Influence of charge transfer strength on emission bandwidth for multiple-resonance emitters via systematically tuning the acceptor-donor assembly

Multiple resonance (MR) thermally activated delayed fluorescence (MR-TADF) materials, categorized for the B/N and carbonyl/amine fragments, are now intensively and widely studied due to their color purity. However, the relationship between the MR molecular structure and full width at half maximum (FWHM) of the emission remains elusive. In order to probe the factor of determining the emission FWHM of the MR emitters, a series of relevant molecules were synthesized where the MR core acceptor moiety, phenylborane or acetophenone, is fixed, while the donor moiety is altered to adjust the charge transfer (CT) strength. These potential MR molecules provide sufficient photophysical data to shed light on the emission bandwidth influenced by CT strength. Meanwhile, OLEDs fabricated using all the studied compounds achieve maximum external quantum efficiencies (EQE(max)) around 7.0-15.3% because of the TADF character induced by the host/guest interaction in the excited state. The results provide further understanding on the fundamentals of the MR emitters.