Dynamic Molecular Conformational Change Leading to Energy Transfer in F8-5% BSP Copolymer Revealed by Single-Molecule Spectroscopy

License
© 2022 Author(s). Published under an exclusive license by AIP Publishing.

Embargo End Date
2023-01-18

Type
Article

Authors
Yan, Hao
Tseng, Tzu-Wei
Omagari, Shun
Hamilton, Iain
Nakamura, Tomonori
Vacha, Martin
Kim, Ji Seon

KAUST Department
Physical Science and Engineering (PSE) Division

Date
2022-01-18

Submitted Date
2021-11-30

Abstract
Polyfluorene-based copolymers such as poly(9,9-dioctylfluorene)-alt-5% (bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (F8-5% BSP) are efficient blue emitting polymers with various electronic phases: F8 blue-emitting glassy phase, F8 ordered more red-emitting β-phase, and F8/BSP charge transfer (CT) state. Polymer light-emitting device performance and color purity can be significantly improved by forming β-phase segments. However, the role of β-phase on energy transfer (ET) among glassy F8, β-phase and F8/BSP CT state is unclear. Herein, we identify dynamic molecular conformation-controlled ET from locally-excited states to either CT state or β-phase in light-emitting copolymers. By conducting single-molecule spectroscopy for single F8-5% BSP chains, we find inefficient intra-chain ET from glassy segments to the CT state, while efficient ET from the glassy to the β-phase. Spontaneous and reversible CT on-off emission is observed both in the presence and absence of the β-phase. The DFT calculations reveal the origin of the on-chain CT state and indicate this CT emission on-off switching behavior could be related to molecule torsional motion between BSP and F8 units. The population of the CT state by ET can be increased via through-space interaction between the F8 block and the BSP unit on a self-folded chain. Temperature-dependent single-molecule spectroscopy confirms such interaction showing a gradual increase in intensity of the CT emission with the temperature. Based on these observations, we propose the dynamic molecular motion-induced conformation change as the origin of the glassy-to-CT energy transfer, and thermal energy may provide the activation for such change to enhance the ET from glassy or β-phases to the CT state.

Citation
Yan, H., Tseng, T.-W., Omagari, S., Hamilton, I., Nakamura, T., Vacha, M., & Kim, J.-S. (2022). Dynamic Molecular Conformational Change Leading to Energy Transfer in F8-5% BSP Copolymer Revealed by Single-Molecule Spectroscopy. The Journal of Chemical Physics. doi:10.1063/5.0080406

Acknowledgements
We acknowledge the UK Engineering and Physical Sciences Research Council (EPSRC), Plastic Electronics Doctoral Training Centre (EP/G037515/1) and Cambridge Display Technology Ltd. for supplying the PFO, F8-5% BSP and PFB polymers. In addition, the research was financially supported by the JSPS KAKENHI grants number 19H02684 (M.V.) and number 21K18927 (M.V.).

Publisher
AIP Publishing

Journal
The Journal of Chemical Physics

DOI
10.1063/5.0080406

Additional Links
https://aip.scitation.org/doi/10.1063/5.0080406

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