KINETIC INVESTIGATION OF SPECIFIC ION-PAIR EFFECTS IN INTERMOLECULAR ELECTRON-TRANSFER

Electron transfer quenching of the electronically excited singlet stat es of 9,10-dicyanoanthracene ((1)DCA) and 9-cyano-anthracene ((1)*CA) by chloride, bromide and iodide ions, in the presence of tetra-n-alky lammonium-ions with different alkyl chain lengths was studied by means of stationary and time-resolved fluorescence measurements. Solvents o f different polarity such as dichloromethane, acetonitrile and acetoni trile/water (1:1) were used. A dependence of the measured quenching co nstants and the electron transfer rate constants derived on the radii of the redox-inert counterions has been found in dichloromethane, but not in acetonitrile and acetonitrile/water, respectively. The quenchin g constants of (1)CA by tetra-n-alkylammoniumbromides and of both acc epters by tetra-n-butylammoniumchloride in dichloromethane were larger than expected according to the free energy of the photoinduced electr on transfer processes. The results have been interpreted in terms of s pecific ion-pair effects by an essentially nonadiabatic electron trans fer model.