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Conjugation in polysiloxane copolymers via unexpected Si-O-Si dπ-pπ overlap, a second mechanism?

Polymer Journal volume 56, pages 577–588 (2024)Cite this article

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Abstract

We previously reported that functionalized phenyl- and vinyl-silsesquioxanes (SQs) and [RSiO1.5]8,10,12 (R = Ph or vinyl) exhibited redshifted absorption and emission, suggesting 3-D conjugation via a cage-centered lowest unoccupied molecular orbital (LUMO). The functionalized [PhSiO1.5]7(OSiMe3)3 with a missing corner and edge-opened, end-capped [PhSiO1.5]8(OSiMe2)2 (double decker, DD) analogs also exhibit emission redshifts, indicating 3-D conjugation. DD [PhSiO1.5]8(OSiMevinyl)2 and R-Ar-Br copolymers exhibit polymerization (DP)-dependent emission λmax and integer charge transfer (ICT) to 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TNCQ). The terpolymer-averaged redshifts all suggest conjugation with two (O-Si-O) endcaps, possibly via a cage-centered LUMO. In assessing conjugation limits, it was anticipated that copolymers of the ladder (LL) SQ, (vinylMeSiO2)[PhSiO1.5]4(O2SiMevinyl), with Br-Ar-Br and without a cage would eliminate LUMO formation and a redshift. The λmax values observed were greater for analogous copolymers, which requires a different explanation. Here, we assess the photophysical behavior of copolymers closer to polysiloxanes, namely, the expanded cage (MeVinylSiO)2[PhSiO1.5]8(OSiMeVinyl)2SQs. Copolymers with Br-Ar-Br exhibit redshifted absorption and emission, which supports conjugation via Si-O-Si bonds rather than cage-centered LUMOs, contrary to traditional views of Si-O-Si copolymers. One- and two-photon photophysical probes showed that XDD copolymers exhibit multiple fluorescence-emitting excited states, in violation of Kasha’s rule stating that emission should occur only from the lowest excited state. Finally, new modeling studies suggested that conjugation derives from Si-O-Si bond dπ-pπ interactions, an unexpected result for polysiloxanes that supports two forms of conjugation.

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Acknowledgements

The Laine and Rebane groups gratefully thank NSF Chemistry for the collaborative research award No. 1610344. Support from the Estonian National Science Foundation grant PRG661 is acknowledged (Ramo and Rebane). The Unno/Liu group is grateful for support from the NEDO project (JPNP06046). Professor Jungsuttiwong thanks NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation [B16F640099] for funding work performed by her team. The work performed at The Georgia Institute of Technology was made possible through the Air Force Office of Scientific Research (AFOSR) under support provided by the Organic Materials Chemistry Program (Grant FA9550-20-1-0353, Program Manager: Dr. Kenneth Caster).

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  1. Department of Materials Science and Engineering, Macromolecular Science and Engineering Center University of Michigan, Ann Arbor, MI, 48109-21236, USA

    Jose Jonathan Rubio Arias, Zijing Zhang & Richard M. Laine

  2. Dept of Chemistry and Chemical Biology, Graduate School of Science and Technology Gunma University, Kiryu, 376-8515, Japan

    Manae Takahashi, Masafumi Unno & Yujia Liu

  3. School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA, 30332, USA

    Paramasivam Mahalingam & Jason Azoulay

  4. Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, 34190, Thailand

    Pimjai Pimbaotham & Siriporn Jungsuttiwong

  5. Dept. of Chemistry, Faculty of Science, Silpakorn University, Nakorn Pathom, 73000, Thailand

    Nuttapon Yodsin

  6. National Institute of Chemical Physics and Biophysics, Tallinn, Estonia

    Matt Rammo & Aleksander Rebane

  7. Montana State University, Bozeman, MT, 59717, USA

    Aleksander Rebane

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Arias, J.J.R., Zhang, Z., Takahashi, M. et al. Conjugation in polysiloxane copolymers via unexpected Si-O-Si dπ-pπ overlap, a second mechanism?. Polym J 56, 577–588 (2024). https://doi.org/10.1038/s41428-024-00899-5

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