Heat, pressure and light-induced interconversion of bisdithiazolyl radicals and dimers

The heterocyclic bisdithiazolyl radical 1b (R1 = Me, R2 = F) crystallizes in two phases. The α-phase, space group P2₁/n, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group P2₁/c, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility measurements on α-1b indicate Curie-Weiss behavior (with Θ = -14.9 K), while the dimer phase β-1b is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of β-1b to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-1a (R1 = Et, R2 = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (β-1b) and 242 K (β-1a) before reverting to the dimer state. Variable-temperature optical measurements on β-1b and β-1a have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol(-1), respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of β-1a and β-1b is symmetry forbidden, while the photochemical transformation is symmetry allowed.