Synthesis, Structures, and Photophysical Properties of π‑Expanded Oligothiophene 8‑mers and Their Saturn-Like C₆₀ Complexes

Two isomers of a multifunctional π-expanded macrocyclic oligothiophene 8-mer, <i>E</i>,<i>E</i>-<b>1</b> and <i>Z</i>,<i>Z</i>-<b>1</b>, were synthesized using a McMurry coupling of a dialdehyde composed of four 2,5-thienylene and three ethynylene units under high dilution conditions. On the other hand, cyclo[8]­(2,5-thienylene-ethynylene) <b>2</b> was synthesized by intramolecular Sonogashira cyclization of ethynyl bromide <b>5</b>. From STM measurements, both <i>E</i>,<i>E</i>-<b>1</b> and <i>Z</i>,<i>Z</i>-<b>1</b> formed self-assembled monolayers at the solid–liquid interface to produce porous networks, and from X-ray analyses of <i>E</i>,<i>E</i>-<b>1</b> and <b>2</b>, both compounds had a round shape with a honeycomb stacked structure. <i>E</i>,<i>E</i>-<b>1</b> formed various fibrous polymorphs due to nanophase separation of the macrorings. <i>E</i>,<i>E</i>-<b>1</b> and <i>Z</i>,<i>Z</i>-<b>1</b> in solution exhibited photochromism upon irradiation with visible and UV light, respectively, and this photoisomerization was confirmed by using STM. Furthermore, amorphous films of <i>Z</i>,<i>Z</i>-<b>1</b> and <i>E</i>,<i>E</i>-<b>1</b> showed photoisomerization, although single crystals, fibers, and square tubes of <i>E</i>,<i>E</i>-<b>1</b> remained unchanged under similar conditions. <i>E</i>,<i>E</i>-<b>1</b> with a 12.5–14.7 Å inner cavity incorporated fullerene C₆₀ in the cavity in solution and the solid state to produce a Saturn-like complex, whose structure was determined by X-ray analysis. <b>2</b> also formed a Saturn-like complex with C₆₀ in the solid state. These Saturn-like complexes are stabilized by van der Waals interactions between the sulfur atoms of 8-mer and C₆₀. The complexes exhibited charge-transfer interactions in the solid state. Like <i>E</i>,<i>E</i>-<b>1</b>, Saturn-like complex <i>E</i>,<i>E</i>-<b>1</b>⊃C₆₀ formed small cube and fiber structures depending on the solvent used, whereas those of Saturn-like complex <b>2</b>⊃C₆₀ were limited due to the rigidity of the macroring of <b>2