Two-dimensional observation of multicolor multistep photoreaction process by using white light excitation covering entire visible region

Multicolor multistep photoisomerization process of azobenzene derivative in photostationary state was observed by newly developed Fourier transform two-dimensional (2D) spectroscopic technique. The photostationary state was generated by excitation using white light covering the entire visible region, and the photoreaction was monitored by measuring photoinduced bleaching and absorption signals detected by white light probe. The wavelength participating in excitation process for the observed signal was clarified by modulation frequency marked by passing the pump white light through a scanning tandem Fabry-Pérot interferometer, and the 2D amplitude and phase spectra were obtained by Fourier transform of the obtained 2D interferogram. By using the system, we succeeded in observing the multicolor multistep process in photoisomerization of Sudan red 7B [1-(4-[Phenylazo]phenylazo)-2-ethylaminonaphthalene, SR7B] that has two azo groups and shows several configurations. It was concluded from the analysis of obtained 2D amplitude and phase spectra that at least two reaction pathways of sequential photoisomerization reactions exist in the photostationary state of SR7B under the white light irradiation. The interpretation is supported by the results of quantum chemical calculations using density functional theory.