Transient absorption spectroscopy: a mechanistic tool for triplet sensitizers and their applications

Transient absorption spectroscopy (TAS) is a well-known technique used to study events that take place upon photoexcitation at short time scales (usually from femtoseconds to nanoseconds and up to milli-seconds) and involve processes such as electron transfer, energy transfer, intersystem crossing, isomerization, proton transfer, exciplex or excimer formation, etc. These events are fundamental steps for the mechanisms of many chemical processes including organic synthetic reactions, photo-polymerization, photodynamic therapy, and thermally activated delayed fluorescence. In this review we thoroughly discuss and critically analyze how to exploit TAS to derive mechanistic insights in these processes. Moreover, some basic principles that can help in easily elucidating and predicting a transient absorption (TA) spectrum are included with previously reported examples. This paper is aimed at facilitating the application of TAS as a tool for mechanistic studies and to provide an overview of the techniques related to scientists and students with diverse scientific backgrounds.