Synthesis and Characterization of a Novel Pillar[5]arene That can Undergo Single-Crystal-to-Single-Crystal (SCSC) Transformation

Organic smart materials are able to change their properties in response to specific stimuli, being a type of materials that are potential to be applied in many fields. The stimuli-responsive abilities often arise from variations of structures at the molecular level. In this study, a new type of skeleton-functionalized pillar[5]arene (N-naphthyl-phenothiazinyl-pillar[5]arene, 6) was successfully synthesized by incorporating a quinoline structure with a naphthalene substitution into the pillar[5]arene skeleton. First, compound 2 was obtained through two-step reactions, and was further reacted with 2-aminothiophenol through an amine/carbonyl condensation followed by substitution to yield a quinone-type thiazine structure of pillar[5]arene 3. Compound 3 was further reduced to a phenol-type structure, i.e. compound 4, by sodium borohydride. Compound 4 was further functionalized with 2-bromo-naphthalene through palladium-catalyzed Buchwald-Hartwig reaction to yield compound 5. In order to improve stability, the hydroxy group of compound 5 was converted to a methoxy group by injecting iodomethane into the acetone reaction solution of compound 5, resulting in compound 6. Structure of compound 6 was characterized by H-1 NMR (nuclear magnetic resonance spectroscopy), C-13 NMR, high-resolution mass spectra (HRMS), and X-ray single-crystal diffraction. The single crystal of this new pillar[5]arene was able to undergo a single-crystal-to-single-crystal (SCSC) conformational transformation under heating, accompanied by a change in stacking mode. Moreover, it is found that changing the guest molecule can adjust the molecular conformation in the grown single crystal. X-ray single-crystal diffraction and thermogravimetric analysis were used to verify this transformation process. This work studies the solvent-induced crystal conformation transition process from the molecular level, which provides an example for the rational construction of stimuli-responsive smart materials.