Yan Hao, Tseng Tzu-Wei, Omagari Shun, Hamilton Iain, Nakamura Tomonori, Vacha Martin, Kim Ji-Seon
School of Advanced Materials, Peking University Shenzhen Graduate School, Peking University, Shenzhen 518055, China.
Department of Materials Science and Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan.
J Chem Phys. 2022 Feb 21;156(7):074704. doi: 10.1063/5.0080406.
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 the 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 the 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 density functional theory 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 ET, and thermal energy may provide the activation for such a change to enhance the ET from glassy or β-phases to the CT state.
基于聚芴的共聚物，如聚（9,9 - 二辛基芴）-alt-5%[双-N,N'-（4 - 丁基苯基）-双-N,N'-苯基-1,4 - 苯二胺]（F8 - 5%BSP），是具有多种电子相的高效蓝色发光聚合物：F8蓝色发光玻璃相、F8有序的更发红光的β相以及F8/BSP电荷转移（CT）态。通过形成β相链段，聚合物发光器件的性能和色纯度可得到显著提高。然而，β相对玻璃态F8、β相和F8/BSP CT态之间能量转移（ET）的作用尚不清楚。在此，我们确定了发光共聚物中从局域激发态到CT态或β相的动态分子构象控制的ET。通过对单个F8 - 5%BSP链进行单分子光谱研究，我们发现从玻璃态链段到CT态的链内ET效率低下，而从玻璃态到β相的ET效率较高。在有和没有β相的情况下均观察到自发且可逆的CT开关发射。密度泛函理论计算揭示了链上CT态的起源，并表明这种CT发射开关行为可能与BSP和F8单元之间的分子扭转运动有关。通过自折叠链上F8嵌段与BSP单元之间的空间相互作用，可以增加通过ET产生的CT态的数量。温度依赖的单分子光谱证实了这种相互作用，表明CT发射强度随温度逐渐增加。基于这些观察结果，我们提出动态分子运动诱导的构象变化是玻璃态到CT态ET的起源，并且热能可能为这种变化提供激活，以增强从玻璃态或β相到CT态的ET。
