Optically Reconfigurable Monolayer of Azobenzene Donor Molecules on Oxide Surfaces

Quantifying Hydrogen-Bonding Interactions in the Self-Assembly of Photoresponsive Azobenzene Amphiphiles at the Air-Water Interface

Amphiphilic azobenzene molecules offer ample scope to design functional supramolecular systems in an aqueous medium that can be controlled by light. Despite their widespread applications in photopharmacology and optoelectronics, the self-assembly pathways and energy landscapes of these systems are not well understood. Here, we report combined molecular dynamics (MD) simulation and surface manometry studies on a specially designed alkylated, meta-substituted azobenzene derivative to quantify the hydrogen-bonding interactions in the self-assembled monolayers of its photoisomers. The z-density profile, radial distribution function, order parameters, and hydrogen bond analyzed using MD simulations corroborated the experimental observations of changes in surface pressure, dipole moment, and thickness of the monolayers. Even a small change in the number of hydrogen bonds in the molecule-molecule and molecule-water interactions causes significant changes in the monolayer properties. These results are fundamentally important for engineering photoresponsive molecules with tailored properties for applications in targeted drug delivery and other industrial applications.

Azobenzene-Based Solar Thermal Fuels: A Review

The energy storage mechanism of azobenzene is based on the transformation of molecular cis and trans isomerization, while NBD/QC, DHA/VHF, and fulvalene dimetal complexes realize the energy storage function by changing the molecular structure. Acting as "molecular batteries," they can exhibit excellent charging and discharging behavior by converting between trans and cis isomers or changing molecular structure upon absorption of ultraviolet light. Key properties determining the performance of STFs are stored energy, energy density, half-life, and solar energy conversion efficiency. This review is aiming to provide a comprehensive and authoritative overview on the recent advancements of azobenzene molecular photoswitch system in STFs fields, including derivatives and carbon nano-templates, which is emphasized for its attractive performance. Although the energy storage performance of Azo-STFs has already reached the level of commercial lithium batteries, the cycling capability and controllable release of energy still need to be further explored. For this, some potential solutions to the cycle performance are proposed, and the methods of azobenzene controllable energy release are summarized. Moreover, energy stored by STFs can be released in the form of mechanical energy, which in turn can also promote the release of thermal energy from STFs, implying that there could be a relationship between mechanical and thermal energy in Azo-STFs, providing a potential direction for further research on Azo-STFs.

Recent Progresses in Nanometer Scale Analysis of Buried Layers and Interfaces in Thin Films by X-rays and Neutrons

In the early 1960s, scientists achieved the breakthroughs in the fields of solid surfaces and artificial layered structures. The advancement of surface science has been supported by the advent of ultra-high vacuum technologies, newly discovered and established scanning probe microscopy with atomic resolution, as well as some other advanced surface-sensitive spectroscopy and microscopy. On the other hand, it has been well recognized that a number of functions are related to the structures of the interfaces, which are the thin planes connecting different materials, most likely by layering thin films. Despite the scientific significance, so far, research on such buried layers and interfaces has been limited, because the probing depth of almost all existing sophisticated analytical methods is limited to the top surface. The present article describes the recent progress in the nanometer scale analysis of buried layers and interfaces, particularly by using X-rays and neutrons. The methods are essentially promising to non-destructively probe such buried structures in thin films. The latest scientific research has been reviewed, and includes applications to bio-chemical, organic, electronic, magnetic, spintronic, self-organizing and complicated systems as well as buried liquid-liquid and solid-liquid interfaces. Some emerging analytical techniques and instruments, which provide new attractive features such as imaging and real time analysis, are also discussed.

Photoisomerizable Guanosine Derivative as a Probe for DNA Base-Pairing in Langmuir Monolayers

Mixtures of azo-functionalized amphiphilic derivatives of guanosine and of amphiphilic derivatives of other DNA nucleobases were deposited at an air-water interface and repeatedly irradiated with light of 340 and 440 nm wavelengths. The consequent switching between cis and trans configurations of the azobenzene moiety caused changes in the surface pressure of the film, which were analyzed using a model based on the two-dimensional Van der Waals equation of state. For mixed films of guanosine and cytidine derivatives, the analysis revealed a significant modification of the strength of intermolecular interaction caused by the optical irradiation, while no such modifications were identified in mixed films involving other nucleobases. The difference is attributed to light-induced breaking of the hydrogen bonding that is established only between specific nucleobases. The results demonstrate that photosensitive nucleoside derivatives can be used as an efficient probe for base-pairing in Langmuir monolayers.

Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer

Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved X-ray reflectivity studies show that the reconfiguration of a densely packed rhenium-azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

Unit Cell Level Thickness Control of Single-Crystalline Zinc Oxide Nanosheets Enabled by Electrical Double-Layer Confinement

Ionic layer epitaxy (ILE) has recently been developed as an effective strategy to synthesize nanometer thick 2D materials with a nonlayered crystal structure, such as ZnO. The packing density of the amphiphilic monolayer is believed to be a key parameter that controls the nanosheet nucleation and growth. In this work, we systematically investigated the growth behavior of single-crystalline ZnO nanosheets templated at the water-air interface by an anionic oleylsulfate monolayer with different packing densities. The thicknesses of ZnO nanosheets were tuned from one unit cell to four unit cells and exhibited good correlation with the width of Zn²⁺ ion concentration zone (the Stern layer) underneath the ionized surfactant monolayer. Further analysis of the nanosheet sizes and density revealed that the nanosheet growth was dominated by the steric hindrance from the surfactant monolayer at lower surface pressure, while the nucleation density became the dominating factor at higher surface pressure. The ZnO nanosheets exhibited a decreasing work function as the thickness reduced to a few unit cells. This research validated a critical hypothesis that the nanosheet growth is self-limited by the formation of a double layer of ionic precursors. This work will open up a new way toward controlled synthesis of novel 2D nanosheets from nonlayered materials with a thickness down to one unit cell.

Photo-switching of a non-ionic azobenzene amphiphile in Langmuir and Langmuir–Blodgett films

The concept of programmable and reconfigurable soft matter has emerged in science in the last few decades and can be realized by photoisomerization of azobenzene derivatives.