A First-Principles Investigation of Structure-Luminescence Activity of Donor-Acceptor-Donor Organic Triad.

Structure-property relationships of different conformers of an organic D-A-D triad are explored to rationalize the structural motif toward photoluminescence activity. In a recent experiment (Chem. Sci. 2017, 8, 2677-2686), Takeda and coworkers revealed that the PTZ-DBPHZ-PTZ (D-A-D) triad exhibited multicolor luminescence properties and thermally activated delayed fluorescence (TADF) emission. We computationally studied the photophysical properties of the conformers of that D-A-D triad to provide a detailed description of the luminescence activity. Our analysis confirms that the twisting of the axial phenothiazine (PTZ) unit to an equatorial position altered the nature of the S1 state from local to a charge transfer state and was responsible for the large red shift in emission (S1) energy. Calculated fluorescence and intersystem crossing (ISC) rate constants suggest that the prompt fluorescence is turned on for axial-axial conformers while it is turned off for others. Fast reverse intersystem crossing (RISC) from triplet CT to the S1 state (3CT1 -> 1CT1), close spacing and effective crossing between 3LE1A, 3CT1 and 1CT1 states cause efficient harvesting of triplet excitons to S1 state, thus enabling TADF emission for equatorial-equatorial conformer.