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

Two new donor (D)–acceptor
(A) type molecules, PXZ-DBTO2 and PXZ-Ph-DBTO2, configured with phenoxazine donor
and dibenzothiophene-S,S-dioxide
acceptor are reported. PXZ-Ph-DBTO2, 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 PXZ-Ph-DBTO2 donor to conjugate
across the D–A bridge, whereas in PXZ-DBTO2 the
lone pairs remain localized on the donor and strongly mix with the
donor π electrons. However, both PXZ-Ph-DBTO2 and PXZ-DBTO2 have the same energy splitting between the charge-transfer
states and local donor triplet states, ΔEST, close to 70 meV. PXZ-DBTO2 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 PXZ-Ph-DBTO2. 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 PXZ-DBTO2 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 PXZ-Ph-DBTO2 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 PXZ-DBTO2.