Photochemical- and pH-switching properties of a new photoelastic ligand based upon azobenzene. Basicity and anion binding

The synthesis of a new cylindrical macrotricyclic ligand (L) based upon two 1,4,7,13-tetramethyl-1,4,7, 10,13,16-hexaazacyclooctadecane rings connected via two azobenzene pillars is described. The ligands displays photoelastic properties: light absorption at 366 nm produces trans --> cis isomerization of the azobenzene moieties with formation of trans-cis (LE-Z) and cis-cis (LZ-Z) isomers. Recovery of the thermodynamically more stable trans-trans (LE-E) isomer can be obtained by thermal back-isomerization or visible light (430 nm) absorption. The isomeric composition of L solution depends upon pH: for pH < 5.5 the LE-E form is the unique species in solution, while increasing amounts of Z species are formed at increasing pH. The protonation constants of LE-E have been determined in 0.1 mol dm(-3) Me4NNO3 solution at 298.1 +/- 0.1 K. The protonated species of L form stable inclusion complexes with anionic species such as [Co(CN)(6)](3-).