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-9yl)phenyl)(4-(10H-phenothiazin-10-yl)phenyl)sulfone and compare its photophysical properties to the parent symmetric D-AD and D'-A-D' molecules. These D-AD type small molecules all show strong TADF. The work reveals how the relative orientations of D-A (D'-A) moieties favour 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 in these particular molecules and shown to be a manifestation of solvatochromism in these materials.

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