Solar overall water-splitting by a spin-hybrid all-organic semiconductor.

Direct solar-to-hydrogen conversion from pure water using all-organic heterogeneous catalysts remains elusive. The challenges are twofold: (i) full-band low-frequent photons in the solar spectrum cannot be harnessed into a unified S₁ excited state for water-splitting based on the common Kasha-allowed S₀ → S₁ excitation; (ii) the H⁺ → H₂ evolution suffers the high overpotential on pristine organic surfaces. Here, we report an organic molecular crystal nanobelt through the self-assembly of spin-one open-shell perylene diimide diradical anions (:PDI^2-) and their tautomeric spin-zero closed-shell quinoid isomers (PDI^2-). The self-assembled :PDI^2-/PDI^2- crystal nanobelt alters the spin-dependent excitation evolution, leading to spin-allowed S₀S₁ → ¹(TT) → T₁ + T₁ singlet fission under visible-light (420 nm~700 nm) and a spin-forbidden S₀ → T₁ transition under near-infrared (700 nm~1100 nm) within spin-hybrid chromophores. With a triplet-triplet annihilation upconversion, a newly formed S₁ excited state on the diradical-quinoid hybrid induces the H⁺ reduction through a favorable hydrophilic diradical-mediated electron transfer, which enables simultaneous H₂ and O₂ production from pure water with an average apparent quantum yield over 1.5% under the visible to near-infrared solar spectrum. Achieving direct solar-to-hydrogen conversion from pure water using solely organic heterogeneous catalysts is still challenging. Here the authors report an all-organic semiconductor catalyst system for overall water splitting under visible to near-infrared light via triplet-triplet annihilation up conversion based on spin coupling. [ABSTRACT FROM AUTHOR]