Dual-state emission triphenylamine-coumarin fluorescent polymorphs: halochromic reversible fluorescence switching and π–π stacking facilitated picric acid sensing

Abstract

Organic fluorescence molecules with external stimuli-responsive properties have received significant interest because of their potentiality in various device applications ranging from optoelectronics to biological sciences. Herein, we designed and synthesized dual-state emission (DSE), triphenylamine (TPA)-integrated, coumarin-based donor–acceptor multifunctional fluorophores (DP2C and DP4C) with an acid responsive pyridine unit. DP2C produced fluorescent polymorphs (yellow (DP2C-Y) and orange (DP2C-O)) and showed tunable solid-state fluorescence (λ_max = 537 and 560 nm). DP2C-O exhibited relatively strong fluorescence (quantum yield (ϕ_f) = 20%) compared with DP2C-Y (ϕ_f = 12%) and DP4C (ϕ_f = 16%), which could be attributed to the higher molecular twist in the solid state without any close π-stacking. Both DP2C and DP4C showed strong fluorescence in the solution state, especially in non-polar toluene and CHCl₃ (ϕ_f = 0.69) compared with a 0.5 M quinine sulphate standard in H₂SO₄. Acid responsive pyridine and DSE were utilized to demonstrate reversible fluorescence switching via protonation and deprotonation upon exposure to trifluoroacetic acid (TFA) and triethylamine/ammonia (NEt₃)/NH₃ in CHCl₃ and solids. TFA addition red shifted the fluorescence with a strong reduction in fluorescence intensity, which was reversed to the initial state by adding NEt₃. Furthermore, DP2C and DP4C exhibited highly selective fluorescence sensing of picric acid (PA). Limit of detection (LOD) studies for PA sensing indicated sensing values of up to 280.7 and 310 nM corresponding to DP2C and DP4C, respectively. Single crystal analysis revealed π-stacked arrangements between DP2C and PA. Computational studies showed intermolecular charge transfer from DP2C to the PA unit in the lowest unoccupied molecular orbital (LUMO), supporting fluorescence quenching. Fluorescent PMMA and PVA polymer thin films were fabricated and demonstrated reversible fluorescence switching and PA sensing, respectively, in thin films, indicating the practical utility of the developed DSE fluorophores.