Excited state geometry of β-carotene influenced by environments: the nature and decisive role of solvent revealing by two-dimensional resonance Raman correlation spectroscopy.

Resonance Raman scattering can be used to investigate the ground and excited state information of carotenoid. It is known that the Dushinsky rotation can significantly influence the resonant Raman intensity of β-carotene (β-car). The excited state geometry revealed by the double components feature of the C = C stretching vibrational modes and the environmental dependence of the Raman intensity for each component remain unknown. We explore the influence of environmental factors on the relative intensity of these two C = C stretching vibration modes and perform two-dimensional resonance Raman correlation analysis to reveal the changes on β-car excited state geometry. The results show that the relative wavelength difference between the 0-0 absorption and the excitation is the key factor that decides the intensity ratio of the two components and that the intensity of each mode is modulated by environmental factors. This modulation is closely related to the excited state geometry and dynamics, effective conjugation length, and electron-phonon coupling constant. It also shows that the asynchronous cross-peaks in the two-dimensional resonance Raman correlation spectrum (2DRRCOS) can effectively characterize the degree of the varied electron-phonon coupling with the changing conditions. These results are not only complementary to the research on the excited states of carotenoids but also applicable to investigate the environmental dependence of Raman intensity for a lot of π-conjugated molecules.