Denis Frath

Our article is available to read at J. Porphyr. Phthalocyanines.

A linear porphyrin-based tecton bearing two 4,4′bipyridinium units (viologens) and two monomethyl-ether triethylene glycol-substituted phenyl substituents at the meso positions was synthesized and characterized. The latter was involved in the redox-triggered formation of linear supramolecular assemblies with cucurbit[8]uril (CB[8]) cavitands in aqueous media. The CB[8]-promoted intermolecular π-dimerization of the viologen cation radicals introduced at the meso positions of the porphyrin platform 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 reduction of the viologen-based tectons.

A PhD position is opened at the Chemistry Laboratory of ENS de Lyon and funded for three years by the French ANR in the frame of the project ChiroSwitch: metamorphic approaches for on-surface switching of chiroptical properties.

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

We herein report the synthesis and magnetic properties of a Ni(II)-porphyrin tethered to an imidazole ligand through a flexible electron-responsive mechanical hinge. The latter is capable of undergoing a large amplitude and fully reversible folding motion under the effect of electrical stimulation. This redox-triggered movement is exploited to force the axial coordination of the appended imidazole ligand onto the square-planar Ni(II) center, resulting in a change in its spin state from low spin (S = 0) to high spin (S = 1) proceeding with an 80% switching efficiency. The driving force of this reversible folding motion is the π-dimerization between two electrogenerated viologen cation radicals. The folding motion and the associated spin state switching are demonstrated on the grounds of NMR, (spectro)electrochemical, and magnetic data supported by quantum calculations.

Our article is available to read at ECS J. Solid State Sci. Technol.

Through electrochemical deposition, photoswitchable single and bilayer molecular junctions based on diarylethene (DAE) and bisthienylbenzene (BTB) layers were fabricated. The electrical characterization of closed and open forms of DAE were investigated by C-AFM for two different layer thicknesses fixed at 2–3 nm and 8–9 nm, i.e. below and above the direct tunneling limit. Both layers switch between high and low conductance modes (« ON » and « OFF » states) when irradiated by UV and visible light. ON/OFF ratios of 2–3 and 200–400 were obtained for 3 nm- and 9 nm-thick DAE MJs, respectively. Next, we prepared 9 nm-thick MJs using a bi-layer system. The first layer (5 nm) is based on BTB oligomers. The second layer (4 nm) is based on DAE oligomers. The impact of this first layer on the switchable properties of the system, and on the photoresponse of the 9 nm-thick DAE-based MJs has been studied. The DAE/BTB bilayer generates new electronic functions combining photoswitching and photorectification. The open form of DAE/BTB shows low conductance and asymmetric I(V) curves while the closed form shows symmetric I(V) curves and high conductance. More importantly, unprecedented ON/OFF current ratios of over 10 000 at 1 volt were reproducibly measured.

Our article is available to read at ECS Adv.

Viologen-based ditopic bis-pyridinyl-triazole bidentate ligands self-assemble in the presence of palladium ions into supramolecular polymers whose structure is imposed by the directed formation of coordination bonds. Light-irradiation of these electron-responsive supramolecular materials triggers a photo-induced electron transfer yielding isolated π-radicals and dimers of radicals. The photoreduction events and the associated dimerization steps trigger a large-scale reorganization occurring within the supramolecular network yielding aggregates or gels depending on the irradiation conditions (power, duration). Detailed electrochemical, spectro-electrochemical and photochemical analyses were conducted to understand the mechanisms at stakes in these light-induced aggregation and gelation.

Our article is available to read at J. Phys. Chem. B.

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.

Our article is available to read at Nano Lett.

Photoactive molecular junctions, based on 4 nm thick diarylethene (DAE) and 5 nm thick bisthienylbenzene (BTB) layers, were fabricated by electrochemical deposition. Total thickness was around 9 nm, that is, above the direct tunneling limit and in the hopping regime. The DAE units were switched between their open and closed forms. The DAE/BTB bilayer structure exhibits new electronic functions combining photoswitching and photorectification. The open form of DAE/BTB shows low conductance and asymmetric I–V curves while the closed form shows symmetric I–V curves and high conductance. More importantly, unprecedented ON/OFF current ratios of over 10 000 at 1 V were reproducibly measured.

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”.