Concentration-Dependent Conformational Isomerization of Fluorenone-Based Polycatenar in 2D Polymorphic Self-Assembly
2020
A fluorenone-based polycatenar consisting of a central 2,7-bis((5-phenylthiophen-2-yl)ethynyl)-9-fluorenone core and −OCH₂– linked 3,4,5-tris(dodecyloxy)benzoate unit at both sides (FTQ) has been synthesized by using Sonogashira coupling reactions as key steps. FTQ molecule exhibits columnar liquid crystalline phase. The molecular self-assembled behavior at the solid/liquid interface is investigated by scanning tunneling microscopy (STM). With the decrease of solution concentration, three nanostructures (parallelogram, wave-like, and double-S patterns) are formed at the 1-phenyloctane/HOPG interface. More importantly, the molecular cis–trans isomerization is observed, resulting from different packing fashions of the side chains. The dimers are the basic building units due to the similar C(sp²)–H···O═C hydrogen bonds formed between the hydrogen atom in the thiophene group and the oxygen atom in the carbonyl group. Density functional theory (DFT) results show that the isomerized structure constructing by the S-shaped conformation is more stable. We speculate that the double-S structure might be in accordance with the molecular conformation in liquid crystalline phase. Our work is of significance to further explore molecular packing mechanism and stability of fluorenone-based polycatenars with liquid crystal property.
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