High resolution spectroscopy of the lowest excited state in ReI tetra-carbonyl complexes

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Abstract The luminescence and absorption properties of [Re(bpy)(CO) 4 ](PF 6 ) and [Re(phen)(CO) 4 ](PF 6 ) are consistent with representation of the lowest excited states as nominally 3 LC with an admixture of 1 CT character. Using high resolution spectroscopic techniques at cryogenic temperatures, such as luminescence line narrowing spectroscopy or spectroscopy in single crystals, the vibrational sideband information which is normally lost in the ‘natural’ solution environment can be observed in the luminescence and absorption spectra. Mixing between the 3 LC and 1 CT excitation (3%) has previously been reported in [Re(bpy)(CO) 4 ](PF 6 ), resulting in metal-ligand sidebands at 184 and 198 cm −1 in the absorption spectrum and a short luminescence lifetime (33.0 μs). In the luminescence spectra (line narrowed) the metal-ligand sidebands are observed at 194 cm −1 . Weak mixing (∼ 1%) of the 1 CT excitation (32 100 cm −1 ) with the 3 LC excitation (22 100 cm −1 ) in [Re(phen)(CO) 4 ](PF 6 ) gives rise to the observation of metal-ligand vibrational sidebands in the luminescence spectrum (204 cm −1 ) and a luminescence lifetime of τ = 295±5 μ s at 20 K. A spin-orbit mixing matrix element of 〈 3 LC| H so | 1 CT〉 for [Re(phen)(CO) 4 ](PF 6 ) of 65 cm −1 is calculated, compared to 261 cm −1 in [Re(bpy)(CO) 4 ](PF 6 ).