Tetrapyrroles at near-ambient pressure: porphyrins and phthalocyanines beyond the pressure gap

Abstract

Many complex mechanisms underlying the fascinating functionalities provided by tetrapyrrolic macrocycles in biochemistry have been already unraveled. Light harvesting, molecular transport, and catalytic conversion are some of the processes performed by tetrapyrrole-based centers embedded in protein pockets. The main function is determined by the single atom species that is caged in the macrocycle, while a finer tuning (band gap, chemical selectivity etc) is granted by the geometric and electronic structure of the tetrapyrrole, including its residues, and by the proximal and distal structures of the protein surroundings that exploit the molecular trans-effect and direct weak interactions, respectively. Hence, a scientific and technological challenge consists in the artificial replication of both structure and functionality of natural reaction centers in 2D ordered arrays at surfaces. Nano-architected 2D metalorganic frameworks can be indeed self-assembled under controlled conditions at supporting surfaces and, in the specific, porphyrin-and phthalocyaninebased systems have been widely investigated in ultra-high vacuum conditions by means of surface science approaches. Deep insight into the geometry, electronic structure, magnetic properties, ligand adsorption mechanisms, and light absorption has been obtained, with the strong experimental constraint of vacuum. Especially in the case of the interaction of tetrapyrroles with ligands, this limit represents a relevant gap with respect to both comparison with natural counterparts from the liquid environment and potential applicative views at both solid-liquid and solid-gas interfaces. Thus, a step forward in the direction of near-ambient pressure is strongly necessary, while maintaining the atomiclevel detail characterization accuracy. Nowadays this becomes feasible by exploiting state-of-the-art experimental techniques, in combination with computational simulations. This review focusses on the latest advances in this direction.

Stabilization and activation of molecular oxygen at biomimetic tetrapyrroles on surfaces: from UHV to near-ambient pressure

“…[2][3][4][5] In this sense, Nature has a lot to teach, since biological processes largely exploit O 2 transportation and activation. A biomimetic approach may provide useful insight into reaction centres based on tetrapyrroles in the case of several reaction schemes, [6][7][8] including O 2 activation. 1,[9][10][11][12][13] 1.1 From Nature to biomimetic heterostacks: molecular-and surface trans-effect Binding and/or activation of small ligands in the biochemistry of enzymes occur by means of a combination of adsorption, charge transfer, and steric effects taking place in a reaction pocket where, in many (but of course not all) cases, a single metal ion is stabilized by a tetrapyrrolic matrix playing the major role (Fig.…”

“…1c). 7,8,14,15 Indeed, the trans-effect describes in general the situation in which two ligands in trans position, binding to the same metal centre, compete for coordination. 14 In the case of supported metallorganic molecules, one of the two ligands is replaced by the supporting surface.…”

Dioxygen at Biomimetic Single Metal-Atom Sites: Stabilization or Activation? The Case of CoTPyP/Au(111)

“…In a biomimetic perspective, single-atom catalysis is a promising route to design novel, cheap, efficient, selective, and stable heterogeneous catalysts [1]. In this context, tetrapyrrolic metallorganic frameworks (MOF) based on porphyrins or phthalocyanines tectons can represent an effective way to mimic the single metal atom active sites typical of many active enzymes or protein vectors that Nature exploits for many reactions, including oxygen conversion and transport [2][3][4][5][6][7][8][9][10]. In the specific case, the oxygen reduction reaction (ORR) is essential for life, as well as for many applicative purposes like e.g.…”

“…The activity and the selectivity of natural systems are mainly determined by the metal species embedded in the active pocket site and, in a number of cases, the metal atom is coordinated in a tetrapyrrolic macrocycle. A fine tuning of the site's properties is granted by the local geometric and electronic configuration of the tetrapyrrole, including its residues: the proximal structure of the protein surroundings induces the well-known molecular trans-effect, while the distal structure operates through direct weak interactions [2,16]. As a matter of fact, the metal centers in biomimetic synthetic MOFs are coordinatively unsaturated, thus yielding available ligation sites for adducts and reactants [17][18][19][20][21][22].…”

“…This pressure gap, separating characterization and applicative environments, often strongly questions the significance of the results obtained by means of fundamental approaches. To this purpose, novel experimental techniques, such as Sum-Frequency Generation Spectroscopy, have been recently developed in order to bridge this gap, allowing investigations in situ and operando up to pressure values close to ambient conditions [2,[24][25][26].…”