Synthesis and near-infrared absorption properties of linearly π-extended squarylium oligomers

Linear squarylium oligomers bearing extended π-conjugation systems were newly synthesized, and their near-infrared (NIR) light-absorbing properties were investigated. Replacement of the iodo substituent in a 5-iodinated indolino-squarylium dye to the 2-hydroxy-3,4-dioxocyclobut-1-en-1-yl functional group via the Pd-catalyzed cross-coupling with a tributylstannylsquarate followed by condensation with a quaternary indolium salt afforded the semi-squarylium-attached squarylium derivative, i.e., the squarylium dimer. The introduction of the semi-squarylium unit gave rise to a significant bathochromic shift towards the NIR region (λabs = 763 nm in CHCl3 at 298 K). Starting from a 5,5′-diiodinated indolino-squarylium, the iterative extension of semi-squarylium units successfully yielded the linearly π-extended trimer and pentamer, that showed absorption maxima at 862 and 940 nm in CHCl3 at 298 K, respectively. Especially, the pentamer exhibited a considerably low optical band gap of 1.1 eV.

Research highlights
► We synthesized novel linearly π-extended squarylium oligomers. These dyes exhibited the light absorption in the NIR regions.
► The MO calculations indicated that the NIR-absorption is based on the ICT mechanism.
► The pentameric squarylium showed a considerably low optical band gap of 1.1 eV.