Factors that Impact Photochemical Cage Escape Yields

Abstract

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored “geminate” recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled “The Cage Effect” is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions

“…The actual quenching rate constant ( k q ) for this series of [Ru(bpy) 3 ](X) 2 photosensitizers could be accessed thanks to the Stern–Volmer relationship ( K sv = k q τ o ), and remarkably, a decrease by two orders of magnitude between the large and noncoordinating BAr F 4 – and the small coordinating TsO – was determined, highlighting the impressive superiority of [Ru(bpy) 3 ](BAr F 4 ) 2 to efficiently transfer its triplet energy to the α,β - unsaturated 2-acyl imidazole substrate 1a . This reactivity could be explained by the higher tendency of the bulky BAr F 4 – counterions to dissociate from their cation to form solvent-separated ion pairs, , thus unshielding the [Ru(bpy) 3 ] 2+ cation in its triplet excited state and facilitating collisions with the substrate for effective triplet–triplet energy transfer. , …”

Factors Controlling Cage Escape Yields of Closed- and Open-Shell Metal Complexes in Bimolecular Photoinduced Electron Transfer

“…The factors that impact cage escape yields are not yet well-defined, and several studies have investigated these yields with respect to the solvent polarity and viscosity, − the driving force for electron transfer, , the spin states, ,,− as well as ionic strength effects. ,,, Pioneers in the field have dedicated tremendous resources to the understanding of cage escape in the 1970s and 1990s, but unfortunately, no conclusive, overarching mechanism has been reached to date.…”