A cyanine-type homolog of the red hair bibenzothiazine chromophore combining reversible proton-sensing with a hydrophobic-to-hydrophilic switching response

Δ2,2’-bi-(2H-1,4-benzothiazine) chromophore of trichochrome pigments occurring in red hair and feathers has been recognized as a source of inspiration for the design of functional chromophores given its excellent photochromic, solvatochromic, and acidichromic properties. Here we report the synthesis of a symmetric homolog of the bibenzothiazine system produced by condensation of 3-phenyl-2H-1,4-benzothiazine with glyoxal that was assigned the cyanine-type 2Z,2′Z-(1,2-ethanediylidene)bis(3-phenyl-2H-1,4-benzothiazine) (1) structure by spectral analysis and DFT calculations. The cyanine showed an intense absorption maximum at ca. 480 nm (ε = 16.070 ± 60 M−1cm−1) in 3:1 methanol/water at pH ≥ 4.0 undergoing a marked bathochromic shift in acid to 610 nm (ε = 22.330 ± 40 M−1cm−1), and further shift to 637 nm in 2.5 M HCl accompanied by a distinct color change from red to deep blue. The bathochromic shift upon exposure to acids was observed on fabrics, paper, polylactic acid films or alginate hydrogels as well as on thin films of the cyanine exposed to acid/basic vapors and proved reversible over several cycles. The acidichromic behaviour was associated to a remarkable hydrophobic-to-hydrophilic switch, that was probed in a filter permeability experiment. Other applications that are presented for the cyanine include the use as pH sensor paper for visual assessment of pHs below 0 and devices for monitoring food freshness, e.g detection of volatile amines generated by fish spoilage or pH changes associated to milk deterioration.