HJ-aggregates of donor-acceptor-donor oligomers and polymers

A vibronic exciton model is developed to account for the spectral signatures of HJ-aggregates of oligomers and polymers containing donor-acceptor-donor (DAD) repeat units. In (DAD)(N) pi-stacks, J-aggregate-promoting intrachain interactions compete with H-aggregate-promoting interchain interactions. The latter includes Coulombic coupling, which arises from "side-by-side" fragment transition dipole moments as well as intermolecular charge transfer (ICT), which is enhanced in geometries with substantial overlap between donors on one chain and acceptors on a neighboring chain. J-behavior is dominant in single (DAD)(N) chains with enhanced intrachain order as evidenced by an increased red-shift in the low-energy absorption band along with a heightened A(1)/A(2) peak ratio, where A(1) and A(2) are the oscillator strengths of the first two vibronic peaks in the progression sourced by the symmetric quinoidal-aromatic vibration. By contrast, the positive H-promoting interchain Coulomb interactions operative in aggregates cause the vibronic ratio to attenuate, similar to what has been established in H-aggregates of homopolymers such as P3HT. An attenuated A(1)/A(2) ratio can also be caused by H-promoting ICT which occurs when the electron and hole transfer integrals are out-of-phase. In this case, the A(1) peak is red-shifted, in contrast to conventional Kasha H-aggregates. With slight modifications, the ratio formula derived previously for P3HT aggregates is shown to apply to (DAD)(N) aggregates as well, allowing one to determine the effective free-exciton interchain coupling from the A(1)/A(2) ratio. Applications are made to polymers based on 2T-DPP-2T and 2T-BT-2T repeat units, where the importance of the admixture of the excited acceptor state in the lowest energy band is emphasized. Published under an exclusive license by AIP Publishing.