Temperature effects on structural order of all-trans-β-carotene

The effects of temperature on the visible absorption and Raman spectra of all-trans-β-carotene dissolved in dimethyl sulfoxide at temperatures ranging from 81 ℃ to 18 ℃ were determined. The bands of the visible absorption and Raman spectra of all-trans-β-carotene showed red blue shifts. The bandwidth of the Raman spectra becomes narrow. Raman scattering cross-section increases as the temperature decreases. The red shift of the absorption spectrum is attributed to the thermal conformational change-induced decrease in the effective conjugation length in all-trans-β-carotene chains. The molecular structural order increases and the π-electron delocalization range is extended as the temperature decreases. The red shift in all-trans-β-carotene can be also attributed to the decrease in the liquid density, and the concomitant decrease in the refractive index is shown by the Lorentz-Lorenz relation. The apparent behavior of the temperature-induced band broadening of CC bonds can be associated with the decrease of difference in C-C and C=C bond lengths, and the shorter vibrational relaxation time. The shoulder observed below 1520 cm-1 shows a red shift. The enhancement of coherent weakly-damped CC stretching vibrations may increase the Raman scattering cross-section.