Vibronic relaxation dynamics at intermediate state of two-pump excitation: Photoisomerization of indocyanine green in ethanol

Abstract

NIR-UV two-pulse correlation measurements of a cyanine dye were conducted to study its trans-to-cis photoisomerization through a multiphoton process. The signal-to-noise ratios of the correlation traces were improved by modulating only the UV pump pulse. The absorbance change of the photo-produced cis isomers was dependent on the NIR-UV interval, and the time constant of the main decay component was 0.1 ps, which was shorter than the reported values of electronic relaxation. The results indicate that the multi-step excitation via the vibrationally excited S_n state is essential for the photoisomerization. By using the vibronic relaxation of the S_n state, we succeeded in controlling the number of cis isomers produced.