studies have shown that several triangle-shaped π-conjugated molecules (also called triangulenes) decorated with nitrogen atoms display a negative ∆E ST , thus violating Hund's rule. Indeed, in these systems the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are marginally overlapping and the lowest energy transition has a dominant multi resonant charge-transfer (MR-CT) character, with the electronic charge being transferred from HOMO to LUMO. In these conditions, the exchange integral, responsible for the ST splitting, is tiny, and the spin polarization correction, typically negligible, may be large enough to revert the ST gap. The family of MR-CT dyes is interesting in several respects. Even systems with the normal order of singlet and triplet states have small ∆E ST , making it possible to thermally populate the S 1 state from the T 1 state. Fluorescent MR-CT dyes are then intensively investigated as emitters featuring thermally activated delayed fluorescence (TADF). They are of special interest for OLED applications, since the reduced polarity of the MR-CT state and the molecular rigidity guarantee for very narrow and marginally solvatochromic emission spectra, properties in demand for emitters for (blue) OLED with high color purity. Here we will not address the family of MR-CT dyes at large, but will focus on the specific issue of ST inversion. The theoretical interest in molecules breaking Hund's rule is backed up by their applicative potential. The reverted ST order, in fact, allows for easy and efficient triplet harvesting in organic light-emitting diodes (OLEDs), increasing their maximum internal efficiency from a disappointing 25% to a reassuring 100%. Spin-polarization, the mechanism responsible for ST-inversion, is due to electron correlation so that conventional TD-DFT approaches to excited states are inadequate to address ST inversion, as it was early pointed out. High-quality, computationally expensive methods have been exploited so far to deal with ST-inversion, including CIS(D), 3,6 SCS-CC2, EOM-CCSD, SCS-ADC(2), 2-4,7,8 CASPT2, 2 SA-CASSCF, 3,8 SC-NEVPT2 3,4,8 and Bethe-Salpeter equation in the GW approximation. In an effort to better understand the physics underlying the ST inversion, we undertook an extensive study of several molecular structures described by the celebrated Pariser-Parr-Pople (PPP) model Hamiltonian, the simplest model for correlated electrons in π-conjugated molecules. The model, proposed in the 1950s, successfully described the behavior of several families of small organic molecules. In the field of π-conjugated polymers, PPP was pivotal in understanding the anomalous behavior of polyacetylene, where correlated electrons are responsible for the appearance of a low-lying dark state, thus solving the puzzle of its non-fluorescent behavior. More recently, density matrix renormalization group (DMRG) approaches allowed to extend the PPP model to deal with graphene-based structures, to address singlet-fission in polyenes and to build models for dark states in polyene and carotenoid systems. Here, we exploit the comparative simplicity of the PPP approach to understand ST inversion in triangulenes and related molecules. Comparing with exact diagonalization results, we critically discuss the validity of the double configuration interactions (CI) and related approaches, typically adopted to deal with all electrons models of triangulenes. The adopted PPP model cannot provide quantitatively accurate results on specific molecules. However, it is a solid and elegant approach that, applied here to a variety of molecular structures, will disclose qualitative trends about the effects of electron correlations in π-conjugated molecules.