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

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

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

KAUST Department
Physical Science and Engineering (PSE) Division


Submitted Date

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.

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

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.).

AIP Publishing

The Journal of Chemical Physics


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