Metal ions drive thermodynamics and photochemistry of the bis(styryl) macrocyclic tweezer.

UV/Vis and NMR spectroscopy were used for the structural elucidation and thermodynamic and photochemical studies of the metal-coordinated crown-containing macrocyclic tweezer (E,E)-1. The bis(styryl) tweezer (E,E)-1 formed two types of complexes with magnesium(II): a 1:1 intramolecular asymmetric sandwich complex [(E,E)-1].Mg(2+) and a 1:2 complex [(E,E)-1].(Mg(2+))(2). In the former case, there is direct cation intramolecular exchange (0.299 s(-1), DeltaG(++)=69.4 kJ mol(-1)) between two parts of the bis(styryl) tweezer (E,E)-1. Addition of barium(II) to the bis(styryl) tweezer (E,E)-1 led to an intramolecular centrosymmetric sandwich 1:1 complex [(E,E)-1].Ba(2+). Irradiation of [(E,E)-1]Ba(2+) afforded reversible intramolecular [2pi+2pi] photocyclization with excellent stereoselectivity and quantitative yield. In contrast, irradiation of [(E,E)-1].(Mg(2+))(2) resulted in reversible stepwise E,Z-isomerization.