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External electric field effects on the fluorescence of methylene-linked pyrene and N,N-dimethylaniline doped in a PMMA polymer film
14 August 1998
Chemical Physics Letters 292 Ž1998. 535–541
External electric field effects on the fluorescence of methylene-linked pyrene and N, N-dimethylaniline doped in a PMMA polymer film Nobuhiro Ohta
a,)
, Takayuki Kanada a , Iwao Yamazaki a , Michiya Itoh
b
a
b
Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060, Japan Department of Physical and Chemical Biodynamics, Graduate School and Faculty of Pharmaceutical Sciences, Kanazawa UniÕersity, Takara-machi, Kanazawa 920, Japan Received 26 February 1998; in final form 8 May 1998
Abstract Fluorescence emitted from the pyrene chromophore is quenched by an electric field and both intra- and intermolecular electron transfer processes from N, N-dimethylaniline ŽDMA. to the excited state of pyrene are suggested as being enhanced by an electric field in a methylene-linked compound of pyrene and DMA doped in a PMMA polymer film. Exciplex fluorescence, which appears as a result of photoinduced electron transfer, shows field-induced enhancement and quenching, depending on the concentration. Quenching of the exciplex fluorescence observed at high concentrations is attributed to a photocarrier generation. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In a methylene-linked electron-donor and -acceptor system composed of carbazole and terephthalic acid methyl ester doped in a PMMA polymer film, exciplex fluorescence, which appears as a result of photoinduced electron transfer, is greatly enhanced by an electric field at low sample concentrations w1x. These field effects markedly depend on the methylene chain length. Fluorescence emitted from the locally excited state of carbazole is also quenched by an electric field. Thus, the initial step of the electron transfer, charge recombination and charge migration are markedly affected by an electric field, depending on the methylene chain length. These findings show )
Corresponding author.
that the investigation of photoinduced electron transfer reactions in methylene-linked donor and acceptor systems is useful to elucidate both intra- and intermolecular photoinduced electron transfer processes as well as their field dependence, even when these systems are doped in a solid film. In the present study, the external electric field effect on the fluorescence of 1-Ž1-pyrenyl.-3-ŽmŽ N, N-dimethylamino.phenyl.propane, denoted by Py-m-DMA, doped in a PMMA polymer film has been examined using field modulation emission spectroscopy. A combination of pyrene with dimethylaniline seems to be one of the best candidates for further studies on the electric field effect and its methylene chain length dependence for photoinduced electron transfer processes, since the excitation dynamics, spin dynamics and their methylene chain
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 7 0 7 - 6
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N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
length dependence following photoinduced electron transfer from dimethylaniline to the exited state of pyrene have been well examined in solution w2–5x. Both the intra- and inter-molecular electron transfer of Py-m-DMA have been found to be enhanced by an external electric field. In comparison with methylene-linked compounds of carbazole and terephthalic acid methyl ester, however, the field-induced enhancement of the exciplex fluorescence of Py-mDMA is small even at low concentrations, suggesting that charge recombination of the radical ion pair of Py-m-DMA produced by photoinduced electron transfer is inefficient, irrespective of the methylene chain length.
measured at the second harmonic of the modulation frequency Žtypically 40 Hz.. The external electric field, which is denoted by F, was applied up to 0.8 MV cmy1 .
3. Results and discussion Fig. 1 shows the electrofluorescence spectra of Py-m-DMA doped in a PMMA polymer film at different concentrations from 0.01 to 10 mol% in the ratio to the monomer unit of PMMA, together with the fluorescence spectra simultaneously observed. These spectra were obtained with a field strength of 0.5 MV cmy1 and with excitation at 321 nm, where
2. Experimental Py-m-DMA was synthesized and purified with the same methods as described elsewhere w6x. The purification of PMMA and the preparation of the Py-mDMA sample, doped in a PMMA polymer film, are the same as those for the methylene-linked compounds of carbazole and terephthalic acid methyl ester w1x. A benzene solution of PMMA and Py-mDMA was poured onto the indium tin oxide ŽITO.coated substrate by a spin coating method to prepare a thin polymer film. A semitransparent aluminum film was deposited on the dried polymer film. The ITO and aluminum films were used as electrodes. All the optical measurements were carried out at room temperature under vacuum conditions. Plots of the electric field-induced change in the absorption intensity and in the fluorescence intensity as a function of wavelength, i.e. the so-called electroabsorption spectrum and electrofluorescence spectrum, respectively, were obtained using a field modulation spectroscopy with the same apparatus as described elsewhere w1,7x. A sinusoidal ac voltage was applied between the electrodes using a function generator combined with a home-made amplifier. For the measurements of the electrofluorescence spectra, a small amount of the ac component of the fluorescence intensity at wavelength l D I F Ž l., synchronized with the applied ac voltage, was detected with a lock-in amplifier as a function of l, together with the total fluorescence intensity Ž I F Ž l... Both the electroabsorption spectra and electrofluorescence spectra were
Fig. 1. Electrofluorescence spectrum Žsolid line. and fluorescence spectrum Ždotted line. of Py-m-DMA doped in a PMMA film at 0.01, 0.5, 2.0, 5.0 and 10.0 mol% Žfrom top to bottom.. These spectra were obtained with a field strength of 0.5 MV cmy1 with excitation at 321 nm. The maximum fluorescence intensity is normalized to unity in every case. Thin dotted lines show D I F s 0.
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
Fig. 2. Plots of D I F r I F of Py-m-DMP at 10 mol% as a function of the square of the applied electric field strength for the pyrene fluorescence at 378 nm Žopen circle. and for the exciplex fluorescence at 455 nm Žsolid circle..
the field-induced change in the absorption intensity relative to the absorption intensity was estimated to be as small as ; 1 = 10y4 at 0.5 MV cmy1 . The field-induced change in the fluorescence intensity is proportional to the square of the applied field strength, as is shown in Fig. 2. As the concentration of Py-m-DMA increases, a broad emission with a maximum at around 450 nm is observed besides the sharp structured emission. The excimer produced by the intermolecular interaction between the pyrene chromophores of Py-m-DMA is considered to give a broad fluorescence with a peak at ; 470 nm, as in the case of pyrene w8x or 1,3-bisŽ1-pyrenyl.propane doped in a PMMA polymer film w9x. As is shown in Fig. 1, however, the peak of the present broad fluorescence is at ; 450 nm, which is different from the peak of the pyrene excimer by ; 20 nm. In connection with the peak shift, it is also noted that a shoulder at ; 470 nm, which seems to appear in the fluorescence spectra shown in Fig. 1 is false, which comes from an incomplete correction for the wavelength dependence of the sensitivity of the spectrometer employed in the electrofluorescence measurements. As mentioned in a previous Letter w8x, the Stark shift of the pyrene excimer in a PMMA polymer film is small. As is shown later, however, the Stark shift of the broad fluorescence of Py-m-
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DMA is large, suggesting that the molecular polarizability of the emitting state of the broad fluorescence of Py-m-DMA is much larger than that of the pyrene excimer. Further, the present electric field effect on the broad fluorescence is different from that of the pyrene excimer with a peak at ; 470 nm. For example, the field-induced enhancement observed in the present experiments at low concentrations was not observed for the pyrene excimer. Furthermore, the electrofluorescence spectra observed in the present experiments do not show any evidence of the presence of different fluorescent complexes, which show different Stark shifts or different field-effects on excitation dynamics from each other. Therefore, the broad emission observed for Py-m-DMA doped in a PMMA film is assigned as the exciplex fluorescence resulting from the electron transfer from the N, N-dimethylaniline chromophore ŽDMA. to the excited state of the pyrene chromophore ŽPY. of Py-mDMA, while the structured emission is assigned as the fluorescence emitted from the locally excited state S 1 of PY w2,3x. Hereafter, the broad and structured emissions are referred to as the exciplex fluorescence and monomer fluorescence, respectively. The emission at low concentrations is dominated by the monomer fluorescence, suggesting that the observed exciplex fluorescence results from intermolecular electron transfer from DMA to the excited state of PY and that intramolecular photoindued electron transfer is inefficient in PMMA films, if it exists. The fact that the exciplex fluorescence relative to the monomer fluorescence becomes stronger with an increase of the Py-m-DMA concentration suggests that the rate, as well as the efficiency, of the intermolecular electron transfer becomes larger by decreasing the donor and acceptor distance. The presence of three types of isomeric conformers has been suggested in the ground state of Py-m-DMA, based on the fluorescence excitation spectra observed in a supersonic jet w6x. In the present experiments, however, the spectra of different isomeric chain conformers were not separated, though different conformers may exist even in a solid polymer film. As is shown in the electrofluorescence spectra in Fig. 1, the field-induced change in fluorescence intensity, i.e. D I F is negative in the monomer fluorescence region of 370–425 nm at any concentration,
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indicating that the quantum yield of the monomer fluorescence decreases in the presence of F. Actually, the electrofluorescence spectra of monomer fluorescence are regarded as given by a linear combination of the fluorescence spectrum with its first derivative spectrum, as will be mentioned later. Then, the field-induced change in intensity relative to the total intensity, which corresponds to DF FrF F was evaluated for the monomer fluorescence from D I FrIF at 378 nm, where the first derivative of the fluorescence spectrum is zero. Here, F F and DF F represent the fluorescence quantum yield and its field-induced change, respectively. Plots of D I FrIF thus obtained are shown in Fig. 3 as a function of the Py-m-DMA concentration. Obviously, F F decreases in the presence of F and the magnitude of the decrease, i.e. DF FrF F increases on increasing the Py-m-DMA concentration. Electrofluorescence spectra of the exciplex fluorescence is also given by a linear combination of the fluorescence spectrum with its first derivative spec-
Fig. 3. Plots of the field-induced change in fluorescence intensity relative to the fluorescence intensity observed at 378 nm Žcircle. and at 455 nm Žsquare. as a function of the Py-m-DMA concentration. These intensities were obtained with a field strength of 0.5 MV cmy1 .
trum, as will be mentioned later. Then, DF FrF F of the exciplex fluorescence was determined by evaluating D I FrIF at 455 nm, where the first derivative of the exciplex fluorescence is nearly zero, as a function of the Py-m-DMA concentration. The results are also shown in Fig. 3. In contrast with the monomer fluorescence, exciplex fluorescence is enhanced by F at low Py-m-DMA concentrations below 0.5 mol%. At high concentrations of Py-m-DMA, on the other hand, exciplex fluorescence is quenched by an electric field and the magnitude of the quenching, i.e. < DF FrF F < becomes larger on increasing the Py-mDMA concentration. It is worth mentioning that the exciplex fluorescence intensity is very weak at low concentrations and that DF FrF F of the exciplex fluorescence below 1 mol% may be much larger than D I FrIF at 455 nm since fluorescence at 455 nm, is considered to be dominated by the monomer fluorescence at these low concentrations. The following scheme is employed to interpret the present results of the field effects, as in the case of methylene-linked carbazole and terephthalic acid methyl ester w1x:
Here, D and A represent DMA and PY, respectively. A) ) is the photoexcited state of PY and k rel is the rate constant of relaxation from A) ) to A) , following which a contact pair D PPP A) is formed. This relaxation process includes both the internal conversion to the lowest excited state of S 1 and the excitation energy migration among different molecules of PY because the excitation spectrum of the exciplex fluorescence is essentially the same as the absorption spectrum of Py-m-DMA. The electron transfer process with a rate constant of k et competes with the radiative process from A) , which gives the structured monomer fluorescence. Dq–Ay shows a radical ion-pair state produced by the electron transfer.
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
Here, it is noted that it is not clear whether the ion-pair state is produced through a ‘unrelaxed and non-fluorescent’ exciplex state w10–12x. ŽDq [Ay . represents the exciplex, which gives a broad fluorescence with a peak at ; 450 nm and k gr is the rate constant of the exciplex formation from the radical ion-pair state. It is obvious that the fluorescent exciplex k cr and k ex represent the rate constants for the charge recombination of the radical ion-pair state and the decay rate constant of the fluorescent exciplex, respectively. It is unlikely that molecules doped in a polymer film can move, but a field-assisted dissociation to free carriers with a rate constant of k dis may have to be considered with the assumption that the charge can move from a molecule to a neighboring molecule in the presence of F. Then the field-induced quenching of the exciplex fluorescence observed at high concentrations of Py-m-DMA is attributed to this dissociating process of the radical ion-pair which leads to a photocarrier generation w10–17x. As the origin of the field-induced quenching of the monomer fluorescence, two possibilities have to be considered: one is the field-induced decrease of the radiative decay rate and the other is the field-induced enhancement of the non-radiative decay rate. In the latter case, the photoinduced electron transfer process is considered to be enhanced by F. It is unlikely that the radiative decay rate of PY ) is seriously affected by F in Py-m-DMA since the electroabsorption and electrofluorescence spectra show that the transition moment of pyrene is nearly unaffected by F for any low-lying transition w7x. Therefore, the quenching of the monomer fluorescence is attributed to the field-induced enhancement of the rate of the electron transfer from DMA to PY ) . The concentration dependence both of the fluorescence spectrum and of the electrofluorescence spectrum shows that the photoinduced intermolecular electron transfer rate and its field-induced change become larger on decreasing the donor-acceptor distance. Even at a low concentration of 0.01 mol%, further, the monomer fluorescence is reduced by F, as is shown in Fig. 1. These results imply that not only the intermolecular electron transfer but also intramolecular electron transfer are enhanced by F, though the intramolecular electron transfer is inefficient or non-existent in a PMMA film at zero field.
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As mentioned above, the field-induced quenching of the monomer fluorescence is attributed to the enhancement of the electron transfer rate. Then, a small enhancement of the exciplex fluorescence in the presence of F, observed at low concentrations of Py-m-DMA, may be attributed to an increase of the exciplex formation yield because of the field-induced increase of the radical ion pair produced by the photoinduced electron transfer process. As the origin of the field-induced enhancement of the exciplex fluorescence, however, other possibilities may also have to be considered, as mentioned below. If a charge recombination of the radical ion pair produced by a photoinduced electron transfer occurs at zero field, the charge recombination may be inhibited by F. As a result, the exciplex fluorescence may be enhanced by F because of a field-induced recovery of the quenched exciplex fluorescence. Note that the charge recombination induces a quenching of the exciplex fluorescence, as a result of the decrease of the exciplex formation yield Žsee scheme 1.. In methylene-linked carbazole and terephthalic acid methyl ester, for example, a field-induced enhancement of the exciplex fluorescence induced by this mechanism was shown to occur efficiently when a methylene chain length is short. The present enhancement of the exciplex fluorescence in Py-mDMA is small in comparison with the methylene-linked compound composed of carbazole and terephthalic acid methyl ester with the same chain length. The large difference seems to imply that the charge recombination process, which is inhibited by F, is negligibly small in Py-m-DMA. Even when the formation yield of the fluorescent exciplex is not changed by F, the fluorescence quantum yield increases when the radiative decay rate is enhanced by F. Therefore, the field effect on the radiative decay rate of the fluorescent exciplex, as well as the field effect on the charger recombination through the methylene chain, must be examined in order to elucidate the field-induced enhancement of the exciplex fluorescence more clearly, though the present field-induced enhancement is attributed to the fieldinduced increase of the fluorescent exciplex as a result of the increase of the electron transfer rate at the moment. As is shown in Fig. 4, the electrofluorescence spectrum of the exciplex at 10 mol% is simulated by
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
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Fig. 4. Simulation of the electrofluorescence spectrum of Py-m-DMA at 10 mol%. The figure shows the electrofluorescence spectrum observed with a field strength of 0.5 MV cmy1 Ždotted line., fluorescence spectrum Žbroken line., the first derivative spectrum of the fluorescence spectrum Žthin solid line. of Py-m-DMA and the spectrum simulated by a linear combination of the fluorescence spectrum with its first derivative spectrum Žthick solid line.. The maximum fluorescence intensity is normalized to unity and arbitrary units were used for the first derivative spectrum.
a linear combination of the fluorescence spectrum with its first-derivative spectrum. As already mentioned, the former is attributed to the field-induced change in F F . On the other hand, the latter comes from the so-called Stark shift, as mentioned below. The energy level shift induced by F depends on the electric dipole moment and molecular polarizability. As a result, the emission spectrum as well as the absorption spectrum are expected to be changed by F. An expression for such a field-induced change in absorption intensity as well as in emission intensity was derived by Liptay and co-workers w18,19x. By assuming that the original isotropic distribution in rigid matrices such as PMMA polymer films is maintained even in the presence of F, the spectral change in the fluorescence intensity given in units of wavenumber, n i.e. D I F Ž n . in the presence of F may be given by the following equation w18–20x: D I F Ž n . s Ž fF .
2
qCn
½
ž
d 2 Ž I F Ž n . rn dn 2
d Ž I F Ž n . rn .
ž . /5
AIF Ž n . q Bn
dn
/ Ž 1.
where f is the internal field factor, A depends on the change in the fluorescence quantum yield, and B and C are given as follows: Bs
D ar2 q Ž D a m y D a . Ž 3cos 2 x y 1 . r10
Cs Ž Dm.
hc 2
ž
5 q Ž 3cos 2 j y 1 .Ž 3cos 2 x y 1 . 30 h 2 c 2
/
Ž 2. Ž 3.
where h is Planck’s constant and c is the speed of light. Here, D m is the difference in electric dipole moment between the ground and excited state, i.e. D m s mg y me and D a is related to the difference in the polarizability tensor D a s a g y a e : D m s < D m < ; D a s 13 Tr Ž D a .
Ž 4.
D a m denotes the diagonal component of D a with respect to the direction of the transition dipole moment, x is the angle between the direction of F and the electric vector of the excitation light and j is the angle between the direction of D m and the transition dipole moment.
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
As is shown in Figs. 1 and 4, the peak of the electrofluorescence spectrum is shifted relative to the fluorescence spectrum and the first derivative of the fluorescence spectrum is necessary for simulation of the electrofluorescence spectrum. This result indicates that the Stark shift comes from a change in molecular polarizability. By evaluating the first-derivative part in the electrofluorescence spectrum and comparing with Eq. Ž1., D a is determined to be 190 ˚ 3 at 10 mol%. Here, it is assumed in units of 4 p´ 0 A that the internal field is the same as the applied field i.e. f s 1 in Eq. Ž1. and that the molecular polarizability is isotropic, i.e. D a m s D a . The monomer fluorescence of PY is also considered to show the Stark shift induced by the change in molecular polarizability between the S 1 state and the ground state of PY. In fact, the electrofluorescence spectrum at 0.01 mol%, where the field-induced change in F F is very small, is not identical with the fluorescence spectrum in shape, as is shown in Fig. 1, implying that the first derivative of the fluorescence spectrum is necessary to simulate the observed electrofluorescence spectrum. Da of PY is regarded ˚ 3 with f s 1 being as small as 15 in units of 4 p´ A in Eq. Ž1., as reported for pyrene doped in a PMMA film w7x. It is noted that both the exciplex fluorescence and monomer fluorescence show a red-shift in the presence of F, indicating that the molecular polarizability in the emitting state is larger than the ground state in both cases. The field-induced enhancement at the initial step of the photoinduced electron transfer may be attributed to the field-induced increase of the transfer integral andror the field-induced change in the free energy change of the electron transfer process. Fluorescence decay measurements, both in the presence and absence of the external electric field, are now in progress to discuss the field effect more quantitatively and to specify the mechanism of the field effect on the electron transfer. Photocurrent measurements are also in progress to confirm the relationship between the field-induced quenching of the exciplex fluorescence and the photocarrier generation.
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Acknowledgements This work was partly supported by a Grant-in-Aid for the Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture.
References w1x N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, Y. Tanimoto, Y. Hatano, M. Yamamoto, H. Kono, J. Phys. Chem. 100 Ž1996. 19295. w2x R. Ide, Y. Sakata, S. Misumi, T. Okada, N. Mataga, Chem. Commun. 1009 Ž1972. . w3x N. Mataga, M. Ottolenghi, in: R. Foster ŽEd.., Molecular Association, Vol. 2, Academic, London, 1979, p. 1. w4x H. Staerk, W. Kuhnle, R. Treichel, A. Weller, Chem. Phys. ¨ Lett. 118 Ž1985. 19. w5x A. Weller, in: V. Balzani ŽEds.., Supramolecular Photochemistry, D. Reidel Publishing, Dordrecht, The Netherlands, 1987, pp. 343. w6x M. Kurono, M. Itoh, J. Phys. Chem. 99 Ž1995. 17113. w7x S. Umeuchi, Y. Nishimura, I. Yamazaki, H. Murakami, M. Yamashita, N. Ohta, Thin Solid Films 311 Ž1997. 239. w8x N. Ohta, S. Umeuchi, T. Kanada, Y. Nishimura, I. Yamazaki, Chem. Phys. Lett. 279 Ž1997. 215. w9x N. Ohta, S. Umeuchi, H. Kawabata, I. Yamazaki, to be submitted. w10x M. Yokoyama, Y. Endo, H. Mikawa, Chem. Phys. Lett. 34 Ž1975. 597. w11x M. Yokoyama, Y. Endo, H. Mikawa, Bull. Chem. Soc. Jpn 49 Ž1976. 1538. w12x M. Yokoyama, Y. Endo, A. Matsubara, H. Mikawa, J. Chem. Phys. 75 Ž1981. 3006. w13x R.B. Comizzori, Photochem. Photobiol. 15 Ž1972. 399. w14x Z.D. Popovic, Chem. Phys. 86 Ž1984. 311. w15x D.S. Weiss, M. Burberry, Thin Solid Films 158 Ž1988. 175. w16x H. Sakai, A. Itaya, H. Masuhara, J. Phys. Chem. 93 Ž1989. 5351. w17x J. Kalinowski, W. Stampor, P.G. Di Marco, J. Chem. Phys. 96 Ž1992. 4136. w18x W. Liptay, in: E.C. Lim ŽEd.., Excited States, Vol. 1, Academic Press, New York, NY, 1974, p. 129. w19x R. Wortmann, K. Elich, W. Liptay, Chem. Phys. 124 Ž1988. 395. w20x S.G. Boxer, J. Deisenhofer, J.R. Norris ŽEds.., The Photosynthetic Reaction Center, Vol. II, Academic Press, San Diego, CA, 1993, p. 179.
Chemical Physics Letters 292 Ž1998. 535–541
External electric field effects on the fluorescence of methylene-linked pyrene and N, N-dimethylaniline doped in a PMMA polymer film Nobuhiro Ohta
a,)
, Takayuki Kanada a , Iwao Yamazaki a , Michiya Itoh
b
a
b
Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060, Japan Department of Physical and Chemical Biodynamics, Graduate School and Faculty of Pharmaceutical Sciences, Kanazawa UniÕersity, Takara-machi, Kanazawa 920, Japan Received 26 February 1998; in final form 8 May 1998
Abstract Fluorescence emitted from the pyrene chromophore is quenched by an electric field and both intra- and intermolecular electron transfer processes from N, N-dimethylaniline ŽDMA. to the excited state of pyrene are suggested as being enhanced by an electric field in a methylene-linked compound of pyrene and DMA doped in a PMMA polymer film. Exciplex fluorescence, which appears as a result of photoinduced electron transfer, shows field-induced enhancement and quenching, depending on the concentration. Quenching of the exciplex fluorescence observed at high concentrations is attributed to a photocarrier generation. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In a methylene-linked electron-donor and -acceptor system composed of carbazole and terephthalic acid methyl ester doped in a PMMA polymer film, exciplex fluorescence, which appears as a result of photoinduced electron transfer, is greatly enhanced by an electric field at low sample concentrations w1x. These field effects markedly depend on the methylene chain length. Fluorescence emitted from the locally excited state of carbazole is also quenched by an electric field. Thus, the initial step of the electron transfer, charge recombination and charge migration are markedly affected by an electric field, depending on the methylene chain length. These findings show )
Corresponding author.
that the investigation of photoinduced electron transfer reactions in methylene-linked donor and acceptor systems is useful to elucidate both intra- and intermolecular photoinduced electron transfer processes as well as their field dependence, even when these systems are doped in a solid film. In the present study, the external electric field effect on the fluorescence of 1-Ž1-pyrenyl.-3-ŽmŽ N, N-dimethylamino.phenyl.propane, denoted by Py-m-DMA, doped in a PMMA polymer film has been examined using field modulation emission spectroscopy. A combination of pyrene with dimethylaniline seems to be one of the best candidates for further studies on the electric field effect and its methylene chain length dependence for photoinduced electron transfer processes, since the excitation dynamics, spin dynamics and their methylene chain
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 7 0 7 - 6
536
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
length dependence following photoinduced electron transfer from dimethylaniline to the exited state of pyrene have been well examined in solution w2–5x. Both the intra- and inter-molecular electron transfer of Py-m-DMA have been found to be enhanced by an external electric field. In comparison with methylene-linked compounds of carbazole and terephthalic acid methyl ester, however, the field-induced enhancement of the exciplex fluorescence of Py-mDMA is small even at low concentrations, suggesting that charge recombination of the radical ion pair of Py-m-DMA produced by photoinduced electron transfer is inefficient, irrespective of the methylene chain length.
measured at the second harmonic of the modulation frequency Žtypically 40 Hz.. The external electric field, which is denoted by F, was applied up to 0.8 MV cmy1 .
3. Results and discussion Fig. 1 shows the electrofluorescence spectra of Py-m-DMA doped in a PMMA polymer film at different concentrations from 0.01 to 10 mol% in the ratio to the monomer unit of PMMA, together with the fluorescence spectra simultaneously observed. These spectra were obtained with a field strength of 0.5 MV cmy1 and with excitation at 321 nm, where
2. Experimental Py-m-DMA was synthesized and purified with the same methods as described elsewhere w6x. The purification of PMMA and the preparation of the Py-mDMA sample, doped in a PMMA polymer film, are the same as those for the methylene-linked compounds of carbazole and terephthalic acid methyl ester w1x. A benzene solution of PMMA and Py-mDMA was poured onto the indium tin oxide ŽITO.coated substrate by a spin coating method to prepare a thin polymer film. A semitransparent aluminum film was deposited on the dried polymer film. The ITO and aluminum films were used as electrodes. All the optical measurements were carried out at room temperature under vacuum conditions. Plots of the electric field-induced change in the absorption intensity and in the fluorescence intensity as a function of wavelength, i.e. the so-called electroabsorption spectrum and electrofluorescence spectrum, respectively, were obtained using a field modulation spectroscopy with the same apparatus as described elsewhere w1,7x. A sinusoidal ac voltage was applied between the electrodes using a function generator combined with a home-made amplifier. For the measurements of the electrofluorescence spectra, a small amount of the ac component of the fluorescence intensity at wavelength l D I F Ž l., synchronized with the applied ac voltage, was detected with a lock-in amplifier as a function of l, together with the total fluorescence intensity Ž I F Ž l... Both the electroabsorption spectra and electrofluorescence spectra were
Fig. 1. Electrofluorescence spectrum Žsolid line. and fluorescence spectrum Ždotted line. of Py-m-DMA doped in a PMMA film at 0.01, 0.5, 2.0, 5.0 and 10.0 mol% Žfrom top to bottom.. These spectra were obtained with a field strength of 0.5 MV cmy1 with excitation at 321 nm. The maximum fluorescence intensity is normalized to unity in every case. Thin dotted lines show D I F s 0.
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
Fig. 2. Plots of D I F r I F of Py-m-DMP at 10 mol% as a function of the square of the applied electric field strength for the pyrene fluorescence at 378 nm Žopen circle. and for the exciplex fluorescence at 455 nm Žsolid circle..
the field-induced change in the absorption intensity relative to the absorption intensity was estimated to be as small as ; 1 = 10y4 at 0.5 MV cmy1 . The field-induced change in the fluorescence intensity is proportional to the square of the applied field strength, as is shown in Fig. 2. As the concentration of Py-m-DMA increases, a broad emission with a maximum at around 450 nm is observed besides the sharp structured emission. The excimer produced by the intermolecular interaction between the pyrene chromophores of Py-m-DMA is considered to give a broad fluorescence with a peak at ; 470 nm, as in the case of pyrene w8x or 1,3-bisŽ1-pyrenyl.propane doped in a PMMA polymer film w9x. As is shown in Fig. 1, however, the peak of the present broad fluorescence is at ; 450 nm, which is different from the peak of the pyrene excimer by ; 20 nm. In connection with the peak shift, it is also noted that a shoulder at ; 470 nm, which seems to appear in the fluorescence spectra shown in Fig. 1 is false, which comes from an incomplete correction for the wavelength dependence of the sensitivity of the spectrometer employed in the electrofluorescence measurements. As mentioned in a previous Letter w8x, the Stark shift of the pyrene excimer in a PMMA polymer film is small. As is shown later, however, the Stark shift of the broad fluorescence of Py-m-
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DMA is large, suggesting that the molecular polarizability of the emitting state of the broad fluorescence of Py-m-DMA is much larger than that of the pyrene excimer. Further, the present electric field effect on the broad fluorescence is different from that of the pyrene excimer with a peak at ; 470 nm. For example, the field-induced enhancement observed in the present experiments at low concentrations was not observed for the pyrene excimer. Furthermore, the electrofluorescence spectra observed in the present experiments do not show any evidence of the presence of different fluorescent complexes, which show different Stark shifts or different field-effects on excitation dynamics from each other. Therefore, the broad emission observed for Py-m-DMA doped in a PMMA film is assigned as the exciplex fluorescence resulting from the electron transfer from the N, N-dimethylaniline chromophore ŽDMA. to the excited state of the pyrene chromophore ŽPY. of Py-mDMA, while the structured emission is assigned as the fluorescence emitted from the locally excited state S 1 of PY w2,3x. Hereafter, the broad and structured emissions are referred to as the exciplex fluorescence and monomer fluorescence, respectively. The emission at low concentrations is dominated by the monomer fluorescence, suggesting that the observed exciplex fluorescence results from intermolecular electron transfer from DMA to the excited state of PY and that intramolecular photoindued electron transfer is inefficient in PMMA films, if it exists. The fact that the exciplex fluorescence relative to the monomer fluorescence becomes stronger with an increase of the Py-m-DMA concentration suggests that the rate, as well as the efficiency, of the intermolecular electron transfer becomes larger by decreasing the donor and acceptor distance. The presence of three types of isomeric conformers has been suggested in the ground state of Py-m-DMA, based on the fluorescence excitation spectra observed in a supersonic jet w6x. In the present experiments, however, the spectra of different isomeric chain conformers were not separated, though different conformers may exist even in a solid polymer film. As is shown in the electrofluorescence spectra in Fig. 1, the field-induced change in fluorescence intensity, i.e. D I F is negative in the monomer fluorescence region of 370–425 nm at any concentration,
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N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
indicating that the quantum yield of the monomer fluorescence decreases in the presence of F. Actually, the electrofluorescence spectra of monomer fluorescence are regarded as given by a linear combination of the fluorescence spectrum with its first derivative spectrum, as will be mentioned later. Then, the field-induced change in intensity relative to the total intensity, which corresponds to DF FrF F was evaluated for the monomer fluorescence from D I FrIF at 378 nm, where the first derivative of the fluorescence spectrum is zero. Here, F F and DF F represent the fluorescence quantum yield and its field-induced change, respectively. Plots of D I FrIF thus obtained are shown in Fig. 3 as a function of the Py-m-DMA concentration. Obviously, F F decreases in the presence of F and the magnitude of the decrease, i.e. DF FrF F increases on increasing the Py-m-DMA concentration. Electrofluorescence spectra of the exciplex fluorescence is also given by a linear combination of the fluorescence spectrum with its first derivative spec-
Fig. 3. Plots of the field-induced change in fluorescence intensity relative to the fluorescence intensity observed at 378 nm Žcircle. and at 455 nm Žsquare. as a function of the Py-m-DMA concentration. These intensities were obtained with a field strength of 0.5 MV cmy1 .
trum, as will be mentioned later. Then, DF FrF F of the exciplex fluorescence was determined by evaluating D I FrIF at 455 nm, where the first derivative of the exciplex fluorescence is nearly zero, as a function of the Py-m-DMA concentration. The results are also shown in Fig. 3. In contrast with the monomer fluorescence, exciplex fluorescence is enhanced by F at low Py-m-DMA concentrations below 0.5 mol%. At high concentrations of Py-m-DMA, on the other hand, exciplex fluorescence is quenched by an electric field and the magnitude of the quenching, i.e. < DF FrF F < becomes larger on increasing the Py-mDMA concentration. It is worth mentioning that the exciplex fluorescence intensity is very weak at low concentrations and that DF FrF F of the exciplex fluorescence below 1 mol% may be much larger than D I FrIF at 455 nm since fluorescence at 455 nm, is considered to be dominated by the monomer fluorescence at these low concentrations. The following scheme is employed to interpret the present results of the field effects, as in the case of methylene-linked carbazole and terephthalic acid methyl ester w1x:
Here, D and A represent DMA and PY, respectively. A) ) is the photoexcited state of PY and k rel is the rate constant of relaxation from A) ) to A) , following which a contact pair D PPP A) is formed. This relaxation process includes both the internal conversion to the lowest excited state of S 1 and the excitation energy migration among different molecules of PY because the excitation spectrum of the exciplex fluorescence is essentially the same as the absorption spectrum of Py-m-DMA. The electron transfer process with a rate constant of k et competes with the radiative process from A) , which gives the structured monomer fluorescence. Dq–Ay shows a radical ion-pair state produced by the electron transfer.
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
Here, it is noted that it is not clear whether the ion-pair state is produced through a ‘unrelaxed and non-fluorescent’ exciplex state w10–12x. ŽDq [Ay . represents the exciplex, which gives a broad fluorescence with a peak at ; 450 nm and k gr is the rate constant of the exciplex formation from the radical ion-pair state. It is obvious that the fluorescent exciplex k cr and k ex represent the rate constants for the charge recombination of the radical ion-pair state and the decay rate constant of the fluorescent exciplex, respectively. It is unlikely that molecules doped in a polymer film can move, but a field-assisted dissociation to free carriers with a rate constant of k dis may have to be considered with the assumption that the charge can move from a molecule to a neighboring molecule in the presence of F. Then the field-induced quenching of the exciplex fluorescence observed at high concentrations of Py-m-DMA is attributed to this dissociating process of the radical ion-pair which leads to a photocarrier generation w10–17x. As the origin of the field-induced quenching of the monomer fluorescence, two possibilities have to be considered: one is the field-induced decrease of the radiative decay rate and the other is the field-induced enhancement of the non-radiative decay rate. In the latter case, the photoinduced electron transfer process is considered to be enhanced by F. It is unlikely that the radiative decay rate of PY ) is seriously affected by F in Py-m-DMA since the electroabsorption and electrofluorescence spectra show that the transition moment of pyrene is nearly unaffected by F for any low-lying transition w7x. Therefore, the quenching of the monomer fluorescence is attributed to the field-induced enhancement of the rate of the electron transfer from DMA to PY ) . The concentration dependence both of the fluorescence spectrum and of the electrofluorescence spectrum shows that the photoinduced intermolecular electron transfer rate and its field-induced change become larger on decreasing the donor-acceptor distance. Even at a low concentration of 0.01 mol%, further, the monomer fluorescence is reduced by F, as is shown in Fig. 1. These results imply that not only the intermolecular electron transfer but also intramolecular electron transfer are enhanced by F, though the intramolecular electron transfer is inefficient or non-existent in a PMMA film at zero field.
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As mentioned above, the field-induced quenching of the monomer fluorescence is attributed to the enhancement of the electron transfer rate. Then, a small enhancement of the exciplex fluorescence in the presence of F, observed at low concentrations of Py-m-DMA, may be attributed to an increase of the exciplex formation yield because of the field-induced increase of the radical ion pair produced by the photoinduced electron transfer process. As the origin of the field-induced enhancement of the exciplex fluorescence, however, other possibilities may also have to be considered, as mentioned below. If a charge recombination of the radical ion pair produced by a photoinduced electron transfer occurs at zero field, the charge recombination may be inhibited by F. As a result, the exciplex fluorescence may be enhanced by F because of a field-induced recovery of the quenched exciplex fluorescence. Note that the charge recombination induces a quenching of the exciplex fluorescence, as a result of the decrease of the exciplex formation yield Žsee scheme 1.. In methylene-linked carbazole and terephthalic acid methyl ester, for example, a field-induced enhancement of the exciplex fluorescence induced by this mechanism was shown to occur efficiently when a methylene chain length is short. The present enhancement of the exciplex fluorescence in Py-mDMA is small in comparison with the methylene-linked compound composed of carbazole and terephthalic acid methyl ester with the same chain length. The large difference seems to imply that the charge recombination process, which is inhibited by F, is negligibly small in Py-m-DMA. Even when the formation yield of the fluorescent exciplex is not changed by F, the fluorescence quantum yield increases when the radiative decay rate is enhanced by F. Therefore, the field effect on the radiative decay rate of the fluorescent exciplex, as well as the field effect on the charger recombination through the methylene chain, must be examined in order to elucidate the field-induced enhancement of the exciplex fluorescence more clearly, though the present field-induced enhancement is attributed to the fieldinduced increase of the fluorescent exciplex as a result of the increase of the electron transfer rate at the moment. As is shown in Fig. 4, the electrofluorescence spectrum of the exciplex at 10 mol% is simulated by
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
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Fig. 4. Simulation of the electrofluorescence spectrum of Py-m-DMA at 10 mol%. The figure shows the electrofluorescence spectrum observed with a field strength of 0.5 MV cmy1 Ždotted line., fluorescence spectrum Žbroken line., the first derivative spectrum of the fluorescence spectrum Žthin solid line. of Py-m-DMA and the spectrum simulated by a linear combination of the fluorescence spectrum with its first derivative spectrum Žthick solid line.. The maximum fluorescence intensity is normalized to unity and arbitrary units were used for the first derivative spectrum.
a linear combination of the fluorescence spectrum with its first-derivative spectrum. As already mentioned, the former is attributed to the field-induced change in F F . On the other hand, the latter comes from the so-called Stark shift, as mentioned below. The energy level shift induced by F depends on the electric dipole moment and molecular polarizability. As a result, the emission spectrum as well as the absorption spectrum are expected to be changed by F. An expression for such a field-induced change in absorption intensity as well as in emission intensity was derived by Liptay and co-workers w18,19x. By assuming that the original isotropic distribution in rigid matrices such as PMMA polymer films is maintained even in the presence of F, the spectral change in the fluorescence intensity given in units of wavenumber, n i.e. D I F Ž n . in the presence of F may be given by the following equation w18–20x: D I F Ž n . s Ž fF .
2
qCn
½
ž
d 2 Ž I F Ž n . rn dn 2
d Ž I F Ž n . rn .
ž . /5
AIF Ž n . q Bn
dn
/ Ž 1.
where f is the internal field factor, A depends on the change in the fluorescence quantum yield, and B and C are given as follows: Bs
D ar2 q Ž D a m y D a . Ž 3cos 2 x y 1 . r10
Cs Ž Dm.
hc 2
ž
5 q Ž 3cos 2 j y 1 .Ž 3cos 2 x y 1 . 30 h 2 c 2
/
Ž 2. Ž 3.
where h is Planck’s constant and c is the speed of light. Here, D m is the difference in electric dipole moment between the ground and excited state, i.e. D m s mg y me and D a is related to the difference in the polarizability tensor D a s a g y a e : D m s < D m < ; D a s 13 Tr Ž D a .
Ž 4.
D a m denotes the diagonal component of D a with respect to the direction of the transition dipole moment, x is the angle between the direction of F and the electric vector of the excitation light and j is the angle between the direction of D m and the transition dipole moment.
N. Ohta et al.r Chemical Physics Letters 292 (1998) 535–541
As is shown in Figs. 1 and 4, the peak of the electrofluorescence spectrum is shifted relative to the fluorescence spectrum and the first derivative of the fluorescence spectrum is necessary for simulation of the electrofluorescence spectrum. This result indicates that the Stark shift comes from a change in molecular polarizability. By evaluating the first-derivative part in the electrofluorescence spectrum and comparing with Eq. Ž1., D a is determined to be 190 ˚ 3 at 10 mol%. Here, it is assumed in units of 4 p´ 0 A that the internal field is the same as the applied field i.e. f s 1 in Eq. Ž1. and that the molecular polarizability is isotropic, i.e. D a m s D a . The monomer fluorescence of PY is also considered to show the Stark shift induced by the change in molecular polarizability between the S 1 state and the ground state of PY. In fact, the electrofluorescence spectrum at 0.01 mol%, where the field-induced change in F F is very small, is not identical with the fluorescence spectrum in shape, as is shown in Fig. 1, implying that the first derivative of the fluorescence spectrum is necessary to simulate the observed electrofluorescence spectrum. Da of PY is regarded ˚ 3 with f s 1 being as small as 15 in units of 4 p´ A in Eq. Ž1., as reported for pyrene doped in a PMMA film w7x. It is noted that both the exciplex fluorescence and monomer fluorescence show a red-shift in the presence of F, indicating that the molecular polarizability in the emitting state is larger than the ground state in both cases. The field-induced enhancement at the initial step of the photoinduced electron transfer may be attributed to the field-induced increase of the transfer integral andror the field-induced change in the free energy change of the electron transfer process. Fluorescence decay measurements, both in the presence and absence of the external electric field, are now in progress to discuss the field effect more quantitatively and to specify the mechanism of the field effect on the electron transfer. Photocurrent measurements are also in progress to confirm the relationship between the field-induced quenching of the exciplex fluorescence and the photocarrier generation.
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Acknowledgements This work was partly supported by a Grant-in-Aid for the Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture.
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