Conjugated polymers with large effective Stokes shift: benzobisdioxole-based poly(phenylene ethynylene)s.

Phenyleneethynylene-based conjugated copolymers using benzo[1,2-d:4,5-d']bis[1,3]dioxole (BDO) in the repeating unit are reported. The electronic structure of the BDO unit imparts a localized HOMO topology while the LUMO is delocalized over the polymer backbone, so that the lowest optical absorption band of the polymer has considerable intramolecular charge transfer character. This contrasts with published donor-acceptor polymers with localized LUMO and delocalized HOMO. The very large Stokes shifts of the monomers, which are due to the small oscillator strength of the lowest optical transition, are largely retained in the polymers as a result of covalently constrained dihedral angles in the substituents (not the backbone), as predicted/explained by calculations.