Tuning the position of unpaired electrons and doublet–quartet gap of the 1,3,5-trimethylenebenzene triradical by nitrogen, phosphorus and arsenic substitution

Substitution of C–H groups in 1,3,5-trimethylenebenzene (1,3,5-TMB) triradical by one, two or three N, P and As atoms is studied using CASSCF/CASPT2 computations with a ANO-RCC triple-ζ basis set. The computations showed that consistently with the previous results, the unpaired electrons in 1,3,5-TMB are highly delocalized, but our computations show that they become localized on the P and As atoms, with a more pronounced localization effect of As. All studied compounds have a quartet ground state, but the results showed in this Letter indicate that the quartet–doublet energy gap is reduced upon substitution. Localization of the unpaired electrons on the ring reduces the average distances between them, which induces a destabilization of the quartet states compared to the doublet. Therefore the position of the unpaired electrons and the doublet–quartet gap can be tuned by chemical methods, which opens the way to fine-tune the properties of the building blocks of organic magnets.