Facile synthesis of triphenylamine and phenothiazine-based Schiff bases for aggregation-induced enhanced emission, white light generation, and highly selective and sensitive copper(ii) sensing

Abstract

Two new Schiff bases, one triphenylamine-based (TPA-SB) and another phenothiazine-based (PTz-SB), were synthesized via facile reactions. Both molecules showed aggregation-induced enhanced emission (AIEE) properties. In fact, an 8-fold increase in the fluorescence intensity was observed for a 20 : 80 THF : water mixture of TPA-SB as compared to that in pure THF. The AIEE nature was further confirmed by dynamic light scattering (DLS) experiment and transmission electron microscopy (TEM) studies. These two moieties were mixed with rhodamine B (Rh-B) dye to obtain efficient white light emission (WLE) in solution and gelatin gel phase. Excellent Commission Internationale d’Eclairage (CIE) coordinates (0.32, 0.34) and (0.31, 0.34) were obtained in the solution and gelatin gel phase, respectively. The correlated color temperatures (CCT) obtained in the two phases (6115 K in solution and 6500 K in gelatin gel) suggest that the system emitted cool white light. The mechanism of WLE as verified through fluorescence titration, lifetime and spectral overlap studies, was found to arise due to Förster resonance energy transfer (FRET) among the three fluorophores. The optical band gaps obtained from UV-visible spectroscopy were 2.82 and 2.59 eV for TPA-SB and PTz-SB, respectively. The SEM and TEM studies revealed that in the aggregated state, TPA-SB exhibited a spherical morphology while PTz-SB showed nanopyramidal morphology. The new molecules were thermally robust as reflected by their TGA and DSC studies. Interestingly, TPA-SB showed remarkable fluorescence quenching response towards copper(II) cations with a limit of detection (LOD) value of 1.8 ppb, while PTz-SB showed a LOD value of 4.8 ppb. The Schiff bases were particularly selective towards Cu(II) ions without interference from ten other common cations. Hence, these easily synthesizable Schiff bases have multiple applicability, viz., in AIEE, WLE and Cu²⁺ sensing.