Structural regulation of NHC-protected copper(i) clusters through substitution for photoluminescence enhancement

Abstract

NHC-protected polynuclear Cu(I) clusters have found various applications in the field of catalysis, bio-medicine and luminescence, and they exhibit remarkable advantages over their mononuclear Cu(I) counterparts. However, the synthesis of these Cu(I) clusters is challenging because of the lack of an effective approach to regulate their nuclearity. Herein, we reported a facile method to prepare Cu(I) clusters with Cu₂, Cu₃ and Cu₄ kernels (Cu₂I₂(NHC^PyOF)₂, Cu₃I₃(bisNHC^Me) and Cu₄I₄(NHC^Py2)₂, respectively) by simply changing the N-substituents on the benzimidazole ring. The structural evolution of the Cu(I) clusters was accompanied by the variation in Cu–C_NHC–Cu bonding modes from the σ-bonding/π-interaction model for Cu₂ to the three-center-two-electron (3c-2e) interaction model for the Cu₄ motif, which could be explained by the synergistic effect of the steric and electronic properties of NHC ligands. DFT and experimental results indicated that the emission of all Cu(I) clusters mainly originated from the intra-molecular ligand-to-metal charge transfer (LMCT) mechanism, especially for Cu₂I₂(NHC^PyOF)₂ in the solid state with a QY over 80%. The stability of the polynuclear Cu(I) complexes was tested and provided excellent results. Cu₂I₂(NHC^PyOF)₂ exhibited admirable processability—it could be incorporated into a PVDF film and fabricated into 3D-printing ink without loss of its luminescence performance. This work presents a deeper understanding of the bonding mode of polynuclear Cu(I) clusters and paves the way for the promising applications of photoelectronic devices.