Organic triradicals molecules to obtain high-spin ground state populations

Recent studies, [1,2,3], have shown that light irradiation can enhance the quartet ground-state population of triradical molecules, making them promising candidates for magnetic sensing and quantum information applications. Here, we focus on π-conjugated triradicals to investigate radical– radical interactions in both ground and excited states through molecular structure tuning. We present a theoretical-computational study of photoactive high-spin triradicals, analyzing the excited-state relaxation pathways responsible for quartet-state population. Excited-state properties are examined using the Pariser–Parr–Pople (PPP) model. To address the strong electron correlation characterizing these systems, we diagonalize the PPP Hamiltonian within a Configuration Interaction (CI) framework and compare the results with a Restricted Active Space CI (RASCI) approach based on tailored active spaces. Given the multiconfigurational nature of the electronic states, we further benchmark our results against ab initio CASSCF-QD-NEVPT2 calculations. Our findings highlight the key role of frontier orbitals and molecular structure in governing radical interactions and demonstrate that the PPP- RASCI approach provides an effective balance between accuracy and computational cost for modelling excited states in organic triradicals. References [1] Wang S. et al., CCS Chem. (2025). (accepted) [2] Wang X. et al., fmre (2026). (accepted) [3] Privitera A. et al., JACS 147 331 (2024)