Influence of Aromatic Environments on the Physical Properties of β-Carotene

The influence of the surrounding aromatic environment on the properties of β-carotene (Car) was investigated for a simple system comprising single-walled carbon nanotubes (SWCNTs) encapsulating Car molecules. Both metallic- and semiconducting-type SWCNTs encapsulating Car were prepared, and their physical properties were investigated using optical measurements and first-principles calculations. The optical absorption peaks of encapsulated Car in metallic and semiconducting SWCNTs were slightly different, which is thought to be caused by the difference in polarizability of the two types of SWCNTs. The Raman frequency of the CC stretching mode of Car in the metallic SWCNTs was 3 cm⁻¹ down-shifted from that in the semiconducting SWCNTs. This down-shift could not be explained by the difference of dielectric environments of the metallic and the semiconducting SWCNTs. One possible origin for the shift is a difference in the amount of charge on the encapsulated Car, which was supported by theoretical calculations. From the results of this study, it can be concluded that the electronic structure of the nanotube walls influences the properties of encapsulated molecules.