Alkyl Chain-Engineered Cyanostilbenes: Integrated Optical, Mechanical, and Mesomorphic Responses from Rigid Cores To...

A series of dicyanodistyrylbenzene derivatives Dn with different length of alkyl chains (C_4 ~ 16) were synthesized to systematically study the effects of alkyl chains on the solid state optical properties, mechanofluorochromic (MFC) and liquid crystal (LC) behaviors. The photophysical properties, time resolved photoluminescence (TRPL), fourier transform infrared spectrometer (FTIR) and X-ray diffraction (XRD) were conducted to explore their MFC response. MFC was fundamentally governed by the switch of excited-state species instead of just XRD or fluorescent lifetime changes. Dn with intermediate alkyl lengths (C₈, C₁₂) show the most contrastive MFC properties. Meanwhile shorter alkyl chains (C₄, C₈) promote the reversibility. However, excessive length (C₁₂, C₁₆) diminishes this effect. The mesomorphic properties were studied by combining differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM) and small-angle X-ray diffraction (SAXD) measurements. Alkyl chains mediate a crystal-to-mesophase transition. Shorter chains (C₄) favor crystalline rigidity, and LC properties were observed in homologs with longer alkyl chains (C_8 - 16). Four phase transitions of melting, S_E to S_C, S_C to S_A phase, and finally clearing were displayed in sequence during heating process. Lamellar structures were adopted with alkyl length correlating to interlayer distance and affecting structure stability. Among them, D8 exhibited the widest mesophase range. These findings establish alkyl chains as multifunctional spacers that comprehensively control optical, mechanical and thermal responses via competing π-π and aliphatic interactions.