ORGANIC DIRADICALS AS OPTICALLY ADDRESSABLE MOLECULAR QUBITS: WHAT THE PARISER-PARR-POPLE MODEL CAN TELL US?

Recent experiments [2] and theoretical predictions [1,3] emphasize the growing importance of luminescent organic diradicals as optically addressable molecular qubits. These systems also show significant promise for magnetic sensing and quantum information science. Our objective is to develop specific π-conjugated diradical molecules for these applications by investigating the radical- radical interactions in both the ground and excited states through molecular structure tuning. We examine the excited-state properties of these systems through a comprehensive study based on the Pariser-Parr-Pople (PPP) model. The strong electron correlation present in these systems complicates the calculation of their excited-state properties. To manage electron correlation more effectively, we diagonalize the PPP Hamiltonian within the Configuration Interaction (CI) approach and compare the results with those obtained by strategically partitioning the PPP molecular orbitals into different active spaces, adopting a restricted active space configuration interaction (RASCI) approach.[4] Due to the multiconfigurational nature of these electronic states, we compare results with ab initio CASSCF, NEVPT2, and QD-NEVPT2. We emphasize the crucial role of the frontier orbitals and the importance of molecular structure in the radical-radical interactions. We underline that the PPP- RASCI approach combines precision and computational efficiency in accurately modelling the excited-state properties of diradical systems, enabling the mapping of excited-state properties in organic diradicals. [1] J. Yuen-Zhou et al. J. Am. Chem. Soc. 146 (2024) 15549. [2] R. Wasielewski et al. J. Am. Chem. Soc. 146 (2024) 27935. [3] Y. R. Poh et al. ACS Cent. Sci. 11 (2025) 116. [4] M. Bedogni et al. J. Chem. Theory Comput. 20 (2024) 8634.