J. Phys. Chem. B 2002, 106, 895-898
895
Distance Dependence of the Electric Field Effect on Photoinduced Electron Tunneling between Cyanine Dye and Viologen through a Fatty Acid Monolayer Takashi Ito,†,‡ Iwao Yamazaki,‡ and Nobuhiro Ohta*,† Research Institute for Electronic Science (RIES), Hokkaido UniVersity, Sapporo 060-0812, Japan, and Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan ReceiVed: July 17, 2001; In Final Form: NoVember 28, 2001
Distance dependence of the electric field effect on the interlayer photoinduced electron tunneling from cyanine dye to viologen through a fatty acid has been examined using various spacer layers of fatty acid, based on the fluorescence measurements. The magnitude of the field-induced change in tunneling rate is shown to increase monotonically with decreasing the donor-acceptor distance.
1. Introduction Long-distance photoinduced electron transfer occurs in a molecular system where donor and acceptor are arranged with a well-defined molecular order, as is well-known in the photosynthetic reaction center.1-3 Langmuir-Blodgett (LB) technique can be used to prepare an artificial molecular system where vectorial photoinduced energy transfer or electron transfer occurs, and the distance dependence of excited state quenching via energy and electron transfer has been investigated.4-12 In LB monolayer films of cyanine dye and viologen with a spacer layer of fatty acid, for example, fluorescence quenching which results from a vectorial photoinduced interlayer electron transfer (PIET) from the excited state of cyanine dye to viologen was demonstrated.7,8 The rate of PIET becomes faster exponentially with shortening the donor-acceptor distance, and the electron tunneling through a fatty acid was proposed. In relation not only to a control of photochemical reactions but also to a development of new materials having photoinduced function of electric property, it is interesting to know how external electric field influences the photochemical reaction in well-ordered molecular systems. In fact, the rate of PIET between oxacarbocyanine and viologen was shown to be enhanced and de-enhanced by an external electric field, depending on the field direction relative to the direction of PIET.13,14 Then, a question arises how electric field effects on PIET depend on the donor-acceptor distance. In the present study, interlayer photoinduced electron-transfer systems composed of oxacarbocyanine and viologen with various fatty acids having different molecular lengths have been prepared, and the spacer distance dependence of the electric field effects on PIET from the excited oxacarbocyanine to viologen has been examined on the basis of the measurements of the electrofluorescence spectra, i.e., plots of the field-induced change in fluorescence intensity as a function of wavelength.
Figure 1. Layer structure of the stacking multilayer LB films composed of mixed monolayers of OCC, VIO, and fatty acid monolayer used for the measurements of the electric field effects. The direction of applied electric field and the direction of electron transfer are shown by an arrow and a half arrow, respectively.
2. Experimental Section A mixture of arachidic acid (AA) and methyl arachidate (MA) whose ratio is 1:1, denoted by AA/MA, was used as a matrix of a mixed monolayer film of N,N′-dioctadecyl-oxacarbocyanine * E-mail:
[email protected]. † Research Institute for Electronic Science (RIES), Hokkaido University. ‡ Graduate School of Engineering, Hokkaido University.
(OCC) or N,N′-dioctadecyl-4,4′-bipyridinium dibromide (VIO). Mixing fractions of OCC or VIO to AA/MA were 2 and 10 mol %, respectively. Two kinds of multilayer films were prepared: one includes VIO (sample 1), and the other does not include VIO (sample 2). Sample 1, whose layer structure is shown in Figure 1, was prepared as follows. First, seven layers of AA were deposited on an aluminum-coated semitransparent
10.1021/jp0127566 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/11/2002
896 J. Phys. Chem. B, Vol. 106, No. 5, 2002
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Figure 3. Plots of ln(If0/IfVIO - 1) (upper), ∆IfVIO/IfVIO (middle), and |∆ket/ket| (lower) as a function of the spacer layer distance. The applied field strength was 1.0 MVcm-1. Figure 2. Fluorescence spectra (upper) and E-F spectra (lower) of OCC in the presence of VIO with different spacer layers of PA, SA, AA, and BA. The spectra in the absence of VIO are also shown by a solid line with a spacer layer of SA. The applied field strength was 1.0 MVcm-1 in every case. All the E-F spectra are taken by normalizing the maximum intensity to unity for the fluorescence spectra simultaneously observed.
quartz substrate, as a precoating and insulating film. One mixed monolayer of OCC was deposited, one fatty acid monolayer was deposited as a spacer, and one mixed monolayer of VIO was deposited; deposition of these three monolayers were repeated for three times with a spacer of four monolayers of AA. Eight layers of AA were deposited for protection, and a semitransparent aluminum film was again coated with evaporation. All the samples of the LB films were deposited as a cadmium salt. 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 32, were used for the optical measurements. As a spacer between OCC and VIO, an LB monolayer film of parmitic acid (PA), stearic acid (SA), arachidic acid (AA), and behenic acid (BA) was used. Mixed monolayers of VIO were deposited as Z type following the deposition of a spacer layer of fatty acids, and other films were deposited as Y type (see Figure 1). The total thickness of the stacking layers (D) was determined by 27.3 × 29 + d0 × 3 in units of Å, where d0 shows the length of each fatty acid: 22.4, 25.0, 27.3, and 29.9 Å, respectively for PA, SA, AA, and BA.15 Sample 2 has the same layer structure as sample 1 except that all the VIO mixed layers are replaced by an AA layer. Aluminum films were used as electrodes. The applied field strength was calculated by V/D with an applied voltage of V. Electroabsorption spectra and electrofluorescence spectra, hereafter abbreviated as E-A and E-F spectra, respectively, were obtained using electric field modulation spectroscopy. All the optical measurements were carried out at room temperature under vacuum conditions. The amount of the field-induced
change in fluorescence intensity and in absorption intensity was measured using a spectrometer equipped with an electric field modulation apparatus for the first harmonic of the modulation frequency (typically 40 Hz). The procedures have been described elsewhere.16,17 A small amount of the ac component of the fluorescence intensity or transmitted excitation light intensity, synchronized with the applied ac voltage, was detected with a lock-in-amplifier. 3. Results and Discussion Figure 2 shows fluorescence and E-F spectra of samples 1 and 2 for various fatty acid spacers. At 2 mol % of the OCC monolayers mixed with AA/MA, fluorescence emitted from an aggregate of OCC (probably dimer) with a peak at ∼16 500 cm-1 a little exists, but most fluorescence is emitted from the OCC monomer, as in the case of the 0.5 mol % mixed monolayers.13,16 In the presence of VIO, fluorescence quenching is observed as a result of PIET from the excited state of OCC to VIO irrespective of the electric field. The magnitude of the quenching becomes larger with shortening the spacer distance, i.e., d0. Plots of ln(If0/IfVIO - 1) as a function of the spacer distance between OCC and VIO are shown in Figure 3. Here, If0 and IfVIO represent the fluorescence intensities of samples 2 and 1, respectively. By using a simple kinetic model, the rate constant of electron transfer (ket) can be written as (Φf0/ΦfVIO -1)/τf0, where Φf0 and ΦfVIO represent the fluorescence quantum yields in the absence of VIO and in the presence of VIO, respectively, and τf0 represents the fluorescence lifetime in the absence of VIO. Therefore, the linear relation between ln(If0/ IfVIO - 1) and d0 in Figure 3 indicates that the electron-transfer rate is proportional to the exponential function of the donoracceptor distance. A similar dependence of the fluorescence quenching on the donor-acceptor distance has been reported in other LB film systems which show a PIET process.7-9 The long distance electron transfer through a fatty acid is usually considered to occur by an electron tunneling mechanism,
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J. Phys. Chem. B, Vol. 106, No. 5, 2002 897
and its rate constant, ket, is usually given by K0 exp(-βd), where d is the donor-acceptor distance and K0 is constant. β is related to the barrier height (φ) for electron tunneling through a fatty acid spacer, i.e., β ) 2(2mφ)1/2/p.18,19 Here, m is the electron mass, and p is the Planck’s constant divided by 2π. The spacer distance dependence of PIET between OCC and VIO agrees well with the one expected from the tunneling mechanism; plots of ket versus d0 give a linear straight line with a slope of 0.23 Å-1. Then, the barrier height of this process, i.e., φ, is evaluated to be 0.2 eV by using d ) d0. In donor-acceptor pairs of other cyanine dyes and viologen with a spacer layer of fatty acid, a similar spacer distance dependence has been observed. The barrier height was reported to be 0.15 eV for a pair of thiacyanine and viologen and 0.7 eV for a pair of indocarbocyanine and viologen.20 The present value of φ between OCC and VIO is the same order of the magnitude as these values. The E-F spectra of sample 2, where PIET does not occur from OCC to VIO because of the absence of VIO, are very different from the ones of sample 1. The former spectra are essentially the same as the first derivative of the fluorescence spectra, indicating that only the Stark shift is induced by a change in electric dipole moment between the ground state and the excited state.13 As an example, the E-F spectrum of sample 2 with a spacer layer of SA is shown in Figure 2. The excitation dynamic of OCC is regarded as nearly unaffected by an external electric field (F). In contrast with sample 2, the E-F spectra of sample 1 are similar in shape to the fluorescence spectra, indicating that fluorescence quantum yield is changed by F (see Figure 2). Actually, the E-F spectra are simulated by a linear combination between the fluorescence spectrum and its first derivative spectrum, indicating that both the Stark shift and the fieldinduced change in fluorescence quantum yield are observed. The magnitude of the quenching of the OCC fluorescence, which occurs in the presence of a VIO mixed layer, significantly depends on the spacer distance, while the magnitudes of the field-induced change in fluorescence intensity relative to the total fluorescence intensity are roughly the same, as is seen in Figure 3, where plots of ln(If0/IfVIO - 1) as well as ∆IfVIO/IfVIO are shown as a function of the spacer distance. Note that ∆IfVIO represents the field-induced change in IfVIO. The E-F spectra in Figure 2, which were obtained with F, whose direction is the same as that of the electron transfer from OCC to VIO, show that fluorescence intensity increases in the presence of F. When electric fields are applied with the opposite direction, the sign of the E-F spectra becomes opposite, though the spectral shapes are essentially the same; fluorescence intensity is deenhanced by F with the opposite field direction. The present results were analyzed by assuming that the field-induced change in fluorescence quantum yield (∆Φf) results from the fieldinduced change in electron-transfer rate constant (∆ket); when electric fields are applied, ket changes to ket + ∆ke, and Φf changes to Φf + ∆Φf. By using the fluorescence lifetime in the presence of VIO (τfVIO), ∆ke is related to ∆ΦfVIO/ΦfVIO by the following equation:17,21
∆ke ) - (∆ΦfVIO/ΦfVIO)/[{1 + (∆ΦfVIO/ΦfVIO)}τfVIO] (1) τfVIO can be given by ΦfVIO/kr, where kr represents the radiative rate constant at the fluorescent state. Further, the following relation holds:
(ΦfVIO)-1 - (Φf0)-1 ) ket/kr
(2)
Figure 4. Plots both of ket (upper) and of |∆ket| in the presence of 1.0 MVcm-1 (lower) as a function of the spacer layer distance.
By combining eqs 1 and 2, ∆ket/ket is given as follows:
∆ket/ket ) -(∆ΦfVIO/ΦfVIO){1 + (∆ΦfVIO/ΦfVIO)}-1{1 (ΦfVIO/Φf0)}-1 (3) Therefore, ∆ket/ket can be evaluated from -(∆IfVIO/IfVIO){1 + (∆IfVIO/IfVIO)}-1{1 - (IfVIO/If0)}-1 with the excitation wavelength where the field-induced change in absorption intensity is negligible. The values of ∆ket/ket thus obtained are linearly proportional to the spacer distance, as shown in Figure 3. The potential barrier height on the electron tunneling is assumed to be changed by an applied electric field. Then, φ is replaced by φ0 + φ′F in the presence of F, where φ0 is the barrier height in the absence of F and φ′ is the first derivative of the barrier height in terms of F; the electron tunneling rate constant, ket(F), is given by K0 exp{-2(2m(φ0 +φ′F)1/2d/p}. Then, ∆ket/ket is nearly given by 2mdφ′F{p(2mφ0)1/2}-1, by assuming that only the barrier height is changed by F. As mentioned above, the present values of ∆ket/ket gives a straight line as a function of the spacer distance, in agreement with the above expectation. From the slope of ∆ket/ket, i.e., 3.4 × 10-4 Å-1, φ′ is evaluated to be 2.9 × 10-10 eV(Vcm-1)-1; i.e., the barrier height increases or decreases, depending on the field direction relative to the direction of the electron transfer. Note that the barrier height increases, e.g., by 0.29 meV in the presence of 1.0 MVcm-1, when the applied field direction is the same as that in Figure 1. By using the fluorescence lifetime of the OCC monomer in the absence of VIO, i.e., τf0 ) 1.0 ns,13 ket as well as ∆ket could be estimated for each spacer. Note that ket is estimated from (If0/IfVIO -1)/τf0. Plots of ket and |∆ket| are shown in Figure 4, as a function of the spacer distance. It is clear that not only ket but also |∆ket| increases monotonically with shortening the donor-acceptor distance. It is concluded that the electron tunneling from OCC to VIO through a fatty acid monolayer is influenced by an electric field and that the barrier height for the tunneling becomes higher and lower, depending on the field direction. It was shown that the magnitude not only of the electron-transfer rate but also of
898 J. Phys. Chem. B, Vol. 106, No. 5, 2002 its field-induced change becomes larger with shortening the donor-acceptor distance. 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. One of the authors (N. O.) is grateful for the Suhara Memorial Foundation and the Iwatani-Naoji Foundation’s Research Grant. 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.; Golddstein, R. A.; Lockhart, D. J.; Middendorf, T. R.; Takiff, L. J. Phys. Chem. 1989, 93, 8280. (3) Fleming, G. R.; van Grandelle, R. Phys. Today 1994, 2, 48. (4) Kuhn, H., Mo¨bius, D., Bu¨cher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, p 577. (5) In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum: New York, 1990. (6) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic: San Diego, 1991. (7) Kuhn, H. J. Photochem. 1979, 10, 111.
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