Unraveling the Side Chain Effects on Chain Conformation of Donor-Accepter Conjugated Polymers*

The less studied chain conformation of donor-accepter conjugated polymers (D-A CPs) hinders the fundamental understanding of their optical and electronic properties. Here, we systematically studied the effect of side-chain length and branch point on the chain conformation of diketopyrrolopyrrole (DPP) and isoindigo (IID) based polymers. Using small-angle neutron scattering (SANS) measurements, we find that chain rigidity of DPP-based polymers increases with side-chain length, which directly correlates with increased charge carrier mobility until the insulating alkyl sidechains are bulky enough to hinder efficient intermolecular charge carrier transport. The chain rigidity of IID-based polymers increases when moving the branch position farther away from the backbone.  The trend of the solution optical bandgap changes is in accordance with an odd-even effect of the branching point. Using atomistic molecular dynamics (MD), we examined how side-chain structure can be modified to control chain conformation which further validated the experimental results. This work fills the fundamental gaps in our knowledge of the most basic relationships between polymer structure and chain conformation.