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

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.