THE DEACTIVATION OF SINGLET EXCITED ALL-TRANS-1,6-DIPHENYLHEXA-1,3,5-TRIENE BY CHARGE-TRANSFER PROCESSES .2. FORMATION AND DYNAMICS OF CHARGE-TRANSFER (CT) INTERMEDIATES

The fluorescence quenching mechanism of singlet excited a]l-trans-l,6- diphenylhexa-1,3,5-triene ((DPH)-D-1) by the electron acceptor molecu le p-dicyanobenzene (p-DCB) was investigated with time-resolved fluore scence and absorption measurements in 5 solvents of different polarity . In the polar solvents (dielectric constant epsilon greater than or s imilar to 10) the interaction between (DPH)-D-1 and p-DCB leads to fo rmation of cationic DPH. In the less polar solvents (epsilon less than or similar to 10) an additional emission in the fluorescence spectrum and a bi-exponential fluorescence decay of 1DPH in the presence of p -DCB is observed. Formation of triplet DPH occurs in every solvent emp loyed. With the assumption that only one CT intermediate is dominant i n a given solvent, rate constants of triplet DPH formation, fluorescen ce (CT emission), and radiationless deactivation (charge recombination , CR) of the respective CT intermediates (solvent-separated ion pairs or exciplexes) and the free energy changes of the corresponding proces ses were determined. The CR rate constant of the CT intermediates exhi bits the most pronounced variation upon changing the solvent. E.g. in acetonitrile solution, it is ca. 50 times larger than in toluene solut ion, whereas the standard free energy change for the CR process - Delt a G(CR) is larger in toluene than in acetonitrile solution. The experi mental results for the CR rate constants are in good agreement with es timations of rate constants in the framework of non-adiabatic electron transfer theory using experimental values far reorganization energies and electronic matrix coupling elements. (C) 1997 Elsevier Science B. V.