Theoretical Study on Vibronically Resolved Spectra of Six Heteroatom-Doped Benzannulated [4]Helicenes: Vibrational Mode Assignments and Substituent Effects

Abstract Helicene derivatives have potential as chiral luminescent materials. However, the effect of substituents on the vibrational structures of their spectra is not yet clear. In this work, we used time-dependent (TD) and time-independent (TI) methods to investigate the influence of six heteroatom substituents (B, N, C, Si, P, and S) on the absorption (ABS), emission (EMI), circular dichroism, and circularly polarized luminescence (CPL) spectra of [4]helicene derivatives. The results show that the peak positions, the number of peaks, and their relative intensities change systematically with the substituent. Clear vibrational fine structures are observed for most derivatives. The only exceptions are the EMI and CPL spectra of the S, Si, and P derivatives. Temperature-dependent calculations show that the Si and P derivatives can recover vibrational structures at low temperature. We also assigned the vibrational modes of the ABS spectra using the TI method. By comparing the dimensionless displacement parameters (δ), we systematically examined the substituent effect on vibronic activity. The assignments show that the displacement of low frequency torsional modes and the number of modes with significant displacement together affect the 0–0 transition intensity. The number of high frequency ring stretching modes determines the richness of the vibrational fine structures. These results provide a direct vibrational assignment basis for understanding the spectral behavior of helicene derivatives. They may also offer theoretical guidance for designing chiral luminescent materials with tunable spectral properties.