Exploring the electronic structure of extended triangulenes: Opening new doors for a fast Reverse Intersystem Crossing

The recent discovery of triangle-shaped compounds bearing an inverted S1-T1 energy gap (INVEST) has represented a promising strategy to harvest the triplet excitons and boost the Reverse Intersystem Crossing (RISC) process in fully organic molecular systems, allowing the enhancement of Organic Light Emitting Diodes (OLEDs) performances. In our previous works, we defined the computational protocol to describe these systems [1-3], entailing the employment of correlated wavefunction methods, and combining quantum-chemistry and group theory we related the optical properties of these molecules to their symmetry [4]. An intriguing design strategy emerging from these studies involved the extension of the molecular backbone by merging two INVEST cores, resulting in a Uthrene-like structure. This leads to a non-zero oscillator strength of the S1 state, thus allowing emission of light, and concomitantly to the energy proximity of the two lowest singlet and triplet excited states, opening the door to multiple RISC channels which can pave the way for a new paradigm for the singlet-triplet conversion.
In this work, we select a series of Uthrene-like extended triangulenes, doped with nitrogen and boron atoms, and model their electronic structures employing correlated wavefunction methods such as SCS-CC2, NEVPT2 and EOM-CCSD. A closer look at the molecular orbitals and electron-correlation descriptors can help in the rationalization of the small energy gap between the two lowest singlet and triplet excited states. In addition, the computation of the non-radiative decay rates (ISC, RISC and internal conversion) can provide a first picture of the spin-conversion pathways occurring in the excited states, to identify the most promising candidate for next-generation OLED applications.