Dye self-association identified by intermolecular couplings between vibrational modes as revealed by infrared spectroscopy, and implications for electron injection

In this Article, we investigate the effects of binding geometry and intermolecular interactions in monolayers of a rhenium-based dye adsorbed to TiO₂. We combine two-dimensional infrared (2D IR) spectroscopy of samples prepared with different dye loadings with density functional theory (DFT) calculations of dye binding energies and vibrational frequencies. Our 2D IR spectra reveal splitting of the CO symmetric stretch mode into two peaks of unequal intensity at high surface coverages, which persists even when samples are washed to remove unadsorbed aggregates. Our DFT calculations indicate that it is unlikely that dye binding geometries account for the shifts in peak frequency observed in our experimental spectra. Instead, we find that the shifts in vibrational frequency and 2D IR peak structure are consistent with coupling of dyes associated on the TiO₂ surface. The relative peak intensities in our 1D and 2D spectra indicate different transition dipole strengths, also a signature of molecular coupling. We show that aggregation of dyes on the surface is energetically favorable. Adsorbate-adsorbate interactions may play an important role in defining surface structure and electronic properties of dye-sensitized solar cells and related organic/inorganic interfaces. Infared spectroscopy is a good means to identify its occurrence, and to begin exploring its effects on phenomena like electron injection kinetics.