Optical absorption in donor-acceptor polymers - alternating vs. random

We investigate in a combined theoretical and experimental study the influence that the specific arrangement, e.g., alternating or random, of donor and acceptor units has on the optical absorption of extended molecules. Because of its important role in low gap polymers we discuss in particular the energetic position of the first electronic transition. We theoretically determine the excitations in extended oligomers with thiophene as the donor and 2,1,3-benzothiadiazole as the acceptor component by using time-dependent density functional theory based on non-empirically tuned range separated hybrid functionals. Corresponding systems are synthesized and theoretical and experimental data are critically compared to each other. We conclude that the influence of the specific arrangement of donor and acceptor monomers on the optical gap is limited and that effects beyond the single molecule level effectively limit the size of the experimentally observed optical gap.