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 solutions of the merocyanine dye DCM (4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-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 after excitation depending on the solvent. The results are interpreted in terms of the rapid formation of an emissive intramolecular charge-transfer state from the emissive initially excited singlet state. A further evolution of the transient spectra includes a blueshift of the gain band during a few tens of picoseconds in solvents having ultrashort solvation time whereas a red-shift is found for solvent of slow response. A relaxation mechanism including vibrational cooling and solvation of the charge-transfer state is discussed to explain the observed time-resolved differential spectra. Other processes such as isomerization or single bond twisting are also discussed.