Photophysics of an Asymmetric Donor-Acceptor-Donor' TADF Molecule and Reinterpretation of Aggregation-Induced TADF Emission in These Materials

We report an in-depth photophysical investigation of an asymmetric donor-acceptor-donor' (D-A-D') thermally activated delayed fluorescence (TADF) molecule (4- (9H-carbazol-9-yl)phenyl) (4- (10 H-phenothiazin-10-yl)-phenyl)sulfone and compare its photophysical properties to the parent symmetric D-A-D and D'-A-D' molecules. These D-A-D type small molecules all show strong TADF. The work reveals how the relative orientations of D-A (D'-A) moieties favor reverse intersystem crossing (rISC) by forming stable charge transfer (CT) states. The key requirement for the efficient TADF emitters is to achieve a very small CT-local triplet state energy splitting, which is shown to be complex in the asymmetric molecule. Throughout the investigations, we show that in the asymmetric D-A-D' system, even though E-CT (D-A) > E-CT (D'-A), no evidence of energy transfer from D-A to A-D' is observed, nor from excited D to D'. This is ascribed to the near orthogonality of the D and D' units and the very strong decoupling of the electrons on the D and A in the CT state. In addition, the possibility of aggregation-induced TADF (AI-TADF) is examined and shown to be a manifestation of solvatochromism in these particular molecules.