Design,Synthesis And Electroluminescent Properties Of Organic Donor-Acceptor Red Molecules With Cyano Group As Ancillary Acceptor

Narrow band-gap luminescent materials including red and near infrared?NIR?materials play a vital role in the field of biological imaging,organic solid laser,fluorescence sensing,organic solar cells,organic light-emitting devices?OLEDs?,etc.However,the red and NIR materials with high photoluminescence?PL?and electroluminescent?EL?efficiency are still relatively scarce.The PL efficiency of narrow band-gap materials is inherently limited by energy gap rule,i.e.,the vibrational wave functional overlap of excited state and ground state increases rapidly with the decreased band gap,which greatly reduces the PL efficiency resulting from the sharply increased non-radiative channel of excited state.The narrow band-gap materials can be divided into fluorescent and phosphorescent materials according to the luminescent mechanism.In the electroluminescence process,the exciton utilization efficiency of fluorescence materials is no more than 25%due to the waste of 75%spin-forbidden triplet excitons transition,which will lead to relatively low EL efficiency of fluorescence materials.The phosphorescent metal complexes show better PL and EL performance than fluorescent narrow band-gap emitters,as a result of the enhanced spin-orbit coupling from heavy atom effect,enabling the full utilization of both singlet and triplet excitons.Therefore,the development of a new generation of narrow band-gap organic fluorescent materials with high efficiency and low cost must conquer the difficulty of energy gap rule and spin statistics.To solve these problems,the strategy of designing donor-acceptor?D-A?molecules is adopted by us.Based on energy level difference between the highest occupied molecular orbital?HOMO?of donor and the lowest unoccupied molecular orbital?LUMO?of acceptor,the band gap can be finely tuned in a wide range according to the choice of donor and acceptor units in donor-acceptor?D-A?molecules.Thus,the rational design of D-A molecules is an effective strategy to obtain high-efficiency red emitters.On the one hand,hybridized local and charge-transfer?HLCT?state of D-A molecule is formed through the combination of locally excited?LE?and charge transfer?CT?state,which has been demonstrated to be in favor of high PL efficiency.On the other hand,the high exciton utilization efficiency of D-A molecules can be achieved by using the mechanism of HOT exciton.The typical characteristic of energy level structure of HOT exciton mechanism is a large energy gap between the first triplet state?T₁?and the second triplet?T₂?state as well as a small energy difference between T₂and S_m?m?1?,together providing an feasible reverse intersystem crossing?RISC?channel for the transformation of high-lying triplet excitons to singlet excitons The combination of HLCT state and HOT exciton mechanism provides an important guarantee for high efficiency narrow band-gap organic fluorescent materials.In this paper,we are mainly focused on the design and synthesis of high-efficiency narrow band-gap organic fluorescent materials with the characteristic of both HLCT state and HOT exciton mechanism,as well as the fabrication of electroluminescent devices with excellent performance.the introduction of auxiliary acceptor into D-A molecular structure is considered into the research.We have achieved the following results:1)Benzo[c][1,2,5]thiadiazole?BZ?and triphenylamine were selected as the acceptor and the donor to construct the D-A backbone?TPA-BZ?.The appropriate twist angle between donor and acceptor in D-A backbone can facilitate the formation of HLCT state.Three molecules?TPA-BZC,TPA-BZO and TPA-BZP?with high luminous efficiency were obtained by incorporation of benzonitrile,anisole and benzene in TPA-BZ backbone.Among them,TPA-BZC exhibits the excellent performance.TPA-BZC film shows a red emission peaking at 640 nm with high photoluminescent quantum yield(?_PL)of 93%.Photophysical characteristic and theoretical calculation suggest that cyano group acting as an ancillary acceptor can effectively decrease band gap and maintain high-efficiency emission,which is beneficial to obtain narrow band-gap deep-red fluorescent materials.The introduction of cyano group into D-A₁-A₂ stucture decreases the energy level of CT state to form HLCT state,which provides a new idea for design of efficient red fluorescent molecules.2)4-?9-?4-?diphenylamino?phenyl?naphtho[2,3-c][1,2,5]thiadiazol-4-yl?benzonitrile?TPA-NZC?,a new NIR material was designed and synthesized with the incorporation of an ancillary acceptor?cyano group?into the D-A molecular backbone of a red emitter TPA-NZP?4,9-diphenylnaphtho[2,3-c][1,2,5]thiadiazole?.Compared with TPA-NZP,TPA-NZC exhibits the NIR emission(?_max=710 nm)with a large redshift?42 nm?and a maintained high?_PL of 17%in film.Moreover,the nondoped OLED based on TPA-NZC emitter exhibits an excellent NIR emission at 702 nm with a maximum EQE of1.2%and the maximum brightness of 757 cd m^-2.Notably,in the device,a high radiative exciton ratio of 35.2%was obtained.The doped device achieves a deep-red emission at656 nm with a maximum EQE of 3.2%and the maximum brightness of 2237 cd m^-2.