Imidazo[1,5-a]pyridine–benzilimidazole conjugated (donor–π–acceptor) greenish-yellow fluorophores and their applications in white light-emitting diodes, acidochromism and anticounterfeiting

Abstract

This work reports the design and synthesis of three D–π–A push–pull fluorophores (BPy-1, BPy-2, and BPy-FL) featuring flexible phenyl or rigid and sterically hindered diethylfluorene moieties. The role of spacer units in modulating their photophysical properties has been analyzed through DFT analysis and experimental findings. All the fluorophores are characterized by excellent intramolecular charge transfer (ICT) and positive solvatochromism with a large Stokes shift (∼7000 cm⁻¹) due to the presence of the N1 functionalized benzilimidazole acceptor and the 1,3-diphenylimidazo[1,5-a]pyridine donor attached through a phenyl spacer in the case of BPy-1 and BPy-2 and a diethyl fluorene spacer in BPy-FL. BPy-1 and BPy-2 show strong greenish-yellow emission at around 520 nm in solution, solid, and thin-film matrices with a good quantum yield (∼70%). BPy-FL exhibits blue emission (λ_max = 458 nm) in THF solution and intense greenish-yellow emission (λ_max = 510 nm) in the solid state with an absolute quantum yield of 93%. The fluorophore BPy-FL shows aggregation-induced emission (AIE) and hybridized local charge transfer (HLCT) characteristics. The DFT analysis depicts the energy matching of high-lying triplet states (T₄–T₅) with the first excited singlet state, which is beneficial for hot exciton harvesting. Due to their strong greenish-yellow emissions in the solid state, these fluorophores are employed as organic downconverter materials in hybrid white light-emitting diodes (LEDs). Among the fabricated white LEDs, the LED consisting of BPy-FL demonstrated excellent luminous efficiency with a high color rendering index (CRI) of ∼90% and CIE coordinates (0.37,0.32) almost identical to the NTSC standard. Due to the bipolar nature of the imidazole ring, all these fluorophores display remarkable on–off–on fluorescence behavior in response to alternate addition of acid and base in solution as well as in a thin film state. These fluorophores are, therefore, successfully utilized as fluorescent pH sensors to detect volatile organic compounds with high acidity (an orange emissive state at pH < 4 and a greenish-yellow emissive state at pH > 4). The sensing mechanism has been studied through ¹H-NMR and DFT analyses. The potential of these fluorophores in anticounterfeiting applications has been studied, and due to intense emission in the solid state of BPy-FL, its application in latent fingerprint (LFP) detection has also been explored.