In vacuo interfacial tetrapyrrole metallation

Abstract

The metallation of tetrapyrroles at well-defined surfaces under ultra-high vacuum conditions represents an unconventional synthesis approach to achieve tetrapyrrole-based metal-organic complexes and architectures. Different protocols, pioneered over the last decade, and now widely applied in several fields, provide an elegant route to metallo-tetrapyrrole systems often elusive to conventional procedures and give access and exquisite insight into on-surface tetrapyrrole chemistry. As highlighted by the functionality of metallo-porphyrins in biological or other environments and by the eminent role of metallo-phthalocyanines in synthetic materials, the control on the metal centres incorporated into the macrocycle is of utmost importance to achieve tailored properties in tetrapyrrole-based nanosystems. In the on-surface scenario, precise metallation pathways were developed, including reactions of tetrapyrroles with metals supplied by physical vapour deposition, chemical vapour deposition or the tip of a scanning tunnelling microscope, and self-metallation by atoms of an underlying support. Herein, we provide a comprehensive overview of in vacuo tetrapyrrole metallation, addressing two-dimensional as well as three-dimensional systems. Furthermore, we comparatively assess the available library of on-surface metallation protocols and elaborate on the state-of-the-art methodology.

On‐Surface Metalation and 2D Self‐Assembly of Pyrphyrin Molecules Into Metal‐Coordinated Networks on Cu(111)

“…for pyridinic nitrogen (−H−) at 400.5 eV, iminic nitrogen (=−) at 398.8 eV, and the cyano nitrogen (−C≡N|) at 399.1 eV binding energy, with a peak area ratio of 2:2:2. The assigned binding energies are consistent with the literature for 2HPyr, free base tetraphenylporphyrin (2HTPP) and cyano‐functionalized molecules at monolayer coverage on various metal surfaces . After annealing to 473 K for 10 min, a narrower N 1s peak is observed which can be deconvoluted into N −Cu (398.9 eV) and C≡ N −Cu (399.8 eV) signals at 4:2 peak area ratio, respectively .…”

“…This is substantially lower than observed for monolayers of 2HTPP on Cu(111), where the activation energy for the rate‐limiting step, i.e . incorporation of the Cu adatom into the macrocycle, has been reported as high as 143 kJ mol −1 and 134 kJ mol −1 , using the same attempt frequency ν 0 = 10 15 s −1 . The smaller cavity of pyrphyrin may lead to a higher metal‐ligand binding energy, but our method determines only the activation energy of the process, which includes also hydrogen release, recombination and desorption.…”

“…For the monolayer of the plain porphyrin‐core species 2H‐porphine and a submonolayer sample of 2H‐phthalocyanine on Cu(111), the onset for metalation was reported at 373 K and 240 K, respectively . There, the low temperature for porphine metalation has also been attributed to an increased interaction with the surface due to a lower height above the surface …”

“…Surface functionalization with macrocycles, such as porphyrin or phthalocyanine, presents a fascinating platform for studying physical and chemical phenomena at interfaces and provides the basis for many applications. These predominantly planar macrocycles can coordinate a metal ion, rendering them interesting model systems to study coordination chemistry at surfaces . In addition, the central metal ions can coordinate an axial ligand at the site opposite to the surface .…”

“…metalation) in‐situ directly at the surface under ultrahigh vacuum (UHV) conditions. The metal atoms can be provided either by the surface (sometimes termed as ‘self‐metalation’) or supplied by co‐deposition through evaporation …”