Synthesis and Photophysical Studies of Copper(I) CAAC Half-Sandwich Complexes as a Highly Modifiable Class of...

Cyclopentadienyls are well-known strong donor ligands and have been successfully employed in catalysis as they tolerate a variety of substituents to adjust their steric and electronic properties. Although such highly modifiable ligands are of great interest for luminescence and photocatalytic applications, studies of Cp^R-containing photoactive transition-metal complexes are quite rare. In this work, we present a structural, electrochemical, and first elaborated photophysical investigation of a series of copper(I) half-sandwich complexes bearing cyclic alkyl(amino)carbenes (CAACs) as chromophore ligands and compare them with [Cu(Cp)(IDipp)] and [Cu(Cp*)(IDipp)] bearing a traditional N-heterocyclic carbene. Furthermore, we present the first molecular structure derived from single-crystal X-ray diffraction of a copper(I) indenyl complex, which can be described as an η² (σ, π)-coordination. The Cu^I half-sandwich complexes show blue-green to orange phosphorescence with a photoluminescence quantum yield of up to 59% and radiative rate constants k_r of up to 4 × 10⁴ s^-1 in the solid state, depending on the substitution pattern of the Cp^R ligand. Our TD/DFT calculations suggest that the emitting excited states are of ³MLCT/LLCT character. We determined the excited-state lifetime of the Cu^I half-sandwich complexes in solution to be as long as 600 ns, which in combination with the large π-surface of the Cp^R ligands allows for Dexter energy transfer for photocatalytic applications. In addition, the chiroptical properties of chiral [Cu(Cp/Cp*)(CAAC^Menthone)] were studied and compared to [CuCl(CAAC^Menthone)], of which we demonstrate that its circular polarized luminescence is the result of excimer formation and not, as previously reported, attributed to the monomeric C₁-symmetric structure.