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 S₁ and S₀ states of β-carotene is stronger than that of lycopene, (b) the diabatic frequency increment of the ν₁ mode is more susceptible to pressure than that of the ν₂ 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.