Characterization of Photoinduced Self-Exchange Reactions at Molecule–Semiconductor Interfaces by Transient Polarization Spectroscopy: Lateral Intermolecular Energy and Hole Transfer across Sensitized TiO₂ Thin Films

Abstract

Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule–semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)−polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO2 or ZrO2 thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer. Self-exchange energy transfer was found to be many orders-of-magnitude faster than hole transfer at the sensitized TiO2 interfaces; for [Ru(dtb)2(dcb)](PF6)2, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine and dcb is 4,4′-(COOH)2-2,2′-bipyridine, anchored to TiO2, the energy-transfer correlation time was θent = 3.3 μs while the average hole-transfer correlation time was ⟨θh+⟩ = 110 ms, under identical experimental conditions. Monte Carlo simulations successfully modeled the anisotropy decays associated with lateral energy transfer. These mesoscopic, nanocrystalline TiO2 thin films developed for regenerative solar cells thus function as active “antennae”, harvesting sunlight and transferring energy across their surface. For the dicationic sensitizer, [Ru(dtb)2(dcb)](PF6)2, anisotropy changes indicative of self-exchange hole transfer were observed only when ions were present in the acetonitrile solution. In contrast, evidence for lateral hole transfer was observed in neat acetonitrile for a neutral sensitizer, cis-Ru(dnb)(dcb)(NCS)2, where dnb is 4,4′-(CH3(CH2)8)2-2,2′-bipyridine, anchored to TiO2. The results indicate that mechanistic models of interfacial charge recombination between electrons in TiO2 and oxidized sensitizers must take into account diffusion of the injected electron and the oxidized sensitizer. The phenomena presented herein represent two means of concentrating potential energy derived from visible light that could be used to funnel energy to molecular catalysts for multiple-charge-transfer reactions, such as the generation of solar fuels