Chirality Induced Spin Selectivity of Photoexcited Electrons in Carbon-Sulfur [n]Helicenes

The Chiral-Induced Spin Selectivity effect (CISS) is ordinarily associated with electron transfer, electron transport or bond polarization in chiral molecules. We explore here CISS associated with inter-system crossing (ISC) in helicene molecules. The relative magnitude of both CISS and ISC is controlled by the strength of the spin-orbit coupling (SOC) associated with either the excited singlet-triplet transition or electron transport through the helix. Depending on the time scale, both mechanisms are connected in photo-induced electron transfer, which is assumed to take place through an initial one-photon singlet-singlet excitation process followed by singlet-triplet mixing, which provides the pathway for subsequent spin polarization. We explore this mechanism computationally, and find an estimate of SOC for helicenes that is consistent with the experimental results obtained for the spin polarization power of molecules using conductance and photo-excitation techniques. Our results show a hitherto unexplored dimension of the CISS effect and connect it to the exciting possibilities of spin-dependent chemistry.