Interfacial electric fields play crucial roles in electrochemistry, catalysis, and solar energy conversion. Understanding of the interfacial electric field effects has been hindered by the lack of a direct spectroscopic method to probe of the interfacial field at the molecular level. Here, we report the characterization of the field and interfacial structure at Au/diisocyanide/aqueous electrolyte interfaces, using a combination of in situ electrochemical vibrational sum frequency generation (SFG) spectroscopy, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. For 1,4-phenylene diisocyanide (PDI), 4,4′-biphenyl diisocyanide (BPDI), and 4,4″-p-terphenyl diisocyanide (TPDI), our results reveal that the frequency of the gold-bound NC stretch mode of the diisocyanide self-assembled monolayer (SAM) increases linearly with the applied potential, suggesting that SFG can be an in situ probe of the strength of the electric field at electrode/electrolyte interfaces. Using DFT-computed Stark tuning rates of model complexes, the electric field strength at the metal/SAM/electrolyte interfaces is estimated to be 108–109 V/m. The linear dependence of the vibrational frequency (and field) with applied potential is consistent with an electrochemical double-layer structure that consists of a Helmholtz layer in contact with a diffused layer. The Helmholtz layer thickness is approximately the same as the molecular length for PDI, suggesting a well-ordered SAM with negligible electrolyte penetration. For BPDI and TPDI, we found that the Helmholtz layer is thinner than the monolayer of molecular adsorbates, indicating that the electrolyte percolates into the SAM, as shown by molecular dynamics simulations of the Au/PDI/electrolyte interface. The reported analysis demonstrates that a combination of in situ SFG probes and computational modeling provides a powerful approach to elucidate the structure of electrochemical interfaces at the detailed molecular level.

Attaching molecular catalysts to metal and semiconductor electrodes is a promising approach to developing new catalytic electrodes with combined advantages of molecular and heterogeneous catalysts. However, the effect of the interfacial electric field on the stability, activity, and selectivity of the catalysts is often poorly understood due to the complexity of interfaces. In this work, we examine the strength of the interfacial field at the binding site of CO 2 reduction catalysts including Re(S-2,2′-bipyridine)(CO) 3 Cl and Mn(S-2,2′-bipyridine)(CO) 3 Br immobilized on Au electrodes. The vibrational spectra are probed by sum frequency generation spectroscopy (SFG), showing pronounced potential-dependent frequency shifts of the carbonyl stretching modes. Calculations of SFG spectra and Stark tuning rates based on density functional theory allow for direct interpretation of the configurations of the catalysts bound to the surfaces and the influence of the interfacial electric field. We find that electrocatalysts supported on Au electrodes have tilt angles of about 65−75°relative to the surface normal with one of the carbonyl ligands in direct contact with the surface. Large interfacial electric fields of 10 8 −10 9 V/m are determined through the analysis of experimental frequency shifts and theoretical Stark tuning rates of the symmetric CO stretching mode. These large electric fields thus significantly influence the CO 2 binding site.

Vibrational sum frequency generation (SFG) spectroscopy has been utilized to study the spatial orientation and alignment of Re(CO) 3 Cl(dcbpy) (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) (or ReC0A) on the (001) and ( 110) surfaces of rutile single-crystalline TiO 2 . The SFG intensity of the CO stretching modes shows an isotropic distribution on the (001) surface and an anisotropic distribution on the (110) surfaces with respect to the in-plane rotation of the crystal relative to the surface normal (or the incident laser beam plane). By combining these results with ab initio SFG simulations and with modeling of ReC0A−TiO 2 cluster binding structures at the density functional theory level, we reveal that the origin of the optical anisotropy for ReC0A on the TiO 2 (110) surface is associated with the binding preference of ReC0A along the [−110] axis. Along this direction, the binding structure is energetically favorable, because of the formation of proper hydrogen bonding between the carboxylate group and passivating water molecules adsorbed on the TiO 2 (110) surface. Simulations of dimers of ReC0A molecules binding close together with full nearest-neighbor effects give a structure that reproduces the experimental SFG polar plot. The tilt angle, defined by the bpy ring angle relative to the surface normal, of the catalyst is found to be 26°for one monomer and 18°for the other, which corresponds to an aggregate at high surface coverage.

Sum frequency generation spectroscopy (SFG) and calculations of SFG spectra based on density functional theory are combined to elucidate the orientation of two Re­(R-2,2′-bipyridine)­(CO)3Cl (R = 4-cyano or 4,4′-dicyano) electrocatalysts when adsorbed on conductive gold surfaces. We find that the electrocatalysts lean on the Au surface to orient the plane of the bipyridine ligand at 63° relative to the surface normal. While the weak binding of the complexes to the gold surface precluded the ability to perform surface immobilized catalysis, homogeneous electrochemical experiments show that the molecular catalysts are active toward the reduction of CO2 to CO and carbonate in the triply reduced state (TOF of 13.3 and 7.2 s–1 for the doubly and singly substituted complexes, respectively). These findings demonstrate the capabilities of the approach of including rigorous spectroscopic and theoretical methods for revealing the conformation and orientation of CO2 reduction catalysts bound to electrode surfaces, which are critical considerations for redox state transitions and catalytic turnover.

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