Excited-State Characters and Dynamics of [W(CO)₅(4-cyanopyridine)] and [W(CO)₅(piperidine)] Studied by Picosecond Time-Resolved IR and Resonance Raman Spectroscopy and DFT Calculations:  Roles of W → L and W → CO MLCT and LF Excited States Revised

The characters, dynamics, and relaxation pathways of low-lying excited states of the complexes [W(CO)₅L] [L = 4-cyanopyridine (pyCN) and piperidine (pip)] were investigated using theoretical and spectroscopic methods. DFT calculations revealed the delocalized character of chemically and spectroscopicaly relevant molecular orbitals and the presence of a low-lying manifold of CO π*-based unoccupied molecular orbitals. Traditional ligand-field arguments are not applicable. The lowest excited states of [W(CO)₅(pyCN)] are W → pyCN MLCT in character. They are closely followed in energy by W → CO MLCT states. Excitation at 400 or 500 nm populates the ³MLCT(pyCN) excited state, which was characterized by picosecond time-resolved IR and resonance Raman spectroscopy. Excited-state vibrations were assigned using DFT calculations. The ³MLCT(pyCN) excited state is initially formed highly excited in low-frequency vibrations which cool with time constants between 1 and 20 ps, depending on the excitation wavelength, solvent, and particular high-frequency ν(CO) or ν(CN) mode. The lowest excited states of [W(CO)₅(pip)] are W → CO MLCT, as revealed by TD-DFT interpretation of a nanosecond time-resolved IR spectrum that was measured earlier in a low-temperature glass (Johnson, F. P. A.; George, M. W.; Morrison, S. L.; Turner, J. J. J. Chem. Soc., Chem. Commun. 1995, 391−393). MLCT(CO) excitation involves transfer of electron density from the W atom and, to a lesser extent, the trans CO to the π* orbitals of the four cis CO ligands. Optical excitation into MLCT(CO) transition of either complex in fluid solution triggers femtosecond dissociation of a W−N bond, producing [W(CO)₅(solvent)]. It is initially vibrationally excited both in ν(CO) and anharmonicaly coupled low-frequency modes. Vibrational cooling occurs with time constants of 16−22 ps while the intramolecular vibrational energy redistribution from the v = 1 ν(CO) modes is much slower, 160−220 ps. No LF excited states have been found for the complexes studied in a spectroscopically relevant range up to 6−7 eV. It follows that spectroscopy, photophysics, and photochemistry of [W(CO)₅L] and related complexes are well described by an interplay of close-lying MLCT(L) and MLCT(CO) excited states. The high-lying LF states play only an indirect photochemical role by modifying potential energy curves of MLCT(CO) states, making them dissociative.