Photo- and Vapor-Controlled Luminescence of Rhombic Dicopper(I) Complexes Containing Dimethyl Sulfoxide

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 <b>Cu</b>_<b>2</b><b>I</b>_<b>2</b><b>-[O,O]</b> and <b>Cu</b>_<b>2</b><b>Br</b>_<b>2</b><b>-[O,O]</b>) 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 <b>Cu</b>_<b>2</b><b>I</b>_<b>2</b><b>-[O,O]</b> and <b>Cu</b>_<b>2</b><b>Br</b>_<b>2</b><b>-[O,O]</b>, 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 (³XLCT and ³MLCT) excited state. Under UV irradiation, the blue phosphorescence of <b>Cu</b>_<b>2</b><b>Br</b>_<b>2</b><b>-[O,O]</b> disappeared uneventfully and no new emission band appeared, whereas the blue phosphorescence of <b>Cu</b>_<b>2</b><b>I</b>_<b>2</b><b>-[O,O]</b> 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 <b>Cu</b>_<b>2</b><b>I</b>_<b>2</b><b>-[O,O]</b> 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 (³CC) emissive state is easily generated by thermal excitation of the ³XLCT and ³MLCT mixed transition state, resulting in the green to yellowish-green emission. In contrast, the Cu···Cu distance in <b>Cu</b>_<b>2</b><b>Br</b>_<b>2</b><b>-[O,O]</b> is considerably longer than that of <b>Cu</b>_<b>2</b><b>I</b>_<b>2</b><b>-[O,O]</b>, which destabilizes the ³CC emissive state, resulting in the nonemissive character