Photochemical Reactivity of W(CO)(4)(phen): Evidence for Prompt One-Electron Transfer in Competition with Vibrational...

Excitation into either the metal-to-ligand charge-transfer, MLCT, band or the ligand field, LF, band of W(CO)(4)(phen) promoted an electron transfer to paraquat, PQ(2+). This process was observed spectroscopically by the appearance of the characteristic bands of the well-known radical cation PQ(*)(+). Both nanosecond and picosecond flash photolysis were used to further examine the electron-transfer reaction. Excitation of W(CO)(4)(phen) in the presence of 10 mM PQ(2+) using a 355 nm, 30 ps pulse revealed the formation of PQ(*)(+) within the pulse. This suggests the rate of electron transfer is above diffusion control and that either there must be some preassociation between the reactants prior to excitation or the reaction is mediated by formation of a solvent radical (in a "hot" electron transfer(?)). A 355 nm, 10 ns pulse was also used to excite W(CO)(4)(phen) in the presence of 10 mM PQ(2+). Again, PQ(*)(+) was formed promptly and persisted for times on the order of microseconds. Subsequently, competition between back electron transfer (tau = 17 &mgr;s) and net reaction was observed. The possibility of electron transfer to "inert" halocarbons was also investigated. Picosecond and nanosecond flash photolysis of W(CO)(4)(phen) using both 355 and 532 nm excitation in a halogenated solvent, C(2)Cl(4), yielded a spectrum indicative of the prompt formation (<30 ps) of reduced C(2)Cl(4). Available estimates of potentials require this to arise from a hot (not vibrationally relaxed) electron transfer.