Photo-
and Vapor-Controlled Luminescence of Rhombic
Dicopper(I) Complexes Containing Dimethyl Sulfoxide
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Abstract
Halide-bridged rhombic dicopper(I)
complexes, [Cu<sub>2</sub>(μ-X)<sub>2</sub>(DMSO)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] (X = I<sup>–</sup>, Br<sup>–</sup>; DMSO = dimethyl sulfoxide; PPh<sub>3</sub> = 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><sub><b>2</b></sub><b>I</b><sub><b>2</b></sub><b>-[O,O]</b> and <b>Cu</b><sub><b>2</b></sub><b>Br</b><sub><b>2</b></sub><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<sub>em</sub> = 435 nm, Φ<sub>em</sub> = 0.19 and 0.14
for <b>Cu</b><sub><b>2</b></sub><b>I</b><sub><b>2</b></sub><b>-[O,O]</b> and <b>Cu</b><sub><b>2</b></sub><b>Br</b><sub><b>2</b></sub><b>-[O,O]</b>, respectively) with a relatively long emission lifetime (τ<sub>em</sub> ∼ 200 μs at 77 K) derived from the mixed halide-to-ligand
and metal-to-ligand charge transfer (<sup>3</sup>XLCT and <sup>3</sup>MLCT) excited state. Under UV irradiation, the blue phosphorescence
of <b>Cu</b><sub><b>2</b></sub><b>Br</b><sub><b>2</b></sub><b>-[O,O]</b> disappeared uneventfully and no
new emission band appeared, whereas the blue phosphorescence of <b>Cu</b><sub><b>2</b></sub><b>I</b><sub><b>2</b></sub><b>-[O,O]</b> rapidly disappeared with simultaneous appearance
of a new green emission band (λ<sub>em</sub> = 500 nm). On further
irradiation, the green emission of the iodide complex gradually changed
to bright yellowish-green (λ<sub>em</sub> = 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><sub><b>2</b></sub><b>I</b><sub><b>2</b></sub><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 Cu<sub>2</sub>(μ-I)<sub>2</sub> core to
make the Cu···Cu interaction more effective. In the
contracted core, the triplet cluster-centered (<sup>3</sup>CC) emissive
state is easily generated by thermal excitation of the <sup>3</sup>XLCT and <sup>3</sup>MLCT mixed transition state, resulting in the
green to yellowish-green emission. In contrast, the Cu···Cu
distance in <b>Cu</b><sub><b>2</b></sub><b>Br</b><sub><b>2</b></sub><b>-[O,O]</b> is considerably longer
than that of <b>Cu</b><sub><b>2</b></sub><b>I</b><sub><b>2</b></sub><b>-[O,O]</b>, which destabilizes
the <sup>3</sup>CC emissive state, resulting in the nonemissive character
