COULOMB-INDUCED INTRAMOLECULAR EXCIPLEX FORMATION IN SEMIFLEXIBLE DONOR-BRIDGE-ACCEPTOR COMPOUNDS IN NONPOLAR-SOLVENTS AS A FUNCTION OF TEMPERATURE

The mechanism of exciplex formation of two donor-bridge-acceptor compo unds, both containing a strong donor - acceptor pair interconnected by a piperidine ring, was studied in nonpolar solvents as a function of temperature. Fluorescence spectra at low temperature in methylcyclohex ane show that in both compounds the exciplex is formed via the harpoon ing mechanism, i.e. long-range charge separation leading to an extende d charge-transfer state, followed by Coulomb-induced folding to the ex ciplex state. The kinetic link between these two emissive states could be established by studying their transient fluorescence curves. Furth ermore, the activation energy of the folding process was estimated fro m the decay rates of the extended charge-transfer state in various sol vents. An interpretation of the observed activation energies in terms of Kramers theory is presented, which gives an indication of the relat ive importance of internal steric effects and solvent viscosity on the dynamics of the folding process.