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The accumulation of photochromic energy acceptor species in A–S dyad Langmuir–Blodgett films
Thin Solid Films 327–329 (1998) 718–721
The accumulation of photochromic energy acceptor species in A–S dyad Langmuir–Blodgett films M. Sakomura a ,*, M. Fujihira b a
Department of Physical Chemistry, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
b
Abstract Fluorescence and absorption spectra were measured on Langmuir–Blodgett (LB) films containing acceptor–sensitizer (A–S) dyads. In the dyads, the length of hydrocarbon bridges interconnecting acceptor (viologen) and sensitizer (pyrene) were synthetically changed. In the LB films, the accumulation of viologen cation radical which can quench the excited pyrene moiety as an energy acceptor was observed during UV irradiation under N2. The rate constant of photoinduced intramolecular electron transfer in the LB films was estimated from the fluorescence intensities by taking the energy transfer quenching into consideration. 1998 Elsevier Science S.A. All rights reserved Keywords: Photo-induced electron transfer; Langmuir–Blodgett films; Dyad
1. Introduction Artificial dyad and triad molecular systems as models of a photosynthetic reaction center were intensively studied by several groups [1–3]. The dynamics of the intramolecular electron transfer processes have been mainly studied in dilute solutions containing the dyad or triad molecules [4–6]. The investigations of randomly dissolved molecular functions are very important but sometimes the highly organized molecular systems like Langmuir–Blodgett (LB) films give intermolecular cooperative phenomena, which could not be expected from the properties of isolated molecules. In our approach to design of molecular photodiodes, we synthesized amphiphilic dyad and triad molecules to construct molecular assemblies using the LB method. Our LB films successfully worked as real devices which generate photo-induced current or voltage [7–11]. For further sophisticated design of artificial molecular photodiodes, information about kinetics of intramolecular electron transfer is indispensable. Distance dependence and intervening medium effects are still unsolved problems in the area of electron transfer kinetics. To investigate the electron transfer * Corresponding author. Tel.: +81 45 3393946; fax: +81 45 3393970; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0749-4
kinetics in LB films, amphiphilic acceptor–sensitizer (A– S) dyads were synthesized (Fig. 1). In these dyads, the length of hydrocarbon bridge interconnecting acceptor (viologen, V2 + ) and sensitizer (pyrene, Py) was synthetically changed. In well oriented LB monolayers, the distances between A and S moieties would be the linear lengths of the hydrocarbon bridges. The stationary fluorescence measurements on LB films containing these A–S dyads describe the distance dependence of the rates of electron transfer from S* to A. The photochemical oxidation of LB films containing 6(1-(6(8)-decyl)pyrene)hexanoic acid in the presence of molecular oxygen was reported previously [12]. The energy transfer quenching of the photoexcited pyrene moieties by the photo-oxidative products strongly diminished the fluorescence intensities of the LB films. Thus it is important to keep the LB films under N2 or an inert atmosphere while the fluorescence spectra measurements are carried out. However, we observed a curious behavior on fluorescence of LB films containing the A–S dyads, that is, the fluorescence intensities of the LB films measured under N2 were weaker than that under air. In this paper, we investigate origin of the quenching under N2 and estimate the rate constant of photoinduced intramolecular electron transfer in LB films containing A– S dyads taking the quenching into consideration.
1998 Elsevier Science S.A. All rights reserved
M. Sakomura, M. Fujihira / Thin Solid Films 327–329 (1998) 718–721
Fig. 1. Structural formulae of A–S dyads and PHA.
2. Experimental details 2.1. Materials The dyads 1–4 and pyrene hexadecanoic acid (PHA) were synthesized in this laboratory. The purity of these compounds was confirmed by NMR and elemental analysis. A poly allyl amine (PAA) solution was a gift by courtesy of Nittobo and used as received. All the other chemicals were of reagent grade and used as received. Pure water from a Milli-Q system (Millipore) was used for preparation of the aqueous subphase.
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intensities of pyrene compounds with increasing the elapsed time after the beginning of the irradiation with UV light under air. The decreasing fluorescence intensity was caused by photo-oxidized pyrene products. Under N2, the decreasing was much inhibited. However, the fluorescence intensities of an A–S dyad (n = 11) measured under the inert atmosphere made by the entirely closed fluorescence cell with the flow of N2 shown in Fig. 2a became weaker while the scanning was repeated four times. After the four-times scan, the fifth scan was made on the same sample under air. As shown in Fig. 2b, the fluorescence intensity became stronger than that of any scan in Fig. 2a although the measurement was carried out under an oxygen existing atmosphere. The absorption spectra were also measured on the same sample as that used in Fig. 2. In Fig. 3, the solid and the dotted curve illustrate the absorption spectra measured immediately after the forth scan in Fig. 2a under N2 and fifth scan in Fig. 2b under air, respectively, and the dashed curve illustrates the absorption difference spectra between them. Before the fluorescence measurements in Fig. 2, the absorption spectra under air were also measured and it was almost the same as the dotted curve in Fig. 3. It is
2.2. Procedures A chloroform solution of a mixture of each of the A–S dyads with arachidic acid (AA) (molar ratio, 1:10) was spread on an aqueous subphase containing 0.3 mM CaCl2 and 0.05 mM NaHCO3 in a Langmuir trough (San-esu Keisoku). A mixed monolayer of PHA and AA (molar ratio, 1:100) was also made by spreading a chloroform solution on an aqueous subphase containing 0.04 mM PAA and 0.05 mM NaHCO3. These monolayers were deposited automatically on non-fluorescent quartz plates (Fujiwara Seisakusho) at the surface pressure of 35 mN/m. The UV-vis absorption and the fluorescence spectra were taken with a Hitachi 220A UV-vis recording spectrophotometer and a Hitachi F-3000 recording fluorescence spectrophotometer, respectively. To protect compounds against photo-oxidation and decomposition, an entirely closed fluorescence cell equipped with quartz windows was employed. The oxygen free atmosphere can be easily obtained by flowing N2 gas into the cell.
3. Results and discussion 3.1. The accumulation of photochromic energy acceptor species in A–S dyad LB films In a previous study, we observed decreasing fluorescence
Fig. 2. Fluorescence spectral changes of the mixed LB film of A–S dyad 3–AA (81 layers). All the spectra exhibit a monomer and an excimer band of pyrene moiety at the wavelength region of 370–430 nm and 430–550 nm, respectively. (a) Four times successive scan under N2. The scanning orders for the curves are as follows from the top: 1st, 2nd, 3rd and 4th. (b) 5th scan under air.
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M. Sakomura, M. Fujihira / Thin Solid Films 327–329 (1998) 718–721
cal with an increase in the bridge chain length of A–S dyads was observed. This result can be explained by the mechanism described above. That is to say, the longer lifetime of the charge separated state of the dyad with the longer bridge chain increases the lateral migration efficiency to form the long-lived separated charge. The long-lived Py + • is well known to be hydroxylated rapidly by H2O. During the transient fluorescence and transient absorption measurements, many pulsed laser lights have to be irradiated to obtain enough data for averaging. Therefore, the accumulation of energy acceptor described here may not be ignored. To clarify the effect, transient absorption and transient fluorescence measurements are now under investigation. 3.3. The distance dependence of electron transfer rate in LB films containing A–S dyads Fig. 3. Absorption spectra measured on the same sample as that used in Fig. 2 (solid and dotted line) and the absorption difference spectra between them (dashed line).
clear from comparison of the absorption difference spectra with the spectra of methyl viologen cation radical solution produced by electrochemical reduction [13] that viologen cation radical accumulated in the LB film during UV irradiation under N2. The produced viologen cation radical can be directly oxidized to viologen by oxygen under air [14]. As shown in Figs. 2 and 3, a fluorescence band of the pyrene moiety overlapped well with an absorption band of viologen cation radical. These results suggest that the photochromic energy acceptor, viologen cation radical, quenched the excited pyrene moiety under N2. The accumulated viologen cation radical was oxidized to viologen with oxygen, and thus recovering of the fluorescence intensity was observed under air. 3.2. The mechanism of the accumulation of viologen cation radical UV irradiation of the V2 + –Py dyad gives V + • –Py + • as a product of the intramolecular electron transfer between V2 + and excited pyrene moiety Py*. But the intramolecular charge recombination is so fast that V + • –Py + • cannot accumulate in the dyad system. For the accumulation of viologen cation radical, an increase in inter-charge distance is essential. Previously [11], very long decaying times (.10 s) of the surface potential after irradiation of the LB films containing A–S dyad or A–S–D triad were observed using a scanning surface potential microscope (SSPM). Such long lifetimes of the separated charges could be interpreted by assuming a succeeding intermolecular charge separation mechanism of lateral migration of photo-created radical anions (A − •) and cations (S + • or D + •) between the dyads or triads following intramolecular charge separation within one dyad or triad. The more efficient accumulation of viologen cation radi-
In order to determine the electron transfer quenching rate from the stationary fluorescence intensity measurements, the fluorescence intensities I should be measured before photochromic energy acceptor species accumulate in LB films. Therefore, we estimated the initial fluorescence intensity I by extrapolation of the stationary fluorescence intensity–time curve. The fluorescence intensity without acceptor I0 was measured on a mixed LB film consisting of PHA and AA (molar ratio, 1:100) deposited on quartz substrate (102 layers). I and I0 were normalized by the absorbance of each LB film. The ratio (I0 − I)/I measures the rate of the electron transfer. This ratio can be related to the rate of electron transfer by the following equation from the theoretical consideration for non-adiabatic electron transfer reaction [15]: log[(I0 − I)=I] = − br=2:303 + const:
(1)
where r and b denote the hydrocarbon chain length between A and S and the slope of r versus log[(I0 − I)/I]. Fig. 4 shows the plot of the logarithm of (Io − I)/I as a function of the carbon number of alkyl chains linking the A and S moieties. The value of b was estimated to be ca. 6.4
Fig. 4. Semilogarithmic plot of the ratio (I0 − I)/I as a function of the carbon number of alkyl chains linking A and S moieties.
M. Sakomura, M. Fujihira / Thin Solid Films 327–329 (1998) 718–721
nm − 1 from the slope of the line in Fig. 4. The slope is much higher than that reported by Kuhn et al. for the multilayer LB film system studied by the stationary fluorescence measurements [16]. The observed linear relationship substantiates favorable orientation of A–S dyads in LB films. The distances between A and S moieties of each dyad were successfully controlled by the length of the alkyl chain linking the two moieties. It is not unusual that electron acceptor changes to energy acceptor due to photochromism. Many organic electron acceptors are colored at the radical state and become energy acceptors. In dilute solutions, the photochromic energy acceptor effect is negligible. But the accumulation of the energy acceptor species should be considered when fluorescence spectroscopy is employed to study electron transfer kinetics of dyad or triad molecules in condensed systems like LB films.
4. Conclusions The accumulation of the photochromic energy acceptor species, viologen cation radical, in LB films containing A–S dyads was observed. The viologen cation radical presumably arose from lateral migration of photo-created charges. The parameter of the distance dependence of the electron transfer rate b was estimated to be 6.4 nm − 1 from the measurements of fluorescence intensities on the LB films by taking the quenching into consideration.
Acknowledgements The dyad compounds in the present work were synthe-
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sized by H. Yamada at the Tokyo Institute of Technology. The use of these compounds is gratefully acknowledged. This work was supported by a Grant-in-Aid for Encouragement of Young Scientists (no. 09750898) from The Ministry of Education, Science, Sports and Culture of Japan.
References [1] N. Mataga, A. Karen, T. Okada, S. Nishitani, N. Kurata, Y. Sakata, S. Misumi, J. Phys. Chem. 88 (1984) 5138. [2] T.A. Moore, D. Gust, P. Mathis, J.C. Mialoeq, C. Chachaty, R.V. Bensasson, E.J. Land, J.C. Doizi, P.A. Liddel, W.R. Lehman, G.A. Nemeth, A.L. Moore, Nature (London) 307 (1984) 630. [3] M.R. Wasielewski, M.P. Niemczyk, W.A. Svec, E.B. Pewitt, J. Am. Chem. Soc. 107 (1985) 5562. [4] N. Liang, J.R. Miller, G.L. Closs, J. Am. Chem. Soc. 112 (1990) 5353. [5] B. Paulson, K. Pramod, P. Eaton, G.L. Closs, J.R. Miller, J. Phys. Chem. 97 (1993) 13042. [6] S.L. Larson, S.M. Hendrickson, S. Ferrere, D.L. Derr, C.M. Elliott, J. Am. Chem. Soc. 117 (1995) 5881. [7] M. Fujihira, K. Nishiyama, H. Yamada, Thin Solid Films 132 (1985) 77. [8] M. Fujihira, H. Yamada, Thin Solid Films 160 (1988) 125. [9] M. Fujihira, M. Sakomura, Thin Solid Films 179 (1989) 471. [10] M. Sakomura, M. Fujihira, Thin Solid Films 243 (1994) 616. [11] M. Fujihira, M. Sakomura, D. Aoki, A. Koike, Thin Solid Films 273 (1996) 168. [12] M. Fujihira, T. Kamei, M. Sakomura, Y. Tatsu, Y. Kato, Thin Solid Films 179 (1989) 485. [13] T. Watanabe, K. Honda, J. Phys. Chem. 86 (1982) 2617. [14] J.A. Farrington, M. Ebert, E.J. Land, K. Fletcher, Biochim. Biophys. Acta 314 (1973) 372. [15] R.A. Marcus, N. Sutin, Biochem. Biophys. Acta 811 (1985) 265. [16] H. Kuhn, J. Photochem. 10 (1979) 111.
The accumulation of photochromic energy acceptor species in A–S dyad Langmuir–Blodgett films M. Sakomura a ,*, M. Fujihira b a
Department of Physical Chemistry, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
b
Abstract Fluorescence and absorption spectra were measured on Langmuir–Blodgett (LB) films containing acceptor–sensitizer (A–S) dyads. In the dyads, the length of hydrocarbon bridges interconnecting acceptor (viologen) and sensitizer (pyrene) were synthetically changed. In the LB films, the accumulation of viologen cation radical which can quench the excited pyrene moiety as an energy acceptor was observed during UV irradiation under N2. The rate constant of photoinduced intramolecular electron transfer in the LB films was estimated from the fluorescence intensities by taking the energy transfer quenching into consideration. 1998 Elsevier Science S.A. All rights reserved Keywords: Photo-induced electron transfer; Langmuir–Blodgett films; Dyad
1. Introduction Artificial dyad and triad molecular systems as models of a photosynthetic reaction center were intensively studied by several groups [1–3]. The dynamics of the intramolecular electron transfer processes have been mainly studied in dilute solutions containing the dyad or triad molecules [4–6]. The investigations of randomly dissolved molecular functions are very important but sometimes the highly organized molecular systems like Langmuir–Blodgett (LB) films give intermolecular cooperative phenomena, which could not be expected from the properties of isolated molecules. In our approach to design of molecular photodiodes, we synthesized amphiphilic dyad and triad molecules to construct molecular assemblies using the LB method. Our LB films successfully worked as real devices which generate photo-induced current or voltage [7–11]. For further sophisticated design of artificial molecular photodiodes, information about kinetics of intramolecular electron transfer is indispensable. Distance dependence and intervening medium effects are still unsolved problems in the area of electron transfer kinetics. To investigate the electron transfer * Corresponding author. Tel.: +81 45 3393946; fax: +81 45 3393970; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0749-4
kinetics in LB films, amphiphilic acceptor–sensitizer (A– S) dyads were synthesized (Fig. 1). In these dyads, the length of hydrocarbon bridge interconnecting acceptor (viologen, V2 + ) and sensitizer (pyrene, Py) was synthetically changed. In well oriented LB monolayers, the distances between A and S moieties would be the linear lengths of the hydrocarbon bridges. The stationary fluorescence measurements on LB films containing these A–S dyads describe the distance dependence of the rates of electron transfer from S* to A. The photochemical oxidation of LB films containing 6(1-(6(8)-decyl)pyrene)hexanoic acid in the presence of molecular oxygen was reported previously [12]. The energy transfer quenching of the photoexcited pyrene moieties by the photo-oxidative products strongly diminished the fluorescence intensities of the LB films. Thus it is important to keep the LB films under N2 or an inert atmosphere while the fluorescence spectra measurements are carried out. However, we observed a curious behavior on fluorescence of LB films containing the A–S dyads, that is, the fluorescence intensities of the LB films measured under N2 were weaker than that under air. In this paper, we investigate origin of the quenching under N2 and estimate the rate constant of photoinduced intramolecular electron transfer in LB films containing A– S dyads taking the quenching into consideration.
1998 Elsevier Science S.A. All rights reserved
M. Sakomura, M. Fujihira / Thin Solid Films 327–329 (1998) 718–721
Fig. 1. Structural formulae of A–S dyads and PHA.
2. Experimental details 2.1. Materials The dyads 1–4 and pyrene hexadecanoic acid (PHA) were synthesized in this laboratory. The purity of these compounds was confirmed by NMR and elemental analysis. A poly allyl amine (PAA) solution was a gift by courtesy of Nittobo and used as received. All the other chemicals were of reagent grade and used as received. Pure water from a Milli-Q system (Millipore) was used for preparation of the aqueous subphase.
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intensities of pyrene compounds with increasing the elapsed time after the beginning of the irradiation with UV light under air. The decreasing fluorescence intensity was caused by photo-oxidized pyrene products. Under N2, the decreasing was much inhibited. However, the fluorescence intensities of an A–S dyad (n = 11) measured under the inert atmosphere made by the entirely closed fluorescence cell with the flow of N2 shown in Fig. 2a became weaker while the scanning was repeated four times. After the four-times scan, the fifth scan was made on the same sample under air. As shown in Fig. 2b, the fluorescence intensity became stronger than that of any scan in Fig. 2a although the measurement was carried out under an oxygen existing atmosphere. The absorption spectra were also measured on the same sample as that used in Fig. 2. In Fig. 3, the solid and the dotted curve illustrate the absorption spectra measured immediately after the forth scan in Fig. 2a under N2 and fifth scan in Fig. 2b under air, respectively, and the dashed curve illustrates the absorption difference spectra between them. Before the fluorescence measurements in Fig. 2, the absorption spectra under air were also measured and it was almost the same as the dotted curve in Fig. 3. It is
2.2. Procedures A chloroform solution of a mixture of each of the A–S dyads with arachidic acid (AA) (molar ratio, 1:10) was spread on an aqueous subphase containing 0.3 mM CaCl2 and 0.05 mM NaHCO3 in a Langmuir trough (San-esu Keisoku). A mixed monolayer of PHA and AA (molar ratio, 1:100) was also made by spreading a chloroform solution on an aqueous subphase containing 0.04 mM PAA and 0.05 mM NaHCO3. These monolayers were deposited automatically on non-fluorescent quartz plates (Fujiwara Seisakusho) at the surface pressure of 35 mN/m. The UV-vis absorption and the fluorescence spectra were taken with a Hitachi 220A UV-vis recording spectrophotometer and a Hitachi F-3000 recording fluorescence spectrophotometer, respectively. To protect compounds against photo-oxidation and decomposition, an entirely closed fluorescence cell equipped with quartz windows was employed. The oxygen free atmosphere can be easily obtained by flowing N2 gas into the cell.
3. Results and discussion 3.1. The accumulation of photochromic energy acceptor species in A–S dyad LB films In a previous study, we observed decreasing fluorescence
Fig. 2. Fluorescence spectral changes of the mixed LB film of A–S dyad 3–AA (81 layers). All the spectra exhibit a monomer and an excimer band of pyrene moiety at the wavelength region of 370–430 nm and 430–550 nm, respectively. (a) Four times successive scan under N2. The scanning orders for the curves are as follows from the top: 1st, 2nd, 3rd and 4th. (b) 5th scan under air.
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M. Sakomura, M. Fujihira / Thin Solid Films 327–329 (1998) 718–721
cal with an increase in the bridge chain length of A–S dyads was observed. This result can be explained by the mechanism described above. That is to say, the longer lifetime of the charge separated state of the dyad with the longer bridge chain increases the lateral migration efficiency to form the long-lived separated charge. The long-lived Py + • is well known to be hydroxylated rapidly by H2O. During the transient fluorescence and transient absorption measurements, many pulsed laser lights have to be irradiated to obtain enough data for averaging. Therefore, the accumulation of energy acceptor described here may not be ignored. To clarify the effect, transient absorption and transient fluorescence measurements are now under investigation. 3.3. The distance dependence of electron transfer rate in LB films containing A–S dyads Fig. 3. Absorption spectra measured on the same sample as that used in Fig. 2 (solid and dotted line) and the absorption difference spectra between them (dashed line).
clear from comparison of the absorption difference spectra with the spectra of methyl viologen cation radical solution produced by electrochemical reduction [13] that viologen cation radical accumulated in the LB film during UV irradiation under N2. The produced viologen cation radical can be directly oxidized to viologen by oxygen under air [14]. As shown in Figs. 2 and 3, a fluorescence band of the pyrene moiety overlapped well with an absorption band of viologen cation radical. These results suggest that the photochromic energy acceptor, viologen cation radical, quenched the excited pyrene moiety under N2. The accumulated viologen cation radical was oxidized to viologen with oxygen, and thus recovering of the fluorescence intensity was observed under air. 3.2. The mechanism of the accumulation of viologen cation radical UV irradiation of the V2 + –Py dyad gives V + • –Py + • as a product of the intramolecular electron transfer between V2 + and excited pyrene moiety Py*. But the intramolecular charge recombination is so fast that V + • –Py + • cannot accumulate in the dyad system. For the accumulation of viologen cation radical, an increase in inter-charge distance is essential. Previously [11], very long decaying times (.10 s) of the surface potential after irradiation of the LB films containing A–S dyad or A–S–D triad were observed using a scanning surface potential microscope (SSPM). Such long lifetimes of the separated charges could be interpreted by assuming a succeeding intermolecular charge separation mechanism of lateral migration of photo-created radical anions (A − •) and cations (S + • or D + •) between the dyads or triads following intramolecular charge separation within one dyad or triad. The more efficient accumulation of viologen cation radi-
In order to determine the electron transfer quenching rate from the stationary fluorescence intensity measurements, the fluorescence intensities I should be measured before photochromic energy acceptor species accumulate in LB films. Therefore, we estimated the initial fluorescence intensity I by extrapolation of the stationary fluorescence intensity–time curve. The fluorescence intensity without acceptor I0 was measured on a mixed LB film consisting of PHA and AA (molar ratio, 1:100) deposited on quartz substrate (102 layers). I and I0 were normalized by the absorbance of each LB film. The ratio (I0 − I)/I measures the rate of the electron transfer. This ratio can be related to the rate of electron transfer by the following equation from the theoretical consideration for non-adiabatic electron transfer reaction [15]: log[(I0 − I)=I] = − br=2:303 + const:
(1)
where r and b denote the hydrocarbon chain length between A and S and the slope of r versus log[(I0 − I)/I]. Fig. 4 shows the plot of the logarithm of (Io − I)/I as a function of the carbon number of alkyl chains linking the A and S moieties. The value of b was estimated to be ca. 6.4
Fig. 4. Semilogarithmic plot of the ratio (I0 − I)/I as a function of the carbon number of alkyl chains linking A and S moieties.
M. Sakomura, M. Fujihira / Thin Solid Films 327–329 (1998) 718–721
nm − 1 from the slope of the line in Fig. 4. The slope is much higher than that reported by Kuhn et al. for the multilayer LB film system studied by the stationary fluorescence measurements [16]. The observed linear relationship substantiates favorable orientation of A–S dyads in LB films. The distances between A and S moieties of each dyad were successfully controlled by the length of the alkyl chain linking the two moieties. It is not unusual that electron acceptor changes to energy acceptor due to photochromism. Many organic electron acceptors are colored at the radical state and become energy acceptors. In dilute solutions, the photochromic energy acceptor effect is negligible. But the accumulation of the energy acceptor species should be considered when fluorescence spectroscopy is employed to study electron transfer kinetics of dyad or triad molecules in condensed systems like LB films.
4. Conclusions The accumulation of the photochromic energy acceptor species, viologen cation radical, in LB films containing A–S dyads was observed. The viologen cation radical presumably arose from lateral migration of photo-created charges. The parameter of the distance dependence of the electron transfer rate b was estimated to be 6.4 nm − 1 from the measurements of fluorescence intensities on the LB films by taking the quenching into consideration.
Acknowledgements The dyad compounds in the present work were synthe-
721
sized by H. Yamada at the Tokyo Institute of Technology. The use of these compounds is gratefully acknowledged. This work was supported by a Grant-in-Aid for Encouragement of Young Scientists (no. 09750898) from The Ministry of Education, Science, Sports and Culture of Japan.
References [1] N. Mataga, A. Karen, T. Okada, S. Nishitani, N. Kurata, Y. Sakata, S. Misumi, J. Phys. Chem. 88 (1984) 5138. [2] T.A. Moore, D. Gust, P. Mathis, J.C. Mialoeq, C. Chachaty, R.V. Bensasson, E.J. Land, J.C. Doizi, P.A. Liddel, W.R. Lehman, G.A. Nemeth, A.L. Moore, Nature (London) 307 (1984) 630. [3] M.R. Wasielewski, M.P. Niemczyk, W.A. Svec, E.B. Pewitt, J. Am. Chem. Soc. 107 (1985) 5562. [4] N. Liang, J.R. Miller, G.L. Closs, J. Am. Chem. Soc. 112 (1990) 5353. [5] B. Paulson, K. Pramod, P. Eaton, G.L. Closs, J.R. Miller, J. Phys. Chem. 97 (1993) 13042. [6] S.L. Larson, S.M. Hendrickson, S. Ferrere, D.L. Derr, C.M. Elliott, J. Am. Chem. Soc. 117 (1995) 5881. [7] M. Fujihira, K. Nishiyama, H. Yamada, Thin Solid Films 132 (1985) 77. [8] M. Fujihira, H. Yamada, Thin Solid Films 160 (1988) 125. [9] M. Fujihira, M. Sakomura, Thin Solid Films 179 (1989) 471. [10] M. Sakomura, M. Fujihira, Thin Solid Films 243 (1994) 616. [11] M. Fujihira, M. Sakomura, D. Aoki, A. Koike, Thin Solid Films 273 (1996) 168. [12] M. Fujihira, T. Kamei, M. Sakomura, Y. Tatsu, Y. Kato, Thin Solid Films 179 (1989) 485. [13] T. Watanabe, K. Honda, J. Phys. Chem. 86 (1982) 2617. [14] J.A. Farrington, M. Ebert, E.J. Land, K. Fletcher, Biochim. Biophys. Acta 314 (1973) 372. [15] R.A. Marcus, N. Sutin, Biochem. Biophys. Acta 811 (1985) 265. [16] H. Kuhn, J. Photochem. 10 (1979) 111.