In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron–hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor–acceptor triphenylamine‐thiadiazol molecule (TPA‐NZP) exhibits fluorescent emission through a hybridized local and charge‐transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high‐efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA‐NZP as the light‐emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep‐red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m−2 and a maximum external quantum efficiency (ηext) of ∼2.8%. The HLCT state is a new way to realize high‐efficiency of EL devices.

Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light‐emitting materials (OLEDS). Charge‐transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi‐equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4‐[2‐(4′‐diphenylamino‐biphenyl‐4‐yl)‐phenanthro[9,10‐d]imidazol‐1‐yl]‐benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record‐setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low‐cost, high‐efficiency fluorescent OLED materials.

For a donor–acceptor (D–A) molecule, there are three possible cases for its low‐lying excited state (S1): a π–π* state (a localized electronic state), a charge‐transfer (CT) state (a delocalized electronic state), and a mixed or hybridized state of π–π* and CT (named here as the hybridized local and charge transfer (HLCT) state). The HLCT state is an important excited state for the design of next‐generation organic light‐emitting diode (OLED) materials with both high photoluminescence (PL) efficiency and a large fraction of singlet exciton generation in electroluminescence (EL). According to the principle of state mixing in quantum chemistry, a series of twisting D–A molecules are designed and synthesized, and their HLCT state characters are verified by both fluorescent solvatochromic experiments and quantum chemical calculations. The CT components in the HLCT state, which greatly affect the molecular optical properties, are found to be enhanced with a decrease of the twist angle of the D–A segment or an increase of the D–A intensity in these twisting D–A molecules. In OLEDs, using these HLCT compounds as the emitting layer, the maximum exciton utilization efficiency is harvested up to 93%. Surprisingly, an exception of Kasha's rule is revealed in some HLCT compounds: restricted internal‐conversion (IC) from the high‐lying triplet state (T2) to the low‐lying triplet T1, and a reopened path of reverse intersystem crossing (RISC) from T2 to S1 or S2, based on the analysis of the excited‐state energy levels and the measurement of the low‐temperature spectrum. RISC from T2 to S1 (S2) as a “hot exciton” channel is believed to contribute to the large proportion of the radiative singlet excitons.

Film-like conjugated microporous polymers (CMPs) are fabricated by the novel strategy of carbazole-based electropolymerization. The CMP film storing a mass of counterions acting as an anode interlayer provides a significant power-conversion efficiency of 7.56% in polymer solar cells and 20.7 cd A(-1) in polymer light-emitting diodes, demonstrating its universality and potential as an electrode interlayer in organic electronics.

Organic fluorescent emitters with narrowband emissions are highly desirable for high‐resolution organic light‐emitting diode (OLED) display technology. In principle, this can be achieved by specifically controlling the intrinsic structural relaxation and vibronic coupling in the excited state. Here, a design strategy to realize narrowband emission of organic fluorescent emitters is proposed by significantly enhancing the low‐frequency vibronic coupling strength (Λ) while simultaneously reducing the high‐frequency Λ of the commonly involved stretching modes. The quinolino‐[3,2,1‐de]acridine‐5,9‐dione (QAO) species is found to be directly associated with this design principle. By introducing single bond‐linked peripheral moieties into the QAO core, the constructed QAO derivatives are shown to exhibit better performance, by achieving a full width at half‐maximum of 23 nm/0.13 eV in toluene for the narrowest band as well as 27 nm/0.15 eV in doped devices, with negligible dependence on the doping concentrations. The maximum external quantum efficiency of the fabricated blue OLED is 17.5%.

510 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.detime-dependent density functional theory (TDDFT). [ 7 ] It aims to reveal the key characteristics of geometry and electronic structure that lead to the high yield of radiative singlet excitons, and provides a new insight into the molecular design for a new generation of organic EL materials.DFT is the most widely used method to describe the groundand excited-state properties from medium to large molecular systems. Taking TPA-NZP as an example, based on the optimized confi guration (DFT/B3LYP/6-31+G (d, p)) of the groundstate (S 0 ), the vertical transition energies were calculated using the methods of TD-B3LYP, [ 8 ] TD-ωB97XD, [ 9 ] and TD-M06-2X, [ 10 ] and further evaluated by EOM-CCSD [ 11 ] for the purpose of comparison. Among these methods, the M06-2X provided the results which were the closest to those data from EOM-CCSD and the experimental values after taking into account basis sets and solvent effects (see Supporting Information). Considering the computational accuracy and cost, the method M06-2X /6-31+G (d, p) was fi nally chosen to describe the excited-state properties of 4CzIPN and TPA-NZP. All the DFT and TDDFT calculations were carried out using the Gaussian 09 package [ 12 ] on a Power Leader workstation.In Figure 1 a is shown the chemical structure of 4CzIPN, which consists of four carbazoles as the donors and a dicyanobenzene core as the acceptor. Because of steric hindrance from the crowded carbazole units, the carbazole units are markedly distorted from the dicyanobenzene plane with twist angles between 63° and 72°. The largely twisted linkage suppresses the coupling between the carbazole donors and the dicyanobenzene acceptor, leading to the spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) at the carbazole units and the dicyanobenzene group. As a comparison, the molecular structure of TPA-NZP (Figure 1 b) is composed of triphenylamine (TPA), which acts as a donor, and naphtho [2,3-c] [1,2,5] thiadiazol (NZ) group, which serves as an acceptor. Compared to 4CzIPN, a slightly reduced twist angle of ≈53° is found between the donor and the acceptor in TPA-NZP, due to the relieved steric hindrance from the repulsion of adjacent hydrogen atoms between NZ and TPA groups. Such a decreased twist angle surely enhances the π-conjugation and electron delocalization between donor and acceptor. As expected, the HOMO of TPA-NZP is nearly delocalized over the whole molecular conjugated backbone, while the LUMO is only localized on the NZ group. Owing to the overlap between HOMO and LUMO determining the Δ E ST value (assuming a major transition confi guration HOMO→LUMO for single electron excitation), it can be concluded that the Recently, Adachi's group reported a series of experimental works on achieving signifi cantly enhanced electroluminescence (EL) quantum yield via a thermally activated delay fl uorescence (TADF) mechanism, in which the lowest triplet excite...

Photoluminescence (PL) efficiency and exciton utilization efficiency are two key parameters to harvest high-efficiency electroluminescence (EL) in organic light-emitting diodes (OLEDs). But it is not easy to simultaneously combine these two characteristics (high PL efficiency and high exciton utilization) into a fluorescent material. In this work, an efficient combination was achieved through two concepts of hybridized local and charge-transfer (CT) state (HLCT) and "hot exciton", in which the former is responsible for high PL efficiency while the latter contributes to high exciton utilization. On the basis of a tiny chemical modification in TPA-BZP, a green-light donor-acceptor molecule, we designed and synthesized CzP-BZP with this efficeient combination of high PL efficiency of η(PL) = 75% in the solid state and maximal exciton utilization efficiency up to 48% (especially, the internal quantum efficiency of η(IQE) = 35% substantially exceed 25% of spin statistics limit) in OLED. The nondoped OLED of CzP-BZP exhibited an excellent performance: a green emission with a CIE coordinate of (0.34, 0.60), a maximum current efficiency of 23.99 cd A(-1), and a maximum external quantum efficiency (EQE, η(EQE)) of 6.95%. This combined HLCT state and "hot exciton" strategy should be a practical way to design next-generation, low-cost, high-efficiency fluorescent OLED materials.

A greatly enhanced proportion of radiative excitons in non-doped blue electroluminescence with a maximum exciton utilizing efficiency (EUE) of 85% is harvested in the orthogonal cyano substituted, charge transfer (CT) emitter TPMCN, in comparison to the localized emission (LE)-like emitter TPM with a low EUE of 16%.