Solvent and temperature effects on the intramolecular fluorescence quenching in dipeptide model compounds .1.

The mechanism of fluorescence quenching through intramolecular electron transfer (ET) was investigated in the dipeptide model compounds DL-N-[4-(diethylamino)alkyl]-2-acetylamio-3-(1-naphthyl)propionamides (1) by stationary and time-resolved fluorescence spectroscopy in non-polar-and polar solvents. An analysis of the observed singlet-exciplex fluorescence and the biexponential decay of monomer fluorescence established that emission quenching in non-polar solvents proceeds through the singlet-exciplex intermediate with a strong charge-transfer character, and that the quenching rate constant increases with a decrease in free-energy change for ET (Delta G(et)) but it is not very sensitive to the variation of methylene chain length in 1. The observations of biexponential emission decay and very weak exciplex emission in polar solvents suggest that a singlet exciplex and/or a solvent-separated radical ion pair become a key intermediate in the quenching process, The finding that the magnitude of Delta G(et), being regarded as a measure of the stability of an intermediate formed, linearly correlates with the logarithm of solvent viscosity is explained in terms of a short-lived singlet exciplex that serves as a precursor of the radical ion pair intermediate. Thus, solvent viscosity effects on the biexponential emission decay processes of 1 with small \Delta G(et)\ in protic polar solvents provide a good criterion for the exclusive operation of a singlet-exciplex mechanism in the case that a short-lived exciplex and a long-lived radical ion pair are produced. It was also found that any of activation parameters for ET fluorescence quenching process in acetonitrile is comparable to the corresponding parameter obtained in methylcyclohexane. This finding may present kinetic evidence for the participation of a singler-exciplex mechanism in the aprotic polar solvent, acetonitrile.