In this review, we highlight the use of organic photoredox catalysts in a myriad of synthetic transformations with a range of applications. This overview is arranged by catalyst class where the photophysics and electrochemical characteristics of each is discussed to underscore the differences and advantages to each type of single electron redox agent. We highlight both net reductive and oxidative as well as redox neutral transformations that can be accomplished using purely organic photoredox-active catalysts. An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.

Organic Photoredox Catalysis

Synthetic Principles for Bandgap Control in Linear pi-Conjugated Systems.

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Bronsted acids, organocatalysts, enzymes, and transition metal complexes.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00018

Dual Catalysis Strategies in Photochemical Synthesis

For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

Tetrathiafulvalenes, oligoacenenes, and their buckminsterfullerene derivatives: the brick and mortar of organic electronics.

p‐Quinodimethane Analogues of Tetrathiafulvalene

The intermolecular contacts between chalcogen atoms and cyano lone pairs were found to stabilize the crystalline state by electrostatic interaction. This interaction is one of the sources of the directionality in crystal paccking of organic molecules and causes the formation of various types of inclusion lattices in the charge-transfer (CT) crystals of I-3. By using highly selective formation of CT crystals with substituted aromatic hydrocarbons, particular isomers such as p-xylene or 2,6-dimethylnaphthalene (2,6-DMN) could be separated from the corresponding isomer mixtures

Clathrate formation and molecular recognition by novel chalcogen-cyano interactions in tetracyanoquinodimethanes fused with thiadiazole and selenadiazole rings

An Absolute Asymmetric Synthesis of the [2 + 2] Cycloadduct via Single Crystal-to-Single Crystal Transformation by Charge-Transfer Excitation of Solid-State Molecular Complexes Composed of Arylolefins and Bis[1,2,5]thiadiazolotetracyanoquinodimethane

The fluorescence quenching mechanism in a highly exothermic region of the Rehm-Weller relationship is shown to be a long-distance electron transfer for producing the geminate radical ion pair with fluorescer radical cation in an electronically excited state and quencher radical anion in the ground state

Quenching mechanism in a highly exothermic region of the Rehm-Weller relationship for electron-transfer fluorescence quenching

Dependence of the fluorescence-quenching rate constant k q , effective quenching distance r q , and free radical yield Φ R on the free enthalpy change ΔG f of full electron transfer has been studied in dichloromethane by using anthracenecarbonitriles as electron-accepting fluorescers and amino- and methoxybenzenes as electron-donating quenchers. On the basis of the present data in dichloromethane and the previous data in acetonitrile, the solvent effects on the electron-transfer fluorescence-quenching mechanism and on the back electron-transfer reaction within the geminate radical ion pairs are elucidated

Tsutomu Miyashi

Papers

p‐Quinodimethane Analogues of Tetrathiafulvalene

Clathrate formation and molecular recognition by novel chalcogen-cyano interactions in tetracyanoquinodimethanes fused with thiadiazole and selenadiazole rings

An Absolute Asymmetric Synthesis of the [2 + 2] Cycloadduct via Single Crystal-to-Single Crystal Transformation by Charge-Transfer Excitation of Solid-State Molecular Complexes Composed of Arylolefins and Bis[1,2,5]thiadiazolotetracyanoquinodimethane

Quenching mechanism in a highly exothermic region of the Rehm-Weller relationship for electron-transfer fluorescence quenching

Solvent effects on photoinduced electron-transfer reactions