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To elucidate the high external quantum efficiency observed for organiclight-emitting diodes using a bisanthracene derivative, BD1, as the emittingmolecule, off-diagonal vibronic coupling constants (VCCs) between the excitedstates of BD1, which govern non-radiative transition rates, were calculatedemploying time-dependent density functional theory. The VCCs were analysedbased on the concept of vibronic coupling density. The VCC calculations suggesta fluorescence via higher triplets (FvHT) mechanism, which entails theconversion of a T$_4$ exciton generated during electrical excitation into anS$_2$ exciton via reverse intersystem crossing (RISC); moreover, the S$_2$exciton relaxes to a fluorescent S$_1$ exciton because of large vibroniccoupling between S$_2$ and S$_1$. This mechanism is valid as long as therelaxation of triplet states higher than T$_1$ to lower states is suppressed.The symmetry-controlled thermally activated delayed fluorescence (SC-TADF) andinverted singlet and triplet (iST) structure, which have been proposed in ourprevious studies, are the special examples of the FvHT mechanism that need highmolecular symmetry. However, BD1 achieves the FvHT mechanism in spite of itsasymmetrical structure. A general condition for the suppression of radiativeand non-radiative transitions in molecules with pseudo-degenerate electronicstructures such as BD1 is discussed. A superordinate concept, fluorescence viaRISC, which includes TADF, SC-TADF, iST structure, and FvHT is also proposed.

Fluorescence via Reverse Intersystem Crossing from Higher Triplet States in a Bisanthracene Derivative