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 φ_trans→cis = 0.48), leading to λ_exc-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 (φ_CB = 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.