Solid-State Effects on the Competition between π-Pairs and C–C σ-Dimers in Bis-1,2,3-dithiazolyl Radical-Based Materials

The presence of either σ-dimers or π-pairs in materials based on π-radicals significantly impacts their magnetic, optical, and charge transport properties. Identifying the factors leading to σ-bonding is thus crucial. The limited occurrence of C–C σ-bonds in bisdithiazolyl-based crystals is here investigated by comparison of N-ethyl and N-methyl pyrazine-bridged derivatives through a combination of high-accuracy correlated wave function calculations in the gas phase and DFT-based periodic calculations. The σ-dimers are found to be only slightly more stable (in a range of ∼5 to ∼7 kcal mol–1) than the corresponding π-dimers in the gas phase. This energy difference is small to compensate for the entropic cost of forming a σ-dimer, which requires specific relative orientations of the π-radicals. In addition, solid-state effects can have a strong impact on the energy difference between the two association modes. The crystal lattice of the ethyl derivative, where the electronic interaction between neighboring pairs of radicals is exceedingly small, favors σ-dimerization. Conversely, the crystal lattice of the methyl derivative hinders σ-dimerization, because the electronic localization of the unpaired π-electrons concomitant to it is constrained by the electronic interactions between neighboring pairs of radicals. These results suggest that less compact packing of the neutral radicals reduces lateral interactions, which in turn favors electron localization of the open-shell π-system to form a σ-bond if the π-stacking provides a suitable relative orientation of the radical pairs. This provides a promising scheme to stabilize single-component molecular organic materials made of neutral radical species.