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 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
(λem = 435 nm, Φem = 0.19 and 0.14
for Cu2I2-[O,O] and Cu2Br2-[O,O], respectively) with a relatively long emission lifetime (τem ∼ 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 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 (λem = 500 nm). On further
irradiation, the green emission of the iodide complex gradually changed
to bright yellowish-green (λem = 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 (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 Cu2Br2-[O,O] is considerably longer
than that of Cu2I2-[O,O], which destabilizes
the 3CC emissive state, resulting in the nonemissive character.