Density functional theory investigation of substituent effects on building blocks of conducting polymers

Substituted heterocyclic dimers were calculated employing density functional theory (DFT) and analyzed with the natural bond orbital method (NBO). Substitution in 3- and 4-positions leads to parallel shifting of HOMO and LUMO but does not reduce energy gaps. For bridged dimers, HOMO-LUMO gaps correlate with pi-electron densities in the carbon backbone and energy gap reductions correlate with the strength of pi-pi* interactions from the backbone to the bridging group. Alternating donor-acceptor groups do not reduce energy gaps and lead to systems with average HOMO and LUMO levels compared to the parent molecules.