Thermally activated delayed fluorescence (TADF) compounds with a flexible donor–acceptor structure suffer from conformational disorder in solid-state, which deteriorates their emission properties as well as OLED performance. Accordingly, TADF materials with predictable solid-state emission properties are highly desirable. In this work, we analyse the relation between the molecular rigidity and solid-state TADF properties. Two TADF compounds, 4,6-bis(2-methyl-4-(10H-phenothiazin-10-yl)phenyl)pyrimidine (PTZ-mPYR) and 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene (4CzPN), with similar emission properties in toluene solutions but different rigidity of the molecular structure were systematically studied. The analysis was supplemented by comparison of solid-state TADF properties of PTZ-mPYR with its analogue 4,6-bis(4-(10H-phenoxazin-10-yl)phenyl)pyrimidine (PXZ-PYR), bearing a more sterically constrained planar electron-donor unit. All compounds showed conformational disorder in diluted polymer films; however its extent directly depended on the molecular structure. Large dispersion of singlet–triplet energy gaps resulted in remarkably prolonged TADF lifetime for PTZ-mPYR with a less sterically constrained donor unit. In contrast, weakened conformational disorder in rigid 4CzPN with sterically crowded donor units was shown to ensure rapid TADF decay with only a threefold lower solid-state rISC rate as compared to toluene. Similarly, selection of a more sterically constrained planar electron-donor unit was also shown to be preferable for lowering the conformational disorder. Our findings are important for the future design of compounds with efficient solid-state TADF as well as for the further application in OLEDs with low external quantum efficiency roll-off.

Utilization of “dark” triplet states is a key task for design of novel emissive organic compounds. Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are two promising pathways to employ triplet excitons and reach high internal quantum efficiencies. In this paper we present a series of carbazole–pyrimidine cored compounds, showing RTP and TADF. A specific energy level scheme was shown to promote intersystem crossing and RTP in carbazole–pyrimidine derivatives with phosphorescence quantum yield values of up to 0.07 in rigid Zeonex films. The modification of the acceptor core with 2-methylthio and dimethylamine fragments allowed us to reduce the singlet–triplet energy gap, enhance the rISC (reverse intersystem crossing) rate and obtain high intensity TADF with a fluorescence quantum yield of up to 0.32. Time resolved fluorescence analysis revealed the presence of conformational disorder of the donor–acceptor core governing TADF properties in solid films. A method to control the conformational disorder by enhancing the host rigidity was demonstrated. Sky-blue electroluminescence was shown for the dimethylamino-modified compound with the most intense TADF. Our results show that carbazole–pyrimidine cored materials are versatile donor–acceptor systems for achieving RTP and TADF via simple structural modifications.

TADF compounds with flexible molecular core show large conformational disorder and emission wavelength instability in solid films.