EXCITED-STATES OF M(II,D6)-4'-PHENYLTERPYRIDINE COMPLEXES - ELECTRON LOCALIZATION

We report spectroscopic and photophysical data of para-substituted phenylterpyridine (ptpy) Ru(II) complexes and molecular orbital studies of the Fe(II), Ru(II), and Os(II) compounds [M(R-ptpy)2]2+, R = H, CH3, OH, OCH3, and Cl. The visible charge-transfer absorption of the [Ru(R-ptpy)2]2+ is almost twice as intense as observed for the corresponding 2,2'-bipyridine (bpy) complex [Ru(bpy)3]2+, and it is red shifted by about 50 nm. The luminescence in solution and in membranes (Nafion, cellophane) is very weak at room temperature, and the luminescence decay time is on the order of a few nanoseconds. In a glass at 77 K, however, the luminescence quantum yield is 0.4 and the decay time 13-mu-s. Excited-state absorption spectra measured at room temperature by laser flash spectroscopy support the interpretation that the first excited state is of the MLCT type. The similarity of the excited-state absorptions to those of the ligand radical anions strengthens the idea that the excited electron is localized on a single ligand. Molecular orbital studies indicate that the nonplanar ligand becomes planar in states corresponding to the (n)1(pi*)1 and the (pi)1(pi*)1 excited configurations and in the MLCT state of the complex. The same holds for the ligand radical anion. Low-lying d states in the [Fe(R-ptpy)2]2+ complexes provide efficient relaxation channels by internal conversion. In the Ru(II) and even more pronounced in the Os(II) complexes, these states lie far above the MLCT state and can be neglected. Thus the low luminescence quantum yield at room temperature is due to low-energy intramolecular vibrations of the nonrigid complex and not to the coupling with d states. Lowering the temperature results in freezing these intramolecular movements and hence in significantly increasing the luminescence quantum yield. The molecular orbital studies indicate that it is reasonable to describe the MLCT state as [(L)Ru(III)(L.-)]2+ because the perpendicular conformation of the two ligands causes all pi-orbitals to be accidentally 2-fold degenerate and therefore a small asymmetric distortion is sufficient to favor the localized situation.