EXCITED SINGLET-STATE REACTION OF PHENOSAFRANINE WITH ELECTRON-DONORS- ROLE OF THE HEAVY-ATOM EFFECT IN TRIPLET INDUCTION

Phenosafranine fluorescence is quenched effectively by halogenated ben zene derivatives in the negative and positive regions of the Gibbs ene rgy change, Delta G(et). The observed quenching constants are compared with the k(q) values calculated using Delta G(#) from the Rehm-Weller expression and k(d) = 2.8 x 10(10) dm(3) mol(-1) s(-1) from the Smolu chowski expression. Good correlation between observed and calculated k (q) values in the exothermic region of Delta G(et) reveals that the re action proceeds here via electron transfer. For positive Delta G(et), the difference between experimental and calculated k(q) values shows t hat quenching occurs via the heavy-atom effect and not via electron tr ansfer. The induced intersystem crossing efficiency in the presence of heavy-atom-substituted quenchers is evaluated using laser flash photo lysis.