Aggregation-induced emission and reversible mechanofluorochromic characteristics of tetra-substituted tetrahydropyrimidine derivatives

Abstract

Organic compounds possessing aggregation-induced emission (AIE) and mechanofluorochromic characteristics are promising in chemo/biosensors and data storage. Here, seven tetra-substituted tetrahydropyrimidine derivatives (TTHPs), products of efficient four-component reactions (4CR), were designed and synthesized for investigation into the AIE and mechanofluorochromic characteristics of TTHPs. It is found that TTHPs show strong AIE: no emission in solutions, but highly emission upon aggregation, with fluorescence quantum yields (ΦF) in solid state up to 80%. TTHPs also show distinct reversible mechanofluorochromic characteristics: upon grinding, TTHPs display a red shift in maximum emission wavelength (λem) up to 46 nm, accompanied by the decrease of ΦF; while upon fuming or heating, the ground powders revert to their initial state. The fluorescence of ground TTHPs 4–6 can recover to their original states spontaneously at room temperature in 1–24 h. The X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) measurements demonstrate that the crystalline TTHPs change into amorphous state by grinding, and revert to initial crystalline state upon fuming or heating. This proves that the mechanofluorochromic characteristics of these TTHPs originates from the reversible transformation between thermodynamically metastable ground powder and thermodynamically stable crystal. This work presents a new and easily synthesized series of materials with interesting AIE and mechanofluorochromic characteristics, which may have great potential in sensing, data storage and self-repair materials.