Electrochemically-driven Pi-extension of porphyrins – PORFUSION
Pi-Extension of porphyrins by oxidative intramolecular C-N fusion
This C-N intramolecular oxidative fusion project based on the porphyrin macrocylce has been investigated since no example in the literature was reported. The C-N fusion products would be all original. Their physico-chemical properties as well as their potential applications should be explored.
Why performing pi-extension of porphyrins by oxidative intramolecular C-N fusion instead of intramolecular oxidative C-C coupling?
Currently, numerous works on oxidative intramolecular C-C fusion of aromatic molecules, in particular for porphyrins, shed light on the impact of the extension of the conjugation path on the electronic properties of these derivatives. <br />For porphyrins, these C-C couplings are generally performed with large amount of oxidizers or with high temperature. Besides, the fused C-C products are easier to oxidize than their precursor which could lead to their overoxidation/degradation and even their air instability.<br />Our project consist in the formation of new C-N+ bonds between a porphyrin and a peripheral imine-type substituent by oxidative fusion in mild conditions (electrochemical oxidation, room temperature). It was expected that the fused C-N products were oxidized at higher potential than their unfused precursors given their cationic nature. Thus the C-N fusion reaction should be more selective and the fused compounds should be more stable. The fused C-N porphyrin derivatives were unknown before this project and could give the unfused precursors by breakage of the C-N link, which will make these fusion-unfusion process reversible. Theoretical calculations have supported some experimental results and helped us in the design of the unfused precursors.
Among the different pathways that we have explored, we started our researches with 5,15-ditolyporphyrin. After bromination and aromatic nucleophilic substitution, numerous nucleophiles (thiopyridine, aminopyridine, thiopyrimidine) have been inserted in the periphery of the porphyrin. Also one or two peripheral quinoline fragments have been grafted thanks to C-C Suzuki cross-coupling reaction. Besides, different porphyrin complexes have been synthesized to determine the more efficient ones for C-N fusion reactions. Finally, after optimization of the experimental conditions, the C-N fusion reactions have been performed by electrosynthesis at controlled potential but also using chemical oxidizers. Purification issues of the cationic derivatives have been overcome with non-usual (for our team) chromatographic methods.
Numerous original precursors have been synthesized. Concerning the C-N fusion, the best results have been obtained with the porphyrins substituted by one or two mercaptopyridinyl fragments. In this case, the electrolysis at controlled potential is very selective since the fused synthesized products exhibits a much higher first oxidation potential than their precursors. For the disubstituted porphyrins, it is possible to prepare first the monofused derivative and then the difused compound. The optical properties show a progressive bathochromic shift of the main absorption bands upon increasing the number of fused substituents.
This project could be extended to other original precursors already mentioned in our project but not synthesized. Other precursors not presented in our project could be prepared such as porphyrin dimers with bridging substituents that can perform one fusion on each porphyrin. Moreover this project can be extended to oxidazable aromatic derivatives other than porphyrins what will considerably increase the scope of our initial project
At the end of this ANR project, two publications have been accepted (Chem. Commun., 2018, 54, 5414-5417, doi: 10.1039/c8cc01375f; Eur. J. Inorg. Chem., 2018, 44, 4834-4841, doi: 10.1002/ejic.201801142), one is under revision and two other publications are in preparation. Furthermore, 5 international communications (1 invited oral, 4 posters) and 13 national communications (3 invited seminars, 2 invited orals, 5 oral communications and 3 posters) have contributed to the broadcasting of the project.
Pi-extension of porphyrins, i.e. fusion of one or several aromatic hydrocarbon(s) or aromatic heterocycle(s) onto the porphyrin periphery by intramolecular chemical oxidative coupling has attracted much attention because of potential applications in Near-IR electroluminescence displays, photovoltaic solar cells, non-linear optical materials, photodynamic therapy and molecular electronics. However, as mentioned in 2013 by Osuka, “New efficient fusion reactions under milder conditions are highly desirable in future developments”. For this purpose, as reported in 2012 by Gryko, “a better understanding of the mechanism of these intramolecular oxidative couplings is needed, specifically which moiety (porphyrin or the second aromatic system) is first attacked by the oxidant”.
Thus this project will take advantage of electrochemistry and theoretical calculations to investigate fusion reaction mechanism with unprecedented peripheral aromatic fragments. Besides, this fusion reaction will be performed by electrosynthesis under mild conditions.
Monsieur Charles DEVILLERS (Institut de Chimie Moléculaire de l'Université de Bourgogne)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ICMUB Institut de Chimie Moléculaire de l'Université de Bourgogne
Help of the ANR 201,864 euros
Beginning and duration of the scientific project: August 2016 - 36 Months