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

Abstract

A computational approach for calculating the distortions in the lowest energy triplet metal to ligand charge-transfer (3MLCT = T0) 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 T0 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 T0 energies.