Photoinduced electron transfer reactions of 2,4,6-triphenylpyrylium: solvent effect and charge-shift type of systems

Abstract

The free energy dependence on the fluorescence quenching of 2,4,6-triphenylpyrylium tetrafluoroborate (TPP) by a variety of halogenated benzenes, has been studied in chloroform. The k_q values calculated using ΔG^‡ from the Levine expression and the k_d value of 4.07×10¹⁰ M^-1 s^-1 from the Smoluchowski expression are in good agreement with the experimental k_q values. The observation of the TPP radical signal at 550 nm and the good correlation of k_q with the ΔG_et values expound the possibility of an electron transfer mechanism. The radical yield and intersystem crossing rate constants are evaluated using the flash photolysis technique. The recombination rate constant (k_b) values obtained from the radical yield values are compared with the k_b values calculated using the semi-classical expression. On the basis of the present data in chloroform and previous data in acetonitrile, the effect of solvent on the free energy of activation, diffusion, radical yield and radical pair recombination are elucidated.