Synthesis of near-infrared absorbing and fluorescent bis(pyrrol-2-yl)squaraines and their halochromic properties

1,3- and 1,2-Bis(pyrrol-2-yl)squaraines were easily and selectively synthesized. 1,2-Squaraines (partially conjugated neutral structure) showed blue-shifted λmax compared to the corresponding 1,3-squaraines (fully conjugated zwitterionic structure). Thiophene-fused 1,3-squaraine showed NIR absorption (781 nm) and fluorescence (833 nm). The λmax of thiophene-fused 1,3-squaraine was red-shifted to 1007 nm after mono-protonation and blue-shifted to 680 nm with di-protonation, suggesting that the mono-cationization of a linear π-conjugated structure bearing two terminal amino groups is an effective strategy for achieving a further red-shift in the λmax. A highly sensitive reusable acid gas sensing textile sensor was fabricated.