High-Nuclear Organometallic Copper(I)-Alkynide Clusters: Thermochromic Near-Infrared Luminescence and Solution Stability

Cu(CF3COO)(2) reacts with tert-butylacetylene (tBuC equivalent to CH) in methanol in the presence of metallic copper powder to give two air-stable clusters, [Cu-15(I)(tBuC equivalent to C)(10)(CF3COO)(5)]center dot tBuC equivalent to CH (1) and [Cu-16(I)(tBuC equivalent to C)(12)(CF3COO)(4)(CH3OH)(2)] (2). The assembly process involves in situ comproportionation reaction between Cu2+ and Cu-0 and the formation of two different clusters is controlled by reactants concentration. The clusters consist of Cu-15 and Cu-16 cores co-stabilized by strong by sigma- and pi-bonded tert-butylethynide and CF3COO- (together with methanol molecule in 2). Their stabilities in solution were confirmed using electrospray ionization mass spectrometry in which the cluster core remains intact for 1 in chloroform and acetone, and for 2 in acetonitrile. Strong thermochromic luminescence in the near infrared (NIR) region was observed in the solidstate. Of particular interest, the emission maximum of 1 is red-shifted from 710 nm at 298 K to 793 nm at 93 K, along with a 17-fold fluorescence enhancement. In contrast, 2 exhibits red shift from 298 to 123 K followed by blue shift from 123 to 93 K. The emission wavelength was correlated with the structural parameters using variable-temperature Xray single-crystal analyses. The rich cuprophilic interaction plays a significant role in the formation of (LMCT)-L-3 (tBuC equivalent to C -> Cu-x) excited state mixed with cluster-centered ((CC)-C-3) characters, which can be considerably influenced by temperature, leading to thermochromic luminescence. The present work provides 1) a new synthetic protocol for the high-nuclear Cu-I-alkynyl clusters; 2) a comprehensive insight into the mechanism of thermochromic luminescence; 3) unusual emissive materials with the characters of NIR and thermochromic luminescence simultaneously.