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

Abstract

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 S 1 state from local to a charge transfer state and was responsible for the large red shift in emission (S 1 ) 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 1 state ( 3 CT 1 → 1 CT 1 ), close spacing and effective crossing between 3 LE 1A , 3 CT 1 and 1 CT 1 states cause efficient harvesting of triplet excitons to S 1 state, thus enabling TADF emission for equatorial−equatorial conformer.