Oxidation of\nWeakly Interacting Diradicals: An Approach\nfor Strong and Tunable Near-Infrared-Absorbing Dyes Based on Small\nChromophores

Near-infrared (NIR)-absorbing dyes are valuable for various\napplications,\nsuch as bioimaging and electronic devices. This work introduces a\nnovel approach for designing NIR dyes, oxidation of weakly coupled\ndiradicals. Our approach features a weak exchange interaction in diradicals,\nwhich potentially leads to bonding/antibonding molecular orbitals\nwith a small energy gap. We found that removing one of two singly\noccupied molecular orbital electrons of the diradicals results in\nan exceptionally narrow frontier orbital energy gap. We examined a\nseries of Blatter radical dimers, and the most weakly coupled diradical\nprepared in this work (<i>ΔE</i><sub>ST</sub> ∼\n0.12 eV) with a molecular weight of 590 Da exhibited a strong NIR\nabsorption band reaching 2200 nm upon one-electron oxidation. The\noptical band gaps of the radical cations strongly correlate to the\nexchange interaction in the precursor neutral species, offering prediction\nand fine-tuning of the optical band gap in the NIR region.