Investigation of photo- and thermal isomerization pathways of azobenzene derivative in photostationary state generated by two-color excitation

The reaction pathways for photo- and thermal isomerization in the photostationary state of Sudan red 7B (SR7B), which has two azo groups in its molecule, were examined by a laboratory-designed two-dimensional millisecond time-resolved transient absorption spectrometer that utilizes two-color cw lasers (532 nm and 650 nm) as pump and a white light as probe. The temporal population behavior of four isomeric forms after the two-color pump lasers were simultaneously switched off was monitored in terms of transient absorption change with the time resolution of 50 ms. It was found that, in the photostationary state generated by the two-color pump scheme, the two-color two-step photoisomerization process and the multistep thermal isomerization processes occurred. The reaction rates of the thermal isomerization processes were also determined. Based on the results, we constructed a framework for the reaction pathway network in the photostationary state of SR7B under two-color cw excitation.