Effect of alkyl length dependent crystallinity for the mechanofluorochromic feature of alkyl phenothiazinyl tetraphenylethenyl acrylonitrile derivatives

Abstract

Three novel alkyl phenothiazinyl tetraphenylethenyl acrylonitrile derivatives (PhC3P4, PhC6P4, and PhC12P4) with alkyl groups of different lengths (–C₃H₇, –C₆H₁₃, –C₁₂H₂₅) were prepared in high yield (>90%) and successfully confirmed using ¹H NMR, ¹³C NMR, and mass spectrometry. Quantum mechanical computation demonstrated significant difference in the conformation of alkyl chains among the compounds, but showed no obvious difference in the migration of electron clouds. Apparent twisted intramolecular charge transfer (TICT) and an aggregation-induced emission (AIE) nature were evidenced in these derivatives. However, totally different mechanofluorochromic properties were found in three compounds, as PhC3P4 showed an obvious red-shifted mechanofluorochromic feature, while PhC6P4 and PhC12P4 exhibited almost no mechanofluorochromic phenomenon. Therefore, X-ray scattering, time-resolved emission-decay behaviour, and differential scanning calorimetry were performed, indicating that the mechanofluorochromic mechanism of PhC3P4 was attributed to the obvious phase transformation from the crystalline to the amorphous state, and a significant increase of the weighted mean lifetime. In comparison, PhC6P4 and PhC12P4 showed an amorphous state in the original sample, and had no apparent change of the aggregated state and weighted mean lifetime after grinding, which might be due to the steric effect of the long alkyl group in the compounds. Reversible mechanofluorochromic behaviour by fuming the ground sample was observed in PhC3P4, making it a promising smart fluorescent material for fluorescence switches and mechanosensors. The discussion on the effect of alkyl length dependent crystallinity for mechanofluorochromic compounds in this work would provide a new path to modify the performance of mechanofluorochromic materials.