Peripherally Metalated Porphyrins with Applications in Catalysis, Molecular Electronics and Biomedicine

Porphyrins are conjugated, stable chromophores with a central core that binds a variety of metal ions and an easily functionalized peripheral framework. By combining the catalytic, electronic or cytotoxic properties of selected transition metal complexes with the binding and electronic properties of porphyrins, enhanced characteristics of the ensemble are generated. This review article focuses on porphyrins bearing one or more peripheral transition metal complexes and discusses their potential applications in catalysis or biomedicine. Modulation of the electronic properties and intramolecular communication through coordination bond linkages in bis-porphyrin scaffolds is also presented.

What does it take to induce equilibrium in bidirectional energy transfers?

Two dyads built with a co-facial slipped bis(zinc(ii)porphyrin), a free base and a bridge, [Zn2]-bridge-[Fb] (bridge = C6H4C[triple bond, length as m-dash]C, 1 and C6H4C[triple bond, length as m-dash]CC6H4, 2), exhibit S1 energy equilibrium [Zn2]* ↔ [Fb]* at 298 K, an extremely rare situation, which depends on the degree of MO coupling between the units. At 77 K, 2 becomes bi-directional due to the two large C6H4-[Zn2] and C6H4-[Fb] dihedral angles.

Is π-Stacking Prone To Accelerate Singlet-Singlet Energy Transfers?

π-Stacking is the most common structural feature that dictates the optical and electronic properties of chromophores in the solid state. Herein, a unidirectional singlet-singlet energy-transfer dyad has been designed to test the effect of π-stacking of zinc(II) porphyrin, [Zn₂], as a slipped dimer acceptor using a BODIPY unit, [bod], as the donor, bridged by the linker C₆H₄C≡CC₆H₄. The rate of singlet energy transfer, k_ET(S₁), at 298 K ( k_ET(S₁) = 4.5 × 10¹⁰ s^-1) extracted through the change in fluorescence lifetime, τ_F, of [bod] in the presence (27.1 ps) and the absence of [Zn₂] (4.61 ns) from Streak camera measurements, and the rise time of the acceptor signal in femtosecond transient absorption spectra (22.0 ps), is faster than most literature cases where no π-stacking effect exists (i.e., monoporphyrin units). At 77 K, the τ_F of [bod] increases to 45.3 ps, indicating that k_ET(S₁) decreases by 2-fold (2.2 × 10¹⁰ s^-1), a value similar to most values reported in the literature, thus suggesting that the higher value at 298 K is thermally promoted at a higher temperature.

Metal-mediated linear self-assembly of porphyrins

Porphyrin derivatives are highly relevant to biological processes such as light harvesting and charge separation. Their aromatic electronic structure and their accessible HOMO-LUMO gap render porphyrins highly attractive for the development of opto- and electro-active materials. Due to the often difficult covalent synthesis of multiporphyrins, self-assembly using metal complexation as the driving force can lead to well defined objects exhibiting a controlled morphology, which will be required to analyse and understand the electronic properties of porphyrin wires. This article presents two assembly approaches, namely by peripheral coordination or by binding to a metal ion in the porphyrin core, that are efficient and well designed for future developments requiring interactions with a surface.

From Models of Hemoproteins to Self-Assembled Molecular Wires

In nature, the various properties of tetrapyrrolic macrocycles are mostly due to their proteic environment and result from an evolutionary process that is difficult to reproduce during the lifetime of a synthetic chemist. Thus, the task of synthetic chemists attempting to reproduce the biological role of porphyrin architectures, which perform functions from catalysis to light harvesting, is complicated. This account describes how a phenanthroline-strapped porphyrin architecture initially designed to afford new hemoprotein models led, over the last two decades, to the preparation of highly linear self-assembled nano-objects inspired by light-harvesting architectures. The approach summarized herein mimics, in a very modest way, the polyvalence of tetrapyrrolic macrocycles found in nature.