Salt-induced charge separation in photoinduced electron transfer reactions. The effect of ion size

The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis, We investigate the exciplex-forming pyrene/p-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethylaniline (Py/DMA) systems in ethyl acetate, The salts selected for this study are tetrabutylammoniun chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenylborate, In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate, An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser hash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic, However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (eta) depend on the A/D system and on the nature of the salt. The measured eta values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates, These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.