Common Trends of Chiral Induced Spin Selectivity and Optical Dichroism with Varying Helix Pitch: A First-Principles Study

Electrons transported through a chiral molecule become spin-polarized; this phenomenon is known as chiral induced spin selectivity (CISS). It has implications for spintronics, for electrochemical and enantioselective reactions, and for electron transfer in biological systems. The CISS-induced spin polarization in simulations and in experiment differs by orders of magnitude, and the detailed underlying mechanism is still an open question. Structure-property relationships can help elucidate this question. For this purpose, the effect of helix pitch is studied for a model helix of 20 carbon atoms for two quantities that have been found to correlate in some experiments: spin-polarization in transmitted electrons and electronic circular dichroism (ECD). We find that even though the chirality of these model helices goes down with increased pitch, ECD and CISS go up, along with UV-Vis and magnetic and electric transition dipole moments. Orbital contributions to the most intense UV-Vis transition do not show a consistent qualitative picture. Tentatively, we can assign the increase in these properties to an increase of the electric polarizability with the spatial extension of these helices by changing pitch.