(Sub)picosecond fluorescence upconversion studies of intermolecular protontransfer of dipyrido[2,3-a : 3 ',2 '-i]carbazole and related compounds

For a few carbazole-related compounds in alcoholic solution, photoinduced s olute-solvent proton-transfer dynamics are studied by means of femto- and p icosecond fluorescence transient measurements. The investigated compounds s how two emission bands, the F-1 band (band maximum between 25 500 and 23 00 0 cm(-1)) that had previously been attributed to the normal solute-solvent complex and the F-2 band (band maximum between 17 200 and 14 400 cm(-1)) th at: had previously been ascribed to the solute-solvent complex in its tauto meric form. Our data show that the Fl band fluorescence decay contains two fast decay components (the first of these has a time constant between 0.6 a nd 0.9 ps, the second has a characteristic time between 6.0 and 11 ps) and a slower decay component with a time constant between 50 and 150 ps, depend ing on the compound and the solvent. The F-2 band shows a fast biexponentia l rise, which occurs at the same rate as the fast initial decay of the F-1 band emission, followed by a slow decay of about 150-250 ps, depending on t he compound and the solvent. The fast decay and rise components of the F-1 and F-2 band emissions, respectively, are discussed as being characteristic of the intermolecular double proton transfer within two distinct "cyclic" solute-solvent complexes. The slower decay component (50-150 ps) in the F-1 band emission is attributed to the decay of the "blocked" solute-solvent c omplex that does not exhibit intermolecular proton transfer. In deuterated small-molecule alcohols, deuteron transfer is found for one cyclic solute-s olvent species only. Its transfer rate appears to be temperature-dependent. The results are suggestive of a thermally averaged deuteron tunneling proc ess in the cyclic solute-solvent complex.