J. Phys. Chem. 1996, 100, 2213-2219
2213
Excited State Dynamics of Phthalocyanine Films V. Gulbinas,† M. Chachisvilis,‡ L. Valkunas,†,§ and V. Sundstro1 m*,‡,⊥ Vilnius Institute of Physics, Gostauto 12, 2600 Vilnius, Lithuania, and Department of Chemical Physics, Lund UniVersity, Box 124, S-22100 Lund, Sweden ReceiVed: July 11, 1995; In Final Form: October 5, 1995X
Femtosecond pump-probe transient absorption measurements were performed for thermally evaporated polycrystalline vanadyl and lead phthalocyanine (VOPc and PbPc) films in order to obtain information about the excitation energy migration and relaxation. The films were shown to be composed of phase II and amorphous material. Fast excitation localization in phase II was concluded from measurement and analysis of the ground and excited state spectra. Comparison of the ground state, difference absorption, and luminescence spectra suggests a small oscillator strength of the electronic transition from the lowest excited state to the ground state. The influence of local heating on the transient spectra is discussed, and the possibility to obtain the excitation decay kinetics free from this influence is proposed. Exciton-exciton annihilation with a time dependent rate (proportional to t-0.5) is observed in both films. This is explained by one-dimensional diffusion-limited annihilation. Linear relaxation times are equal to 28 ( 6 and 42 ( 8 ps and approximate intermolecular excitation hopping times of 0.1 ÷ 0.4 and 0.02 ÷ 0.08 ps were determined for VOPc and PbPc, respectively.
Introduction At high excitation intensities, phthalocyanine (Pc) structures of various molecular arrangements such as thermally evaporated crystalline and amorphous films or phthalocyanines dispersed in polymer matrices demonstrate ultrafast nonlinear annihilation which determines the main relaxation channel on a fast time scale.1-6 Moreover, by means of subpicosecond kinetic measurements of H2-phthalocyanine, Greene and Millard1 showed that the exciton-exciton annihilation rate is time-dependent (∼t-1/2). Later the same time dependence was observed by Terasaki et al.5 in (t-Bu)1.1 VOPc doped into polystyrene, and we observed it in colloidal particles of AlClPc.6 Ho and Peyghambarian2 found that the polycrystalline AlClPc film possesses two extremely fast routes of ground state recovery, i.e., a fast singlet-singlet annihilation which determines the subpicosecond kinetics at high excitation intensities and the linear relaxation on the picosecond time scale due to excitonphonon coupling. Hole burning experiments of Williams et al.7 by means of 130 fs pulses established the presence of a hole in the spectrum of a nearly amorphous AlFPc film, coincident with the spectrum of the pump pulse when pump and probe pulses overlap temporally and both have parallel polarization. These results support the suggestion of rapid (subpicosecond) relaxation of the exciton population to the bottom of the excited state manifold. From ultrafast evolution of the transient absorption spectra of AlCl- and AlF-phthalocyanines Williams et al.3 suggested that a subgap state is present, which is occupied during the first picosecond after film excitation by a 100 fs pulse. Recently, the excited state dynamics of vanadyl phthalocyanines in various molecular arrangements were examined by Terasaki et al.5 and found to be strongly dependent on the molecular arrangement. Though these works1-7 clarified aspects of the excitation energy relaxation, the complexity of the physical phenomena †
Vilnius Institute of Physics. ‡ Lund University. § E-mail address:
[email protected]. ⊥ E-mail address:
[email protected]. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2213$12.00/0
taking place during the initial stages after excitation also became quite apparent and a general model which describes the electronic relaxation in phthalocyanines does not exist yet. The fact that several processes appear to take place simultaneously on the femtosecond to picosecond time scale makes an unambiguous interpretation of all experimental results difficult. In the literature one can consequently find apparent contradictions between experimental results and some suggested models. New, better models of excitation energy relaxation in phthalocyanines should include as many as possible of the observed phenomena. Local heating of the material, for instance, is one of the additional effects caused by relaxation which frequently has been ignored. As it was shown in our previous investigations,4,8 this effect largely determines the dynamics of absorption spectra of the VOPc film excited by picosecond light pulses. Femtosecond experiments on H2Pc films1 demonstrate the local heating effect as well, but it is not so strong under these experimental conditions due to the lower excitation energy density. An analysis of phthalocyanine transient absorption spectra and their femtosecond to picosecond dynamics is performed in this work, in order to provide a detail examination of exciton relaxation pathways with a special emphasis toward the mechanisms of the energy migration. This paper reveals a complexity of the processes under consideration and demonstrates the possibility to obtain the exciton dynamics, unperturbed by other processes, by wavelength selcetive monitoring. The obtained results and the methodology used can be applied for other phthalocyanines and other polymorphic molecular materials. The possibility to obtain information about the dimensionality of the exciton diffusion and annihilation by analyzing the excitonexciton annihilation dynamics is demonstrated. Materials and Methods VOPc films of the thickness 30 nm and PbPc films of the thickness 50 nm were vapor-deposited onto silica substrates held at 20 °C. The absorption spectra of the samples were recorded on a Beckman spectrophotometer (Model UV 5240). Timeresolved measurements of the light induced transient absorption © 1996 American Chemical Society
2214 J. Phys. Chem., Vol. 100, No. 6, 1996
Gulbinas et al.
Figure 1. Absorption spectra of the VOPc and PbPc films. Dashed line is the luminescence spectrum of the VOPc film.
were carried out by means of a femtosecond spectrometer with 200 fs pulses. The 70 ps infrared pulses generated by a continuous wave (cw) mode-locked Nd:YAG laser were compressed in a fiber-grating compressor to ∼3 ps fwhm. Subpicosecond pulses (300-400 fs duration) at 590 nm were generated by a R6G dye laser, pumped by the second harmonic of the YAG laser. The dye laser pulses were compressed in a fiber-prism compressor and amplified in a two-stage dye amplifier pumped by a regenerative Nd:YAG amplifier. The amplified 590 nm pulses of 10 µJ energy, 200 fs duration, and 1 kHz repetition rate were used for the excitation of samples and for continuum generation. The continuum was used as probe light covering the 400-870 nm spectral region. A spectrometer with 2 ps time resolution and 525 nm wavelength excitation pulses based on a low repetition rate passively mode locked, feedback controlled Nd:glass laser was used to measure the VOPc difference spectrum in the IR region (800-970 nm). The fluorescence spectra were measured by exciting films with the second harmonic (532 nm) of low repetition rate 30 ps duration passively mode-locked Nd:YAG laser pulses. Results and Discussion Absorption Spectra. Absorption spectra of the phthalocyanine films under consideration are depicted in Figure 1. The VOPc and PbPc films have a wide Q band with three distinguished maxima. Similar spectral characteristics have been observed in numerous other observations2,7,9-13 and assigned to the polycrystalline phase II. Some differences in the wavelengths of peaks and their absorbance ratio are connected with metal atom of the molecules as well as with conditions of film production, determining the quality of crystalline structure, the size of crystallites and residues of other phases. Recently Yamashita et al.14 concluded by means of X-ray diffraction on vapor deposited VOPc films that the quality of phase II in such films is not high; it is a mixture of phase II and amorphous material. This point of view is also supported by Huang and Sharp,13 who explained the central component of the Q band in a polycrystalline VOPc film by residual amorphous material. Thus, it is likely that the VOPc and PbPc films under consideration also contain some amount of amorphous material. Fluorescence Spectra. The fluorescence spectrum of the VOPc film is presented in Figure 1. It has a maximum at 905 nm and is similar to that of (t-Bu)1,4VOPc presented by Huang and Sharp,13 only red shifted by approximately 10 nm. The fluorescence spectrum of the PbPc film was too weak to be measured. Transient Absorption Spectra. Transient absorption spectra of VOPc and PbPc films measured at different delays after
Figure 2. Transient difference absorption spectra of the VOPc (a) and PbPc (b) films at various delays after excitation at 600 nm by 200 fs, 1.2 mJ/cm2 energy density pulses. Circles in a are data measured with a picosecond spectrometer. Inserts show absorption changes induced by stationary heating of the films by 50 K.
excitation are shown in Figure 2. The transient spectra of both films are similar. Both possess a long wavelength bleaching, in good correlation with the longest wavelength absorption band and a broad induced absorption in the 400-550 nm region. In the 600-700 nm interval both VOPc and PbPc have weak spectral changes, but PbPc has a more pronounced induced absorption at early times. In addition to the decay of the main induced absorption and bleaching bands, the shape of the spectra changes in the course of time. At short delay times (shorter than 3 ps) only weak spectral modifications occur, while at long delay times (150 and 200 ps) qualitative changes in the difference spectra are evident. The short time scale spectral evolution appears most clearly as a blue shift of the red edge of the induced absorption band and some additional changes in other spectral regions. For instance, the induced absorption at 770 nm at t ) 0 turns into a bleaching during this period. The evolution dynamics at different wavelengths of the VOPc film is shown in Figure 3. It is evident that at 605 nm the induced absorption changes its sign during the first few picoseconds, reflecting a shift of the induced absorption band toward shorter wavelengths. In comparison with the induced absorption at 500 nm, the decay dynamics of the bleaching at 830 nm is slightly faster. The evolution dynamics in the PbPc film is very similar to that of VOPc (not shown). It has been shown by Ho et al.2 that a very fast energy redistribution occurs in AlFPc already within the first 55 fs after the excitation. It has been shown as well3 that the induced absorption in the energy gap between the Q and Soret bands of AlClPc and AlFPc films has a 200 fs delay in comparison with the bleaching of the Q band. It was explained by a short-lived subgap state which is occupied during the relaxation process.
