Photoisomerization of Bis(tridentate) 2,6-Bis(1H-pyrazol-1-yl)pyridine Ligands Exhibiting a Multi-anthracene Skeleton.

A novel molecular design is described where two peripheral moieties made of 2,6-bis(1H-pyrazol-1-yl)pyridine are linked through multi-1,8-diethynylanthracene moieties. The optimized synthesis of the three isostructural analogues 1 a, 1 b, and 1 c, containing the anthraquinone, anthracene, and 10-methoxyanthracene units, respectively, is reported. The resulting spatial face-to-face arrangement of the peripheral anthracene rings enables to trigger the intramolecular [4+4] photocycloaddition affording the isomers P1 b and P1 c, which can be thermally cleaved back to the original anthracene derivatives 1 b and 1 c, respectively. Single-crystal X-ray diffraction studies confirm the expected molecular structures of compounds 1 a-1 c as well as of their corresponding isomers P1 b and P1 c. The spectral, optical, and electrochemical properties of all synthesized compounds are investigated and discussed.