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Visible light photocatalysis has evolved into a promising mild and sustainable strategy to access radicals. This field unlocks formerly challenging or even previously inaccessible organic transformations. In this review, an overview of some lesser-known modes of photochemical activation of organic molecules and several emerging techniques within the versatile field of visible light photocatalysis are discussed. These are illustrated by selected photocatalytic reactions, with particular attention given to the reaction mechanism. 1 Introduction 2 Advanced Photoactivation Modes 2.1 Photoinduced Hydrogen-Atom Transfer 2.2 Proton-Coupled Electron Transfer 2.3 Electron Donor-Acceptor Photoactivation of Organic Substrates 2.4 Excited-State Transition Metal Catalysis 3 Emerging Techniques 3.1 Dual Catalysis 3.2 Excited Radical Ion Photocatalysis 3.3 Upconversion Strategies and Other Two-Photon Mechanisms 3.4 Red and Near-Infrared Photocatalysis 4 Conclusions and Outlook [ABSTRACT FROM AUTHOR]

Visible light photocatalysis has established itself as a promising sustainable and powerful strategy to access reactive intermediates, i.e. radicals and radical ions, under mild reaction conditions using visible light irradiation. This field enables the development of formerly challenging or even previously inaccessible organic transformations. In this tutorial review, an overview of the essential concepts and techniques of visible-light-mediated chemical processes and the most common types of photochemical activation of organic molecules, i.e. photoredox catalysis and photosensitization, are discussed. Selected photocatalytic alkene functionalization reactions are included as examples to illustrate the basic concepts and techniques with particular attention given to the understanding of their reaction mechanisms. 1 Introduction 2 Photocatalysts 3 Photophysical and Electrochemical Properties 3.1 Excited-State Energy 3.2 Ground-State Redox Potentials 3.3 Excited-State Redox Potentials 3.4 Local Absorbance Maximum for Lowest Energy Absorption 3.5 Excited-State Lifetime 3.6 [Ru(bpy)3 ]2+ as a Case Study 3.7 Basic Laws and Equations of Photochemistry and Photocatalysis 3.8 Common Terminology in Photochemistry and Photocatalysis 4 Activation Modes in Photocatalysis 4.1 Photoinduced Electron Transfer 4.2 Photoinduced Energy Transfer 5 Conclusions and Outlook [ABSTRACT FROM AUTHOR]