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 (<b>1</b>), pyridine (<b>2</b>), pyrazine (<b>3</b>), and pyrimidine (<b>4</b>) 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 (Δ<i>E</i>_S-T from -1.39 kcal·mol^-1 to -0.16 kcal·mol^-1 across <b>1</b> to <b>4</b>), and results in a concomitant red shift of maximum absorption in the UV-vis-NIR spectrum from <b>1</b> to <b>4</b>. Diradicals <b>2</b>-<b>4</b> represent the first stable nitrogen-centered non-Kekulé diradicals incorporating electron-deficient heteroaromatic spacers.