Chemistry with Red and Near Infrared Light

Abstract

Opportunities are explored to initiate controlled chemistry by photolysis with long-wavelength visible and near infrared light. In one direction, bimolecular reactions are induced by exciting collisional pairs in a solid matrix to energy surfaces well below reactant dissociation limits. Examples discussed include product specific (including stereospecific) photo-oxidation of small alkenes and alkynes by NO₂, and cycloaddition reactions of singlet SO and singlet O₂. Conducting the chemistry in rare gas matrices allows us to elucidate elementary reaction steps by FT-IR spectroscopy of trapped intermediates, and to gain insight into the dynamics of transients by wavelength-dependent laser photochemistry. In the case of olefin epoxidations, stereochemical details of reaction paths are uncovered by chemical trapping of transient oxirane biradicals in their nascent conformation. State-specific reactions of singlet excited SO and singlet O₂ with hydrocarbons illustrate how photons deep in the near infrared can be used for controlled chemical synthesis. In parallel work, excited state redox reactions in homogeneous and colloidal semiconductor solutions are explored that pertain to chemical storage of near infrared quanta and conversion into electrical energy. Examples are studies on chemical storage of singlet delta molecular oxygen (O₂(¹Δ)), a metastable near infrared energy carrier, its excited state redox chemistry in aqueous solution, and direct reduction of O₂(¹Δ), at a semiconductor electrode. Using very sensitive time resolved optical techniques, new mechanistic insight is gained on singlet O₂ reactions, and on photo-oxidation of halide at the semiconductor-solution interface.