Solution-processable benzothiadiazole/triphenylamine-based hybridized local and charge-transfer (HLCT) hyper-structured molecular red emitters for OLEDs

Abstract

The insolubility and severe self-aggregation tendency of hybridized localized and charge-transfer (HLCT) small molecular emitters have limited their application in solution-processed organic light-emitting diodes (OLEDs), particularly in red OLEDs. Herein, a series of solution-processable novel red hyper-structured molecular (HSM) HLCT emitters, CRA–MTBP_(X)–mCP_(8−X) (X = 8, 7, 6, 4, 2, 1 or 0), are designed. The benzothiadiazole/triphenylamine-based HLCT unit (MTBP) and the host unit (mCP) at different molar ratios were hooked up to the inactive calix[4]resorcinarene (CRA) core via efficient “Click” reactions, respectively. CRA–MTBP_(X)-mCP_(8−X) exhibited good thermal stability with a glass transition temperature (T_g) of above 120 °C and a decomposition temperatures (T_d) exceeding 363 °C. UV-Vis, PL, and CV results demonstrated that these HSMs inherit the photophysical and electrochemical properties of the corresponding functional units. Spin-coating CRA–MTBP_(X)–mCP_(8−X) as light-emitting layers, all the non-doped diodes showed orange to red emission. Among them, a CRA–MTBP₍₈₎–mCP₍₀₎-based device emitted red light, with an EL emission peak (λ_EL) at 651 nm and CIE coordinates of (0.63, 0.36), a maximum external quantum efficiency (EQE_max) of 1.35% with low roll-off and a maximum luminance (L_max) of 2831 cd m⁻². Furthermore, the doped device based on CRA–MTBP₍₈₎–mCP₍₀₎ using CBP as the host emitted bright red light with a λ_EL at 612 nm, an EQE_max of 3.27%, and a L_max of 4362 cd m⁻². These results indicate that the construction of HSMs by introducing red HLCT units could be an effective strategy for structuring solution-processable HLCT red emitters.