Effect of End Groups on the Raman Spectra of Lycopene and β-Carotene under High Pressure

The Raman spectra of all-trans-lycopene in n-hexane were measured under high pressure, and the results compared with those of β-carotene. The different pressure effects on Raman spectra are analyzed taking into account the different structures of lycopene and β-carotene molecules. It is concluded that: (a) the vibronic coupling between the S(1) and S(0) states of β-carotene is stronger than that of lycopene, (b) the diabatic frequency increment of the ν(1 )mode is more susceptible to pressure than that of the ν(2) mode for lycopene, and (c) β-rings rotation can relieve the pressure effect on the C=C bond length in β-carotene. This work provides some insights for elucidating the structural and environmental effects on Raman spectra of carotenoids.