Unravelling the Key Driving Forces of the Spin Transition in π-Dimers of Spiro-biphenalenyl-Based Radicals.

Spiro-biphenalenyl (SBP) boron radicals constitute an important family of molecules for the preparation of functional organic materials. The building blocks of several SBP-based crystals are π-dimers of these radicals, in which two phenalenyl (PLY) rings face each other and the other two PLYs point away from the superimposed PLYs. The dimers of ethyl-SBP and butyl-SBP undergo a spin transition between a diamagnetic and a paramagnetic state upon heating, while other dimers exhibit paramagnetism at all temperatures. Here, we present a computational study aimed at establishing the driving forces of the spin-transition undergone by ethyl-SBP at ∼140 K. The ground state of the π-dimers below 140 K is a singlet state in which the SBP unpaired electrons are partially localized in the superimposed PLYs. Above 140 K, the unpaired electrons are localized in the nonsuperimposed PLYs. These high-temperature structures are exclusively governed by the ground triplet state because the open-shell singlet with the unpaired electrons localized in the nonsuperimposed PLYs does not feature any minimum in the potential energy surface of the system. Furthermore, we show that the electrostatic component of the interaction energy between SBP radicals in the π-dimers is more attractive in the triplet than in the singlet, thereby partially counteracting the bonding and dispersion components, which favor the singlet. This electrostatic stabilization of the triplet state is a key driving force of the spin transition of ethyl-SBP and a key factor explaining the paramagnetic response of the π-dimers of other SBP-based crystals.