Investigation of the excited state structure of DCM via ultrafast electronic pump/vibrational probe

Time resolved visible pump, infrared probe transient absorption measurements of the solutes 4-dicyanomethylene-2-methyl-6-(p-(dimethylamino) styryl)-4H-pyran (DCM) and its isotopomer DCM-d(6) are employed to probe the dynamics of charge transfer state formation in dimethyl sulfoxide (DMSO) and acetonitrile (MeCN). We observe a two stage charge transfer (CT): the first step is an instrument-response-limited charge separation to the dicyanomethylene group, and the second involves a structural evolution of the dimethylamino group. Theoretical calculations and isotopic substitution indicate that the observed vibration is due to the dimethylamino group twisting out of plane, stabilizing the charge separation.