A heterocycle fusing strategy for simple construction of efficient solution-processable pure-red thermally activated delayed fluorescence emitters

The development of solution-processable pure-red thermally activated delayed fluorescence (TADF) emitters remains a challenging task in the field of organic light-emitting diodes (OLEDs). Here two strong electron acceptor moieties, acenaphtho[1,2-b][1,2,5]oxadiazolo[3,4-e]pyrazine (ANOP) and acenaphtho[1,2-b][1,2,5]thiadiazolo[3,4-e]pyrazine (ANTP), were designed and synthesized by fusing 1,2,5-oxadiazole or 1,2,5-thiadiazole on an acenaphtho[1,2-b]pyrazine unit through a simple and catalyst-free dehydration cyclization reaction. Combined with rigid electron-donating units, 9,9-dimethyl-9,10-dihydroacridine (DMAC) and 2,7-dimethyl-10Hspiro[acridine-9,9 '-fluorene] (MeFAC), four solution-processable pure-red TADF emitters, ANOP-DMAC, ANOP-MeFAC, ANTP-DMAC, and ANTP-MeFAC, were designed and prepared through catalyst-free nucleophilic substitution reactions. The rational molecular design principles endow all the emitters with small singlet-triplet energy gaps (Delta E-ST), prominent TADF character, red emission, and moderate phi(PL)s. The optical, electrochemical, and film morphological properties of all the emitters were systematically investigated. As a consequence, the ANTP-DMAC-based OLED realized a maximum external quantum efficiency of 6.8% with a peak wavelength at 630 nm and Commission International de l'Eclairage 1931 (CIE1931) coordinates of (0.59, 0.40), which is among one of the highest device performances for solution-processed pure-red TADF OLEDs.