1,10-Phenanthroline-5-carboxylic acid is a functionalized derivative of 1,10-phenanthroline, a classic bidentate nitrogen-containing heterocyclic ligand. Its structure features a carboxylic acid group at the 5-position, which provides a versatile handle for further chemical modification, coordination, or covalent attachment to other molecular frameworks. This compound is primarily utilized as a key building block in the synthesis of advanced functional materials. Its most prominent application is in the field of coordination chemistry and materials science, where it serves as a precursor for constructing metal-organic complexes, particularly those involving ruthenium, iridium, and other transition metals. These complexes are extensively studied and employed as emitters, sensitizers, or charge-transport materials in organic light-emitting diodes (OLEDs), dye-sensitized solar cells (DSSCs), and other optoelectronic devices. The carboxylic acid group facilitates its incorporation into larger molecular architectures or its anchoring onto metal oxide surfaces (e.g., TiO₂ in solar cells). Additionally, due to its well-defined chelating properties and synthetic utility, it finds use as a specialized organic synthesis intermediate in academic and industrial research for developing novel ligands, sensors, and catalysts.