Synthesis of Near-Infrared Absorbing Bisquarylium Dyes Bearing Unsymmetrically Extended π-Conjugation Structures

Novel cationic bisquarylium dyes 3 and 4 were prepared by the reaction of benzothiazolinosquarylium dye 2 with a series of 4-substituted 3-hydroxy or 3-ethoxycyclobut-3-ene-1,2-diones. These dyes exhibit large and intenseabsorptions in the near-infrared region with varying absorption maxima in the range of 771 to 815 nm.