Differing Isomerization Kinetics of Azobenzene-Functionalized Self-Assembled Monolayers in Ambient Air and in Vacuum

Meta-Substituted Asymmetric Azobenzenes: Insights into Structure-Property Relationship

This article presents a comprehensive investigation into the functionalization of methoxyphenylazobenzene using electron-directing groups located at the meta position relative to the azo group. Spectroscopic analysis of meta-functionalized azobenzenes reveals that the incorporation of electron-withdrawing units significantly influences the absorption spectra of both E and Z isomers, while electron-donating functionalities lead to more subtle changes. The thermal relaxation process from Z to E result in almost twice as prolonged for electron-withdrawing functionalized azobenzenes compared to their electron-rich counterparts. Computational analysis contributes a theoretical understanding of the electronic structure and properties of meta-substituted azobenzenes. This combined approach, integrating experimental and computational techniques, yields significant insights into the structure-property relationship of meta-substituted asymmetrical phenolazobenzenes.

Characteristics and long-term kinetics of an azobenzene derivative and a donor-acceptor Stenhouse adduct as orthogonal photoswitches

Light-triggered molecular switches are extensively researched for their applications in medicine, chemistry and material science and, if combined, particularly for their use in multifunctional smart materials, for which orthogonally, i.e. individually, addressable photoswitches are needed. In such a multifunctional mixture, the switching properties, efficiencies and the overall performance may be impaired by undesired mutual dependences of the photoswitches on each other. Within this study, we compare the performance of the pure photoswitches, namely an azobenzene derivative (Azo) and a donor-acceptor Stenhouse adduct (DASA), with the switching properties of their mixture using time-resolved temperature-dependent UV/VIS absorption spectroscopy, time-resolved IR absorption spectroscopy at room temperature and quantum mechanical calculations to determine effective cross sections, switching kinetics as well as activation energies of thermally induced steps. We find slightly improved effective cross sections, percentages of switched molecules and no increased activation barriers of the equimolar mixture compared to the single compounds. Thus, the studied mixture Azo + DASA is very promising for future applications in multifunctional smart materials.

Arylazopyrazole-Modified Thiolactone Acrylate Copolymer Brushes for Tuneable and Photoresponsive Wettability of Glass Surfaces

Photoswitches have long been employed in coatings for surfaces and substrates to harness light as a versatile stimulus to induce responsive behavior. We previously demonstrated the viability of arylazopyrazole (AAP) as a photoswitch in self-assembled monolayers (SAMs) on silicon and glass surfaces for photoresponsive wetting applications. We now aim to transfer the excellent photophysical properties of AAPs to polymer brush coatings. Compared to SAMs, polymer brushes offer enhanced stability and an increase of the thickness and density of the functional organic layer. In this work, we present thiolactone acrylate copolymer brushes which can be post-modified with AAP amines as well as hydrophobic acrylates, making use of the unique chemistry of the thiolactones. This strategy enables photoresponsive wetting with a tuneable range of contact angle change on glass substrates. We show the successful synthesis of thiolactone hydroxyethyl acrylate copolymer brushes by means of surface-initiated atom-transfer radical polymerization with the option to either prepare homogeneous brushes or to prepare micrometer-sized brush patterns by microcontact printing. The polymer brushes were analyzed by atomic force microscopy, time-of-flight secondary ion spectrometry, and X-ray photoelectron spectroscopy. Photoresponsive behavior imparted to the brushes by means of post-modification with AAP is monitored by UV/vis spectroscopy, and wetting behavior of homogeneous brushes is measured by static and dynamic contact angle measurements. The brushes show an average change in static contact angle of around 13° between E and Z isomer of the AAP photoswitch for at least five cycles, while the range of contact angle change can be fine-tuned between 53.5°/66.5° (E/Z) and 81.5°/94.8° (E/Z) by post-modification with hydrophobic acrylates.

Dynamic Wetting of Photoresponsive Arylazopyrazole Monolayers is Controlled by the Molecular Kinetics of the Monolayer

Smart surfaces that can change their wettability on demand are interesting for applications such as self-cleaning surfaces or lab-on-a-chip devices. We have synthesized arylazopyrazole (AAP) phosphonic acids as a new class of photoswitchable molecules for functionalization of aluminum oxide surfaces. AAP monolayers were deposited on α-Al₂O₃(0001) and showed reversible E/Z photoswitching that can trigger contact angle changes of up to ∼10°. We monitored these changes on the macroscopic level by recording the contact angle while the monolayer was switched in situ. On the molecular level, time-dependent vibrational sum-frequency generation (SFG) spectroscopy provided information on the kinetic changes within the AAP monolayer and the characteristic times for E/Z switching. In addition, vibrational SFG at different relative humidity indicates that the thermal stability of the Z configuration is largely influenced by the presence of water which can stabilize the Z state and hinder E → Z switching of the AAP monolayer when it is wetted with H₂O. Having established the switching times on the molecular scale, we additionally measured the dynamic contact angle and show that the time scales of the substrate and droplet dynamics can be extracted individually. For that, we report on a relaxation model that is solved analytically and is verified via a comparison with simulations of a Lennard-Jones system and with experimental data. The slower E to Z switching in the presence of the droplet as compared to the vapor phase is rationalized in terms of specific interactions of water with the exposed AAP moieties.

Direct Color Observation of Light-Driven Molecular Conformation-Induced Stress

Although usually complex to handle, nanomechanical sensors are exceptional, label-free tools for monitoring molecular conformational changes, which makes them of paramount importance in understanding biomolecular interactions. Herein, a simple and inexpensive mechanical imaging approach based on low-stiffness cantilevers with structural coloration (mechanochromic cantilevers (MMC)) is demonstrated, able to monitor and quantify molecular conformational changes with similar sensitivity to the classical optical beam detection method of cantilever-based sensors (≈4.6 × 10^-3  N m^-1 ). This high sensitivity is achieved by using a white light and an RGB camera working in the reflection configuration. The sensor performance is demonstrated by monitoring the UV-light induced reversible conformational changes of azobenzene molecules coating. The trans-cis isomerization of the azobenzene molecules induces a deflection of the cantilevers modifying their diffracted color, which returns to the initial state by cis-trans relaxation. Interestingly, the mechanical imaging enables a simultaneous 2D mapping of the response thus enhancing the spatial resolution of the measurements. A tight correlation is found between the color output and the cantilever's deflection and curvature angle (sensitivities of 5 × 10^-3  Hue µm^-1 and 1.5 × 10^-1  Hue (°)^-1 ). These findings highlight the suitability of low-stiffness MMC as an enabling technology for monitoring molecular changes with unprecedented simplicity, high-throughput capability, and functionalities.

Advances and opportunities in the exciting world of azobenzenes

Azobenzenes are archetypal molecules that have a central role in fundamental and applied research. Over the course of almost two centuries, the area of azobenzenes has witnessed great achievements; azobenzenes have evolved from simple dyes to 'little engines' and have become ubiquitous in many aspects of our lives, ranging from textiles, cosmetics, food and medicine to energy and photonics. Despite their long history, azobenzenes continue to arouse academic interest, while being intensively produced for industrial purposes, owing to their rich chemistry, versatile and straightforward design, robust photoswitching process and biodegradability. The development of azobenzenes has stimulated the production of new coloured and light-responsive materials with various applications, and their use continues to expand towards new high-tech applications. In this Review, we highlight the latest achievements in the synthesis of red-light-responsive azobenzenes and the emerging application areas of photopharmacology, photoswitchable adhesives and biodegradable materials for drug delivery. We show how the synthetic versatility and adaptive properties of azobenzenes continue to inspire new research directions, with limits imposed only by one's imagination.

Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation

N-heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field-effect transistors. Here, we investigated the electronic properties of 1,3,8,10-tetraazaperopyrene derivatives adsorbed on Au(111) using a complementary experimental approach, namely, scanning tunneling spectroscopy and two-photon photoemission combined with state-of-the-art density functional theory. We find signatures of weak physisorption of the molecular layers, such as the absence of charge transfer, a nearly unperturbed surface state, and an intact herringbone reconstruction underneath the molecular layer. Interestingly, molecular states in the energy region of the sp- and d-bands of the Au(111) substrate exhibit hole-like dispersive character. We ascribe this band character to hybridization with the delocalized states of the substrate. We suggest that such bands, which leave the molecular frontier orbitals largely unperturbed, are a promising lead for the design of organic-metal interfaces with a low charge injection barrier.

Measurement of the conformational switching of azobenzenes from the macro- to attomolar scale in self-assembled 2D and 3D nanostructures

We compare the cis–trans conformational switching of commercial azobenzene molecules in different chemical environments, ranging from isolated molecules in liquid to attomolar-2D and macro-scale 3D self-assembled structures.

Molecular Photoswitching in Confined Spaces

In nature, light is harvested by photoactive proteins to drive a range of biological processes, including photosynthesis, phototaxis, vision, and ultimately life. Bacteriorhodopsin, for example, is a protein embedded within archaeal cell membranes that binds the chromophore retinal within its hydrophobic pocket. Exposure to light triggers regioselective photoisomerization of the confined retinal, which in turn initiates a cascade of conformational changes within the protein, triggering proton flux against the concentration gradient, providing the microorganisms with the energy to live. We are inspired by these functions in nature to harness light energy using synthetic photoswitches under confinement. Like retinal, synthetic photoswitches require some degree of conformational flexibility to isomerize. In nature, the conformational change associated with retinal isomerization is accommodated by the structural flexibility of the opsin host, yet it results in steric communication between the chromophore and the protein. Similarly, we strive to design systems wherein isomerization of confined photoswitches results in steric communication between a photoswitch and its confining environment. To achieve this aim, a balance must be struck between molecular crowding and conformational freedom under confinement: too much crowding prevents switching, whereas too much freedom resembles switching of isolated molecules in solution, preventing communication.

In this Account, we discuss five classes of synthetic light-switchable compounds—diarylethenes, anthracenes, azobenzenes, spiropyrans, and donor–acceptor Stenhouse adducts—comparing their behaviors under confinement and in solution. The environments employed to confine these photoswitches are diverse, ranging from planar surfaces to nanosized cavities within coordination cages, nanoporous frameworks, and nanoparticle aggregates. The trends that emerge are primarily dependent on the nature of the photoswitch and not on the material used for confinement. In general, we find that photoswitches requiring less conformational freedom for switching are, as expected, more straightforward to isomerize reversibly under confinement. Because these compounds undergo only small structural changes upon isomerization, however, switching does not propagate into communication with their environment. Conversely, photoswitches that require more conformational freedom are more challenging to switch under confinement but also can influence system-wide behavior.

Although we are primarily interested in the effects of geometric constraints on photoswitching under confinement, additional effects inevitably emerge when a compound is removed from solution and placed within a new, more crowded environment. For instance, we have found that compounds that convert to zwitterionic isomers upon light irradiation often experience stabilization of these forms under confinement. This effect results from the mutual stabilization of zwitterions that are brought into close proximity on surfaces or within cavities. Furthermore, photoswitches can experience preorganization under confinement, influencing the selectivity and efficiency of their photoreactions. Because intermolecular interactions arising from confinement cannot be considered independently from the effects of geometric constraints, we describe all confinement effects concurrently throughout this Account.

Optically Triggered Control of the Charge Carrier Density in Chemically Functionalized Graphene Field Effect Transistors

Field effect transistors (FETs) based on 2D materials are of great interest for applications in ultrathin electronic and sensing devices. Here we demonstrate the possibility to add optical switchability to graphene FETs (GFET) by functionalizing the graphene channel with optically switchable azobenzene molecules. The azobenzene molecules were incorporated to the GFET channel by building a van der Waals heterostructure with a carbon nanomembrane (CNM), which is used as a molecular interposer to attach the azobenzene molecules. Under exposure with 365 nm and 455 nm light, azobenzene molecules transition between cis and trans molecular conformations, respectively, resulting in a switching of the molecular dipole moment. Thus, the effective electric field acting on the GFET channel is tuned by optical stimulation and the carrier density is modulated.

Polysilsesquioxane Nanowire Networks as an "Artificial Solvent" for Reversible Operation of Photochromic Molecules

Efficient isomerization of photochromic molecules often requires conformational freedom and is typically not available under solvent-free conditions. Here, we report a general methodology allowing for reversible switching of such molecules on the surfaces of solid materials. Our method is based on dispersing photochromic compounds within polysilsesquioxane nanowire networks (PNNs), which can be fabricated as transparent, highly porous, micrometer-thick layers on various substrates. We found that azobenzene switching within the PNNs proceeded unusually fast compared with the same molecules in liquid solvents. Efficient isomerization of another photochromic system, spiropyran, from a colorless to a colored form was used to create reversible images in PNN-coated glass. The coloration reaction could be induced with sunlight and is of interest for developing "smart" windows.

Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties

The ligand, bis-β-diketone with an azobenzene bridge (4,4'-(4,4,4-trifluoro-1,3-butanedione)azobenzene, H 2 L), was prepared for the synthesis of a series of dinuclear lanthanide complexes with the formula [Ln 2 L 3 (DMSO) 4 ] (Ln = Eu3+, Gd3+, Tb3+, and DMSO = dimethyl sulfoxide). X-ray crystallographic analysis reveals that the three complexes are triple-stranded dinuclear structures formed by three bis-β-diketonate ligands with two lanthanide ions (Ln3+). The trans-to-cis photoisomerization rates of the azobenzene group of the three [Ln 2 L 3 (DMSO) 4 ] complexes in ethanol and acetonitrile solutions are similar to those of the pure H 2 L ligand and other azobenzene-containing mononuclear lanthanide complexes, but the trans-to-cis quantum yields (Φt→c = 10-3) are 1 order of magnitude smaller. The first-order rate constant for the cis-to-trans thermal isomerization at 50 °C of the H 2 L ligand is similar to those of azobenzene derivatives, while those for the [Ln 2 L 3 (DMSO) 4 ] complexes (k iso = 10-4 s-1) are higher than those of the mononuclear azobenzene-containing lanthanide complexes. Furthermore, as the lanthanide ionic radius becomes smaller from Eu3+ to Gd3+ to Tb3+, the thermal isomerization rate constant decreases and the half-life increases. All these results are proposed to arise from the rigidity at both ends of the azo group by coordination to the dinuclear lanthanide ions and the different isomerization mechanisms. These are the first examples of bis-β-diketonate dinuclear lanthanide complexes with an azobenzene bridge and help illustrate the mechanism of azobenzene isomerization.

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 of azobenzene-substituted alkanethiolates on Au(111) substrates in the context of work function variation: the effect of structure and packing density

Photoisomerization of a series of custom-designed, azobenzene-substituted alkanethiolate (AT) self-assembled monolayers (SAMs) on Au(111) substrates was studied in the context of work function variation, using Kelvin probe measurements as a transduction technique. These SAMs featured variable packing density (by ∼14%; due to the odd-even effects) and, as an option, were additionally decorated with the electron donating/withdrawing -CH3 and -CF3 tail group, respectively, which induce additional dipole moments. The efficiency of photoisomerization and the respective extent of work function variation (ΔΦ) were found to be quite low and independent of the packing density in the SAMs, within the given odd-even packing density variation. They could only be increased, up to ca. 40 meV for ΔΦ, by mixing the azobenzene-substituted ATs with shorter "matrix" molecules, which were introduced for a partial release of the sterical constraints. The ΔΦ values for the SAMs decorated with the -CH3 and -CF3 tail groups were found to be lower than those for the monolayers without such a decoration, which correlated well with the theoretical estimates for the change of the dipole moment of the relevant molecules upon the photoisomerization.

Re- and Preconfigurable Multistable Visible Light Responsive Surface Topographies

Light responsive materials that are able to change their shape are becoming increasingly important. However, preconfigurable bistable or even multi-stable visible light responsive coatings have not been reported yet. Such materials will require less energy to actuate and will have a longer lifetime. Here, it is shown that fluorinated azobenzenes can be used to create rewritable and pre-configurable responsive surfaces that show multi-stable topographies. These surface structures can be formed and removed by using low intensity green and blue light, respectively. Multistable preconfigured surface topographies can also be created in the absence of a mask. The method allows for full control over the surface structures as the topographical changes are directly linked to the molecular isomerization processes. Preliminary studies reveal that these light responsive materials are suitable as adaptive biological surfaces.

Reversible Photoswitching and Isomer-Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present herein the study of a three-dimensional shape-persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units that is adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lost their switching ability, different isomers of the upright standing arm of the tetramer were obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface was dependent on the isomeric state-trans or cis-of the upright oriented azobenzene group. Hence, molecular mobility can be modulated by its isomeric state, which suggests that molecular growth processes could be controlled by external stimuli.

Mapping reversible photoswitching of molecular resistance fluctuations during the conformational transformation of azobenzene-terminated molecular switches

We report a direct mapping and analysis of electrical noise in azobenzene-terminated molecular monolayers, revealing reversible photoswitching of the molecular resistance fluctuations in the layers. In this work, a conducting atomic force microscope combined with a homemade spectrum analyzer was used to image electrical current and noise at patterned self-assembled monolayers (SAMs) of azobenzene-terminated molecular wires on a gold substrate. We analyzed the current and noise imaging data to obtain maps of molecular resistances and amount of mean-square fluctuations in the resistances of the regions of trans-azobenzene and a cis/trans-azobenzene mixture. We revealed that the fluctuations in the molecular resistances in the SAMs were enhanced after the trans-to-cis isomerization, while the resistances were reduced. This result could be attributed to enhanced disorders in the molecular arrangements in the cis-SAMs. Furthermore, we observed that the changes in the resistance fluctuations were reversible with respect to repeated trans-to-cis and cis-to-trans isomerizations, indicating that the effects originated from reversible photoswitching of the molecular structures rather than irreversible damages of the molecules. These findings provide valuable insights into the electrical fluctuations in photoswitchable molecules, which could be utilized in further studies on molecular switches and molecular electronics in general.

Photoionization and trans-to-cis isomerization of β-cyclodextrin-encapsulated azobenzene induced by two-color two-laser-pulse excitation

Azobenzene (1) and the complex resulting from the incorporation of 1 with cyclodextrin (1/CD) are attractive for light-driven applications such as micromachining and chemical biology tools. The highly sensitive photoresponse of 1 is crucial for light-driven applications containing both 1 and 1/CD to reach their full potential. In this study, we investigated the photoionization and trans-to-cis isomerization of 1/CD induced by one- and two-color two-laser pulse excitation. Photoionization of 1/CD, which was induced by stepwise two-photon absorption, was observed using laser pulse excitation at 266nm. Additionally, simultaneous irradiation with 266 and 532nm laser pulses increased the trans-to-cis isomerization yield (Υ_t→c) by 27%. It was concluded that the increase in Υ_t→c was caused by the occurrence of trans-to-cis isomerization in the higher-energy singlet state (S_n), which was reached by S₁→S_n transition induced by laser pulse excitation at 532nm. The results of this study are potentially applicable in light-driven applications such as micromachining and chemical biology tools.

Optical differential reflectance spectroscopy for photochromic molecules on solid surfaces

Optical reflectance of thin adsorbates on solid surfaces is able to reveal fundamental changes of molecular properties compared to bulk systems. The detection of very small changes in the optical reflectance required several technical improvements in the past decades. We present an experimental setup that is capable of high-quality measurements of submonolayers and ultrathin layers of photochromic molecules on surfaces as well as quantifying their isomerization kinetics. By using photomultipliers as detectors, an enhancement of the signal-to-noise ratio by a factor of three with a total reduction of light exposure on the sample by at least four orders of magnitude is achieved. The potential of the experimental setup is demonstrated by a characterization of the photoswitching and thermal switching of a spirooxazine derivate on a bismuth surface.

Delocalized versus localized excitations in the photoisomerization of azobenzene-functionalized alkanethiolate SAMs

Self-assembled monolayers of azobenzene-functionalized alkanethiolates form molecular ensembles with preferential orientation and significant excitonic coupling among the azobenzene chromophores. We have studied their optical switching with differential reflectance and two-photon-photoemission spectroscopy tuning the excitation wavelength through the excitonically broadened S₂ absorption band. While the effective isomerization cross-section increases towards shorter wavelengths, the fraction of cis molecules in the photostationary state decreases. We attribute this observation to the absorption of the cis isomer in the SAM. The photoisomerization in the SAM thereby follows the behavior of non-interacting chromophores in solution, despite the formation of H-aggregates. Our study thus reveals that photoswitching occurs via localized excitations while strongly excitonically coupled, delocalized states do not contribute significantly.

Understanding the effects of packing and chemical terminations on the optical excitations of azobenzene-functionalized self-assembled monolayers

In a first-principles study based on many-body perturbation theory, we analyze the optical excitations of azobenzene-functionalized self-assembled monolayers (SAMs) with increasing packing density and different terminations, considering for comparison the corresponding gas-phase molecules and dimers. Intermolecular coupling increases with the density of the chromophores independently of the functional groups. The intense [Formula: see text] resonance that triggers photo-isomerization is present in the spectra of isolated dimers and diluted SAMs, but it is almost completely washed out in tightly packed architectures. Intermolecular coupling is partially inhibited by mixing differently functionalized azobenzene derivatives, in particular when large groups are involved. In this way, the excitation band inducing the photo-isomerization process is partially preserved and the effects of dense packing partly counterbalanced. Our results suggest that a tailored design of azobenzene-functionalized SAMs which optimizes the interplay between the packing density of the chromophores and their termination can lead to significant improvements in the photo-switching efficiency of these systems.

Adsorption and switching properties of nitrospiropyran on Bi(1 1 4)

Spiropyrans are prototype molecular switches, which undergo a reversible photoinduced ring-opening/-closure reaction between the closed three-dimensional spiropyran (SP) and the open, planar merocyanine (MC) form. In solution the SP isomer is the thermodynamically stable form. Using high resolution electron energy loss spectroscopy, we resolve a thermally-activated irreversible ring-opening reaction of nitrospiropyran resulting in the MC form for coverages above one monolayer. Thus, the situation found in solution is reversed for the adsorbed molecules, since the MC form is more stable due to the modified energetics by the presence of the substrate. In addition, illumination with blue light (445 nm) induced also the ring-opening, while the photostimulated back-reaction could not be observed. The photoisomerization is driven by a substrate-mediated process, i.e. a charge transfer from the substrate into molecular states. The situation changes completely in the monolayer regime. Neither a thermally-assisted nor a photoinduced ring-opening reaction has been identified. We ascribe the suppression to sterical effects stabilizing the SP form due to the surface structure of Bi(1 1 4), which consists of straight atomic rows separated by rough valleys.

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.