Photorheological action plots

Wavelength-dependent photochemistry is often probed in solution via photochemical action plots using optical parametric oscillators (OPOs). These studies have shown that wavelength-dependent photochemical reactivity and absorption spectra are misaligned for a range of photochemical reactions. Crosslinked soft matter materials are often obtained via light-driven curing. Herein we introduce the concept of photorheological action plots for crosslinking reactions to assess their response to an identical number of photons at each wavelength. We employ the radical initiator aminobis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO-NH$_2$) for photocuring in a polyethylene glycole diacrylate (PEG-DA) resin. We demonstrate that different evaluation methodologies of the time-dependent rheological measurements lead to similar photorheological action plots, showing a mismatch between the absorbance of the photoinitiator and the photorheological response. Critically, the photorheological action plot does not fully resemble the solution action plot of BAPO-NH$_2$ initiated bulk methyl methacrylate (MMA) photopolymerization. Our study thus demonstrates that the most effective curing wavelength for cross-linking cannot (at least in the current case) be derived from the photoinitiator's absorbance spectrum nor a solution-based photochemical action plot.