The Metal-to-Ligand Charge-Transfer Luminescences of Ruthenium(II)-Polypyridine-Tetraam(m)ine Complexes. Support for the Interconvertibility of Optical and Thermal (Kinetic) Franck-Condon Parameters

This is the first report of the metal-to-ligand charge-transfer (MLCT) emission spectra of the bis-ethylenediamine- and tetraamminebipyridineruthenium(II) complexes. The emission maxima occur at 800 and 840 nm, respectively, at 77 K in DMSO/H2O glasses. The MLCT excited states of these complexes have short lifetimes (less than 50 ns), and as a consequence, the emission intensities are very small. The energies of the emission maxima are very close to those expected on the basis of the difference in reduction potentials of the metal and ligand ((delta)E1/2) and the values of the reorganizational free energies ((chi)r) of the vertical transitions estimated from the electron-transfer self-exchange reactions of the complexes (hmax(em)(approximately equal to) F(delta)E1/2 - (chi)r, where F is Faradayʹs constant). The low energy of the emissions is in large part a consequence of the substantial contributions of the reorganizational free energies.