Bis-pyrene based reversibly multi-responsive fluorescence switches

Abstract

A bis-pyrene derivative m-Py-BP was designed and synthesized, where two pyrenes units were connected by pyridine-2,6-dicarbonyl linkers, displaying multi-responsive fluorescence switching behaviors. In solution, the quenched fluorescence emission of m-Py-BP by HCl was recovered with the addition of NH₃·H₂O. In the solid state, m-Py-BP exhibited vapor, mechano and thermo-induced fluorescence variations. The as-prepared sample showed green fluorescence (G form) with emission maxima at 483 nm. Upon mechanical shearing, the fluorescence of G form was transformed into a yellow color (Y form) by disrupting the crystalline structures. Vapor or heating induced the transformation from Y to G form. Powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), time-resolved fluorescence (TRF) and scanning electron microscopy (SEM) supported the corresponding changes in supramolecular structures and morphologies. Interestingly, m-Py-BP showed acid–base responsive fluorescence switching behavior. The HCl could quench the fluorescence of Y form to DY (dark yellow) form, which could be recovered to Y form in the presence of NH₃·H₂O or converted to DG (dark green) form upon treatment with CHCl₃ solvent vapor. Furthermore, the DG form could become DY form by mechanical shearing or convert to G form upon treatment with NH₃·H₂O vapor. The halochromic fluorescence should be related to the protonation of m-Py-BP molecules by acid treatment. The protonated pyridine became a strong electron acceptor, resulting in fluorescence quenching. m-Py-BP could construct a multi-responsive and recyclable fluorescence switching system.