Excited-state dynamics of fac-[ReI(L)(CO)3(phen)]+ and fac-[ReI(L)(CO)3(5-NO2-phen)]+ (L = imidazole, 4-ethylpyridine; phen=1,10-phenanthroline) complexes

he nature and dynamics of the lowest excited states of fac-[Re-1(L)(CO)(3)(phen)](+) and fac-[Re-1(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 Re-1 --> phen (MLCT)-M-3 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" v(CO) vibrations. The lowest excited states of the 5-NO2-phen complexes also have (MLCT)-M-3 character; the larger upward v(CO) shifts accord with much more extensive charge transfer from the Re-1(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 v(NO2) band in the preresonance Raman spectrum of [Re-1(4-Etpy)(CO)(3)(5-NO2-phen)](+). The Re-1 --> 5-NO2-phen (MLCT)-M-3 excited states are very short-lived: 7.6, 170, and 43 ps for L = Cl-, 4-Etpy, and imH, respectively, in CH3CN solutions. The (MLCT)-M-3 excited state of [Re-1(imH)(CO)(3)(5-NO2-phen)](+) is even shorter-lived in MeOH (15 ps) and H2O (1.3 ps). In addition to (MLCT)-M-3, excitation of [Re-1(imH)(CO)(3)(5-NO2-phen)](+) populates a (LLCT)-L-3 (imH --> 5-NO2-phen) excited state. Most of the (LLCT)-L-3 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, [Re-1(im(.))(CO)(3)(5-NO2-phen)(.-)](+).