Irradiation of acetonitrile solutions of the bis-crown ether E,E-2,7-bis[2-(6,7,9,10,12,13,15,16-octahydro-5,8,11,14,17-pentaoxabenzocyclopentadecen-2-yl)vinyl]-benzo[1,2-d;3,4-d′]bisthiazole (hereafter, 1) gives efficient E → Z photoisomerization (initial φtrans→cis = 0.48), leading to λexc-dependent quasi-photostationary states composed primarily of E,Z and E,E isomer mixtures. Further irradiation gives [2 2]-cycloadducts of 1. In the presence of Ba2 ions, essentially quantitative formation of 2 : 2 complexes, 12·(Ba2 )2 controls the photochemical outcome. E → Z photoisomerization of the ligand is entirely suppressed and efficient intramolecular [2 2]-photocycloaddition in the complexes leads to cyclobutane dimers of 1 (φCB = 0.26). The reactivity of 1 in the presence of Mg2 ions for which 1 : 2 complex formation dominates gives both cis–trans photoisomerization and enhanced photocycloaddition.
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Vol. 82 • No. 4