Benzil-imidazole blue fluorophores and their applications in blue/white light-emitting diodes, sensing and anticounterfeiting

Abstract

A series of benzilimidazole-based donor–π–acceptor (D–π–A) blue fluorophores, functionalized at the N1 position of the imidazole ring are designed and synthesized. In solution, solid, and thin-film matrices, the luminophores exhibit intense blue emission with a good quantum yield (∼70%) and excellent thermal stability, and show excellent intramolecular charge transfer (ICT) with a large Stokes’ shift (∼8000 cm⁻¹). Due to the excellent fluorescence characteristics in the solid phase, these synthesized fluorophores are utilized for the fabrication of hybrid white and blue light-emitting diodes (LEDs) and anticounterfeiting applications. The fabricated blue and white LEDs exhibited high luminous efficiency of radiation values with CIE coordinates that are nearly matched with the NTSC standard and a high color rendering index (CRI) of 86%. Latent fingerprint (LFP) imaging has been effectively implemented using fluorophores. They can detect weak or damaged fingerprints, which traditional techniques can miss, and also, the LFP is photostable and temperature-resistant. Additionally, specific structural functionalities of the fluorophores enable them to interact selectively with picric acid (PA) over other nitroaromatic compounds (NACs). The sensing mechanism and selective interactions between the fluorophores and PA are analyzed using nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations. These fluorophores exhibit superior sensing abilities towards PA compared to other NACs, as evidenced by their high quenching constant (K_sv) of 6.6 × 10⁶ M⁻¹ and ultralow detection limit of 400 ppb compared to other recently reported fluorophores for PA detection. Furthermore, due to the bipolar nature of the imidazole ring, these molecules are found to show remarkable on–off–on fluorescence behavior in response to acid–base vapor interaction and, thus, can be a probable candidate for vapor sensing.