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

Abstract

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 state...