Effect of Positional Substitution on the Optical Response of Symmetrically Disubstituted Azobenzene Derivatives

Combined Liposome-Gold Nanoparticles from Honey: The Catalytic Effect of Cassyopea® Gold on the Thermal Isomerization of a Resonance-Activated Azobenzene

Gold nanoparticles (AuNPs) have been synthesized directly inside liposomes using honey as a reducing agent. The obtained aggregates, named Cassyopea® Gold due to the method used for their preparation, show remarkable properties as reactors and carriers of the investigated AuNPs. A mean size of about 150 nm and negative surface charge of -46 mV were measured for Cassyopea® Gold through dynamic light scattering and zeta potential measurements, respectively. The formation of the investigated gold nanoparticles into Cassyopea® liposomes was spectroscopically confirmed by the presence of their typical absorption band at 516 nm. The catalytic activity of the combined liposome-AuNP nanocomposites was tested via the thermal cis-trans isomerization of resonance-activated 4-methoxyazobenzene (MeO-AB). The kinetic rate constants (kobs) determined at 25 °C in the AuNP aqueous solution and in the Cassyopea® Gold samples were one thousand times higher than the values obtained when performing MeO-AB cis-trans conversion in the presence of pure Cassyopea®. The results reported herein are unprecedented and point to the high versatility of Cassyopea® as a reactor and carrier of metal nanoparticles in chemical, biological, and technological applications.

Voltage-Gated Ion Channels: Structure, Pharmacology and Photopharmacology

Voltage-gated ion channels are transmembrane proteins responsible for the generation and propagation of action potentials in excitable cells. Over the last decade, advancements have enabled the elucidation of crystal structures of ion channels. This progress in structural understanding, particularly in identifying the binding sites of local anesthetics, opens avenues for the design of novel compounds capable of modulating ion conduction. However, many traditional drugs lack selectivity and come with adverse side effects. The emergence of photopharmacology has provided an orthogonal way of controlling the activity of compounds, enabling the regulation of ion conduction with light. In this review, we explore the central pore region of voltage-gated sodium and potassium channels, providing insights from both structural and pharmacological perspectives. We discuss the different binding modes of synthetic compounds that can physically occlude the pore and, therefore, block ion conduction. Moreover, we examine recent advances in the photopharmacology of voltage-gated ion channels, introducing molecular approaches aimed at controlling their activity by using photosensitive drugs.

Cationic Azobenzenes as Light-Responsive Crosslinkers for Alginate-Based Supramolecular Hydrogels

Azobenzene photoswitches are fundamental components in contemporary approaches aimed at light-driven control of intelligent materials. Significant endeavors are directed towards enhancing the light-triggered reactivity of azobenzenes for such applications and obtaining water-soluble molecules able to act as crosslinkers in a hydrogel. Here, we report the rational design and the synthesis of azobenzene/alginate photoresponsive hydrogels endowed with fast reversible sol-gel transition. We started with the synthesis of three cationic azobenzenes (AZOs A, B, and C) and then incorporated them in sodium alginate (SA) to obtain photoresponsive supramolecular hydrogels (SMHGs). The photoresponsive properties of the azobenzenes were investigated by UV-Vis and 1H NMR spectroscopy. Upon irradiation with 365 nm UV light, the azobenzenes demonstrated efficient trans-to-cis isomerization, with complete isomerization occurring within seconds. The return to the trans form took several hours, with AZO C exhibiting the fastest return, possibly due to higher trans isomer stability. In the photoresponsive SMHGs, the minimum gelation concentration (MGC) of azobenzenes was determined for different compositions, indicating that small amounts of azobenzenes could induce gel formation, particularly in 5 wt% SA. Upon exposure to 365 nm UV light, the SMHGs exhibited reversible gel-sol transitions, underscoring their photoresponsive nature. This research offers valuable insights into the synthesis and photoresponsive properties of cationic, water-soluble azobenzenes, as well as their potential application in the development of photoresponsive hydrogels.

Supramolecular Complexation of Azobenzene Dyes by Cucurbit[7]uril

This report describes cucurbit[7]uril (CB7) complexation of azobenzene dyes that have a 4-(N,N'-dimethylamino) or 4-amino substituent. Absorption and NMR data show that CB7 encapsulates the protonated form of the azobenzene and that the complexed dye exists as its azonium tautomer with a trans azo conformation and substantial quinoid resonance character. Because CB7 complexation stabilizes the dye conjugate acid, there is an upward shift in its pKa, and in one specific case, the pKa of the protonated azobenzene is increased from 3.09 to 4.47. Molecular modeling indicates that the CB7/azobenzene complex is stabilized by three major noncovalent factors: (i) ion-dipole interactions between the partially cationic 4-(N,N'-dimethylamino) or 4-amino group on the encapsulated protonated azobenzene and the electronegative carbonyl oxygens on CB7, (ii) inclusion of the upper aryl ring of the azobenzene within the hydrophobic CB7 cavity, and (iii) a hydrogen bond between the proton on the azo nitrogen and CB7 carbonyls. CB7 complexation enhances azobenzene stability and increases azobenzene hydrophilicity; thus, it is a promising way to improve azobenzene performance as a pigment or prodrug. In addition, the striking yellow/pink color change that accompanies CB7 complexation can be exploited to create azobenzene dye displacement assays with naked eye detection.

Theoretical Study on Photothermal Properties of Azobenzene Sulfonate/Magnesium-Aluminum Hydroxide Composite Dye

The assembly of various azo dyes and pigments with inorganic layered materials could develop new types of intercalation materials. The electronic structures and photothermal properties of composite materials (AbS^--LDH) constituted by azobenzene sulfonate anions (AbS^-) and Mg-Al layered double hydroxide (LDH) lamella were theoretically studied at the M06-2X/def2-TZVP//M06-2X/6-31G(d,p) level using density functional theory and time-dependent density functional theory. Meanwhile, the influences of LDH lamella on the AbS^- in AbS^--LDH materials were investigated. The calculated results showed that the addition of LDH lamella could lower the isomerization energy barrier of CAbS^- anions (CAbS^- stands for cis AbS^-). The thermal isomerization mechanisms of AbS^--LDH and AbS were related to the conformational change of the azo group, out-of-plane rotation and in-plane inversion. The LDH lamella could reduce the energy gap of the n → π* and π → π* electronic transition and lead to a red-shift in the absorption spectra. When a polar solvent DMSO was applied, the excitation energy of the AbS^--LDHs was increased, making its photostability stronger than in nonpolar solvent and solvent-free.

Effect of stacking interactions on charge transfer states in photoswitches interacting with ion channels

The activity of ion channels can be reversibly photo-controlled via the binding of molecular photoswitches, often based on an azobenzene scaffold. Those azobenzene derivatives interact with aromatic residues of the protein via stacking interactions. In the present work, the effect of face-to-face and t-shaped stacking interactions on the excited state electronic structure of azobenzene and p-diaminoazobenzene integrated into the NaV1.4 channel is computationally investigated. The formation of a charge transfer state, caused by electron transfer from the protein to the photoswitches, is observed. This state is strongly red shifted when the interaction takes place in a face-to-face orientation and electron donating groups are present on the aromatic ring of the amino acids. The low-energy charge transfer state can interfere with the photoisomerization process after excitation to the bright state by leading to the formation of radical species.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Photo-Electroswitchable Arylaminoazobenzenes

Azobenzenes appended with a redox-active arylamino group (redox auxiliary, RA) are prepared and shown to undergo fast, complete, and catalytic Z→E azo isomerization upon electron loss from the RA unit of the azobenzene. The RA-azo structures can be reversibly (E→Z→E)_n cycled by sequential photo- and electrostimulation. Due to the robust nature of the RA^•+-azo radical cation chain carrying species, initiation of electron transfer (ET) catalysis occurs at low levels (1.0-0.04 mol %) of catalytic loading and is effective even at Z-RA-azo concentrations of 10^-4-10^-5 M, yielding TONs (turnover numbers) of 100-2300 under such dilute conditions. The RA-azo Z→E conversion is demonstrated using chemical oxidation (redox switching), electrochemical oxidation (electro switching), and photochemical oxidation (photoredox switching). The Z→E acceleration is shown to be at least 2 × 10⁹-fold for RA-azo 5. DFT calculations on methyl yellow suggest that a N-centered radical cation of the RA group stabilizes the Z→E N-N twist transition state of the RA^•+-azo, yielding a large reduction in the barrier for RA^•+-azo compared to neutral RA-azo. The RA-azo structure class has nanomechanical features that can be toggled with photo- and electrostimulation, the latter offering a quick switch for complete Z→E conversion.

Fluorescent Azobenzene-Containing Compounds: From Structure to Mechanism

The reversible photoisomerization of azobenzenes has been extensively studied to construct systems with optical responsiveness; however, this process limits the luminescence of these compounds. Recently, there have been many efforts to design and synthesize fluorescent azobenzene compounds, such as inhibition of electron transfer, inducing aggregation, and metal-enhancement, which make the materials ideal for application in fluorescence probes, light-emitting devices, molecular detection, etc. Herein, we review the recently reported progress in the development of various fluorescent azobenzenes and summarize the possible mechanism of their fluorescence emission. The potential applications of these materials are also discussed. Finally, in order to guide research in this field, the existing problems and future development prospects are discussed.

Binding of azobenzene and p-diaminoazobenzene to the human voltage-gated sodium channel Nav1.4

The activity of voltage-gated ion channels can be controlled by the binding of photoswitches inside their internal cavity and subsequent light irradiation. We investigated the binding of azobenzene and p-diaminoazobenzene to the human Na_v1.4 channel in the inactivated state by means of Gaussian accelerated molecular dynamics simulations and free-energy computations. Three stable binding pockets were identified for each of the two photoswitches. In all the cases, the binding is controlled by the balance between the favorable hydrophobic interactions of the ligands with the nonpolar residues of the protein and the unfavorable polar solvation energy. In addition, electrostatic interactions between the ligand and the polar aminoacids are also relevant for p-diaminoazobenzene due to the presence of the amino groups on the benzene moieties. These groups participate in hydrogen bonding in the most favorable binding pocket and in long-range electrostatic interactions in the other pockets. The thermodinamically preferred binding sites found for both photoswitches are close to the selectivity filter of the channel. Therefore, it is very likely that the binding of these ligands will induce alterations in the ion conduction through the channel.

Effect of backbone and end-group regioisomerism on thermomechanical properties of vanillin-based polyurethane networks

Monomer composition, geometry, cross-link density, and cross-link distribution are the primary determinants of material properties in thermosetting networks.

Spectral Tuning and Photoisomerization Efficiency in Push-Pull Azobenzenes: Designing Principles

This work demonstrates how push–pull substitution can induce spectral tuning toward the visible range and improve the photoisomerization efficiency of azobenzene-based photoswitches, making them good candidates for technological and biological applications. The red-shifted bright ππ* state (S₂) behaves like the lower and more productive dark nπ* (S₁) state because less potential energy along the planar bending mode is available to reach higher energy unproductive nπ*/S₀ crossing regions, which are responsible for the lower quantum yield of the parent compound. The stabilization of the bright ππ* state and the consequent increase in isomerization efficiency may be regulated via the strength of push–pull substituents. Finally, the torsional mechanism is recognized here as the unique productive route because structures with bending values attributable to the inversion mechanism were never detected, out of the 280 ππ* time-dependent density functional theory (RASPT2-validated) dynamics simulations.

Activation of Sirtuin 2 Inhibitors Employing Photoswitchable Geometry and Aqueous Solubility

Because isoenzymes of the experimentally and therapeutically extremely relevant sirtuin family show high similarity, addressing the unique selectivity pocket of sirtuin 2 is a promising strategy towards selective inhibitors. An unrelated approach towards selective inhibition of isoenzymes with varied tissue distribution is targeted drug delivery or spatiotemporal activation by photochemical activation. Azologization of two nicotinamide‐mimicking lead structures was undertaken to combine both approaches and yielded a set of 33 azobenzenes and azopyridines that have been evaluated for their photochemical behaviour and bioactivity. For some compounds, inhibitory activity reached the sub‐micromolar range in their thermodynamically favoured E form and could be decreased by photoisomerization to the metastable Z form. Besides, derivatization with long‐chain fatty acids yielded potent sirtuin 2 inhibitors, featuring another intriguing aspect of azo‐based photoswitches. In these compounds, switching to the Z isomer increased aqueous solubility and thereby enhanced biological activity by up to a factor of 21. The biological activity of two compounds was confirmed by hyperacetylation of sirtuin specific histone proteins in a cell‐based activity assay.

Negative phototactic behaviour of crystals on a glass surface

We demonstrate that visible light irradiation can drive negative phototactic behavior of azobenzene crystals, which have an amoeba-like crawling motion.

Emissive Azobenzenes Delivered on a Silver Coordination Polymer

Azobenzene has become a ubiquitous component of functional molecules and polymeric materials because of the light-induced trans → cis isomerization of the diazene group. In contrast, there are very few applications utilizing azobenzene luminescence, since the excitation energy typically dissipates via nonradiative pathways. Inspired by our earlier studies with 2,2'-bis[ N,N'-(2-pyridyl)methyl]diaminoazobenzene (AzoAM oP) and related compounds, we investigated a series of five aminoazobenzene derivatives and their corresponding silver complexes. Four of the aminoazobenzene ligands, which exhibit no emission under ambient conditions, form silver coordination polymers that are luminescent at room temperature. AzoAE pP (2,2'-bis[ N,N'-(4-pyridyl)ethyl]diaminoazobenzene) assembles into a three-dimensional coordination polymer (AgAAE pP) that undergoes a reversible loss of emission upon the addition of metal-coordinating analytes such as pyridine. The switching behavior is consistent with the disassembly and reassembly of the coordination polymer driven by displacement of the aminoazobenzene ligands by coordinating analytes.

Azobenzene-based solar thermal fuels: design, properties, and applications

Development of renewable energy technologies has been a significant area of research amongst scientists with the aim of attaining a sustainable world society. Solar thermal fuels that can capture, convert, store, and release solar energy in the form of heat through reversible photoisomerization of molecular photoswitches such as azobenzene derivatives are currently in the limelight of research. Herein, we provide a state-of-the-art account on the recent advancements in solar thermal fuels based on azobenzene photoswitches. We begin with an overview on the importance of azobenzene-based solar thermal fuels and their fundamentals. Then, we highlight the recent advances in diverse azobenzene materials for solar thermal fuels such as pure azobenzene derivatives, nanocarbon-templated azobenzene, and polymer-templated azobenzene. The basic design concepts of these advanced solar energy storage materials are discussed, and their promising applications are highlighted. We then introduce the recent endeavors in the molecular design of azobenzene derivatives toward efficient solar thermal fuels, and conclude with new perspectives on the future scope, opportunities and challenges. It is expected that continuous pioneering research involving scientists and engineers from diverse technological backgrounds could trigger the rapid advancement of this important interdisciplinary field, which embraces chemistry, physics, engineering, nanoscience, nanotechnology, materials science, polymer science, etc.

Photoactivated Polymeric Bilayer Actuators Fabricated via 3D Printing

4D printing is an emerging additive manufacturing technology that combines the precision of 3D printing with the versatility of smart materials. 4D printed objects can change their shape over time with the application of a stimulus (i.e., heat, light, moisture). Light driven smart materials are attractive because light is wireless, remote, and can induce a rapid shape change. Herein, we present a method for fabricating polymeric bilayer actuators via 3D printing which reversibly change their shape upon exposure to light. The photoactive layer consists of a poly(siloxane) containing pendant azobenzene groups. Two different photoactive polymers were synthesized, and the photomechanical effect displayed by the bilayers was evaluated. These bilayers exhibit rapid actuation with full cycles completed within seconds, and photo generated stresses ranging from 1.03 to 1.70 MPa.

Kinetics and Energetics of Thermal Cis-Trans Isomerization of a Resonance-Activated Azobenzene in BMIM-Based Ionic Liquids for PF₆-/Tf₂N- Comparison

BMIM PF₆ (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf₂N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF₆- and Tf₂N-. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF₆ and in BMIM Tf₂N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF₆- and Tf₂N- anions during the cis-trans isomerization.

Smart nanocomposite hydrogels based on azo crosslinked graphene oxide for oral colon-specific drug delivery

A safe and efficient nanocomposite hydrogel for colon cancer drug delivery was synthesized using pH-sensitive and biocompatible graphene oxide (GO) containing azoaromatic crosslinks as well as poly (vinyl alcohol) (PVA) (GO-N=N-GO/PVA composite hydrogels). Curcumin (CUR), an anti-cancer drug, was encapsulated successfully into the hydrogel through a freezing and thawing process. Fourier transform infrared spectroscopy, scanning electron microscopy and Raman spectroscopy were performed to confirm the formation and morphological properties of the nanocomposite hydrogel. The hydrogels exhibited good swelling properties in a pH-sensitive manner. Drug release studies under conditions mimicking stomach to colon transit have shown that the drug was protected from being released completely into the physiological environment of the stomach and small intestine. In vivo imaging analysis, pharmacokinetics and a distribution of the gastrointestinal tract experiment were systematically studied and evaluated as colon-specific drug delivery systems. All the results demonstrated that GO-N=N-GO/PVA composite hydrogels could protect CUR well while passing through the stomach and small intestine to the proximal colon, and enhance the colon-targeting ability and residence time in the colon site. Therefore, CUR loaded GO-N=N-GO/PVA composite hydrogels might potentially provide a theoretical basis for the treatment of colon cancer with high efficiency and low toxicity.

TD-DFT based fine-tuning of molecular excitation energies using evolutionary algorithms

An evolutionary de novo design method is presented to fine-tune the excitation energies of molecules calculated using time-dependent density functional theory (TD-DFT).

An Optimized Glutamate Receptor Photoswitch with Sensitized Azobenzene Isomerization

A new azobenzene-based photoswitch, 2, has been designed to enable optical control of ionotropic glutamate receptors in neurons via sensitized two-photon excitation with NIR light. In order to develop an efficient and versatile synthetic route for this molecule, a modular strategy is described which relies on the use of a new linear fully protected glutamate derivative stable in basic media. The resulting compound undergoes one-photon trans-cis photoisomerization via two different mechanisms: direct excitation of its azoaromatic unit and irradiation of the pyrene sensitizer, a well-known two-photon sensitive chromophore. Moreover, 2 presents large thermal stability of its cis isomer, in contrast to other two-photon responsive switches relying on the intrinsic nonlinear optical properties of push-pull substituted azobenzenes. As a result, the molecular system developed herein is a very promising candidate for evoking large photoinduced biological responses during the multiphoton operation of neuronal glutamate receptors with NIR light, which require accumulation of the protein-bound cis state of the switch upon repeated illumination.

Nitric Oxide Catalysis of Diazene E/Z Isomerization

Nitric oxide is an efficient catalyst for the cis-trans (E/Z) isomerization of diazenes. We compare the effect of room temperature solutions bearing low concentrations of nitric oxide, nitrogen dioxide, or oxygen on the rate of cis-trans isomerization, CTI, of the alkene bond in stilbene and on the azo double bond in azobenzene, as well as in four azo derivatives as measured by UV-vis spectroscopy. These rate enhancements can be as large as 3 orders of magnitude for azobenzene in solution. A mechanism is proposed where catalysis is promoted by the interaction of the nitric oxide with the diazene nitrogen lone pairs. Density functional theory, B3LYP/6-311++g** suggests that the binding of NO to the diazene should be weak and reversible but that its NO adduct has an E/Z isomerization barrier of 7.5 kcal/mol.

Anion-tunable control of thermal Z→E isomerisation in basic azobenzene receptors

Herein, we report that thermal Z→E isomerisation of simple azobenzene urea derivatives is selectively and predictably controlled by anion binding. The rate of this process depends strictly on the anion concentration and its binding affinity to the Z-isomer of the azobenzene host, i.e. increased rate constants are observed for higher anion concentration as well as for more strongly bound guests. The origin of this phenomenon is attributed to the electron density transfer from the anion to the host π-system, resulting in increased repulsion between the lone electron pairs in the N=N bond.

Near-infrared light activated azo-BF2 switches

Increasing the electron density in BF2-coodinated azo compounds through para-substitution leads to a bathochromic shift in their activation wavelength. When the substituent is dimethyl amine, or the like, the trans/cis isomerization process can be efficiently modulated using near infrared light. The electron donating capability of the substituent also controls the hydrolysis half-life of the switch in aqueous solution, which is drastically longer for the cis isomer, while the BF2-coodination prevents reduction by glutathione.

Hybrid chromophore/template nanostructures: a customizable platform material for solar energy storage and conversion

Challenges with cost, cyclability, and/or low energy density have largely prevented the development of solar thermal fuels, a potentially attractive alternative energy technology based on molecules that can capture and store solar energy as latent heat in a closed cycle. In this paper, we present a set of novel hybrid photoisomer/template solar thermal fuels that can potentially circumvent these challenges. Using first-principles computations, we demonstrate that these fuels, composed of organic photoisomers bound to inexpensive carbon-based templates, can reversibly store solar energy at densities comparable to Li-ion batteries. Furthermore, we show that variation of the template material in combination with the photoisomer can be used to optimize many of the key performance metrics of the fuel-i.e., the energy density, the storage lifetime, the temperature of the output heat, and the efficiency of the solar-to-heat conversion. Our work suggests that the solar thermal fuels concept can be translated into a practical and highly customizable energy storage and conversion technology.

Photoisomerization in different classes of azobenzene

Azobenzene undergoes trans→cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis→trans isomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Luminescence of Eu(DBM)3Phen-doped in azobenzene-containing copolymers--effects of absorption overlapping of two components

Poly(N-isopropylacrylamide)-b-poly[6-[4-(4-methylphenyl-azo)phenoxy]hexylmethacrylate] (PNIPAM(32)-b-PAzoMM(8)) and poly(N-isopropylacrylamide)-b-poly[6-[methyl(4-nitrophenyldiazenyl)phenyl]aminohexylmethacrylate] (PNIPAM(32)-b-PAzoNO(10)) were prepared by reversible addition-fragmentation chain transfer (RAFT) radical polymerization. The optical properties of Eu(DBM)(3)Phen (Eu, Europium; DBM, dibenzoylmethide; Phen, 1,10-phenanthroline)-doped azobenzene-containing copolymer vesicle solutions were studied by UV-vis and fluorescence spectroscopy. When the electronic transition bands of azobenzene and Eu(DBM)(3)Phen overlapped at about 350 nm in PNIPAM(32)-b-PAzoMM(8), the fluorescent emission intensity at 612 nm of Eu(DBM)(3)Phen could be modulated by irradiation with UV and visible light. However, when the absorption of the azobenzene-moiety red-shifted to 477 nm in PNIPAM(32)-b-PAzoNO(10), the luminescence intensity of Eu(DBM)(3)Phen was not affected any longer. The difference between these two systems was possibly caused by the energy allocation in the luminescence process, which was discussed in detail.

pH-Dependent cis --> trans isomerization rates for azobenzene dyes in aqueous solution

Azobenzenes can function as molecular switches driven by their unusual cis <--> trans photoisomerization properties. The stability of an azobenzene-based switch depends on its rate of thermal relaxation, which is known to depend on the solvent environment, but few kinetic studies in aqueous media have been reported. We use nanosecond UV laser flash photolysis-transient absorption spectroscopy to measure thermal cis --> trans isomerization rates for mono- and disubstituted p-aminoazobenzenes and p-hydroxyazobenzenes in water at 23 degrees C over the pH range of 4 to 11. Observed absorption transients are fit to first-order relaxation rate constants between 10(5) and 10(1) s(-1), which is generally much faster than in nonpolar solvents, and the relaxation rates vary systematically and predictably with pH as the equilibrium shifts to ionized forms of the dyes that isomerize much more rapidly. Acid ionization constants for these dyes determined from our kinetic mechanism are compared with the pH dependence of their equilibrium UV-vis spectra. New kinetics results may enable pH control of azobenzene-based molecular switching times.

Synthesis of novel amphiphilic azobenzenes and X-ray scattering studies of their Langmuir monolayers

We report a simple synthetic route to novel symmetrical alkylated and acylated amphiphilic 4,4'-diaminoazobenzene dyes, with their optical axis perpendicular to the amphiphilic direction of the molecule. Three different substitution patterns are reported, two of which are highly amphiphilic. At the air-water interface, the amphiphilic azobenzenes form noncrystalline but stable Langmuir films that display an unusual reversible monolayer collapse close to 35 mN/m. The structures and phase transitions were studied by X-ray reflectivity (XR) and grazing-incidence X-ray diffraction, both utilizing synchrotron radiation. Compression beyond the collapse point does not change the XR data, showing that the film is unchanged at the molecular level, even at areas less than half of that of the collapse. This leads to the conclusion that few macroscopic collapse sites are responsible for reversibly removing large amounts of material from the interface.