We have characterized the covalent binding of the CO(2) reduction electrocatalyst ReC0A (Re(CO)(3)Cl(dcbpy) (dcbpy =4,4'-dicarboxy-2,2'-bipyridine)) to the TiO(2) rutile (001) surface. The analysis based on sum frequency generation (SFG) spectroscopy and density functional theory (DFT) calculations indicates that ReC0A binds to TiO(2) through the carboxylate groups in bidentate or tridentate linkage motifs. The adsorbed complex has the dcbpy moiety nearly perpendicular to the TiO(2) surface and the Re exposed to the solution in a configuration suitable for catalysis.

The role of the interface-bound charge-separated pair (IBCSP) in electron transfer at molecule/inorganic semiconductor interface remains poorly understood despite the importance of its counterpart in the solution-phase charge-separation process. To probe their role, we have compared the dynamics of electron transfer from C343 to TiO2, SnO2, and ZnO nanocrystalline thin films. The decay of the C343 excited state and the formation of oxidized C343 are measured by transient visible absorption to follow the rate of charge separation across the interface to form the IBCSPs. The dissociation of IBCSPs to form free electrons in semiconductor is probed by the free-carrier absorption in the mid-IR. For C343 on TiO2 and SnO2, the rate of the overall electron-transfer process is determined by the rate of the initial charge separation across the interface to form the IBCSP, which dissociates with negligible lifetime. The charge-separation rates are instrument-response-function limited (<150 fs) and ∼1 ps for C343 on TiO2 and SnO2, respectively. The faster electron-transfer rate to TiO2 than to SnO2 is attributed to the higher density of conduction-band states in the former. For C343 on ZnO, the initial charge-separation rate across the interface was shown to be instrument-response limited (<150 fs), but the IBCSP dissociates in ∼12 ps, which determines the overall electron transfer rate. We discuss possible reasons for the different lifetimes of IBCSP intermediates in interfacial electron transfer from C343 to TiO2, SnO2, and ZnO.

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.

We report the average molecular orientation of a series of rhenium bipyridyl CO 2 reduction catalysts adsorbed onto a rutile TiO 2 (001) single crystal surface. The molecular conformation of these catalysts on electrode surfaces is expected to affect their overall catalytic efficiency, since the catalytic Re center must be free to coordinate a CO 2 molecule in working systems. Phase-sensitive vibrational sum frequency generation spectroscopy (PS-VSFG) and ab initio SFG simulation and conformation search were used to determine the molecular tilt angles of five such complexes, Re(L n A)(CO) 3 Cl [L n A = 2,2′-bipyridine-4,4′-(CH 2 ) n -COOH, n = 0−4] (abbreviated as ReCnA), as a function of the length of their anchoring groups. Results show that both the short and long axes of the catalytic bipyridine ring tilt further toward the TiO 2 (001) surface as the number of methylene groups in the anchoring groups increases. The increasing tilt angles are shown to correlate with an increasing conformational difference between the two anchoring groups as they lengthen, caused by the free rotation of σ-bonds between −CH 2 − groups. This study demonstrates an effective approach to manipulate the spatial orientation of heterogeneous electrocatalysts on TiO 2 semiconductor surfaces by varying the length of the molecular linkers.

The vibrational relaxation dynamics of a rhenium bipyridyl CO 2 -reduction catalyst chemisorbed to a model Au electrode surface have been investigated using broadband time-resolved vibrational sum frequency generation (VSFG) spectroscopy. IR pump-VSFG probe spectra indicate a biexponential decay of the excited vibrational population, consisting of an ultrafast initial relaxation followed by complete recovery of the ground vibrational state within tens of picoseconds. The ultrafast decay is assigned to rapid population equilibration between the strongly coupled CO stretching modes of the rhenium bipyridyl complex (τ υ−υ = 0.26 ps), whereas the slower decay is assigned to population relaxation from these coupled modes to lower frequency modes within the molecule (T 1 = 14.8 ps). This study demonstrates the ability of time-resolved VSFG to selectively monitor the vibrational energy relaxation processes at the electrode surface in relevant electrocatalytic systems.