A single crystal X-ray structurally characterized amide–imine conjugate (GAN) derived from gallic acid and naphthalene selectively recognizes La³⁺ ions via TURN ON fluorescence through ESIPT and CHEF mechanisms. GAN can detect as low as 23.93 × 10⁻⁹ M La³⁺ ions and image intracellular La³⁺ ions in live HeLa and SiHa cells under a fluorescence microscope in a time- and concentration dependent manner. The corresponding [La(III)–GAN] complex is established as an efficient catalyst for the synthesis of benzimidazole derivatives from o-phenylenediamine and substituted benzaldehydes. Moreover, GAN is very useful for enrichment of La³⁺ in ethyl acetate medium.