Effects of Ortho-Phenyl Substitution on the rISC Rate of D–A Type TADF Molecules

Two new donor (D)–acceptor (A) type molecules, <b>PXZ-DBTO2</b> and <b>PXZ-Ph-DBTO2</b>, configured with phenoxazine donor and dibenzothiophene-<i>S</i>,<i>S</i>-dioxide acceptor are reported. <b>PXZ-Ph-DBTO2</b>, with a phenyl group introduced at the ortho position of PXZ, was used to probe the effects of the congested aryl substitution on the molecular conformation and electronic coupling toward the acceptor core, as well as the thermally activated delayed fluorescence behavior. The highly twisted donor–acceptor configurations of these two molecules were confirmed by X-ray analysis. Different D–A conformations stemmed from the steric interactions between the phenyl group and acceptor core, which leads the nitrogen lone pair electrons of the <b>PXZ-Ph-DBTO2</b> donor to conjugate across the D–A bridge, whereas in <b>PXZ-DBTO2</b> the lone pairs remain localized on the donor and strongly mix with the donor π electrons. However, both <b>PXZ-Ph-DBTO2</b> and <b>PXZ-DBTO2</b> have the same energy splitting between the charge-transfer states and local donor triplet states, Δ<i>E</i>_ST, close to 70 meV. <b>PXZ-DBTO2</b> exhibits a far more efficient thermally activated delayed fluorescence due to nearly 2 orders of magnitude faster reverse intersystem crossing rate as compared to that of <b>PXZ-Ph-DBTO2</b>. Detailed photophysical analysis of both molecules indicates that the presence of the phenyl group on the donor disrupts the π–π*/n−π* orbital mixing across the N–C bridge that plays a fundamental role in the excited state dynamics and vibronic coupling governing the reverse intersystem crossing rate and thus the efficiency of thermally activated delayed fluorescence. Devices employing <b>PXZ-DBTO2</b> as an emitting dopant gave an external quantum efficiency (EQE) of 16.7% (42 cd m^–2) and a limited efficiency roll-off (15.7% at 1000 cd m^–2), whereas the device based on <b>PXZ-Ph-DBTO2</b> produced a maximum EQE up to 20.6%, but with a significant efficiency roll-off (8.8% at 1000 cd m^–2) ascribed to the much faster reverse intersystem crossing rate of <b>PXZ-DBTO2