Unlike conventional thermally activated delayed fluorescence chromophores, boron-centered azatriangulene-like molecules combine a small excited-state singlet-triplet energy gap with high oscillator strengths and minor reorganization energies. Here, using highly correlated quantum-chemical calculations, we report this is driven by short-range reorganization of the electron density taking place upon electronic excitation of these multi-resonant structures. Based on this finding, we design a series of π-extended boron- and nitrogen-doped nanographenes as promising candidates for efficient thermally activated delayed fluorescence emitters with concomitantly decreased singlet-triplet energy gaps, improved oscillator strengths and core rigidity compared to previously reported structures, permitting both emission color purity and tunability across the visible spectrum.

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We have investigated the origin of the S1‐T1 energy levels inversion for heptazine, and other N‐doped π‐conjugated hydrocarbons, leading thus to an unusually negative singlet‐triplet energy gap (ΔEST<0 ). Since this inversion might rely on substantial doubly‐excited configurations to the S1 and/or T1 wavefunctions, we have systematically applied multi‐configurational SA‐CASSCF and SC‐NEVPT2 methods, SCS‐corrected CC2 and ADC(2) approaches, and linear‐response TD‐DFT, to analyze if the latter method could also face this challenging issue. We have also extended the study to B‐doped π‐conjugated systems, to see the effect of chemical composition on the results. For all the systems studied, an intricate interplay between the singlet‐triplet exchange interaction, the influence of doubly‐excited configurations, and the impact of dynamic correlation effects, serves to explain the ΔEST<0 values found for most of the compounds, which is not predicted by TD‐DFT.

In a theoretical study, we characterized the nature of the key excited states involved in the TADF process of donor–acceptor compounds and showed that light emission is enhanced by dynamic fluctuations of the donor–acceptor torsion resulting from flat torsional potentials.

The full harvesting of both singlet and triplet excitons can pave the way towards more efficient molecular light-emission mechanisms (i.e., TADF or Thermally Activated Delayed Fluorescence) beyond the spin statistics limit. This TADF mechanism benefits from low (but typically positive) singlet-triplet energy gaps or ∆E ST . Recent research has suggested the possibility of inverting the order of the energy of lowest singlet and triplet excited-states, thus opening new pathways to foster light emission without any energy barrier through triplet to singlet conversion, which is systematically investigated here by means of theoretical methods. To this end, we have selected a set of heteroatom-substituted triangle-shaped molecules (or triangulenes) for which ∆E ST < 0 is predicted. We successfully rationalize the origin of that energy inversion, and the reasons for which theoretical methods might produce qualitatively inconsistent predictions depending on how they treat n-tuple excitations (e.g., the large contribution of double excitations for all the ground-and excited-states involved). Unfortunately, the TD-DFT method is unable to deal with the physical effects driving this behaviour, which prompted us to the use here of more sophisticated ab initio methods such as SA-CASSCF, SC-NEVPT2, SCS-CC2, and SCS-ADC(2).