Theoretical Design of Low Band Gap Conjugated Polymers through Ladders with Acetylenic Crosspieces

Building ladder polymers provides an alternative way to design intrinsically low band gap conjugated polymers. Using conventional conjugated polymers, polyacetylene, polydiacetylene and polytriacetylene as sidepieces, we propose a series of novel types of ladder polymers with conjugated acetylenic blocks (−C⋮C−) m serving as crosspieces. Periodic boundary conditions density functional calculations are performed to investigate the effect of the acetylenic cross-couplings between the two sidepieces. We predict the band gap of the acetylenic coupled (AC) ladder polymers can be reduced to less than 0.3 eV without doping, significantly reduced compared with the parent sidepiece polymers at the same theoretical level. The effect of varying the length and spacing of the crosspieces along the sidepieces is investigated in order to maximize the effect of the gap reduction. Unusual electronic and optical properties are expected from these series of ladder conjugated polymers.