Aggregation-induced enhanced emission (AIEE), pH sensing and selective detection of sulfuric acid of novel imidazole-based surrogates made via microwave-assisted synthesis

Abstract

A novel series of imidazole derivatives (4a–d) was synthesized via a microwave-assisted synthesis and whose structures were elucidated using ¹H and ¹³C NMR, ESI-HRMS, and FT-IR spectroscopy. Photophysical characterization revealed absorption peaks around 305 nm and 327–365 nm, with strong photoluminescence (PL) emission maxima ranging between 435 and 453 nm. Aggregation-induced enhanced emission (AIEE) behavior was observed in THF/H₂O mixtures, where 4a–c showed maximal fluorescence at certain solvent ratios, indicating aggregate formation. 4d disclosed a wavelength shift from 408 to 460 nm along with an enhanced emission, which is attributed to restricted intramolecular rotation (RIR), as confirmed by viscosity studies. Fluorescence lifetime and dynamic light scattering (DLS) measurements further supported the aggregation process, with particle sizes between 100 nm and 720 nm. Density functional theory (DFT) calculations validated electronic conjugation, showing HOMO–LUMO bandgaps (ΔE) of approximately 2.01–2.23 eV for 4a–d. Compound 4a exhibited the largest HOMO–LUMO energy gap of 2.23 eV, indicating greater electronic stability and enhanced emission efficiency upon aggregation. In contrast, compound 4d, with the smallest energy gap of 2.01 eV, suggests higher reactivity and better sensitivity to aggregation phenomena. This characteristic renders 4d particularly advantageous for sensing applications where rapid responsiveness to environmental variations is critical. pH sensing studies demonstrated the stability of 4a–d over a broad pH range, with 4d showing a 47 nm red shift in highly acidic conditions besides a selectivity for sulfuric acid detection. Investigation of sulfuric acid detection limit was studied, revealing a capacity for 4a–d to detect an acidic concentration as low as 16.5 μM. Stability and practical applicability of 4d compound as a sensor are further confirmed through reversibility and repeatability tests which reveal the possibility to regenerate the imidazole derivative even after several uses.