Chemically Validating Slow Photon-Enhanced Photoreactivity in Photonic Crystals

The role of slow photons in enhancing photochemical processes within photonic crystals has been widely proposed but remains experimentally elusive due to confounding effects from the intrinsic photoactivity of constituent materials. Here, we present a chemical strategy to directly probe slow photon effects using SiO 2 opal photonic crystals constructed from non-photoabsorbing and chemically inert building blocks. Our approach notably employs a chromogenic molecule as a sensitive probe whose photobleaching behavior serves as an indicator of local light intensity, enabling the direct interrogation of light–matter interactions within the photonic crystal. By tuning the photonic bandgap via controlled structural periodicity, we demonstrate that photobleaching efficiency and reaction kinetics are enhanced by up to 1.4-fold and 1.8-fold for substrate-supported and three-dimensional opals, respectively, when their photonic bandgap aligns with the molecule’s absorption band. Control experiments with disordered SiO 2 assemblies and off-resonant opals confirm that the enhancement originates from slow photon generation rather than from molecular surface adsorption or other light-scattering effects. This study provides the first chemical evidence that slow photons can amplify local optical fields and promote molecular transformations. Our findings deepen the understanding of light trapping via slow photon generation, offering valuable insights for enhancing light-to-chemical conversion in sustainable photochemistry and photocatalysis.