Role of the Geometry Restriction and Quasi‐Degeneracy of the Excited States in Thermally Activated Delayed Fluorescence: A Density Functional Theory Study of Carbzolyl‐Bispyridinylmethanone Derivatives

Abstract

AbstractWe theoretically investigated the photophysical and excited‐state properties of carbazolyl‐bispyridinylmethanone (Cbz‐PyM) derivatives as thermally activated delayed fluorescence (TADF) emitters, for example, the energies of the low‐lying singlet and triplet excited states, their natures, energy differences between them, intersystem crossing (ISC), and fluorescence rate constants. Our results match reasonably well the experimental data; the difference between the theoretical and experimental values provides a useful insight into the role of structural restriction in the solid‐state film, which particularly has a distinctive impact on the singlet excited states. In addition, via a comparison between molecules, either with one or two carbazolyl subunits, we found that a quasi‐degeneracy in the excited states can enhance the ISC, fluorescence, and hence TADF efficiency. We also proposed a molecular design strategy for these degeneracies in the excited states.