The UV-Vis absorption spectra of the PCs across the four solvents are shown in Figures , with the absorption maxima and molar absorptivity collated in Table and reflected graphically in Figure . If there is a large change in the permanent dipole moments between ground and excited states, solvatochromism can be observed. For the majority of the PCs investigated, and particularly for the transition metal complexes, minimal changes in the absorption profile were detected. Molecules with a small permanent dipole moment in the ground state (close to zero) often display negligible absorption solvatochromism. CT states, and consequently, small variations in ε can be observed, as may be the case for the changes in spectra for 4CzIPN and 2CzPN (Figure and S25). In photocatalysis, the reaction rate is dependent on the number of photons absorbed by the PC, which is governed by e. Modern excitation sources, such as Kessil LEDs, emit light over a narrow range, with a spectrum that is Gaussian in nature. By contrast, compact fluorescence light (CFL) sources