Picosecond Laser Studies of Photoinduced Electron Transfer in Porphyrin-Quinone and Related Model Systems

Abstract

Mechanisms of photoinduced electron transfer (ET) in porphyrin-quinone model systems have been investigated by means of ps laser photolysis method. Transient absorption spectra were measured with a microcomputer-controlled double beam mode-locked Nd³⁺:YAG laser photolysis system [1]. Second harmonic or third harmonic was used for excitation. The fluorescence decay times were determined by the same laser photolysis system with a streak camera. For example, the ps transient absorption spectra of EEP (ethyletioporphyrin)-TQ (toluquinone) system in acetone, where the fluorescence of EEP is completely quenched, are very similar to the S_n←S₁ absorption spectrum of EEP itself. This result indicates that the photoinduced ET or ion-pair state of EEP-TQ system in acetone is deactivated to the original ground state very rapidly without producing dissociated ions and we are observing only the S1 state before deactivation by collisional interaction with TQ. The decay time of this transient absorbance was obtained to be τ_obs~70 ps which was in an approximate agreement with the value (τ_calc~80 ps) estimated from the relation τ_calc=τ₀/(1+k_gτ₀[TQ])where τ₀ is the lifetime of S₁ state of EEP and kq is the bimolecular rate constant of the quenching by TQ. Contrary to the above results, the ps transient absorption spectra in benzene are quite different from the S_n←S₁ spectra of EEP and are similar to that of porphyrin cation.