Picosecond dynamics of Re f polypyridine 3 MLCT excited states of [Re(Etpy)(CO) 3 (dmb)] + and [Re(Cl)-(CO) 3 (bpy)] were investigated by time-resolved UV-vis, resonance Raman, and IR spectroscopy. Raman bands due to NN •intraligand vibrations of the excited molecules increase with time during the first 15-20 ps after excitation. The time constant of 6 ( 2 ps was estimated for the increase of areas of excited-state Raman bands of [Re(Etpy)(CO) 3 (dmb)] + . The growth of Raman bands is accompanied by an increase of the near-UV transient absorption band at 375 nm, which corresponds to a ππ* transition of the dmb •ligand of the 3 MLCT excited state [Re II (Etpy)(CO) 3 (dmb •-)] + . These effects are attributed to structural reorganization during vibrational cooling, during which the electronic dipole moment and/or vibrational overlap integrals increase. IR bands due to CO stretching vibrations and some of the Raman bands undergo dynamical upward shift and narrowing, that occur with time constants between 1 and 11 ps, manifesting cooling of anharmonically coupled low-frequency vibrational modes. These observations demonstrate that relaxation dynamics of 3 MLCT excited states of metal-polypyridine complexes extend into the picosecond time domain. It follows that many important ultrafast photochemical processes of metal polypyridine complexes, such as electron injection into semiconductors, actually occur from unequilibrated, vibrationally excited states.

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.

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