In general, the homoleptic and heteroleptic bis-diimine copper complexes typically manifest a wide MLCT band spanning between 450 -700 nm in solution (in the visible part of the spectrum). Electronic factors, such as extending the  conjugation as observed in [Cu(bcp)2] +83 induce a red shift in the MLCT band (labs = 475 nm). Diphosphines, being effective -acceptors, contributes in lowering the HOMO energy of the complex, therefore inducing a blue shift in the absorption maxima to 360 -450 nm, in contrast to the homoleptic bis-diamine copper (I) complexes. The pronounced interaction of the ligand with copper also confers significant ligand character to the HOMO. Hence, the transition from HOMO to LUMO (* of the ligand) has a mixed MLCT and LLCT (ligand to ligand charge transfer) character, contingent to the character of the diphosphines.