}793
Measurements of the electric-"eld-induced change in #uorescence decay pro"le of a mixture of ethylcarbazole and dimethylterephthalate in a PMMA polymer "lm Yoshinobu Nishimura *, Iwao Yamazaki , Nobuhiro Ohta Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan
Abstract Electric "eld e!ects on photoinduced intermolecular electron transfer between ethylcarbazole (ECz) and dimethylterephthalate (DMTP) doped in a PMMA "lm have been investigated by using a picosecond time-correlated singlephoton counting system combined with a bipolar sample bias, which enables the direct measurements of the "eldinduced change in #uorescence decay pro"le. The di!erence of the lifetime in the presence and absence of applied electric "elds was successfully measured. The obtained results show that the electron transfer rate is enhanced by an electric "eld in agreement with those by the steady-state #uorescence measurements. 2000 Elsevier Science B.V. All rights reserved. Keywords: Photoinduced electron transfer; Electric "eld e!ect; Time-resolved decay measurement
1. Introduction Extensive attention has been paid to the mechanisms of electron transfer reactions for many years [1,2]. Recently, "eld e!ect on electron transfer rate was found in the experiment of monolayer containing a photosynthetic center pigment [3,4]. Thus, the electric "eld e!ect on dissociation or recombination process of the produced radical-ion pair can be expected, if a radical-ion pair possesses fairly a large dipole moment. We have been studying the electron transfer reactions of methylene-linked (n"3, 20) carbazole and terephthalic acid methyl ester (abbreviated as D-(n)-A) [5] with reference to mixtures of ethylcarbazole (ECz) and dimethylterephthalate (DMTP) [6] in PMMA "lms. Photoexcitation of carbazole shows exciplex #uorescence in PMMA "lms with increasing the dopant concentration together with the #uorescence from the carbazole. Fluorescence from both carbazole and the exciplex was quenched at high concentration of D-(n)-A or ECz when
an external electric "eld was applied. The "eld-induced quenching of the exciplex #uorescence is proved to be the free carrier generation by the "eld-assisted dissociation of an electron}hole ion-pair state [7,8]. In the previous article [9], we reported the "rst detailed con"rmation of "eld-induced acceleration of electron transfer rate in methylene linked compounds. The aim of the present study is to directly measure changes in #uorescence lifetime of ECz with and without the external "eld in the presence of DMTP. Time-correlated single-photon counting method combined with an electromodulation apparatus was used to determine the quenching rate constant of the #uorescence state of ECz by intermolecular interaction. In addition, the steadystate #uorescence measurements also carried out with a series of an applied "eld to determine the quenching e$ciency of ECz. The consistency of the results obtained by two di!erent methods will be discussed.
2. Experimental * Corresponding author. Fax: #81-11-709-2037. E-mail address: [email protected] (Y. Nishimura)
The samples were prepared using the methods reported previously [6]. PMMA polymer "lms that
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 0 7 - X
792
Y. Nishimura et al. / Journal of Luminescence 87}89 (2000) 791}793
contain mixtures of ECz (1 mol%) and DMTP (3 or 5 mol%) were made by spin coating. The concentration was de"ned by molar ratio of a dopant relative to the monomer unit of PMMA. Hereafter, concentration expression refers to that of DMTP unless otherwise stated. Thin "lms were formed on the ITO-coated quartz substrate with an Al semitransparent electrode on top. The thickness of the polymer "lm was typically 0.7 lm. The steady-state electro#uorescence spectra were measured using modulation spectroscopy with the same apparatus as those reported in previous papers [6]. Fluorescence decay curves were measured by using a femtosecond pulse laser and a single-photon counting system combined with an electromodulation apparatus [9]. The decay curves were measured with an excitation wavelength of 294 nm. The #uorescence decays were measured at 370 nm, where ECz emission is dominant. Criteria for the best "t were s-square and the Durbin}Watson parameters obtained by nonlinear regression [10].
3. Results and discussion Fig. 1 shows the #uorescence spectrum and the electromodulated #uorescence spectrum of ECz (1 mol%) with excitation at 294 nm in the presence of DMTP (5 mol%). Photoinduced electron transfer from the excited singlet state of ECz, hereafter abbreviated as ECzH, to adjacent DMTP occurs followed by the formation of exciplex. The #uorescence spectrum attributed to the emission from ECzH contained two characteristic peaks at 355 and 370 nm, while the exciplex had a broad peak at &450 nm. As also shown in Fig. 1, electromodulated #uorescence spectrum was obtained by irradiation at 294 nm with a strength of 0.52 MV/cm. ECz emission decreased in the presence of an electric "eld, F [6]. The exciplex emission showed di!erent behavior from ECz
Fig. 1. Fluorescence spectra (dotted line) and electro#uorescence spectra (solid line) of a mixture of ECz (1 mol%) and DMTP (5 mol%) with a strength of 0.52 MV/cm.
Fig. 2. (a) Fluorescence decay curves of ECz emission doped in a PMMA "lm measured in the absence (solid line) and in the presence (dotted line) of an external electric "eld of 0.84 MV/cm. (b) *I (t), which is the di!erence of decay curves at zero "eld and $ 0.84 MV/cm, is denoted by a thin solid line, besides the decay curves at zero "eld (thick solid line).
emission when F was applied. This phenomenon is successfully explained in terms both of Stark shift and a change in intensity of exciplex emission; the observed spectrum is allowed to be simulated by the exciplex emission spectrum and its "rst derivative. *I at the peak $ wavelength of the exciplex spectrum, which has a positive value, indicates that F enhances the exciplex emission [7]. Fig. 2(a) shows #uorescence decay curves observed at 370 nm in the presence and absence of F, being denoted by I (t) and I (t), respectively. These decay curves $ $ were multiexponential pro"les, probably arising from inhomogeneous environments. To examine a change in #uorescence decay induced by F, *I (t) was introduced $ to elucidate these di!erences; i.e. *I (t)"I (t)!I (t). $ $ $ As shown in Fig. 2(b), *I (t) remained positive value $ within whole measured time region and gave maximum count delayed by about 1 ns compared to I (t). These $ suggest that the lifetime of ECz emission was changed by F, considering disagreement of *I (t) with either $ I (t) or I (t). These behaviors were also observed in $ $
Y. Nishimura et al. / Journal of Luminescence 87}89 (2000) 791}793
793
Table 1 Fitting parameters for di!erent electric "elds using multi-exponential analysis. Normalized preexponential factors are indicated in parenthesis. (a) 3 mol% and (b) 5 mol% of DMTP F
q /ns
q /ns
q /ns
(a)
0 0.87
0.98 (0.37) 0.94 (0.38)
3.82 (0.51) 3.76 (0.50)
8.77 (0.12) 8.58 (0.12)
(b)
0 0.84
0.23 (0.23) 0.16 (0.26)
1.24 (0.37) 1.10 (0.35)
3.97 (0.35) 3.82 (0.35)
q /ns
11.7 (0.05) 11.6 (0.04)
Unit in MV/cm.
the case of 3 mol%. Thus, it was con"rmed that the rate constant of #uorescence decay in the presence of F changes in comparison to no external "eld, since no change was found in #uorescence lifetime of ECz at 0.1 mol% (data not shown). Fluorescence decay curves could be analyzed by multiexponential function, A exp(t/q ), where A and q deG G G G G note preexponential factor and lifetime, respectively. Subscript i refers to the number of components which was 3 and 4 for 3 and 5 mol%, respectively. This indicates that the higher the concentration the broader the distribution of the distance between ECz and DMTP in a polymer "lm, leading to more complicated pro"le of a decay curve. Table 1 shows representative lifetimes and preexponential factors for 3 and 5 mol% in the ascending order of lifetime. Note that A is unchanged despite G q depends on F. q in 5 mol% is shorter than that in G 3 mol%. The lifetime of every component in both 3 and 5 mol% decreased in the presence of F, and F caused a decrease in q in 5 mol% more e!ectively than that in G 3 mol%; nearly 30% decrement for 5 mol% as compared to 4% for 3 mol%. The average lifetime, q , was introduced to evaluate the "eld dependence of #uorescence decay; q was de"ned by A q / A . The average rate constant of electron transG G G G G fer is expressed as k "1/q !1/q where q denotes the lifetime of ECz in the absence of an electron acceptor; i.e. q is 12.5 ns [6]. *k , a "eld-induced change in k , was also de"ned as *k "k (F)!k (F"0). Fig. 3 shows *k as a function of F. Field dependence of *k is quadratic, that is consistent with methylene linked compounds and explained by a change in *G induced by F [6]. *k in 5 mol% showed a remarkable change with F, as compared to that in 3 mol%. This is also con"rmed from q listed in Table 1. *k reported by previous paper G [6], which was obtained by steady-state measurement, is found to be fairly good agreement with present results regarding both the concentration dependence and the magnitude within experimental errors. *I /I , which was obtained by steady-state measure$ $ ment of ECz emission as a function of F, is also plotted
Fig. 3. Plots of *k and *I /I of a mixture of ECz (1 mol%) $ $ and DMTP as a function of applied electric "eld strength. The concentration of DMTP was (a) 3 and (b) 5 mol%.
in Fig. 3. The quadratic dependence of *I /I on F $ $ can be interpreted in terms of a relationship, *k "!(*I /I )/q , in the case of *I /I ;1. Conse $ $ $ $ quently, we conclude that the present apparatus successfully observed changes in lifetime of ECzH induced by F.
Acknowledgements This work was supported by Grant-in-aids for the Scienti"c Research from the Ministry of Education, Science, Sports and Culture of Japan (Nos. 10440163 and 09740507).
References [1] D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259. [2] P.F. Barbara, T.J. Meyer, M.A. Ratner, J. Chem. Phys. 100 (1996) 13148. [3] S.G. Boxer, in: J. Deisenhofer, J.R. Norris (Eds.), The Photosynthetic Reaction Center Vol. II, Academic Press, San Diego, CA, 1993, p. 179. [4] C.C. Moser, R.J. Sension, A.Z. Szarka, S.T. Repinec, R.M. Hochstrasser, P.L. Dutton, Chem. Phys. 197 (1995) 343. [5] N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, Y. Tanimoto, Y. Hatano, M. Yamamoto, H. Kono, J. Phys. Chem. 100 (1996) 19295. [6] N. Ohta, M. Koizumi, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Chem. Phys. 100 (1996) 16466. [7] N. Ohta, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Phys. Chem. B 102 (1998) 3784. [8] Y. Nishimura, N. Ohta, M. Yamamoto, I. Yamazaki, Molec. Cryst. Liq. Cryst. 315 (1998) 181. [9] Y. Nishimura, I. Yamazaki, M. Yamamoto, N. Ohta, Chem. Phys. Lett. 307 (1999) 8. [10] N. Boens, N. Tamai, I. Yamazaki, T. Yamazaki, Photochem. Photobiol. 52 (1990) 911.
Measurements of the electric-"eld-induced change in #uorescence decay pro"le of a mixture of ethylcarbazole and dimethylterephthalate in a PMMA polymer "lm Yoshinobu Nishimura *, Iwao Yamazaki , Nobuhiro Ohta Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan
Abstract Electric "eld e!ects on photoinduced intermolecular electron transfer between ethylcarbazole (ECz) and dimethylterephthalate (DMTP) doped in a PMMA "lm have been investigated by using a picosecond time-correlated singlephoton counting system combined with a bipolar sample bias, which enables the direct measurements of the "eldinduced change in #uorescence decay pro"le. The di!erence of the lifetime in the presence and absence of applied electric "elds was successfully measured. The obtained results show that the electron transfer rate is enhanced by an electric "eld in agreement with those by the steady-state #uorescence measurements. 2000 Elsevier Science B.V. All rights reserved. Keywords: Photoinduced electron transfer; Electric "eld e!ect; Time-resolved decay measurement
1. Introduction Extensive attention has been paid to the mechanisms of electron transfer reactions for many years [1,2]. Recently, "eld e!ect on electron transfer rate was found in the experiment of monolayer containing a photosynthetic center pigment [3,4]. Thus, the electric "eld e!ect on dissociation or recombination process of the produced radical-ion pair can be expected, if a radical-ion pair possesses fairly a large dipole moment. We have been studying the electron transfer reactions of methylene-linked (n"3, 20) carbazole and terephthalic acid methyl ester (abbreviated as D-(n)-A) [5] with reference to mixtures of ethylcarbazole (ECz) and dimethylterephthalate (DMTP) [6] in PMMA "lms. Photoexcitation of carbazole shows exciplex #uorescence in PMMA "lms with increasing the dopant concentration together with the #uorescence from the carbazole. Fluorescence from both carbazole and the exciplex was quenched at high concentration of D-(n)-A or ECz when
an external electric "eld was applied. The "eld-induced quenching of the exciplex #uorescence is proved to be the free carrier generation by the "eld-assisted dissociation of an electron}hole ion-pair state [7,8]. In the previous article [9], we reported the "rst detailed con"rmation of "eld-induced acceleration of electron transfer rate in methylene linked compounds. The aim of the present study is to directly measure changes in #uorescence lifetime of ECz with and without the external "eld in the presence of DMTP. Time-correlated single-photon counting method combined with an electromodulation apparatus was used to determine the quenching rate constant of the #uorescence state of ECz by intermolecular interaction. In addition, the steadystate #uorescence measurements also carried out with a series of an applied "eld to determine the quenching e$ciency of ECz. The consistency of the results obtained by two di!erent methods will be discussed.
2. Experimental * Corresponding author. Fax: #81-11-709-2037. E-mail address: [email protected] (Y. Nishimura)
The samples were prepared using the methods reported previously [6]. PMMA polymer "lms that
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 0 7 - X
792
Y. Nishimura et al. / Journal of Luminescence 87}89 (2000) 791}793
contain mixtures of ECz (1 mol%) and DMTP (3 or 5 mol%) were made by spin coating. The concentration was de"ned by molar ratio of a dopant relative to the monomer unit of PMMA. Hereafter, concentration expression refers to that of DMTP unless otherwise stated. Thin "lms were formed on the ITO-coated quartz substrate with an Al semitransparent electrode on top. The thickness of the polymer "lm was typically 0.7 lm. The steady-state electro#uorescence spectra were measured using modulation spectroscopy with the same apparatus as those reported in previous papers [6]. Fluorescence decay curves were measured by using a femtosecond pulse laser and a single-photon counting system combined with an electromodulation apparatus [9]. The decay curves were measured with an excitation wavelength of 294 nm. The #uorescence decays were measured at 370 nm, where ECz emission is dominant. Criteria for the best "t were s-square and the Durbin}Watson parameters obtained by nonlinear regression [10].
3. Results and discussion Fig. 1 shows the #uorescence spectrum and the electromodulated #uorescence spectrum of ECz (1 mol%) with excitation at 294 nm in the presence of DMTP (5 mol%). Photoinduced electron transfer from the excited singlet state of ECz, hereafter abbreviated as ECzH, to adjacent DMTP occurs followed by the formation of exciplex. The #uorescence spectrum attributed to the emission from ECzH contained two characteristic peaks at 355 and 370 nm, while the exciplex had a broad peak at &450 nm. As also shown in Fig. 1, electromodulated #uorescence spectrum was obtained by irradiation at 294 nm with a strength of 0.52 MV/cm. ECz emission decreased in the presence of an electric "eld, F [6]. The exciplex emission showed di!erent behavior from ECz
Fig. 1. Fluorescence spectra (dotted line) and electro#uorescence spectra (solid line) of a mixture of ECz (1 mol%) and DMTP (5 mol%) with a strength of 0.52 MV/cm.
Fig. 2. (a) Fluorescence decay curves of ECz emission doped in a PMMA "lm measured in the absence (solid line) and in the presence (dotted line) of an external electric "eld of 0.84 MV/cm. (b) *I (t), which is the di!erence of decay curves at zero "eld and $ 0.84 MV/cm, is denoted by a thin solid line, besides the decay curves at zero "eld (thick solid line).
emission when F was applied. This phenomenon is successfully explained in terms both of Stark shift and a change in intensity of exciplex emission; the observed spectrum is allowed to be simulated by the exciplex emission spectrum and its "rst derivative. *I at the peak $ wavelength of the exciplex spectrum, which has a positive value, indicates that F enhances the exciplex emission [7]. Fig. 2(a) shows #uorescence decay curves observed at 370 nm in the presence and absence of F, being denoted by I (t) and I (t), respectively. These decay curves $ $ were multiexponential pro"les, probably arising from inhomogeneous environments. To examine a change in #uorescence decay induced by F, *I (t) was introduced $ to elucidate these di!erences; i.e. *I (t)"I (t)!I (t). $ $ $ As shown in Fig. 2(b), *I (t) remained positive value $ within whole measured time region and gave maximum count delayed by about 1 ns compared to I (t). These $ suggest that the lifetime of ECz emission was changed by F, considering disagreement of *I (t) with either $ I (t) or I (t). These behaviors were also observed in $ $
Y. Nishimura et al. / Journal of Luminescence 87}89 (2000) 791}793
793
Table 1 Fitting parameters for di!erent electric "elds using multi-exponential analysis. Normalized preexponential factors are indicated in parenthesis. (a) 3 mol% and (b) 5 mol% of DMTP F
q /ns
q /ns
q /ns
(a)
0 0.87
0.98 (0.37) 0.94 (0.38)
3.82 (0.51) 3.76 (0.50)
8.77 (0.12) 8.58 (0.12)
(b)
0 0.84
0.23 (0.23) 0.16 (0.26)
1.24 (0.37) 1.10 (0.35)
3.97 (0.35) 3.82 (0.35)
q /ns
11.7 (0.05) 11.6 (0.04)
Unit in MV/cm.
the case of 3 mol%. Thus, it was con"rmed that the rate constant of #uorescence decay in the presence of F changes in comparison to no external "eld, since no change was found in #uorescence lifetime of ECz at 0.1 mol% (data not shown). Fluorescence decay curves could be analyzed by multiexponential function, A exp(t/q ), where A and q deG G G G G note preexponential factor and lifetime, respectively. Subscript i refers to the number of components which was 3 and 4 for 3 and 5 mol%, respectively. This indicates that the higher the concentration the broader the distribution of the distance between ECz and DMTP in a polymer "lm, leading to more complicated pro"le of a decay curve. Table 1 shows representative lifetimes and preexponential factors for 3 and 5 mol% in the ascending order of lifetime. Note that A is unchanged despite G q depends on F. q in 5 mol% is shorter than that in G 3 mol%. The lifetime of every component in both 3 and 5 mol% decreased in the presence of F, and F caused a decrease in q in 5 mol% more e!ectively than that in G 3 mol%; nearly 30% decrement for 5 mol% as compared to 4% for 3 mol%. The average lifetime, q , was introduced to evaluate the "eld dependence of #uorescence decay; q was de"ned by A q / A . The average rate constant of electron transG G G G G fer is expressed as k "1/q !1/q where q denotes the lifetime of ECz in the absence of an electron acceptor; i.e. q is 12.5 ns [6]. *k , a "eld-induced change in k , was also de"ned as *k "k (F)!k (F"0). Fig. 3 shows *k as a function of F. Field dependence of *k is quadratic, that is consistent with methylene linked compounds and explained by a change in *G induced by F [6]. *k in 5 mol% showed a remarkable change with F, as compared to that in 3 mol%. This is also con"rmed from q listed in Table 1. *k reported by previous paper G [6], which was obtained by steady-state measurement, is found to be fairly good agreement with present results regarding both the concentration dependence and the magnitude within experimental errors. *I /I , which was obtained by steady-state measure$ $ ment of ECz emission as a function of F, is also plotted
Fig. 3. Plots of *k and *I /I of a mixture of ECz (1 mol%) $ $ and DMTP as a function of applied electric "eld strength. The concentration of DMTP was (a) 3 and (b) 5 mol%.
in Fig. 3. The quadratic dependence of *I /I on F $ $ can be interpreted in terms of a relationship, *k "!(*I /I )/q , in the case of *I /I ;1. Conse $ $ $ $ quently, we conclude that the present apparatus successfully observed changes in lifetime of ECzH induced by F.
Acknowledgements This work was supported by Grant-in-aids for the Scienti"c Research from the Ministry of Education, Science, Sports and Culture of Japan (Nos. 10440163 and 09740507).
References [1] D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259. [2] P.F. Barbara, T.J. Meyer, M.A. Ratner, J. Chem. Phys. 100 (1996) 13148. [3] S.G. Boxer, in: J. Deisenhofer, J.R. Norris (Eds.), The Photosynthetic Reaction Center Vol. II, Academic Press, San Diego, CA, 1993, p. 179. [4] C.C. Moser, R.J. Sension, A.Z. Szarka, S.T. Repinec, R.M. Hochstrasser, P.L. Dutton, Chem. Phys. 197 (1995) 343. [5] N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, Y. Tanimoto, Y. Hatano, M. Yamamoto, H. Kono, J. Phys. Chem. 100 (1996) 19295. [6] N. Ohta, M. Koizumi, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Chem. Phys. 100 (1996) 16466. [7] N. Ohta, S. Umeuchi, Y. Nishimura, I. Yamazaki, J. Phys. Chem. B 102 (1998) 3784. [8] Y. Nishimura, N. Ohta, M. Yamamoto, I. Yamazaki, Molec. Cryst. Liq. Cryst. 315 (1998) 181. [9] Y. Nishimura, I. Yamazaki, M. Yamamoto, N. Ohta, Chem. Phys. Lett. 307 (1999) 8. [10] N. Boens, N. Tamai, I. Yamazaki, T. Yamazaki, Photochem. Photobiol. 52 (1990) 911.