Non-Kekulé Bi-Blatter Diradicals: Phenyl versus Electron-Deficient Heteroaromatic Linkers

A series of stable non-Kekulé 1,2,4-benzotriazinyl-based diradicals bridged by phenyl (1), pyridine (2), pyrazine (3), and pyrimidine (4) groups have been successfully isolated. Combined experimental and theoretical analyses confirm that all diradicals exhibit open-shell singlet ground states and display a strong bridge dependence. As the number of nitrogen atoms increases, the enhanced electron-deficient character of the linker systematically reduces the singlet-triplet energy gap (ΔE_S-T from -1.39 kcal·mol^-1 to -0.16 kcal·mol^-1 across 1 to 4), and results in a concomitant red shift of maximum absorption in the UV-vis-NIR spectrum from 1 to 4. Diradicals 2-4 represent the first stable nitrogen-centered non-Kekulé diradicals incorporating electron-deficient heteroaromatic spacers.