The abnormal solvatochromism, high-contrast mechanochromism and internal mechanism of two AIEE-active β-diketones

Abstract

Tradition β-diketones are rarely used as fluorescent emitters and mechanochromic materials. Here, TPED2F and TPED2T with tiny structural differences (Furan unit and thiophene unit) and the same asymmetric β-diketone skeleton were designed and synthesized. Both of them show abnormal solvatochromism, water-sensitive fluorescence emission, marked aggregation-induced enhanced emission (AIEE) activity and high-contrast mechanochromism. Furthermore, crystal TPED2T displays inferior quantum efficiency (0.08) and hypsochromic-shift fluorescence emission (493 nm) in contrast with crystal TPED2F presenting 0.20 quantum efficiency and 516 nm emission maxima. However, ground and cured TPED2T exhibit deeper bathochromic-shift (54 nm and 59 nm in turn) than ground (37 nm) and cured (1 nm) TPED2F. In addition, ground TPED2F show reversible mechanochromism by heating, but mechanochromism of ground TPED2T is irreversible whether heating or solvent fuming. Based on 1H NMR, 13C NMR, UV–vis absorption spectra, emission spectra, XRD, DSC, single crystal analysis and theoretical calculation, the abnormal solvatochromism and water-sensitive fluorescence emission are attributed to enol-keto tautomerism and solvent polarity induced conformation changes of excited states, while heavy atom effect and wide band gap respectively lead to inferior quantum efficiency and hypsochromic-shift emission for crystal TPED2T compared with TPED2F. Moreover, excellent crystallization ability and close melting point before/after grinding endow TPED2F with thermal reversible mechanochromism. The amorphous state provided grinding and melting may contain distinct local energy traps, resulting in different fluorescence emission. Finally, existence forms of TPED2F and TPED2T depend on intramolecular hydrogen bonds, conjugate structure and small relative energy.