Effect of pressure and solvent on Raman spectra of all-trans-β- carotene

The ground state Raman spectra of all-trans-β-carotene in n-hexane and CS₂ solutions are measured by simultaneously changing the solvent environment and molecular structure under high hydrostatic pressure. The diverse pressure dependencies of several representative Raman bands are explained using a competitive mechanism involving bond length changes and vibronic coupling. It is therefore concluded that (a) the in-phase C=C stretching mode plays an essential role in the conversion of energy from S₁ to S₀ states in carotenoids, (b) internal conversion and intramolecular vibrational redistribution can be accelerated by high pressure, and (c) the environmental effect, but not the structural distortion or π-electron derealization, is responsible for the spectral properties of a given carotenoid species. These findings revealed the potential of high pressure in exploring the nature of the biological functions of carotenoids.