Narrow band red emitting phosphors play a vital role in high performance smart white LEDs. In this context, a series of new Eu(iii) complexes have been synthesized with neutral ligands (C1-functionalised phenanthro-imidazole-based ancillary ligands substituted with phenyl moieties (m-CF3, p-CF3, p-CH3and 1-naphthyl and 2-naphthyl group)) and dibenzoylmethane (DBM) as an anionic ligand. All the newly synthesized Eu-complexes showed extremely narrow band red emission due to the electric dipole transition (5D0-7F2, both in solid and thin film) with high quantum efficiency. A combined experimental and theoretical study indicates that the energy transfer from the ligand to the metal ion is complete. A temperature dependent photoluminescence (PL) study in solution reveals that the red emission is retained (only the ED (5D0-7F2) intensity decreases with increasing temperature). Red LEDs and hybrid white LEDs were fabricated by using a near UV LED chip with europium complexes and a near UV/blue LED conjugated with a yellow organic dye and europium complex mixture, respectively. Hybrid white LEDs (Eu(DBM)3Phen-pCN-pCF3+ yellow organic dye) showed superior CRI (84%) and CIE (x= 0.36,y= 0.39) values (near UV based) and CRI (82%) and CIE (x= 0.34,y= 0.36) values (blue LED based). In addition, the studied Eu-complexes herein showed excellent reversible on-off-on luminescence behavior with exposure to acid-base vapours. Detailed spectroscopic investigation reveals that the protonation of the Eu-complexes (ligands) plays a key role in the on-off-on luminescence. © The Royal Society of Chemistry 2020.