Analytical expressions for the photoisomerisation (Equation S4) and photocycloaddition (Equation S5) were derived and used to model the reaction yield over time and over a range of wavelengths. The molar absorptivity can be experimentally determined by recording an absorbance spectrum of both the reactant and photoproduct at known concentrations. If the mismatch between reactivity and absorbance can be entirely explained by the backward and forward rate coefficients, it should be possible to match the model to experimentally obtained action plot data. The reaction yield at any point in time was therefore modelled over a range of wavelengths, as shown in Figure for the conversion of trans pyrene-chalcone (trans-PyChal) into cis pyrene-chalcone (cis-PyChal) (Figure ) and shown in Figure for conversion of two trans-PyChal monomers into a pyrene-chalcone dimer ((PyChal)2) (Figure ). Details for the parameters used can be found in Supporting Information Section S10. Figure shows good agreement between the model and the data due to the high level of overlapping absorbance between the cis and trans isomers. Despite the identical conjugation of the π-electron system, differences in the geometry for each isomer influences the dipole moment and the degree of π-electron delocalisation for each, affecting their absorbance profiles. For the cis-PyChal, this results in a blueshift in the absorbance maximum when compared to trans-PyChal, with a large amount of overlapping absorbance between the two. Contrary to the case of isomerisation, reactivity predicted by Equation S5 for the photocycloaddition (Figure ) is minimally enhanced at longer wavelengths due to competing photocycloaddition and photocycloreversion reactions, yet the extent of this minimal enhancement is not sufficient to explain the experimental data presented in Figure . Even with a large difference in quantum yield between the forward and backward reaction, this effect cannot sufficiently explain the observed mismatch due to the lack of overlap between dimer and monomer absorbance at longer wavelengths, which occurs as the formation of the dimer reduces the degree of conjugation resulting in a more drastic change in the frontier orbital energies between the two species. When considering the wavelength region of 400 nm to 450 nm, the molar absorptivity for the dimer only varies slightly and the effect of photocycloreversion is relatively wavelength-independent over this region. Only at wavelengths below 400 nm does the change in molar absorptivity of the dimer become significant, which is then reflected by the model.