Electronic and Photophysical Properties of Platinum(II) Biphenyl Complexes Containing 2,2′-Bipyridine and 1,10-Phenanthroline Ligands

Pt(bph)(bpy) and Pt(bph)(phen), where bph is the 2,2'-biphenyl dianion, bpy is 2,2'-bipyridine, and phen is 1,10-phenanthroline, crystallize in the space groups I4(1)/a and P2(1)/c, respectively, in two different configurations as X-shaped and bowed (B). The distance between Pt centers is 3.5 angstrom indicative of pi-pi stacking. The complexes are optically active, absorb light at 440 rim, and emit in the solid state at room temperature and in the solid glass phase at 77 K. The emission maxima for both in the glass occur near 581 nm but are red-shifted to similar to 700 nm in the solid state. Both complexes exhibit solvatochromism in nitrile-based solvents with the Pt(bph)(phen) complex showing greater excited state dipole character compared to the Pt(bph)(bpy) derivative. Frontier orbitals for the HOMO determined by DFT calculations contain electronic contributions from the biphenyl ligand and the platinum center. The LUMO orbitals primarily reside on the diimine ligands. TDDFT calculations indicate the low-energy transitions occur from the metal/bph combination to the diimine ligand.