Picosecond relaxation of 3MLCT excited states of [Re(Etpy)(CO)3(dmb)]+ and [Re(Cl)(CO)3(bpy)] as revealed by time-resolved resonance raman, UV-vis, and IR absorption spectroscopy

Picosecond dynamics of Re --> polypyridine (MLCT)-M-3 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 pipi* transition of the dmb(.)- ligand of the (MLCT)-M-3 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 3MLCT 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.