Red-emitting ionic fluorophores: anion-dependent tunable fluorescence, chlorinated solvent sensing, white light emission and latent fingerprinting

Abstract

Developing organic fluorescent molecules with multi-functional properties such as stimuli responses, fluorescence tuning, sensing, white light emission and bioimaging is highly desirable because of their application potential. Herein, we synthesized a triphenylamine (TPA)-based red-emitting ionic fluorophore, (E)-4-(4-(diphenylamino)styryl)-1-alkylpyridin-1-ium (TPA-P), with varied alkyl chain lengths (TPA-P-1 (methyl), TPA-P-7 (heptyl) and TPA-P-14 (tetradecyl)) and utilized them for fluorescence tuning, white light emission, chlorinated solvent sensing and latent fingerprinting applications. TPA-P ionic fluorophores showed anion-dependent tunable solid-state fluorescence from 564 to 671 nm. Single crystal structural studies suggested the anion exchange and modulation of molecular packing in the solid state. Further, TPA-P-1 revealed distinctly different colors and fluorescences towards sodium dodecyl sulphate (SDS) anions in an aqueous medium. Limit of detection (LOD) analysis indicated a sensing response up to 0.8 μM of SDS. TPA-P fluorophores also exhibited sensitive color changes in chlorinated solvents. TPA-P exhibited distinguishable fluorescence in ortho-dichlorobenzene (o-DCB) and meta-dichlorobenzene (m-DCB) isomers. The ionic nature of red-emitting fluorophores was exploited to generate a stable white light-emitting solution by mixing with blue fluorescent polystyrene sulphonate (PSS). CIE chromaticity indicated 0.32, 0.33 color coordinates, which are very close to pure white light emission. The red fluorescence and lipophilic alkyl chain of the TPA-P-14 ionic fluorophore were utilized for latent fingerprinting via powder dusting, which showed well-defined finger marks up to level 3 information. Thus, the quaternization of pyridine nitrogen in TPA π-conjugated fluorophores red shifted the fluorescence, and ionic functionality was further utilized to tailor functional fluorescence properties.