METALLO-PORPHYRIN CONJUGATES FOR ARTIFICIAL PHOTOSYNTHESIS OR SINGLE-ATOM CATALYSIS

The aim of the PhD project was the synthesis and characterization of novel metallo-porphyrin conjugates for applications in artificial photosynthesis and single-atom catalysis. Exploiting the synthetic versatility of porphyrins, redox-active residues (tyrosine or phenol analogues) were introduced at the meso-positions of the macrocycle. The resulting free-base porphyrins were metalated with different metal salts, affording ZnII- and SnIV-porphyrin derivatives. The photochemical properties of the SnIV-porphyrin conjugates were investigated in collaboration with Prof. Natali (University of Ferrara, IT). Photophysical studies were done in organic solution and in the presence of pyrrolidine as external base, with the aim of activating a photoinduced PCET. This work was inspired by a previously reported SnIV-porphyrin/tyrosine system having the redox-active unit in the apical position, which was able to undergo PCET but generated a short-lived diradical species. Peripheral conjugation of the redox-active moiety was explored to slow down diradical recombination. However, in the newly synthesized SnIV-porphyrin/phenol conjugates, PCET occurred only from the triplet excited state and competed with a much faster bimolecular quenching process, indicating that increasing the distance between the porphyrin macrocycle and the redox-active unit significantly slows electron-transfer kinetics despite favourable thermodynamics. The project was further extended to biomimetic transmembrane systems during a six-month stay in the laboratory of Prof. Bonnet (Leiden University, NL). A SnIV-porphyrin/amino-acid conjugate was used as building block to prepare a transmembrane peptide by solid-phase peptide synthesis and manual coupling. The peptide and a model analogue, were embedded into asymmetric liposomes and studied under blue-light irradiation. Electron transfer was monitored by the characteristic absorption band of the reduced electron acceptor loaded in the bulk aqueous phase. The experiments showed that reduction mainly originated from leakage of the electron donor from the inner compartment to the outer one, followed by direct photoreaction in the bulk aqueous phase between donor and acceptor. In addition, two shorter peptides were synthesized to investigate photoinduced PCET in organic solvents. In parallel, porphyrins bearing pendant phenol ligands deposited on two-dimensional surfaces were investigated as models for single-atom catalysis, in collaboration with Prof. Vesselli (University of Trieste, IT). These studies demonstrated that second-sphere proton-responsive pendant groups modulate the reactivity and stability of isolated metal centres, influencing electronic communication between the metal, ligand environment and supporting surface, and provided insights into structure–function relationships relevant to heterogeneous catalysis.