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.

Scheme 1. Synthesis of 4a–d.

Fig. 1. Normalized UV/vis absorption (A) (C_M = 10⁻⁶ M solid lines) and emission (B) (C_M = 10⁻⁷ M dashed lines) spectra of compounds 4a–d recorded in THF (absorption maxima were used as the excitation wavelengths).

Fig. 2. (A) Emission spectra of 4d in THF/water mixtures (0–100%); (B) plot of maximum emission intensity of 4dversus water fraction; (C–F) dynamic light-scattering (DLS) spectra of 4a–d in THF/water, at specific f_w ratio.

Fig. 3. (A) Powder XRD pattern of the synthesized (black graph) and THF/water, f_w 90% (blue graph) of 4d and (B) fluorescence lifetime decay profiles of 4d in THF/water, f_w %; 0 (red graph) and 90 (blue graph).

Fig. 4. The optimized structures and molecular orbital amplitude plots of HOMO and LUMO energy levels for 4a–d computed using the B3LYP/6-31G* basis set.

Fig. 5. Emission spectra of 4d (1 × 10⁻⁷ M) in different THF-buffer mixture, 9 : 1, v/v, λ_exc = 365 nm.

Fig. 6. (a) Emission spectra of 4d (1 × 10⁻⁶ M in THF) in various acid solutions (excitation wavelength: 365 nm). (b) Linear correlation of I₀/I vs. the concentrations of H₂SO₄ in the range 0.5 × 10⁻⁵ to 1.75 × 10⁻⁴ M.

Fig. 7. (a) Emission spectra of 4d (1 × 10⁻⁶ M in THF) in different acidic and basic solutions (excitation wavelength: 365 nm). (b) Change of the emission intensity at ∼450 nm upon adding H₂SO₄ and NaOH (excitation wavelength: 365 nm).