Ultrafast charge transfer dynamics in the excited state of DCM measured by a 6-fs UV pulse laser

The photoexcited dynamics of a laser dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), in an acetonitrile solution and a dimethyl sulfoxide (DMSO) solution were studied by pump–probe spectroscopy with a 6-fs ultraviolet laser. UV light excites the DCM solved in any of acetonitrile or DMSO into locally excited (LE) state (via S₀→S₂ excitation), then the LE state relaxes to a charge transfer state. In the previous study, the mechanism of transition from the LE state to the intramolecular charge transfer (ICT) state was not clear just known to proceed faster than 100 fs. Our present work has clarified the primary process determining the time constant (LE→ICT state) to be 50 fs. Spectral analysis of the transient difference-absorption spectrum has shown that the locally excited state has a stimulated emission peak at 2.9 eV.