Impact of connectivity on the electronic structure of 3D carbazole-bridged conjugated systems

Three-dimensional non-fullerene acceptors (3D-NFAs) provide a versatile platform for regulating optoelectronic properties through molecular topology; however, isolating topological effects from chemical composition remains challenging. Herein, two topological isomers, D1 and D2, are designed by integrating identical Y6-inspired terminal units onto a carbazole-centered scaffold via distinct molecular connectivity, resulting in X-shaped and S-shaped architectures, respectively. Both acceptors exhibit broad optical absorption extending into the near-infrared region (up to ca. 890 nm) with comparable optical bandgaps. Despite identical chemical composition and conjugation length, distinct electronic structures and aggregation behaviours arising from topology-dependent conjugation pathways are observed. Combined experimental investigations and density functional theory calculations reveal that molecular connectivity modulates frontier energy levels and excited-state transition manifolds through topology-dependent conjugation pathways and through-space interactions. Both molecules display nonlinear optical absorption of comparable magnitude, consistent with their similar total oscillator strengths, while topology mainly affects the distribution of low-energy excited states. When incorporated into organic solar cells, the S-shaped acceptor D2 delivers a higher power conversion efficiency, whereas the X-shaped D1 exhibits a higher open-circuit voltage (0.904 V), reflecting topology-dependent energetic alignment. This work demonstrates that molecular topology serves as an effective and independent design parameter for three-dimensional non-fullerene acceptors, offering fundamental insight into topology-driven structure–property relationships in multidimensional organic semiconductors.