Excited-State Dynamics offac-[Re^I(L)(CO)_3(phen)]^+ and fac-[Re^I(L)(CO)_3(5-NO_(2-)phen)]^+(L = Imidazole, 4-Ethylpyridine; Phen = 1,10-Phenanthroline) Complexes

Abstract

The nature and dynamics of the lowest excited states of fac-[Re^I(L)(CO)_3(phen)]^+ and fac-[Re^I(L)(CO)_3(5-NO_2-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^I → 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-NO_(2-)phen complexes also have ^3MLCT character; the larger upward ν(CO) shifts accord with much more extensive charge transfer from the Re^I(CO)_3 unit to 5-NO_(2-)phen in these states. Transient visible absorption spectra indicate that the excited electron is delocalized over the 5-NO_(2-)phen ligand, which acquires radical anionic character. Similarly, involvement of the -NO_2 group in the Franck−Condon MLCT transition is manifested by the presence of an enhanced ν(NO_2) band in the preresonance Raman spectrum of [Re^I(4-Etpy)(CO)_3(5-NO_(2-)phen)]^+. The Re^I → 5-NO_(2-)phen ^3MLCT excited states are very short-lived:  7.6, 170, and 43 ps for L = Cl-, 4-Etpy, and imH, respectively, in CH_3CN solutions. The ^3MLCT excited state of [Re^I(imH)(CO)_3(5-NO_(2-)phen)]^+ is even shorter-lived in MeOH (15 ps) and H_2O (1.3 ps). In addition to ^3MLCT, excitation of [Re^I(imH)(CO)_3(5-NO_(2-)phen)]^+ populates a ^3LLCT (imH → 5-NO_(2-)phen) excited state. Most of the ^3LLCT population decays to the ground state (time constants of 19 (H_2O), 50 (MeOH), and 72 ps (CH_3CN)); in a small fraction, however, deprotonation of the imH^(•+) ligand occurs, producing a long-lived species, [Re^I(im^•)(CO)_3(5-NO_(2-)phen)^(•-)]^+.