Dye Self-Association Identified by Intermolecular Couplings between Vibrational Modes As Revealed by Infrared Spectroscopy, and Implications for Electron Injection

Abstract

In this Article, we investigate the effects of binding geometry and intermolecular interactions in monolayers of a rhenium-based dye adsorbed to TiO 2 . 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 TiO 2 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 dyesensitized 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.

Structural heterogeneity and dynamics of dyes on TiO2: implications for charge transfer across organic–inorganic interfaces

“…The Re-based dyes with various bridge lengths are shown in Figure 1a, and the different carboxylic acid binding motifs that we consider are shown in Figure 1b-f. These are the same binding motifs that we considered previously 35 and are analogous to the most stable binding modes of formic acid to anatase. 29 Significant increases in the lifetime broadening (LB) of the dye LUMO, indicative of stronger dye-surface electronic coupling, occur with decreasing bridge length (Table 1).…”

2D attenuated total reflectance infrared spectroscopy reveals ultrafast vibrational dynamics of organic monolayers at metal-liquid interfaces

“…These studies demonstrated the usefulness of multi-dimensional spectroscopy regarding the resolution of intermolecular interactions, 17,24 vibrational dephasing and relaxation, as well as spectral diffusion from samples at interfaces. 16,19,49 For instance, variations in spectral diffusion and vibrational relaxation dynamics have been discussed between octahedral rheniumcarbonyl photocatalysts at silica interfaces and in bulk solution, 16,49 revealing that spectral diffusion at surfaces is slower than in bulk solution.…”

“…Multi-dimensional signals from monolayer-forming samples have been reported previously obtained in transmission 2D IR 17,49,60 or with 2D SFG 24,27 spectroscopy. These studies demonstrated the usefulness of multi-dimensional spectroscopy regarding the resolution of intermolecular interactions, 17,24 vibrational dephasing and relaxation, as well as spectral diffusion from samples at interfaces.…”

“…Regarding spectroscopic investigations of MLs at interfaces, central points of interest are represented by details of binding configurations or influences of the environmental interactions on molecular properties as compared to the case of isolated molecules in bulk solution. [16][17][18][19] This is because the structure of molecules in MLs is often influenced by intermolecular interactions within the densely packed aggregate. 20,21 In addition, solvent molecules directly at the interface are likely to exhibit differences in their properties such as hydrogen-bonding networks and dipolar orientation as compared to the bulk solvent.…”

“…be a useful tool to unravel changes in molecular structure and dynamics, which occur down to the femtosecond time scale. [16][17][18][19] In particular, multi-dimensional techniques 17,[24][25][26][27] have been developed in recent years as powerful methods to tackle the issues mentioned above. These techniques are sensitive to, e.g., structural heterogeneity and flexibility, energy transfer, or intermolecular interactions, signatures of which are challenging to extract unambiguously from conventional (1D) vibrational spectra.…”