Efficient Through-Space Charge Transfer in Heavy-Atom-Free Photosensitizers via Conformational Preorganization for Photocatalytic Polymerization

Abstract

Developing heavy-atom-free photosensitizers capable of efficient triplet-state generation relies on a delicate balance between molecular flexibility and structural integrity. Here, we present a design strategy based on conformational preorganization that stabilizes through-space charge-transfer (TSCT) configurations, thereby promoting spin-orbit charge transfer intersystem crossing (SOCT-ISC). Through systematic modification of the donor segments, we demonstrate that a moderately flexible bridging unit enables excited-state conformational relaxation, leading to the formation of quasi "faceto-face" TSCT geometries, whereas structurally rigid or incomplete analogues impede efficient TSCT. This study highlights the synergistic roles of ground-state preorganization and excited-state conformational adaptability in controlling TSCT formation and populating the triplet state. The resulting photosensitizer achieves efficient triplet-state generation and exhibits high photocatalytic polymerization performance (a monomer conversion rate of 86 ± 2% at 0.01 mol%) without requiring external electron donors. This work provides a generalizable molecular design paradigm for constructing heavy atom-free photosensitizers through rational conformational control.