Determining the Quantum Efficiency for Activation of an Organometallic Photoinitiator for Cationic Polymerization

We present a new laboratory experiment on the photochemistry of organometallic η⁵,η⁶-mixed-sandwich compounds, which is suitable for both the physical chemistry and inorganic chemistry laboratory. Specifically, students use 1,10–phenanthroline to trap the intermediate formed when [CpFe(isopropylbenzene)]PF₆ is photolyzed with 458 nm light from an argon–krypton laser. Using quantitative UV–vis spectroscopy students then determine the quantum yield of arene ring displacement from the mixed-sandwich complex. The photogenerated intermediate, [CpFe(solvent)₃]⁺ is an efficient initiator for the polymerization of epoxides, dicyanate esters, pyrrole, styrene, dioxolenes, and acrylates. The efficiency with which the intermediate is photochemically formed is therefore an important property in determining its commercial utility. Although the experiment can easily be done in one 3–4 hour laboratory period, it readily lends itself to broadening into a multi-week research project. In this mode, students can examine how changing the nature of the solvent and the irradiation wavelength affect the quantum yield of reaction. Such information can be used to reveal the role of the solvent in the reaction and the electronic excited states from which reaction occurs. Furthermore, in the inorganic laboratory, students could be asked to synthesize [CpFe(isopropylbenzene]PF₆ before looking at its photochemical behavior, thus gaining experience in working under air-free conditions.