Design, Synthesis and Electroluminescence Applications of Pyrazine Derivatives-Based Long-wavelength Thermally Activated Delayed Fluorescent Emitters
基於吡嗪衍生物的長波長熱活化延遲熒光發光材料的設計、合成及電致發光應用
Student thesis: Doctoral Thesis
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Detail(s)
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| Award date | 25 Sept 2024 |
Link(s)
| Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(c4d05525-97d6-4758-8a6a-faf44cfee7b9).html |
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| Other link(s) | Links |
Abstract
Long-wavelength including red to near-infrared (NIR) (600-1100 nm) thermally activated delayed fluorescence-based organic light-emitting diodes (TADF-OLEDs) are highly desired due to their versatile applications. However, efficient long-wavelength TADF-OLEDs are still rarely reported because of the “energy-gap law”. In this thesis, three series of electron donor-acceptor (D-A) type long-wavelength TADF emitters were developed and applied in OLEDs.
First, an electron-deficient pyrimidine (Pm) ring was attached respectively at different positions of a strong D-A framework to yield two isomeric orange-red TADF emitters, oPDM and pPDM. With the same basic D-A backbone, the two emitters show similarly good photoluminescence (PL) properties, including narrow singlet−triplet energy offsets (ΔESTs) and high photoluminescence quantum yields (ΦPLs) in doped films. Surprisingly, the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in aggregate states and thus different carrier mobilities. As a result, the orange-red OLED employing oPDM as an emitter achieved almost twice as high maximum external quantum efficiency (EQEmax, 28.2%) compared with that of a pPDM-based OLED (11.8%).
Second, the conventional triphenylamine (TPA) donor was decorated with spirobifluorene (SBF) unit(s) to extend π-conjugation and increase the stereoscopic effect in emitters. The decorated donors were combined with a phenylquinoxaline-6,7-dicarbonitrile (QCN) acceptor and developed two new deep red (DR) TADF emitters, named SBFA-QCN and DSBFA-QCN. Benefiting from the stronger intramolecular charge transfer (ICT) and extended π-conjugations, as well as reduced inter-chromophore quenching, SBFA-QCN and DSBFA-QCN show smaller ΔESTs and higher ΦPLs in both doped and non-doped films, compared with the TPA-based counterpart (TPA-QCN). Therefore, SBFA-QCN and DSBFA-QCN present red EL with higher EQEmaxs of 22.9% and 22.8% in doped devices than TPA-QCN (17.9%) and DR EL with higher EQEmaxs of 5.9% and 5.3% compared with TPA-QCN (4.3%) in non-doped devices.
Last, another acceptor dibenzo[a,c]phenazine-11,12-dicarbonitrile (DCN) was exploited by fusing an additional naphthalene unit to the QCN segment, and it was employed to construct a new DR/NIR TADF emitter (DSBFA-DCN). The more extended π-conjugation and stronger ICT of DSBFA-DCN provide much longer-wavelength PL emission and smaller ΔEST in doped film compared with DSBFA-QCN. The rigidified molecular configuration also gives raise to decent ΦPL in both doped and non-doped films, thus bringing DR and NIR EL in doped and non-doped devices with respective EQEmaxs of 23.0% and 2.6%.
First, an electron-deficient pyrimidine (Pm) ring was attached respectively at different positions of a strong D-A framework to yield two isomeric orange-red TADF emitters, oPDM and pPDM. With the same basic D-A backbone, the two emitters show similarly good photoluminescence (PL) properties, including narrow singlet−triplet energy offsets (ΔESTs) and high photoluminescence quantum yields (ΦPLs) in doped films. Surprisingly, the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in aggregate states and thus different carrier mobilities. As a result, the orange-red OLED employing oPDM as an emitter achieved almost twice as high maximum external quantum efficiency (EQEmax, 28.2%) compared with that of a pPDM-based OLED (11.8%).
Second, the conventional triphenylamine (TPA) donor was decorated with spirobifluorene (SBF) unit(s) to extend π-conjugation and increase the stereoscopic effect in emitters. The decorated donors were combined with a phenylquinoxaline-6,7-dicarbonitrile (QCN) acceptor and developed two new deep red (DR) TADF emitters, named SBFA-QCN and DSBFA-QCN. Benefiting from the stronger intramolecular charge transfer (ICT) and extended π-conjugations, as well as reduced inter-chromophore quenching, SBFA-QCN and DSBFA-QCN show smaller ΔESTs and higher ΦPLs in both doped and non-doped films, compared with the TPA-based counterpart (TPA-QCN). Therefore, SBFA-QCN and DSBFA-QCN present red EL with higher EQEmaxs of 22.9% and 22.8% in doped devices than TPA-QCN (17.9%) and DR EL with higher EQEmaxs of 5.9% and 5.3% compared with TPA-QCN (4.3%) in non-doped devices.
Last, another acceptor dibenzo[a,c]phenazine-11,12-dicarbonitrile (DCN) was exploited by fusing an additional naphthalene unit to the QCN segment, and it was employed to construct a new DR/NIR TADF emitter (DSBFA-DCN). The more extended π-conjugation and stronger ICT of DSBFA-DCN provide much longer-wavelength PL emission and smaller ΔEST in doped film compared with DSBFA-QCN. The rigidified molecular configuration also gives raise to decent ΦPL in both doped and non-doped films, thus bringing DR and NIR EL in doped and non-doped devices with respective EQEmaxs of 23.0% and 2.6%.
