Advancing conPET: Nanosecond transient absorption spectroscopy to validate how ‘radical’ the photocatalyst is

The consecutive photoinduced electron transfer (conPET) mechanism has been debated for more than a decade; diffusion-limited kinetics is central to it. The synthetic scope is not in dispute, nor are the extreme redox potentials invoked, but the identity of the species performing the chemistry remains unresolved. Excited organic radicals decay within picoseconds, orders of magnitude faster than diffusion-controlled bimolecular electron transfer requires, and this brevity makes absorption of a second photon, the most questionable step of the cycle, difficult to sustain. Because a reagent combining a strongly reducing potential with a longer lifetime would activate a far wider substrate range, attention has turned to alternatives. It has recently been proposed that the short-lived excited radical ion is, in some systems, converted into a persistent species such as a Meisenheimer complex, a solvated electron, or a side-product impurity, any of which could act as the true super-reducing photoreagent. Time-resolved spectroscopy (femtosecond and nanosecond transient absorption, pulse radiolysis, time-correlated single photon counting, time-resolved Raman), magnetic resonance, electrochemistry, and coupled techniques have been used to interrogate such intermediates. Since the contested step is ultimately one of lifetime (long-lived species), nanosecond transient absorption spectroscopy, which resolves precisely the long-lived species at issue, is well placed to settle the question. This perspective sets out the utility of transient absorption spectroscopy for conPET and the considerations governing reliable data interpretation, since experimental design and accurate spectral assignment determine whether the underlying photophysical and photochemical pathways are correctly resolved. It closes with an outlook on in situ ns-TAS coupled to electrochemistry.