In line with this second approach, Hamze et al. subsequently reported a series of linear Cu I complexes exhibiting near-quantitative photoluminescence (PL) quantum yields (PL) and s-to-ms lifetimes at room temperature, each coordinated by a CAAC ligand and a N-heterocyclic amide. It is noteworthy that besides the linear geometry that eliminated the propensity for the Cu I system to undergo Jahn-Teller distortion, these authors also screened a series of strongly -donating and relatively strong -accepting CAACs that were able to render the lowest unoccupied molecular orbital (LUMO) localized on the empty p orbital of the carbene. Furthermore, through their choice of an electron-rich and rigid carbazolide or diphenylamide ligand trans to the -accepting CAAC in a co-planar arrangement, the complexes were optimized for effective ligand-to-ligand charge-transfer (LLCT) mediated by the bridging Cu I center (Figure ). Because of the ~3.7 Å separation between the ligands, the S1-T1 energy gaps (EST) were remarkably small for most of the complexes, in the 30-100 meV range, resulting in efficient thermally activated delayed fluorescence (TADF), long-lived PL, and high PL. A follow up study from Shi et al. reported a series of carbazolide Cu I complexes coordinated to cyclic monoamido-aminocarbenes and diamidocarbenes that showed similar LLCT emissions. Separately, Li et al. revealed a dual emissive Cu I complex ligated by N-heterocyclic carbenes (NHC) and carbazolide ligands that exhibited both fluorescence and phosphorescence, while Hockin et al. described another dual emissive Cu(I) complex supported by an acenaphthoimidazolylidene-based NHC with the two emissions originating from LC and LLCT transitions. However, since the NHCs in Li et al.'s study are poorer π-acceptors than the CAACs, and the dihedral angles between the mesityl and the 2,6-diisopropylphenyl NHCs with the carbazolide are 33.9º and 89.9º, respectively, the luminescence from their photoexcited Cu I complexes were proposed to arise from singlet and triplet LE states localized on the carbazolide (Figure ), an assignment supported by the vibronic progression in their spectra. 63 Our team had been interested in the development of metal complexes composed of earth-abundant first-row transition elements as photocatalysts and luminophores. We were especially keen to capitalize on the extensive developments in stable carbenes since Arduengo's pioneering work on N-heterocyclic carbenes. The original imidazolidine-based carbenes were originally thought to be purely -donors, although Hu et al. later discovered some modest p-accepting behavior. Subsequently, CAACs and other "abnormal" N-heterocyclic carbenes including triazolylidene-based mesoionic carbenes (MICs) containing different substituents were developed, which allowed the orthogonal tuning of -donation and -accepting abilities of the Nheterocyclic carbenes. We intended to explore the modulation of the emissive states by using the strongly π-accepting and synthetically facile "abnormal" MIC ligands. We hoped to prepare highly luminescent Cu I complexes with tuneable emissive behaviour by balancing the electronic effects of the substituents on the π-accepting MICs with the donor properties of disparate amide ligands. Since the triazolylidene MICs are versatile and readily synthesized using Cu I -catalyzed click chemistry, we sought to use sterically hindering groups and polyaromatic rings to vary the degree of conjugation with the triazole moiety and hence the πaccepting properties of this ligand and thus the energy of the LUMO. Moreover, by choosing substituents with different electronicdonor strengths, we should be able to adjust the σ-donor properties of the MICs.