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 ( . . 2017, 8, 2677-2686), Takeda and co-workers 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 S state from local to a charge transfer state and was responsible for the large red shift in emission (S) 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 S state (CT → CT), close spacing and effective crossing between LE, CT and CT states cause efficient harvesting of triplet excitons to S state, thus enabling TADF emission for equatorial-equatorial conformer.