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 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.
Highlights
The development of efficient deep blue polymer light-emitting diodes (PLEDs) is the bottleneck for high-performance display applications because of the requirement for electroluminescence emission (EL) to have Commission International de L’Eclairage (CIE) (x, y) coordinates both less than 0.15.1–3 the operational stability and color purity for blue PLEDs are important parameters
We propose the dynamic molecular motion-induced conformation change as the origin of the glassy-to-charge transfer (CT) energy transfer (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
We propose that the enhanced CT emission at higher temperature is a result of conformational changes that either lead to the formation of the intra- or inter-chain CT states and/or to increased efficiency of energy transfer from the glassy or β-phase segments to the lower-energy CT trap states
Summary
The development of efficient deep blue polymer light-emitting diodes (PLEDs) is the bottleneck for high-performance display applications because of the requirement for electroluminescence emission (EL) to have Commission International de L’Eclairage (CIE) (x, y) coordinates both less than 0.15.1–3 the operational stability and color purity for blue PLEDs are important parameters. The BSP moieties act as efficient exciton formation sites, resulting in a broad CT-like emission in glassy conformation, which is not suitable for deep blue PLEDs. by introducing around 5% β-phase segments into the copolymer, the PL emission significantly shifted to higher energy β-phase emission with 38% residual CT emission.. By introducing a 5% β-phase to make similar fractions as BSP units, we find the β-phase sample has a Huang–Rhys factor S of 0.40, which is similar to that of β-phase PFO (0.38) This indicates that excitons formed locally on glassy F8 segments undergo efficient energy transfer to β-phase. We assume that the thermal energy is the trigger for CT on–off emission by providing the activation energy for dynamic molecular conformation changes
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