Probing the Roles of Temperature and Cooperative Effects in Chirality-Induced Spin Selectivity: Photoelectron Spin Polarization in Helical Tetrapyrroles

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

本研究旨在探讨分子振动和分子间相互作用在螺旋四吡咯（TPBT）分子单层中手性诱导自旋选择性（CISS）机制中的作用。我们测量了由TPBT功能化的Cu(111)表面发射的光电子的自旋极化，并分析了其随温度和表面覆盖度的变化关系。通过密度泛函理论（DFT）计算，我们确定了振动模式的能量和温度依赖性，这些模式能够改变分子的螺距和/或分子半径。综合数据表明，在TPBT层中，分子振动对于CISS的作用并不显著。在自旋极化测量过程中，通过逐步热脱附分子从表面移除，创建了亚单分子层覆盖。尽管自旋极化与表面覆盖度呈非线性关系，但这种行为完全可以通过表面功能化时光电子产率的改变来解释，因此不构成CISS中存在协同效应的证据。