A pyridine-based small-molecule receptor (L) for the specific recognition of TNP was synthesised and characterised by ¹H NMR, ¹³C NMR and FT-IR spectra, and the optical properties were studied by UV-Vis absorption and PL spectra. The results showed that the fluorescence of L was strongly quenched by picric acid accompanied by an obvious colour change from transparent to yellow, which indicated that the receptor L can serve as a small-molecule sensor for TNP. Crystal structure determination and theoretical calculation of the corresponding host–guest complex L·TNP were also performed.