Small-molecule and Polymer Based on Carbazole/Quinoline Derivatives Exhibiting AIEE and TADF Properties for Efficient Solution-processable Nondoped Blue OLEDs

A novel organic carbazolyl-quinoline derivative with D-pi-A structure, 9-[4-(quinoline-8-yl)phenyl]-9Hcarbazole (CPBA-Qx), was designed and synthesized, in which carbazole was used as electron donor (D), quinoline as the electron acceptor (A) and phenyl spacer as pi-bridge. Based on the designed blue-emitting small-molecule CPBA-Qx, a blue-emitting polymer, poly[3-phenyl-9-(4-(quinoline-8-yl)phenyl)-9H-carbazole] (poly(CPBAQx-P)), was designed and synthesized by Suzuki coupling copolymerization with donor as the main chain and acceptor as the side chain according to the molecular designing strategy of TADF polymer. Aggregation-induced enhanced emission (AIEE) performances of CPBA-Qx and poly(CPBAQx-P) were examined by studying the PL emission behaviour of their diluted mixtures in THF/H2O with different water fractions (f(w)). Compared with pure solution, the study showed that the PL intensity and quantum yield (Phi(PL)) of CPBA-Qx and poly(CPBAQx-P) are significantly enhanced by 2. 10 times in the aggregate state, exhibiting typical AIEE characteristics. Meanwhile, both of them exhibited obviously delayed fluorescence with the lifetime of 1.25 and 1.09 mu s, accompanied by prompt fluorescence of 3.78 and 3.41 ns. For instance, the temperature-dependent transient photoluminescence spectra measured for CPBA-Qx and poly(CPBAQx-P) displayed enhanced delayed fluorescence upon the temperature rising from 100 K to 300 K, indicative of their TADF nature. The excellent TADF characteristics were closely related to extremely narrow singlet-triplet state energy splitting (Delta E-ST, CPBA-Qx 0.11 eV and poly(CPBAQx-P) 0.16 eV). As a consequence, using CPBA-Qx and (poly(CPBAQx-P) as emitters, the turn-on voltage (V-on) and threshold voltage (Vth) of solution-processable nondoped OLEDs were 3.26/12 V and 2.91/11 V, respectively. The maximum external quantum efficiency (EQE(max)) and Commission Internationale De L'Eclairage (CIE) coordinates were 5.24% and (0.19, 0.12), 8.39% and (0.17, 0.10), respectively. Moreover, the non-doped OLED based on poly(CPBAQx-P) can retain high EL efficiency (EQE=7.42%) at 1000 cd.m(-2), with small efficiency roll-off, a narrow full width at half maximum (FWHM=59 nm) and CIE y <= 0.1. To sum up, poly(CPBAQx-P) as an emitter has great potential in solution-processed non-doped OLEDs, which lays experimental foundation for the application of AIEE-TADF polymer in OLEDs.