Investigation of Hydrogen Bonding in Donor–Acceptor Conjugated Polymer Mesoporous C ₃ N ₅ , C ₃ N ₆ , and C ₃ N ₇ Nanostructures for Solar Hydrogen Production

Organic semiconductors present a promising alternative for solar-hydrogen production (SHP) due to their cost-effectiveness and environmentally friendly nature. However, their availability is limited, and they often exhibit lower efficiency than inorganic semiconductors. This inefficiency is primarily attributed to their intrinsic Frenkel excitons with high binding energy, which restrict charge separation and transport. This study explores hydrogen bonding interactions in donor–acceptor conjugated polymer heterojunctions (PHJs) incorporating mesoporous C3N5, C3N6, and C3N7 nanostructures. The fluorine (–F) atoms in poly(5,6-difluoro-4-methyl-7-(7-methyl-9,9-dioctyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazole) and the amino (−NH2) groups in C3N5, C3N6, and C3N7 facilitate hydrogen bonding interactions, ensuring strong interfacial contact. These interactions enhance charge separation and light absorption, improving photocatalytic performance. Experimental results demonstrate that incorporating the donor unit into the polymer structure enhances light capture ability and improves charge transport. Among the tested materials, the strongest electron-donating donor–acceptor unit, poly(5,6-difluoro-4-methyl-7-(7-methyl-9,9-dioctyl-9H-fluoren-2 yl)benzo[c][1,2,5]thiadiazole), exhibits the highest light absorption and charge separation efficiency. This optimized structure significantly enhances SHP, achieving an impressive hydrogen evolution rate of 2992.7 μmol g–1 h–1. These findings provide valuable insights into the development of organic semiconductor-based photocatalysts, contributing to the advancement of renewable hydrogen production.