So far, we have established the covalent bonding pattern of our networks and their permanent micro-and mesoporosity. Further, we make use of their intrinsic π-aromaticity and heteroatom content by exposing these polymers to a trigger: corrosive HCl vapors. In principle, SNPs have two Lewis-basic sites that can be protonated -triazine-ring nitrogens and thiophene sulfur atoms. SP-BTT plays a particular role in this study, since it contains thiophene-moieties only (Scheme S5). The as-received powders of SNPs show a rapid color change and marked red-shift of the absorption edge in solid-state UV-Vis spectra (Figure , S18-22, Video S1). These values correspond to a decrease of the direct optical bandgap by 0.45 to 0.66 eV and indirect optical bandgap by 0.42 to 0.76 eV according to the Kubelka-Munk function (Figure , S18-S22). The most sensitive network, SNP-NDT1, is triggered by hydrochloric acid concentrations as low as 54 ppm (dosing limit of the current setup, see SI); a response that can still be distinguished by the naked eye within 30 seconds (Figure , Table ). In contrast, sulfur-containing SP-BTT does not show any appreciable color change when exposed to HCl vapors, hence, no marked changes in UV-Vis absorption (Figure ) and in optical bandgaps (Figure ), respectively. One conceivable explanation for this phenomenon could be the difference in basicity between the triazine and the thiophene moiety. While 1,3,5-substituted triazines are readily protonated at the pyridinic ring-nitrogen, thiophenes and oligothiophenes are protonated by superacids only at the 2-position. Notably, this coloration can be fully re-set when the HCl-treated polymer is exposed to NH3 vapors from 24% aqueous ammonia solution (Figure ). This on/off response to HCl/NH3 vapors can be triggered at least five consecutive times (as shown for SNP-NDT1, Figure ) with no apparent losses in the intensity of the optical response.