Revealing the initial steps in homogeneous photocatalysis by time-resolved spectroscopy

Photocatalysis attracts currently intense research since it can provide efficient routes for generating solar fuels and allows to apply sunlight for an environmentally friendly synthesis of valuable chemical compounds. Accordingly, in future photocatalysis may contribute significantly to a sustainable economy. However, up to now photocatalysis has made it only into some niche applications. The reasons are manifold including too low yields, insufficient stability, and scarce availability of the precious metals and rare earths used in most cases. The design of better systems is the goal of many research activities. They call for a detailed knowledge of the individual steps and the microscopic mechanisms. Time-resolved spectroscopy is a powerful tool to improve our understanding of the individual steps of a photocatalytic process and of the efficiencies and losses associated with them. This allows to address specific weaknesses of the components of a photocatalytic system and to pursue a rational design of the corresponding compounds. In this review an overview is given about what insights can be gained by time-resolved spectroscopy referring mostly to our own results while it has to be stressed that many other groups are also highly successfully working in this area. We restrict ourselves to homogeneous systems which are often easier to analyze and focus on the primary steps occurring after optical excitation. This includes intramolecular relaxation and intersystem crossing in the photosensitizer as well as the first electron transfer step resulting from the interaction of the sensitizer with other components of the system. Ultrafast pump-probe spectroscopy turns out to be particularly helpful in analyzing new photosensitizers based on abundant metals, i.e. copper and iron. These sensitizers can suffer from short lifetimes of the metal-to-ligand charge transfer states which are typically involved in the intermolecular charge transfer processes. The latter are investigated on the pico- to microsecond timescale by quenching experiments making use of a streak camera and by pump-probe spectroscopy applying a YAG-laser system for excitation. The experiments with the streak camera allow to discriminate between oxidative and reductive pathways and to determine the corresponding bimolecular quenching rates which are compared to their diffusion limit to obtain a measure for the quenching efficiency. By applying transient absorption spectroscopy, it is furthermore possible to observe appearing charge transfer products and to determine their concentrations. In this way the efficiency of the electron transfer itself can be deduced and the relevance of lossy quenching events can be estimated.