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

Abstract

Cu(CF₃ COO)₂ reacts with tert-butylacetylene (tBuC≡CH) in methanol in the presence of metallic copper powder to give two air-stable clusters, [Cu^I₁₅ (tBuC≡C)₁₀ (CF₃ COO)₅ ]⋅tBuC≡CH (1) and [Cu^I₁₆ (tBuC≡C)₁₂ (CF₃ COO)₄ (CH₃ OH)₂ ] (2). The assembly process involves in situ comproportionation reaction between Cu²⁺ and Cu⁰ and the formation of two different clusters is controlled by reactants concentration. The clusters consist of Cu₁₅ and Cu₁₆ cores co-stabilized by strong by σ- and π-bonded tert-butylethynide and CF₃ COO^- (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 solid-state. 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 X-ray single-crystal analyses. The rich cuprophilic interaction plays a significant role in the formation of ³ LMCT (tBuC≡C→Cu_x ) excited state mixed with cluster-centered (³ CC) 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.