Halide-bridged rhombic dicopper(I) complexes, [Cu2(μ-X)2(DMSO)2(PPh3)2] (X = I(-), Br(-); DMSO = dimethyl sulfoxide; PPh3 = triphenylphosphine), were synthesized, the iodide complex of which exhibited interesting photochromic luminescence driven by photoirradiation and by exposure to DMSO vapor in the solid state. Single-crystal X-ray diffraction measurements revealed that the iodo and bromo complexes (abbreviated Cu2I2-[O,O] and Cu2Br2-[O,O]) were isomorphous, and that the two DMSO ligands were coordinated to the Cu(I) ion via the O atom in both complexes. Both complexes exhibited bright blue phosphorescence at room temperature (λ(em) = 435 nm, Φ(em) = 0.19 and 0.14 for Cu2I2-[O,O] and Cu2Br2-[O,O], respectively) with a relatively long emission lifetime (τ(em) ~ 200 μs at 77 K) derived from the mixed halide-to-ligand and metal-to-ligand charge transfer ((3)XLCT and (3)MLCT) excited state. Under UV irradiation, the blue phosphorescence of Cu2Br2-[O,O] disappeared uneventfully and no new emission band appeared, whereas the blue phosphorescence of Cu2I2-[O,O] rapidly disappeared with simultaneous appearance of a new green emission band (λ(em) = 500 nm). On further irradiation, the green emission of the iodide complex gradually changed to bright yellowish-green (λ(em) = 540 nm); however, this change could be completely suppressed by lowering the temperature to 263 K or in the presence of saturated DMSO vapor. The initial blue phosphorescence of Cu2I2-[O,O] was recovered by exposure to DMSO vapor at 90 °C for a few hours. IR spectroscopy and theoretical calculations suggest that the DMSO ligand underwent linkage isomerization from O-coordination to S-coordination, and both the occurrence of linkage isomerization and the removal of DMSO result in contraction of the rhombic Cu2(μ-I)2 core to make the Cu···Cu interaction more effective. In the contracted core, the triplet cluster-centered ((3)CC) emissive state is easily generated by thermal excitation of the (3)XLCT and (3)MLCT mixed transition state, resulting in the green to yellowish-green emission. In contrast, the Cu···Cu distance in Cu2Br2-[O,O] is considerably longer than that of Cu2I2-[O,O], which destabilizes the (3)CC emissive state, resulting in the nonemissive character.
