In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow‐Core Photonic Crystal Fiber Microreactors

Hollow‐core photonic crystal fibers (HC‐PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette‐based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen‐evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT‐simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady‐state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber‐based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future. Catalytic intermediates can be observed on microliter volumes using in situ fiber‐based spectroscopy. The hydrogen‐evolving reaction driven by the combination of cobaloxime hydrogen evolution catalysts with [Ru (bpy)3]2+ photosensitizers reveal the formation of a transient CoI species and a steady‐state CoII species.