A comprehensive understanding of ultrafast excited-state dynamics of fac-[Re(MQ+)(CO)3(dmb)]2+ (MQ+ = N-methyl-4,4‘-bipyridinium, dmb = 4,4‘-dimethyl-2,2‘-bipyridine) was achieved by combining several time-resolved investigations:  visible and IR absorption, resonance Raman, and emission. Optical excitation of fac-[Re(MQ+)(CO)3(dmb)]2+ populates a Re → dmb 3MLCT (MLCT = metal-to-ligand charge transfer) excited state which undergoes dmb•- → MQ+ interligand electron transfer (ILET) to form a Re → MQ+ 3MLCT excited-state fac-3[ReII(MQ•)(CO)3(dmb)]2+. ILET rates were measured in a series of solvents by time-resolved visible absorption spectroscopy. Time constants range from 8 to 18 ps. Picosecond time-resolved resonance Raman and IR spectroscopies have revealed that ILET is accompanied by a large structural reorganization of the MQ and Re(CO)3 moieties. The MQ• ligand attains a quinoidal structure while positive shifts of ν(CO) absorption bands indicate shortening of C⋮O bonds due to a decrease of electron density on Re upon ILET. Hence, a relatively large reorganization energy is implicated. Both Raman and IR bands undergo a solvent-dependent dynamic blue shift and narrowing on a picosecond time scale, showing that the ILET product fac-3[ReII(MQ•)(CO)3(dmb)]2+ is initially formed “hot”highly excited in low-frequency modes that are anharmonically coupled to the intra-MQ• and ν(CO) vibrations. Moreover, it is shown that the Re → dmb 3MLCT precursor state remains vibrationally excited on a time scale comparable with that of ILET. Three kinds of convoluted vibrational dynamics related to ILET are thus indicated:  (i) cooling of the precursor state alongside ILET, (ii) an “instantaneous” change in the frequencies of high-frequency vibrations upon ILET, and (iii) cooling of the ILET product. The ILET rate does not correlate with any relevant solvent property (solvent function, relaxation time, LUMO energy, ionization potential). Apparently, the only way the solvent affects the ILET rate is through changing the driving force. ILET is much faster than expected from conventional electron-transfer theories. Analysis in terms of Marcus and Jortner−Bixon theories shows that the electronic coupling through the Re atom is relatively large, ≥130 cm-1, making ILET (partly) adiabatic. Its unexpectedly fast rate is attributed to a strong involvement of intramolecular vibrational modes of the precursor state.

Early excited-state dynamics of [Cr(CO)(4)(bpy)] were studied in a CH(2)Cl(2) solution by picosecond time-resolved IR spectroscopy, which made it possible to characterize structurally the individual species involved and to follow separately the temporal evolution of the IR bands due to the bleached ground-state absorption, the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct, and two (3)MLCT states. It was found that the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct is formed alongside population of two (3)MLCT states during the first picosecond after excitation at 400 or 500 nm by a branched evolution of the optically populated excited state. Vibrationally relaxed (3)MLCT excited states are unreactive, decaying directly to the ground state on a picosecond time scale. The photoproduct is long-lived, persistent into the nanosecond time domain. Changing the excitation wavelength from 400 to 500 nm strongly increases the extent of the bleach recovery and decreases the yield of the photoproduct formation relative to the initial yield of the population of the unreactive (3)MLCT states. The photochemical quantum yield of CO dissociation also decreases with increasing excitation wavelength (Víchová, J.; Hartl, F.; Vlcek, A., Jr. J. Am. Chem. Soc. 1992, 114, 10903). These observations demonstrate the relationship between the early dynamics of optically populated excited states and the overall outcome of a photochemical reaction and identify the limiting role of the branching of the initial excited-state evolution between reactive and relaxation pathways as a more general principle of organometallic photochemistry.