Inverting the singlet-triplet energy gap in triangle-shaped molecules: a matter of Symmetry and correlation

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 for exciton formation. This TADF mechanism benefits from low, but typically positive, singlet-triplet energy gaps (?EST). Recent results have suggested a possible inversion of the singlet-triplet excited states in triangle-shaped molecules (or triangulenes) opening a new pathway to improve the efficiency of organic lightemitting materials. This inversion originates from the interplay between the electron exchange interaction and electron correlation effects, the latter arising from the presence of doubly excited configurations driving the stabilization of S1 with respect to T1 and the consequent energy inversion. The first part of this seminar is purely “computational”.

We will explore a set of triangulenes with different sizes and doping patterns to show how the conventional TimeDependent Density Functional Theory (TD-DFT) is unable to tackle the S1-T1 inversion and relying on correlated wavefunction methods, e.g. SCS-CC2, SCSADC(2), CASSCF, NEVPT2[1,2], or double-hybrid functionals[3], is the key to properly account for the electron correlation effects leading to the negative ?EST. In the second part, we will shed light on the role that the symmetry has in determining the optical features of the triangulene systems, i.e. negative ?EST and oscillator strength, and finally 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.
Gaetano Ricci is PhD student at Laboratory for Computational Modeling of Functional Materials, Namur Institute of Structured Matter, Université de Namur (Belgium).
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