When further adding larger amounts of TFA a new behavior was observed, which manifested itself in a very strong red shift and distinct intense absorption bands with intense maxima at PAT chromophore that is now also protonated at the central thioindigo fragment, most probably at the carbonyl function. It is referred to as 1-E-H + CO for distinction from the aniline-protonated species 1-E-H + N. This assignment was possible by a related protonation experiment with the mesitylene-substituted PAT 4, which showed the same absorption shift behavior upon addition of an excess of TFA despite the absence of an aniline substituent (see Supporting Information).