Structural dependence of the optical properties of narrow bandgap semiconductors with orthogonal donor–acceptor geometries

We discuss donor–acceptor conjugated polymers where the acceptor moieties are orthogonal to the donor-based backbone direction through the synthesis and study of a new class of materials (including six oligomers and three polymers) based on 4H-cyclopentadithiophene (CPDT) with an imine functionality introduced at the CPDT bridgehead position. Absorption spectroscopy provides information on the influence of structure on the optical properties. We paid special attention to the energies and oscillator strengths of the low-energy transitions and how they correlate with chain length. When the orthogonally conjugated materials are compared to a more traditional polymer, where the donor and acceptor fragments are in series along the backbone direction, fundamental differences in the optical properties are observed. Quantum-mechanical studies of the geometric structure, electronic structure, and excited-state vertical transitions using density functional theory unravel the interplay of structural design and resulting optoelectronic properties. Our findings underline that the magnitude and orientation relative to the backbone long axis of the transition dipole moment is key in designing narrow optical-gap materials with large absorption cross-sections and oscillator strengths.