Photocharging of semiconductor materials : database, quantitative data analysis, and application in organic synthesis

Photocharging or photodoping is a process in which electrons are accumulated in a semiconductor upon bandgap excitation followed by quenching of the photogenerated holes by reductants. In semiconductors with excess of electrons, negative charge is compensated by cations, of which the most ubiquitous is H+. Photocharging of semiconductors has been studied since 1980s both from a fundamental perspective and application—as source of electrons and protons for reduction of organic compounds in the dark, solar-to-electric energy conversion, and recently also in the design of autonomous microswimmers. In this review, experimental data collected over 40 years of research are summarized and quantified. Maximum specific concentration of electrons stored in 1 g of a semiconductor, maximum average number of electrons stored per semiconductor particle, initial rate of photocharging, and initial rate of discharging are calculated for six classes of semiconductor materials, Ti-, Zn-, Cd-, In-, W-based and graphitic carbon nitrides. Dependence of these parameters on material specific surface area, particle volume, and other properties is analyzed and trends are derived. A public database of photocharged materials is created to facilitate design of high-performing materials with photocharging function, their application as rechargeable reductants in organic synthesis and development of devices

Dichloromethylation of enones by carbon nitride photocatalysis

Small organic radicals are ubiquitous intermediates in photocatalysis and are used in organic synthesis to install functional groups and to tune electronic properties and pharmacokinetic parameters of the final molecule. Development of new methods to generate small organic radicals with added functionality can further extend the utility of photocatalysis for synthetic needs. Herein, we present a method to generate dichloromethyl radicals from chloroform using a heterogeneous potassium poly(heptazine imide) (K-PHI) photocatalyst under visible light irradiation for C1-extension of the enone backbone. The method is applied on 15 enones, with γ,γ-dichloroketones yields of 18–89%. Due to negative zeta-potential (−40 mV) and small particle size (100 nm) K-PHI suspension is used in quasi-homogeneous flow-photoreactor increasing the productivity by 19 times compared to the batch approach. The resulting γ,γ-dichloroketones, are used as bifunctional building blocks to access value-added organic compounds such as substituted furans and pyrroles

Light as a tool in organic photocatalysis : multi-photon excitation and chromoselective reactions

Over the past decades, photoredox catalysis has developed to the big mature field of chemistry. Especially in the field of organic synthesis, more and more sustainable alternatives to conventional synthesis are being developed. Nonetheless, most research activities are focused on the development of new synthetic pathways, utilizing photons as a source of energy for breaking and building of new chemical bonds. Indeed, photons are traceless reagents in the photocatalytic system, however the full power of which is usually overlooked. The intensity and energy of photons are important parameters that regulate the thermodynamic limits of photocatalytic reaction, and therefore control its reactivity and/or selectivity. Herein, we overview light as a powerful instrument in hands of organic chemist and discuss such emerging concepts as wavelength-dependent, multi-photon, and chromoselective organic photocatalytic reactions. We discuss relevant cases, where selection of the irradiation mode, light intensity (one-, two- photons) or wavelength (UV, blue, green) has a crucial role on the outcome of the photocatalytic event. Thus, control over irradiation conditions influence the reaction mechanism and offers an access to highly reactive species that are otherwise hardly available and. This allows for the unique chemical events to happen, such as deep reduction reactions, typical for alkali metals. The principles of these concepts and their applications are reviewed for both homogeneous and heterogeneous photocatalysts

Identification of the structure of triethanolamine oxygenation products in carbon nitride photocatalysis

Triethanolamine (TEOA) is one of the most commonly used sacrificial agents in photocatalysis. Due to its more complex structure compared to, for example, ethanol, and its sacrificial role in photocatalysis, it gives a mixture of products. The structures of these molecules are not usually analyzed. Herein, we obtain and isolate the products of TEOA and N-tert-butyl diethanolamine oxygenation under photocatalytic conditions with ≈15 % yield, and followingly characterized them by NMR and mass spectroscopy. The reaction is mediated by potassium poly(heptazine imide) (K-PHI) in the presence of O2 and affords formyl esters of β-hydroxyethylene formamides from the corresponding ethanolamines

Increasing Profitability of Ethanol Photoreforming by Simultaneous Production of H₂ and Acetal

Often, H2 is produced photocatalytically at the expense of sacrificial agents. When a sacrificial agent is selectively oxidized, this allows coupling of H2 production with synthesis of value-added organic compounds. Herein, it is argued that the conversion of bioethanol into 1,1-diethoxyethane with simultaneous H2 production increases the economic viability of photocatalysis and suggests a semiconductor material that is the most relevant for this purpose

Visible Light Driven Reductive (Cyclo)Dimerization of Chalcones Over Heterogeneous Carbon Nitride Photocatalyst

Single-electron reduction of chalcones to the respective radical anions is a useful technique to activate these molecules toward subsequent transformations. Herein, a metal-free photocatalytic version of chalcones reduction in the presence of triethanolamine as a convenient electron donor and using heterogeneous carbon nitride visible-light photocatalyst is presented. The reaction proceeds via a long-lived radical species of the heterogeneous organic semiconductor. The scope of the reaction was studied, and regioselectivity of the chalcone radicals coupling was investigated. (1) Ten chalcones gave selectively polysubstituted cyclopentanoles with 31–73% isolated yield; (2) Two chalcones bearing electron-donor groups, 4-MeOC6H4 and 2-thienyl, gave selectively the β-ketodienes in 42% and 53% isolated yield, respectively; (3) Pentafluorophenyl substituted chalcone gave exclusively the product of the radicals coupling followed by hydrogen transfer from triethanolamine–hexane-1,6-dione in 65% isolated yield. Reductive cross cyclodimerization of a mixture of two different chalcones proceeded regioselectively with the formation of one product out of four possible. The mechanism was investigated by cyclic voltammetry and linear sweep voltammetry and suggests that the reaction proceeds through proton-coupled electron transfer