David Écija

Porphyrin polymers merge the photophysical, electrochemical, sensory and catalytic properties of porphyrins with the processability and robustness of polymeric materials, offering versatile and synergetic platforms for optoelectronics and catalysis. Here we report an on-surface strategy for the atomically precise polymerization of porphyrins via regioselective indenyl coupling. Deposition of an indenyl-functionalized porphyrin on Au(111) and sequential annealing drives a stepwise transformation: cyclodehydrogenation at 100 °C planarizes the precursor into π-extended porphyrins, while annealing above 125 °C triggers carbon–carbon homocoupling through five-membered moieties, forming short oligomers. Heating to 150 °C promotes chain growth into one-dimensional polymers with a 1.1 eV bandgap and extended π delocalization, alongside a minor fraction of cyclic trimers featuring a 1.1 eV HOMO–LUMO gap with both local and global aromaticity. The methodology also proceeds for Co-metallated precursors, confirming that indenyl coupling is also compatible for metalloporphyrins, establishing a general route to design π-extended porphyrin polymers.

π‐Extended porphyrinoids are versatile platforms for the emerging field of π‐electron magnetism in conjugated systems. However, achieving controlled bond‐selective transformations that enable tuning of their electronic ground states remains a major challenge. Here, we propose a protocol for covalent bond dissociation in π‐extended porphyrinoids at carbonitrile‐Au substituents. By applying bias pulses with an STM tip positioned close to the molecule, the ligand can be selectively cleaved, resulting in chemisorption of the porphyrinoid onto the Au(111) surface. BR‐STM provides structural insights into the bonding configurations before and after the scission, while STS in combination with DFT+CASCI calculations reveals the impact of the cleavage on the molecule's electronic properties. Whereas the intact molecule exhibits a closed‐shell electronic structure, removal of the ligands induces a change in its electronic ground state, leading to the emergence of a Kondo resonance upon cleavage of the CN─Au ligands. In addition, DFT + vdW simulations show that carbonitrile─Au coordination weakens the carbon─carbon bond connecting the ligand to the molecular core, thereby facilitating its cleavage through tip‐induced manipulation. Overall, these results provide insights into the bonding nature of the carbonitrile─Au interaction and demonstrate how the bond scission induces a transition from closed‐shell to open‐shell states.

π‐Extended porphyrinoids are versatile platforms for the emerging field of π‐electron magnetism in conjugated systems. However, achieving controlled bond‐selective transformations that enable tuning of their electronic ground states remains a major challenge. Here, we propose a protocol for covalent bond dissociation in π‐extended porphyrinoids at carbonitrile‐Au substituents. By applying bias pulses with an STM tip positioned close to the molecule, the ligand can be selectively cleaved, resulting in chemisorption of the porphyrinoid onto the Au(111) surface. BR‐STM provides structural insights into the bonding configurations before and after the scission, while STS in combination with DFT+CASCI calculations reveals the impact of the cleavage on the molecule's electronic properties. Whereas the intact molecule exhibits a closed‐shell electronic structure, removal of the ligands induces a change in its electronic ground state, leading to the emergence of a Kondo resonance upon cleavage of the CN─Au ligands. In addition, DFT + vdW simulations show that carbonitrile─Au coordination weakens the carbon─carbon bond connecting the ligand to the molecular core, thereby facilitating its cleavage through tip‐induced manipulation. Overall, these results provide insights into the bonding nature of the carbonitrile─Au interaction and demonstrate how the bond scission induces a transition from closed‐shell to open‐shell states.

This Review highlights recent advances in the on-surface synthesis of low-dimensional antiaromatic carbon nanostructures, covering molecules, polymers, and networks, their atomic-scale characterization, and emerging opportunities.

Controlling oxygen‐mediated connectivity at metal surfaces offers a powerful route to engineer covalent and metal–organic architectures with atomic precision. Here, we demonstrate tunable oxygen‐based connectivity in carbaporphyrinoid polymers on Au(111) under ultrahigh vacuum (UHV) conditions. Initial thermal activation of hydroxyl‐functionalized dicarbahemiporphyrazine precursors induces carbon–carbon coupling via a [3+3] cycloaromatization, affording one‐dimensional polymers equipped with hydroxyl‐terminated units. From this intermediate two distinct oxygen‐based linkages can be achieved: further annealing induces dehydrogenative coupling of adjacent polymers through the hydroxyls groups to yield dibenzo‐p‐dioxane (O‐heterocyclic) bridges that covalently connect the carbaporphyrinoid macrocycles, whereas dosing cobalt produces extended metal–organic networks stabilized by two‐fold O···Co···O coordination motifs accompanied by cobalt‐metalated macrocycles. Scanning tunneling microscopy (STM) and non‐contact atomic force microscopy (nc‐AFM) directly resolve both the coordinated networks and the O‐heterocyclic bridging motifs. Complementary DFT and Free energy calculations elucidate the reaction pathways, while x‐ray photoelectron spectroscopy (XPS) reveals characteristic chemical shifts that account for C–O–C formation and suggest Co···O coordination. This controllable oxygen‐based connectivity provides a versatile platform to tune structure and electronic properties in porphyrinoid‐based polymers.

Controlling oxygen‐mediated connectivity at metal surfaces offers a powerful route to engineer covalent and metal–organic architectures with atomic precision. Here, we demonstrate tunable oxygen‐based connectivity in carbaporphyrinoid polymers on Au(111) under ultrahigh vacuum (UHV) conditions. Initial thermal activation of hydroxyl‐functionalized dicarbahemiporphyrazine precursors induces carbon–carbon coupling via a [3+3] cycloaromatization, affording one‐dimensional polymers equipped with hydroxyl‐terminated units. From this intermediate two distinct oxygen‐based linkages can be achieved: further annealing induces dehydrogenative coupling of adjacent polymers through the hydroxyls groups to yield dibenzo‐p‐dioxane (O‐heterocyclic) bridges that covalently connect the carbaporphyrinoid macrocycles, whereas dosing cobalt produces extended metal–organic networks stabilized by two‐fold O···Co···O coordination motifs accompanied by cobalt‐metalated macrocycles. Scanning tunneling microscopy (STM) and non‐contact atomic force microscopy (nc‐AFM) directly resolve both the coordinated networks and the O‐heterocyclic bridging motifs. Complementary DFT and Free energy calculations elucidate the reaction pathways, while x‐ray photoelectron spectroscopy (XPS) reveals characteristic chemical shifts that account for C–O–C formation and suggest Co···O coordination. This controllable oxygen‐based connectivity provides a versatile platform to tune structure and electronic properties in porphyrinoid‐based polymers.

π-Electron magnetic compounds on surfaces have emerged as a powerful platform for probing spin interactions at the atomic scale. In particular, on-surface synthesis has become a powerful method for fabricating open-shell, carbon-based materials whose magnetic interactions, mediated by covalent bonding, have been systematically explored in recent years [1]. On the other hand, coordination chemistry on surfaces has been shown to promote long-range ordering through the formation of metal-organic frameworks. However, understanding its potential to mediate spin exchange interactions in carbon-based open-shell systems remains in its early stages.

Indenyl coupling refers to the reaction where molecular precursors equipped with indenyl ligands promote the formation of C-C bonds, thanks to deprotonation of indenyl groups and subsequent homocoupling. This leads to extended π- conjugated polymers along a polyacetylene backbone, expanding the electronic delocalization, which is crucial to obtain π-conjugated systems with broad applications in fields such as spintronics, sensing, optoelectronics and photovoltaics, among others.

The on-surface synthesis of atomically precise carbon nanomaterials has emerged as a powerful strategy to overcome the limitations of solution-based chemistry, enabling the fabrication of low-dimensional polymers and networks with unprecedented structural control. In particular, the development of single-layer two-dimensional (2D) covalent organic frameworks (COFs) remains a central challenge due to their promising applications in a variety of fields, as sensing, catalysis, and (opto)electronics. [1,2]

Gem-dibromide functional group has been revealed as a powerful moiety to steer polymerization and intra- and intermolecular reactions on surfaces. In this talk, I will revise our recent endeavours regarding the formation of 0D species, 1D and 2D nanoarchitectures, exploiting benzenoid and non-benzenoid constituents, and embedding all-carbon or heteroatomic-based nanoforms.