Thianthrene and acridan-substituted benzophenone or diphenylsulfone: Effect of triplet harvesting via TADF and phosphorescence on efficiency of all-organic OLEDS

Two new asymetric D-A-D' type compounds containing benzophenone and diphenylsulfone as electron accepting core, and acridan and thianthrene as electron donating moieties were design as multifunctional materials. The influence of different acceptor and donor units on thermally activated delayed fluorescence and room temperature phosphorescence was investigated and discussed. The combination of different donors with benzophenone unit resulted in the compound with effective frontier orbital separation, as well as the sustainment of high radiative rate. These statements are both experimentally and theoretically supported using time-resolved photoluminescence spectroscopy and DFT calculations. Deoxygenation experiment revealed higher triplet exdton contribution in the emission of benzophenone derivative, as compared to that of diphenylsulfone-based compound. The films of molecular mixtures of these emitters with 1,3-bis(N-carbazolyl)benzene demonstrated temperature-dependent photoluminescence. The estimated energy gaps between the lowest singlet and triplet states were found to be 0.06 eV and 0.27 eV for benzophenone and diphenylsulfone derivatives, respectively. Due to the relatively high singlet-triplet energy splitting of the diphenylsulfone-based compound, triplet harvesting was detected not only via temperature-stimulated fluorescence but also via room temperature phosphorescence. Displaying effect of triplet harvesting on efficiency of all-organic blue and green OLEDs based on thianthrene and acridan-substituted benzophenone or diphenylsulfone derivatives demonstrated very different maximum external quantum efficiencies of 1.4 and 22.2% respectively.