Chitosan as a Sustainable Loading-Density-Tunable Scaffold for Switching Energy- and Electron-Transfer Pathways in Porphyrin Photocatalysis

Support architectures critically govern the molecular organization and excited-state pathways of organic chromophores. Chitosan, one of the most widely studied sustainable, biomass-derived polysaccharides, has typically served as a low-loading, aggregation-suppressing host for photofunctional chromophores such as porphyrins. By contrast, high chromophore loading remains largely unexplored because it is generally assumed to cause detrimental aggregation-induced quenching. Here, this sustainable biopolymer is redefined as an active, high-density scaffold for controlling the excited-state branching of covalently attached platinum(II) meso-tetrakis(4-carboxyphenyl)porphyrin (PtTCPP). PtTCPP was grafted onto partially acetylated chitosan over a broad loading range of 0.050–1.00 equivalents per glucosamine unit. At low loading (<0.25 eq), the chitosan scaffold preserved predominantly monomer-like PtTCPP photophysics and promoted Type II energy transfer to oxygen. Increasing the loading (≥0.25 eq) promoted J-type PtTCPP assembly, suppressed singlet-oxygen generation, and stabilized a long-lived, oxygen-sensitive triplet component associated with electron-transfer-dominant photochemistry. Cocatalyst-free photocatalytic hydrogen evolution served as a functional readout of this loading-induced pathway switching, reaching 74.1 mmol g -1 h -1 at 0.50 eq, with an apparent quantum yield of 0.4% at 405 nm. These findings establish chromophore loading density on chitosan as a practical design lever for directing excited-state branching between energy-transfer and electron-transfer pathways, repositioning a widely used sustainable biopolymer from a passive dispersion matrix to an active scaffold for aggregation-enabled photochemistry and solar-driven hydrogen production.