Hydrogen bonded helices: Synthesis, crystal structure and self-assembled microtubes

Dicarboxylic bola-shaped compounds 1–3, possessing phthalyl head groups and diol spacers are synthesized and characterized. Keeping phthalyl head group common for all three diester-dicarboxylic acids, the spacer moiety is systematically altered by two and four carbon atoms in 1 and 2, 3, respectively. The flexible spacer moiety ethane-1,2-diol in compound 1 is replaced by cis-but-2-ene-1,4-diol and 1,4-butane diol in 2 and 3, respectively, to study the effect on the morphology of the microcrystal grown on them. Thus compound 2 and 3 though posses four carbon atoms in their respective spacer moiety, they differ by their rigidity. The single crystal X-ray structure obtained for 1, 2 and 3 indicates the formation of self-assembled single stranded helical structure mediated through O–H…O interaction of the end carboxylic acids. Interestingly compound 1 self-assembled into microtubes in ethanol:water solvent mixture. The solvent and the O–H…O; C–H…O interaction combinedly play crucial role in molecular self-assembly process and defines the morphology for 1 into “microtube” whereas 2 and 3 forming “bar” fails to produce such tubular texture though their respective crystal structure shows single stranded helices. The role of weak C–H...O interaction, incorporation of rigid spacer and various other factors such as polarity of the solvents are discussed in detail to explore the difference in the morphology.