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 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 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 ³LE_1A, ³CT₁ and ¹CT₁ states cause efficient harvesting of triplet excitons to S₁ state, thus enabling TADF emission for equatorial-equatorial conformer.