A series of blue-green-yellow-red emitting Cu(I) complexes: Molecular structure and photophysical performance

In this work, we designed a series of [Cu(NN)(PPh₃)₂]BF₄ complexes with different optical edge values and emission colors from blue to red, where NN and PPh₃ denoted a diamine ligand and triphenylphosphine, respectively. Six NN ligands with various conjugation chains (short π chain, modest π chain and long π chain) were selected. A systematical comparison between these Cu(I) complexes was performed, so that the correlation between NN structure and [Cu(NN)(PPh₃)₂] photophysical performance was tentatively discussed. Their single crystal structure was found consistent with literature ones, forming a typical tetrahedral coordination geometry. Density functional theory calculation indicated that their onset electronic transition showed a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. Detailed analysis on photophysical parameters suggested that the absorption edge of [Cu(NN)(PPh₃)₂]BF₄ complex was controlled by conjugation length in diamine ligand. A wide absorption edge needed a short conjugation chain in diamine ligand. Similar tendency was found for their emission spectra. In addition, a long conjugation chain in diamine ligand widened emission spectra obviously. Emission dynamics showed slim correlation with diamine ligand conjugation length since the excited state was controlled mainly by dynamic procedure and steric factor of diamine ligands.