Establishing PQ-ERA photoclick reactions with unprecedented efficiency by engineering of the nature of the phenanthraquinone triplet state

The light-induced photocycloaddition of 9,10-phenanthrenequinone (PQ) with electron-rich alkenes (ERA), known as the PQ-ERA reaction, is a highly attractive photoclick reaction characterized by high selectivity, external non-invasive control with light and biocompatibility. The conventionally used PQ compounds show limited reactivity, which hinders the overall efficiency of the PQ-ERA reaction. To address this issue, we present in this study a simple strategy to boost the reactivity of the PQ triplet state to further enhance the efficiency of the PQ-ERA reaction, enabled by thiophene substitution at the 3-position of the PQ scaffold. Our investigations show that this substitution pattern significantly increases the population of the reactive triplet state (³ππ*) during excitation of 3-thiophene PQs. This results in a superb photoreaction quantum yield (Φ_P, up to 98%), high second order rate constants (k₂, up to 1974 M⁻¹ s⁻¹), and notable oxygen tolerance for the PQ-ERA reaction system. These results have been supported by both experimental transient absorption data and theoretical calculations, providing further evidence for the effectiveness of this strategy, and offering fine prospects for fast and efficient photoclick transformations.