ULTRAFAST INTRAMOLECULAR CHARGE-TRANSFER IN THE MEROCYANINE DYE DCM

Time-resolved transient absorption and gain spectra with subpicosecond laser excitation are reported for weakly polar and highly polar solut ions of the merocyanine dye DCM ylene-2-methyl-6-(p-dimethylamino-styr yl)-4-pyran) in the 380-800 nm spectral range. A temporary isosbestic point is observed in the transient gain spectra during 3 to 18 ps afte r excitation depending on the solvent. The results are interpreted in terms of the rapid formation of an emissive intramolecular charge-tran sfer state from the emissive initially excited singlet state. A furthe r evolution of the transient spectra includes a blueshift of the gain band during a few tens of picoseconds in solvents having ultrashort so lvation time whereas a red-shift is found for solvent of slow response . A relaxation mechanism including vibrational cooling and solvation o f the charge-transfer state is discussed to explain the observed time- resolved differential spectra. Other processes such as isomerization o r single bond twisting are also discussed.