Extended conjugation of thienyl-benzodithiophene-based polymers: towards narrow-bandgap conjugated polymer networks

Conjugated polymer networks are widely used in (opto-)electronic applications due to their aromatic backbones, which enable tunable bandgaps and electrical conductivity. While symmetric building blocks are commonly employed to dictate network topology, stepwise introduction of asymmetric monomer building blocks remains largely unexplored. Here, we report a strategy to incorporate a C₂ symmetric tetrafunctional benzodithiophene monomer into macromolecular structures via the orthogonal post-functionalization of a linear polymer bearing aldehyde side groups with small aromatic amines as linkages. Depending on the amine functionality, this approach yields either imine-capped linear polymers or crosslinked conjugated networks. Efficient conversion was confirmed by Fourier transform infrared spectroscopy and (solid-state) nuclear magnetic resonance spectroscopy and high conversion was achieved in all cases. The resulting (opto-)electronic properties, including terahertz transmission modulation (-ΔE/E), bandgap and energy levels, were investigated using UV/Vis spectroscopy, optical pump-THz probe spectroscopy, cyclic voltammetry, and density functional theory calculations. Network formation led to a reduction in the optical bandgap from 2.36 eV to 1.82 eV and an approximate doubling of the terahertz transmission modulation. Notably, we observed a pronounced spatial separation of the highest occupied molecular orbital and lowest unoccupied molecular orbital densities along orthogonal axes on the corresponding finite model compound. This work establishes orthogonal post-functionalization as a viable route to novel conjugated polymer architectures with tunable electronic properties, offering new design principles for next-generation (opto-)electronic materials.