Recently, the use of transition metal based chromophores as photo-induced single-electron transfer reagents in synthetic organic chemistry has opened up a wealth of possibilities for reinventing known reactions as well as creating new pathways to previously unattainable products. The workhorses for these efforts have been polypyridyl complexes of Ru(ii) and Ir(iii), compounds whose photophysics have been studied for decades within the inorganic community but never extensively applied to problems of interest to organic chemists. While the nexus of synthetic organic and physical-inorganic chemistries holds promise for tremendous new opportunities in both areas, a deeper appreciation of the underlying principles governing the excited-state reactivity of these charge-transfer chromophores is needed. In this Tutorial Review, we present a basic overview of the photophysics of this class of compounds with the goal of explaining the concepts, ground- and excited-state properties, as well as experimental protocols necessary to probe the kinetics and mechanisms of photo-induced electron and/or energy transfer processes.

Time-resolved absorption spectroscopy on the femtosecond time scale has been used to monitor the earliest events associated with excited-state relaxation in tris-(2,2'-bipyridine)ruthenium(II). The data reveal dynamics associated with the temporal evolution of the Franck-Condon state to the lowest energy excited state of this molecule. The process is essentially complete in approximately 300 femtoseconds after the initial excitation. This result is discussed with regard to reformulating long-held notions about excited-state relaxation, as well as its implication for the importance of non-equilibrium excited-state processes in understanding and designing molecular-based electron transfer, artificial photosynthetic, and photovoltaic assemblies in which compounds of this class are currently playing a key role.

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Molecular dynamics occurring in the earliest stages following photo-induced charge transfer were investigated. Femtosecond time-resolved absorption anisotropy measurements on [Ru(bpy) 3 ] 2ϩ , where bpy is 2,2Ј-bipyridine, reveal a time dependence in nitrile solutions attributed to initial delocalization of the excited state over all three ligands followed by charge localization onto a single ligand. The localization process is proposed to be coupled to nondiffusive solvation dynamics. In contrast, measurements sampling population dynamics show spectral evolution associated with wave packet motion on the excited state surface that is independent of solvent. The results therefore reveal two important contributions to the evolution of charge transfer states in condensed phase, one that is strongly coupled to the surrounding environment and another that follows a potential internal to the molecule.Photo-induced charge transfer processes are central to a wide range of important physical and chemical phenomena. Perhaps best known is the photo-induced charge separation that occurs in photosynthetic reaction centers, yielding a transmembrane potential gradient that ultimately drives the production of adenosine triphosphate (1). In materials science, light absorption by wide band gap semiconductors results in the formation of conduction band electrons that are the basis for photoelectron conversion in photovoltaic devices (2). The surrounding medium can be an important factor in directing the course of chemical dynamics in condensed phase. When the dynamics of the chemical process in question are slow relative to the response of the medium, a quasi-steady-state condition exists between the chromophore and its environment, and the overall effect of the solvent is largely thermodynamic. Recently, there has been considerable interest both experimentally and theoretically in the dynamics of solvation, that is, when the chemistry and medium response to that chemistry occur on comparable time scales (3)(4)(5)(6)(7)(8). Under these circumstances, the role of solvent dynamics in the kinetics and mechanism of excited-state evolution can be profound. Transition metal complexes in particular can exhibit charge transfer transitions between the metal center and ligand(s) bound to that metal, a process that necessarily involves charge redistribution on the periphery of the chromophore. Because the ligands are interacting with the solvent, such compounds provide a convenient platform for examining how solvation dynamics couple to photo-induced charge transfer events.Tris-(2,2Ј-bipyridine)ruthenium(II), abbre-, is the prototype for a large class of chromophores whose study has formed the basis for most of the transition metal-based solar energy conversion schemes of the past two decades (9, 10). [Ru(bpy) 3 ] 2ϩ has been extensively studied, and much is known about the properties of its lowest energy triplet metal-to-ligand charge transfer ( 3 MLCT) excited state. The quantum yield for the formation of this state is near unity (11,12)...

Transition metal catalysis has traditionally relied on organometallic complexes that can cycle through a series of ground-state oxidation levels to achieve a series of discrete yet fundamental fragment-coupling steps. The viability of excited-state organometallic catalysis via direct photoexcitation has been demonstrated. Although the utility of triplet sensitization by energy transfer has long been known as a powerful activation mode in organic photochemistry, it is surprising to recognize that photosensitization mechanisms to access excited-state organometallic catalysts have lagged far behind. Here, we demonstrate excited-state organometallic catalysis via such an activation pathway: Energy transfer from an iridium sensitizer produces an excited-state nickel complex that couples aryl halides with carboxylic acids. Detailed mechanistic studies confirm the role of photosensitization via energy transfer.