Enhancing Reverse Intersystem Crossing with Extended Inverted Singlet-Triplet (X−INVEST) systems

The discovery of triangular-shaped organic molecules bearing an inverted singlet-triplet (INVEST) energy gap opened the way for a new strategy to increase the internal quantum efficiency of organic light-emitting diodes by enhancing the Reverse Intersystem Crossing (RISC) process thanks to a downhill process. However, all the molecular systems showing a negative ΔEST suffer from both a vanishing Spin-Orbit Coupling between the lowest singlet (S1) and triplet (T1) excited states and high energy differences with higher-lying singlet and triplet excited states, precluding their involvement in the spin conversion process. In this work we proposed a new design strategy entailing the extension of the triangulene cores by connecting two INVEST triangulene units to form Uthrene- and Zethrene-like systems, doped with N and B atoms. A detailed inspection of the resulting molecular orbital distribution allowed rationalizing the electronic structure properties obtained employing wavefunction-based methods (NEVPT2, EOM-CCSD, SCS-CC2), showing how the Uthrene-like architecture can give origin to the energy proximity between the lowest singlet and triplet excited states, in some cases leading to their energy inversion. By feeding a kinetic model with the non-radiative rate constants calculated from first principle we showed how the Extended INVEST (X−INVEST) design strategy can open new pathways to boost the spin conversion process and the population of the emissive S1 excited state.