A Superior Polypyridine Ligand for Platinum(II) Lumaphors. Methyl Substituents with a Large Stereoelectronic Influence

This research deals with the synthesis and characterization of a new series of platinum(II) polypyridine complexes
that incorporate a relatively rigid and hydrophobic ligand. The parent complex Pt(php)Cl+, where php denotes
2-(2‘-pyridyl)-1,10-phenanthroline, resembles Pt(trpy)Cl+, where trpy denotes 2,2‘:6‘,2‘ ‘-terpyridine, but is photoluminescent in solution. Hence php derivatives should prove to be superior tags and/or spectroscopic probes for
biological systems. A theoretical analysis reveals some of the advantages of php over trpy as a platform. Due to
a ligand π system with a relatively small HOMO−LUMO gap, the emission from Pt(php)Cl+ exhibits significant
vibrational structure and a mixed 3π−π*/3d−π* orbital parentage. In deoxygenated dichloromethane solution the
php complex exhibits an emission quantum yield of 3.1 × 10-3 and an excited-state lifetime of 0.23 μs at room
temperature. However, methyl groups have an unusually strong stereoelectronic influence, particularly at the 5,6-positions of the phenanthroline moiety. The platinum(II) complex with 2-(2‘-pyridyl)-3,5,6,8-tetramethyl-1,10-phenanthroline is the best emitter with an emission yield of 0.055 and a lifetime of 9.3 μs in dichloromethane.
Strongly donating solvents like dimethylformamide are potent quenchers of the emission. The methods of
characterization used include absorption and emission spectroscopies, electrochemistry, and, in the case of [Pt{2-(2‘-pyridyl)-4,7-dimethyl-1,10-phenanthroline}Cl]O3SCF3, X-ray crystallography. Another intriguing finding is that
methyl substituents have preferred orientations with respect to the phenanthroline ligand.