Conjugated polymers have emerged as a promising class of photocatalysts due to their tunable electronic structures, cost-effectiveness, and processability. Unlike traditional inorganic photocatalysts, these organic materials offer flexibility in molecular design, enabling precise control over optical and electronic properties. Their applications span water splitting, carbon dioxide reduction, and pollutant degradation, driven by advancements in bandgap engineering and charge separation strategies.

A key advantage of conjugated polymers is their ability to absorb visible light, making them suitable for solar-driven processes. Graphitic carbon nitride (g-C3N4) is one of the most studied polymeric photocatalysts, with a bandgap around 2.7 eV, allowing absorption of wavelengths up to 460 nm. The material’s layered structure provides a high surface area and active sites for redox reactions. Covalent organic frameworks (COFs) represent another class, where periodic porosity and crystallinity enhance charge transport and reactant diffusion. By modifying their building blocks, COFs can achieve bandgaps ranging from 1.5 to 3.0 eV, covering a broad spectrum of solar energy.

Bandgap engineering is critical for optimizing light absorption and redox potentials. In g-C3N4, copolymerization with electron-rich or electron-deficient monomers adjusts the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. For instance, incorporating benzene rings narrows the bandgap to 2.4 eV, extending absorption into the visible range. Similarly, COFs can be tailored by selecting specific linkers and nodes. Imine-linked COFs exhibit bandgaps near 2.2 eV, while β-ketoenamine-linked variants achieve 1.9 eV, improving visible-light utilization. Heteroatom doping, such as sulfur or phosphorus in g-C3N4, further modifies electronic properties by introducing mid-gap states that enhance charge carrier generation.

Charge separation efficiency is another decisive factor in photocatalytic performance. Conjugated polymers often suffer from rapid electron-hole recombination, which limits their activity. Strategies to mitigate this include structural modifications and heterojunction formation. In g-C3N4, creating nitrogen vacancies or carbon dopants introduces defect states that trap electrons, prolonging carrier lifetimes. Morphological control, such as exfoliating bulk g-C3N4 into ultrathin nanosheets, reduces charge migration distances and exposes more active sites. For COFs, constructing donor-acceptor systems within the framework promotes intramolecular charge transfer. A COF combining electron-donating triphenylamine and electron-accepting benzothiadiazole units demonstrates enhanced charge separation, leading to higher photocatalytic efficiency.

Composite formation with conductive materials or other semiconductors further improves charge separation. Conjugated polymers paired with carbon nanotubes or reduced graphene oxide exhibit accelerated electron transport due to the conductive network. In g-C3N4/graphene hybrids, graphene acts as an electron sink, reducing recombination rates. Similarly, covalent integration of COFs with metal-organic frameworks (MOFs) creates synergistic effects where the MOF facilitates charge separation while the COF provides photocatalytic activity.

In water splitting, conjugated polymers function as both light absorbers and reaction catalysts. g-C3N4 modified with cobalt-based co-catalysts achieves hydrogen evolution rates exceeding 100 µmol h⁻¹ under visible light. The polymer’s conduction band, positioned around -1.1 eV vs. NHE, provides sufficient driving force for proton reduction. For oxygen evolution, coupling g-C3N4 with iridium oxide boosts hole extraction, enabling overall water splitting in some systems. COFs with integrated bipyridine metal complexes mimic natural photosynthesis, where the metal centers act as active sites for water oxidation.

CO2 reduction leverages the tunable redox potentials of conjugated polymers. g-C3N4 with embedded single-atom nickel sites converts CO2 to CO with selectivity over 90%. The polymer’s conduction band aligns favorably with the CO2/CO redox potential, while nickel sites lower the activation barrier. COFs with built-in porphyrin units achieve methanol production, as the porphyrin’s metal center facilitates multi-electron transfer. By adjusting the linker design, the COF’s band structure can be optimized to match specific CO2 reduction pathways.

Pollutant degradation benefits from the oxidative power of photoexcited holes in conjugated polymers. g-C3N4 degrades organic dyes like rhodamine B within hours under visible light, with hydroxyl radicals as the primary reactive species. Sulfur-doped variants show improved activity due to enhanced charge separation and broader light absorption. COFs with tetrathiafulvalene units exhibit superior degradation rates for pharmaceuticals, attributed to efficient hole transfer to adsorbed pollutants.

Despite these advances, challenges remain in stability and scalability. Some conjugated polymers suffer from photodegradation or poor recyclability. Cross-linking strategies and protective coatings are being explored to enhance durability. Large-scale synthesis of uniform COFs also requires further development to ensure consistent photocatalytic performance.

Future research may focus on integrating machine learning for polymer design, enabling rapid screening of optimal structures for specific reactions. Advances in operando characterization techniques will provide deeper insights into reaction mechanisms at the molecular level. With continued progress, conjugated polymers could become a mainstream solution for sustainable photocatalysis, complementing or surpassing conventional inorganic materials in efficiency and versatility.