Substitutional flexibility and molecular pinning in porphyrin-based interfaces sensitive to NO2

Porphyrins may easily meet the diverse requirements for their employment in a variety of molecular-based nanodevices, extending from gas sensing to photochemistry, due to their remarkable chemical and structural flexibility. In this context, self-assembled layers of cobalt β-octaethylporphyrins and meso-tetraphenylporphyrins on inert metal substrates exhibit distinct periphery-driven conformations, with the macrocycle adsorbed in either a planar or saddle-shaped configuration (PC and SSC, respectively). Here, by means of x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine spectroscopy (NEXAFS) supported by density functional theory (DFT) calculations, we demonstrate that the adsorbate behaviour on the chemically more reactive copper surface is diametrically different, as the strong molecular pinning stabilizes the macrocycle of the differently substituted molecules in a planar conformation. At the same time, the peripheral substitution in the cobalt porphyrins shows a minor effect in the charge donation to the unoccupied macrocycle molecular orbital, but it has a stronger impact on the stabilization of the Co 3dz2-based orbital. Moreover, the reduction of the Co(II) species, occupying the molecular core, is also ascribed to the molecular pinning. All these features are crucial for the metal-porphyrin array exploitation in nanotechnology applications, as their reactivity may be retained while the optoelectronic properties are fully tunable. Indeed, when the organic layer is exposed to hazardous nitrogen dioxide (NO2), the low-valence cobalt metallic center selectively interacts with even small amounts of gas. This coordination induces the macrocycle's ring distortion in tetraphenylporphyrin, while the octaethylporphyrin maintains a flat conformational geometry.