Observations on the low-energy limits for metal-to-ligand charge-transfer excited-state energies of ruthenium(II) polypyridyl complexes.

The 77 K emission spectral maxima of bis(bipyridine)ruthenium(II) complexes are found to approach a limit at energies below about 14,000 cm(-1). There is also evidence for related low-energy excited-state limits in some other classes of ruthenium polypyridyl complexes. The shapes of the vibronic sidebands found in these limits differ from those of complexes that emit at higher energies. These low-energy excited states are not simple "charge-transfer" excited states and are analogous to ππ* excited states. The observations are consistent with effective ground state/excited state mixing matrix elements in the range of (5-10) × 10(3) cm(-1) for ruthenium polypyridine complexes.