SOLVENT EFFECTS ON THE STRUCTURE OF FLUORESCENT EXCIPLEXES IN RIGIDLY-BRIDGED, FLEXIBLY-BRIDGED, AND NON-BRIDGED DONOR-ACCEPTOR SYSTEMS

The solvent dependence of the 'exciplex' emission frequency is compared for two electron Donor-Acceptor pairs that are either separate molecular entities, bridged by a flexible trimethylene chain or by a (semi)rigid piperidine ring. It is concluded that in solvents of intermediate and high polarity the emissive state has a structure involving a rather large DA separation in all cases. For the rigidly bridged systems this structure is dictated by the chair conformation of the piperidine ring, for the flexibly bridged systems it implies that in the emissive state the trimethylene chain adopts a (partly) extended conformation and for the non-bridged systems this 'exciplex' structure appears closer to that of a solvent separated ion pair (SSIP) than that of a contact ion pair (CIP). In nonpolar media the exciplex structure for flexibly bridged and non-bridged systems appears to involve a much smaller DA distance, compatible with a folded structure or a CIP. Under these conditions the rigidly bridged systems either show no exciplex emission at all (in case the charge separated state is thermodynamically inaccessible) or at much higher energies than the flexibly and non-bridged systems. Temperature dependent studies suggest that for flexibly linked systems containing sufficiently strong DA pairs the folded exciplex observed in nonpolar media is formed via a 'harpooning' mechanism involving long-range charge separation followed by electrostatically driven folding.