Interfacial Charge-Transfer Pathways: Evidence for Marcus-Type Inverted Electron Transfer in Metal Oxide Semiconductor/Inorganic Dye Systems

Under high driving force conditions, solution-phase chargetransfer processes are often characterized by Marcus-type inverted reactivity, i.e., a decrease in rate with an increase in thermodynamic driving force.1 In contrast, interfacial charge-transfer reactions tend to exhibit either Marcus normal region behavior or asymptotically constant rate behavior.2 The usual explanation for such behavior, at least at metal/solution interfaces, is the availability of a range of filled states below the Fermi level and a range of empty states above the Fermi level of the electrode, such that lower driving force reaction pathways are always available. If the lower driving force pathways are faster, then asymptotic rather than inverted kinetic behavior will be encountered. With semiconductor/solution interfaces, however, the existence of a finite band gap eliminates states immediately above the Fermi level (p-type semiconductors) or immediately below the Fermi level (n-type semiconductors) and opens up the possibility of inverted interfacial reactivity. Indeed, inverted behavior has often been invoked to explain the otherwise surprisingly slow back-electron-transfer reactivity typically encountered at inorganic-dye-sensitized metal oxide semiconductor/ solution interfaces.3 Nevertheless, in the few cases where driving force effects at the semiconductor/dye interface have been systematically investigated, either by manipulating band edge energetics or by manipulating molecular dye redox potentials, either Marcus normal region behavior or driving-force-independent kinetic behavior has been observed.2,4-7 We now report the direct observation of inverted interfacial electron-transfer reactivity at a molecular-dye-modified semiconductor/solution interface. The systems examined were classic ruthenium tris(polypyridine) complexes (homoleptic complexes), electrostatically bound to tin dioxide nanoparticles.8 Photoexcitation of the complexes leads to rapid electron injection into the semiconductor particle, followedsin the absence of an external electrical connections by back electron transfer, as shown in eqs 1 and 2.