A series of smart luminescent Eu(iii) complexes (C1 = Eu(DBM)3ligand 1, C2 = Eu(DBM)3ligand 2, C3 = Eu(DBM)3ligand 3, C4 = Eu(DBM)3ligand 4, C5 = Eu(DBM)3ligand 5) were synthesized by using C1-functionalized phenanthro-imidazole derivatives as the neutral ligands and dibenzoylmethanate (DBM) as the anionic ligand. All the neutral ligands and their corresponding Eu(iii) complexes were structurally characterized by spectroscopic methods. Photoluminescence (PL) emission spectra of the ligands showed blue to bluish-green emission, whereas their corresponding Eu complexes showed multi-color emission in solution and red emission in the solid state (solid as well as neat thin film). The energy transfer mechanism between the ligand and the Eu(iii) ion is explained based on both theoretical (density functional theory (DFT) and time-dependent-DFT) and experimental study. Red light-emitting diodes (LEDs) were fabricated by integrating a near-ultraviolet (NUV)_ LED chip (∼395 nm) with Eu(III) complexes and all of the LEDs showed excellent color purity. A hybrid white LED was fabricated by conjugating the mixed phosphor dye (ligand and complex) with near UV LED, showing superior white light emission characteristics (CCT (5298), CRI (80%), CIE (x = 0.337, y = 0.343)). In addition, dual emission Eu(iii) complexes were explored for thermo-sensor (in the range 303-353 K) applications. Paper-based acid-base vapour sensors (exposure leads to a switch in the emission color (bluish-green to red)) using these complexes were explored and can also be used for anti-counterfeiting applications. This journal is © The Royal Society of Chemistry.