Computational modeling of the triplet metal-to-ligand charge-transfer excited-state structures of mono-bipyridine-ruthenium(II) complexes and comparisons to their 77 K emission band shapes

摘要

A computational approach for calculating the distortions in the lowest energy triplet metal to ligand charge-transfer (~3MLCT = T _0) excited states of ruthenium(II)-bipyridine (Ru-bpy) complexes is used to account for the patterns of large variations in vibronic sideband amplitudes found in the experimental 77 K emission spectra of complexes with different ancillary ligands (L). Monobipyridine, [Ru(L)_4bpy] ~(m+) complexes are targeted to simplify analysis. The range of known emission energies for this class of complexes is expanded with the 77 K spectra of the complexes with (L)_4 = bis-acetonylacetonate (emission onset at about 12 000 cm~(-1)) and 1,4,8,11-tetrathiacyclotetradecane and tetrakis-acetonitrile (emission onsets at about 21 000 cm~(-1)); no vibronic sidebands are resolved for the first of these, but they dominate the spectra of the last two. The computational modeling of excited-state distortions within a Franck-Condon approximation indicates that there are more than a dozen important distortion modes including metal-ligand modes (low frequency; lf) as well as predominately bpy modes (medium frequency; mf), and it simulates the observed 77 K emission spectral band shapes of selected complexes very well. This modeling shows that the relative importance of the mf modes increases very strongly as the T_0 energy increases. Furthermore, the calculated metal-centered SOMOs show a substantial bpy-π-orbital contribution for the complexes with the highest energy T0. These features are attributed to configurational mixing between the diabatic MLCT and the bpy ~3ππ* excited states at the highest T_0 energies.