Synthesis of novel unsymmetrical squarylium dyes absorbing in the near-infrared region

Novel unsymmetrical squarylium dyes which absorb in the near-infrared region were synthesized by a stepwise procedure via intermediate 4-substituted 3-hydroxycyclobut-3-ene-1,2-diones. By introducing benz[c,d]indoline or benzo[b]pyran moieties at one end as an electron-donating component, the SQ dyes exhibited their absorption maxima at 739-821 nm with large molar absorption coefficients (log epsilon=4.96-5.18) in CHCl3. The X-ray analysis for one of these dyes was also examined to confirm the overall structure of the unsymmetrical SQ dye.