Excited-State Dynamics of <i>fac</i>-[Re^I(L)(CO)₃(phen)]⁺ and <i>fac</i>-[Re^I(L)(CO)₃(5-NO₂-phen)]⁺ (L = Imidazole, 4-Ethylpyridine; Phen = 1,10-Phenanthroline) Complexes

The nature and dynamics of the lowest excited states of fac-[ReI(L)(CO)3(phen)]+ and fac-[ReI(L)(CO)3(5-NO2-phen)]+ [L = Cl-, 4-ethyl-pyridine (4-Etpy), imidazole (imH); phen = 1,10-phenanthroline] have been investigated by picosecond visible and IR transient absorption spectroscopy in aqueous (L = imH), acetonitrile (L = 4-Etpy, imH), and MeOH (L = imH) solutions. The phen complexes have long-lived ReI → phen 3MLCT excited states, characterized by CO stretching frequencies that are upshifted relative to their ground-state values and by widely split IR bands due to the out-of-phase A‘(2) and A‘ ‘ ν(CO) vibrations. The lowest excited states of the 5-NO2-phen complexes also have 3MLCT character; the larger upward ν(CO) shifts accord with much more extensive charge transfer from the ReI(CO)3 unit to 5-NO2-phen in these states. Transient visible absorption spectra indicate that the excited electron is delocalized over the 5-NO2-phen ligand, which acquires radical anionic character. Similarly, involvement of the -NO2 group in the Franck−Condon MLCT transition is manifested by the presence of an enhanced ν(NO2) band in the preresonance Raman spectrum of [ReI(4-Etpy)(CO)3(5-NO2-phen)]+. The ReI → 5-NO2-phen 3MLCT excited states are very short-lived: 7.6, 170, and 43 ps for L = Cl-, 4-Etpy, and imH, respectively, in CH3CN solutions. The 3MLCT excited state of [ReI(imH)(CO)3(5-NO2-phen)]+ is even shorter-lived in MeOH (15 ps) and H2O (1.3 ps). In addition to 3MLCT, excitation of [ReI(imH)(CO)3(5-NO2-phen)]+ populates a 3LLCT (imH → 5-NO2-phen) excited state. Most of the 3LLCT population decays to the ground state (time constants of 19 (H2O), 50 (MeOH), and 72 ps (CH3CN)); in a small fraction, however, deprotonation of the imH•+ ligand occurs, producing a long-lived species, [ReI(im•)(CO)3(5-NO2-phen)•-]+.