Subphthalocyanine-based photocatalysts for photocatalytic hydrogen production

Photocatalytic water splitting systems are consists of three main components: a specific photocatalyst (typically a metal oxide-based semiconductor) that uses solar energy to convert water to H2, and a cocatalyst, usually noble metal nanoparticles, that serve as active catalytic centers. In addition, an electron donor material (SED) is used to regenerate photocatalysts, optimizing the reduction reaction of water to H2 only. Although many inorganic or organic semiconductor-based photocatalysts have been used and promising results have been obtained in photocatalytic water splitting systems, most of them fail to meet important criteria for photocatalytic systems such as high light collecting capacity, and efficient charge separation. To overcome such difficulties, various strategies such as heterojunction based systems (p-n junction), Z-scheme configuration, and dye sensitization of semiconductors have been developed to both improve the light-harvesting ability and optimize the band structure. The development of dye-sensitized photocatalysts inspired by dye-sensitized solar cells is recognized as one of the most promising approaches to realize photocatalytic water splitting under visible light irradiation. To date, many dyes have been integrated into metal oxide-based semiconductors and used in catalytic applications. On the other hand, the biggest disadvantage of dye-sensitive systems comes from the planar conjugated structures of the dyes used. Because the light absorption capacity is related to the conjugation in organic dyes, while the planar π conjugated systems absorb the long wavelength of the solar emission, it also brings the aggregation problem by causing π-π interactions between the molecules. Aggregation reduces the efficiency of the system by causing the recombination of charges formed by light, especially in electron transfer mechanisms. In this sense, the dyes to be used to increase the catalytic performance of the catalyst in photocatalytic systems should both absorb at long wavelengths, have the appropriate HOMO-LUMO energy level, and not aggregate. Subphthalocyanines (SubPc) are structures consisting of three isoindole units with a boron atom in the center. These aromatic compounds with a conical structure, unlike general aromatic compounds due to their geometry, don't show aggregation in organic solvents. In the UV-Vis spectra of subphthalocyanines, two characteristic severe bands are observed, namely the Q (500-700 nm) and the Soret (300 nm) band. The absorption ranges of SubPc can be shifted to a longer wavelength according to the desired application area by donor or acceptor groups connected to its peripheral location. Due to these properties, SubPcs are used in many fields, especially light-emitting diodes, photodynamic therapy, and photovoltaics as light-absorbing molecules. On the other hand, there are quite a limited number of studies in the literature in which SubPc compounds are used for photocatalytic H2 production. In this master's thesis study, SubPc molecules with different groups connected in axial and peripheral positions were synthesized. New subphthalocyanine compounds were obtained from SubPc-TiO2 photocatalysts by binding to TiO2, which is used as a semiconductor, with the carboxyl group they contain. The hydrogen production performance of SubPc-TiO2 photocatalysts from water has been tested in photocatalytic systems containing triethanol amine as an electron donor.