Isomeric Squaraine-Based [2]Pseudorotaxanes and [2]Rotaxanes: Synthesis, Optical Properties, and Their Tubular Structures in the Solid State

On the basis of formation of [2]pseudorotaxane complexes between triptycene-derived tetralactam macrocycles 1a and 1b and squaraine dyes, construction of squaraine-based [2]rotaxanes through clipping reactions were studied in detail. As a result, when two symmetrical squaraines 2d and 2e were utilized as templates, two pairs of isomeric [2]rotaxanes 3-ab and 4a-b as diastereomers were obtained, owing to the two possible linking modes of triptycene derivatives. It was also found, interestingly, that when a nonsymmetrical dye 2g was involved, there existed simultaneously three isomers of [2]rotaxanes in one reaction due to the different directions of the guest threading. The (1)H NMR and 2D NOESY NMR spectra were used to distinguish the isomers, and the yield of [2]rotaxane 5a with the benzyl group in the wider rim of the host la was found to be higher than that of another isomer 5b with an opposite direction of the guest, which indicated the partial selection of the threading direction. The X-ray structures of 3b and 4a showed that, except for the standard hydrogen bonds between the amide protons of the hosts and the carbonyl oxygen atoms of the guests, multiple pi center dot center dot center dot pi stacking and C-H center dot center dot center dot pi interactions between triptycene subunits and aromatic rings of the guests also participated in the complexation. Crystallographic studies also revealed that the [2]rotaxane molecules 3b and 4a further self-assembled into tubular structures in the solid state with the squaraine dyes inside the channels. In the case of 4a, all the nonsymmetrical macrocyclic molecules pointed in one direction, which suggests the formation of oriented tubular structures. Moreover, it was also found that the squaraines encapsulated in the triptycene-derived macrocycles were protected from chemical attack, and subsequently have potential applications in imaging probes and other biomedical areas.