Photoredox catalysis

Photoredox catalysis is a branch of catalysis that harnesses the energy of light to accelerate a chemical reaction via single-electron transfer events. This area is named as a combination of 'photo-' referring to light and redox, a condensed expression for the chemical processes of reduction and oxidation. In particular, photoredox catalysis employs small quantities of a light-sensitive compound that, when excited by light, can mediate the transfer of electrons between chemical compounds that would usually not react at all. Photoredox catalysts are generally drawn from three classes of materials: transition-metal complexes, organic dyes, and semiconductors. While organic photoredox catalysts were dominant throughout the 1990s and early 2000s, soluble transition-metal complexes are more commonly used today. Photoredox catalysis is a branch of catalysis that harnesses the energy of light to accelerate a chemical reaction via single-electron transfer events. This area is named as a combination of 'photo-' referring to light and redox, a condensed expression for the chemical processes of reduction and oxidation. In particular, photoredox catalysis employs small quantities of a light-sensitive compound that, when excited by light, can mediate the transfer of electrons between chemical compounds that would usually not react at all. Photoredox catalysts are generally drawn from three classes of materials: transition-metal complexes, organic dyes, and semiconductors. While organic photoredox catalysts were dominant throughout the 1990s and early 2000s, soluble transition-metal complexes are more commonly used today. Study of this branch of catalysis led to the development of new methods to accomplish known and new chemical transformations. Photoredox catalysts are usually far less toxic than traditional reagents used to generate free radicals, such as organotin compounds. Furthermore, photoredox catalysts generate potent redox agents when exposed to light, they are unreactive under normal conditions. Thus, transition-metal complex photoredox catalysts are more attractive than stoichiometric redox agents such as quinones. The properties of transition-metal photoredox catalysts depend on the ligands and metal and can be modified for different purposes. Photoredox catalysis is often applied to generate known reactive intermediates in a novel way and has led to the discovery of new organic reactions, such as the first direct functionalization of the β-arylation of saturated aldehydes. While the D3-symmetric transition-metal complexes used in many photoredox-catalyzed reactions are chiral, enantioenriched photoredox catalysts has only led to low levels of enantioselectivity in a photoredox-catalyzed aryl-aryl coupling reaction, suggesting that the chiral nature of these catalysts is still poor at transmitting stereochemical information. While synthetically useful levels of enantioselectivity has not been achieved using chiral photoredox catalysts alone, enantioselectivity have been obtained through the synergistic combination of photoredox catalysis with chiral organocatalysts such as secondary amines and Brønsted acids. The activity of a photoredox catalyst can be described in three steps. First, the catalyst in its ground state, where the electrons are distributed among the lowest-energy combination of states, absorbs light and moves into a long-lived excited state, where the electrons are not distributed among the lowest-energy combination of available states. Second, the excited photocatalyst interacts by an outer sphere electron transfer process to 'quench' the excited state which activates another component of the chemical reaction. Finally, a second single electron transfer occurs to return the catalyst to its original oxidation state and electron arrangement. Photoexcitation is initiated by absorption of a photon, promoting an electron from the highest occupied molecular orbital (HOMO) of the photocatalyst to another spin-allowed state. Since the ground state of the photocatalyst is a singlet state (a state with no total electron spin), the photon absorption excites the catalyst to another singlet state of higher energy. This excitation is realized as a metal-to-ligand charge transfer, where the electron moves from an orbital centered on the metal (e.g. a d orbital) to an orbital localized on the ligands (e.g. the π* orbital of an aromatic ligand). While absorption of a photon can occur for any energy corresponding to the difference between two singlet states, the excited electronic state relaxes to the lowest energy singlet excited state through internal conversion, a process where energy is dissipated as vibrational energy rather than as electromagnetic radiation. This singlet excited state can relax further by two distinct processes: the catalyst may fluoresce, radiating a photon and returning to the singlet ground state, or it can move to the lowest energy triplet excited state (a state where two unpaired electrons have the same spin) by a second non-radiative process termed intersystem crossing. Direct relaxation of the excited triplet to the ground state, termed phosphorescence, requires both emission of a photon and inversion of the spin of the excited electron. This pathway is slow because it is spin-forbidden so the triplet excited state has a substantial average lifetime. For the common photocatalyst, tris-(2,2’-bipyridyl)ruthenium chloride (also known as Ru(bipy)32+), the lifetime of the triplet excited state is approximately 1100 ns. This is long enough for other relaxation pathways (specifically, electron-transfer pathways) to occur before decay of the catalyst to its ground state. The long-lived triplet excited state accessible by photoexcitation is both a more potent reducing agent and a more potent oxidizing agent than the ground state of the catalyst. In other words, the catalyst more readily gives up or accepts an electron from an external source. The catalyst excited state is a stronger reductant because its highest energy electron has been excited to an even higher energy state through the photoexcitation process. Similarly, the excited catalyst is a stronger oxidant because one of the catalyst's lowest energy orbitals, which is fully occupied in the ground state, is only singly occupied after photoexcitation and thus, available for an external electron to occupy. Since organometallic photocatalysts consist of a coordinatively saturated metal complex, i.e. a structure that cannot form any additional bonds, electron transfer cannot take place by an inner sphere mechanism through a direct bond of the metal complex to another reagent. Instead, electron transfer must take place via an outer sphere process, where the electron tunnels between the catalyst and another molecule. Marcus' theory of outer sphere electron transfer predicts that such a tunneling process will occur most quickly in systems where the electron transfer is thermodynamically favorable (i.e. between strong reductants and oxidants) and where the electron transfer has a low intrinsic barrier.