Vapor-triggered Green-to-Yellow Luminescence Conversion due to the Variation of Ligand Orientations in Tetranuclear Copper(I) Complex

The reaction of 3,6-ditert-butyl-1,8-bis(diphenylphosphino)-9-methyl-9H-carbazole (L) with CuBr resulted in the isolation of tetranuclear copper(I) complex Cu4Br4L2 as two colorless crystal morphs, i.e., green-emitting 1G and yellow-emitting 1Y. As demonstrated by X-ray crystallography, the Cu4Br4 moiety in both 1G and lY adopts the same chair conformations. When L is bonded perpendicularly to the Cu-4 plane, 1G with green emission is obtained, while it gives a yellow emission of 1Y once the L is parallelly bonded to Cu-4 plane. Theoretical computational studies suggest that the variation in ligand orientation results in a different degree of structural distortion in triplet state and thus different luminescent energy. Particularly, 1Y undergoes dramatic structural distortion from the ground (S-0) to triplet excied state (T-1). Interestingly, 1G can be converted into 1Y upon exposed to saturated hexane vapor, which would return to 1G upon exposure to acetonitrile vapor. As demonstrated experimentally and theoretically, the reversible luminescence transformation between 1G and 1Y is ascribed to the variation of ligand L orientations.