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 4a2- to 7a2- 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.