We closely examined the chemical composition of the polymer backbone during this on/off response to verify that the interaction of the polymer with corrosive HCl is indeed a reversible chemisorption process. Fourier-transform infrared (FT-IR) spectra of pristine and HCl-treated samples show characteristic C3N3-ring vibrations around 1500 cm -1 and 1360-1370 cm -1 , along with the triazine-ring breathing mode at 805-820 cm -1 (Figure -33). Moreover, there are no indicative signals in the OH-and NH-region around 3000 cm -1 that would be a tell-tale sign of irreversible hydrolysis of the polymers. Similarly, FT-IR spectra of the pristine and HCl treated SP-BTT polymer show C-S vibrations of the thiophene ring at 810 cm -1 as well as aromatic C-C stretching at 1342 cm -1 (Figure ). A comparison of FT-IR spectra over five cycles of exposure to HCl/NH3 gases shows that all reported networks remain chemically stable (Figure ). Previous DFT calculations confirm that the Lewis basic N-atoms of the triazine moieties are the preferred site of protonation. This rapid and fully-reversible detection of HCl vapors at low concentrations makes SNPs suitable candidates for naked-eye sensors. Moreover, we monitored the response of SNP-NDT1 to different acids, such as hydrobromic, nitric, sulfuric, trifluoromethylsulfonic (TFMSA), and acetic acid. Initial exposure for 1 min revealed color changes only in case of HCl. Longer exposure for up to 3 days ensured complete diffusion of acid vapors throughout the material. Accordingly, we also observed marked color change in case of HBr and HNO3, whereas vapors of TFMSA and acetic acid caused only slight shifts of the UV-Vis absorption edge (Figure ). Interestingly, sulfuric acid shifted the absorption edge of SNP-NDT1 towards the blue region by ~12 nm; an electron induction effect into the polymer network which probably stems from a strong interaction of the SO4 2-anion with the polymer network, analogous to charge-transfer complexes formed between sulfate and molecular electron acceptors. Overall, we observe that soft conjugate bases (e.g. AcO -) cause a smaller UV-Vis shift than hard conjugate bases (e.g.