Trying to Crack the Shell: Investigating the Magnetic Properties and Electronic Structures of Redox-Active Ester-Substituted BODIPY-Benzoquinone Diradicaloid.

We report the synthesis of a BODIPY 8, which bears ester substituents at the 2,6-positions and is coupled to a benzoquinone unit through an ethanediylidene linker. The hydroxyphenyl precursor, 7, was obtained via Knoevenagel condensation and oxidized chemically or electrochemically to yield the benzoquinone-BODIPY 8 conjugate. The magnetic properties of 8 were probed by EPR spectroscopy, VT 1H NMR spectroscopy, and DFT/TDDFT calculations. The benzoquinone-BODIPY 8 exhibited a predominantly closed-shell singlet ground state, with the open-shell singlet and triplet states being higher in energy, consistent with the absence of a room-temperature EPR signal in solution. Electrochemistry and spectroelectrochemistry revealed stepwise oxidation from the dianionic precursor [7]2- to a mixed-valence anion radical [7]-•, featuring a broad IVCT band at 1980 nm (TDDFT-predicted at 1790 nm), before full conversion to BODIPY 8. These results demonstrate fine-tuning of diradical character and charge-transfer behavior in BODIPY chromophores through conjugation and redox modulation. We also discovered the formation of an unexpected blue-colored BODIPY dimer 9 upon treatment of compound 8 with methanol. In this dimer, two BODIPY units are connected by a single sp3-sp3 carbon bond located at the carbon atoms directly attached to the α-position of the BODIPY core.