Time-resolved infrared absorption spectroscopy applied to photoinduced reactions: how and why.

Time-resolved infrared (IR) spectroscopy is a widely used technique in the investigation of photoinduced reactions, given its capabilities of providing structural information about the presence of intermediates and the reaction mechanism. Despite the fact that it is used in several fields since the '80s, the communication between the different scientific communities (photochemists, photobiologists, etc.) has been to date quite limited. In some cases, this lack of communication happened-and still happens-even inside the same scientific community (for instance between specialists in ultrafast ps/fs IR and those in "fast" ns/µs/ms IR). Even more surprising is the difficulty of non-specialists to understand the potential of time-resolved IR spectroscopy, despite the fact that IR spectroscopy is normally taught to all chemistry and material science students, and to several biology and physics students. This tutorial review aims at helping to solve these issues, first by providing a comprehensive but reader-friendly overview of the different techniques, and second, by focusing on five "case studies" (from photobiology, gas-phase photocatalysis, photochemistry, semiconductors and metal-carbonyl complexes). We are confident that this approach can help the reader-whichever is its background-to understand the capabilities of time-resolved IR spectroscopy to study the mechanism of photoinduced reactions.