Wavelength-Selectivity in Step-Growth Photopolymerizations Mapped via Photochemical Action Plots

[2 + 2] Photocycloadditions are one of the most important classes of photochemical reactions and are often employed in soft matter materials science. However, the wavelength resolved investigation of [2 + 2] based photopolymerizations has not been examined. Herein, we initially introduce a photoreactive alkene (nitrovinyl pyrene), which displays strong absorption in the visible light spectrum, extending to the green light (λ > 500 nm) region. Chromophore conversion and cycloaddition yield were quantified through a photochemical action plot using 1H NMR spectroscopy. A local maximum in the wavelength dependent quantum yield was observed close to 520 nm, revealing a strong mismatch between absorptivity and reactivity, i.e. higher values are observed at excitation wavelengths that are red-shifted relative to the absorption maximum. To ultimately map the wavelength dependence of a photopolymerization based on the introduced chromophore, we subsequently establish a size-exclusion chromatography (SEC)-based method to map the nitrovinyl pyrene species depletion as a function of wavelength, introducing an SEC-based photochemical action plot methodology. We next modify our chromophore with a second nitrovinyl group, making it a photoreactive bifunctional monomer, leading to a further redshift in its absorbance maximum and an increased quantum yield at 550 nm. We finally apply our SEC-based photochemical action plot methodology to map the wavelength-dependent [2 + 2] photopolymerization of the dinitrovinyl chromophore and demonstrate a significant effect of monochromatic wavelength on the product distribution of the individual chains, highlighting how changes in the electronic properties─particularly the size of the conjugated system─of multifunctional chromophore monomers can be exploited to control photopolymerizations via irradiation wavelength.