Raman and ROA spectra of (-)- and (+)-2-Br-hexahelicene: Experimental and DFT studies of a π-conjugated chiral system

The Raman optical activity (ROA) spectra of both enantiomers of 2-Br-hexahelicene in chloroform solution have been measured in the range 1700-300 cm-1. Density functional theory (DFT) calculations accurately reproduce the observed features. The most intense ROA features are also the most intense Raman features, in the region 1350-1400 cm-1, and correspond to the so-called D-modes, which play a major role in coronene and other PAHs (polycyclic aromatic hydrocarbons). Together with a detailed analysis of the normal mode structure, the polarizability tensors for the intense Raman features are investigated and related to the principal characteristics of helicene systems, namely, chirality and π-conjugation. Through electron-phonon coupling analysis, we propose a mechanism that justifies the intense ROA signals.