Vectorial Photoinduced Electron Transfer in Phytochlorin[60]Fullerene

J. Phys. Chem. B 2000, 104, 6371-6379

6371

Vectorial Photoinduced Electron Transfer in Phytochlorin-[60]Fullerene Langmuir-Blodgett Films Nikolai V. Tkachenko,*,† Elina Vuorimaa,† Tero Kesti,† Alexander S. Alekseev,§ Andrei Y. Tauber,†,‡ Paavo H. Hynninen,‡ and Helge Lemmetyinen† Institute of Materials Chemistry, Tampere UniVersity of Technology, P.O. Box 541, FIN-33101 Tampere, Finland, Department of Chemistry, UniVersity of Helsinki, P.O. Box 55, FIN-00014 UniVersity of Helsinki, Finland, and General Physics Institute, Russian Academy of Science, VaViloV St. 38, 117942 Moscow, Russia ReceiVed: January 18, 2000; In Final Form: April 17, 2000

A novel molecular electron donor-acceptor (DA) dyad, composed of a phytochlorin donor and a [60]fullerene acceptor, was used for the preparation of solid molecular films capable of performing vectorial photoinduced electron transfer (VPET). Being mixed with octadecylamine at concentrations of 50 mol % and lower, the DA compounds form a stable monolayer, which can be transferred onto a solid substrate. Thus prepared Langmuir-Blodgett (LB) monolayer films are characterized by uniform orientation of the DA molecules and, consequently, can undergo VPET. This was confirmed by time-resolved Maxwell displacement charge (TRMDC) measurements. The rate constant for the electron transfer was ca. 109 s-1 as estimated from the fluorescence lifetime measurements. The majority of the charge transfer states of the DA molecules (>60%) recombined with a time constant of ca. 30 ns, being almost independent of the DA concentration in the concentration range from 2 to 50 mol %, as revealed from TRMDC experiments. Therefore, VPET is most probably an intramolecular reaction. The dependences of the TRMDC signal amplitude on the DA concentration and on the density of the excitation energy indicated that an energy transfer takes place prior to the electron transfer. A variation in the charge recombination rate was observed when a static bias voltage was applied across the films. An estimation of the charge displacement distance across the film gave a value of ca. 0.5 nm.

1. Introduction Design of artificial molecular devices is a challenging goal for modern science and technology. One of the particular targets is modeling of natural photoreaction centers. Great efforts have been undertaken in molecular design mimicking its antenna functions and primary photoinduced charge separations.1-3 A variety of molecular systems have been synthesized and proved to perform light harvesting,4 charge separation,1,2 and charge transport functions.5,6 Potentially, these molecular functions can be used to build up macroscopic molecular devices by assembling molecules into ordered arrays. Practical methods to achieve the goal are not well developed yet, although several approaches, such as the Langmuir-Blodgett (LB) technique7 and molecular self-assembling,8,9 have been successfully applied, and promising results were obtained.6,10 The discovery of fullerenes by Kroto and co-workers11 had a major impact on the design of the DA molecular systems.12-18 The C60 fullerene appeared to be an attractive acceptor with moderate electron-accepting properties comparable with those of benzo- and naphthoquinones, being capable of the reversible acceptance of up to six electrons.18 As the C60 molecule consists of solely 60 carbon atoms, it is essentially hydrophobic compared with the quinone type acceptors. This property allows the researchers to construct LB films of fullerene derivatives with polar substituents and to study their photochemical properties in solid phases.19,20 However, there are no reports so

far on the photochemical studies of LB films composed of tetrapyrrole-fullerene DA molecules. Recently, a phytochlorin-fullerene DA dyad was synthesized,21 and its photochemistry was carefully studied in solutions.22 The dyad absorbs photons and converts the energy of light into electrostatic energy by performing intramolecular electron transfer. The molecule consists of a hydrophobic fullerene unit covalently linked to a phytochlorin unit which possesses a hydrophilic propionic acid residue. Such a design provides a good possibility of using the dyad to fabricate ordered molecular films by means of the LB technique. Thus produced films would contain DA dyads with a uniform orientation on a macroscopic scale. Therefore, a macroscopic photoinduced vectorial charge transfer (CT) can be expected. In the frame of the present study, the principal method for the investigation of the vectorial CT is the measurement of timeresolved Maxwell displacement charge (TRMDC).23-27 The method gives direct information about the charge shift in the direction perpendicular to the plane of the film. A high sensitivity of the method allows us to study a single layer of the DA compound. A nanosecond time resolution was enough to measure the charge recombination time. Analysis of the dependences of the TRMDC signal on the density of the DA molecule surface, excitation energy density, and external electric field provide information about the DA molecule arrangement and their functioning in the film. 2. Methods and Materials

†

Tampere University of Technology. ‡ University of Helsinki. § Russian Academy of Science.

Materials. Chloroform of analytical-reagent grade (Merck) was used for solution preparation. Octadecylamine (ODA) was

10.1021/jp000235x CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000

6372 J. Phys. Chem. B, Vol. 104, No. 27, 2000 of 99% grade (Sigma). The phytochlorin-fullerene dyad (PF) was prepared as described earlier21 by 1,3-dipolar cycloaddition28 of an azomethine ylide, generated in situ from Nmethylglycine and 3-formyl-3-dethylphytochlorin across a junction of two 6-rings in C60. Film Deposition. The LB films were prepared with a LB 5000 double barrier trough (KSV Instruments, Helsinki, Finland). The films were deposited on quartz plates for optical studies and on quartz plates coated by an ITO semitransparent thin electrode for TRMDC measurements. The plates were cleaned before the film deposition using a standard procedure.7 A 0.6 mM phosphate buffer (pH ) 7) was used as a subphase. The deposition rates were 8-10 mm/min for downward and 7-8 mm/min for upward directions. The films were deposited at a constant pressure of 30 mN/m for PF/ODA mixtures and 24-28 for 100% PF films. Before the deposition of the DA layers, all the substrates were precoated with nine layers of ODA. This was needed to isolate the DA layers from the semiconductor ITO film. The same procedure was used to prepare films for optical measurements. In addition, the DA layers prepared for TRMDC measurements were coated with 10 ODA layers to prevent any interaction of the DA molecules with the top electrode. Spectroscopy Study. Absorption spectra of the samples were recorded by a Shimadzu UV-2501PC spectrophotometer. Steady state fluorescence spectra were measured with a Fluorolog 3 fluorimeter (SPEX Inc.) and were corrected to the instrument wavelength sensitivity. Fluorescence decay curves were measured using a time-correlated single-photon-counting technique with the instrument described elsewhere.29 In short, samples were excited at 590 nm, decays were recorded in the 620-800 nm wavelength range, and instrumental response (fwhm) was ca. 100 ps. To obtain decay-associated spectra (DAS), decays were collected at different wavelengths with a constant averaging time (2 min, typically) and globally fitted to multiexponentials, and preexponential factors (spectra) were corrected in accordance with the sensitivity spectrum of the microchannel plate photomultiplier (Hamamatsu R3809U-50) provided by the manufacturer. Time-Resolved Maxwell Displacement Charge Measurements. The arrangement of the samples is shown schematically in Scheme 1, and the apparatus used is described elsewhere.23-26 The excitation wavelength was 532 nm (second harmonic of the Nd:YAG Q-switched laser), and the time resolution of the instrument was 15 ns (determined by the excitation pulse width). The sample structure for the photoelectric measurements was as follows: ITO/ODA insulating layers/PF active layer/ODA insulating layers/InGa liquid-metal drop electrode. Therefore, the TRMDC signals were produced by charge motions inside the PF layer and could not be affected by a semiconductordye interaction. The active PF layers were deposited in either an upward or downward direction, thus producing samples with complementary donor-acceptor orientations, as shown in Scheme 1. The samples had extremely low conductivity (Rs > 1012 Ω) and could be treated as pure capacitors (typical Cs was 100-200 pF). The preamplifier input resistance was Rin ) 100 MΩ or greater, and therefore, in a time scale shorter than 10 ms, the TRMDC measurements were done in a photovoltage mode (RinCs > 10 ms), which means that the measured signals are directly proportional to the charge displacement. The high resistance of the sample allows one to apply a constant bias voltage, Ubias, without a danger of the sample damage (there was no static current flow across the sample). Thus, an influence of the external electric field on the ET property of the sample

Tkachenko et al SCHEME 1: Arrangement of the Samples for the TRMDC Measurements

can be studied. In this study, the applied bias voltage varied in the range from -1 to +1 V, which corresponds to an electric field less than 2 × 106 V/m. 3. Results 3.1. LB Films. The surface pressure-mean molecular area (mma) isotherms of the PF/ODA mixtures at different molar concentrations and those of the pure substances are presented in Figure 1a. The pure PF molecules formed a stable monolayer on the water surface, which collapsed at a pressure of about 30 mN/m. Mixing with ODA improved the monolayer properties, isotherms became sharper, and the collapse pressure increased. The dependence of the mean molecular area vs PF concentration measured at 30 mN/m was almost linear (Figure 1b). The estimated average area of the PF molecule on the water surface was about 0.9 nm2. Only a single layer of 100% PF film could be transferred from the water surface onto a solid support, and the transfer ratio was less than 0.7. Therefore mixed films were prepared and studied. Up to 50% concentration (here and later through the text the concentrations are given in molar %) of PF in the ODA matrix, the layers could be transferred onto a solid support with the transfer ratio close to 1. A single layer could be successfully covered with a multilayer ODA film, or a multilayer film of PF/ODA mixture could be deposited. In the latter case a slight decrease in the transfer ratio was observed after deposition of more than eight layers from the same Langmuir film. However, the films had uniform color and did not exhibit any properties that could indicate film inhomogeneity. 3.2. Optical Properties of the Films. Absorption spectra of the films deposited on quartz slides were similar to each other, and the optical densities increased with increasing the PF concentrations. The spectrum of the 50% monolayer film is shown in Figure 2a, where a distinct Q-band absorption of the phytochlorin part of the PF compound can be seen. The Soret band is observed as a stepwise increase in the absorbance at the blue region. A gradual increase in the absorption when going to the shorter wavelengths can be explained by the absorption of the fullerene part and by the sample scattering. The latter is

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Figure 1. (a) Isotherms for PF/ODA mixtures; concentrations are marked on the figure. (b) Dependence of the mean molecular area (mma) on the molar concentration, c, of PF; mma’s were obtained at the surface pressure of π ) 30 mN/m for all the films except for the 50% (π ) 28 mN/m) and 100% (π ) 25 mN/m) films; the solid line presents a linear fit of the data.

Figure 2. (a) Absorption spectrum of the 1:1 PF/ODA mixed monolayer film (on both sides of the slide). (b) Optical density of the monolayer films at 680 nm as a function of the PF surface density. The solid line is a linear fit, OD ) 0.0104n, where density, n, is given in nm-2.

caused by domain structures of the monolayer films of the ODA molecules, which were used as the bottom layers and as the matrix molecules. An important characteristic of the film is the surface density of the chromophore molecules. The density, n, was calculated on the basis of the mean molecular area, s, at the film deposition pressure (30 mN/m), and the molar fraction of PF, c, as n ) c/s. The absorption of the monolayer films (on both sides of the slide) at 680 nm (Q-band) is shown in Figure 2b as a function of the chromophore density. The dependence can be approximated by a straight line. Despite the fact that transfer ratio showed a good deposition of the multilayer films, the absorption of the film did not increase linearly with the increasing number of layers. Typically, after 3-4 dippings the increase in the absorption per layer was less than half of that for the first layer. At least two distinct bands can be seen in the fluorescence spectra of the PF films (Figure 3a): a sharp emission band at 680 nm and a broad one in the 700-800 nm region. The fluorescence decay curves monitored at these two bands were clearly different (Figure 3b). In solutions, three emitting species were formed during the relaxation of the PF excited state:22 a singlet excited state of phytochlorin, P*F, a singlet excited state of fullerene, PF*, and an intramolecular exciplex, (PF)*. These three species had different emission maxima. In toluene the emission maximum for phytochlorin was at ca. 680 nm. Fullerene C60 and its derivatives have the emission maximum at the 700-740 nm region. The expected wavelength range for the exciplex emission, (PF)*, is at 740-850 nm.22

The fluorescence decays for the PF films were measured in the wavelength range of 630-800 nm, and the DAS were fitted using a three-exponential approximation (Figure 3c). Although applicability of the exponential fitting to a fluorescence decay for solid LB films is somewhat controversial,27,30 the used threeexponential model has a certain physical meaning and helps us to follow the principal excitation relaxation processes. The shortest-lived component, 58 ps for the 15% sample, has the emission band around 680 nm and can be attributed to the fluorescence of the singlet excited state of the phytochlorin chromophore. The fitted lifetime is shorter than the time resolution of the instrument and, therefore, should be considered as the upper limit of the real lifetime. In the case of PF in solutions, the lifetime of the singlet excited state of phytochlorin chromophore was shorter than 1 ps.22 The 190 ps component has an emission maximum at 710 nm, which fits well the fluorescence of the fullerene moiety. The third component has a lifetime of 820 ps and a broad maximum at 730-770 nm, which is attributed to the fluorescence of the exciplex. The fluorescence properties of the films were rather similar for all the samples with different PF concentrations (in the range 2-50%). The lifetimes, as fitted with three-exponentials, were 40-60 ps, 200-300 ps, and 0.8-1.5 ns. The weighted mean square deviations, χ2, were 1.3-1.4 (global fits). A fourexponential fit gave χ2 ≈ 1.2 and mainly resulted in a splitting of the longest-lived component into two components with lifetimes of roughly 0.5 and 2 ns. 3.3. Photoelectric Measurements. The PF layers deposited in different directions should have different orientation for the donor and the acceptor with respect to the ITO electrode.

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Figure 4. Photovoltage responses of the 20% PF/ODA monolayer films deposited in upward and downward directions and measured at different time scales. Figure 3. Fluorescence of the 15% PF sample: (a) the steady state spectrum; (b) the decays at 680 and 750 nm; the dotted line is the instrument response; (c) the decay-associated spectra (DAS) calculated by the three-exponential approximation; lifetimes are indicated in the figure.

Consequently, the photovoltage responses should have opposite signs, but otherwise the signals should be identical. This phenomenon is illustrated in Figure 4 for two 20% monolayer films, where the photovoltage responses for the samples deposited in the upward and downward directions are shown at different time scales. The measurements were carried out at five different time domains, covering the time range from nano- to milliseconds. A reasonable approximation of the data can be obtained with a five-exponential fitting. The summary of the results is presented in the Table 1 (note the time range, from 30 ns to 2 ms, covering almost 5 orders of magnitude). For all the samples, the dominating component (approximately 60% of the signal amplitude) had a lifetime in the 20-50 ns range. The next component had a lifetime close to 1 µs and a relative intensity of about 20% of the signal amplitude. The remaining signal had an intensity of roughly 20% of the initial value, and it relaxed “multiexponentially” in a micro- to millisecond time

TABLE 1: TRMDC Decay Lifetime Constants, τ, and Corresponding Preexponential Factors, a, Calculated for Monolayer 20% Films Deposited in Upward and Downward Directions; arel Is the Relative Intensity of the Component down

up

no.

τ

a, mV

arel

τ

a, mV

arel

1 2 3 4 5

34 ns 330 ns 3.6 µs 48 µs 0.9 ms

26.4 7.8 4.4 2.1 1.2

0.63 0.19 0.11 0.05 0.03

38 ns 530 ns 2.6 µs 35 µs 1.9 ms

-19.4 -7.9 -4.4 -3.4 -2.5

0.52 0.21 0.12 0.09 0.07

domain. We supposed that the nanosecond component presents the main path of the CT state relaxation, and therefore we concentrate our research efforts on the investigation of the sample properties in the nanosecond time domain. The shapes of the photovoltage signals were independent of the excitation energy in a wide range of the energy density. Thus the amplitude of the photovoltage response alone was used to characterize the dependence of the photoresponse on the excitation energy density. The dependence is shown in Figure 5 for the 5% and 20% PF films (the dependences were measured in a wider energy range, but the scales are selected to show

Vectorial Electron Transfer in LB Films

Figure 5. 5. Dependence of the photovoltage signal amplitude on the excitation energy density for the 5% and 20% films. Solid lines show the fits to eq 2; fit parameters are indicated in the plot.

Figure 6. Photovoltage responses of the 5% and 20% PF/ODA monolayer films. The excitation energy density for 5% films was 12fold that for the 20% films.

both curves simultaneously). At the low excitation densities the signal amplitudes increased linearly with the increase of the energy. With the further increase of the excitation energy, a saturation of the signal amplitude was observed. The photovoltage response amplitudes varied for different sample concentrations, but the shapes of the signals were almost independent of the concentrations. Figure 6 presents the photovoltage responses for the 5% and 20% monolayer films. To achieve approximately the same signal intensities, the excitation energy for the 5% samples was 12-fold that used for the 20% samples. It is essential that the lifetime of the shortestlived component is almost insensitive to the DA dyad concentration. The amplitudes of the photovoltage responses can be used to compare the efficiencies of ET for samples at different DA concentrations. To provide similar conditions for different samples, the dependence of the photovoltage response should be measured at excitation densities lower than the saturation density. The latter is lower for samples with higher concentrations, but the signal at low concentrations decreased to the level of the noise. Therefore, the samples were measured at different excitation densities below the saturation, and their intensities were recalculated to the level corresponding to the same excitation density in all cases assuming a linear dependence of the signal on the density. The results are shown in Figure 7 in a double logarithmic plot. It is important to note that the reduction of the surface density of the DA molecules to onefifth decreases the signal intensity to almost 1/100, which is clearly a nonlinear dependence. An external electric field affects differently the ET in the films with ordered insulated DA molecules compared with the ET in the films containing disordered aggregated chromophore assemblies.25-27 The influence of a bias voltage, Ubias, on the

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Figure 7. Dependence of the photovoltage response amplitude on the surface density of the PF dyad. Measurements were carried out at different excitation densities and recalculated to the same density for all the samples (see text for details). The dotted line is the linear fit of the data, and the solid line is the fit to the third power of the PF concentration.

Figure 8. Photovoltage decay curves at different bias voltages for the 20% PF sample deposited in an upward direction.

Figure 9. Dependence of the fastest lifetime on the bias voltage for the 20% PF films. Lines present linear fits of the data.

photoresponse of the 20% sample is shown in Figure 8. One obvious effect of the external electric field is a change in the signal level. The change was independent of the deposition directions and can be attributed to an intermolecular charge migration.26,27 However, the variable part of the photoresponse was almost independent of the Ubias, indicating the presence of intramolecular CT processes. The intramolecular CT processes dominated the photovoltage response for all the samples in the nanosecond time domain. A careful comparison of the decay curves recorded at different bias voltages revealed a small effect of the Ubias on the lifetime of the fast component in the nanosecond range. The decays were fitted at all Ubias, and the resulting time constants were plotted as a function of Ubias, as shown in Figure 9. The positive potential (vs ITO electrode) applied to the sample deposited in the upward direction increased the CT recombination time constant, whereas for the samples deposited in the downward

6376 J. Phys. Chem. B, Vol. 104, No. 27, 2000 direction the positive potential reduced the recombination time. In relation to the orientation the positive potential applied to the fullerene side increased the CT state lifetime. 4. Discussion 4.1. Structure of the Films. Crucial points in the mixed chromophore/matrix films prepared using the LB technique are the density of the chromophores and their orientation and aggregation. The dependence of the mean molecular area on the PF concentration suggests that the area occupied by the PF molecule on the water surface at the liquid-condensed part of the isotherm is ca. 0.9 nm2 (Figure 1). This value is in agreement with the values reported for fullerene and chlorin-like molecules.19,20,27,31 Absorption spectra of the films revealed the characteristic phytochlorin bands (at 420 and 670 nm) in the visible region and the fullerene band in the UV region (Figure 2). The slope of the concentration dependence of absorption at the Q-band (Figure 2b) suggests a molar absorptivity of ca. 30 000 M-1 cm-1. This value is somewhat lower than the molar absorptivity obtained in a chloroform solution (680 ) 44 600 M-1 cm-1).22 The absorption decrease was accompanied by a broadening (19 nm bandwidth in the films vs 15 nm in chloroform) and a red shift of the Q-band λmax (678 nm in the films vs 672 nm in chloroform). However, the oscillator strength of the Q-band was nearly the same in the films and in the solutions. Thus we may conclude that, in a wide range of concentrations, the monolayer films can be transferred onto the solid support without any loss of PF. We propose that the PF molecules are oriented on the water surface so that the 7-propionic acid residue and 131-oxo group of the phytochlorin part stick into the water and the strongly hydrophobic fullerene moiety protrudes into the air. This orientation is well confirmed by the TRMDC measurements (Figures 4 and 6), which show the electron displacement from the phytochlorin donor to the fullerene acceptor. The photovoltage response proves the vectorial photoinduced electron transfer to take place. Usually chromophores tend to aggregate when they are placed in a rigid matrix such as ODA.26,32 Lipids with saturated hydrocarbon chains form tightly packed crystal-like domains and, therefore, have poor miscibility with other molecules. The aggregation of the DA molecules often leads to a significant reduction in the ET efficiency and promotes intermolecular ET reactions.26,27 This seems not to be the case in the PF/ODA films. Although the recombination of the photoinduced charges is a complex process, the major part of the molecules recombines with the time constant of ca. 30 ns. This time constant is almost independent of the concentration of the DA molecules and of the direction of the film deposition. It is also quite insensitive to the external electric field. Therefore, we can conclude that the phenomenon is the intramolecular charge recombination. Consequently, aggregation, if it exists, does not affect the charge separation and the recombination processes in the films. Two factors may result in the low degree of aggregation of the PF molecules in the ODA matrix. First, because fullerene is a bulky and highly hydrophobic molecule, it can prevent tight packing (aggregation) of the phytochlorin moieties. Second, at the water surface, the propionic acid residue of the phytochlorin moiety may interact with the amine heads of the matrix molecules, thus improving the miscibility of PF with ODA. 4.2. Excited States of PF. The TRMDC measurements support the CT function of the PF dyad in the films. The formation of the CT state is not time resolved. Certain indirect methods and time-resolved fluorescence data can be used to

Tkachenko et al obtain some information about the processes occurring prior to the CT reaction. The samples were excited at the wavelength of 532 nm in the TRMDC measurements. This should mainly result in a population of the lowest singlet excited state of the phytochlorin chromophore, since its extinction coefficient is higher than that of the fullerene part (in toluene the ratio of the phytochlorin/fullerene extinction coefficients is 8 at 532 nm). In the time-resolved fluorescence measurements the samples were excited at 590 nm. At this wavelength, the difference between the extinction coefficients of phytochlorin and fullerene is smaller (the extinction coefficients ratio is 2.3). The fluorescence of the phytochlorin part is shown in the steady state spectrum (Figure 3a) and is resolved in time as the short-lived component in DAS (Figure 3c). All the longer-lived fluorescence components possessed the spectra characterized by a broad band in the NIR wavelength range. One can suggest that the fluorescence in the NIR range is produced by the singlet excited state of the fullerene moiety and by the intramolecular exciplex, similar to that formed by the PF in solutions.22 Alternatively, the NIR fluorescence can be a consequence of the aggregation of the phytochlorin chromophore. In the latter case, it is not clear why the most red-shifted fluorescing species had the longest lifetime. Therefore, on the basis of a solution study we suggest that the NIR fluorescence is produced by the singlet excited state of the fullerene part and by the exciplex. In this case, the component with a lifetime of ca. 200 ps could be attributed to the fluorescence of the fullerene moiety, PF*. The longest-lived and the most red-shifted component can be associated with the exciplex (PF)*.22 The exciplex was found to be a precursor of the CT state in solution. Consequently, the time constant of the CT state formation is the lifetime of the exciplex, and it is somewhat shorter than 1 ns. Thus, the simplest kinetic scheme can be presented as follows,

where also the intramolecular energy transfer, P*F f PF*, is taken into account as it was in solutions. According to the above model, the lifetimes τF and τET, and the value of (τP)-1 + (τPF)-1 can be determined from the timeresolved fluorescence measurements. For the 15% sample, the values (τP)-1 + (τPF)-1 ≈ 1/50 ps, τF ≈ 200 ps, and τET ≈ 800 ps are obtained (Figure 3). The recombination lifetime, τCR ≈ 30 ns, is obtained from the time-resolved photovoltage measurements. Unfortunately, the above model does not explain all the phenomena observed in the TRMDC measurements, the concentration dependence of the photovoltage signals, for example. In fact, the model does not take into account the main distinction between the LB films and the solutions: the high density of the PF molecules. An average center-to-center distance between the PF molecules in the 20% film is ∼1.3 nm. Under such conditions, the intermolecular energy transfer can play an important role among the photoprocesses of the DA molecules. 4.3. Intermolecular Energy Transfer. The energy transfer is usually discussed in terms of exciton hopping from one site to another. For the Fo¨rster type energy transfer, the exciton hopping rate is proportional to the sixth power of the distance. For a two-dimensional film with a homogeneous distribution of the chromophores, the hopping rate is proportional to the third power of the chromophore surface density, n3. The hopping rate determines the number of sites that the exciton can visit before the relaxation (yielding, for example, the CT state). This

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number defines the size of the antenna subsystem, which contributes to the exciton delivery to an active DA molecule. Therefore, the third-order dependence of the photovoltage signal on the chromophore density, U ≈ n3, is expected. Two lines are drawn in Figure 7. The dotted line represents the linear fit, Uout ) 2.4n, and the solid line the third-order dependence, Uout ) 7.4n3 (where n is the surface density in nm-2 and Uout is measured in mV). Obviously the third-order dependence fits better to the measurements. This model is valid under the following conditions: (1) the relative number of the molecules that can undergo electron transfer, i.e., the active DA molecules, is much smaller than the number of the molecules involved in the antenna subsystem and (2) the density of the active DA molecules is independent of the concentration. The latter statement apparently is not reliable in the present case. One can expect that the number of the active DA molecules increases with the increase of the concentration. This would result in dependence higher than third order. However, the quantitative verification of the dependence order is not possible, and therefore the condition (2) will not be discussed. The condition (1) is of major importance because the DA molecules act also as antenna molecules. Consequently, the probability that a PF molecule undergoes an ET reaction in the film is low. Here the term “probability” means that different PF molecules in the film have different capabilities of undergoing an ET reaction; that is, the DA molecules are not identical with each other in the sense of their photochemical properties. The energy transfer can influence the value of the saturation excitation density, Is (Figure 5). For independent DA molecules a relationship between the photovoltage signal amplitude, U, and the excitation density, I, is given by

U ) Uo(1 - exp(-I/Is)) ) Uo(1 - exp(-σI/hV))

(2)

where σ is the absorption cross-section of the chromophore at the excitation wavelength, Is is the saturation excitation density (Is ) hV/σ), and Uo is the saturated signal amplitude, or the amplitude observed when all the DA molecules were excited. Therefore, if the absorption of the photon and the following charge transfer reaction is a single-molecule process (as for PF in solutions), Is should be independent of the film concentration and should be completely determined by the chromophore crosssection σ. In the present study, Is was clearly different for the samples of different concentrations. In the framework of this model, the saturated signal amplitude, Uo, is proportional to the surface density of the DA molecules. The ratio of the densities for the 20% and 5% films is approximately 2.5. Thus the expected ratio of the saturated signal amplitudes should be 2.5 as well. However, the ratio of the experimentally observed signal amplitudes at the excitation densities higher than Is was about 10. The determination of the saturation dependence for a single sample is relatively accurate, as the excitation density can be changed precisely by using a set of gray filters. Therefore the model given by eq 2 can be verified. It fits the data reasonably well at energies less than the saturation energy (I < Is). At higher excitation densities a gradual increase in the signal was observed even when the excitation energy exceeded the saturation level. Therefore, the model of the independent DA molecules does not describe satisfactorily the behavior of the PF films. Alternatively, one can consider excitation energy dependence in the presence of the intermolecular energy transfer. If an active DA molecule traps the excitation from the antenna consisting of N neighboring molecules, the efficient absorption cross-

section for the ET reaction is given by σeff ) Nσ. Because the value N depends on concentration, also σeff is concentration dependent. An increase in the concentration results in an increase of the efficient antenna size, which leads to a decrease in the saturation energy density. In theory, a quantitative analysis of the dependence, σeff vs n, might give information about the size of the antenna subsystem and about the surface distribution of the molecules. The calculations involve the cross-section of the molecules at the excitation wavelength (532 nm), the absolute value of the excitation density, and the interelectrode distance. Unfortunately, the accuracy of the determination of these parameters was not good enough to perform this type of analysis. A positive qualitative conclusion, however, is that the distribution of the chromophores in the films is rather homogeneous, as the energy transfer rate increases with the increase of the concentration. Another conclusion is that a relatively small number of the DA molecules undergo the ET reaction, whereas the majority of the molecules act only as an antenna. 4.4. Influence of the External Electric Field. Because the CT state has a significant electrostatic dipole moment, its energy depends on the external electric field. This dependence can be observed experimentally as an influence of the bias voltage on the rate of the ET reaction.33 When a DA molecule is placed in a static electric field, E, the energy of the CT state contains the work done for shifting the charge e by the distance equal to the donor-acceptor separation. If the potentials at the initial and final charge positions are U1 and U2, respectively, the work done is

A ) e(U2 - U1) ) e∆U

(3)

where ∆U ) U2 - U1. Thus, the free energy of the CT reaction is

∆G ) ∆Go + e∆U

(4)

where ∆Go is the free energy of the CT in the absence of the external electric field. Under the conditions of a homogeneous dielectric medium between the electrodes, the potential difference is

∆U ) Ubias(d/D)

(5)

where D is the distance between the electrodes, Ubias is the applied bias voltage, and d is the charge displacement in the direction perpendicular to the electrode plane. Thus

∆G ) ∆Go + (d/D)eUbias ) ∆Go + aUbias

(6)

where a ) ed/D. The difference in the free energy caused by the bias voltage is relatively small, in the present study. The thickness, D, of a 21-layer film can be estimated7 to be 21 × 2.4 nm ≈ 50 nm, and a reasonable charge displacement could be 0.5-1 nm. Thus aUbias ≈ 0.01-0.02 eV, which is less than kBT (0.026 eV at room temperature). According to the ET theory,34 the rate of the electron transfer is given by

ket ) K exp[-(∆G + λ)2/4λkBT)]

(7)

where λ is the reorganization energy and K is a coefficient. Then

ket(Ubias) ) K exp[-(∆Go + λ + aUbias)2/4λkBT)] (8)

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Assuming a small contribution of the bias voltage (|aUbias| , |∆Go + λ|), one can obtain

ket(Ubias) ≈ K exp[-(∆Go + λ)2/4λkBT] × exp[-(∆Go + λ)aUbias/2λkBT] ) ket0 exp[-(∆Go + λ)aUbias/2λkBT] ≈ ket0[1 - (∆Go + λ)aUbias/2λkBT] ) ket0[1 - deUbias(∆Go + λ)/2DλkBT]

(9)

where ket0 ) K exp[-(∆Go + λ)2/4λkBT)] is the electron transfer rate at Ubias ) 0. Therefore, a reasonable presentation for the τ1 vs Ubias dependencies shown in Figure 9 is

τ1(Ubias) ) τ10(1 + RUbias)

(10)

where the factor R ) (ed/D)(λ + ∆Go)/2λkBT determines the sensitivity of the ET time constant to the external electric field, τ10 is the time constant at Ubias ) 0, and the condition |RUbias| , 1 was employed. The sensitivity factor, R, depends on (∆Go + λ)/λ. When -∆Go ) λ, the rate of ET is insensitive to the field. The sign of the effect of Ubias on τ1 depends on the sign of the (∆Go + λ) term. For the “normal” regime ∆Go + λ > 0, and the bias voltage applied in the direction of the ET reaction decreases the time constant of the reaction. For the “inverted” regime ∆Go + λ < 0, and the bias voltage has an opposite effect. In the present work the charge recombination rate was studied. The sign of the influence of the bias voltage on the recombination time constant indicates the normal regime, ∆Go + λ > 0. The values of ∆Go and λ are not well known for the system under investigation, and the estimation of the term (∆Go + λ)/λ cannot be done explicitly. Assuming a high reorganization energy (λ > |∆Go| or (∆Go + λ)/λ ≈ 1), which seems to be reasonable for the ET reaction in a rigid medium,35 one obtains

τ1(Ubias) ≈ τ10[1 + (d/D)eUbias/2kBT ]

(11)

or the sensitivity factor is R ≈ ed/2kBTD. Employing this approximation, one can estimate the CT distance. The experimentally determined value for the sensitivity was R ) 0.18 V-1 (20% sample, data shown in Figure 9), and assuming that D ) 50 nm (approximate thickness of the 21 layers of ODA), one can obtain the CT displacement d close to 0.5 nm in the direction perpendicular to the film plane. This value is the shortest possible for the CT distance for the following reasons: (1) the DA molecules can be tilted, (2) the interelectrode distance can be greater than the estimated film thickness, as the InGa liquid electrode might have an oxide layer on top, and (3) as far as ∆G is negative for the reaction to occur, (∆Go + λ)/λ < 1, and the influence of Ubias is overestimated in eq 11. However the value of d ) 0.5 nm seems to be reasonable for the CT distance for the PF compound. 4.5. Probability of the Electron Transfer. The knowledge of the charge displacement during the ET reaction allows one to estimate the CT efficiency. For the 20% sample the saturation voltage measured after the preamplifier was Uo ) 1.1 V. Thus the voltage created on the electrodes was Uin ≈ 20 mV (the gain factor is 50). The sample capacitance was C ) 100 pF, which gives for the displacement charge a value Q ) UinC ) 2 × 10-12 C. This charge was induced by the 0.5 nm electron shift inside the capacitor with the 50 nm interelectrode distance. Therefore, the total number of DA molecules in the CT state needed to create this charge is NCT ) (Q/e)(D/d) ) 1.25 × 109. The area of the electrode was roughly Sel ≈ 1 mm2, and the

density of the acting DA molecules was nCT ) NCT/Sel ≈ 10-3 nm-2. The total surface density of the PF molecules for the 20% film is n ) 0.6 nm-2. Thus the relative amount of DA molecules capable of performing the light-induced ET is ca. 0.2%. This is a surprisingly low value. Nevertheless, this value is in agreement with the studies of the PF dyad in solution.22 The PF compounds were studied by means of femto- and picosecond transient absorption and fluorescence techniques, and the CT state was observed only in polar solvents, e.g., benzonitrile. No evidence for the occurrence of the ET reaction in nonpolar toluene was found. The dielectric permittivity of the LB films made of fatty acids and related compounds is roughly 2.5.7 Thus, the films are usually treated as a nonpolar medium. From this viewpoint, the ET properties of the PF dyad in toluene and in the LB film should be similar. Obviously, if the probability of the ET reaction in toluene is 0.2%, the ET cannot be detected by the optical transient pump-probe method. The reason that only 0.2% of the DA molecules are capable of performing the light-induced ET reaction should be discussed. Apparently, the distribution of different PF conformations has to be considered. An important state in the excitation relaxation pathway is the exciplex state, (PF)*, which is the precursor of the CT state. The exciplex energy is lower than the energies of the locally exited states, P*F (1.85 eV) and PF* (1.73 eV). If the red fluorescence limit (840 nm) is used to estimate the energy of (PF)*, a value of 1.48 eV is obtained. If the energy of the CT state in the film is between 1.5 and 1.7 eV, the ET reaction is possible only from the locally excited state and its efficiency is low, because the exciplex formation is extremely fast, 0.9 ps in toluene.22 However, if for some reason the exciplex is not formed in some of the DA molecules (because of steric restrictions in the film, for example), the CT state may be formed instead. Evidently, that is a phenomenon with low probability. The low concentration of the active DA molecules opens channels for the intermolecular excitation energy transfer. The latter increases the quantum efficiency of the ET at high concentrations of PF. For example, photovoltage response of the 20% sample was 2 orders of magnitude higher than that of the 2% sample, whereas the density of the PF molecules for the 20% sample is approximately 5 times higher than that of the 2% sample. The concentration of 2% is probably the lower limit where the energy transfer takes place. Then the quantum efficiency for the ET for the 2% sample should be close to the relative amount of the molecules capable of undergoing ET; thus the efficiency is g0.2%. However, for the 20% sample the increase in the signal amplitudes suggests that the quantum efficiency of the ET is ca. 20 times higher than that for the 2% sample, being g4%. It is not clear which state is the most efficient for the intermolecular energy transfer. The average distance between the chromophores is roughly 1.3 nm (center-to-center) for the film with a PF concentration of 20%. Thus one can expect the time constant of the Fo¨rster-type energy transfer to be ∼5 ps between the phytochlorin-like molecules (the Fo¨rster critical radius is ∼4 nm, and the singlet state lifetime is 6 ns).27,36 In solutions the energy of the singlet excited state of the phytochlorin part of the dyad is transferred to the singlet excited state of its fullerene part, P*F f PF*, in less than 1 ps. This is shorter than the phytochlorin-phytochlorin energy transfer time. In the films, the fluorescence decay time of the P*F state at 670 nm was ca. 50 ps, which is long enough to allow tens of molecules to act as an antenna subsystem. The PF* state has a longer lifetime (∼30 ps in toluene and, presumably, ∼200 ps

Vectorial Electron Transfer in LB Films in the film), but the efficiency of the energy transfer between two fullerene units is much lower compared with that between phytochlorin units. The distance 1.3 nm is compatible with the chromophore size, and the non-Fo¨rster mechanism and/or the cluster-like chromophore assemblies may enhance the energy transfer rate gradually. The role of the exciplex in the ET process is not evident at the moment. In most cases, the ET reactions for the DA molecules involving a chlorin-like donor occur directly from the locally excited state.1,2 Consequently, an exciplex might be formed due to a specific phytochlorin-fullerene interaction, inducing an extra loss of energy prior to the charge transfer state. On the other hand, the fullerene unit plays an important role in the film formation properties of the PF dyad. We considered the PF dyad to be an attractive compound for the development of molecular optoelectronic devices utilizing the LB technique. 5. Conclusions The PF/ODA LB films perform vectorial photoinduced electron transfer, although the relative amount of the PF molecules undergoing the ET reaction is low, 0.2%. The lifetime of the intramolecular charge transfer state is ca. 30 ns, and it is independent of the concentration of the DA molecules. There is no evidence for the strong aggregation of the phytochlorin chromophores, typical of the LB films in rigid matrixes. Probably the fullerene moiety prevents the aggregation and plays an important role in the film formation properties of the PF compound. Acknowledgment. The work was supported by the Academy of Finland, the National Programme on Material and Structure Research, and the Technology Development Centre of the Nanotechnology Programme. References and Notes (1) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (2) Gust, D.; Moore, T. A. Topics in Current Chemistry; SpringerVerlag: Berlin Heidelberg, 1991; Vol. 159. (3) Naito, K.; Sakurai, M.; Egusa, S. J. Phys. Chem. A 1997, 101, 2350. (4) Gregory, P.; Patten, V.; Shreve, A. P.; Lindsey, J. S.; Donohoe, R. J. J. Phys. Chem. B 1998, 102, 4209. (5) Wang, Y.; Suna, A. J. Phys. Chem. B 1997, 101, 5627. (6) Tkachenko, N. V.; Hynninen, P. H.; Lemmetyinen, H. Chem. Phys. Lett. 1996, 261, 234. (7) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York and London, 1990. (8) Ulman, A. Chem. ReV. 1996, 96, 533. (9) Imahori, H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata Y. Bull. Chem. Soc. Jpn. 1999, 72, 485.

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