Non-Equilibrium Proton Gradients for Photoenergy Conversion

Biological systems use proton gradients to convert light into chemical potential with remarkable efficiency, offering a blueprint for synthetic approaches to light-to-ionic energy conversion. Inspired by this principle, light-responsive molecular switches have emerged as key building blocks for artificial systems that generate and sustain proton gradients under illumination. This review surveys recent advances in light-to-ionic energy conversion, focusing on three mechanistically distinct strategies: (i) light-driven active proton transport, (ii) excited-state proton separation via bipolar membrane interfaces, and (iii) metastable-state proton gradient formation. Each approach offers a different means of coupling photochemical events to directional ion transport or sustained pH gradients, highlighting trade-offs in reversibility, device complexity, and photovoltage output. While overall electrochemical performance remains modest, these systems offer compelling advantages for applications that demand soft, transparent, or biointegrated platforms. Conceptually, they broaden the landscape of molecular energy systems, bridging synthetic photochemistry, proton-coupled transport, and device-level functionality. Looking ahead, advances in molecular design, nanostructured interfaces, and hybrid architectures will be essential to transform these systems from proof-of-concept demonstrations into functional components for next-generation energy harvesting and conversion applications.