9896
J. Phys. Chem. C 2008, 112, 9896–9902
Photoinduced Electron Transfer in Langmuir-Blodgett Monolayers of Double-Linked Phthalocyanine-Fullerene Dyads Heli Lehtivuori,*,† Tatu Kumpulainen,† Alexander Efimov,† Helge Lemmetyinen,† Aiko Kira,‡ Hiroshi Imahori,‡,§ and Nikolai V. Tkachenko† Department of Chemistry and Bioengineering, Tampere UniVersity of Technology, P.O. Box 541, 33101 Tampere, Finland, and Department of Molecular Engineering, Graduate School of Engineering, and Institute for Integrated Cell-Material Sciences, Kyoto UniVersity, Kyotodiagaku-katsura, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed: March 28, 2008; ReVised Manuscript ReceiVed: April 17, 2008
Photoinduced electron transfer (ET) in films of the double-linked free-base phthalocyanine-fullerene dyads (Pc-C60) was studied in solid Langmuir-Blodgett (LB) films using femtosecond pump-probe and microsecond flash-photolysis methods. Excitation of the phthalocyanine (Pc) moiety of the dyad results in a rapid (0.8 ps) intramolecular ET from phthalocyanine to fullerene. The formed charge separated (CS) state relaxes in 35 ps, yielding an intermolecular CS state, which relaxes to the ground state in the microsecond time domain. The surface organization of the Pc-C60 dyad monolayers was studied by atomic force microscopy for both pure Pc-C60 films and Pc-C60 mixed films with matrix molecules, octadecylamine (ODA). This study together with the estimations done based on surface pressure-mean molecular area isotherms reveals that the dyads tend to aggregate rather than form a homogeneous mixture with the ODA molecules. This enforces intermolecular interactions and leads to the formation of a long-lived intermolecular CS state. Introduction For the past decade, the chemical and physical properties of porphyrin-fullerene dyads have been under active investigation1–7 owing to their promising applications in photovoltaic devices, such as solar cells. A great number of donor-acceptor (DA) dyads have been synthesized and studied by using porphyrins as donors and fullerenes as acceptors. Phthalocyanines,8–10 being structurally and chemically related to porphyrins and having an advantage of a higher absorption at the red part of the spectrum, are much lesser studied as electron donors in DA systems.11–15 The covalently linked phthalocyanine-fullerene (Pc-C60) dyad compounds were described first by Hirsch et al.16 However, the electrochemical investigation of the dyads did not confirm any charge transfer from Pc to C60. An intermolecular electron transfer (ET) between fullerenes and phthalocyanines was reported by Ito et al.8 In covalently linked DA systems in solutions, the charge transfer states with lifetimes of 3 ns have been reported for the first time by Guldi et al.17 Nevertheless, for an effective use of such molecules in photovoltaic applications, a longer charge separated (CS) state lifetime is necessary in the solid state to avoid recombination and to increase the chances of the positive and negative charges to reach the electrodes.12,18–20 A traditional approach to DA systems is based on connecting the phthalocyanine and fullerene by a single linker.12,13,17,19 Double-linked Pc-C60 dyads are attractive as materials for photovoltaic applications, since they allow intimate contact between the electron donor, phthalocyanine, and the electron acceptor, fullerene, to promote an efficient charge separation. Knowledge of Pc-C60 excited electronic states and relaxation * To whom correspondence should be addressed. E-mail: heli.lehtivuori@ tut.fi. † Tampere University of Technology. ‡ Department of Molecular Engineering, Kyoto University. § Institute for Integrated Cell-Material Sciences, Kyoto University.
processes in films is needed for understanding their functions. In this study, the photophysical characterization of a free-base double-linked Pc-C60 dyad in the solid state is carried out, pointing out the occurrence of photoinduced ET. Observed longliving intermolecular CS states (lifetime in microseconds) in films opens a possibility to collect the charges at suitable electrodes and to use such DA systems in organic photovoltaic applications. The Langmuir-Blodgett (LB) technique is a well-known method to construct oriented multilayer thin film structures.21 In this paper, the LB film formation properties of double-linked free-base Pc-C60 dyads are described in detail. The photochemical properties of the dyad films were studied by using femtosecond pump-probe,22 microsecond flash-photolysis, and nanosecond time-resolved Maxwell displacement charge (TRMDC)23,24 methods. A long-living intermolecular photoinduced ET was found in solid LB films of Pc-C60 dyads. Experimental Section Materials. Chloroform and toluene of analytical grade (Merck) were used for solution preparation. Octadecylamine (ODA) was of 99% grade (Sigma). The double-linked freebase phthalocyanine-fullerene dyad, referred to as Pc-C60 (see Figure 1), was synthesized as described elsewhere.15 A stock solution of Pc-C60 films was prepared by dissolving the compound in chloroform (0.3 mM). The mixed films were prepared from ODA and dyad chloroform solutions at concentration of 1 mg mL-1. For spreading solutions, the concentrations of compounds were equal to or less than 1.0 mM. Film Preparation. A KSV minitrough and LB 5000 apparatus (KSV Instrument Ltd., Helsinki, Finland) with areas of 240 and 735 cm2, respectively, were utilized for recording the surface pressure-mean molecular area (mma) isotherms and for film depositions. A phosphate buffer (0.5 mM Na2HPO4 and 0.1 mM NaH2PO4 in ion-exchanged MilliQ water with pH 7)
10.1021/jp8026918 CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
Photoinduced ET in Double-Linked Pc-C60 Films
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9897
Figure 2. Isotherms of 100, 40, 30, 10, and 0 mol % Pc-C60 dyad in an ODA matrix. The inset shows Pc-C60 limiting areas as a function of dyad fraction in the ODA matrix. Figure 1. Double-linked free-base phthalocyanine-fullerene dyad.
was used as the subphase. Films for spectroscopic studies were deposited onto 1 or 0.5 mm thick quartz substrates which were cleaned by using a standard procedure.21 For AFM measurements, glass slides were used. Glasses were cleaned first in chloroform for 30 min in an ultrasonic bath and then were left in NaOH (0.001 M) solution overnight. For the photoelectrical measurements, glass slides covered by a semitransparent indium tin oxide (ITO) electrode with a sheet resistance of approximately 10 Ω per square were used. The glass slides with ITO electrodes were cleaned in an ultrasonic bath first in acetone and then in chloroform. Eleven or twelve layers of ODA were deposited onto the ITO slides to prevent interactions between the active dyad layers and the ITO electrode. After the active layer deposition, 20 ODA layers were deposited onto the sample in order to prevent interactions between the active dyad molecules and the InGa electrode. Atomic Force Microscope (AFM) Measurements. AFM measurements were carried out using a Digital Instrument Nanoscope III instrument (Digital Instruments, Santa Barbara, CA) in the tapping mode. All AFM experiments were done using Veeco NanoprobeTM tips (model RTESP). The scan rate for the samples was 2.0 Hz, and the resonance frequency for the tips was 250 kHz. Spectroscopic Measurements. The steady-state absorption spectra of dyad mono- and multilayers were recorded by using a Shimadzu UV-3600 spectrophotometer. The pump-probe method was used to measure transient absorption spectra in a sub-picosecond-nanosecond time domain. The measurements were carried out using the instrument described previously.22 In brief, the second harmonic (420 nm) of the amplified Ti:sapphire mode-locked pulses (50 fs) was used for the excitation. A white continuum generated by the pulses at a fundamental wavelength (840 nm) was used to form the probe and reference beams. The system operated at a 10 Hz repetition rate, and 100 pulses were averaged at each delay time to improve the signal-to-noise ratio (S/N). The typical time resolution of the instrument was 150 fs (FWHM). The transient spectra were recorded by a charge-coupled device (CCD) detector coupled with a monochromator in the visible and near-infrared (NIR) ranges. The total internal reflection measurement method was used for the flash-photolysis of thin films in the nano- to microsecond time scale. For this purpose, the samples were deposited on 0.5 mm thick quartz plates with the side faces polished at 45°. The
monitoring light entered and exited the plate through the side faces polished at 45°. The monitoring beam propagated in a 0.5 mm thick quartz plate at an incident angle ∼45° to the plate plane, which was higher than the critical angle for the total internal reflection. The beam was reflected around 30 times from the plate surface covered by the studied layers. With this method, it is possible to obtain transient absorption spectra from samples with only one or few LB monolayers. The absorption of the 100 mol % LB monolayer of Pc-C60 at the Q-band was 0.015 (see the Supporting Information), but 30 reflections increase the efficient absorption by 1.5 orders of magnitude. Laser flashphotolysis experiments were carried out in a modified Luzchem laser flash system (mLFP-111, Luzchem Co.) using a Ti:sapphire laser (pumped by the second harmonic of a Nd:YAG laser), which provided 10 ns pulses at 400 nm (second harmonic) for excitation. The maximum excitation power density was ∼2 mJ cm-2. A continuous Xe lamp (Oriel Simplesity Arc Source) was used as a monitoring light. The monitoring beam intensity was recorded by using a digitizing oscilloscope (Tektronix, TDS3032B, 300 MHz). The system was controlled by a personal computer. The sample box was deoxygenated by nitrogen purging for 30 min prior to measurements. All the measurements were carried out at room temperature. Transient Photovoltage Measurements. The photoinduced electron transfer in the films was studied with the time-resolved Maxwell displacement charge (TRMDC) method as described elsewhere.23,24 In brief, the samples were excited by 10 ns pulses at 725 nm generated by a Ti:sapphire laser pumped by the second harmonic of a Q-switched Nd:YAG laser (532 nm). The instrument time resolution was 10 ns. The photoactive layers were insulated from the electrodes, and therefore, the measured TRMDC signals are due to the photoinduced electron displacement inside the dyad layer in the direction perpendicular to the plane of the film.20,24–27 Results Film Properties. The Langmuir film surface pressure-mma isotherms were measured for the 100 mol % dyad films and for the mixed films in an ODA matrix. The isotherms are presented in Figure 2. The isotherm of the pure dyad monolayer has a smooth start of the pressure rise and a gentle collapse. The obtained limiting area, which was extrapolated from the sharpest part of the isotherm curve to the zero surface pressure, is 83 Å2. The mean molecular area obtained for 100 mol % Pc-C60 is much smaller than could be expected for the dyad on the
9898 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Figure 3. Absorption spectra of 1, 19, 43, 71, 99, and 127 layers of the 10 mol % Pc-C60 dyad in an ODA matrix on a quartz plate (solid lines) and 15 µM Pc-C60 in toluene (dashed line). The inset shows the absorption maxima at 690 nm as a function of the number of layers. The linear increase indicates that equal amounts are deposited in each deposition cycle.
water surface.20,24 The mean molecular areas for the mixed films as a function of the dyad fraction are shown in the inset of Figure 2. The data were approximated by a linear dependence, which yields limiting areas of 84 and 18 Å2 for the 100 mol % dyad and ODA, respectively. From this dependence, one can estimate the relative coverage of dyads to be 34% for the 10 mol % layer. The compressing rate used for the Langmuir film was 15 cm2 min-1, and the 100 mol % Pc-C60 monolayer films were deposited at surface pressure of 20 mN m-1. The transfer ratios in the emersion were ∼1. The absorbance of the 100 mol % Pc-C60 LB monolayer at the Q-band was 0.015 (film on one side of the glass plate; see the Supporting Information, Figure 1), which was sufficient for flash-photolysis measurements with the total internal reflection method. Fast photoinduced processes taking place in solid films can be investigated using the femtosecond pump-probe method. This, however, requires samples with an absorbance of 0.5-1.0, which is by many orders of magnitude higher than the absorbance of an LB monolayer. The deposition of 100 mol % Pc-C60 multilayers on a solid substrate was not possible by using the LB method, because the transfer ratio was too low already for the second layer. To achieve multilayer deposition, dyads were mixed with supporting matrix molecules, ODA. The 10 mol % Pc-C60 multilayer films were prepared at a compressing rate of 5 mm min-1 and deposited at a surface pressure of 25 mN m-1. The transfer ratios in emersion and immersion were roughly 1. Multilayer LB film deposition of 10 mol % Pc-C60 was found to be possible for up to more than 100 layers without any noticeable drop in the optical quality of the film. Absorption spectra of the films at the 10 mol % concentration and in toluene are presented in Figure 3. The maximum absorbance of the films at the Q-band is roughly 0.8 (films on both sides of the glass plate). For the pump-probe measurements, the film can be only on one side of the plate. Therefore, the film was washed away from the other side of the plate. The absorbance of the sample used for pump-probe measurements was thus 0.4 at the Q-band maximum. The absorption maximum of the multilayer sample is at 350 nm. The absorption in the UV region is mainly due to C60 and to the Soret band of the phthalocyanine. In films, the relative intensity of this band is 2 times higher than that of the Q-band,
Lehtivuori et al. while in solution they are about equal. For the free-base phthalocyanine in solution, the Q-band at 710 nm is split in two parts. In films, the Q-band is broadened and can be presented as a sum of two Gaussian bands at 675 and 724 nm, having 55 and 19 nm bandwidths, respectively (see the Supporting Information, Figure 1). The absorption maximum of the Q-band is also shifted to the blue. The blue-shift and broadening of the Q-band in thin films arise from aggregation, as predicted by the molecular exciton model for dyes in close proximity.28–30 AFM Studies of the LB Monolayers of Pc-C60. AFM measurements in air at room temperature were performed for LB monolayers on glass plates (Figure 4). For the microscopy measurements, the dyad monolayers with concentrations of 10 and 100 mol % were deposited via the LB method onto glass substrates. The deposition conditions were the same as those previously described. AFM images of the Pc-C60 dyad with two different concentrations in films are presented in Figure 4. The image of the 100 mol % Pc-C60 (Figure 4b) shows more uniform surface morphology on the glass plate than that of the 10 mol % Pc-C60 (Figure 4a). Presumably, relatively smooth dark areas in Figure 4a are the regions filled by a monolayer of ODA molecules exclusively, whereas the lighter “bubblelike” areas are the regions occupied by dyads mostly. The topology of these regions resembles that of the 100 mol% Pc-C60 layer (Figure 4b). The ratio of areas covered by two different types of molecular aggregates in Figure 4a correlates with the coverage calculated based on the isotherm measurements. For the 10 mol % Pc-C60, the coverage obtained from the AFM image (lighter “bubblelike” area) is 30%, while the calculated coverage from the isotherms is roughly 34%. It seems that the reason for this island formation is the strong attractive interaction between the dyads that exceeds by far their attractive interaction with the ODA. In Figure 4a, bright dyad aggregates consist of small domain structures of 70 ( 20 nm size, whereas aggregates of 100 mol % Pc-C60 exhibit partial domain structures of 50 ( 10 nm size (Figure 4b). In conclusion, the AFM image of the 100 mol % film shows single type of arrangement, whereas the 10 mol % film has clear phase separated structure; that is, the phase separation occurs in mixed dyad/ODA films. However, the packing inside the dyad domains seems to be the same for both films. Time-Resolved Absorption. Pump-Probe. In the pumpprobe measurements, a sample was excited at 420 nm and timeresolved spectra were collected in the wavelength ranges from 500 to 770 nm and from 840 to 1080 nm (laser fundamental is at 800 nm). The pump-probe measurements were first carried out at different excitation energies in the range 0.2-1.1 mJ cm-2 (see the Supporting Information, Figure 2). The decays with higher excitation densities have a clear fast component (
