Introducing of an Unexplored Aza-BODIPY Diradicaloids with 4-(2,6-Ditert-butyl)phenoxyl Radicals Located in 1,7-Positions of the Aza-BODIPY Core

We have prepared and characterized two diradicaloid systems 5a and 5b that originated from the oxidation of a 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPY core. The aza-BODIPY diradicaloids were characterized by a large array of experimental and computational methods. The diamagnetic closed-shell state was postulated as the ground state in solution and a solid-state with the substantial thermal population originating from both open-shell diradical and open-shell triplet states observed at room temperature. Transient absorption spectroscopy indicates fast (<10 ps) excited state deactivation pathways associated with the target compounds’ diradical character in solution at room temperature. Variable-temperature ¹H NMR spectra indicate the solvent dependency of the diradical character in 5a and 5b. The diradicaloids could be stepwise reduced to the mixed-valence radical-anion and dianion states upon consequent single-electron reductions. Similarly, deprotonated 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPYs can be oxidized to the diradicaloid form. Both mixed-valence and dianionic forms exhibit an intense absorption in the NIR region. Density functional theory (DFT) and time-dependent DFT calculations were used to explain the transformations in the UV-Vis-NIR spectra of all target compounds.