Establishing design rules for emissive materials as next generation emitters for organic light-emitting diodes: A computational perspective from the inversion of the singlet-triplet energy gap

The inversion of the lowest singlet and triplet excited state energy gap, in fully organic triangle-based compounds, can give rise to a new exergonic pathway to enhance the Organic Light Emitting Diodes (OLEDs) performance, going beyond the novel yet promising Thermally Activated Delayed Fluorescence (TADF) mechanism. If, on one hand, the origin of this inversion, arising from the interplay between exchange and electron correlation effects, has been extensively investigated in last years, identifying the wavefunction methods as key to predict the excited-state inversion, on the other hand a proper picture of the structure-property relationships characterizing these systems is still missing. In this work, we thus assess the effect of different symmetry point groups (D3h, C2v, C3h and C3v) on the orbital localization to shed light on the role that the symmetry has in determining the optical features of the triangulene systems (on both S1-T1 inversion and oscillator strengths). The presence of the C_2 axis and the σ_v plane (as it happens for the D3h, C2v and C3v groups) turned out to be critical for ensuring the proper orbital localization aimed at minimizing the exchange interaction and therefore favouring the inversion. In particular, adopting a C2v (and its subgroups) symmetry, either through the proper doping pattern, by introducing substituents, or by merging two triangulene cores, is the only way to conciliate a negative ΔEST and a non-zero oscillator strength. Finally, we gathered the lessons learnt from this analysis to establish a series of design rules, aimed at helping the identification of inverted singlet-triplet (INVEST) emitters for applications in the next generation of OLEDs.