Kinetics, Thermodynamics, and Dynamics in Organosilane Self-Assembly

Organosilane self-assembly is a widely studied template-free approach to design organic–inorganic hybrids structured at the nanometer scale. The main emphasis has been focused so far on novel precursor architectures and sol–gel preparation methods to drive the self-assembly. This feature attempts for the first time a thermodynamic, kinetic, and dynamic description of the organosilane supramolecular assembly. Condensation and hydrolysis rates are the main kinetic parameters impacting the self-assembly, while organic moiety, alkoxy head, temperature, or relative humidity determine essentially the energetic contributions of the self-association, and therefore, form part of a thermodynamic description. In terms of dynamics, the gradual conversion of the isotropic precursor into a cross-linked hybrid nanostructure was assessed by time-resolved infrared spectroscopy combined with small-angle X-ray scattering. To reveal the mechanism of self-assembly, our system is simplified to the main ingredients: <i>n</i>-dodecyltrimethoxysilane (C₁₂H₂₅Si­(OCH₃)₃) as a model organosilane building block and a photoacid generator ((C₁₂H₂₅)₂Φ₂I⁺ SbF₆^–), deposited as a photolatent micrometric film. UV light governs the sol–gel polymerization kinetics through the controlled liberation of Brönsted superacids

Water-Catalyzed Low-Temperature Transformation from Amorphous to Semi-Crystalline Phase of Ordered Mesoporous Titania Framework

In this paper, the phase transformation in water at low temperature from amorphous TiO₂ to semi-crystalline anatase is reported. Our approach is an environmentally friendly low energy consumer process that requires no specific devices or instrumentation. The phase transition occurs even at room temperature. However, the higher the temperature is, the better the crystallinity is. Crystallization of amorphous titania occurs through a rearrangement of the TiO₆^2– octahedral units in amorphous TiO₂. The phase transformation is catalyzed by water, which adsorbs on the titania surface to form bridges between the surface OH groups of different octaedra. The obtained titania samples have been used for the photodegradation of methyl orange. Because of the formation of anatase, mesoporous TiO₂ exhibits a photocatalytic activity after treatment in water. However, the activity is lower than that of the standard photocatalysts because the TiO₂ treated during 1 h in water at 120 °C has degraded 85% of methyl orange within 240 min compared to 45 min for P25

Block Copolymer Self-Assembly in Mesostructured Silica Films Revealed by Real-Time FTIR and Solid-State NMR

Over the past ten years, understanding the self-assembly process within mesostructured silica films has been a major concern. Our characterization approach relies on two powerful and complementary techniques: in situ time-resolved FTIR spectroscopy and ex situ solid-state NMR. As model systems, three silica/surfactant films displaying various degrees of mesostructuration were synthesized using an amphiphilic block copolymer (PEO-<i>b</i>-PPO-<i>b</i>-PEO) via a UV light induced self-assembly process. The key idea is that the hydration state of the hydrophobic PPO chain is expected to be different depending upon whether the sample is amorphous (blend) or mesostructured (segregated). With real-time FTIR experiments, we show that the methyl deformation mode can act as a signature for the PPO microenvironment so as to trace the progressive copolymer self-association throughout the irradiation time. In ¹H solid-state NMR, the dependence of the ¹H chemical shift on the PPO hydration state has been exploited to evidence the extent of mesostructuration

Periodic Mesostructured Silica Films Made Simple Using UV Light

Recent research has shown that silica/surfactant self-assembly combined with photoacid-catalyzed sol–gel polymerization can direct the formation of disordered mesoporous silica films. Using only a mixture of photoacid generator (PAG), amphiphilic surfactant (PEO-<i>b</i>-PPO-<i>b</i>-PEO), and methoxy oligomeric precursor, this original UV-driven method is fast and obviates the need of sol preparation and volatile compounds. Here, we report a rational basis to promote a disorder-to-order transition through the control of relative humidity (RH) and irradiance. Above a threshold RH of 35%, a long-range organization is promoted by greater interactions between the water-enriched silica phase and the PEO block. Alternatively, mesophase rearrangement and ordering are achieved by decreasing the irradiance below a limiting value (150 mW/cm²) so as to minimize the condensation rate. These two strategies show that the fundamental driving force for the creation of well-ordered hybrid mesostructures can be described both on thermodynamic and kinetic grounds. Subsequently, these optimized reaction conditions are exploited to tailor the final mesophase (hexagonal, lamellar, cubic) at different template/inorganic ratios. In a last part, the photoinduced mesostructuration mechanism is elucidated for model lamellar films using X-ray diffraction and Fourier transform infrared spectroscopy

Structural Effects in the Indanedione Skeleton for the Design of Low Intensity 300–500 nm Light Sensitive Initiators.

Newly synthesized indanedione derivatives combined with an iodonium salt, <i>N</i>-vinylcarbazole, amine, phenacyl bromide, or 2,4,6-tris­(trichloromethyl)-1,3,5-triazine have been used as photoinitiating systems upon very low visible light intensities: blue lights (e.g., household blue LED bulb at 462 nm) or even a halogen lamp exposure. One of them (ID2) is particularly efficient for cationic, radical and thiol–ene photopolymerizations as well as for the synthesis of interpenetrated polymer networks (IPNs). It can be useful to overcome the oxygen inhibition. ID2 based photoinitiating systems can also be selected for the reduction of Ag⁺ and the in situ formation of Ag(0) nanoparticles in the synthesized polymers. The (photo)­chemical mechanisms are studied by electron spin resonance spin trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques

Comparative Study of SWCNT Fluorination by Atomic and Molecular Fluorine

Single-wall carbon nanotubes (SWCNTs) are fluorinated around 200 °C with molecular fluorine (F₂) and xenon difluoride (XeF₂) as fluorination agents. In this latter case, fluorination is carried out by atomic fluorine F^• generated by the thermal decomposition of gaseous XeF₂ on the nanotube surface. XeF₂ treatment results in stoichiometries from CF_0.05 to CF_0.32, and F₂ treatment gives compositions in the range CF_0.04 and CF_0.37. Transmission electronic microscopy (TEM), solid state Nuclear Magnetic Resonance (NMR), Raman scattering and Optical Absorption (AO) studies demonstrate that different fluorination mechanisms occur using molecular fluorine (F₂) and atomic fluorine (F^•). Atomic fluorine results in less sample damage and a more homogeneous fluorine distribution over the SWCNT surface than F₂. This is explained via DFT calculations showing that HF catalyzed F₂ deposition necessarily leads to highly fluorinated domain formation whereas F^• addition occurs spontaneously at the initial species arrival site

Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface

Single crystals of thiophene–phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas–liquid interface by using solvent–antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′-diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm², which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry <i>Cc</i> for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent–antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas–solution interface are discussed