Controlling Crystal Structures and Multiple Thermo- and Vapochromic Behaviors of Benzimidazole-Based Squaraine Dyes...

Organic micro- and nanostructures are expected to be promising candidates for micro- and nanophotonic materials with desirable properties owing to their low cost, flexible molecular design, and tunable self-assembly. Among these candidates, well-known squaraine dyes (SQs) have rarely been investigated because of their nonfluorescent properties in the solid state and because their optical behavior varies with changes in morphology. In this contribution, two novel 1,2-SQs, SQM and SQB, with strong bright-yellow to red fluorescence emission in the crystalline state, were designed and structured at the molecular level and by solvent adjustment. Their self-assembly behavior was studied, and it was revealed that the SQM assembly provided 1D microrods, whereas 1D microrods (Z-SQB⋅CH₂ Cl₂ ) and 2D microplates (E-SQB⋅2 CH₃ OH) could be obtained from SQB assemblies through a solution-based self-assembly method. The varied assembly behaviors of these SQs were attributed to different π-π stacking interactions that resulted in different molecular conformations and packing modes. These assemblies exhibited distinct optical properties, and in particular, SQB⋅solvent assemblies showed multiple thermo- and vapochromic effects. Thus, the SQB assemblies are potential fluorescent sensors for organic solvent vapors. More importantly, favorable optical-waveguide properties were observed in these SQ-based microstructures.