Probing the\nRoles of Temperature and Cooperative Effects\nin Chirality-Induced Spin Selectivity: Photoelectron Spin Polarization\nin Helical Tetrapyrroles

We investigate the roles of molecular vibrations and\nintermolecular\ninteractions in the mechanism underlying chirality-induced spin selectivity\n(CISS) in monolayers of helical tetrapyrrole (TPBT) molecules. The\nspin polarization of photoelectrons emitted from TPBT-functionalized\nCu(111) surfaces was measured as a function of the temperature and\nthe surface coverage. We employed DFT calculations to determine the\nenergy and temperature-dependent population of vibrational modes which\nvary either the molecular pitch and/or the molecular radius. In combination,\nthe data demonstrate that molecular vibrations do not play a significant\nrole for CISS in the TPBT layers. Submonolayer coverages were created\nby gradual thermal desorption of the molecules from the surface during\nthe spin polarization measurements. While the spin polarization scales\nnonlinearly with the surface coverage, this behavior can be rationalized\nentirely through changes of the photoelectron yield upon surface functionalization,\nand therefore represents no evidence for cooperative effects involved\nin CISS.