Understanding Wavelength-Dependent Photopolymerizations via Nano-Second Resolved Transient Spectroscopy

In the past decade, systematic wavelength-dependent studies of photopolymerizations have revealed that the absorption spectrum alone does not predict radical generation and subsequent monomer conversion. In particular, excitation within the weak absorption tail of certain photoinitiators exhibited disproportionately high polymerization efficiencies. Such behavior suggests that postexcitation processes may play a critical role, challenging the widespread practice of evaluating photoinitiator performance primarily based on spectral overlap of the light source with molar absorptivity. Here, we investigate this behavior using the type I germanium-based photoinitiator Ivocerin (bis(4-methoxybenzoyl)diethylgermanium) as a model system. By combining steady-state monomer conversion analysis with time-resolved spectroscopy and kinetic modeling, we quantify the extent to which radical formation probability and radical persistence contribute to polymerization efficiency as a function of monochromatic wavelength. We demonstrate a direct correlation between the transient excited-state dynamics of Ivocerin and the macroscopic polymerization efficiency across the wavelength spectrum. Enhanced polymerization efficiency at longer wavelengths emerges from the interplay of radical generation and lifetime, suggesting a mechanistic framework that effectively explains the observed discrepancy between molar absorptivity and reactivity in wavelength-dependent photopolymerizations.