Influence of temperature on the vibration and electron absorption spectra 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 ranging from 81 to 18 °C were determined. The bands of the visible absorption and Raman spectra of all-trans-β-carotene show red and blue shifts, respectively. Changes in the shape of the spectral feature are observed. Raman scattering cross-section increases as 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 structure order increases and the π-electron delocalization extends as temperature decreases. The bandwidth of the electronic energy gap becomes narrow, which causes the slight shift of visible absorption spectrum towards longer wavelengths. An increase in effective conjugation length causes a blue shift of the Raman band. The decreasing difference between CC and CC bond lengths and longer vibrational relaxation time in low temperature makes the line width narrow. The formation of the triplet excited state of all-trans-β-carotene contributes to the asymmetrical growth in the shape of the spectral feature of the (CC bonds) stretching modes. The shoulder observed below 1520 cm−1 shows a blue shift. The enhancement of coherent weakly damped CC stretching vibrations increases the Raman scattering cross-section.