MECHANISM OF ASSOCIATIVE PHOTOINDUCED LIGAND SUBSTITUTION IN W(CO)4PHEN UNDER METAL-TO-LIGAND CHARGE-TRANSFER EXCITATION - SUBNANOSECOND TRANSIENT SPECTRA AND SOLVENT AND WAVELENGTH DEPENDENCE EVIDENCE FOR RAPID PRIMARY PROCESSES

Quantum yields for the photochemical reaction Of W(CO)4phen were evaluated under both ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) excitation in a variety of conditions. Upon LF excitation, the quantum yield increased from 0.011 to 0.017 when the concentration of (n-Bu)3P was increased from 0.015 to 0.36 M. This is interpreted as the indication of a minor associative pathway operating in competition with the known dissociative pathway. Excitation into different regions of the MLCT envelope revealed a complex wavelength and solvent dependence of the quantum yield. The quantum yield increases from 0.0024 to 0.0061 and from 0.0079 to 0.010 in solutions of (n-Bu)3P in dichloromethane and tetrachloroethylene, respectively, on changing the irradiation wavelength from 488.0 to 514.5 nm. A similar wavelength change led to only a slight increase in quantum yield when the solvent was toluene. There was no demonstrable wavelength dependence in pyridine. This solvent dependence of the quantum yield is uncorrelated with the solvatochromism of MLCT bands. Excited-state absorption (ESA) was observed within 20 ps of excitation into either the LF (355-nm excitation) or the MLCT (532-nm excitation) band. Both the shape of the initial ESA envelopes and their evolution over time are distinguishably different upon change of excitation wavelength from 355 to 532 nm. The ESA is assigned to MLCT states Of W(CO)4phen (the ESA bands resembled the spectra of analogous one-electron-ligand-reduction products). These data imply the presence of two pathways for CO substitution, dissociative from the LF states and associative from the MLCT ones. Partition between these distinct pathways is controlled by early events which compete with the vibrational and electronic relaxation of the initial excited states. We suggest that two excited states, already distinguishable at 20 ps after excitation, include one which has relaxed in a way making it susceptible to nucleophilic attack.