Semi‐Organic Artificial Photosynthetic System with Engineered Phenoxazinone Derivatives for Photocatalytic Hydrogen Production with Broadened Near‐Infrared Light Harvesting

Natural photosynthetic systems utilize complex pigment-protein assemblies for light harvesting across a broad spectral range from UV to near-infrared, enabling efficient photogeneration and charge separation. Conventional photocatalysts, however, primarily absorb UV (<380 nm) and visible light (380-780 nm), resulting in suboptimal spectral utilization. This study introduces a semi-organic artificial photosynthetic system that integrates molecularly engineered phenoxazinone derivatives with H-doped rutile TiO₂ (H-TiO₂) nanorods. Bis(Triphenylamine)Phenoxazinone (BTP) features a phenoxazinone core with two triphenylamine donor groups, enabling light absorption up to 800 nm. Modifying BTP with an additional malononitrile group (MBTP) extends absorption into the NIR region up to 1200 nm. Optimized semi-organic catalysts with MBTP nanobelts and H-TiO₂ nanorods showed an excellent photocatalytic hydrogen evolution rate of 29.4 mmol g^-1 h^-1 and 60.4 µmol g^-1 h^-1 under UV-vis and NIR irradiation, respectively. Femtosecond transient absorption (fs-TA) spectroscopy showed rapid electron injection from the photoexcited phenoxazinone derivatives to the H-TiO₂ conduction band, indicating efficient charge carrier dynamics. Photoelectrochemical measurements confirmed improved charge transport and reduced recombination in the MBTP-based system, attributed to the stronger internal electric field and increased dipole moment from the malononitrile modification. These findings highlight the potential of tailored semi-organic systems for high-efficiency solar-to-hydrogen conversion.