| Abstract | The study has developed two hemi-isoindigo (HID)-based polymers for printed flexible resistor-type nitrogen oxide (NO₂) sensors: poly[2-ethylhexyl 3-((3′",4′-bis(dodecyloxy)-3,4-dimethoxy-[2,2′:5′,2′"-terthiophen]-5-yl)methylene)-2-oxoindoline-1-carboxylate] (P1) and poly[2-ethylhexyl 2-oxo-3-((3,3′",4,4′-tetrakis(dodecyloxy)-[2,2′:5′,2′"-terthiophen]-5-yl)methylene)indoline-1-carboxylate] (P2). These polymers feature thermally removable carbamate side chains on the HID units, providing solubility and creating molecular cavities after thermal annealing. These cavities enhance NO₂ diffusion, and the liberated unsubstituted amide ─C(═O)NH─ groups readily form robust double hydrogen bonds (DHB), as demonstrated by computer simulations. Furthermore, both polymers possess elevated highest occupied molecular orbital (HOMO) energy levels of −4.74 and −4.77 eV, making them highly susceptible to p-doping by NO₂. Gas sensors fabricated from P1 and P2 films, anneal under optimized conditions to partially remove carbamate side chains, exhibit remarkable sensitivities of +1400% ppm⁻¹ and +3844% ppm⁻¹, and low detection limit (LOD) values of 514 ppb and 38.9 ppb toward NO₂, respectively. These sensors also demonstrate excellent selectivity for NO₂ over other gases. |
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