Journal
MACROMOLECULES
Volume 45, Issue 5, Pages 2257-2266Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ma202673q
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Funding
- German Research Council (DFG)
- State of Baden-Wurttemberg through the Center for Functional Nanostructures [C 3.02]
- Fonds der Chemischen Industrie
- Karlsruhe Institute of Technology (KIT)
- Excellence Initiative for leading German universities
- Ministry for Science and Arts of the state of Baden-Wurttemberg
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The excited states and dynamics of the three triplet radical photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone, Bz), 2,4,6-trimethylbenzoin (2-hydroxy-1-mesity1-2-phenylethanone, TMB), and mesitil (1,2-bis(2,4,6-trimethylphenyl)-1,2-ethanedione, Me)-employed in our previous studies for quantifying net initiation efficiencies in pulsed laser polymerization with methacrylate monomers [Voll, D.; Junkers, T.; Bamer-Kowollik, C. Macromolecules 2011, 44, 2542-2551]-are investigated via both femtosecond transient absorption (TA) spectroscopy and density functional theory (DFT) methods to elucidate the underlying mechanisms causing different initiation efficiencies when excited at 351 nm. Bz and TMB are found to have very similar properties in the calculated excited states as well as in the experimentally observed dynamics. After excitation into the first excited singlet state (S-1) Bz and TMB undergo rapid intersystem crossing (ISC). The ISC can compete with ultrafast internal conversion (IC) processes because an excited triplet state (T-n) of nearly the same energy is present in both cases. ISC is therefore the dominating depopulation channel of S-1, and subsequent alpha-cleavage to produce radicals takes place on the picosecond time scale. In contrast, Me is excited into the second excited singlet state (S-2). In this case no isoenergetic triplet state is available, which inhibits ISC from competing with ultrafast deactivation processes. ISC is therefore assigned to be a minor deactivation channel in Me. Employing these findings, quantitative photoinitiation efficiency relations of Bz, TMB, and Me obtained by pulsed laser polymerization can be directly correlated with the relative TA intensities found in the femtosecond experiments. The ISC efficiency is thus a critical parameter for evaluating the overall photoinitiation efficiency and demonstrates that the employment of the herein presented method represents a powerful tool for attaining a quantitative picture on the suitability of a photoinitiator.
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