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Subject of the thesis - Campagne de thèses 2022, sujet SL-DRF-22-0797

Development of photoredox catalysts based on chemically assembled composites of quantum dots and metallic nanoparticles behaving as nano-reservoirs of electrons

Published on 14 February 2022
Scientific context
During the last decade photoredox catalysis emerged has an efficient strategy to perform selective organic chemistry reactions in more environmental friendly conditions [1], by using visible light to enable the reaction instead of harsh reactants and/or thermal heating. The majority of the reported reactions relies on molecular photocatalysts, but few groups [2–5], including ours [2], demonstrated few years ago that colloidal semi-conductor nanoparticles (colloidal quantum dots, named hereafter “QD”) can act as efficient photocatalysts. Nowadays, the use of QD as photocatalysts in photoredox catalysis is increasing fastly6, due to their tunable optical properties and to their easy separation from the targeted organic products. In such photoredox catalysis reactions, the QD absorb the light and, from their excited state (1st exciton), transfer charges (electrons and holes) to the surrounding organic electron donors and acceptors, so enabling both reduction and oxidation steps necessary for the photocatalytic cycle (see Scheme 1 below)

Unfortunately most of the photogenerated electrons and holes recombine inside the photoexcited QD, instead of performing the desired reactions (typical quantum yields are < 1%, [2,4,5]) and the photocatalytic systems have to be illuminated for a quite long period of time (typically few hours) for completing the chemical reactions. So, it would be highly desirable to modify QD photocatalysts in a way that would prevent charges recombination.

Recent work in CEA-CAMPE and I.Néel-OPTIMA – proof of concept
During the last 3 years our teams started to explore together an original and modular strategy consisting of associating CdSe@ZnS QDs with gold nanoparticles (named hereafter AuNP) by creating a covalent links between some ligands of the two types of nanoparticles (see Scheme 2 below). We expected that AuNP could act as electrons sinks and so avoid the charges recombination.
We introduced in the ligands shells of both QD and AuNP some ligands bearing alkynes and azides moieties, then we optimized the conditions of the chemical coupling of these moieties by Huisgen 1,3 dipolar cycloaddition (click-chemistry), and we obtained QD-AuNP composites (See Fig.1 left panel) well dispersed in water solution.

Then we tested the photocatalytic activity of such CdSe@ZnS-AuNP composites and observed that they are much more efficient photocatalysts (10 times faster) than CdSe@ZnS QD alone for the redox reactions we previously reported [2] (see Figure 1 right panel).

Figure 1. Left panel: FESEM imaging of CdSe@ZnS-AuNP composites assembled in our labs by using the click-chemistry reaction. Right panel: monitoring of the photocatalytic oxidation of an electron donor (8oxodG, initial concentration 120 μM) with CdSe@ZnS-AuNP composites as photocatalysts.

Moreover when illuminating such QD-AuNP composites in the presence of organic electron donors (without acceptors), we observed that photogenerated electrons accumulate into AuNP (several thousands of electrons per particle) and that the corresponding charge decays very slowly (in fews hours) after stopping the illumination. So AuNP behave more like a nano-reservoir for electrons instead of a sink in such composites. This is very promising for photocatalysis because 1/ it can acilitate multi-electrons reactions, 2/ the Fermi level of the electrons in the AuNP nano-reservoirs can be raised (or lowered) by illuminating more (or less) the photocatalytic system.

Proposed work for the PhD thesis
The proposed PhD will consist in three complementary main tasks :

Task A – Photocatalysis of reactions useful in organic synthesis:
Test and adapt the CdSe@ZnS- AuNP composites and find the right experimental conditions for new challenging selective photoredox reactions in organic synthesis, starting with the reaction of allylation/cyclisation or aromatic amines published by Pr. Renaud in 2021 in collaboration with our groups [7] in the framework of the French-Swiss ANR/FNS project PhotoRedoQs.

Task B – Coupling QDs and metalNP in organic solvents:
Explore coupling reactions of AuNP and QD in organic solvents (Acetonitrile, DCM) in order to obtain QD-AuNP composite dispersed in these solvents. The objective is to broaden the range of accessible experimental conditions for the organic chemistry reaction studied in Task A and to facilitate task C (see below).

Task C – Extend the QD-metalNP family to improve charges accumulation:
Extend the scope of the study of QD-metalNP composites. i) to variable sizes, variable shape or other type (silverNP) of metalNP. ii) to cadmium-free low gap quantum dots (typically InP@ZnS and/or CuInS2). The two aims of this task are: 1/ to remove this heavy toxic metal from the composites and 2/ to investigate the accumulation of charges when associating QD and metalNP having different energy levels compared with the reference CdSe@ZnS-AuNP composites.

For this work, the PhD student will share his time equally between the two labs and benefit from:

1/ The experience acquired by the CEA-SyMMES-CAMPE group in the synthesis, functionalization and photocatalytic activity studies of QD photocatalysts and the corresponding facilities (synthesis lab equipped with gloves box, illumination setups, UPLC-MS-MS spectrometer for monitoring the photocatalytic reaction in coll. with dr. J.-L. Ravanat, NMR, EPR and fluorescence spectroscopies).

2/ The experience acquired by the I-Néel-PLUM-OPTIMA group in the synthesis of colloid and their functionalization, the characterization of colloidal systems [8] (the team is equipped by DLS and FCS equipments, zeta potential analysis, Raman-SERS characterization set-up and SEM and TEM microscope) and a synthesis lab for the preparation of composite materials by coupling reactions.

[1] Marzo, L.; Pagire, S. K.; Reiser, O.; König, B. Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis? Angew. Chem. Int. Ed. 2018, 57 (32), 10034–10072.
[2] Chauviré, T.; Mouesca, J.-M.; Gasparutto, D.; Ravanat, J.-L.; Lebrun, C.; Gromova, M.; Jouneau, P.-H.; Chauvin, J.; Gambarelli, S.; Maurel, V. Redox Photocatalysis with Water-Soluble Core– Shell CdSe-ZnS Quantum Dots. J. Phys. Chem. C 2015, 119 (31), 17857–17866.
[3] Pal, A.; Ghosh, I.; Sapra, S.; König, B. Quantum Dots in Visible-Light Photoredox Catalysis: Reductive Dehalogenations and C–H Arylation Reactions Using Aryl Bromides. Chem. Mater. 2017, 29 (12), 5225–5231.
[4] Caputo, J. A.; Frenette, L. C.; Zhao, N.; Sowers, K. L.; Krauss, T. D.; Weix, D. J. General and Efficient C–C Bond Forming Photoredox Catalysis with Semiconductor Quantum Dots. J. Am. Chem. Soc. 2017, 139 (12), 4250–4253.
[5] Zhang, Z.; Edme, K.; Lian, S.; Weiss, E. A. Enhancing the Rate of Quantum-Dot-Photocatalyzed Carbon-Carbon Coupling by Tuning the Composition of the Dot’s Ligand Shell. J Am Chem Soc 2017, 139, 4246–4249.
[6] Yuan, Y.; Jin, N.; Saghy, P.; Dube, L.; Zhu, H.; Chen, O. Quantum Dot Photocatalysts for Organic Transformations. J. Phys. Chem. Lett. 2021, 12 (30), 7180–7193.
[7] Colson, E.; Andrez, J.; Dabbous, A.; Dénès, F.; Maurel, V.; Mouesca, J.-M.; Renaud, P. Tropane and Related Alkaloid Skeletons via a Radical [3+3]-Annulation Process. 2021.
[8] Barbara, A.; Dubois, F.; Ibanez, A.; Eng, L. M.; Quémerais, P. SERS Correlation Spectroscopy of Silver Aggregates in Colloidal Suspension: Quantitative Sizing Down to a Single Nanoparticle. J. Phys. Chem. C 2014, 118 (31), 17922–17931.

Profile required for applying
Master 2 track in Nanosciences or Chemistry or Physical Chemistry. Useful (but not mandatory) skills: synthesis of nanoparticles, fluorescence and UV-visible absorption spectroscopies, chromatography (HPLC, GC), magnetic resonance, microscopy techniques.

Supervision and contacts
Dr. Vincent Maurel, CEA-Grenoble IRIG/SyMMES/CAMPE  (tel. 04 38 78 35 98)
Dr. Fabien Dubois, Ins.Néel Grenoble PLUM/OPTIMA  (tel. 04 76 88 74 10)

Starting of the PhD
October 2022.

Deadlines for applying to fellowships
March 24th, 2022 (LANEF Labex, Grenoble), 
March 31st, 2022 (EDCSV doctoral school, Grenoble-Alpes Univ.)