Temperature effect on electronic and vibrational properties of β-carotene aggregates in aqueous ethanol solution

Raman spectra of beta-carotene aggregates are measured in ethanol-water solution at different temperatures. The aim is to study in detail the Raman spectra of these aggregates in ethanol/water solution at room temperature and their changes with temperature. When the temperature is reduced from 295 K to 77 K, the Raman band is almost unchanged. The CC stretching mode of the beta-carotene aggregate shows a relatively small frequency shift compared to the organic solvent due to the frequency shift and the change of the resonance condition. By fitting the experimental results, it was shown that the prominent peak at 1522 cm(-1) can be deconvoluted into two closely spaced modes. The frequency shifts of these two modes show similar trends, and their resonance enhancements are different. The 0-0 and 0-1 bands of the beta-carotene aggregates behave differently before and after the solution phase transition by background extraction when lowering the temperature. The solution phase transition also leads to a dramatic fluorescence quenching of the beta-carotene aggregates. In the solid state, the fluorescence enhancement of the 0-1 band can be found, while the 0-0 band shows a decreasing trend. The results showed that the beta-carotene aggregates retained weakly coupled aggregates at 77 K while exhibiting H-like temperature dependence in ethanol/water solution.