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Efficient fatigue of excimer fluorescence by an electric field and generation of electroluminescence of 1,3-bis-(1-pyrenyl)propane in a PMMA polymer film
10 September 1999
Chemical Physics Letters 310 Ž1999. 397–404 www.elsevier.nlrlocatercplett
Efficient fatigue of excimer fluorescence by an electric field and generation of electroluminescence of 1,3-bis-ž11-pyrenyl /propane in a PMMA polymer film Nobuhiro Ohta
a,)
, Hiroshi Kawabata
a,b
, Shiro Umeuchi b, Iwao Yamazaki
b
a
b
Research Institute for Electronic Science, Hokkaido UniÕersity, Sapporo 060-0812, Japan Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060-8628, Japan Received 10 May 1999; in final form 21 June 1999
Abstract Excimer fluorescence of 1,3-bis-Ž1-pyrenyl.propane ŽPy-Py. doped in a PMMA polymer film with a peak at ; 470 nm is quenched by an external electric field extremely well. It is suggested that a field-assisted charge separation of Py-Py followed by efficient charge transports occurs only in the excimer with a sandwich-type configuration. The electroluminescence spectrum which is similar to the excimer fluorescence is generated at high fields without photoirradiation, probably by a radiative recombination of the hole–electron pair through a short methylene chain. A good correlation is found between the efficient fatigue of the excimer fluorescence and the generation of the electroluminescence. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In polymethylene-linked systems of electron donors and acceptors, magnetic field effects on exciplex fluorescence depend markedly on the methylene chain length in solution w1–3x. The chain length dependence results from an increasing influence of the exchange interaction with decreasing the chain length of the radical ion pair produced by the photoinduced electron transfer. The chain length dependence was also found for the electric field effects on
) Corresponding author. Fax: q81-11-706-4970; e-mail: [email protected]
fluorescence of methylene-linked carbazole and terephthalic acid methyl ester doped in a PMMA polymer film w4x. The observed dependence was interpreted in terms of the chain length dependence of charge recombination through a methylene chain. Thus, photoexcitation dynamics of chain molecules, where two reactants are attached to chain ends w5x, is very different in certain cases from that in a freely diffusing unlinked system not only in solution but also in solid state. In the present study, electric field effects on fluorescence of a methylene-linked compound where two pyrene chromophores are attached to the chain ends have been examined in a polymer film. As mentioned in our previous paper w6x, the fluorescence of pyrene doped in a PMMA polymer film is markedly
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 1 5 - 5
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affected by an electric field, depending on the dopant concentration. The various components of the fluorescence of pyrene show a different field dependence of quantum yield from each other. At high dopant concentrations where excimer fluorescence is observed, for example, excimer fluorescence with a peak at ; 415 nm shows the field-induced enhancement, whereas both monomer fluorescence emitted from the locally excited state and excimer fluorescence with a peak at ; 470 nm are quenched by an electric field. By comparing the field dependence of the fluorescence of methylene-linked pyrene chromophores with that of bare molecules of pyrene, the role the methylene chain plays in the excitation dynamics and its electric field effect is discussed. A crucial difference of the field effects on excimer fluorescence between pyrene and methylene-linked compound seems to give an important guide for the design of new organic materials which show efficient electroluminescence.
tion intensity or emission intensity was detected with a lock-in amplifier ŽSR830, SRS. at the second harmonic of the modulation frequency. The applied field strength was evaluated from the applied voltage divided by the film thickness. Hereafter, the strength of the externally applied field is represented in rms value. In our previous papers w4,6,7x, plots of the change in absorption intensity and in fluorescence intensity as a function of wavelength were denoted by electroabsorption spectrum and electrofluorescence spectrum, respectively. In the present study, two kinds of field-induced change in emission intensity have been observed. One is the change in intensity of emission induced by photoexcitation, and the other is the field-induced change in intensity of emission observed without photoirradiation. In the present Letter, ‘electrofluorescence’ ŽE-F. spectrum is used for the former change, while ‘electroluminescence’ ŽEL. spectrum is used for the latter.
3. Results and discussion 2. Experimental 1,3-bis-Ž1-pyrenyl.propane ŽMolecular Probe., hereafter denoted by Py-Py, was used without further purification. A PMMA polymer film which contains Py-Py was cast on the ITO-coated quartz substrate by a spin coating method. Then, a semitransparent aluminum ŽAl. film was deposited on the polymer film. ITO and Al films were used as electrodes. The thickness of the polymer film was determined with a nanospecrAFT system ŽM3000, nanometrics.. The thickness was typically 0.3 mm. A concentration of Py-Py relative to the monomer unit of PMMA up to 10 mole% was employed in the present study. All the optical spectra were measured at room temperature under vacuum conditions. The electricfield-induced change in absorption and emission spectra were measured using electric field modulation spectroscopy with the same apparatus as mentioned elsewhere w4,7x. A modulation in absorption intensity or emission intensity was induced by a sinusoidal ac voltage with a modulation frequency of 40 Hz. By using a function generator ŽSG-4311, Iwatsu. combined with an amplifier, ac voltages up to 100 V were applied. Hereafter, the applied field is denoted by F. The field-induced change in absorp-
Fig. 1 shows fluorescence spectra and E-F spectra of Py-Py doped in a PMMA polymer film at various concentrations. These spectra were obtained with the excitation wavelength where the field-induced change in absorption intensity is negligibly small. At a low concentration of 0.1 mole%, sharp structured fluorescence emitted from the locally excited state of pyrene chromophore is observed Žsee Fig. 1.. Hereafter, this emission component is denoted by monomer fluorescence. The monomer fluorescence of Py-Py shows a very strong 0–0 band in comparison with pyrene because of the breakdown of the molecular symmetry w8,9x. The observed E-F spectrum of the monomer at 0.1 mole% is given by a linear combination of the fluorescence spectrum with its first derivative spectrum, suggesting that electric fields induce a slight change in the intramolecular dynamics of Py-Py in addition to the Stark shift induced by a change in molecular polarizability between the emitting state and the ground state. Note that pyrene doped in a PMMA polymer film at a concentration of 1 mole% shows only the Stark shift w7x. It is worth mentioning that the E-F spectra which give the same shape as the fluorescence spectrum show the field-induced change in fluorescence quantum yield w4x.
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Fig. 1. Fluorescence spectra Ždotted line. and E-F spectra Žsolid line. of Py-Py doped in a PMMA polymer film at 0.1, 1.0, 2.0 and 5.0 mole% Žfrom top to bottom.. The applied field strength is shown in the figure at each concentration. The excitation wavelength was 319.5 nm at 0.1 mole%, 321 nm at 1.0 mole%, and 322 nm at 2.0 and 5.0 mole%. The maximum fluorescence intensity is normalized to unity in every case.
At moderately high concentrations, broad excimer fluorescence with a peak at ; 470 nm appears. The excimer fluorescence relative to the monomer fluorescence increases monotonically with increasing concentration, indicating that the excimer observed in the absence of the electric field is mainly intermolecular in nature when Py-Py molecules are doped in a polymer film. The intensity ratio of the peak of the intramolecular excimer relative to the monomer peak is considered to be sensitive to microviscosity
w10,11x. At high concentrations, the E-F spectrum of monomer of Py-Py gives a shape that is quite similar to the monomer fluorescence Žsee Fig. 1., indicating that the quantum yield of the monomer fluorescence markedly decreases in the presence of F. Excimer fluorescence with a peak at ; 470 nm is also quenched by F, whereas the excimer fluorescence with a peak at ; 415 nm is enhanced by F. These results agree well with those of pyrene w6x; the quantum yield both of the monomer fluorescence
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and of the first-excimer fluorescence of pyrene is decreased by F, whereas the yield of the second-excimer fluorescence is enhanced by F. Here, the first excimer and the second excimer are considered to give a broad emission with a peak at ; 470 and ; 415 nm, respectively. The first excimer is regarded as having a sandwich-type configuration w12,13x, while the second excimer is regarded as having a partially overlapping type of configuration w14x. Even at a high concentration of 10 mole%, similar field effects were observed on each fluorescence of Py-Py; monomer fluorescence and the first-excimer fluorescence are quenched by F, whereas the second-excimer fluorescence is enhanced by F. These results show that the formation yields both of the
first excimer and of the second excimer are changed by F in different manners, as in the case of pyrene. In contrast with pyrene, however, Py-Py shows a remarkable field strength dependence of E-F spectra, as will be mentioned below. In pyrene, the magnitude of D I FrIF is proportional to the square of the applied field strength at any monitoring wavelength of fluorescence in every concentration below 20 mole%, even when the field strength was increased up to more than 1 MVrcm. Note that I F and D I F show the fluorescence intensity and its field-induced change, respectively. The applied field strength dependence of D I F of Py-Py is conclusively different from that of pyrene. Plots of D I FrIF of Py-Py observed at 10 mole% as a function of the fourth powers of the applied field strength
Fig. 2. Plots of D I F rIF of Py-Py doped in a PMMA polymer film at 10 mole% with the monitoring wavelengths of 377 nm Žclosed squares., 415 nm Žclosed circles. and 470 nm Žopen circles., as a function of the fourth power of the applied field strength. Photoexcitation was made at 322 nm.
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are shown in Fig. 2. The monitoring wavelengths of 377, 415 and 470 nm correspond to the peaks of the monomer fluorescence, the second-excimer fluorescence and the first-excimer fluorescence, respectively. D I FrIF at 470 nm is nearly proportional to the fourth power of the applied field strength. At 377 nm, D I FrIF is nearly proportional to the square of the applied field strength when the field strength is weak, but D I FrIF is nearly proportional to the fourth power of the applied field strength in the high-field region. At 415 nm, D I FrIF increases with increasing
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field strength in the low-field region, but the magnitude of D I FrIF decreases and the field-induced quenching occurs with further increasing the field strength. The E-F spectrum given in Fig. 3 clearly shows that the field strength dependence of D I FrIF depends on the emission wavelength. Note that the E-F spectra of pyrene observed at 10 or 20 mole% are essentially independent of the applied field strength in contrast with Py-Py. As the field strength is increased, the E-F spectrum becomes closer in shape to the fluorescence spectrum, indicating that
Fig. 3. E-F spectra of Py-Py doped in a PMMA polymer film at 10 mole% with various applied field strengths. The maximum fluorescence intensity is normalized to unity in every case, and the applied field strengths are shown in the figure. Dotted line shows the fluorescence spectrum.
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only the first-excimer fluorescence of Py-Py is quenched by F very efficiently and that both the monomer fluorescence and the second-excimer fluorescence are not affected by F so efficiently as the first-excimer fluorescence. The fact that the magnitude of D I FrIF at 377 nm is nearly proportional to the square of the applied field strength in the lowfield region implies that D I FrIF of the monomer is proportional to the square of the applied field strength. The second-excimer fluorescence also seems to show the field dependence where D I FrIF is proportional to the square of the applied field strength. The fourth-power dependence of D I FrIF at 377 and 415 nm in the high-field region probably comes from the superposition of the first-excimer fluorescence, whose D I FrIF becomes dominant at high fields. Efficient fatigue of the first-excimer fluorescence of Py-Py by F, i.e., roughly the fourth-power dependence of the quenching by F, was observed not only at 10 mole% but also at lower concentrations, e.g., 2 or 5 mole%. The efficient fatigue of the first-excimer fluorescence of Py-Py by F may be attributed to the field-assisted generation of a hole–electron pair followed by efficient charge transport among Py-Py molecules. Note that similar phenomena of photoluminescence fatigue is induced by electrochemical doping in a conducting polymer w15x. It is unlikely that charge separations occur from the monomer or the second excimer since D I FrIF of these components is regarded as proportional to the square of the applied field strength, in contrast with the first excimer. In any sample of Py-Py doped in a PMMA polymer film at 10 mole%, the electroluminescence was observed without photoirradiation, when the field strength was increased up to 0.8 MVrcm. The EL intensity increases with increasing applied field strength. The EL spectrum observed with an applied voltage of 30 V, where the field strength was 1 MVrcm, is shown in Fig. 4, together with the normal fluorescence spectrum observed at zero field. In contrast with the normal fluorescence spectrum, the monomer fluorescence is not observed in the EL spectrum Žsee Fig. 4., and the EL spectrum of Py-Py corresponds to the first-excimer fluorescence. As mentioned above, only the first-excimer fluorescence shows an efficient fatigue in the presence of F.
Fig. 4. EL spectrum Žsolid line. and fluorescence spectrum Ždotted line. of Py-Py doped in a PMMA polymer film at 10 mole%.
Thus, the first excimer shows a good correlation between the efficient fatigue of fluorescence and the efficient generation of EL. This correlation may be applicable in other materials for examining the efficiency of the EL generation, based on the measurements of the E-F spectra. The EL appears only when ITO is positively charged and Al is negatively charged, suggesting that the EL is attributed to a recombination of the pairs of hole and electron which are injected from ITO and Al, respectively. It is worth mentioning that the detection of EL was very difficult at concentrations of Py-Py below 5 mole%, indicating that the generation efficiency of EL depends on the dopant concentration. In single crystals of pyrene, the EL spectrum was reported to be different from the fluorescence spectrum w16x. In Py-Py, however, the EL spectrum is similar in shape to the fluorescence spectrum of the first excimer, as mentioned above. It is noted that the total bandwidth of the EL spectrum is larger than the normal fluorescence spectrum Žsee Fig. 4., which is not unusual in organic materials w17x. A crucial difference between Py-Py and pyrene suggests that the methylene chain plays a significant role in the efficient generation of EL as well as in the efficient fatigue of fluorescence in the presence of F. At a high concentration of 10 mole% of Py-Py, hopping migrations of holes andror electrons injected from ITO and Al, respectively, seem to produce intramolecular hole–electron pairs. Once a
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charge carrier Že.g., an electron. localizes on a molecule Ži.e., it is trapped., a charge of opposite polarity Ža hole. injected from the counter electrode may become similarly localized on that same molecule, i.e., at the counterpart of the pyrene chromophores of Py-Py. The produced intramolecular electron–hole pair may eventually recombine, leading to the emissive first excimer, when the pair has enough energy to form the excimer. The efficiency of the hole–electron recombination seems to be significantly enhanced by a linkage of two pyrene chromophores with a short methylene chain because the intramolecular hole–electron pairs are induced by the linkage. A generation of efficient EL was shown in single crystals of anthracene w18,19x. The efficiency of the EL generation in pyrene crystals was reported to be more than two orders of magnitudes lower than anthracene w16x. The present results imply that the efficiency of the EL generation is enhanced by a linkage of pyrene chromophores with a short methylene chain. The intensity of EL depends on the injection of both holes and electrons from the electrodes, carrier mobility, hole–electron recombination and radiative efficiency w20x. The barrier height for the injection of a hole or electron seems to be the same both for pyrene and for Py-Py, as far as these compounds are doped in a PMMA film, and the current generated in the present organic films seem to be charge-space limited, implying that the current depends on the carrier mobility. As mentioned above, the efficient fatigue of the first-excimer fluorescence of Py-Py by F is attributed the efficient charge transport in the film. Similarly, the efficient generation of EL in Py-Py may be attributed to an efficient carrier mobility; the mobility is much more efficient for Py-Py than for pyrene in PMMA polymer films. Thus, the carrier mobility is considered to be remarkably enhanced by a linkage with a short methylene chain between pyrene chromophores. As mentioned above, the efficiency of the charge recombination seems to be also enhanced by a linkage with a short methylene chain, as a result of the presence of the intramolecular hole–electron pairs suitable for recombination. Thus, a linkage of chromophores with a short methylene chain seems to induce the enhancement both of the carrier mobility and of the efficiency of the charge recombination.
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The monomer fluorescence emitted from the locally excited state is negligible in the EL spectra of Py-Py Žsee Fig. 4., in contrast with the normal fluorescence spectra. If there is no relation at all between the recombination site and the emission site, the generated EL seems to give the same spectrum as the fluorescence spectrum following the optical transition. The presence of the difference between the normal fluorescence spectrum and the EL spectrum suggests that the hole–electron recombination site is strongly related to an EL emission site, i.e., the EL emission site is considerably limited.
Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture of Japan ŽNo. 10440163..
References w1x H. Staerk, W. Kuhnle, R. Treichel, A. Weller, Chem. Phys. ¨ Lett. 118 Ž1985. 19. w2x Y. Tanimoto, N. Okada, M. Itoh, K. Iwai, K. Sugioka, F. Takemura, R. Nakagaki, S. Nagakura, Chem. Phys. Lett. 136 Ž1987. 42. w3x Y. Tanimoto, Y. Fujiwara, R. Nakagaki, in: S. Nagakura, H. Hayashi, T. Azumi ŽEds.., Dynamic Spin Chemistry, Kodansha, Tokyo, 1998, Chap. 3. w4x N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, Y. Tanimoto, Y. Hatano, M. Yamamoto, H. Kono, J. Phys. Chem. 100 Ž1996. 19295. w5x T. Kanaya, Ph.D. Thesis, Kyoto University, Kyoto, 1981. w6x N. Ohta, S. Umeuchi, T. Kanada, Y. Nishimura, I. Yamazaki, Chem. Phys. Lett. 279 Ž1997. 215. w7x S. Umeuchi, Y. Nishimura, I. Yamazaki, H. Murakami, M. Yamashita, N. Ohta, Thin Solid Films 311 Ž1997. 239. w8x A. Nakajima, Bull. Chem. Soc. Jpn. 44 Ž1971. 3272. w9x J.H. Krech, S.L. Rose-Pehrsson, Appl. Spectrosc. 50 Ž1996. 1528. w10x K.A. Zachariasse, W. Kuhnle, Z. Phys. Chem., N.F. 101 ¨ Ž1976. 267. w11x K.A. Zachariasse, Chem. Phys. Lett. 57 Ž1978. 429. w12x E.A. Chandross, C.J. Demster, J. Am. Chem. Soc. 92 Ž1970. 3586. w13x Y.C. Wang, H. Morawetz, J. Am. Chem. Soc. 98 Ž1975. 3611. w14x I. Yamazaki, N. Tamai, T. Yamazaki, J. Phys. Chem. 91 Ž1987. 3572.
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w15x S. Hayashi, K. Kaneto, K. Yoshino, Solid State Commun. 61 Ž1987. 249. w16x J.G. Basurto, Z. Burshtein, Mol. Cryst. Liq. Cryst. 31 Ž1975. 211. w17x C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 56 Ž1990. 799.
w18x W. Helfrich, W.G. Schneider, Phys. Rev. Lett. 14 Ž1965. 229. w19x W. Helfrich, W.G. Schneider, J. Chem. Phys. 44 Ž1966. 2902. w20x D.V. Khramtchenkov, H. Bassler, V.I. Arkhipov, J. Appl. ¨ Phys. 79 Ž1996. 9283.
Chemical Physics Letters 310 Ž1999. 397–404 www.elsevier.nlrlocatercplett
Efficient fatigue of excimer fluorescence by an electric field and generation of electroluminescence of 1,3-bis-ž11-pyrenyl /propane in a PMMA polymer film Nobuhiro Ohta
a,)
, Hiroshi Kawabata
a,b
, Shiro Umeuchi b, Iwao Yamazaki
b
a
b
Research Institute for Electronic Science, Hokkaido UniÕersity, Sapporo 060-0812, Japan Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060-8628, Japan Received 10 May 1999; in final form 21 June 1999
Abstract Excimer fluorescence of 1,3-bis-Ž1-pyrenyl.propane ŽPy-Py. doped in a PMMA polymer film with a peak at ; 470 nm is quenched by an external electric field extremely well. It is suggested that a field-assisted charge separation of Py-Py followed by efficient charge transports occurs only in the excimer with a sandwich-type configuration. The electroluminescence spectrum which is similar to the excimer fluorescence is generated at high fields without photoirradiation, probably by a radiative recombination of the hole–electron pair through a short methylene chain. A good correlation is found between the efficient fatigue of the excimer fluorescence and the generation of the electroluminescence. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In polymethylene-linked systems of electron donors and acceptors, magnetic field effects on exciplex fluorescence depend markedly on the methylene chain length in solution w1–3x. The chain length dependence results from an increasing influence of the exchange interaction with decreasing the chain length of the radical ion pair produced by the photoinduced electron transfer. The chain length dependence was also found for the electric field effects on
) Corresponding author. Fax: q81-11-706-4970; e-mail: [email protected]
fluorescence of methylene-linked carbazole and terephthalic acid methyl ester doped in a PMMA polymer film w4x. The observed dependence was interpreted in terms of the chain length dependence of charge recombination through a methylene chain. Thus, photoexcitation dynamics of chain molecules, where two reactants are attached to chain ends w5x, is very different in certain cases from that in a freely diffusing unlinked system not only in solution but also in solid state. In the present study, electric field effects on fluorescence of a methylene-linked compound where two pyrene chromophores are attached to the chain ends have been examined in a polymer film. As mentioned in our previous paper w6x, the fluorescence of pyrene doped in a PMMA polymer film is markedly
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 1 5 - 5
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affected by an electric field, depending on the dopant concentration. The various components of the fluorescence of pyrene show a different field dependence of quantum yield from each other. At high dopant concentrations where excimer fluorescence is observed, for example, excimer fluorescence with a peak at ; 415 nm shows the field-induced enhancement, whereas both monomer fluorescence emitted from the locally excited state and excimer fluorescence with a peak at ; 470 nm are quenched by an electric field. By comparing the field dependence of the fluorescence of methylene-linked pyrene chromophores with that of bare molecules of pyrene, the role the methylene chain plays in the excitation dynamics and its electric field effect is discussed. A crucial difference of the field effects on excimer fluorescence between pyrene and methylene-linked compound seems to give an important guide for the design of new organic materials which show efficient electroluminescence.
tion intensity or emission intensity was detected with a lock-in amplifier ŽSR830, SRS. at the second harmonic of the modulation frequency. The applied field strength was evaluated from the applied voltage divided by the film thickness. Hereafter, the strength of the externally applied field is represented in rms value. In our previous papers w4,6,7x, plots of the change in absorption intensity and in fluorescence intensity as a function of wavelength were denoted by electroabsorption spectrum and electrofluorescence spectrum, respectively. In the present study, two kinds of field-induced change in emission intensity have been observed. One is the change in intensity of emission induced by photoexcitation, and the other is the field-induced change in intensity of emission observed without photoirradiation. In the present Letter, ‘electrofluorescence’ ŽE-F. spectrum is used for the former change, while ‘electroluminescence’ ŽEL. spectrum is used for the latter.
3. Results and discussion 2. Experimental 1,3-bis-Ž1-pyrenyl.propane ŽMolecular Probe., hereafter denoted by Py-Py, was used without further purification. A PMMA polymer film which contains Py-Py was cast on the ITO-coated quartz substrate by a spin coating method. Then, a semitransparent aluminum ŽAl. film was deposited on the polymer film. ITO and Al films were used as electrodes. The thickness of the polymer film was determined with a nanospecrAFT system ŽM3000, nanometrics.. The thickness was typically 0.3 mm. A concentration of Py-Py relative to the monomer unit of PMMA up to 10 mole% was employed in the present study. All the optical spectra were measured at room temperature under vacuum conditions. The electricfield-induced change in absorption and emission spectra were measured using electric field modulation spectroscopy with the same apparatus as mentioned elsewhere w4,7x. A modulation in absorption intensity or emission intensity was induced by a sinusoidal ac voltage with a modulation frequency of 40 Hz. By using a function generator ŽSG-4311, Iwatsu. combined with an amplifier, ac voltages up to 100 V were applied. Hereafter, the applied field is denoted by F. The field-induced change in absorp-
Fig. 1 shows fluorescence spectra and E-F spectra of Py-Py doped in a PMMA polymer film at various concentrations. These spectra were obtained with the excitation wavelength where the field-induced change in absorption intensity is negligibly small. At a low concentration of 0.1 mole%, sharp structured fluorescence emitted from the locally excited state of pyrene chromophore is observed Žsee Fig. 1.. Hereafter, this emission component is denoted by monomer fluorescence. The monomer fluorescence of Py-Py shows a very strong 0–0 band in comparison with pyrene because of the breakdown of the molecular symmetry w8,9x. The observed E-F spectrum of the monomer at 0.1 mole% is given by a linear combination of the fluorescence spectrum with its first derivative spectrum, suggesting that electric fields induce a slight change in the intramolecular dynamics of Py-Py in addition to the Stark shift induced by a change in molecular polarizability between the emitting state and the ground state. Note that pyrene doped in a PMMA polymer film at a concentration of 1 mole% shows only the Stark shift w7x. It is worth mentioning that the E-F spectra which give the same shape as the fluorescence spectrum show the field-induced change in fluorescence quantum yield w4x.
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Fig. 1. Fluorescence spectra Ždotted line. and E-F spectra Žsolid line. of Py-Py doped in a PMMA polymer film at 0.1, 1.0, 2.0 and 5.0 mole% Žfrom top to bottom.. The applied field strength is shown in the figure at each concentration. The excitation wavelength was 319.5 nm at 0.1 mole%, 321 nm at 1.0 mole%, and 322 nm at 2.0 and 5.0 mole%. The maximum fluorescence intensity is normalized to unity in every case.
At moderately high concentrations, broad excimer fluorescence with a peak at ; 470 nm appears. The excimer fluorescence relative to the monomer fluorescence increases monotonically with increasing concentration, indicating that the excimer observed in the absence of the electric field is mainly intermolecular in nature when Py-Py molecules are doped in a polymer film. The intensity ratio of the peak of the intramolecular excimer relative to the monomer peak is considered to be sensitive to microviscosity
w10,11x. At high concentrations, the E-F spectrum of monomer of Py-Py gives a shape that is quite similar to the monomer fluorescence Žsee Fig. 1., indicating that the quantum yield of the monomer fluorescence markedly decreases in the presence of F. Excimer fluorescence with a peak at ; 470 nm is also quenched by F, whereas the excimer fluorescence with a peak at ; 415 nm is enhanced by F. These results agree well with those of pyrene w6x; the quantum yield both of the monomer fluorescence
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and of the first-excimer fluorescence of pyrene is decreased by F, whereas the yield of the second-excimer fluorescence is enhanced by F. Here, the first excimer and the second excimer are considered to give a broad emission with a peak at ; 470 and ; 415 nm, respectively. The first excimer is regarded as having a sandwich-type configuration w12,13x, while the second excimer is regarded as having a partially overlapping type of configuration w14x. Even at a high concentration of 10 mole%, similar field effects were observed on each fluorescence of Py-Py; monomer fluorescence and the first-excimer fluorescence are quenched by F, whereas the second-excimer fluorescence is enhanced by F. These results show that the formation yields both of the
first excimer and of the second excimer are changed by F in different manners, as in the case of pyrene. In contrast with pyrene, however, Py-Py shows a remarkable field strength dependence of E-F spectra, as will be mentioned below. In pyrene, the magnitude of D I FrIF is proportional to the square of the applied field strength at any monitoring wavelength of fluorescence in every concentration below 20 mole%, even when the field strength was increased up to more than 1 MVrcm. Note that I F and D I F show the fluorescence intensity and its field-induced change, respectively. The applied field strength dependence of D I F of Py-Py is conclusively different from that of pyrene. Plots of D I FrIF of Py-Py observed at 10 mole% as a function of the fourth powers of the applied field strength
Fig. 2. Plots of D I F rIF of Py-Py doped in a PMMA polymer film at 10 mole% with the monitoring wavelengths of 377 nm Žclosed squares., 415 nm Žclosed circles. and 470 nm Žopen circles., as a function of the fourth power of the applied field strength. Photoexcitation was made at 322 nm.
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are shown in Fig. 2. The monitoring wavelengths of 377, 415 and 470 nm correspond to the peaks of the monomer fluorescence, the second-excimer fluorescence and the first-excimer fluorescence, respectively. D I FrIF at 470 nm is nearly proportional to the fourth power of the applied field strength. At 377 nm, D I FrIF is nearly proportional to the square of the applied field strength when the field strength is weak, but D I FrIF is nearly proportional to the fourth power of the applied field strength in the high-field region. At 415 nm, D I FrIF increases with increasing
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field strength in the low-field region, but the magnitude of D I FrIF decreases and the field-induced quenching occurs with further increasing the field strength. The E-F spectrum given in Fig. 3 clearly shows that the field strength dependence of D I FrIF depends on the emission wavelength. Note that the E-F spectra of pyrene observed at 10 or 20 mole% are essentially independent of the applied field strength in contrast with Py-Py. As the field strength is increased, the E-F spectrum becomes closer in shape to the fluorescence spectrum, indicating that
Fig. 3. E-F spectra of Py-Py doped in a PMMA polymer film at 10 mole% with various applied field strengths. The maximum fluorescence intensity is normalized to unity in every case, and the applied field strengths are shown in the figure. Dotted line shows the fluorescence spectrum.
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only the first-excimer fluorescence of Py-Py is quenched by F very efficiently and that both the monomer fluorescence and the second-excimer fluorescence are not affected by F so efficiently as the first-excimer fluorescence. The fact that the magnitude of D I FrIF at 377 nm is nearly proportional to the square of the applied field strength in the lowfield region implies that D I FrIF of the monomer is proportional to the square of the applied field strength. The second-excimer fluorescence also seems to show the field dependence where D I FrIF is proportional to the square of the applied field strength. The fourth-power dependence of D I FrIF at 377 and 415 nm in the high-field region probably comes from the superposition of the first-excimer fluorescence, whose D I FrIF becomes dominant at high fields. Efficient fatigue of the first-excimer fluorescence of Py-Py by F, i.e., roughly the fourth-power dependence of the quenching by F, was observed not only at 10 mole% but also at lower concentrations, e.g., 2 or 5 mole%. The efficient fatigue of the first-excimer fluorescence of Py-Py by F may be attributed to the field-assisted generation of a hole–electron pair followed by efficient charge transport among Py-Py molecules. Note that similar phenomena of photoluminescence fatigue is induced by electrochemical doping in a conducting polymer w15x. It is unlikely that charge separations occur from the monomer or the second excimer since D I FrIF of these components is regarded as proportional to the square of the applied field strength, in contrast with the first excimer. In any sample of Py-Py doped in a PMMA polymer film at 10 mole%, the electroluminescence was observed without photoirradiation, when the field strength was increased up to 0.8 MVrcm. The EL intensity increases with increasing applied field strength. The EL spectrum observed with an applied voltage of 30 V, where the field strength was 1 MVrcm, is shown in Fig. 4, together with the normal fluorescence spectrum observed at zero field. In contrast with the normal fluorescence spectrum, the monomer fluorescence is not observed in the EL spectrum Žsee Fig. 4., and the EL spectrum of Py-Py corresponds to the first-excimer fluorescence. As mentioned above, only the first-excimer fluorescence shows an efficient fatigue in the presence of F.
Fig. 4. EL spectrum Žsolid line. and fluorescence spectrum Ždotted line. of Py-Py doped in a PMMA polymer film at 10 mole%.
Thus, the first excimer shows a good correlation between the efficient fatigue of fluorescence and the efficient generation of EL. This correlation may be applicable in other materials for examining the efficiency of the EL generation, based on the measurements of the E-F spectra. The EL appears only when ITO is positively charged and Al is negatively charged, suggesting that the EL is attributed to a recombination of the pairs of hole and electron which are injected from ITO and Al, respectively. It is worth mentioning that the detection of EL was very difficult at concentrations of Py-Py below 5 mole%, indicating that the generation efficiency of EL depends on the dopant concentration. In single crystals of pyrene, the EL spectrum was reported to be different from the fluorescence spectrum w16x. In Py-Py, however, the EL spectrum is similar in shape to the fluorescence spectrum of the first excimer, as mentioned above. It is noted that the total bandwidth of the EL spectrum is larger than the normal fluorescence spectrum Žsee Fig. 4., which is not unusual in organic materials w17x. A crucial difference between Py-Py and pyrene suggests that the methylene chain plays a significant role in the efficient generation of EL as well as in the efficient fatigue of fluorescence in the presence of F. At a high concentration of 10 mole% of Py-Py, hopping migrations of holes andror electrons injected from ITO and Al, respectively, seem to produce intramolecular hole–electron pairs. Once a
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charge carrier Že.g., an electron. localizes on a molecule Ži.e., it is trapped., a charge of opposite polarity Ža hole. injected from the counter electrode may become similarly localized on that same molecule, i.e., at the counterpart of the pyrene chromophores of Py-Py. The produced intramolecular electron–hole pair may eventually recombine, leading to the emissive first excimer, when the pair has enough energy to form the excimer. The efficiency of the hole–electron recombination seems to be significantly enhanced by a linkage of two pyrene chromophores with a short methylene chain because the intramolecular hole–electron pairs are induced by the linkage. A generation of efficient EL was shown in single crystals of anthracene w18,19x. The efficiency of the EL generation in pyrene crystals was reported to be more than two orders of magnitudes lower than anthracene w16x. The present results imply that the efficiency of the EL generation is enhanced by a linkage of pyrene chromophores with a short methylene chain. The intensity of EL depends on the injection of both holes and electrons from the electrodes, carrier mobility, hole–electron recombination and radiative efficiency w20x. The barrier height for the injection of a hole or electron seems to be the same both for pyrene and for Py-Py, as far as these compounds are doped in a PMMA film, and the current generated in the present organic films seem to be charge-space limited, implying that the current depends on the carrier mobility. As mentioned above, the efficient fatigue of the first-excimer fluorescence of Py-Py by F is attributed the efficient charge transport in the film. Similarly, the efficient generation of EL in Py-Py may be attributed to an efficient carrier mobility; the mobility is much more efficient for Py-Py than for pyrene in PMMA polymer films. Thus, the carrier mobility is considered to be remarkably enhanced by a linkage with a short methylene chain between pyrene chromophores. As mentioned above, the efficiency of the charge recombination seems to be also enhanced by a linkage with a short methylene chain, as a result of the presence of the intramolecular hole–electron pairs suitable for recombination. Thus, a linkage of chromophores with a short methylene chain seems to induce the enhancement both of the carrier mobility and of the efficiency of the charge recombination.
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The monomer fluorescence emitted from the locally excited state is negligible in the EL spectra of Py-Py Žsee Fig. 4., in contrast with the normal fluorescence spectra. If there is no relation at all between the recombination site and the emission site, the generated EL seems to give the same spectrum as the fluorescence spectrum following the optical transition. The presence of the difference between the normal fluorescence spectrum and the EL spectrum suggests that the hole–electron recombination site is strongly related to an EL emission site, i.e., the EL emission site is considerably limited.
Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture of Japan ŽNo. 10440163..
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