Optical properties of S2 and S3 excited states of protonated schiff-base retinal chromophores in TPA, ECD and ROA

The electronic transitions of the protonated Schiff base of 1l-cis-retinal (PSB11) and protonated Schiff bases of all-trans retinal (PSBT) for the second or higher electronic excited states are hard to be observed experimentally, due to weak intensities of electronic state excitations. In this paper, we propose visualizations method to investigate these weak electronic state transitions of PSB11 and PSBT, using two-photon absorption (TPA), electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra. Because of the resonance excitations of PSB11 and PSB11 in TPA, the transition intensity of the third electronic state is significantly enhanced, which are much larger than that of S1 and S2 electronic transitions. The charge transfer and electron-hole coherence of these electronic transitions in each step in TPA are visualized with charge difference density and transition density matrix. Also, the strong absorptions of S1 and S2 electronic excited states are observed with ECD spectra, and the physical mechanism of electric and magnetic interactions for these electronic transitions are revealed by visualization method. The large intensity of ROA at S3 excited state results from transition electric and magnetic dipole interactions, not from transition electric dipole and transition electric quadrupole interactions. Our results provide a new visualization method to study the optical properties of biological system using TPA and ECD spectra.