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Multi-functional switches of ditopic ligands with azobenzene central bridges at a molecular scale.

Ligands are designed to have ditopic bipyridine terminal groups linked through photochromic azobenzene central units, which exhibit multi-switchable properties by different external stimuli. The molecule can switch between cis-and trans-conformations at their bipyridine terminal groups upon protonation and at their central azobenzene units upon irradiation of photons. As a result, the system shows four different isomeric states: cis-TRANS, trans-TRANS, cis-CIS and trans-CIS. The four conformers are switched and are visualized by scanning tunneling microscopy at the solid-liquid interface, which gives a direct demonstration of the multi-functional switches at a molecular level.