Abnormal Enhancement of the Photoisomerization Process in a <i>trans</i>-Nitroalkoxystilbene Dimer Sequestered in β‑Cyclodextrin Cavities

We report on the synthesis and the photophysical properties of a <i>trans</i>-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φ_f), the Stokes shift, and the quantum yield for the <i>trans</i>-to-<i>cis</i> photoisomerization (Φ_<i>t→c</i>) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φ_f increases together with the decrease of Φ_<i>t→c</i>. This solvent-induced reverse behavior mainly stems from the progressive stabilization of a highly polar twisted internal charge transfer state (TICT) at excited singlet level which opens a competing channel to photoisomerization. In the presence of hydroxylic substrates (i.e., alcohols or water), fluorescence of DPNS is strongly quenched due to a hydrogen bonding interaction at excited state. The efficiency of the process is clearly correlated to the H-bond donor ability of the quencher. In aqueous solution, the major formation of a 2:1 host–guest complex with β-cyclodextrins (β-CD) prevents the quenching by H₂O and leads to a 50-fold increase of the fluorescence signal together with a strong band blue-shift with respect to that of the free chromophore. This latter effect was rationalized in terms of a severe reduction of the solvent-induced stabilization of the TICT state. As a consequence, the <i>trans</i>-to-<i>cis</i> photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φ_<i>t→c</i> even though DPNS is sequestered in β-CD cavities

A Tris(triphenylphosphine)ruthenium(II) Complex as a UV Photoinitiator for Free-Radical Polymerization and <i>in Situ</i> Silver Nanoparticle Formation in Cationic Films

The characterization and the photochemical investigation of a Ru^II complex (Ru­(PPh₃)₃Cl₂) having phosphine ligands are reported. DFT calculations and ESR spin trapping experiments revealed for the first time that the photodecomposition of the complex is governed by a homolytic cleavage of the P–(C₆H₅) bond generating phenyl radicals Ph^• which are able to initiate the free radical polymerization of acrylate monomers. The addition of a H-donor HD plays a key role in the cationic photopolymerization of epoxides: (i) the reaction efficiency is enhanced using [Ru­(PPh₃)₃Cl₂]/HD and (ii) the <i>in situ</i> formation of Ag nanoparticles is observed in the presence of Ru­(PPh₃)₃Cl₂/HD/AgSbF₆ according to the following reactions: Ph^•/DH hydrogen abstraction yielding a D^• radical and oxidation of D^• by the silver salt

Enhancement of Two-Photon Initiating Efficiency of a 4,4′-Diaminostyryl-2,2′-bipyridine Derivative Promoted by Complexation with Silver Ions

We report on the two-photon-induced polymerization (TPIP) ability of a new class of free radical two-photon initiator based on a cationic silver­(I) complex incorporating 4,4′-diaminostyryl-2,2′-bipyridine (DES) derivatives as ligands. Coordination with Ag⁺ induces a strong increase of the charge transfer character at excited state, which enhances the two-photon absorption properties of the complex with respect to that of the free ligand. A comparative analysis of the photophysical properties of DES and DES₂Ag⁺ shows that the presence of silver cation increases the efficiency for the generation of radical cations (DES^•+), which can be used as hydrogen abstractor in free radical photopolymerization. We show that the addition of an aliphatic amine used as hydrogen donor also opens a parallel route for the regeneration of DES. The improvement of the two-photon polymerization efficiency is then evidenced by the fabrication of microstructures. We finally demonstrate that the use of DES₂Ag⁺ as two-photon initiator offers new opportunities for the fabrication of functional nanostructures composed of metal–polymer nanocomposites

STM Studies of Self-Assembled Tetrathiafulvalene (TTF) Derivatives on Graphene: Influence of the Mode of Deposition

The conformations and the self-assembly process of tetrathiafulvalene (TTF) derivatives functionalized by lateral alkylthio chains deposited on graphene/SiC(0001) in ultrahigh vacuum (UHV) and at the solid–liquid interface are studied by scanning tunneling microscopy (STM). The study in UHV evidences a “molecular fastener” effect induced by the increase of van der Waals interactions between the alkylthio side chains which forces the major part of the molecules to self-organize in π–π stacked edge-on conformation. The study at the solid–liquid interface reveals a drastically different behavior with molecules lying flat on the surface as the solvent is involved in the stabilization of the molecular layer. This work raises a burning issue concerning the choice of the deposition method for graphene functionalization with such molecules

Paprika, Gallic Acid, and Visible Light: The Green Combination for the Synthesis of Biocide Coatings

A performing photoinitiating system based on paprika spice was developed (i) to efficiently initiate, according to a green photoinduced process, the cationic polymerization of a biosourced and renewable monomer, e.g., gallic acid, and (ii) to synthesize environmentally friendly antibacterial coatings in a reduced time. A decrease of 100% of the adhered bacteria was demonstrated upon visible light illumination without any remaining live bacteria

Biomimetic Cryptic Site Surfaces for Reversible Chemo- and Cyto-Mechanoresponsive Substrates

Chemo-mechanotransduction, the way by which mechanical forces are transformed into chemical signals, plays a fundamental role in many biological processes. The first step of mechanotransduction often relies on exposure, under stretching, of cryptic sites buried in adhesion proteins. Likewise, here we report the first example of synthetic surfaces allowing for specific and fully reversible adhesion of proteins or cells promoted by mechanical action. Silicone sheets are first plasma treated and then functionalized by grafting sequentially under stretching poly(ethylene glycol) (PEG) chains and biotin or arginine-glycine-aspartic acid (RGD) peptides. At unstretched position, these ligands are not accessible for their receptors. Under a mechanical deformation, the surface becomes specifically interactive to streptavidin, biotin antibodies, or adherent for cells, the interactions both for proteins and cells being fully reversible by stretching/unstretching, revealing a reversible exposure process of the ligands. By varying the degree of stretching, the amount of interacting proteins can be varied continuously

Enhancement of Acid Photogeneration Through a Para-to-Meta Substitution Strategy in a Sulfonium-Based Alkoxystilbene Designed for Two-Photon Polymerization

This contribution reports on the synthesis and the photochemical behavior of two new sulfonium-based photoacid generators (PAGs). We demonstrate that a para-to-meta substitution of a methyl (<i>p</i>-cyanobenzyl) sulfonium group in a 4-alkoxystilbene core strongly influences the photodissociation efficiency of the PAGs and leads to an increase of the quantum yield for acid generation by a factor 2.4. This substantial effect, which was also corroborated by a reactivity enhancement in cationic photopolymerization, is assigned to the modulation of the electronic interaction between two low lying excited states whose energy gap is strongly influenced by this substitution effect. Moreover, it was found that the position of the sulfonium moiety hardly affects the two-photon absorption properties of these push–pull chromophores. By the two-photon fabrication of microstructures, we finally show the potential use of the meta derivative as cationic two-photon initiator