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).