Elucidating the early steps in photoinitiated radical polymerization via femtosecond pump-probe experiments and DFT calculations

Wolf, T. J. A., Voll, D., Barner-Kowollik, C., & Unterreiner, A. N. (2012) Elucidating the early steps in photoinitiated radical polymerization via femtosecond pump-probe experiments and DFT calculations. Macromolecules, 45(5).

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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-mesityl-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.; Barner-Kowollik, C. Macromolecules2011, 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 α-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. © 2012 American Chemical Society.

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ID Code: 99332
Item Type: Journal Article
Refereed: Yes
Additional Information: Cited By :17
Export Date: 5 September 2016
Correspondence Address: Barner-Kowollik, C.; Preparative Macromolecular Chemistry, Center for Functional Nanostructures (CFN), Institut für Technische Chemie und Polymerchemie, Engesserstr. 18, 76128 Karlsruhe, Germany; email: christopher.barner-kowollik@kit.edu
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Keywords: Critical parameter, Deactivation channel, Deactivation process, Density functional theory methods, DFT calculation, Excited singlet state, Excited triplet state, Femtosecond transient absorption, Femtoseconds, Initiation efficiency, Internal conversions, Intersystem crossing, Methacrylate monomers, Photo-initiator, Photoinitiation, Picosecond time scale, Pulsed laser polymerization, Pump-probe experiments, Radical photoinitiators, Triplet state, Ultra-fast, Underlying mechanism, Density functional theory, Dynamics, Experiments, Optical pumping, Polymerization, Pulsed lasers, Excited states
DOI: 10.1021/ma202673q
ISSN: 00249297
Divisions: Current > Schools > School of Chemistry, Physics & Mechanical Engineering
Current > Institutes > Institute for Future Environments
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Deposited On: 22 Sep 2016 04:50
Last Modified: 30 Sep 2016 03:05

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