Fluorene‐Based Oligomers for Highly Efficient and Stable Organic Blue‐Light‐Emitting Diodes

Organic light-emitting diodes (OLEDs) have great potential for applications such as flat panel displays and solid-state lighting. Many greenand red-light-emitting organic materials have been developed for full-color displays but highly efficient and stable blue-light-emitting organic materials are still rare. Due to the large band-gap of blue-light emitters, it is difficult to inject holes from the anode as well as electrons from the cathode in blue-light OLEDs. Promising candidates for blue-light-emitting materials include fluorene-based polymers and oligomers, due to their high photoluminescent quantum yield (PLQY), good thermal stability, and excellent solubility. In this communication, we report a series of new blue-light-emitting materials consisting of oligofluorenyl blocks and electron-donating/electron-withdrawing groups, which facilitate the injection and transport of both holes and electrons. The device performance of undoped and doped devices indicates that they are very promising for OLEDs, with essential elements of high efficiency, good stability, and color purity for pure-blue emission. The target blue-light-emitting compounds were synthesized through three key steps (Scheme 1). First, 4-iodoaniline was coupled with carbazole through anUllmann coupling reaction. In our first attempt, where N,N-dimethylacetamide (DMAc) was used as solvent, the reaction solution was heated at 160 8C for 24 h, and 76% yield of compound A was achieved. In our second attempt with diphenyl ether as solvent, the temperature was raised to 190 8C and the reaction yielded 92% of desired product. Compound A was then coupled with dibromo-oligofluorene (B1–B3), catalyzed by Pd(OAc)2, to afford compounds C1 to C3, which were eventually end-capped with cyanophenyl groups through a Suzuki coupling reaction. The three coupling reactions produced an overall yield of 55%, 37%, and 33% for D1, D2, and D3, respectively. The physical properties of the compounds are summarized in Table 1. All three compounds show high photoluminescent quantum yields, ranging from 59% to 64% in chloroform solution. With the number of fluorene units increasing from 1 to 3, the optical properties change remarkably. The optical bandgaps determined from the l0–0 absorption-band edge are 2.78, 2.81, and 2.89 eV for D1, D2 and D3, respectively. The absorption maximum in the UV-vis spectrum ofD3 in chloroform solution is blue-shifted by 9 nm, and the peak of photoluminescence (PL) spectrum of the thin film is largely blue-shifted by 33 nm compared with D1, as shown in Figure 1a. The OLED device performances also depend on the number of fluorene units, which will be described below. The preparation of the electroluminescent devices is briefly described as follows (see Experimental section): poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and the light-emitting layer were spin-coated from solutions onto indium tin oxide (ITO) surfaces sequentially. 1,3,5-tris(phenyl-2-benzimidazolyl)benzene (TPBI) and the cathode were thermally deposited in vacuum of 1 10 5 Pa. The device performances are summarized inTable2.For theundopeddevices (Fig. 2), the emissions from D1, D2, and D3 are peaked at 493 nm, 493nm, and 446nm, with Commission Internationale de L’eclairage (CIE) color coordinates of (0.176,0.397), (0.148,0.280), and (0.150,0.148), respectively. The emission from D3 is in the limit of pure-blue emission, with CIE xþ y< 0.30. The turn-onvoltage (at1 cdm ) for all thesedevices is 3.0V, almost as low as the optical band-gaps of the blue emitters. The highest brightness is 23965 cdm 2 for D1, 38327 cdm 2 for