Strategic Design of Hemi‐Isoindigo Polymer for a Highly Sensitive and Selective All‐Printed Flexible Nitrogen Dioxide Chemiresistive Sensor

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.