Chiral spin selectivity and chiroptical activity in helical molecules.

Chiral structures, breaking spatial inversion symmetry, exhibit non-zero chiroptical activity (COA) due to the coupling between their electric and magnetic responses under external electromagnetic fields, an effect absent in achiral systems. Non-magnetic chiral structures also exhibit Chiral-Induced Spin Selectivity (CISS), primarily detected in two terminal measurements in the linear regime, where spin selection emerges without external magnetic influence. Despite the different origins of these physical phenomena, our model captures the relevant physics required to address CISS as an intrinsic molecular effect with the basic ingredients: (i) chirality/inversion asymmetry, (ii) meV atomic spin-orbit coupling, and (iii) decoherence as a source of reciprocity breaking. In this work, we derived how the electronic system couples with polarized electromagnetic radiation to yield a spin-dependent polarization rotation power, quantified through the Rosenfeld tensor, predicting characteristic spin signatures in the COA. The model also predicts that a net spin polarization manifests in the molecular terminations that have been surmised as an explanation for chiral species separation of racemic mixtures and interactions with surface magnetic domains. A recent sensitive spectroscopic measurement of electron transfer in donor-acceptor complexes is consistent with the standalone CISS effect.