DFT investigation of band gap and conductivity in pristine and doped Polyacetylene oligomers: A new extrapolation approach

The electronic properties of polyacetylene (PA) isomers and their doped derivatives were investigated using density functional theory (DFT) at the PM3/B3LYP/def2-TZVP level. By studying a series of oligomers with increasing conjugation length, we determined the asymptotic band gaps to be 1.26 eV for trans-PA and 2.01 eV for cis-PA, rationalizing the higher conductivity of the trans-isomer. A novel nonlinear extrapolation method was developed to accurately predict the band gap of the infinite polymer from a minimal set of oligomer calculations. Furthermore, we demonstrate a dramatic band gap reduction—up to 97 %—upon p-doping via oxidation or bromination, and n-doping via deprotonation or reduction, confirming the transition to a metal-like state. The insulating nature of polytetrafluoroethylene (PTFE) was concurrently verified. Our findings provide a robust computational framework for screening optimal dopants to maximize PA conductivity.