12072
J. Phys. Chem. 1995, 99, 12072-12075
Ultrafast Electron-Transfer and Recombination Processes on Copper Phthalocyanine SoliWater Interface As Revealed by Picosecond Regular Reflection Spectroscopy Musubu Ichikawa, Hiroshi Fukumura, and Hiroshi Masuhara" Department of Applied Physics, Osaka University, Suita, Osaka 565, Japan Received: March 7, 1995; In Final Form: May 16, 1995@
Picosecond absorption spectra of the copper phthalocyanine solidwater interface were obtained by analyzing transient regular reflection spectra, on which photoexcitation relaxation dynamics at the interface was elucidated. Photoelectron ejection from the phthalocyanine solid into water was induced rapidly, and the formed phthalocyanine cation-electron geminate pair recombined with a time constant of 10 ps. The vibrationally excited, electronically ground state was produced by dissipation of electronic energy through the recombination.
Introduction Studies on photodynamics of solifliquid interfaces have received much attention, because of its fundamental importance and high potential for future applications such as solar energy conversion and photocataly~is.'-~ It is very important to investigate photophysical and photochemical primary processes occurring on the interfaces, for example, interfacial electron transfer (ejection) and the following recombination, decomposition, and dissociation of adsorbed molecules, and so on. For this purpose, time-resolved transient UV-visible absorption spectroscopy is considered to have a high potential and be useful, since the technique can detect electronic structural changes of excited state accompanied with conformational changes, identify produced transient species, and determine rate constants of relaxation p r o c e ~ s e s . ~ Particularly, -~ in the case of organic solifliquid systems, taking into consideration of the variety of transient species produced by photoexcitation, measurement of transient absorption spectra is indispensable and fruitful for elucidation of photodynamics of the interfaces. Recently, we have developed a new time-resolved spectroscopic system for measurement of transient absorption spectra of organic solids, the picosecond regular reflection spectroscopic ~ y s t e m .As ~ one of the advantages of this system, analysis of reflection spectra with the Kramers-Kronig transformation provides the transient extinction coefficient change (Ak) spectra of organic solid surfaces and solifliquid interfaces in the picosecond time region, where the Ak spectrum corresponds to the transient absorption difference one. We have already reported interesting photodynamics of phthalocyanine solids by the picosecond regular reflection ~pectroscopy.~ Under vacuum, photophysical primary process in the compressed pellet of copper phthalocyanine p-form (p-CuPc) microcrystalline powder is controlled by exciton-exciton annihilation and the large amount of the excitons created by photoexcitation decays through the mutual annihilation immediately after excitation. Namely, the photoexcitation energy is rapidly converted to thermal energy, and elevation of local temperature was proved by measuring the hot band due to the vibrationally excited, electronically ground state.7 Furthermore, excitation energy migration in organic solids can be directly observed by the picosecond regular reflection spectroscopy system combined with polarized light. We discussed the photothermal conversion dynamics in terms of excitation energy migration process.s
* To whom all correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, July 15, 1995.
Another potential application of the time-resolved regular reflection spectroscopy is to investigate photodynamics at solid liquid interface; however, no such report has been given as far as we know. In the present paper, we have studied picosecond photodynamics of a p-CuPc solidwater interface. It is clearly demonstrated that electrons are ejected into water from the p-CuPc solid and the Pc cation-electron geminate pair is formed. Furthermore, its recombination results in a temperature elevation providing the hot band.
Experimental Section A compressed pellet of p-CuPc was prepared for the regular reflection spectroscopy as described elsewhere.' Water was purified by deionization and distillation. Methylviologen (MV2+)dichloride hydrate (Tokyo Chemical Ind.) was purified by several times recrystallization from methanol. The refractive index and extinction coefficient spectra of the p-CuPc pellet in the electronic ground state were measured by the regular reflection spectroscopic apparatus and analyzed by the Avery meth~d.~ The picosecond regular reflection spectroscopy system and the analysis method of transient reflectance change (r)spectra have been described in detail el~ewhere.~ A third harmonic of Nd3+:YAG laser was used for excitation. A picosecond whitelight continuum as the probe light was incident on the sample pellet at 1.4" to the normal, whose setup was regarded as normal incidence. The sample pellet was placed in a rectangular quartz cell of 2 mm path length filled with water or MV2+ aqueous solution, and then the liquid was bubbled with nitrogen gas. The reflected light from the pellevwater interface was distinguished from the reflection at the cell surface by tilting the pellet to the cell. The transient extinction coefficient change (Ak) spectrum, which corresponds to a transient absorption difference spectrum, was obtained from the r spectrum using the Kramers-Kronig transformation. At the analysis of the r spectrum, refractive index of water was assumed to a constant of 1.333.1° All measurements were performed at room temperature (23 "C).
Results and Discussion The time-resolved extinction coefficient change (Ak) spectra of p-CuPc pellevwater system at the excitation intensity of 1.4 mJlcm2 are shown in Figure 1. The positive Ak peak around 530 nm and the negative Ak peaks due to the ground-state depopulation around 600 and 700 nm were observed immediately after excitation. Corresponding to the decay of Ak
0022-365419512099-12072$09.00/0 0 1995 American Chemical Society
Letters
J. Phys. Chem., Vol. 99, No. 32, 1995 12073
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Figure 1. Time-resolved extinction coefficient change (Ak) spectra of p-CuPc pelledwater system at the excitation intensity of 1.4 mJ/ cm2. Transient spectrum (dotted line) of p-CuPc under vacuum at 0 ps after excitation at the excitation intensity of 1.5 mJ/cm2.
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Figure 2. Responses of transient extinction coefficient change (Ak) of p-CuPc pelledwater system at 530 (0)and 760 nm (0). peak at 530 nm, the Ak band at 670 and 760 nm rose and the spectral shape of the bleaching component was changed. The decay time of late components was long. As seen in Figure 2, the fast decay at 530 nm corresponded to the rise at 760 nm; thus, the late component was formed from the primary transient species. First, we discuss an assignment of the short-lived transient species with Ak maximum at 530 nm observed in water. The transient spectrum at 0 ps after excitation is compared to transient one obtained under vacuum, where the delay time is the same and the excitation intensity is almost equal (1.5 d l cm2). The spectrum under vacuum (the dotted line Figure 1) was already assigned to absorption from the exciton state of the p-CuPc solid.7 As seen in Figure 1, there are some differences between the spectra in water and under vacuum, namely, the Ak peak around 530 nm in water was slightly blue shifted and the shape of bleaching bands around 550-730 nm was different. Furthermore, it was already reported that the Ak band at 530 nm under vacuum, the exciton decayed through
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Figure 3. Excitation intensity dependence on response of transient extinction coefficient change (Ak) of B-CuPc pelledwater system at 530 nm. The solid and open circles represent the Ak responses at the excitation intensity of 1.4 and 0.5 mJ/cm2,respectively. exciton-exciton annihilation under this excitation condition, namely, the response of Ak at 530 nm under vacuum was dependent on excitation inten~ity.~Accordingly, excitation intensity dependence of the response was also examined in the present solidwater system; however, the normalized time profiles at 530 nm (at the excitation intensity of 1.4 and 0.5 d / c m 2 ) were the same as presented in Figure 3. Thus, the Ak response is independent of excitation intensity in the B-CuPc/ water system, and we should consider that a different transient species was formed on the surface of the B-CuPc solid by photoexcitation in water. According to the literature," one possible transient state with absorption at 530-540 nm is Pc cation. In this literature, all the Pc chromophore in film were oxidized chemically in solution and its absorption maximum was measured to be 570 nm. It is worth noting that this reference spectrum has nothing to do with bleaching of the neutral Pc; however, taking into the bleaching in the present spectra, the Ak maximum of Pc cation could appear around 530 nm. Furthermore, it is considered that the ionization of Pc solids is possible. Actually, ionization potential of CuPc chromophore in vacuum is about 5.05 eV, while the excitation energy (355 nm) is 3.49 eV. The difference will be canceled by solvation energy of electron by water and a Coulombic interaction in the geminate pair of the charged surface and solvated electron. Hence, the Ak peak at 530 nm can be assigned to the Pc cation, and we have come to consider that photoelectron ejection takes place at the interface. If photoelectron ejection leading to formation of the Pc cation occurs at the interface, a counter-charged species, which is an aqueous electronI2 or a Pc anion,I3 must be produced. Unfortunately, we could not detect supporting transient spectra. To examine electron ejection leading to formation of Pc cation, we measured transient Ak spectra of P-CuPc/water system by adding MV2+ (0.7M) as an electron scavenger. The spectral shape and its time evolution of this system was independent of concentration of MV2+ and did not show drastic changes compared to the system without the electron scavenger. Hence, it is concluded that the photophysical and photochemical primary process of this system was not changed, and MV2+ acts only as an electron scavenger. The Ak spectrum of the p-CuPcIH20 with MV2+ system at 330 ps after excitation and that of the p-CuPclH20 system at the same delay were presented in Figure 4 without normalization, and a difference spectrum, which was
Letters
12074 J. Phys. Chem., Vol. 99, No. 32, 1995
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Figure 4. Transient extinction coefficient change (Ak) spectra of p-CuPc pellet/MV2+(0.7 M)/water system (solid line) and p-CuPc pelledwater system (dashed line). The difference spectrum (dotted line) was obtained by subtracting the solid line the dashed line. The delay time is 330 ps after excitation.
obtained by subtracting the Ak spectrum with MV2+ from that without MV2+, was also shown in Figure 4. As seen in the Figure 4, their spectra were slightly different only around 500600 nm. The first electronic transition band of MV2+ in water starts from 336 nm; hence the probe and excitation (355 nm) lights are not absorbed by MV2+. Furthermore, we confirmed that reactions tuming out permanent products did not occur.14 According to ref 15, on the contrary, the absorption band of methylviologen monocation radical (MV’+) in water appears around 600 nm, and the two characteristic peaks come into view at 550 and 605 nm. Despite the weak signal, the difference spectrum shown in Figure 4 clearly shows two peaks at 550 and 610 nm and is in good agreement with an absorption spectrum of MV’+ in water. Thus, we consider that photoejection of electron at the /3-CuPclwater interface is confirmed spectroscopically. The decay at 530 nm did not depend on concentration of MV2+ (from 0.1 to 0.7 M) as described above. Therefore, it was hardly believed that a direct electron transfer from Pc to MV2+ occurred at the interface. A large part of ejected electrons recombined with the Pc cation rapidly as seen in Figure 3. Hence, the detected MV’+ could be generated only by scavenging the electron escaping from the recombination, and we conclude that the electron is directly ejected infowafer. This is the first demonstration that electron ejection into water and formation of CuPc cation occurred on the Pc/water interface upon photoexcitation. Decay dynamics of the cations, as shown in Figure 3, was independent of the initial surface density of cation (concentration); hence the decay process was a unimolecular one. There are two explanations for the excitation-intensity-independent decay: (1) The cation formed a geminate pair with the ejected electron, and the decay is caused by recombination of the pair competing wi€h dissociation giving a solvated electron. ( 2 )The ejected electron does not have a specific interaction with the parent p-CuPc cation (formation of the geminate pair), and disappearance of the cation band results from diffusion of its plus charge (hole) into the bulk CuPc solid followed by trapping in the defect. In case 1, the surface density of the hole (N) at time instance t is given by
N(t) = NOexp(-k,,f)
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Figure 5. Response of transient extinction coefficient change (Ak) of p-CuPc pelledwater system at 530 nm. Solid and dotted curves are simulated, which are computed with impulse response as single exponential (solid line) and as error function (dotted line). The solid curve is calculated with the rate constant of 0.1 ps-I, while the dotted curves are done with the diffusion coefficient of 1, 10, lo2, lo3, lo4, and lo5 nm2/psin ordering belong the arrow.
where NOis the initial density, kg represents the recombination rate constant of the cation-electron geminate pair, which is assumed to be time independent. On the other hand, in case 2 the decay originates from the hole diffusion, hence the cation distribution function R(x,t) is the solution of the diffusion equation with a photogeneration term of the cation, where x is the distance from the surface to the bulk. However, we have no information of depth dependence of the cation generation term at the present stage. Consequently, the generation term was ignored and initial condition was first assumed to R(x,t=O) = d(x) at this time. Then, the “detectable” cation density is given byI6
~ ( t=)N , erf[’/,a(~t)-’’~l
(2)
where D is the diffusion coefficient of the hole, which is assumed to be a constant, and a is the effective probing depth, which represents 13/4nIAI = 30 nm at 530 nm, where A is the complex refractive index at the wavelength. The experimental Ak response was convoluted with the pump (Zp)and probe (Is) pulses. That is
where A is the scaling factor. The respective fitting results were shown in Figure 5, where both pulse widths were assumed to be the Gaussian function of 25 ps fwhm. The solid line in Figure 5, which employed the impulse response of eq 1, fitted to the experimental results excellently with the parameter kgr = 1/(10 ps). On the other hand, the dotted lines obtained from eq 2 do not match the experimental data with the parameter D of any value. Though the fitting results of the parameter D = lo4 and lo5 nm2/ps were better without the slow tail after 50 ps than the others, their D values are higher than the reported diffusion coefficient of hole in metallo Pc single crystals with factor of about lo4lO5.I7 Depth dependence of the cation-generation term was ignored in case 2 at this time. If the generation term is incorporated, the slow tail of characteristic behavior of diffusion,
Letters however, will come out to great extent. Thus, we conclude that the decay dynamics of the cations is controlled by the geminate recombination, and its lifetime is 10 ps. Corresponding to the decay of the cation, the long lifetime species, which has Ak peaks at 670 and 760 nm, was produced. These spectral features were similar to the spectrum observed in vacuum which was already assigned to the vibrationally excited, electronically ground state of P-CUPC.~ The free energy of the geminate pair dissipate into thermal energy by the geminate recombination; hence the spectra in the later stage can be assigned to the “hot band”. In addition, the decay of these bands was faster than that of “hot band” observed in vacuum. This fact can be explained by efficient dissipation of thermal energy through diffusion into water. Thus, the spectra obtained in the long delay time region were assigned to the vibrationally excited, electronically ground state.
Conclusion We have measured photodynamics of the P-CuPc solidwater interface by picosecond regular reflection spectroscopy. It is clearly shown here for the first time that the cation-electron geminate pair is formed at the interface by photoexcitation and its geminate recombination completes with a lifetime of 10 ps. Picosecond regular reflection spectroscopy is a powerful tool for an elucidation of photodynamics of solifliquid interfaces.
Acknowledgment. The authors thank Otsuka Electronics for their generous afford of multichannel photodiode arrays. The present work was partly supported by a grant-in-aid on priority-
J. Phys. Chem., Vol. 99, No. 32, 1995 12075 area-research “Photoreaction Dynamics” from the Japanese Ministry of Education, Science, and Culture (No. 06239101).
References and Notes (1) Organic Phototransformations In Nonhomogeneous Media; Fox, M., Ed.; ACS Symposium Series 278; American Chemical Society: Washington, DC, 1985. (2) (a) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (b) Kalyanasundaram, K.; Gratzel, M.; Pelizzetti, E. Coord. Chem. Rev. 1986, 69, 57. (3) Nozik, A. Annu. Rev. Phys. Chem. 1978, 29, 189. (4) Thomas, J. K. J . Phys. Chem. 1987, 91, 267. (5) Nosaka, Y.; Miyama, H.; Terauchi, M.; Kobayashi, T. J . Chem. Phvs. 1988. 92. 255. A,; Roggers, M. A. J.; Webber, S. E. Chem. Phys. Lett. 1991. ‘ (6)177, Shand,’M. 11. (7) Ichikawa, M.; Fukumura, H.; Masuhara, H. J . Phys. Chem. 1994, 98, 12211. ( 8 ) Ichikawa, M.; Fukumura, H.; Masuhara, H.; Koide, A,; Hyakutake, H. Chem. Phys. Lett. 1995,232, 346. (9) Fukumura, H.; Yoneda, Y.; Takahashi, H.; Masuhara, H. In Abstracts of Japan Symposium on Photochemistry; Kyoto, 1990; p 47. (10) Kagaku Binran, 3rd ed.; Maruzen: Tokyo, 1984; p 11-535. (1 1) Green, J. M.; Faulkner, L. R. J. Am. Chem. Sac. 1983, 105, 2950. (12) Migus, A.; Gaudel, Y.; Martin, J. L.; Antonetti, A. Phys. Rev. Lett. 1987, 58, 1559. (13) Debacker, M. G.; Deleplanque, 0.;van Vlierberge, B.; Sauvage, F. X. Laser Chem. 1988, 8, 1. (14) The absorption spectrum of a a-CuPc evaporated film before and after immersion in the MVZCaqueous solution was not changed. Though the crystal form is different from the present transient study, we believe that the nonreactivity is independent of the crystal form. (15) Kaneko, M.; Motoyoshi, J.; Yamada, A. Nature 1980, 285, 468. (16) Auston, D. H.; Shank, C. V. Phys. Rev. Lett. 1974, 32, 1120. (17) Cox, G. A.; Knight, P. C. J . Phys. C; Solid State Phys. 1974, 7, 146.
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