Despite their ubiquity, self-assembled
monolayers (SAMs) of thiols
on coinage metals are difficult to study and are still not completely
understood, particularly with respect to the nature of thiol–metal
bonding. Recent advances in molecular electronics have highlighted
this deficiency due to the sensitivity of tunneling charge-transport
to the subtle differences in the overall composition of SAMs and the
chemistry of their attachment to surfaces. These advances have also
challenged assumptions about the spontaneous formation of covalent
thiol–metal bonds. This paper describes a series of experiments
that correlate changes in the physical properties of SAMs to photoelectron
spectroscopy to unambiguously assign binding energies of noncovalent
interactions to physisorbed disulfides. These disulfides can be converted
to covalent metal–thiolate bonds by exposure to free thiols,
leading to the remarkable observation of the total loss and recovery
of length-dependent tunneling charge-transport. The identification
and assignment of physisorbed disulfides solve a long-standing mystery
and reveal new, dynamic properties in SAMs of thiols.
It is established that electron transmission through chiral molecules depends on the electron's spin. This phenomenon, termed the chiral-induced spin selectivity (CISS), effect has been observed in chiral molecules, supramolecular structures, polymers, and metal-organic films. Which spin is preferred in the transmission depends on the handedness of the system and the tunneling direction of the electrons. Molecular motors based on overcrowded alkenes show multiple inversions of helical chirality under light irradiation and thermal relaxation. The authors found here multistate switching of spin selectivity in electron transfer through first generation molecular motors based on the four accessible distinct helical configurations, measured by magnetic-conductive atomic force microscopy. It is shown that the helical state dictates the molecular organization on the surface. The efficient spin polarization observed in the photostationary state of the right-handed motor coupled with the modulation of spin selectivity through the controlled sequence of helical states, opens opportunities to tune spin selectivity on-demand with high spatio-temporal precision. An energetic analysis correlates the spin injection barrier with the extent of spin polarization.
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