Investigation of excited-state charge transfer with structural change in compounds containing anilino subunits by subpicosecond spectroscopy

Subpicosecond absorption spectroscopy and picosecond streak camera fluorometry are applied to the study of the photoinduced processes in three classes of compounds for which molecular torsion with charge localization leading to an emissive or non-emissive twisted internal charge transfer state was proposed. The compounds are classified according to their steady-state fluorescence properties: compounds having a single band without solvatochromism or a single band with solvatochromism and compounds exhibiting dual fluorescence in polar solvents. The selected compounds are di- and tri-phenylmethane and amino-rhodamine dyes for the first class, the laser dye DCM for the second class and a substituted triphenylphosphine for the third class. These compounds contain at least one electron-donating anilino-group able to rotate. In all cases, the transient spectroscopy gives evidence for an excited-state relaxation involving the formation of an intermediate state. The changes observed in the kinetics by changing the solvent viscosity and polarity and upon donor protonation bring support to a mechanism involving photoinduced charge transfer with structural change. The kinetics are tentatively discussed within the context of theories of electron transfer reactions in the condensed phase.