EVIDENCE FOR A HYDROGEN-BONDED DIMER AS A FLUORESCENCE QUENCHING CHANNEL IN 4,5-DIPHENYLIMIDAZOLE

Fluorescence studies of the three substituted imidazoles, 2-phenyl-, 4,5-diphenyl and 2,4,5-triphenyl- in a variety of solvents, including pH dependence, suggest that a hydrogen bonded dimer is a contributing radiationless decay channel in 4,5-diphenylimidazole, which is excluded in the other two derivatives. The acid/base dependence of the fluorescence of this molecule indicates an unexpectedly acidic hydrogen at the 2-position in the excited singlet state. In alkaline media the increased fluorescence, phi-F = 0.68, due to the molecular anion, is accompanied by approximately 1500 cm-1 lowering of the emitting state, that is attributed to increased planarity. The absence of the fluorescence dependence with pH for 2-phenyl and 2,4,5-triphenylimidazole suggests that an adiabatic proton transfer is occurring in the excited singlet of 4,5-diphenylimidazole.