Denis Frath

Our article is available to read at Eur. J. Org. Chem.

The search for simple, low-cost, versatile, easily accessible, stimuli-responsive, highly emissive molecular fluorophores emitting both in solution and in the solid-state has prompted us to investigate the optical properties of a series of synthetically accessible salicylaldehyde derivatives possessing a π-conjugated moiety at their 4-position. These dyes are mainly known as synthetic intermediates but can also display sizeable Excited-State Intramolecular Proton Transfer (ESIPT) fluorescence owing to the presence of a 6-membered H-bonded ring in their structure. The photophysical properties of these compounds have been studied in solution (multiple solvents) and in the solid-state, as doped in PMMA films, KBr pellets or as powders leading to the observation of a pronounced fluorosolvatochromism. Emission wavelengths in the range 400–654 nm, along with photoluminescence quantum yields up to 76 % were recorded. Modification of the spacer (ethynyl, vinyl or direct connection) involved the π-delocalization triggers major differences in terms of maximum emission wavelength and fluorescence quantum yields in the various media studied. All photophysical observations are rationalized by first-principle calculations.

Our article is available to read at ChemElectroChem.

The functionalization of electrodes by the reduction of diazonium cations generated in situ from 4-(2,3-diethylthieno[3,4-b]pyrazine-5-yl)aniline has been investigated. The thienopyrazine unit of this molecule is a precursor of n-dopable π-conjugated oligomers. Electrochemical reduction of diazonium cation coats the electrode with organic layers. Raman, IR, and XPS analyses show that their composition corresponds to that of the starting monomer, while AFM scratching measurements indicate thicknesses below 10 nm. The electrochemical responses of various reversible redox couples on the modified electrodes show that the attached layer is insulating in the positive potential range but can be n-doped at negative potential and switches to a conductive state. Moreover, oligo(4-(2,3-diethylthieno[3,4-b]pyrazine-5-yl)phenyl) can be selectively grafted onto gold nanoparticles (AuNPs) by plasmon-induced diazonium reduction. A 10–20 nm-thick organic layer is easily grafted onto each gold nanoparticle by visible-light illumination in a few minutes without any reducing agent or molecular photocatalyst. This result is attributed to the transfer of hot electrons from the excited plasmonic NPs to the diazonium, confirms that localized surface plasmon resonance can induce nanolocalized electrochemical reactions, and contributes to the emerging field of “plasmonic electrochemistry”.

Our article is available to read at Inorg. Chem.

A metal-induced self-assembly strategy is used to promote the π-dimerization of viologen-based radicals at room temperature and in standard concentration ranges. Discrete box-shaped 2:2 (M:L) macrocycles or coordination polymers are formed in solution by self-assembly of a viologen-based ditopic ligand with cis-[Pd(en)(NO₃)₂], trans-[Pd(CH₃CN)₂(Cl)₂], or [Pd(CH₃CN)₄(BF₄)₂]. Changing the redox state of the bipyridium units involved in the tectons, from their dicationic state to their radical cation state, results in a reversible “inflation/deflation” of the discrete 2:2 (M:L) macrocyclic assemblies associated to a large modification in the size of their inner cavity. Viologen-centered electron transfer is also used to trigger a dissociation of the coordination polymers formed with tetrakis(acetonitrile)Pd(II), the driving force of the disassembling process being the formation of discrete box-shaped 2:2 (M:L) assemblies stabilized by π-dimerization of both viologen cation radicals.

Our article is available to read at J. Am. Chem. Soc.

Thin layers of diarylethene oligomers (oligo(DAE)) were deposited by electrochemical reduction of a diazonium salt on glassy carbon and gold electrodes. The layers were fully characterized using electrochemistry, XPS, and AFM, and switching between open and closed forms using light was evidenced. Solid-state molecular junctions (MJs), in which a C-AFM tip is used as the top contact, were fabricated with total layer thicknesses fixed at 2–3 nm and 8–9 nm, i.e. below and above the direct tunneling limit. DAE was then photoswitched between its open and closed forms. Oligo(DAE) MJs using the open form of DAE are highly resistive while those with DAE in the closed form are more conductive. ON/OFF ratios of 2–3 and 200–400 were obtained for 3-nm- and 9-nm-thick DAE MJs, respectively.

Our article is available to read at Nanoscale

Ligands are designed to have ditopic bipyridine terminal groups linked through photochromic azobenzene central units, which exhibit multi-switchable properties by different external stimuli. The molecule can switch between cis-and trans-conformations at their bipyridine terminal groups upon protonation and at their central azobenzene units upon irradiation of photons. As a result, the system shows four different isomeric states: cis–TRANS, trans–TRANS, cis–CIS and trans–CIS. The four conformers are switched and are visualized by scanning tunneling microscopy at the solid–liquid interface, which gives a direct demonstration of the multi-functional switches at a molecular level.

Our article is available to read at Electrochim. Acta

The present paper reports on the formation and on the electrochemical/spectroscopic characterization of inclusion complexes formed in aqueous media between cucurbit[7 or 8]urils cavitands (CB[7], CB[8]) and a rigid four-pointed star-shaped viologen-appended porphyrin tecton. The formation of discrete 4:1 pseudo-rotaxane-like caviplexes, involving threading of CB[n] rings on the rigid viologen-based star’s branches has been demonstrated by nuclear magnetic resonance and mass spectrometry measurements. Then, the photo- and redox-triggered formation of 2D supramolecular assemblies involving CB[8]s and the four electron reduced tectons as key building elements, has been established on the ground of in-depth electrochemical and spectroscopic analyses supported by quantum calculations. The CB[8]-promoted intermolecular π-dimerization of the viologen cation radicals introduced at the meso positions of the porphyrin plateform has been brought to light through the diagnostic signatures of the 1:2 host-guest ternary caviplexes formed between viologen and CB[8] and by spectroscopic data collected after electrochemical, chemical or photochemical reduction of the viologen-based tectons. The CB[8] hosts not only proved useful to promote the redox-triggered formation of supramolecular assemblies, it was also found to prevent the chemical reduction of the porphyrin ring in aqueous media and its subsequent conversion into phlorin products.

Denis Frath obtained his PhD in chemistry from the University of Strasbourg in 2013 (laboratory of Dr. Gilles Ulrich and Raymond Ziessel). His doctoral work, supported by a MENRT scholarship, focused on hydrogen production using light and the synthesis of new fluorescent dyes. He then joined the Matsuda laboratory to work on photoresponsive self-assembled monolayers as a JSPS postdoctoral fellow at Kyoto University (2013-2015). After that, he worked on redox-active molecular layers for electronic devices at Paris Diderot University (2016-2017) and binuclear phthalocyanine metallic complexes for catalytic reactions at CNRS in Lyon (2018).

Since 2018, he is CNRS researcher at the Chemistry Laboratory of ENS Lyon where he is head of the Supramolecular Chemistry and Chemical Biology group. His research interests are directed towards π‐conjugated molecules with optical and electrochemical properties, their supramolecular self-assembly and functional materials with switchable properties (gels, liquid crystals, surfaces…). In 2024, he was awarded the SP2P Young Researcher Prize (French Chemical Society).