Characterization of low-energy electronic excited states of substituted group 8 metallocenes
De Mello, Jennifer Sinclair
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Photoinitiated polymerization has found uses in photolithography, coatings, and curing of adhesives. Typical photoinitiators generated cationic or radical species that initiated polymerization. Recently, it has been shown that photochemically generated anions are capable of initiating polymerization. Reinecke’s salt (trans-Cr(NH3)2(NCS)4-) has been shown to undergo ligand substitution releasing a thiocyanate anion. It was shown that this anion could induce polymerization of ethyl 2-cyanocrylate (CA), a common adhesive. In an effort to look at new classes of anionic photoinitiators, ferrocene (Fc), ruthenocene (Rc), and their benzoyl derivatives were studied. The presence of the electron-withdrawing substituent introduces metal-to-ligand charge transfer (MLCT) character into the excited states of the parent metallocenes. This charge-separated excited state led to relatively efficient metal-ring bond cleavage in the benzoylferrocenes. Interestingly, the benzoylruthenocenes did not undergo metal-ring bond cleavage and instead undergo inefficient photooxidation. The parent metallocenes are also susceptible to photooxidation and follow this pathway more efficiently than the benzoylruthenocenes owing to the ease of oxidizing the metal center. Electronic absorption and resonance Raman spectral studies revealed that adding the benzoyl group to a cyclopentadienyl ring of osmocene (Oc) caused a significant perturbation of electronic properties. The lowest energy electronic excited states in Oc, which are primarily ligand field in nature, acquired MLCT character in benzoylosmocene (BOc). Both Oc and BOc are relatively photoinert (disappearance quantum yield <10-3) in methanol. Irradiating the metallocenes in neat CA initiated the anionic polymerization of the electrophilic monomer. This photoinitiated process resulted from the population of a charge-transfer-to-solvent (CTTS) excited state, which generated the monomer radical anion as the actual initiating species. In an effort to enhance the CTTS mechanism, decamethyl derivatives of ferrocene and ruthenocene were also studied but found to thermally react with CA. Comparison of the present results with those reported previously for the corresponding ferrocene and ruthenocene compounds revealed some interesting similarities and differences in the excited state behavior of the group 8 metallocenes.