Alkyl length dependent mechanofluorochromism of AIE-based phenothiazinyl fluorophenyl acrylonitrile derivatives

Four alkyl phenothiazinyl fluorophenyl acrylonitrile derivatives (PhC3F, PhC6F, PhC10F, and PhC12F) with different length of alkyl chains were synthesized in high yield. The compounds showed obvious aggregation-induced emission (AIE) and alkyl length dependent mechanofluorochromic (MFC) properties. PhC3F with propyl chain and PhC12F with dodecyl chain showed obvious and reversible MFC behaviours, while PhC6F with hexyl chain and PhC10F with decyl chain exhibited little MFC property. Small and wide-angle X-ray scattering indicated the MFC behaviour of PhC3F and PhC12F were attributed to the phase transformation from crystalline to amorphous state, while PhC6F maintained excellent crystalline performance before and after grinding, which was also evidenced by differential scanning calorimetry result. Quantum mechanical computations further revealed PhC6F had high molecular polarity, which greatly improved its crystalline performance, and made its original aggregated morphology hard to damage, then brought about little MFC diversification. PhC10F showed the highest molecular polarity and least MFC behaviour. We concluded that obvious MFC feature derived from appropriate crystalline performance, favourable molecular polarity and energy gap.