Achieving Conformational Control in Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence Emitters by Functionalization of the Central Core

Three new symmetrical donor–acceptor–donor (D–A–D)-type molecules were prepared with phenothiazine (PTZ) as electron donors and 9,9-dimethylthioxanthene (TX) as the electron acceptor. The PTZ groups are attached at different positions on the acceptor core: either ortho or meta to the sulfur of TX. The molecules have been characterized by X-ray crystallography, in-depth photophysical studies, and theoretical calculations. This series provides new insights into how molecular functionalization and intramolecular charge transfer determines the singlet–triplet gap ΔEST. Two of the molecules have weak D/A decoupling and a relatively large ΔEST of 0.52 eV which prohibits the upconversion of triplet excitons to the singlet state, showing strong room-temperature phosphorescence (RTP). When the TX acceptor strength is enhanced by the attachment of benzoyl substituents a very small ΔEST of <0.01 eV is observed. In this case, excitons in the triplet state can be efficiently upconverted to the singlet state via reverse intersystem crossing (RISC) resulting in thermally activated delayed fluorescence (TADF). TADF and RTP are unambiguously assigned by distinctive photophysical data, notably a comparison of degassed and aerated luminescence spectra, temperature-dependent time-resolved fluorescence decays, and the power dependence of the intensity of delayed emission (for the TADF emitter).