Organo Eu(iii) based narrowband red emitting components demonstrate dynamic character and have extraordinary performance in the smart optoelectronic device, sensing, and biomedical fields. In this report, a series of unique Eu(iii) complexes have been synthesized using a new class of ancillary neutral thiabendazole ligands (TBZ, in which the N1 position is functionalized with benzyl and N-alkylated carbazole as pendant moieties). Dibenzoyl methane 2-theonyltrifluoroacetone are used as anionic ligands to saturate the coordination sphere. A computational study reveals that the TBZ based neutral ligands are better energy harvesters than other analogous neutral ligands. All the freshly synthesized TBZ based Eu-complexes exhibit outstanding narrowband red emission due to an electric dipole transition (5D0 → 7F2) in the solid state, solution, and thin film. Theoretical analysis (TD-DFT) and an experimental study (phosphorescence spectra) indicate that the energy transfer from the ligand to the Eu(iii) ion is comprehensive. The Eu(iii) complexes are potentially applied as red phosphors for the fabrication of red light-emitting diodes (LEDs) and hybrid white LEDs. The LEDs were fabricated by spreading Eu(iii) over UV LED/blue LED chips conjugated with a yellow dye with the proper concentration. The fabricated red LEDs showed high luminous efficiency of radiation values. The blue LED-based hybrid white LEDs showed superior performance with a high color rendering index CRI (83%) and a CIE (x = 0.37, y = 0.34) close to the NTSC standard. Moreover, the present Eu complexes showed outstanding alterable on-off-on luminescence performance in response to contact with acid and base vapors. The intense red emission of the complexes gives rise to anti-counterfeiting applications. To explore the biocompatibility of the complexes, it was further demonstrated that the TBZ based Eu(iii) complexes have potential applications for the biolabeling of cells. Owing to the less cytotoxic behavior of Eu(iii) complexes, they can be used as potential red emitters (markers) for bioimaging studies. They also show great performance as cytoplasmic staining reagents due to their intense red emission. © 2022 The Royal Society of Chemistry.