In the compendium of porphyrinoids, corroles have experienced increasing attention in the past two decades, starting from the disclosure in 1999 of simple routes for the synthesis of meso- triarylcorroles.
Since then, they have been successfully used in various contexts ranging from catalysis to dye-sensitized solar cells.
Peripheral modifications are important to modulate corrole properties and make them suitable for these applications. In this view, the synthetic availability of triarylcorroles has allowed a more detailed investigation of the corrole ring functionalization. Research under this heading will focus in particular on the synthesis and functionalization of new free base- and metallo-corrole derivatives.
The synopsis presented here reports a concise overview of the design, synthesis, structural, photophysical, electrochemical, and molecular modelling characterization of:
1. Expanded Corroles by β-fused aromatic rings.
2. Design and synthesis of new Phthalocyanine-Corrole hybrid conjugates for application in light harvesting systems.
This thesis consists of a general introduction and two chapters.
Introduction
In the introduction, a general description of corrole structure, reactivity and photophysical properties is presented. The main synthetic strategies to achieve symmetric and asymmetric corrole derivatives are briefly discussed, as well as their applications in different research fields. Lastly, general objectives of the present thesis are there highlighted.
Chapter 1
Among the different corrole functionalizations, we focused on the fusion of aromatic substituents at the macrocyclic β-positions. It is well known that in the case of porphyrins, the introduction of fused π-conjugated units strongly modifies the electronic character of the resulting chromophores, allowing porphyrin systems with unique optical and electrochemical properties.
Starting from the same substrate, a variety of π-extended corroles were obtained using two different synthetic pathways. The first methodology exploited a one-pot reaction of 2,3-diaminocorroles (or 2,3,17,18-tetraaminocorroles) with different diones, affording new corrole derivatives with β-fused pyrazino rings.
X-ray crystallographic characterization and electronic absorption spectra highlight interesting features of these derivatives. An easy demetallation procedure allowed us to obtain the corresponding free base derivatives, opening the door to the preparation of different metal complexes, with the possible modulation of the related photophysical characteristics and potentially making these species suitable for optoelectronic applications.
The obtained results prompted us to investigate the development of other synthetic routes leading to π-expanded corroles with annulated pyrazino units. These groups are interesting because it has been shown that heterocycles containing a pyrrolo[1,2-a]pyrazino moiety can play an important role in pharmacological terms.
In this reaction, already performed on porphyrins, corrole reveals once again an unusual reactivity pattern, affording the unsubstituted pyrrolopyrazino substituent by an unprecedented reaction pathway. All the new derivatives were fully characterized, by means of 1H NMR, UV-vis, electrochemical and X-ray techniques.
Chapter 2
Chapter 2 focuses on the design, synthesis and photophysical studies of new functional materials for light harvesting applications.
In the last decade great attention has been focused on the preparation of porphyrin derivatives characterized by an expanded aromatic system, which causes a decrease in the HOMO-LUMO gap that leads to improved harvesting of solar energy in a broad spectral region. These molecules are very promising as dyes in the study of photoinduced electron and energy transfer. Most of these artificial systems are made up of oligomeric species, where the porphyrin (or phthalocyanine) units are linked by bridges, such as for example ethynes, that allow macrocyclic conjugation. To the best of our knowledge, no multicomponent systems based on corrole-phthalocyanine (Corr-Pc) have been described.
In the first section of this Chapter, we successfully synthesized two different corrole-phthalocyanine covalent dyes. The presence of a greater number of methoxy moieties in the corrole framework was expected to improve the electron-donor properties of the macrocycle and enhance the electron transfer to the linked zinc phthalocyanine bearing electron-withdrawing groups. The newly prepared multicomponent systems have been studied performing spectral, computational and electrochemical measurements. Transient absorption spectroscopy showed the appearance of a fast-forming and short-living charge separated states constituted by the radical cation of corrole and radical anion of the phthalocyanine.
In the second part of the chapter, we have prepared a donor-acceptor ensemble built on metal-ligand coordination between a zinc(II) phthalocyanine derivative, bearing electron withdrawing groups at its periphery, and an electron-donor corrole, featuring a pyridyl moiety in position 10. Binding studies were performed by means of NMR and photophysical techniques, in order to determine thermodynamic and kinetic constants.
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