Chiral N-Heterocyclic Carbene On-Surface Induced Electronic Spin Filtration

It is well established that transport of electrons through chiral molecules can depend on their spins. However, it is not yet comprehensible how to relate molecular structure and size to spin selectivity. N-heterocyclic carbenes (NHCs) are a unique class of small organic molecules with electronic tunability generating huge structural and applicative diversity and covalently bind to metal surfaces to form a robust self-assembled monolayer (SAM). Here, we demonstrate that this family of molecules exhibits high spin selectivity, despite their small size, while being extremely stable on metal surfaces. We have fabricated chiral NHC thin films on the metal surfaces and utilized them as electron spin filters for the first time. The (S,S)-SINpEt and (R,R)-SINpEt thin films on Au-Ni surfaces showcase a high electronic spin polarization (SP) of up to 60%. The chiral NHC thin films exhibit electrochemical stability under continuous voltametric interrogation for 50 cycles (within a -0.3 to 0.3 V vs Ag/AgCl voltage window) in an aqueous solution (pH = 13), which enables the utilization of spin-polarized electrons to enhance the oxygen reduction reaction (ORR). The ORR with chiral SINpEt thin films of different thicknesses indicates the importance of the coherency of the electrons in the ORR reaction. The binding, orientation of the chiral SINpEt molecules, and their molecular coverage on the Au surface have also been investigated using quantum chemical calculations, which ensure a good agreement with the experimental observations. This finding introduces a new class of small chiral molecules as a useful source of electronic spin-polarization for various applications.