Bi-Blatter diradicals: conformation and substituent dependent high-spin materials†

Abstract

Reactions of bi-benzo[e][1,2,4]triazines connected through the C(3′)–C(6) and C(3′)–C(7) bonds with PhLi lead to bi-Blatter diradicals. DFT calculations revealed two conformations of the essentially planar diradical core, anti and syn, with opposite spin ground states and comparable populations at ambient temperature equilibrium. This indicates that solution spectroscopic and electrochemical analyses involve mixtures of two conformers and thermally populated spin states. Consequently, analysis of variable temperature EPR data for the diradicals in polystyrene solid solutions included the presence of two conformers and gave the singlet–triplet energy gap for each of them ΔE_S–T of 1.02(5) and −0.45 kcal mol⁻¹ for the C(3′)–C(6) connected diradical, and 1.78(3) and −0.10 kcal mol⁻¹ for its C(3′)–C(7) isomer. The observed trend in ΔE_S–T is consistent with that obtained with DFT methods. Optical spectroscopy revealed broad absorption bands tailing to 1150 nm, presumably due to the presence of the open-shell singlet species in each diradical. Both diradicals exhibit four quasi-reversible one-electron redox processes separated by 0.13–0.27 V. The structure of the two diradicals was confirmed with a single crystal XRD analysis. Results for this prototypical pair of isomeric diradicals suggest a simple and efficient access to an unusual family of chemically stable, dynamic Blatter diradicals with the controllable overall concentration of the triplet species through a judicious choice of substituents. DFT calculations indicate that the ΔE_S–T of each conformer and the ratio of the conformers is affected by substituents at the C(3), C(6′)/C(7′), and N(1)/N(1′) positions and the concentration of the triplet species can be varied in a range of 50–77% at 298 K.