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Abstract

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.