Role of charge-transfer interactions in photoreactions. VIII. Fluorescence quenching and induction of intersystem crossing and photoisomerization by inorganic anions

The relevance of intersystem crossing catalysis in the fluorescence quenching of n-styrylphenanthrenes (n-StPh, with n = 1, 2, 3, 9) by iodide, thiocyanate and bromide anions has been investigated. Quenching rate constants (k(q)) were found to decrease with increasing the free-energy change for the electron transfer process [k(q)(I-)> k(q)(SCN-)> k(q)(Br-)]. In parallel, the quantum yield for the trans --> cis photoisomerization (phi(c)) has also been measured. The addition of inorganic anions (X-) to StPhs caused, in general, an increase of phi(C). Determination of phi(C) values as a function of anion concentration allowed 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 triplet population. In the case of I-, a transient attributed to I* was observed, even though the anion of the aromatics, expected for the electron transfer process, did not appear.