Toposelective on-surface synthesis of curved π-extended oligomers based on bowl-shaped aromatics

Curved π-extended systems based on bowl-shaped porphyrinoids are attractive because they exhibit unachievable properties in their planar counterparts. However, their synthesis typically yields oligomeric structures without topological control, a prerequisite for engineering materials with precise optoelectronic and physical properties. Here, we report a novel strategy to obtain exclusively syn topoisomers from ortho-dihalogenated subphthalocyanines (SubPcs) making use of on-surface synthesis methods. Using low-temperature scanning tunnelling microscopy (STM), supported by ab initio calculations, we show that the morphology and electronic structure of the products strongly depend on the halogen substituent in the precursor. Upon annealing up to 475 K, Br-substituted precursors yield covalently fused dimers and trimers sharing four- and six-membered rings, respectively, whereas F-substituted SubPcs form metal-organic chains mediated by F⋯Au and N⋯Au coordination. In contrast, Cl-substituted precursors, representing an intermediate case, require higher activation temperatures than their Br analogues, resulting in more irregular structures. In all cases, a single syn topology is observed that demonstrates the surface induced selectivity. In the covalent architectures, scanning tunnelling spectroscopy (STS) supported by Density Functional Theory (DFT) reveals that both the HOMO and LUMO are significantly perturbed upon oligomerization, resulting in modulation of the semiconducting energy gap consistent with effective π-delocalization. Altogether, these findings demonstrate that surface-assisted synthesis enables the toposelective construction of π-systems and highlight the decisive role of halogen selection in governing the formation of structurally and electronically defined architectures.