KINETICS OF INTERSYSTEM ELECTRON-TRANSFER WITHIN TRIPLET RADICAL-ION PAIRS ON SILICA STUDIED BY DIFFUSE-REFLECTANCE LASER FLASH-PHOTOLYSIS - BELL-SHAPED ENERGY-GAP DEPENDENCE ON THE SURFACE

Photoinduced electron transfer (ET) reactions of quinones (A) and tert iary aromatic amines (D) both adsorbed onto porous silica (14 nm pore size) were studied by diffuse-reflectance laser flash photolysis techn ique. Both diffusion-controlled dynamic and Perrin type static quenchi ng of (3)A by D were observed. Static quenching results in formation o f triplet radical ion pairs (RIPs). RIPs decay via an intersystem back electron transfer (ET). The ET kinetics are discussed in terms of two formalisms: a first-order law with Gaussian distribution on the free energy or by a fractal-like analysis with time dependent first-order r ate constant k(t) = k'(f)t(-h). The heterogeneity constant, h, increas es with the increase in average rate constant of reaction in accordanc e with the theoretical predictions for lower dimensional and fractal m edia. The back ET is a reaction-controlled process at early times and a diffusion-controlled one at times longer than 0.5 mu s. The dependen ce of the average rate constant of back ET and of k(t) at early times on the ET free energy is bell-shaped. This can be quantitatively descr ibed in terms of the single quantum mode model of the nonadiabatic ET theory with a higher value of the reorganization energy of environment (0.9 eV) as compared to that in moderately polar solvents (other para meters being the same). The bell-shaped energy gap dependence demonstr ates that adsorbed RIPs appear to experience a strongly polar environm ent.