Influence of Side Chain Interdigitation on Strain and Charge Mobility of Planar Indacenodithiophene Copolymers.

Indacenodithiophene (IDT) copolymers are a class of conjugated polymers that have limited long-range order and high hole mobilities, which makes them promising candidates for use in deformable electronic devices. Key to their high hole mobilities is the coplanar monomer repeat units within the backbone. Poly(indacenodithiophene-benzothiadiazole) (PIDT _C16 -BT) and poly(indacenodithiophene-thiapyrollodione) (PIDT _C16 -TPD _C1 ) are two IDT copolymers with planar backbones, but they are brittle at low molecular weight and have unsuitably high elastic moduli. Substitution of the hexadecane (C ₁₆ ) side chains of the IDT monomer with isocane (C ₂₀ ) side chains was performed to generate a new BT-containing IDT copolymer: PIDT _C20 -BT. Substitution of the methyl (C ₁ ) side chain on the TPD monomer for an octyl (C ₈ ) and 6-ethylundecane (C _13B ) afford two new TPD-containing IDT copolymers named PIDT _C16 -TPD _C8 and PIDT _C16 -TPD _C13B , respectively. Both PIDT _C16 -TPD _C8 and PIDT _C16 -TPD _C13B are relatively well deformable, have a low yield strain, and display significantly reduced elastic moduli. These mechanical properties manifest themselves because the lengthened side chains extending from the TPD-monomer inhibit precise intermolecular ordering. In PIDT _C16 -BT, PIDT _C20 -BT and PIDT _C16 -TPD _C1 side chain ordering can occur because the side chains are only present on the IDT subunit, but this results in brittle thin films. In contrast, PIDT _C16 -TPD _C8 and PIDT _C16 -TPD _C13B have disordered side chains, which seems to lead to low hole mobilities. These results suggest that disrupting the interdigitation in IDT copolymers through comonomer side chain extension leads to more ductile thin films with lower elastic moduli, but decreased hole mobility because of altered local order in the respective thin films. Our work, thus, highlights the trade-off between molecular packing structure for deformable electronic materials and provides guidance for designing new conjugated polymers for stretchable electronics.
Competing Interests: The authors declare no competing financial interest.
(© 2022 The Authors. Published by American Chemical Society.)