ROLE OF CHARGE-TRANSFER INTERACTIONS IN PHOTOREACTIONS .8. FLUORESCENCE QUENCHING AND INDUCTION OF INTERSYSTEM CROSSING AND PHOTOISOMERIZATION BY INORGANIC ANIONS

The relevance of intersystem crossing catalysis in the fluorescence qu enching of n-styrylphenanthrenes (n-StPh, with n = 1, 2, 3, 9) by iodi de, thiocyanate and bromide anions has been investigated. Quenching ra te constants (k(q)) were found to decrease with increasing the free-en ergy change for the electron transfer process [k(q)(I-)> k(q)(SCN-)> k (q)(Br-)]. In parallel, the quantum yield for the trans --> cis photoi somerization (phi(c)) has also been measured. The addition of inorgani c anions (X-) to StPhs caused, in general, an increase of phi(C). Dete rmination of phi(C) values as a function of anion concentration allowe d the quantum yield of the complexed excited molecules (StPh..X-) to be determined (phi(CE)lim). Laser flash photolysis experiments showed that the fluorescence quenching was accompanied by an enhancement of t riplet population. In the case of I-, a transient attributed to I was observed, even though the anion of the aromatics, expected for the el ectron transfer process, did not appear.