(Sub)picosecond Fluorescence Upconversion Studies of Intermolecular Proton Transfer of Dipyrido[2,3-a:3‘,2‘-i]carbazole and Related Compounds

For a few carbazole-related compounds in alcoholic solution, photoinduced solute−solvent proton-transfer dynamics are studied by means of femto- and picosecond fluorescence transient measurements. The investigated compounds show two emission bands, the F 1 band (band maximum between 25 500 and 23 000 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 ) that had previously been ascribed to the solute−solvent complex in its tautomeric form. Our data show that the F 1 band fluorescence decay contains two fast decay components (the first of these has a time constant between 0.6 and 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, depending on the compound and the solvent. The F 2 band shows a fast biexponential 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 the 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 complex that does not exhibit intermolecular proton transfer. In deuterated small-molecule alcohols, deuteron transfer is found for one cyclic solute−solvent species only. Its transfer rate appears to be temperature-dependent. The results are suggestive of a thermally averaged deuteron tunneling process in the cyclic solute−solvent complex.