Synthesis of Anthracene-Based Cyclic π-Clusters and Elucidation of their Properties Originating from Congested Aromatic Planes.

Synthesis and properties of anthracene-based cyclic π-clusters 1 and 2 , which possess two and four anthracene units, respectively, are discussed. The optimal cyclization conditions were determined based on a nickel(0)-mediated reaction that afforded 1 as the major product. Bringing two anthracene planes in close proximity in a face-to-face manner resulted in red-shifted absorption owing to the narrowing of the HOMO-LUMO gap. Cyclic π-cluster 1 exhibits multi-stimuli responsiveness due to high π-congestion. For example, photoirradiation on 1 affords its photoisomer 1 ' having C-C bonds that are longer than 1.65 Å, which can undergo thermal reversion to 1 under gentle heating. This enabled mechanochromism of photoisomer 1 ' (colorless) to 1 (red). Photoisomerization was also observed in the crystalline state, accompanied by crystal jumping or collapsing, that is, the photosalient effect.