Ultrafast Excited-State Dynamics of [Re(L)(CO)₃(bpy)]ⁿ Complexes: Involvement of the Solvent

Ultrafast excited-state dynamics of [Re(L)(CO)₃(bpy)]ⁿ (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2′-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FlUC) and UV−vis transient absorption (TA) with ∼100 fs time resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF₄ ionic liquid. The complexes show a very broad fluorescence band at 540−550 nm at zero time delay, which decays with 100−140 fs (depending on L) by intersystem crossing (ISC) to a ππ* intraligand (³IL) and a Re(L)(CO)₃ → bpy charge-transfer (³CT) excited states. A second emission decay component (1.1−1.7 ps), apparent in the red part of the spectrum, is attributed to ³IL → ³CT conversion, leaving phosphorescence from the lowest ³CT state as the only emission signal at longer time delays. The triplet conversion is slower in DMF than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are due to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV−vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to ³IL → ³CT conversion as well as electronic and vibrational relaxation, and a ∼15 ps rise of only the 373 nm ππ*(bpy^•−) band, which slows down to ∼1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative in other classes of metal-based molecular complexes.