Photocatalysis with stable, efficient and inexpensive metal-free catalysts is one of the most promising options for non-polluting energy production. This review article covers the state-of-the-art development of various effective metal-free polymeric photocatalysts with large π-conjugated units for chemical transformations including water splitting, CO₂ and N₂ reduction, organic synthesis and monomer polymerisation. The article starts with the catalytic mechanisms of metal-free photocatalysts. Then a particular focus is on the rational manipulation of π-conjugation enlargement, charge separation, electronic structures and band structures in the design of metal-free polymeric photocatalysts. Following the design principles, the selection and construction of functional units are discussed, as well as the connecting bonds and dimensions of π-conjugated polymeric photocatalysts. Finally the hot and emerging applications of metal-free polymeric photocatalysts for photocatalytic chemical transformations are summarized. The strategies provide potential avenues to address the challenges of catalyst activity, selectivity and stability in the further development of highly effective metal-free polymeric photocatalysts.

For highly sensitive detection of 4-nitrophenol (4-NP) in the environment, a novel pyridine diketopyrrolopyrrole-functionalized graphene oxide (PDPP–GO) composite was constructed for the first time by an improved Hummers' method. Herein, PDPP was completely dissolved in sulfuric acid (6 mol L⁻¹) and reacted with GO, promoting PDPP evenly adhering to the GO surface. Moreover, the specific surface area increased from 15.51 to 22.033 m² g⁻¹. Infrared spectroscopy and X-ray photoelectron spectroscopy simultaneously demonstrated that PDPP was bound to GO by the strong intermolecular hydrogen bonding and π–π stacking conjugation. During the cyclic voltammetry test, the PDPP–GO coated glassy carbon electrode (PDPP–GO/GCE) direct electrochemical sensor gave expression to the best electrocatalytic activity for 4-nitrophenol detection than GO/GCE and bare GCE. Under optimization conditions, the as-prepared PDPP–GO/GCE sensor brought out remarkable sensitivities of 18.54 (0.5–50 μM) and 6.61 μA μM⁻¹ cm⁻² (50–163 μM) in the linear detection of 4-NP. Besides, a low detection limit of 0.10 μM, reliable long-term stability, excellent selectivity, and reproducibility were obtained. In the real sample test, the PDPP–GO/GCE demonstrated sensitive and reliable determination.