Design of Stable Diradicals Based on Group 13 Dyes of N₂O₂-Bis(phenolate) Aza-Dipyrromethene and Aza-BODIPY

We have prepared a series of strongly fluorescent N₂O₂-bis (phenolate) aza-dipyrromethene dyes incorporating boron (aza-BODIPY), aluminum (aza-ALDIPY), gallium (aza-GADIPY), and indium (aza-INDIPY). These compounds were oxidized both chemically and electrochemically to give diradical molecules 4b-7b with an open-shell singlet ground state and a thermally accessible triplet excited state. The ground state behavior of these diradical molecules was confirmed by a wide array of magnetic spectroscopies, including variable-temperature ¹H NMR, EPR, and SQUID. X-ray crystallography confirmed the N₂O₂ coordination sphere in these systems. DFT and TDDFT quantum chemical calculations were used to determine their electronic structures, nature of the excited states, diradical character (y₀), and spin density distributions for both open-shell singlet and triplet states. Spectroelectrochemical studies on the deprotonated dianions 4a^2- to 7a^2- showed the formation of mixed-valent radical anion species with intense absorption and broad intervalence charge transfer bands in the near-infrared region. Upon further oxidation of the mixed-valent radical anion species, diradical molecules 4b-7b were generated.