What Is the Real-Time Atomistic Mechanism Behind Chirality-Induced Spin Selectivity in Donor-Chiral Bridge-Acceptor Molecules?

Abstract

Chiral-induced spin selectivity (CISS) has been experimentally observed in photo-excited donor-chiral bridge-acceptor (D-Bχ-A) molecules [Science 382, 197-201 (2023)]. However, the microscopic mechanism underlying CISS in such chiral systems remains elusive. Here we develop a quantum dynamical model that precisely maps the atomic structure of binaphthyl-type bridge dimers in isolated D-Bχ-A molecules and introduce a geometric spin-orbit coupling (SOC) mechanism to unveil the intrinsic origin of CISS in axially chiral systems. During photo-excited electron transport along the twisted pathways, the geometric SOC coupling strength exceeds the intrinsic coupling of light atoms by one to two orders of magnitude, readily producing observable high spin polarizations. The resulting spin polarization comprises two components: the CISS-associated polarizations along and perpendicular to the chiral axis are intrinsic to axial chirality, requiring neither external fields nor spin-superexchange transfer, while a non-Abelian curvature correction provides a rigorous mathematical definition of the chiral axis direction. Our calculated polarization components, chirality dependence, and relative magnitudes (30-40\%) quantitatively match time-resolved electron paramagnetic resonance measurements. This geometric SOC framework offers a self-consistent and general physical picture of CISS in axially chiral molecules and provides explicit theoretical guidance for the design of chiral spintronic devices.