Minimization of Solid - State Conformational Disorder in Donor – Acceptor TADF compounds
Thermally activated delayed fluorescence (TADF) compounds with flexible donor-acceptor structure suffer from conformational disorder in solid-state, deteriorating the 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 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 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 less sterically constrained donor unit. On the contrary, weakened conformational disorder in rigid 4CzPN with sterically crowded donor units was shown to ensure rapid TADF decay with only threefold lower solid-state rISC rate as compared to toluene. Similarly, selection of 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.