Tuning H- and J-Aggregate Behavior in, π-Conjugated Polymers via Noncovalent Interactions

We study how tailoring noncovalent interactions through the regiochemistry of common donor-acceptor polymers can alter their excited-state electronic properties. Specifically, we compare two regioregular analogues (referred to as P1 and P2) of the widely used donor-acceptor copolymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b:4,5-b']dithiophene-co-3-fluorothieno [3,4-b]-thiophene-2-carboxylate], which differ only in the orientation of a fluorine atom on the thieno[3,4-b]-thiophene acceptor units with respect to the benzodithiophene unit. We show that in thin films, this subtle change along the polymer backbone results in a transformation from typical H-like to an unusual HJ-like aggregate behavior, indicating significant differences in intrachain exciton coupling along the polymer backbone and interchain coupling between polymer chains. We also use electroabsorption spectroscopy to relate the dominant coupling mechanisms to differences in the excess polarization volume of the excitons in P1 and P2. Overall, our results show that seemingly small changes in noncovalent interactions along a polymer backbone can have profound effects on the exciton electronic structure and dominant electronic coupling mechanisms in the solid state (intrachain vs interchain). Furthermore, these results provide evidence that interchain and intrachain coupling interactions may be correlated with electrostatic properties of the excitons in terms of their excess polarization volume.