Mechanism and dynamics of interligand electron transfer in fac-[Re(MQ(+))(CO)(3)(dmb)](2+). An ultrafast time-resolved visible and IR absorption, resonance raman, and emission study (dmb = 4,4 '-dimethyl-2,2 '-bipyridine, MQ(+) = N-methyl-4,4 '-bipyridinium)

作者：Liard DJ^*; Bu**y M; Farrell IR; Matousek P; Towrie M; Vlcek A

来源：Journal of Physical Chemistry A, 2004, 108(4): 556-567.

DOI：10.1021/jp036822a

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 (MLCT)-M-3 (MLCT = metal-to-ligand charge transfer) excited state which undergoes dmb(.-) --> MQ(+) interligand electron transfer (ILET) to form a Re --> MQ(+) (MLCT)-M-3 excited-state fac-(3)[Re-II(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 v(CO) absorption bands indicate shortening of CdropO 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)[Re-II(MQ(.))(CO)(3)(dmb)](2+) is initially formed "hot" - highly excited in low-frequency modes that are anharmonically coupled to the intra-MQ(.) and v(CO) vibrations. Moreover, it is shown that the Re --> dmb (MLCT)-M-3 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, greater than or equal to130 cm(-1), making ILET (partly) adiabatic. Its unexpectedly fast rate is attributed to a strong involvement of intramolecular vibrational modes of the precursor state.

• 出版日期2004-1-29