New tricks and emerging applications from contemporary azobenzene research

The time-resolved photoelectron spectrum of trans-azobenzene and its relationship with the photoisomerization mechanism: A surface hopping simulation with determination of Dyson orbitals

We performed a simulation of the time-resolved photoelectron spectrum (TRPES) of trans-azobenzene after ππ∗ excitation in a mixed quantum-classical framework. The electronic structure of the molecule and of its cation was obtained with a semiempirical multireference configuration interaction approach, the nonadiabatic dynamics was simulated with the surface hopping algorithm, and the TRPES was determined by computing the relevant Dyson orbitals. According to our results, the TRPES obtained with a probe photon energy of 6.0 eV shows two bands, the one at higher energy being due to the S2 electronic state of the molecule and the other one to the S1 state. For both bands, the main contribution to the spectrum originates from the D0 state of the cation. Our simulated TRPES is in very good agreement with the available experimental data, both in terms of the energetic positions of the bands and their lifetimes. Our findings confirm that the wavelength dependence of the photoisomerization quantum yields of trans-azobenzene is not due to a peculiar nonadiabatic decay process, as previously inferred from the experimental TRPES. Rather, the reason for the violation of Kasha's rule is that different regions of the nuclear phase space are explored in S1, whether this state is populated by direct excitation or by the radiationless decay of S2.

Computational Rational Design of Bridgehead Nitrogen Heterocyclic Azobenzene Photoswitches

Azobenzenes are proven to be one of the most successful molecular photoswitches applied across different fields such as organic chemistry, material science, cosmetics, and pharmaceuticals. Such a widespread implementation is possible because of their photochromic properties contingent upon the substitution pattern and aryl-core nature. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatics turned out to be a good strategy to access compounds with improved photoswitching properties, as well as to expand molecular diversity. One of the challenges related to the design of new azobenzene photoswitches is that it often includes the synthesis of large libraries of compounds due to limited methods for prediction of their properties. Herein, we present a computationally driven workflow for the design and synthesis of a novel class of azobenzene photoswitches, heteroaryl azobenzenes with N-bridgehead heterocycles─pyrazolo[1,5-a]pyrimidine and 1,2,4-triazolo[1,5-a]pyrimidine. A small library of heteroaryl photoswitches was synthesized, and their photochemical properties were evaluated. Subsequently, these results were used to validate a computational approach, which included the in silico evaluation of a large library of designed photoswitch candidates leading to the synthesis of a new photoswitch with improved spectral properties, red-shifted λmax values.

Unusual one dimensional cascade effect in the thermal and photo-induced switching of azobenzene derivatives on a graphite surface

Harnessing cooperative switching opens possibilities for engineering the responses of molecular films to external triggers and provides opportunities to control the directionality of switching/reactions and design novel nanostructures. Here, we demonstrate a one dimensional (1D) cascade effect in the thermal- and photo-induced switching of azobenzene derivatives deposited on a graphite surface. Upon thermal- and photo-induction, molecules switch between their geometric states (trans and cis) along a selected lattice within the assembly. We explore the switching at the molecular level using atomic force microscopy (AFM) and scanning tunneling microscopy (STM) and reveal that the 1D cascading effect proceeds along the lattice direction where the inter-molecular interaction is the strongest. Theoretical calculations and experiments reveal a cascading effect of up to 350 molecules for photo-induced and 530 molecules for thermal-induced switching along a given lattice.

Azo-Bridged Dextran: A Photoresponsive Sustainable Material with Photo-Tunable Mechanical Properties

We report on the synthesis, characterization, and properties of dextran polymers, which are covalently bridged/cross-linked by azobenzene moieties. The reversible photoactivity of the azo moiety is retained in the polymers, and the kinetics of the E/Z isomerization depend on the dextran/azobenzene ratio. Together with the simple preparation, our approach provides convenient access to photoresponsive sustainable materials. Moreover, based on the water-soluble polymers, we have prepared photoresponsive hydrogels, which soften upon UV irradiation. Our findings are based on spectroscopy (UV/vis, IR, and NMR/DOSY), size exclusion chromatography, and rheology.

Remarkable Increase in the Rate of Trans-Cis Photoisomerization of Os(II)-Terpyridine Complexes via Oxidation and Reduction

Luminescent homoleptic Os(II)-terpyridine complexes comprising stilbene-appended naphthalene, anthracene, and pyrene motifs are designed in this work, and their photophysical, electrochemical, and photoisomerization behaviors are extensively investigated. All complexes exhibit intense spin-allowed singlet metal-to-ligand charge transfer (1MLCT) bands in the visible (496-500 nm) and weaker spin-forbidden singlet-to-triplet 3MLCT transitions in the 600-700 nm range. They display moderate emission at room temperature with lifetimes in the range of 84.5-112.5 ns. Electrochemical studies reveal a reversible Os2+/Os3+ oxidation couple within 0.93-0.96 V, alongside multiple reversible or quasi-reversible reduction peaks associated with terpyridine units in between -1.10 and -1.85 V. The stilbene motifs facilitate reversible trans-cis photoisomerization under alternative treatment with visible and UV light, enabling the complexes to function as photomolecular switches in the near-infrared domain. Interestingly, a remarkable increase in the rate of photoisomerization has been achieved via oxidation as well as reduction of the complexes, which, in turn, induces multistep switching involving reversible oxidation-reduction and trans-cis isomerization. Computational investigations are also conducted on all three conformations {trans-trans (t-t), trans-cis (t-c), and cis-cis (c-c)} of the complexes to gain insight into their electronic structures and for accurate assignment of their absorption and emission spectral bands.

Photoacids and Photobases: Applications in Functional Dynamic Systems

Brønsted photoacids and photobases are a unique class of molecules that undergo a major change in their pKa values between their ground and excited states, resulting in donating or accepting a proton, respectively, but only after light excitation. This property of photoacids/photobases makes them an attractive tool for light-gating various dynamic processes. Here, we review the use of this property to manipulate functional dynamic systems with light. We discuss how a proton transfer event that can happen upon light excitation from a photoacid to a chemical moiety of a certain system or, vice versa, from the system to a photobase, can result in a shift in the equilibrium of the system, resulting in some dynamicity. We detail various systems, including self-assembly processes of nanostructures, self-propulsion of droplets, catalysis for hydrogen evolution or CO2 capturing, nanotechnological devices based on enzymatic processes, and changes in proton-conducting ionophores and materials. We detail the basic guidelines for using Brønsted photoacids and photobases in a desired system and conclude with the current technological gaps in further using these molecules.

Research on advanced photoresponsive azobenzene hydrogels with push-pull electronic effects: a breakthrough in photoswitchable adhesive technologies

Smart materials that adapt to various stimuli, such as light, hold immense potential across many fields. Photoresponsive molecules like azobenzenes, which undergo E-Z photoisomerization when exposed to light, are particularly valuable for applications in smart coatings, light-controlled adhesives, and photoresists in semiconductors and integrated circuits. Despite advances in using azobenzene moieties for stimuli-responsive adhesives, the role of push-pull electronic effects in regulating reversible adhesion remains largely unexplored. In this study, we investigate for the first time photo-controlled hydrogel adhesives of azobenzene with different push-pull electronic groups. We synthesized the monomers 4-methoxyazobenzene acrylate (ABOMe), azobenzene acrylate (ABH), and 4-nitroazobenzene acrylate (ABNO2), and examined their effects on reversible adhesion properties. By incorporating these azobenzene monomers into acrylamide, dialdehyde-functionalized poly(ethylene glycol), and [3-(methacryloylamino)propyl]-trimethylammonium chloride, we prepared ABOMe, ABH, and ABNO2 ionic hydrogels. Our research findings demonstrate that only the ABOMe ionic hydrogel exhibits reversible adhesion. This is due to its distinct transition state mechanism compared to ABH and ABNO2, which enables efficient E-Z photoisomerization and drives its reversible adhesion properties. Notably, the ABOMe ionic hydrogel reveals an outstanding skin adhesion strength of 360.7 ± 10.1 kPa, surpassing values reported in current literature. This exceptional adhesion is attributed to Schiff base reactions, monopole-quadrupole interactions, π-π interactions, and hydrogen bonding with skin amino acids. Additionally, the ABOMe hydrogel exhibits excellent reversible self-healing capabilities, significantly enhancing its potential for injectable medical applications. This research underscores the importance of integrating multifunctional properties into a single system, opening new possibilities for innovative and durable adhesive materials.

Exciting Novel Polyaspartates: Design, Synthesis, and Photo-Responsive Behavior in Solution and Lyotropic Liquid Crystalline Phase Upon Irradiation with Visible Light

Many polypeptides form stable, helical secondary structures enabling the formation of lyotropic liquid crystalline (LLC) phases. Contrary to the well-studied polyglutamate, their counterparts based on polyaspartates exhibit a much lower helix inversion barrier. Therefore, the helix sense is not solely dictated by the chirality of the amino acid used, but additionally by the nature and conformation of the polymer sidechain. In this work, polymers responsive to irradiation with visible light are designed achieving conformational transitions from helix-to-coil and helix-to-helix. The synthesis and the application as LLC mesogens of several (co-)polyaspartates bearing ortho-fluorinated azobenzene (FAB) as a photochromic group are presented. Many of the obtained polymers undergo changes in their secondary structure upon E-Z-isomerization of the FAB-containing sidechain. Of special interest are copolymers that exhibit photo-responsive helix inversion without loss of their helical secondary structure. These copolymers form stable LLC phases in helicogenic solvents, where the effect of photo-switching on the macroscopic behavior is studied by NMR spectroscopy. Especially, the irradiation of the different LLC phases of the helix inversion polymers displays a change in the LLC order experienced by the solvent. These peculiar properties are promising for future applications as photo-responsive alignment media for structure elucidation in NMR.

A cautionary tale of basic azo photoswitching in dichloromethane finally explained

Many studies of azobenzene photoswitches are carried out in polar aprotic solvents as a first principles characterization of thermal isomerization. Among the most convenient polar aprotic solvents are chlorinated hydrocarbons, such as DCM. However, studies of azobenzene thermal isomerization in such solvents have led to spurious, inconclusive, and irreproducible results, even when scrupulously cleaned and dried, a phenomenon not well understood. We present the results of a comprehensive investigation into the root cause of this problem. We explain how irradiation of an azopyridine photoswitch with UV in DCM acts not just as a trigger for photoisomerization, but protonation of the pyridine moiety through photodecomposition of the solvent. Protonation markedly accelerates the thermal isomerization rate, and DFT calculations demonstrate that the singlet-triplet rotation mechanism assumed for many azo photoswitches is surprisingly abolished. This study implies exploitative advantages of photolytically-generated protons and finally explains the warning against using chlorinated solvent with UV irradiation in isomerization experiments.

Investigation of Electrocatalytic Oxidative Coupling of Arylamines for the Selective Synthesis of Azo Aromatics

An electrochemical oxidative coupling of anilines to selectively provide azo aromatic compounds by using mild reaction conditions has been investigated. Further, the generality and feasibility of the developed protocol were showcased by synthesizing differently substituted azo compounds in moderate to good isolated yields. Additionally, different control experiments were performed to investigate the reaction mechanism. These experiments indicated the formation of hydrazobenzene as an intermediate while ruling out the possibility of nitrobenzene or nitroso-benzene formation during this transformation.

Strategies to control humidity sensitivity of azobenzene isomerisation kinetics in polymer thin films

Azobenzenes are versatile photoswitches that garner interest in applications ranging from photobiology to energy storage. Despite their great potential, transforming azobenzene-based discoveries and proof-of-concept demonstrations from the lab to the market is highly challenging. Herein we give an overview of a journey that started from a discovery of hydroxyazobenzene's humidity sensitive isomerisation kinetics, developed into commercialization efforts of azobenzene-containing thin film sensors for optical monitoring of the relative humidity of air, and arrives to the present work aiming for better design of such sensors by understanding the different factors affecting the humidity sensitivity. Our concept is based on thermal isomerisation kinetics of tautomerizable azobenzenes in polymer matrices which, using pre-defined calibration curves, can be converted to relative humidity at known temperature. We present a small library of tautomerizable azobenzenes exhibiting humidity sensitive isomerisation kinetics in hygroscopic polymer films. We also investigate how water absorption properties of the polymer used, and the isomerisation kinetics are linked and how the azobenzene content in the thin film affects both properties. Based on our findings we propose simple strategies for further development of azobenzene-based optical humidity sensors.

Discovery of Novel Nicotinamide Derivatives by a Two-Step Strategy of Azo-Incorporating and Bioisosteric Replacement

Azobenzene moieties can serve as active fragments in antimicrobials and exert trans/cis conversions of molecules. Herein, a series of novel nicotinamide derivatives (NTMs) were developed by employing a two-step strategy, including azo-incorporating and bioisosteric replacement. Azo-incorporation can conveniently provide compounds that can be easily optically interconverted between trans/cis isomers, enhancing the structural diversity of azo compounds. It is noteworthy that the replacement of the azo bond with a 1,2,4-oxadiazole motif through further bioisosteric replacement led to the discovery of a novel compound, NTM18, which made a breakthrough in preventing rice sheath blight disease. A control effect value of 94.44% against Rhizoctonia solani could be observed on NTM18, while only 11.11% was determined for boscalid at 200 mg·L-1. Further mechanism validations were conducted, and the molecular docking analysis demonstrated that compound NTM18 might have a tight binding with SDH via an extra π-π interaction between the oxadiazole ring and residue of D_Y586. This work sets up a typical case for the united applications of azo-incorporating and bioisosteric replacement in fungicide design, posing an innovative approach in structural diversity-based development of pesticides.

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.

Detour to success: photoswitching via indirect excitation

Photoswitchable molecules that undergo nanoscopic changes upon photoisomerisation can be harnessed to control macroscopic properties such as colour, solubility, shape, and motion of the systems they are incorporated into. These molecules find applications in various fields of chemistry, physics, biology, and materials science. Until recently, research efforts have focused on the design of efficient photoswitches responsive to low-energy (red or near-infrared) irradiation, which however may compromise other molecular properties such as thermal stability and robustness. Indirect isomerisation methods enable photoisomerisation with low-energy photons without altering the photoswitch core, and also open up new avenues in controlling the thermal switching mechanism. In this perspective, we present the state of the art of five indirect excitation methods: two-photon excitation, triplet sensitisation, photon upconversion, photoinduced electron transfer, and indirect thermal methods. Each impacts our understanding of the fundamental physicochemical properties of photochemical switches, and offers unique application prospects in biomedical technologies and beyond.

Cleavable azobenzene linkers for the design of stimuli-responsive materials

The azo linkage (NN) is one of the very few functional groups in organic chemistry that exhibits sensitivity towards thermal, chemical, photochemical, and biological stimuli. Consequently, this property has given rise to a distinct class of responsive materials. For example, thermal sensitivity has led to generation of free radical initiators useful in curing and polymerization applications. Chemically-induced cleavage has aided the development of self-immolative polymers and reactive scaffolds for proteomics applications. Photo-isomerization capability has given rise to photo-responsive systems. Azobenzene cleavage in biologically reducing environments, such as that of the colon, and under tumor hypoxia conditions has led to diagnostic, therapeutic, and delivery materials. Such conditions have also allowed for control over formation (assembly) and disruption (disassembly) of micellar nanoparticles. The aim of this review article is to look beyond the prevalent photosensitivity aspect of the aromatic azo compounds and draw attention to the azo scission reaction as a trigger of the change in the structure and properties of organic materials. Thus, the main discussion begins with the mechanism of the reductive cleavage. Then, its application in the design of molecules that can be activated as drugs and fluorescent sensors, (nano)materials with potential to release active substances, and polymers with side-chain and main-chain self-immolative capacity is discussed. Finally, the status and future challenges in this field are discussed.

Azobenzene-Based Conjugated Polymers: Synthesis, Properties, and Biological Applications

Conjugated polymers (CPs) have been developed quickly as an emerging functional material with applications in optical and electronic devices, owing to their highly electron-delocalized backbones and versatile side groups for facile processibility, high mechanical strength, and environmental stability. CPs exhibit multistimuli responsive behavior and fluorescence quenching properties by incorporating azobenzene functionality into their molecular structures. Over the past few decades, significant progress has been made in developing functional azobenzene-based conjugated polymers (azo-CPs), utilizing diverse molecular design strategies and synthetic pathways. This article comprehensively reviews the rapidly evolving research field of azo-CPs, focusing on the structural characteristics and synthesis methods of general azo-CPs, as well as the applications of charged azo-CPs, specifically azobenzene-based conjugated polyelectrolytes (azo-CPEs). Based on their molecular structures, azo-CPs can be broadly categorized into three primary types: linear CPs with azobenzene incorporated into the side chain, linear CPs with azobenzene integrated into the main chain, and branched CPs containing azobenzene moieties. These systems are promising for biomedical applications in biosensing, bioimaging, targeted protein degradation, and cellular apoptosis.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

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.

The effect of substituent position and solvent on thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes: kinetic, thermodynamic, and spectral approaches

Kinetic and thermodynamic parameters have been investigated for the thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes with alkyl substituents (Me, t-Bu, and Adm) at positions 1 (dyes 2) and 2 (dyes 3) of the tetrazole moiety in two solvents of different polarity, acetonitrile (MeCN) and toluene. The experimental results show crucial dependence of these parameters on a substituent position in the tetrazole moiety and on a solvent. For dyes 2, Eact and ΔH‡ are lower in MeCN than in toluene that results in a high increase in the lifetimes of the Z isomers: from milliseconds in MeCN to minutes in toluene. For dyes 3, the difference in Eact and ΔH‡ in the two solvents is opposite: Eact and ΔH‡ are by more than 20 kJ mol-1 higher in MeCN, nevertheless, the rate constants for 3 in toluene are comparable with those in MeCN at the ambient temperature and the difference in the behavior is determined by the value of negative entropy of activation. Quantum-chemical calculations of the thermal Z‒E isomerization show the possibility of the process to occur via crossing from the S0 to the thermally induced T1 state. The contribution of this path is highest for 3 in toluene. The analysis of the absorption spectra demonstrates that for the E isomers, the n‒π* and π‒π* transitions are within the long-wavelength absorption band and their positions relative each other are opposite in the solvents: the n‒π* transition is blue-shifted relative to the π‒π* transition in MeCN and is red-shifted in toluene.

Photoswitchable Azoheteroarene-Based Chelating Ligands: Light Modulation of Properties, Aqueous Solubility and Catalysis

We report the design and synthesis of eight photoswitchable phenylazopyridine- and phenylazopyrazole-based molecular systems as chelation-type light-controllable ligands. Besides the studies on fundamental photoisomerization behaviour, the ligands were also screened for light-tuneable properties such as photochromism, phase transition, and solubility, especially in the aqueous medium. This investigation demonstrates how the modulation of aqueous solubility can be achieved through photoisomerization and can further be utilized towards controlling the amount of catalytically active Cu(I) species in the aqueous conditions. Through this approach, light control over the catalytic activity was achieved for Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reactions, along with a partial recovery of the catalytically active species.

Photoisomerization of 2-Arylazoimidazoles under Visible Light: Identifying a Predictive Tool to Anticipate and Tune Likely Photoswitching Performance and Cis Half-Life

Azopyrazoles are an emerging class of photoswitches, whereas analogous azoimidazole-based switches are unable to draw much attention because of their short cis half-lives, poor cis-trans photoreversion yields, and toxic ultraviolet (UV) light-assisted isomerization. A series of 24 various aryl-substituted N-methyl-2-arylazoimidazoles were synthesized, and their photoswitching performances and cis-trans isomerization kinetics were thoroughly investigated experimentally and theoretically. Para-π-donor-substituted azoimidazoles with highly twisted T-shaped cis conformations showed nearly complete bidirectional photoswitching, whereas di-o-substituted switches exhibited very long cis half-lives (days-years) with nearly ideal T-shaped conformations. This study demonstrates how the electron density in the aryl ring affects cis half-life and cis-trans photoreversion via twisting of the NNAr dihedral angle that can be used as a predictive metric for envisaging and tuning the likely switching performance and half-life of any given 2-arylazoimidazole. By applying this tool, two better-performing azoimidazole photoswitches were engineered. All switches permitted irradiation by violet (400-405 nm) and orange (>585 nm) light for forward and reverse isomerization, respectively, and displayed comparatively high quantum yields and impressive resistance to photobleaching.

Red-light photoswitching of indigos in polymer thin films

Through simple synthetic derivatisation, the parent indigo dye becomes a red-light E-Z photoswitch exhibiting negative photochromism and tuneable thermal isomerisation kinetics. These attributes make indigo derivatives extremely attractive for applications related to materials and living systems. However, there is a lack of knowledge in translating indigo photoswitching dynamics from solution to solid state - the environment crucial for most applications. Herein, we study the photoswitching performance of six structurally distinct indigo derivatives in five polymers of varying rigidity. Three key strategies are identified to enable efficient photoswitching under red (660 nm) light: (i) choosing a soft polymer matrix to minimise its resistance toward the isomerisation, (ii) creating free volume around the indigo molecules through synthetic modifications, and (iii) applying low dye loading (<1% w/w) to inhibit aggregation. These strategies are shown to improve both photostationary state distributions and the thermal stability of the Z isomer. When all three strategies are implemented, the isomerisation performance (>80% Z form in the photostationary state) is nearly identical to that in solution. These findings thus pave the way for designing new red-light photochromic materials based on indigos.

Red-shifted and pH-responsive imidazole-based azo dyes with potent antimicrobial activity

A novel route is described to obtain 2-aminoimidazole azo dyes with a unique substituent pattern in the heteroaryl unit that provides halochromic properties, exhibiting vibrant colours that change from magenta to deep blue. Potent antimicrobial properties against infectious yeasts were demonstrated. No cytotoxicity was detected for concentrations lower than 16 μg mL^-1.