We propose a simple procedure that restores the ionization potential theorem as the sole tuning criterion for both the long- and short-range Fock exchange of the range-separated hybrid functional. The procedure works by screening out an opposing effect of the short-range Fock fraction at long range, through the 1/ εr dielectric correction in combination with a popular continuum solvation model. Our method proves to be a consistent and accurate way of tuning for both the isolated and solvated molecules.

Three novel green thermally activated delayed fluorescence (TADF) materials, (5,5′-bis­(4-(9,9-dimethylacridin-10­(9H)-yl)­phenyl)-2,2′-bipyrimidine (22bpmAc), 10,10′-([2,5′-bipyrimidine]-2′,5-diylbis­(4,1-phenylene))­bis­(9,9-dimethyl-9,10-dihydroacridine) (25bpmAc), and 2,2′-bis­(4-(9,9-dimethylacridin-10­(9H)-yl)­phenyl)-5,5′-bipyrimidine (55bpmAc)) based on bipyrimidine cores as the new electron-accepting units and acridine as the electron-donating unit were designed, synthesized, and applied as green emitters in TADF devices. The TADF emitters were designed to have different nitrogen orientations in the bipyrimidine core. Two emitters, 25bpmAc and 55bpmAc, showed higher quantum efficiencies and narrower emission spectra relative to 22bpmAc. The former emitters have restricted geometry by hydrogen-bonding interactions on a central core, whereas the latter emitter does not have hydrogen bonding. The intramolecular hydrogen-bonding interaction suppressed a nonradiative mechanism in the 25bpmAc and 55bpmAc TADF emitters. The TADF device fabricated with 55bpmAc showed a maximum external quantum efficiency of 24.9% for the color coordinates (0.22, 0.46). This work demonstrates the importance of intramolecular hydrogen-bonding interactions for designing highly efficient TADF emitters to suppress nonradiative mechanism. .

An accurate prediction of chemical shifts (δ) to elucidate molecular structures has been a challenging problem. Recently, noble machine learning architectures achieve accurate prediction performance, but the difficulty of building a huge chemical database limits the applicability of machine learning approaches. In this work, we demonstrate that the prior knowledge gained from the simulation database is successfully transferred into the problem of predicting an experimentally measured δ. Although both simulation and experimental databases are vastly different in chemical perspectives, reliable accuracy for δ is achieved by additional training with randomly sampled small numbers of experimental data. Furthermore, the prior knowledge allows us to successfully train the model on the more focused chemical space that the experimental database sparsely covers. The proposed approach, the knowledge transfer from the simulation database, can be utilized to enhance the usability of the local experimental database.

If thermally activated delayed fluorescence (TADF) in organic molecules operates on the premise of a minimal change between the singlet and triplet excited states, persistent room-temperature phosphorescence (PRTP) seems to rely on distinct changes among the excited states. Understanding the nature of such changes at the density functional theory level requires that the accuracy be preserved in the geometries of both the ground and excited states. We first show that adding dielectric effects is necessary not only for identifying the relevant charge-transfer excited states but also for calculating the geometries, nearly free of the size consistency error inherent in the optimally tuned range-separated hybrid functional. We use PTZ-BzPN (phenothiazine bonded to benzophenone), a conformationally bistable molecule known to alternate between PRTP and TADF, to investigate different roles played by individual molecular parameters, namely, the reorganization energy, singlet–triplet energy gap, and spin–orbit coupling. We find that PRTP is active when both the reorganization energy and singlet–triplet gap are finite, which is realized by the spin-adaptive geometry of a carbonyl linkage that switches on orbital orthogonality in the singlet and off in the triplet. With increasing dielectric constant, the forward intersystem crossing rate remains constant. In TADF, on the other hand, the forward and reverse intersystem crossing rates decrease to converge with each other.