“On−Off” Au(I)···Cu(I) Interactions in a Au(NHC)₂ Luminescent Vapochromic Sensor

Reaction of the Au(I) N-heterocyclic carbene (NHC) compound [Au(im(CH2py)2)2]PF6 with 2 equiv of [Cu(MeCN)4]PF6 affords the tricationic compound [Au(im(CH2py)2)2(Cu(MeCN)2)2](PF6)3 (1), which exhibits blue luminescence (λmax = 462 nm). Reaction of 1 with either liquid MeOH or MeOH vapor affords [Au(im(CH2py)2)2(Cu(MeOH))2](PF6)3 (2), which produces green luminescence (λmax = 520 nm) under UV excitation. The molecular structures of 1·2MeCN and 2·2MeOH·2Et2O were determined by single-crystal X-ray diffraction. Compound 1 contains a linearly coordinated [Au(NHC)2]+ core in which each picolyl side arm bridges a [Cu(MeCN)2]+ center. The Au···Cu separations are long at 4.596 Å. Compound 2 exhibits two short Au···Cu interactions of 2.7195(7) Å, with the Au(NHC)2 core acting as an additional ligand toward each copper center to complete its tetrahedral coordination mode. Exposure of 2 to atmosphere produces a partial loss of MeOH accompanied by a luminescence color change to yellow (λmax = 543 nm). The uptake and loss of MeOH vapor is rapid and reversible. Exposure of 2 to vacuum affords complete loss of MeOH, and the luminescence changes to yellow-orange (λmax = 573 nm). Treatment of 2 with MeCN vapor regenerates 1. The interconversion of 1 and 2 was confirmed by powder X-ray diffraction. Compound 1 also reacts with acetone and H2O vapors, leading to species that produce yellow-orange (λmax = 591 nm) and green (λmax = 519 nm) emission, respectively. Compounds 1 and 2 are examples of molecular vapochromic materials that exhibit large changes in the emission though ligand substitution reactions between the solid complex and solvent vapors. The dramatic color change likely results from the “on−off” Au···Cu interactions induced by the ligand exchange reaction.