9930
J. Phys. Chem. B 2001, 105, 9930-9934
Photoinduced Charge Separation and Recombination Processes in Fine Particles of Oligothiophene-C60 Dyad Molecules Mamoru Fujitsuka, Akito Masuhara, Hitoshi Kasai, Hidetoshi Oikawa, Hachiro Nakanishi, Osamu Ito,* Takashi Yamashiro,† Yoshio Aso,† and Tetsuo Otsubo† Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai, 980-8577 Japan ReceiVed: April 26, 2001
Photoinduced charge separation and recombination processes in fine particles of octathiophene-C60 and dodecathiophene-C60 dyad molecules were investigated by the subpicosecond laser photolysis method. Sizes of the fine particle samples were estimated to be 60-140 nm using SEM observation and dynamic light scattering measurements. Charge separated states were generated within 1 ps upon the 150 fs laser excitation in both fine particle samples of the dyad molecules. Generation yields of the charge separated states were about a half of those of the corresponding dyad molecules in polar solvents, indicating the existence of the competitive deactivation pathways of the singlet excited states, such as exciton migration in each fine particle. On the other hand, the charge separated states deactivated via the two-step-decay processes. Direct charge recombination in a dyad molecule and indirect charge recombination after migration of radical cation (hole) and anion (electron) in fine particles were observed as fast and slow decay components, respectively. The faster charge recombination rates than those of previously reported for aniline-fullerene dyad cluster will result from high hole-mobility in the oligothiophene-moieties of the present dyad molecules composing fine particles.
Introduction
CHART 1
Recently, functionalizations of fullerenes with donor molecules have been attempted by several groups.1-7 For fullerene molecules linked with donor, fast charge separation (CS), and slow charge recombination (CR) have been reported.4c As for donors of the dyad molecules, aromatic amino compounds,2 carotenoid,3 porphyrins,4 tetrathiafulvalenes,5 oligothiophenes,6 and so on7 have been employed. These fullerene-donor linked molecules on the electrode exhibit excellent photovoltaic effects upon photoirradiation.4d,6d These interesting properties can be attributed to high efficiency of the CS process in the fullerenedonor linked molecules. 1-7 Thomas et al. reported the CS and CR processes in cluster of aniline-C60 dyad molecule.8 They showed that the decay lifetime of the CS state was as long as 60 µs, which resulted from the migration of the radical anion to the adjacent dyad molecules in the cluster. These studies on clusters are interesting since they give important information on the CS and CR processes in the crystalline phase, which would be expected to be much different from those in the isolated dyad molecules in solutions. Furthermore, studies on the CS and CR processes in the crystalline phase seem to be indispensable for design of the photoactive devices in which donor-acceptor linked molecules are aligned or packed densely like crystalline samples. In the previous papers, we reported that the photophysical and photochemical properties of C60-fine particles (FP), which were successfully generated by the reprecipitation method, were quite different from those of isolated C60 in solution: Migration and localization of the excited states are important processes governing the properties and reactivities of C60 in the FP.9 In † Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-hiroshima, 739-8527, Japan.
the present paper, we applied the reprecipitation method to oligotihiophene-C60 dyad molecules (nT-C60, Chart 1). The CS and CR processes of nT-C60 dyad molecules in solution have been studied by pico- and nanosecond laser flash photolysis methods in our previous reports.6b,c We found that the CS and CR processes in FP of nT-C60 dyad molecules were much different from those of previously reported cluster of the anilineC60 dyad molecules.8 Experimental Section Materials. Syntheses of dihexyltetrathiophene-C60 dyad (4TC60), tetrahexyloctathiophene-C60 dyad (8T-C60), and hexahexyldodecathiophene-C60 dyad (12T-C60) were described in the previous paper.6a Other chemicals were of the best commercial grade available. Apparatus. The subpicosecond transient absorption spectra were observed by the pump and probe method. The samples were excited with a second harmonic generation (SHG, 388 nm) of output from a femtosecond Ti:sapphire regenerative amplifier seeded by SHG of a Er-doped fiber laser (Clark-MXR CPA2001 plus, 1 kHz, fwhm 150 fs). The excitation light was depolarized. A white continuum pulse generated by focusing
10.1021/jp0115796 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/21/2001
Oligothiophene-C60 Dyad Molecules
J. Phys. Chem. B, Vol. 105, No. 41, 2001 9931
Figure 2. Absorption spectra of the (8T-C60)FP water dispersion (solid line) and 8T-C60 in benzonitrile solution (dot line).
Figure 1. (a) SEM image of (8T-C60)FP. (b) Size-distribution of the (8T-C60)FP water dispersion.
the fundamental light on a flowing H2O cell was used as a monitoring light. The visible monitoring light transmitted through the sample was detected with a dual MOS detector (Hamamatsu Photonics, C6140) equipped with a polychromator (Acton Research, SpectraPro 150). For detection of the nearIR light, an InGaAs linear image sensor (Hamamatsu Photonics, C5890-128) was employed as a detector. The spectra were obtained by averaging on a microcomputer. Nanosecond transient absorption measurements were carried out using a SHG (532 nm) of a Nd:YAG laser as an excitation source. The probe light from a pulsed Xe lamp was detected with a Ge-avalanche photodiode equipped with a monochromator after passing through the sample in quartz cell (1 cm × 1 cm). Sample solutions were deaerated by bubbling Ar-gas through the solutions for 15 min. Details of the transient absorption measurements were described in our previous papers.6b,c Steady-state absorption spectra in the visible and near-IR regions were measured on a Jasco V530 spectrophotometer. SEM observations were carried out using Hitachi S-900. The samples for the SEM observations were coated by Pt-spattering in order to avoid charge-up phenomena. Dynamic light scattering apparatus (Otsuka Electronics, DLS-7000) was used to estimate the size of the fine particles dispersed in water. Results and Discussion Preparation of Fine Particles and Ground-State Properties. Fine particle samples of nT-C60 (abbreviated as (nT-C60)FP) were prepared by the reprecipitation method:10 A THF solution of nT-C60 (1.0 mM, 0.2 mL) was injected using a syringe to 10 mL of water under continuous stirring at room temperature. A yellowish orange sample was obtained as a transparent water dispersion of (8T-C60)FP. The generation of fine particles was confirmed by SEM observation; an example of SEM pictures for (8T-C60)FP is shown as Figure 1a. Granular-type fine particles with ∼100 nm of the diameter were confirmed. It is interesting to note that C60 is also reported to form similar granular-type fine particles
(averaged diameter ) 270 nm), which is confirmed to have a crystalline structure.9 Furthermore, from the dynamic light scattering measurement of the (8T-C60)FP water dispersion, the averaged diameter was estimated to be 140 nm (Figure 1b), which is in good agreement with the results of the SEM observations (Figure 1a). As for 12T-C60, the reprecipitation method gave an orange transparent water-dispersion sample, for which similar granular-type fine particles were confirmed by the SEM observations. The averaged diameter of (12T-C60)FP was estimated to be 120 nm from the dynamic light scattering measurement. On the other hand, 4T-C60 formed FP at such low concentration that transient absorption study of (4T-C60)FP did not give reliable data. The averaged diameter of (4T-C60)FP was estimated to be 62 nm, which was smaller than the average diameters from other FP samples. This finding will indicate that the insufficiently hydrophobic oligothiophene-moiety of 4T-C60 results in an unstable FP formation. Figure 2 shows an absorption spectrum of water dispersion of (8T-C60)FP as well as that of the dyad molecule in benzonitrile solution. The absorption spectrum of 8T-C60 in benzonitrile showed a clear absorption band at 450 nm, which can be attributed to the 8T-moiety of 8T-C60, as well as a characteristic absorption band of the C60-moiety at 704 nm. The absorption spectrum of 8T-C60 was interpreted from the superposition of absorptions of 8T- and C60-model compounds, indicating that the interaction between the 8T- and C60-moieties is small in the dyad molecule in the solution. In the case of the absorption spectrum of the water dispersion of (8T-C60)FP, on the other hand, the absorption band became broad and the peak showed a red shift to 460 nm compared to that of the 8T-C60 molecule in benzonitrile. Similar band-broadening and red-shift of the absorption peak were also observed with (12T-C60)FP: the 12T-C60 molecule in benzonitrile showed an absorption peak at 457 nm, while (12T-C60)FP showed a peak at 463 nm. Since oligothiophenes have a stiff backbone, interaction between the oligothiophene- and C60-moieties seems to be weak in the oligothiohene-C60 dyad molecules. On the other hand, in FP sample, the oligothiophene-moiety of the dyad molecule is considered to be located at the closest position of the C60-moiety of other dyad molecules, for which the substantial interactions are expected. These interactions would result in the red-shift and broadening of the absorption bands of the FP samples, as in the case of the absorption spectra of the C60-doped oligothiophene/polythiophene films.11 Photoinduced Charge Separation and Recombination Processes. A weak fluorescence band due to the 8T-moiety was observed around 545 nm for (8T-C60)FP upon excitation. In the case of (12T-C60)FP, a weak fluorescence band appeared around 570 nm. These fluorescence bands are located at somewhat
9932 J. Phys. Chem. B, Vol. 105, No. 41, 2001
Figure 3. (a) Transient absorption spectra of the (8T-C60)FP water dispersion observed by 388 nm laser excitation. (b) Absorption-time profiles at 850 nm of (8T-C60)FP (solid line) and 8T-C60 in benzonitrile (dot line) after laser pulse (Y-axis: 0.05/div). (c) Absorption-time profile at 850 nm of (8T-C60)FP after laser pulse (Y-axis: 0.02/div).
longer wavelength-sides compared to the corresponding oligomers in solutions (542 and 560 nm for 8T and 12T, respectively). The fluorescence lifetimes were estimated to be 1012 s-1 in FP. In the case of 8T-C60 molecule in benzonitrile, the CS state showed a rise for a time over 10 ps after initial quick rise, in
Fujitsuka et al.
Figure 4. (a) Schematic energy diagram for CS processes of (8TC60)FP. Numbers indicate energy levels in eV unit relative to the ground state. (b) Supposed intra- and intermolecular CS processes in the FP samples.
Figure 5. (a) Transient absorption spectra of (12T-C60)FP water dispersion observed by 388 nm laser excitation. (b) Absorption-time profiles at 870 nm of (12T-C60)FP (solid line) and 12T-C60 in benzonitrile (dot line) after laser pulse (Y-axis: 0.05/div). (c) Absorption-time profile at 870 nm of (12T-C60)FP after laser pulse (Y-axis: 0.02/div).
which the initial quick rise is due to absorption of 18T*-C60 (Figure 3b): The generation rate constant of 8T•+-C60•- in benzonitrile was estimated to be 1.1 × 1011 s-1. It should be noted that the absorption band due to the CS state of (8T-C60)FP is small when compared to that of the 8T-C60 molecule in benzonitrile (Figure 3b), even though ground-state absorbance
Oligothiophene-C60 Dyad Molecules
J. Phys. Chem. B, Vol. 105, No. 41, 2001 9933
TABLE 1: Free-Energy Changes and Rate Constants of the CS and CR Processes of (nT-C60)FP and NT-C60 (n ) 8, 12) dyad (8T-C60)FP (12T-C60)FP 8T-C60b 12T-C60b
solvent water water benzonitrile benzonitrile
-∆GCS/eVa
kCS/s-1
0.91 0.95 0.88 0.92
>10 >1012 1.1 × 1011 1.6 × 1011 12
-∆GCR/eV a
kCRfast/s-1
kCRslow/s-1
1.33 1.25 1.36 1.28
3.5 × 10 3.6 × 1011 2.9 × 1010 2.0 × 1010
2.9 × 107 4.4 × 107 2.2 × 106 1.9 × 106
11
2 + + a -∆G CR ) Eox - Ered + ∆Gs, -∆GCS ) ∆E0-0 - (-∆GCR), ∆Gs ) e /(4π0)[(1/(2R ) + 1/(2R ) - 1/Rcc)(1/s) - (1/(2R ) + 1/(2R ))(1/r)] where ∆E0-0 is the energy of the 0-0 transition of nT; Eox and Ered are the first oxidation potential of nT (0.76 and 0.67 V vs Ag/AgCl for 8T-C60 and 12T-C60, respectively) and the first reduction potential of C60 in benzonitrile (-0.63 V for both dyads), respectively. 0 is vacuum permittivity. R+ and R- are radii of 8T (14.5 Å) or 12T (21.6 Å) and C60 (4.2 Å), respectively, Rcc is center-to-center distance between the two moieties (19.0 and 26.6 Å for 8T-C60 and 8T-C60, respectively); s and r are static dielectric constants of solvents used for the rate measurements and the redoxpotential measurements. b The rate constants for the CR processes were cited from ref 6c.
of the (8T-C60)FP and 8T-C60 samples was matched at the excitation wavelength (388 nm). The smaller absorbance of (8T•+-C60•-)FP can be attributed to the existence of competitive deactivation pathway(s) in the singlet excited FP samples such as the exciton migration. On the basis of the fact that 8T-C60 molecule forms the CS state at 0.99 of quantum yield,6c the generation yield of the CS state of (8T-C60)FP was estimated to be 0.56, indicating that the exciton migration in (8T-C60)FP also occurs within 1 ps. In the case of (12T-C60)FP, the CS state was also generated within 1 ps upon laser excitation, which was confirmed by the absorption band at 870 nm due to the radical ion pair (12T•+C60•-)FP (Figure 5a). This finding indicates that the rate constant for the CS process in (12T-C60)FP is also >1012 s-1, while the 12T-C60 molecule generates the CS state in benzonitrile with 1.6 × 1011 s-1 of the rate constant (dot line in Figure 5b). The fast CS processes in FP samples will indicate the contribution of intermolecular CS processes in FP (Figure 4b), since the C60and oligothiophene-moieties will be at a closer position in FP than the dyad molecule in solution: While the center-to-center distances of the oligothiophene- and C60-moieties of 8T-C60 and 12T-C60 dyad molecules were estimated to be 19.0 and 26.6 Å, respectively,6c a short center-to-center of 6.5 Å distance is expected for the dyad molecules in FP at the minimum. The short distance between the oligothiophene- and C60-moieties in the FP samples is supported by the fact that the ground-state absorption spectra showed spectral changes due to the strong interaction between them as indicated above. The effect of solvent polarity is considered to be small in the comparison of (nT-C60)FP in H2O with nT-C60 molecule in benzonitrile, since both solvents are highly polar; thus, the difference of free-energy changes for the CS processes in H2O and in benzonitrile is quite small (
