External Electric Field Effects on Absorption and Fluorescence Spectra

maximum at 460 nm (∼21 730 cm-1). There is no doubt that the absorption spectrum at 20 mol % is mainly due to an aggregate or molecular complex of O...
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J. Phys. Chem. B 1997, 101, 10213-10220

10213

External Electric Field Effects on Absorption and Fluorescence Spectra of Monomer and Dimer of Oxacarbocyanine in Langmuir-Blodgett Films Nobuhiro Ohta,* Takashi Ito, Shigetoshi Okazaki,† and Iwao Yamazaki Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: April 16, 1997; In Final Form: September 11, 1997X

The dimer of oxacarbocyanine (OCC) formed in mixed Langmuir-Blodgett (LB) films shows very different electric field effects from the monomer both in the absorption spectrum and in the fluorescence spectrum. The magnitude of the change in electric dipole moment between the S1 excited state and the ground state of the dimer along the normal to the surface is much larger than that of the monomer. The fluorescence quantum yield of the OCC dimer also shows a remarkable electric field effect. The sign of the field-induced change in the quantum yield of the dimer fluorescence becomes opposite when the direction of the applied electric field is inverted, suggesting that the excitation dynamics of the dimer that is affected by an external electric field is a vectorial process along the normal to the surface. The process affected by an electric field is attributed to the interlayer electron transfer between the excited state of the OCC dimer and the cadmium salt of the fatty acid deposited in contact with each other.

1. Introduction Molecular complex and aggregate may show very different behaviors from the individual molecules both in optical spectra and in excitation dynamics. In bacterial reaction centers, for example, a pair of bacteriochlorophylls, called as the special dimer, is considered to play a significant role in the initial step of photoinduced electron transfer.1-3 In order to elucidate the difference of electronic structure between the individual molecules and molecular complex, application of an external electric field seems to be very useful because spectral shift and/or spectral broadening, i.e., the socalled Stark effect, are induced by a change in electric dipole moment or in molecular polarizability between the excited state and the ground state. In fact, this effect has been successfully applied in molecular spectroscopy, and the electronic structure and level structure of various types of molecules have been elucidated.4-6 Application of external electric fields is also very useful to investigate the excitation dynamics as well as its electric field effects since the excitation dynamics is well related to both the electronic structure and the level structure, which can be well affected by an electric field. Electric field effects are especially suited for the optical measurements of ultrathin films with a well-defined molecular order, since the relation between the applied field and the molecular alignment is well defined and both the first- and the second-order Stark effects are expected to be definitely observed with a relatively weak applied voltage. The Langmuir-Blodgett (LB) technique is one of the most excellent methods for preparation of such a thin film,7 and the electrochromism has been successfully applied to the LB films in order to study electric properties of the embedded chromophores or layer structures.8-12 LB monolayers are especially suitable for study of molecular complex or aggregate, since they provide a highly ordered environment for the molecules, similar to that of a solid matrix, leading to an easy formation of complex or aggregate. In the present study, electronic structure and dynamics of the excited state of oxacarbocyanine embedded in LB films has been † Present address: Laboratory of Molecular Biophotonics, Hamakita 434, Japan. X Abstract published in AdVance ACS Abstracts, November 15, 1997.

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examined by measuring the absorption and fluorescence spectra both in the absence and in the presence of external electric field. In molecular assemblies of LB monolayer films, aggregation or complex formation is enhanced and can be avoided only by dilution with inert molecules that serve to separate the chromophores. Therefore, a difference both of electronic property and of excitation dynamics between monomer and molecular complex of oxacarbocyanine has been examined by comparing the results at different concentrations of oxacarbocyanine in mixed LB films. 2. Experimental Section N,N′-dioctadecyloxacarbocyanine perchrolate (Nippon Kanko Shikiso), hereafter denoted by OCC, was used without further purification. Commercially available arachidic acid (AA) and methyl arachidate (MA) were purified by recrystallization from acetone. All the samples of the LB films were deposited as a cadmium salt. A mixture of AA and MA whose ratio is 1:1, denoted by AA/MA, was used as a matrix. In the present study, two mixing ratios of 0.5 mol % and 20 mol % between OCC and AA/MA were used. LB stacking multilayer films used for the optical measurements were prepared as follows: At first, seven layers of AA were deposited on the quartz substrate coated by a semitransparent aluminum film. Then, six mixed monolayers (for both absorption and fluorescence measurements at 0.5 mol %) or two mixed monolayers (for fluorescence measurements at 20 mol %) or five mixed monolayers (for absorption measurements at 20 mol %) composed of OCC and AA/MA were deposited with a spacer composed of three layers of AA between the adjacent mixed layers. Finally, seven layers of AA were postcoated in every case. All the monolayer films of AA or OCC mixed with AA/MA were deposited as Y-type. Note that LB films deposited on one side of the substrate were wiped off and that only the LB films deposited on the other side, whose total number of the deposited monolayers were 35, 19, or 31, were employed. The monolayer films of a mixture of OCC and AA/MA as well as the films of AA were deposited at a surface pressure of 25 mN/m. The thickness of each layer is 27.3 Å,13 yielding a total thickness of the LB film of 956 Å at © 1997 American Chemical Society

10214 J. Phys. Chem. B, Vol. 101, No. 49, 1997

Ohta et al. num film was again coated with evaporation. These aluminum films were used as electrodes. External electric fields were applied up to 1.2 MV cm-1 in rms by using a function generator (FG 350 or SG 4311, Iwatsu) and a homemade power amplifier. All the optical measurements were carried out under vacuum conditions at room temperature. The field-induced change in fluorescence intensity and in absorption intensity was measured by using a spectrometer equipped with an electric field modulation apparatus for the first harmonic, 1f, and for the second harmonic, 2f, of the modulation frequency (typically 20 Hz). The procedures are the same as described elsewhere.12,14 Picosecond time-resolved fluorescence decay and timeresolved fluorescence spectra were measured by using a femtosecond pulsed laser combined with a single photon counting system equipped with a microchannel plate photomultiplier.15 The laser system was a mode-locked Titanium: Sapphire laser pumped by an argon ion laser combined with a pulse picker. The second harmonic of the laser light was used for excitation. 3. Results and Discussion

Figure 1. Molecular structure of N,N′-dioctadecyloxacarbocyanine perchrolate (OCC) and schematic illustration of LB multilayer films used in the present experiments.

0.5 mol % and 519 and 846 Å at 20 mol %. The molecular structure of OCC and the schematic representation of the sample are shown in Figure 1. Following the deposition of the above-mentioned multilayer films on the aluminum coated plate, a semi-transparent alumi-

Absorption spectra of OCC in mixed LB films at a low concentration of 0.5 mol % and at a high concentration of 20 mol % are shown in Figure 2. Electroabsorption spectra observed at the first harmonic of the modulation frequency (1f) with a field strength of 1.0 MV cm-1 having the field direction shown in Figure 1 and the first derivatives of the absorption spectrum are also shown in Figure 2. The absorption spectrum at 0.5 mol % is nearly the same as the spectrum in diluted solution, e.g., at 10-6 M in chloroform solution, and this absorption spectrum is assigned as the one of the OCC monomer. At high concentrations of OCC in mixed LB films, a complex formation of OCC dye chromophores is known by a reduction in intensity of the absorption maximum at wave-

Figure 2. Electroabsorption spectra obtained at the first harmonic of the modulation frequency, 1f, with a field strength of 1 MV cm-1 (top), the first derivatives of the absorption spectra (middle), and the absorption spectra (bottom) in the S0 f S1 region of OCC in LB films. A broken line in the top shows the spectra simulated by a linear combination of the absorption spectrum and the first derivative spectrum. The concentration of OCC is 0.5 mol % (left) and 20 mol % (right).

Oxacarbocyanine length 495 nm (∼20 200 cm-1) and the appearance of a new maximum at 460 nm (∼21 730 cm-1). There is no doubt that the absorption spectrum at 20 mol % is mainly due to an aggregate or molecular complex of OCC formed in LB films. As demonstrated by Kuhn and Mo¨bius,16 the absorption spectrum of a mixed monolayer of OCC and cadmium arachidate depends on the mixing ratio, and monomer and dimer are in equilibrium in the monolayer. Then, a large part of the absorption intensity observed at 20 mol % is attributed to the dimer of OCC. As a linear external electric field effect on the absorption spectrum, two possibilities can be pointed out; one is the firstorder Stark shift induced by a change in dipole moment (∆µ) between the ground state (S0) and the excited state; another is the field-induced change in transition moment. The former effect will give the 1f electroabsorption spectrum whose shape is the same as the first derivative of the absorption spectrum, while the latter effect induces a change in absorption intensity. In the present case, further, it is also inevitable to consider the spectral overlap of different species of OCC which show different absorption and/or electroabsorption spectra from each other, i.e., the coexistence of monomer and dimer must be considered especially at high concentrations, e.g., at 20 mol %. The observed electroabsorption spectra are not the same as the first derivative of the absorption spectra both at 0.5 and 20 mol %, as is shown in Figure 2. When only the first-order Stark effect is concerned, the integrated intensity of the electroabsorption spectra must be zero, even when the absorption spectra of more than two species overlap, since the first-order Stark effect induces only the spectral shift. As is seen in Figure 2, the integrated intensity of the electroabsorption spectrum at 0.5 mol % is not zero, and the absorption intensity is a little reduced by external electric fields with the field direction shown in Figure 1. Actually, the electroabsorption spectrum at 0.5 mol % could be simulated by a combination of the absorption spectrum with its first derivative spectrum. The simulated spectrum is also shown in Figure 2. The magnitude of ∆µ of the monomer between S0 and the S1 absorbed state is evaluated to be 0.035 D along the normal to the surface, based on the contribution of the first derivative,12 and the field-induced change in the absorption intensity of the monomer is evaluated to be 0.06% of the zero-field intensity at a field strength of 1.0 MV cm-1. Hereafter, the magnitude of ∆µ along the normal to the surface is denoted by ∆µF. Unfortunately, the electroabsorption spectra at 20 mol % could not be simulated by a simple combination of the observed absorption spectrum with its first derivative spectrum. This is probably due to the coexistence of monomer and dimer, which give different spectra and different field effects from each other. Besides the monomer and dimer, further, other species such as a higher aggregate seems to exist a little at 20 mol %. The fact that the field-induced change in the absorption intensity at around 19 500 cm-1 is much smaller than that at around 20 500 cm-1 suggests that ∆µF of the monomer is much smaller than that of the dimer (cf. electroabsorption spectrum and the first derivative spectrum at 20 mol % shown in Figure 2). In comparison of the electroabsorption spectrum with the simulated one in the high wavenumber region, ∆µF following the transition from S0 to S1 of the dimer is evaluated to be 0.35 D. Thus, ∆µF of the OCC dimer is found to be one-order of magnitude larger than that of the monomer. The absorption intensity of the dimer is also found to be changed by an electric field by about 0.2% of the zero-field intensity at 1.0 MV cm-1. The absorption intensity decreases in the presence of the electric field with the direction shown in Figure 1, whereas the intensity

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10215 increases with the opposite field direction. The simulated spectrum with which the above-mentioned values were obtained is also shown in Figure 2. As the origin of the remarkable difference in ∆µF between monomer and dimer, two possibilities may be pointed out; one is a real change in magnitude of ∆µ of chromophores by a complex formation, and another is an orientation factor of OCC chromophores in LB films, i.e., the direction cosines connecting the axis of ∆µ of OCC chromophores with the axis of electric field may be changed by a complex formation. In the former case, ∆µF changes following the complex formation without any change in the molecular orientation of OCC chromophores. In the latter case, ∆µF is changed through the change in the orientation of OCC relative to the surface layer, even when both the direction and the total magnitude of ∆µ of the chromophore remain unchanged. As will be mentioned below, the present films are regarded as belonging to the latter one; the remarkable difference in ∆µF between monomer and dimer is attributed to the orientation factor of OCC chromophores in LB films. In N,N′-dioctadecylselenacarbocyanine, denoted by S20, which is a dye with selenium instead of oxygen and otherwise identical with OCC, the total magnitude of ∆µ was evaluated to be 0.35 D based on the measurements of the Stark shift on hole burning in a polymer film.17,18 The value of ∆µF of this compound in mixed LB monolayer films was reported to be as small as 0.11 D. Then, it was concluded that S20 chromophores in mixed LB films are lying almost flat on the layer surface. The present value of ∆µF of the OCC dimer, i.e., 0.35 D, agrees very well with the above-mentioned total magnitude of ∆µ of S20. The magnitude of ∆µ following excitation into S1 of carbocyanine was theoretically estimated to be 0.6 D,16 based on the simple free electron model. This value is quite close to the value determined for the dimer. The results of S20 and the theoretical work imply that the magnitude of ∆µ of OCC chromophore is not changed so largely by a complex formation and that OCC may lie flat on the surface of the LB films under certain conditions. It is also noted that the magnitude of ∆µ following excitation into S1 of oxacyanine was theoretically determined to be 0.2 D,19 based on the free electron model. This value agrees well with the magnitude of ∆µ determined for the J-aggregate of oxacyanine in LB films.12 Thus, the insensitiveness of the magnitude of ∆µ of OCC to the complex formation seems to be also supported by the results of oxacyanine. Then, it is concluded that the in-plane short axis of the OCC chromophore (z axis in Figure 1) in dimer is directed along the normal to the surface with the in-plane long axis parallel to the surface and that the present value of ∆µF of the dimer (0.35 D) corresponds to the total magnitude of ∆µ of OCC. ∆µF of the monomer determined in the present experiments (0.035 D) is one order of magnitude smaller than that expected as the total magnitude of ∆µ of OCC. Such a small value of ∆µF suggests that OCC chromophores lie almost flat on the surface in monomer, as in the case of S20. The observed electroabsorption spectra indicate that the absorption spectra both of the monomer and of the dimer are blue-shifted with the field direction shown in Figure 1, indicating that the direction of ∆µ deposited in the LB films is opposite to the field direction given in Figure 1. Then, the direction of ∆µ of OCC chromophores following the transition from S0 to S1 is regarded as opposite to the z direction. Electrofluorescence spectra of OCC in mixed LB films at 0.5 and 20 mol % were also measured at 1f with excitation at 441 and at 436 nm, respectively, where the field-induced change in absorption intensity is negligibly small. The results with a field strength of 1.0 MV cm-1 having the field direction shown

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Ohta et al.

Figure 3. Electrofluorescence spectra of OCC in LB films observed at the first harmonic of the modulation frequency, 1f, with a field strength of 1.0 MV cm-1 (upper solid line), the first derivatives of the fluorescence spectrum (upper broken line), and the fluorescence spectra (lower solid line). The concentration of OCC is 0.5 mol % (left) and 20 mol % (right). The fluorescence spectrum at 20 mol % is decomposed into the monomer fluorescence spectrum and the dimer fluorescence spectrum given by a dotted line.

in Figure 1 are shown in Figure 3, together with the fluorescence spectrum and its first derivative spectrum. Note that the firstorder Stark shift induced by a change in electric dipole moment between S0 and the fluorescent state is given by the first derivative of the fluorescence spectrum, as in the case of the first-order Stark shift in absorption spectrum.12,14 The fieldinduced change in fluorescence intensity, denoted by ∆IF at 1f is linearly proportional to the field strength. As an example, plots of ∆IF at 1f observed at 580 nm (∼17 240 cm-1) are shown in Figure 4a. The 1f electrofluorescence spectrum at 0.5 mol % is very similar in shape to the first derivative spectrum (Figure 3), indicating that the field effect on the monomer fluorescence at 0.5 mol % is mainly attributed to the first-order Stark shift of the fluorescence spectrum. ∆µF of the monomer is evaluated to be 0.02 D from the Stark shift of the fluorescence spectrum at 0.5 mol %. This value is smaller than the above-mentioned ∆µF evaluated from the Stark shift of the absorption spectrum by a factor of about 2, implying that the tilt angle of the inplane short axis of the OCC chromophore of the absorbed molecule from the layer plane is different from the fluorescent molecule. In contrast with the 1f spectrum at 0.5 mol %, the 1f electrofluorescence spectrum at 20 mol % is very different from the first derivative spectrum, and the integrated intensity of the electrofluorescence spectrum is largely negative. When the field direction was inverted, the spectral shape was essentially the same, but the sign of ∆IF changed in both cases. Thus, the quantum yield of the dimer fluorescence decreases in the presence of F with the field direction shown in Figure 1, whereas the quantum yield of the dimer fluorescence is increased by F with the opposite direction. Here and hereafter, externally applied electric field is denoted by F. The electrofluorescence spectra observed at 2f of the modulation frequency are also shown in Figure 5. These spectra were observed with the same excitation wavelength as the 1f spectra, i.e., at 441 and 436 nm, respectively, since the field-induced change in the absorption intensity at 2f is also negligibly small at these excitations. It is noted that ∆IF at 2f is proportional to the square of the field strength in both cases, as is shown in Figure 4b.

Figure 4. Plots of ∆IF/IF of OCC at 20 mol % against the electric field strength at 1f (a) and at 2f (b) observed at 580 nm.

Before we discuss the electric field effect of the fluorescence quantum yield of OCC, excitation dynamics of OCC in mixed monolayer films is discussed, based on the time-resolved fluorescence measurements in the absence of F. Figure 6 shows the time-resolved fluorescence spectra of OCC at 0.5 mol % and at 20 mol % observed with excitation at 415 nm. The timeresolved spectra at 0.5 mol % are nearly independent of the

Oxacarbocyanine

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Figure 5. Electrofluorescence spectra of OCC in LB films at 0.5 mol % (left) and 20 mol % (right) observed at the second harmonic of the modulation frequency, 2f, together with the fluorescence spectra given by a dotted line. In both cases, the applied field strength was 1.0 MV cm-1 and the maximum fluorescence intensity is normalized to unity.

Figure 6. Time-resolved fluorescence spectra of OCC in LB films at 0.5 mol % (left) and at 20 mol % (right).

monitoring time-window, indicating that the monomer fluorescence is dominant at 0.5 mol %. Actually, the intensity at 550 nm relative to the maximum intensity at ∼510 nm a little increase after 5 ns, implying that OCC dimer exists slightly even at a low concentration of 0.5 mol %. The time-resolved spectra at 20 mol % are also similar in shape to the monomer spectrum in the initial stage of time, indicating that monomer exists even at a high concentration of 20 mol %. As the passage of time, another emission which shows peaks at ∼550 and ∼580 nm strongly appears, and this emission is assigned as the dimer fluorescence. Figure 7 shows the fluorescence decays of OCC at 0.5 and 20 mol % observed at 510, 580, and 630 nm. A nonexponential decay was observed in every case. The lifetime of the slowly decaying portion of the fluorescence of 0.5 mol % observed at 510 nm, which can be assigned as the lifetime of monomer, is 1.6 ns, while the lifetime of the slow component at 20 mol % observed at 580 nm, which is probably the lifetime of the OCC dimer, is as large as 6.5 ns. Thus, the lifetime of the dimer is

Figure 7. Fluorescence decays of OCC in LB films at 0.5 mol % observed at 510 nm (a) and at 20 mol % observed at 510 nm (b), 580 nm (c) and 630 nm (d). The excitation pulse shape is also shown in the figure. The maximum intensity is normalized to 5000 in every case.

regarded as much longer than that of the monomer, indicating that the emitted photon density of the monomer fluorescence concentrates at the initial stage of time in comparison with the dimer fluorescence. This is probably the reason why the fluorescence at 20 mol % is dominated by the monomer fluorescence at the initial stage of time, but there is no doubt that the dimer is dominant at 20 mol % (cf. absorption spectra at 0.5 and 20 mol % in Figure 2). A fast decaying portion of the monomer fluorescence at 20 mol % observed at 510 nm

10218 J. Phys. Chem. B, Vol. 101, No. 49, 1997 indicates that the energy transfer from monomer to dimer occurs with a lifetime of about 0.6 ns. A fast decaying portion of the monomer fluorescence at 0.5 mol % observed at 510 nm is probably due to the energy transfer from monomer to the dimer which is distributed in LB films slightly. The time-resolved spectra shown in Figure 6 demonstrate that energy transfer occurs so efficiently at 20 mol % from the excited state of OCC monomer to dimer. In the time-resolved spectra at 20 mol %, the intensity at 630 nm relative to the peak intensity of the dimer at 550 or 580 nm becomes stronger after several ns following excitation, implying that fluorescence emitted from the species other than monomer or dimer also exists at 20 mol %. In fact, the fluorescence decay observed at 630 nm is different from that at 580 nm and shows a long component with a lifetime of about 7.7 ns. This lifetime, which is a little longer than the lifetime of dimer determined from the slowly decaying portion of the fluorescence at 580 nm (6.5 ns), may be assigned as the lifetime of OCC excimer or higher aggregates of OCC. A fast decaying portion of the fluorescence at 20 mol % observed at 580 nm may result from the energy transfer from the photoexcited dimer to excimer or higher aggregates of OCC. Note that the lifetime of the fast decaying portion of the fluorescence at 580 nm is much shorter than that at 630 nm, as is shown in Figure 7. As mentioned above, fluorescence of OCC excimer or/and higher aggregate is also observed at 20 mol %, but these emissions are very weak. Then, the total fluorescence at 20 mol % can be regarded as a mixture of the monomer fluorescence and the dimer fluorescence. By combining the time-resolved fluorescence spectra at 20 mol % with the fluorescence spectrum at 0.5 mol %, the steady-state fluorescence spectrum at 20 mol % was decomposed into the sum of the monomer spectrum and the dimer spectrum, as is shown in Figure 3. Then, the observed 1f electrofluorescence spectra at 20 mol % were simulated by a combination of the fluorescence spectrum with its first derivative spectrum of monomer and dimer, i.e., by using the following equation:

∆IF ) C1(dIFmonomer/dνj) + C2(dIFdimer/dνj) + C3IFmonomer + C4IFdimer Here, IFmonomer and IFdimer represent the intensity of the monomer and dimer fluorescence whose spectra are given in Figure 3, and C1 - C4 and νj represent the coefficients and wavenumber, respectively. Actually, the observed spectra could be simulated well by assuming that C3 ) 0, i.e., the field-induced change in the fluorescence quantum yield of the monomer is negligibly small. This assumption agrees well with the result at 0.5 mol %, where the 1f spectra are essentially the same in shape as the first derivative spectra and the quantum yield of the monomer fluorescence is regarded as unaffected by F. The field-induced change in the quantum yield of the dimer fluorescence, i.e., C4, was determined by using ∆IF/IF at 580 nm, where the first derivative of the dimer fluorescence spectrum is zero. With the field direction shown in Figure 1 at a field strength of 1.0 MV cm-1, for example, C4 was determined to be -0.014. These results indicate that fluorescence of the OCC dimer is well quenched by F, in contrast with the monomer fluorescence. When the direction of F is inverted, the 1f electrofluorescence spectra become opposite in sign with essentially the same shape, indicating that the dimer fluorescence is enhanced by F with the direction opposite to that shown in Figure 1. C1 ) -1.69 and C2 ) -5.92 were employed to obtain the simulated spectrum shown in Figure 8. These coefficients correspond to 0.1 D and 0.35 D of ∆µF between S0 and the fluorescent state

Ohta et al.

Figure 8. Electrofluorescence spectrum of OCC at 20 mol % observed at 1f with a field strength of 1.0 MV cm-1 (solid line) and the simulated spectrum (thick broken line) obtained by a linear combination of the dimer fluorescence spectrum (dotted line), its first derivative spectrum (thin broken line), and the first derivative of the monomer fluorescence spectrum (chain line).

of monomer and dimer, respectively. The above value of ∆µF for the dimer is the same as the one evaluated from the electroabsorption spectrum at 20 mol %. On the other hand, ∆µF of the monomer thus obtained is much larger than that evaluated from the electroabsorption spectrum at 0.5 mol % (0.035 D) or the electrofluorescence spectrum at 0.5 mol % (0.02 D). The OCC monomer fluorescence spectrum at 20 mol % observed in the early stage of the time-resolved spectra is similar in shape to the monomer spectrum at 0.5 mol %. However, the peak position of the former spectrum is red-shifted by about 8 nm, in comparison with the monomer spectrum at 0.5 mol %, indicating that OCC molecules which emit the monomer fluorescence strongly interact with other OCC molecules at 20 mol %. Then, the above-mentioned difference in ∆µF for the monomer between 20 mol % and 0.5 mol % seems to show that the molecular plane of the OCC chromophore is tilted from the layer plane with increasing OCC concentration because of an increasing interaction among OCC molecules, even when the monomer fluorescence is emitted. As mentioned previously, the field-induced change in the absorption intensity of the OCC dimer, which corresponds to the change in transition moment, was estimated from the electroabsorption spectrum to be as small as ∼0.2% of the zero field intensity at 1.0 MV cm-1. It is worth mentioning that the field-induced change in absorption intensity is related to the field-induced change in radiative decay rate. The amount of the field induced change in the fluorescence quantum yield of the dimer estimated above is much larger than that expected from this field-induced change in transition moment, indicating that the field-induced change in radiative decay rate is too small to interpret the observed field-induced change in the fluorescence quantum yield of the dimer. Therefore, it is concluded that excitation dynamics of the OCC dimer in LB films is well affected by F and that the rate of this process is enhanced by F with the field direction giVen in Figure 1. Note that the rate constant is decreased by F with the opposite direction. In contrast with the OCC dimer, excitation dynamics of the OCC monomer is considered to be unaffected by an electric field. The 2f electrofluorescence spectra at 0.5 mol % are nearly the same in shape as the fluorescence spectrum irrespective of

Oxacarbocyanine the field direction (see Figure 5), indicating that the quantum field of the OCC monomer fluorescence decreases in the presence of F, but this quadratic field effect is very small; the field-induced change in the fluorescence intensity at 1.0 MV cm-1 is about 2 × 10-4 of the zero-field intensity. The 2f electrofluorescence spectrum at 20 mol % is nearly the same in shape as the dimer fluorescence spectrum, indicating that the quadratic field effect on the dimer fluorescence is much larger than that of the monomer fluorescence. The dimer fluorescence is also quenched by F as the quadratic field effect, irrespective of the field direction. The magnitude of ∆IF/IF in the high wavenumber region, which corresponds to ∆ΦF/ΦF of the monomer fluorescence, is much smaller than that in the low wavenumber regions, as is shown in Figure 5. Here, fluorescence quantum yield and its field-induced change are denoted by ΦF and ∆ΦF, respectively. Thus, the quadratic field effect on the monomer fluorescence is very small both at 0.5 mol % and at 20 mol %. Based on the measurements of ∆IF/IF at 580 nm both at 1f and 2f, ∆ΦF/ΦF of the OCC dimer is given by AF + BF2, where A ) 1.4 × 10-8 V-1 cm and B ) -3.5 × 10-15 V-2 cm2, with the field direction shown in Figure 1 and with units of V cm-1 for F. The negative sign of A and B indicates that the fieldinduced quenching of fluorescence occurs with the field direction shown in Figure 1 both as the linear field effect and as the quadratic effect. When the field direction was inverted, the sign of only A was changed, as mentioned above. The experimental results were analyzed by assuming that the rate of only the nonradiative process is affected by F, i.e., knr (F) with a field strength of F is written as knr(F ) 0) + RF + βF2, where R and β are the coefficients. By using the radiative rate constant, kr, and knr(F), the fluorescence quantum yield with a field of F, (ΦF(F)), is given by kr/(kr + knr(F)). The difference between the fluorescence quantum yield in the presence of F and the yield in the absence of F, i.e., ΦF(F) - ΦF(F ) 0), is written as ∆ΦF. Then, ∆ΦF divided by ΦF(F ) 0) is nearly given by (RF + βF2)/(kr + knr(F ) 0)), since knr(F ) 0) . |RF + βF2|. The term of kr + knr(F ) 0) may correspond to the inverse of the average lifetime defined as ∫IF(t) dt/IF(t ) 0), i.e., τjF. Here, IF(t) represents the time-dependent fluorescence intensity. Then, ∆ΦF/ΦF is represented as follows:

∆ΦF/ΦF ) -(RF + βF2)τjF The average fluorescence lifetime of the dimer was estimated to be 0.67 ns from the fluorescence decay at 20 mol % observed at 580 nm. By using the above-mentioned values of A and B, R and β of the OCC dimer are evaluated to be 21 s-1 V-1 cm and 5.2 × 10-6 s-1 V-2 cm2, respectively. Note that knr(F) knr(F ) 0) is given by RF + βF2. The direction of the applied electric field is a matter of definite importance in the electric field effects on the excitation dynamics of the OCC dimer, suggesting that the dynamics of the dimer which is affected by an electric field is a vectorial process along the field direction. As the vectorial dynamics, interlayer electron transfer between the photoexcited dimer of OCC and the cadmium salt of the arachidic acid deposited in contact with the OCC layer may be pointed out, as in the case of chromophoric thiacyanine in LB films.14 In relation to the present field effects on the OCC dimer fluorescence, preliminary results of the electric field effects on fluorescence are briefly discussed for molecular assemblies composed of mixed monolayers of OCC and mixed monolayers of N,N′-dioctadecyl-4,4′-bipyridinium ion (VIO) separated by a spacer layer of AA. In this system, interlayer vectorial electron transfer occurs from the excited state of OCC to VIO,

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10219 and the fluorescence quantum yield of OCC changes in the presence of external electric field as a result of the field-induced change in the electron transfer rate.20 Fluorescence quantum yield is depressed or enhanced by an electric field, depending on the field direction, as in the present case. When the electric field is applied along the same direction as the electron transfer from OCC to VIO, for example, fluorescence quantum yield increases, i.e., the electron transfer rate is depressed by the electric field. Similar field dependence was also observed in photoinduced vectorial electron transfer from thiacyanine to VIO in LB films.21 The present field-direction dependence of the OCC dimer fluorescence is completely opposite to that for the vectorial electron transfer from OCC or thiacyanine to VIO, if the electron transfer occurs from the excited state of the OCC dimer to the cadmium salt of fatty acid deposited in contact with the OCC layer. In the present system, therefore, it seems that electron transfer occurs from a cadmium salt of fatty acid to the excited state of the OCC dimer, i.e., photoinduced hole transfer seems to occur from the OCC dimer along the field direction shown in Figure 1. 4. Conclusion Monomer and dimer of OCC which coexist in LB mixed monolayer films show very different electric field effects both on the absorption spectrum and on the fluorescence spectrum. The orientation of the OCC chromophore relative to the layer surface seems to be different in the monomer from that in the dimer. Excitation dynamics of the OCC monomer distributed in LB films is regarded as nearly independent of the applied electric field, though the first-order Stark shift induced by a change in electric dipole moment between S0 and S1 is observed both in the absorption spectrum and in the fluorescence spectrum. On the other hand, excitation dynamics of the dimer in LB films is well affected by an external electric field, in addition to the Stark shift. The linear and quadratic field effects on the fluorescence quantum yield of the dimer indicate that the field dependence of the rate of the process which is affected by an external electric field is given by knr(F ) 0) + RF + βF2 with R ) ∼20 s-1 V-1 cm and β ) ∼5 × 10-6 s-1 V-2 cm2 with units of V cm-1 for F. The nonradiative process which is affected by an electric field is attributed to the electron transfer between the excited state of the dimer and the cadmium salt of fatty acid which is deposited in contact with each other. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (No. 08454169). References and Notes (1) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180, 385; Nature 1985, 318, 618. (2) Boxer, S. G.; Goldstein, R. A.; Lockhart, D. J.; Middendorf, T. R.; Takiff, L. J. Phys. Chem. 1989, 93, 8280. (3) Fleming, G. R.; Grandelle, R. van Phys. Today 1994, February, 48. (4) Buckingham, A. D. in Medical Technical Publishing Company International ReView of Science: Physical Chemistry, Series 1, Ramsay, D. A., Ed.; Butterworth: London, 1972; Vol. 3, p 73. (5) Hochstrasser, R. Acc. Chem. Res. 1973, 6, 263. (6) Liptay, W. in Excited States, Lim, E. C., Ed.; Academic Press: New York, 1974; p 129. (7) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (8) Bu¨cher, H.; Wiegand, J.; Snavely, B. B.; Beck, K. H.; Kuhn, H. Chem. Phys. Lett. 1969, 3, 508. (9) Bu¨cher, H.; Kuhn, H. Z. Naturforsch. 1970, 25b, 1323.

10220 J. Phys. Chem. B, Vol. 101, No. 49, 1997 (10) Blinov, L. M.; Dubinin, N. V.; Mikhnev, L. V.; Yudin, S. G. Thin Solid Films 1984, 120, 161. (11) Nishikawa, S.; Tokura, Y.; Koda, T.; Iriyama, K. J. J. Appl. Phys. 1986, 25, L701. (12) Ohta, N.; Okazaki, S.; Yamazaki, I. Chem. Phys. Lett. 1994, 229, 394. (13) Fromherz, P.; Oelschla¨gel, U.; Wilke, W. Thin Solid Films 1988, 159, 421. (14) Ohta, N.; Nomura, T.; Yamazaki, I. Chem. Phys. Lett. 1995, 241, 195. (15) Yamazaki, I.; Tamai, N.; Kume, H.; Tsuchiya, H.; Oba, K. ReV. Sci. Instrum. 1985, 56, 1187.

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