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

The free energy dependence on the fluorescence quenching of 2,4,6-triphenyl pyrylium tetrafluoroborate (TPP) by a variety of halogenated benzenes, has been studied in chloroform. The k(q) values calculated using Delta G double dagger from the Levine expression and the k(d) value of 4.07x10(10) M-1 s( -1) from the Smoluchowski expression are in good agreement with the experim ental k(q) values. The observation of the TPP radical signal at 550 nm and the good correlation of k(q) with the Delta G(et) values expound the possib ility 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 yi eld values are compared with the k(b) values calculated using the semi-clas sical expression. On the basis of the present data in chloroform and previo us data in acetonitrile, the effect of solvent on the free energy of activa tion, diffusion, radical yield and radical pair recombination are elucidate d.