Chiroptical inversion in helical Si-Si bond polymer aggregates.

To elucidate the factors involved in the chiroptical properties of polymer aggregates composed of helical building blocks, a series of rigid rod helical poly[alkyl-(S)-2-methylbutylsilane]s (achiral alkyl side chains = ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl) have been investigated. It was found that the chiroptical sign in the circular dichroism (CD) spectra of the polysilane aggregates depends on the achiral side chain length and cosolvent fraction. Concerning the achiral side chains, the n-propyl group was of a critical length for solvent-dependent chiroptical inversion on aggregation. This unique side chain length-dependent chiroptical inversion was theoretically predictable by using the novel approach of combining the cholesteric hard-core model and exciton chirality method. The latter was also investigated theoretically by Gaussian 03 (TD-DFT, B3LYP, 6-31G(d) basis set) calculations applied to two spatially arranged helical Si-Si bonded decamer models.