Spectroelectrochemistry as a powerful technique for porphyrins/corroles derivatives electro-characterization: Fundamentals and some examples
Graphical abstract
Introduction
Electrochemical measurements can provide details about the mechanisms involved in coupled redox reactions of a given system. Logically, the elucidation of these mechanisms will depend both on the complexity of the system involved, as well as on the relative rates of reaction. In many cases, the information that can be obtained exclusively by the voltammetric processes involved is quite limited, and the complementation of the information by spectroscopic techniques becomes an extremely desirable tool. It is easy to understand that the combination of both techniques will lead to results that are complementary and broader when compared with the data obtained using them in independent form [1], [2].
The advantages of using coupled techniques have generated a series of approaches aimed at providing more spectroscopic details of the intermediates or products electrogenerated during the redox process. The spectroelectrochemical technique allows the in-situ spectroscopic investigation of electrogenerated compounds and this can enable the study of processes which occur in shorter times and also establish the reversibility of these reactions [3]. This allows the construction, and better understanding of the mechanism and, mainly, to provide more precise information about the structures of the electrogenerated intermediates.
In this review, the most relevant spectroelectrochemical results of the last few years are presented, the corresponding data were compiled and the redox processes involved for porphyrins and corroles are discussed [4], [5]. Porphyrins are diprotic, while corroles are triprotic species. This means that corroles can form very strong bonds with a wide number of transition metals in higher oxidation states. In recent years, researchers were able to demonstrate the differentiated characteristics of corroles, such as their chemical reactivity for advanced application in catalysis, in synthetic chemistry (in reactions involving oxo, imido and nitrido transfer agents), for catalytic reduction of oxygen or for hydrogen production from water. Additional applications are their use as imaging agents, as oxygen sensing, for prevention of heart diseases and as carriers of theragnostic for tumor targeting. An overview about the technique and the experimental aspects involved are discussed. Although this contribution focuses on only two types of macrocycles, the spectroelectrochemical technique for tetrapyrrole derivatives has proved to be effective for many studies in this area. In this contribution we intend to illustrate some fundamentals, general approaches, impact of UV–vis-NIR and FTIR/Raman spectroelectrochemical techniques [6] in the elucidation of the redox chemistry of porphyrins and corroles, followed by a compilation of the most relevant examples in the area in the period of 2015 to 2021.
Section snippets
UV–vis-NIR and FTIR analysis
The first experiments dealing with spectroelectrochemistry occurred at the beginning of the 1960′s, but only at the end of the 1980′s appeared the first paper describing an Optical Transparent Thin Layer Electrode (OTTLE) [7], [8], [9], [10]. These tiny cells present differentiated characteristics due to their geometry. From the point of view of the electrochemical process, the distance between the electrodes (typically of the order of 50 to 150 nm) is smaller than the semi-infinite
General aspects
In general, porphyrins and corroles are tetrapyrrolic macrocycles, both formed by four pyrrole units, but which differ in only one bond, with porphyrin having bridge bonds between pyrroles and corroles having a direct pyrrole-pyrrole connection (Fig. 2a). This subtle change in the macrocyclic ring causes these two classes of compounds to have distinct structural, photophysical, photobiological and redox properties.
For example, the π-aromatic character of the porphyrin ring assures strong
Free-base derivatives
Porphyrins and corroles have a rich redox chemistry, both in relation to the tetrapyrrole ring (can form cationic or anionic radical species in solution) as well as according to the substituents present in the macrocycle structure. In general, clear changes are observed in the electronic spectra of these derivatives, for example, in the typical Soret band transition [26]. The formation of oxidized and reduced species can be easily identified by applying potential in the cathodic or anodic range
Concluding remarks
Corrole chemistry is quite new. While porphyrins can be considered widely studied molecules, corroles are still under constant discovery. There is much to be learned and explored in this area. Despite the apparent similarity, porphyrins promote different coordination modes when compared with corrole derivatives, which favors binding to transition metals, even lanthanides and actinides, which favors the formation of a huge range of compounds. The studies carried out so far indicate great
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Sao Paulo Research Foundation (FAPESP processes 2014/50867-3 and 2017/13137-5), the National Council for Scientific and Technological Development (CNPq Processes 409150/2018-5, 403210/2021-6, 305458/2021-3, 311847/2018-8), FAPERGS (21/2551-0002114-4) and INCT/Fapesp (Process 465389/2014-7).
Author contributions
B. Dozza, B.M. Rodrigues, I. Tisoco and V.B. de Souza conceived and carried out a bibliographic review. B.A. Iglesias contributed to the porphyrin/corrole spectroelectrochemical applications. L. Angnes conducted the introduction and fundamentals of spectroelectrochemical techniques. B.A. Iglesias and L. Angnes wrote the manuscript. All authors read and approved the manuscript.
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