Engineering Ordered Small-Molecule Assemblies for Photocatalytic Hydrogen Evolution

Abstract

The pursuit of solar-driven hydrogen (H2) evolution via photocatalysis is central to sustainable energy conversion and storage. Among emerging systems, ordered small-molecule assemblies offer unique opportunities for tuning electronic structure and intermolecular organization, yet their performance is fundamentally constrained by the strong binding energy of Frenkel excitons (~0.3–1.0 eV), which favors rapid geminate recombination over effective charge separation. Rather than being governed by a single dominant factor such as internal electric fields, charge separation in these systems arises from multiple competing and cooperating mechanisms, including intermolecular orbital coupling, packing-dependent polarization, interfacial energetics, and exciton-state evolution. However, in many reported studies, these effects are strongly coupled, making their individual contributions difficult to distinguish. This review critically examines how molecular design, supramolecular order, and interfacial integration influence exciton dynamics and charge transport in ordered small-molecule assemblies. At the molecular level, we reassess dipole engineering strategies and highlight the interplay between polarity, steric geometry, and orbital alignment. At the supramolecular level, we analyze how packing modes, crystallinity, and morphology govern collective electronic properties and exciton dissociation pathways. At the interfacial level, we discuss how donor–acceptor interactions and hybrid architectures introduce additional driving forces for charge separation. Particular emphasis is placed on identifying the conditions under which different mechanisms dominate, as well as the limitations of commonly used descriptors such as molecular dipole moment. By integrating insights across these scales, this review provides a critical perspective on current structure–property relationships and outlines experimentally testable directions for improving molecular photocatalysts for solar hydrogen production.