Elettra | ALOISA Seeding the vertical growth of coordination polymers on rutile via pyridyl/M(II)-TTPs axial binding Tetrapyrrolic macrocycles such as porphyrins and phthalocyanines are versatile chelating agents capable of accommodating a wide range of metal ions. Their planar M–N₄ cores, particularly in metal(II) tetraphenylporphyrins (M-TPPs), often provide one or two axial coordination sites, allowing for vertical linkage. Such supramolecular assemblies can support recursive bilayer stacking, ideal for optoelectronic applications where molecular-level control over vertical charge transport and orientation is essential. Axial coordination of nitrogenbased ligands such as pyridines to the central metal atom offers a route to controlled vertical assembly. In particular the bifunctional N,N'-Di(4-pyridyl)- 1,4,5,8-naphthalenetetracarboxydiimide (DPNDI) ligand offers dangling pyridyl group that may further coordinate to the axial site of a next molecular porphyrin layer. While solution-based axial coordination of pyridyl-functionalized acceptors to porphyrins is well-documented, this approach suffers from poor surface homogeneity and lateral disorder if transferred onto surfaces for technological applications such as photoelectronic devices and chemical sensors. In contrast, vacuum deposition offers superior control of molecular orientation for creating pure, homogeneous and laterally coherent layers. Technologically, the rutile-TiO2(110) surface is an ideal substrate for controlling the growth, stacking, and orientation of porphyrins. In fact, achieving coherent 2D columnar growth of multilayer porphyrin or donor–acceptor bilayers on such material enhance electron transfer upon excitation, due to direct vertical intermolecular interactions. However, such a substrate poses a challenge, as its high reactivity may deactivate the axial coordination sites of the adsorbed porphyrins via charge transfer or chemical bonding. In this work, we focused on Zn-, Co-, and Cu-TPPs as representative macrocycles offering distinct axial coordination capabilities. Zn(II) allows for single axial coordination (square pyramidal geometry), Co(II) permits dual coordination (octahedral), and Cu(II), affected by Jahn–Teller distortion, typically lacks accessible axial sites. The DPNDI linker was deposited atop M-TPP monolayers under UHV conditions. Structural and chemical analysis was carried out using X-ray Photoemission Spectroscopy (XPS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS). Complementary Density Functional Theory (DFT) simulations were performed using pyridine (Py) as a proxy for DPNDI to reduce computational complexity. N 1s XPS spectra of M-TPPs monolayers show a single component for all the M-TPPs, associated with the pyrrolic nitrogen of the macrocycle. When DPNDI is deposited onto Zn- and Co-TPP monolayers in a 1:1 stoichiometric ratio, XPS revealed two new distinct N 1s (Fig. 1 a). Difference XPS spectra distinctly reveal the two well-separated peaks, closely matching the characteristic imide and pyridyl signatures of the DPNDI multilayer, indicating preserved molecular identity. In contrast, the difference spectrum for Cu-TPP is dominated by a single component that closely resembles that of low-coverage DPNDI films directly adsorbed on TiO2(110) (on which we observed partial reduction and altered spectral signatures) indicating a preferential interaction of DPNDI with the substrate. NEXAFS measurements at the N K-edge (Fig. 1 b) showed a strong linear dichroism of the LUMO π* resonances (~398.7 eV) for DPNDI on Zn- and Co-TPP, which indicates standing-up orientation of the former molecule (Fig. 1 c) —an expected outcome of axial coordination—whereas the signal is nearly isotropic for the Cu-TPP system, resembling that of DPNDI ML directly adsorbed on the TiO2(110) (with characteristic satellite at ~400.5 eV), implying the lack of coordination and likely intercalation among porphyrins, in agreement with the photoemission results. This net contrast highlights how the metal center’s electronic structure governs the ability to coordinate ligands under ultrahigh vacuum conditions on reactive oxide surfaces and the role of NEXAFS as a key technique for molecular orientation. In order to have a deeper insight into the coordination behavior of the four different M-TPPs, we performed preliminary theoretical investigations of the Py/M-TPP adducts in the gas phase. DFT simulations predict strong coordination for pyridine to Zn-TPP and Co-TPP (binding energies ~0.8–1.0 eV) and significantly weaker binding for CuTPP and Ni-TPP (0.3–0.5 eV), consistent with the metal–ligand chemistry observed in solution. In-plane M–N₄ bond lengths increased marginally upon coordination, while axial M–N(Py) bond distances ranged from 2.1–2.5 Å. When placed on TiO2(110), coordinated Zn-TPP and Co-TPP show clear elongation of metal–substrate bonds by ~0.8 Å (Fig. 1 d), reflecting a surface trans effect that weakens metal– oxide interactions in favor of axial ligand binding, while Cu- and Ni-TPPs exhibit negligible structural changes, consistent with their weaker coordination capacity, in agreement with the experimental observations. This study establishes the feasibility of constructing vertically coordinated, laterally ordered donor–acceptor heterostructures on TiO2(110) via axial pyridyl–metal linkages. Zn-TPP and Co-TPP retain their axial coordination capability despite the substrate's reactivity, supporting upright DPNDI alignment and enabling multilayer stacking. The resulting molecular films are structurally coherent, electronically tunable, and suitable for theoretical modeling of charge and exciton transport. This molecule-by-molecule growth strategy opens new pathways for developing supramolecular materials tailored for nanoscale optoelectronic, spintronic, and sensing applications. Elettra Highlights 2024–2025 102
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