Oxidation of Weakly Interacting Diradicals: An Approach for Strong and Tunable Near-Infrared-Absorbing Dyes Based on Small Chromophores

Near-infrared (NIR)-absorbing dyes are valuable for various
applications,
such as bioimaging and electronic devices. This work introduces a
novel approach for designing NIR dyes, oxidation of weakly coupled
diradicals. Our approach features a weak exchange interaction in diradicals,
which potentially leads to bonding/antibonding molecular orbitals
with a small energy gap. We found that removing one of two singly
occupied molecular orbital electrons of the diradicals results in
an exceptionally narrow frontier orbital energy gap. We examined a
series of Blatter radical dimers, and the most weakly coupled diradical
prepared in this work (ΔEST ∼
0.12 eV) with a molecular weight of 590 Da exhibited a strong NIR
absorption band reaching 2200 nm upon one-electron oxidation. The
optical band gaps of the radical cations strongly correlate to the
exchange interaction in the precursor neutral species, offering prediction
and fine-tuning of the optical band gap in the NIR region.