A light-emitting mechanism for organic light-emitting diodes: molecular design for inverted singlet–triplet structure and symmetry-controlled thermally activated delayed fluorescence

The concepts of symmetry-controlled thermally activated delayed fluorescence (SC-TADF) and inverted singlet–triplet (iST) structure are proposed. Molecules that can exhibit SC-TADF or have an iST structure can be employed as light-emitting molecules in organic light-emitting diodes. The molecular symmetry plays crucial roles in these concepts since they are based on the selection rules for the electric dipole transition, intersystem crossing, and nonradiative vibronic (electron-vibration) transitions. In addition to the symmetry conditions for the SC-TADF and iST molecules, the molecules should have small diagonal and off-diagonal vibronic coupling constants for suppressing vibrational relaxations and nonradiative vibronic transitions, respectively, and a large transition dipole moment for the fluorescence process. Analyses using the vibronic coupling and transition dipole moment densities are employed to reduce the vibronic coupling constants and to increase the transition dipole moment. The preferable point groups in the development of SC-TADF and iST molecules are discussed on the basis of the ratios of forbidden pairs of irreducible representations. It is found that the existence of the inversion symmetry is preferable for designing SC-TADF and iST molecules. On the basis of these guiding principles, we designed some anthracene and pyrene derivatives as candidate iST molecules. Their electronic structures, spin–orbit couplings, transition dipole moments, and vibronic couplings are discussed.