Photocatalysis reactions generally proceed through one of two distinct pathways: photoinduced electron transfer (PET), commonly termed photoredox catalysis, and photoinduced energy transfer (PEnT) (Figure ). Both scenarios are initiated by photoexcitation of the PC to generate PC*. Providing that the lifetime of its excited state is sufficiently long-lived (on the order of nanoseconds), diffusion of the substrate and PC* to form an encounter complex occurs competitively to other radiative or non-radiative decay pathways. The rate of the photocatalytic reaction is generally assumed to be dependent upon the concentration of PC*, which is itself correlated with the molar absorptivity e of the PC at the wavelength(s) of irradiation as well as its excited-state lifetime.