X-ray induced ultrafast charge transfer in thiophene-based conjugated polymers controlled by core-hole clock spectroscopy.

We explore ultrafast charge transfer (CT) resonantly induced by hard X-ray radiation in organic thiophene-based polymers at the sulfur K-edge. A combination of core-hole clock spectroscopy with real-time propagation time-dependent density functional theory simulations gives an insight into the electron dynamics underlying the CT process. Our method provides control over CT by a selective excitation of a specific resonance in the sulfur atom with monochromatic X-ray radiation. Our combined experimental and theoretical investigation establishes that the dominant mechanism of CT in polymer powders and films consists of electron delocalisation along the polymer chain occurring on the low-femtosecond time scale.

Highly Reactive Diazenyl Radical Species Evidenced during Aryldiazonium Electroreduction.

We report the experimental reassessment of the widely admitted concerted reduction mechanism for diazonium electroreduction. Ultrafast cyclic voltammetry was exploited to demonstrate the existence of a stepwise pathway, and real-time spectroelectrochemistry experiments allowed visualization of the spectral signature of an evolution product of the phenyldiazenyl radical intermediate. Unambiguous identification of the diazenyl species was achieved by radical trapping followed by X-ray structure resolution. The electrochemical generation of this transient under intermediate energetic conditions calls into question our comprehension of the layer structuration when surface modification is achieved via the diazonium electrografting technique as this azo-containing intermediate could be responsible for the systematic presence of azo bridges in nanometric films.

Enlightening the Alkali Ion Role in the Photomagnetic Effect of FeCo Prussian Blue Analogues.

FeCo Prussian blue analogues of general formula AxCoy[Fe(CN)6]z are responsive, non-stoichiometric materials whose magnetic and optical properties can be reversibly switched by light irradiation. However, elucidating the critical influence of the inserted alkali ion, A+, on the material's properties remains complicated due to their complex local structure. Here, by investigating soluble A ⊂ [Fe4-Co4] cyanido cubes (A = K, Rb, and Cs), both accurate structural and electronic information could be obtained. First, X-ray diffraction analyses reveal distinct interactions between the inserted A+ ions and the {Fe4-Co4} box, which impacts the structural distortion in the cubic framework. These distortions vanish, and a displacement of the small K+ ion from a corner toward the center is observed, as a cobalt corner CoIIHS is oxidized to CoIIILS. Second, cyclic voltammetry experiments performed at variable temperatures show distinct splitting of the CoIIHS ⇔ CoIIILS peak potentials for the different A+ cations, which can be qualitatively linked to different thermodynamic (standard potentials) and kinetic (energy barriers) parameters associated with the structural reorganization accompanying this redox-coupled spin state change. Moreover, for the first time, photomagnetism was investigated in frozen solution to avoid effects of intermolecular interactions. The results show that the metastable state is stabilized following the trend K > Rb > Cs. The outcome of these studies suggests that the interaction of the inserted alkali ions with the cyanide cage and the structural changes accompanying the electron transfer impact the stability of the photoinduced state and the relaxation temperature: the smaller the cation, the higher the structural reorganization and the associated energy barrier, and the more stable the metastable state.

Switchable Hydrogel-Gated Organic Field-Effect Transistors.

Stimuli-responsive hydrogels represent a class of materials capable of reversibly switching their morphological and physicochemical characteristics. An ultrathin poly(acrylic acid) film (ca. 6 nm) grafted onto the gate of a p-type EGOFET is studied, and the correlation between the swelling state of the hydrogel and the transistor output characteristics is presented. The hydrogel-related swelling process occurring in basic medium causes an increase in threshold voltage due to the abrupt and intense increase of the negative charge density on the gate electrode. The variation of the drain current during the in situ modification of the pH electrolyte allows a quantitative analysis of the hydrogel switching kinetics. This work shows not only the relevance of EGOFET as an analytical tool in the broad sense, i.e., able to follow in real time phase transition processes of stimuli-responsive materials, but also the relevance of using a hydrogel for field-effect-based (bio)detection according to the ability of such material to overcome the well-known Debye length problematics.

Localized reduction of graphene oxide by electrogenerated naphthalene radical anions and subsequent diazonium electrografting.

Herein, we describe a new localized functionalization method of graphene oxide (GO) deposited on a silicon oxide surface. The functionalization starts with the reduction of GO by electrogenerated naphthalene radical anions. The source of reducers is a microelectrode moving close to the substrate in a typical scanning electrochemical microscopy (SECM) configuration. Then, the recovery of electronic conductivity upon reduction enables the selective electrochemical functionalization of the patterns. The illustrative example is the electrografting of reduced-GO with a diazonium salt bearing a protonated amino group that can further immobilize gold nanoparticles by simple immersion. This study opens new routes for the construction of multifunctional patterned surfaces.

Synthesis of π-conjugated 2,2:6',2"-terpyridine-substituted oligomers based on 3,4-ethylenedioxythiophene.

Dissymmetric π-conjugated monomers and oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units and bearing terpyridine end groups were synthesized in good yields through Vilsmeyer-Haak formylation followed by a reaction with 2-acetylpyridine in basic media or, for the longest oligomers, direct C-H bond arylation. They have a low HOMO-LUMO gap and are easily oxidized at low potentials. Upon complexation with cobalt(II) and iron(II) they yield new hybrid materials that can be used in various applications ranging from photovoltaics to spintronics.

Thermo-induced electromagnetic coupling in gold/polymer hybrid plasmonic structures probed by surface-enhanced raman scattering.

This paper describes a general stepwise strategy combining diazonium salt, surface-initiated atom transfer radical polymerization (SI-ATRP), and click chemistry for an efficient gold surface functionalization by poly(N-isopropylacrylamide) (PNIPAM) brushes and gold nanoparticle assemblies. We designed by this way a new plasmonic device made of gold nanoparticles separated from a gold film through a thermoresponsive polymer layer. This organic layer responds to temperature variations by conformational changes (with a characteristic temperature called the lower critical solution temperature, LCST) and is therefore able to vary the distance between the gold nanoparticles and the gold film. The optical properties of these stimulable substrates were probed by surface-enhanced raman scattering (SERS) using methylene blue (MB) as a molecular probe. We show that an increase of the external temperature reversibly induces a significant enhancement of the MB SERS signal. This was attributed to a stronger interaction between the gold nanoparticles and the gold substrate. The temperature-responsive plasmonic devices developed in this paper thus provide a dynamic SERS platform, with thermally switchable electromagnetic coupling between the gold nanoparticles and the gold surface.

A general approach combining diazonium salts and click chemistries for gold surface functionalization by nanoparticle assemblies.

This paper describes a general stepwise strategy combining diazonium salt and click chemistries for an efficient gold surface functionalization by gold nanoparticles. The procedure first involves the strong covalent bonding to gold electrodes of OH-terminated aryl layers derived from the electroreduction of the parent diazonium salts. The following step consists in transforming the OH end-groups to azides in order to obtain "clickable"-active gold surfaces, which could further be used as versatile platforms for the subsequent grafting of acetylene-bearing molecules. The practical interest of the gold surfaces functionalized by this stepwise strategy was evidenced through the self-assembly of surface-enhanced Raman scattering (SERS)-active gold nanoparticles. SERS activity was shown to be amplified by the presence of a very strong local electric field confinement between the particles and the gold surface.