Fine-tuning the emissions of highly efficient thermally activated delayed fluorescence emitters with different linking positions of electron-deficient substituent groups

To further develop a simple method for emission fine-tuning, two novel thermally activated delayed fluorescence (TADF) emitters, 5′-(10H-phenoxazin-10-yl)-[1,1′:3′,1″-terphenyl]- 2′,4,4″- tricarbonitrile (mPTBC) and 2′-(10H-phenoxazin-10-yl)-[1,1′:3′,1″- terphenyl]-4,4″,5′-tricarbonitrile (oPTBC) have been designed and synthesized by employing exactly the same functional units, a ter-benzonitrile and a phenoxazine (PXZ). By adjusting the linking position of the two electron-deficient benzonitrile arms from a meta-position to an ortho-position with the cyano core, the LUMO energy levels of mPTBC and oPTBC are varied to −2.98 and −3.13 eV, respectively; while the HOMO energy levels remain similar. In devices, the new TADF emitters successfully exhibit similar maximum efficiencies, but different-color emissions with a spectral peak at 516 nm for mPTBC and a spectral peak at 540 nm for oPTBC. These results prove the simple modification of adjusting the linking position of electron-deficient substituent groups in the molecules can effectively fine-tune the emissions of TADF emitters but without significantly changing other key performance.