Robust Packing Patterns and Luminescence Quenching in Mononuclear [Cu(II)(phen)2] Sulfates

Three mixed-ligand Cu(II) complexes with compositions [Cu(phen)(2)(SO4)]center dot CH3OH (1), [Cu(phen)(2)(SO4)]-(H2O)(2)(dmf) (2), and [Cu(phen)(2)H2O](SO4)(H2O)(4) (3), where phen = 1,10-phenanthroline and dmf = N,N'-dimethylformamide, were prepared and studied. These compounds belong to the landscape of the mononuclear Cu(phen)(2) sulfates, and the solvated complexes undergo frequent anion/water exchange at the metal center in aqueous solutions. Complexes are similar by the metal trigonal bipyramidal coordination geometry but differ by the mode of enclathration and number of protic and aprotic solvent guest molecules being accommodated in the crystal lattice. Crystal packing in 1-3 is determined by the robust supramolecular patterns that consist of stacking interactions between the planar extended phen fragments. These are observed in all three solids regardless of the interplay of other noncovalent interactions, including rather strong hydrogen bonds. The dual luminescence is detected at 580 and 470 nm for both crystals of phen and 3. Detailed analysis of singlet and triplet excitations in phen and 3 is performed by time-dependent density functional methods. Fluorescence is predicted with a low quantum yield at 386 nm, and dual phosphorescence from n-pi* and pi-pi* triplet states is predicted at 523 and 496 nm. Emission quenching was demonstrated for 3 and explained by nonradiative decay involving supramolecular stacking and low-lying metal-centered states.