Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters

Chen-Han Lu; Chun-Yen Lin; Shi-Xian Zeng; Yu-Pin Chou; Chia-Hsun Chen; Yi-Hung Liu; Jiun-Haw Lee; Chung-Chih Wu; Ken-Tsung Wong

doi:10.1021/acsami.3c05754

Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters

ACS Appl Mater Interfaces. 2023 Jul 26;15(29):35239-35250. doi: 10.1021/acsami.3c05754. Epub 2023 Jul 17.

Authors

Chen-Han Lu¹, Chun-Yen Lin², Shi-Xian Zeng², Yu-Pin Chou¹, Chia-Hsun Chen², Yi-Hung Liu², Jiun-Haw Lee¹, Chung-Chih Wu¹, Ken-Tsung Wong^{2

3}

Affiliations

¹ Department of Electrical Engineering, Graduate Institute of Electronics Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan.
² Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.
³ Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan.

PMID: 37459567
DOI: 10.1021/acsami.3c05754

Abstract

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ_// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQE_max) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQE_max of 15.4% and CIE coordinates of (0.14, 0.09). The EQE_max of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S₁ state to give lower ΔE_ST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Keywords: deep-blue emitter; emitting dipole orientation; macrocyclic donor; organic light-emitting diode; thermally activated delayed fluorescence.