Mechanofluorochromic properties of tert-butylcarbazole-based AIE-active D-π-A fluorescent dye

Abstract

Two new non-planar carbazole derivative DBTNC and NC were designed and synthesized. The D-π-A type conjugated compounds showed obvious intramolecular charge transfer (ICT) properties, which were evidenced by theoretical calculations and spectral analysis. Both of them exhibited evident AIE characteristic with high fluorescence quantum yield in the solid state with the fluorescence quantum yields of 0.37 and 0.33, respectively. DBTNC and NC were quite weakly fluorescent in THF, while a significant AIE effect were observed in water/THF mixtures with AIE factors of 10.7 and 9.7, respectively. Interestingly, DBTNC showed reversible mechanofluorochromism (MFC). The as-prepared solid of DBTNC emitted strong yellowish-green light and could be transformed into the powders emitting orange-yellow light after grinding with the fluorescence quantum yield decreased to 0.35, and the fluorescence could recover the original station when the ground powders were heated or fumed with DCM for 1 min. The powder XRD and DSC studies suggested that the mechanofluorochromic properties of DBTNC originating not from chemical changes but from the transformation of a crystalline state into an amorphous state. We deduced that tert-butyl can lead to the incompact packing of luminophore, which was more sensitive to external stimuli, so that multi-color solid-state luminescence can be afforded during mechanofluorochromic process. Herein, we provided the strategy for fabricating new mechanofluorochromic dyes via the introduction of non-planar π-skeletons and steric effect, which may possess potential applications in mechanosensors and anti-faked materials.