Theoretical insight of polypyrrole ammonia gas sensor

Density Functional Theory (DFT) and TD-DFT calculation have been performed to investigate the response mechanism of polypyrrole towards ammonia as sensor. Geometric and electronic properties of oligopyroles up to nine repeating units are evaluated theoretically, and the calculated properties are extrapolated for the polymer (polypyrole) through 2nd order polynomial fit. Hydrogen bonds between ammonia and oligopyrole are about 10–11 kcal mol−1 (7–8.5 kcal mol−1 BSSE corrected) in strength. Interaction of ammonia with the oligopyrrole causes certain geometric features to change (for example ∠C1C2N3C5) which results in decrease in resistance for the movement of electron over the oligomer backbone. The reduction in the resistance is also measured by perturbation in electronic properties including ionization potential (I.P), electron affinity (E.A.), HOMO, LUMO, band gap and λmax. E.A. and band gap (HOMO to LUMO) also support the sensing ability of nPy oligomers towards ammonia. Band gaps decrease while LUMO energies for oligopyroles increase upon interaction with NH3. Ammonia donates electron to the LUMO (electron acceptor) of nPy oligomers and increases its electronic clouds density therefore E.A. of nPy decreases. Moreover the extended conjugation in the oligopyrole backbone upon complexation with ammonia, combined with other electronic and geometric properties illustrate the potential of undoped oligopyrole as sensor for ammonia.