Azobenzenes: Photoswitching and Their Chemical Sensor Application

Abstract

Azobenzene is a well-known dye that undergoes fast trans-cis photoisomerization and has been widely studied and used in the development of organic functional materials. Due to its selective isomeric distribution in the excited state, azobenzene has been used as a photoswitch in the storage of information on a molecular level, photo-controllable catalysis, solar light harvesting, photo-pharmacology, optical-to-mechanical energy conversion, molecular electronic, and photonic devices. Furthermore, the characteristic and distinguishable photoelectronic properties of trans and cis azobenzene have been recently used in the sensing properties of different ions and the recognition of molecules. Here, we are going to review the recent literature where different intermolecular forces show the supramolecular properties under the stimuli of photo-light.

1. Introduction

Azobenzene is one of the old known chemicals among few, with which people are fascinated due to its impressive color and have been using it rigorously as a dye. Over the last two centuries, we have witnessed a tremendous change in the application pattern of azobenzene, initially known as a dye, but now evolved as a superpowered tiny engine that could do varieties of works [1]. Thus, recently azobenzene has shown tremendous utility in every day of our life. The design of materials is used for many purposes, viz, solar light harvesting, optical-to-mechanical energy conversion, molecular electronic and photonic devices, in medicinal chemistry, photo-pharmacology, drug delivery, etc. The presence of two benzene rings connected by -N=N- bond makes them have a special photophysical property and has been widely used for light-responsive materials. Many probes have been developed as chemical sensors for the detection of varieties of ions and molecules [2, 3].

Furthermore, azobenzenes have been used as photoswitch materials due to their light-responsive reversible trans ⇆ cis photoisomerization. The change in the configuration of azobenzene opens up a new avenue to explore its physical, chemical, biological, and material properties, since it causes changes in geometries, conformations, and assemblies of molecules [4, 5]. Molecular switches enable the storage of information on a molecular level and since the difference of energy is approximately 47 KJ.mol⁻¹, modulation of the above-mentioned properties could thus be easily achieved reversibly, according to our choice by using noninvasive and versatile stimuli such as light. Azobenzene undergoes symmetry-allowed excitation to π-π* (S₀ → S₂) transitions, followed by rapid relaxation to the S₁ state. Then, there is isomerization of trans-azobenzene along with movement from the Frank-Condon region and S₁ → S₀ relaxation. Over the process, there is a decrease in the intensity of the π-π* band with a blue shift and an increase in the intensity of the symmetry-forbidden n-π* band (S₀ → S₁). The trans → cis photoisomerization leads to nonplanar form with the increase in the dipole moment of cis form. The reverse transformation from cis to trans proceeds via either the photochemical n-π* transition or the thermal process. Depending on the substituents present in the benzene rings, the kinetics of the photoisomerization reaction is determined. Both fast and slow reactions are useful for different applications. Photoswitching properties of azobenzene have been studied for decades, but the last few decades have made this photo-intertransformation, a hot research area, since by appending suitable substituents on azobenzene made it a smart material [6, 7]. Thus, wide applications in energy harvesting, sensing [8], electronics [9], biology [10], soft robotic systems [11], etc., have been explored.

Due to wide application in diagnostics, imaging, and environmental protection concerns, the development of receptors for selective recognition of anions is one of the hot topics in modern science and many colorimetric and fluorescent materials have been developed [12, 13, 14]. There is growing interest to develop probes for ionic or molecular recognition, which are controlled by external stimuli like light. Light changes the conformation or configuration of the probes, and thus binding affinities could be controlled. In the last two decades, many photoswitchable probes, viz, azobenzene [15], spiropyran [16], diarylethene [17], etc., have been developed, which we found to have many applications in materials science and biology. The azobenzene photoswitch is particularly interesting where a planar trans form changes to a nonplanar cis form, and during these interconversions, there is a dramatic change in the dipole moment of the compounds (Figure 1). Due to easy synthesis, robustness during prolonged exposure to light, and easy functionalization, azobenzenes are most extensively used in the development of materials.

Figure 1.

Structure of trans and cis azobenzene and their interconversion initiated by photoirradiation. Characteristic UV-visible absorbance of trans (black colored) and cis azobenzene (blue colored). Reprinted (adapted) with permission from Ref. [4]. Copyright © 2024, Wiley.

2. Azobenzene photoswitching in sensing application

Jiang et al. reported the synthesis of an azobenzene-containing phenyl-1,2,3-triazole motif photoswitchable foldamer 1, which showed a controllable change in binding affinity for anions. Thus, compound xx upon photoirradiation with 339 nm shows an 80% descent in the π-π* transition and increment in the n-π* transition and irradiation at the visible light, band of the π-π* transition reverts back. Both trans and cis-configurations of compound 1 were confirmed by two-dimensional nuclear magnetic resonance (2D NMR) experiments. The addition of chloride ion (Cl-) from tetrabutylammonium chloride (Bu₄NCl) in the ground state suggested an interaction with the inner cavity of the foldamer that participates in hydrogen bonding, as suggested by the proton nuclear magnetic resonance (¹H-NMR) analysis. Furthermore, titration with other ions also showed a similar kind of interaction (1:1), and their binding constant was found to be 70 M⁻¹ for Cl⁻, 22 M⁻¹ for Br⁻, 38 M⁻¹ for HSO₄⁻, 11 M⁻¹ for I⁻, and 10 M⁻¹ for NO₃⁻ ions. The cis configuration dramatically enhances the interaction and here the presence of a triazole ring has more significant contributions, as suggested by the ¹H-NMR. Thus, binding constants were found to increase up by 290 M⁻¹ for Cl⁻, 87 M⁻¹ for Br⁻, 66 M⁻¹ for HSO₄⁻, 31 M⁻¹ for I⁻, and 21 M⁻¹ for NO₃⁻ ions. Thus, binding affinities are highly dependent on the size and shape of anions (Figure 2) [18].

Figure 2.

Photoswitch 1 and a different binding model proposed for its two photoisomers to anions.

Bandyopadhyay et al. recently developed a macrocyclic azobenzene-based fluorescent receptor 2 that very selectively recognizes ATP (adenosine triphosphate) in the 2-trans form. There was a sevenfold increment of fluorescence emission observed when 2-trans interacted with ATP (10 equivalent), on the other hand, ADP (adenosine diphosphate) and GDP (guanosine diphosphate) showed an increment only 3.5 times only. But upon irradiation by 366 nm, it causes photoisomerization to 2-cis to show altered selectivity to GTP (guanosine triphosphate). Interestingly, 2-trans exists in the protonated form through the pyridinium nitrogen atom, even in the neutral condition. Receptor 2-trans showed strong affinity with ATP⁴⁻ due to high Coulomb interaction between the negatively charged oxygen atom of the anions and the positively charged protonated form of the receptor. Thus, due to different binding affinities of various phosphorylated species in their trans and cis form, system 2 was used for monitoring enzymatic phosphorylation reaction with tyrosine kinase enzymes (Figure 3) [19].

Figure 3.

Photomodulation of the receptors 2 and 3.

The rate of interconversion between trans and cis configurations depends upon many factors. The half-life of a few cis -compounds lasts up to weeks but the half-life of some compound could instantly revert back. Both fast and slow individual interconversion processes have been utilized for many purposes. The change in pH modulates the stability and the presence of substituent at the ortho-position increases the stability of cis –isomer [20, 21]. Furthermore, the type of solvent (polar or nonpolar) and intermolecular H-bonding also affect the stability of the cis isomer. Bandyopadhyay et al. reported the synthesis of macrocyclic azobenzenes (2 and 3, Scheme 2) that underwent exclusive photo-transformation to cis -form. The addition of Cu²⁺ and continuous exposure to ultraviolet (UV) radiation make the cis compound stable. But, interestingly, on ceasing of UV light, the cis compound quickly gets converted back to the trans form. The Cu²⁺ ions also bind to the initial trans compounds with the binding constant of 9.9 × 102 M⁻¹, as determined by ¹H NMR. Irradiation with UV light to this Cu²⁺ complex led to slow photoisomerization to cis form [22]. However, on ceasing of the UV light source, the cis form immediately goes back to the initial trans-Cu²⁺ complex within 85 minutes with first-order rate constant of 1.75 × 10⁻⁵ min⁻¹, unlike the original cis complex (t_1/2 = 92 days at 298 K). The Ag + ion was also found to exhibit the same behavior but slower than the Cu²⁺ ion.

The concept of photoswitching has been widely used in biological gated systems to timely open and close the ion channels in a controlled fashion for regulating vital functions of nerve transmission, osmotic regulation, ion homeostasis, muscle excitation, growth, and development. Natural stimuli, such as variations in pH, light, chemical messengers, and membrane potential differences, are what cause the physiological systems’ aforementioned regulating activity. So, misregulation in the above stimuli causes numerous diseases, collectively called channelopathies. Recently, a significant effort has been made to mimic the natural ion transport systems. And in this direction, the azobenzene-based photoswitch has proved to play a significant role. Thus, binding with ions in a supramolecular concept to use twisted cis compound as an anion carrier has been widely used. Yet, intramolecular hydrogen-bound trans-azobenzene diamide system 4, which was recently developed and synthesized by Talukdar et al., provides a thermally stable inactive state in the cis -conformation and creates an effective photoswitchable chloride carrier [23]. Thus, in compound 4-trans, the amide N-H₁ and C_Ar-H₂ create the anion-binding site. In the lipid bilayer membrane, receptor 4 was found to facilitate the transport of Cl⁻ through the formation of a sandwich complex with a chloride ion. Nevertheless, poor anion binding and transport resulted from a change in the planarity and closeness of the anion-recognizing groups caused by the photoisomerization of receptor 4 at 365 nm. Different substituents were taken based on lipophilicity, as per the Lipinski rule and it was found that 4-(trifluoromethyl) phenyl aminoformyl and a propionamide groups provided the best half-maximal effective concentration (EC₅₀) = 0.199 μM and Hill coefficient of n ~ 2 when measured across EYPC-LUVs⊃HPTS (egg yolk phosphatidylcholine-large unilamellar vesicles-pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid)) (Figure 4).

Figure 4.

Design and working principle of light regulatory synthetic ion transporter. Chemical structures of receptors trans-4 and cis-4.

Bandyopadhyay et al. demonstrated a photoswitchable assembly-disassembly process through the photoisomerization of trans and cis interconversion. Thus, azobenzene compound 5 was synthesized, which has a suitable binding pocket that binds to Al³⁺ ions that set up a linear polymeric assembly with the trans form of photoswitch. But photoirradiation of the above-mentioned complex generated a cis form where intramolecular chelation disrupts the assembly. However, the coordination assembly regenerated when the trans complex was irradiated with visible light [24]. The development of assembly was the characteristic property of the Al³⁺ complex (confirmed by dynamic light scattering (DLS) showing the average hydrodynamic diameter, d of 448 nm.) since the original azobenzene did not show any aggregation behavior. The binding constant for the association of Al³⁺ ions with trans-5 was calculated to be 162 M⁻¹ (from ¹H NMR) with 1:1 interaction (from Jobs Plot). Furthermore, investigation by using ¹H NMR confirms that terminal N-atoms participated with the Al³⁺ ion (Figure 5).

Figure 5.

Putative complexation mode of the Al³⁺ ion with the trans and cis forms of 5.

Kalow et al. developed and used a series of azobenzene boronic acids for reversible diol binding via photochemical isomerization reaction. It was observed that the trans-azobenzene having ortho-boronic acid substituent weakly binds with diols to form boronic esters, but the isomerization of the azobenzene to its cis isomer enhances diol binding. Furthermore, a correlation between diol binding and the photostationary state was investigated, which demonstrated that change in the wavelengths of irradiation yielded different quantities of the bound diol. This pattern was observed with most of the diols including cyclic diols, nitrocatechol, biologically relevant compounds, and polyols. The thermodynamics of esterification was explained by competition experiments and computational studies which suggested that the changes in the binding affinity originate from the stabilization by intramolecular H-bonding interactions between the boronic acid O-H and proximal heteroatoms, and the destabilization of the trans-azobenzene by steric interactions (Figure 6). Authors demonstrated the application of the above photoswitchable boronic acid-ester equilibria to a dynamic covalent hydrogel made by diol-substituted amine-terminated 4-arm poly(ethylene glycol) (PEG) and found that boronic esters can be stiffened with red light [25].

Figure 6.

The equilibrium that results from photoisomerization of the photoswitch between ortho-substituted azobenzene boronic acids and diols, where the cis isomer has a greater binding affinity with diols than the trans isomer. The cis binding constant is 4 times larger than the trans binding constant when the diol is pinacol.

Langaton et al. synthesized ortho-chloro azobenzene 9, with electron-deficient iodo-triazole motifs, the first halogen bonding photoswitchable anion receptors, wherein it was demonstrated that chloride binding may be reversibly modulated by the red and blue light. The trans-form of compound 9, which binds weakly with Cl-, appeared to show a 50-fold enhancement in chloride-binding affinity observed for the cis -isomer. The irradiation of 9 affords long cis isomer lifetimes (half-life of ~1 week), which was advantageous for switching binding applications. Furthermore, dicationic iodotriazolium system 1-Me-BarF (), trans-10 was synthesized by methylation and subsequent anion exchange and it was found to serve as a more promising host for chloride and was enhanced relative to trans-9. But, the comparatively switchable anion affinity in cis was maintained but diminished albeit significantly (K_i^trans = 349 M⁻¹, K_i^cis = 945 M⁻¹, K_cis/K_trans = 2.7, DMSO (dimethyl sulfoxide)-d₆). This may be argued due to charge repulsion in the cis -isomer where both binding sites are in close proximity. System 10 showed unprecedented photo-regulated halogen bonding mediated halide abstraction catalysis for a Friedel-Crafts alkylation and Mukaiyama aldol reaction. Here also, the stronger binding cis isomers acted as the more potent catalytic species (Figure 7) [26].

Figure 7.

Photoswitchable anion receptors 9 and 10.

Daṃbrowa et al. synthesized a new tetra-meta-substituted azobenzene trans-11-based receptor for oxoanions, in particular, tetrahedral H₂AsO₄⁻ and H₂PO₄⁻. The photoirradiation of compound trans-11 led to cis − 11, which was evaluated by ¹H NMR spectroscopy. Titration with anions revealed that nonplanar cis −11 was a better host for anions than trans-11, which was argued from the greater ability of four amide NH protons in the cis -state to cooperatively bind with the oxyanions. Furthermore, the planar trans-11 was found to show poorer binding affinities, since its binding groups are too distant from each other and cannot cooperatively bind anions. Interestingly, meta-substituted in compound cis −11 was shown to significantly increase the thermal stability and very resistance of the cis -state to rapid decay triggered by anion binding (Figure 8) [27].

Figure 8.

Photoisomerization of receptor 11.

Jarosz et al. developed an efficient cesium carbonate (Cs₂CO₃)-templated macrocyclization strategy to synthesize chiral photoresponsive macrocyclic azobenzene compound 12 with protected sucrose. Green light (530 nm) irradiation causes the photoisomerization to long-lived cis −12 (25 days), which undergoes photoswitching back to trans-12 by using blue light (410 nm). Both compounds bind with potassium and other alkali metal cations but the cis −12 exhibits higher binding affinity and selectivity for cation than the trans-12 (K_cis/K_trans ≤ 4.1). ¹H NMR titrations and density functional theory (DFT) calculations confirmed that, in addition to the polyether residues, the sucrose ring oxygen and azobenzene nitrogen atoms also markedly contribute to cation recognition (Figure 9) [28].

Figure 9.

Photoswitching of host 12 and complexes of 12 with 10 equiv. of alkali metal triflates in methyl cyanide (MeCN) (50 μM) at 298.0 ± 0.1 K using green light (LED 530 nm) and blue light (LED 410 nm), respectively. Stability constants K_a (M⁻¹) for complexes of hosts trans-12 and cis −12 with alkali metal cations.

Flood et al. synthesized a unique bis-azobenzene system trans-trans-13 that contains aryl-triazole foldamers and has a helical structure in the ground state. Irradiation of light 365 nm changes the configuration to cis -trans-13 and cis-cis −13, and changes to random coil structure. Thus, by irradiating the system with 365 and 436 nm light, it is possible to destabilize and stabilize the helix. The probe trans-trans-13 was found to show good affinity with the chloride ion having an association energy of ΔG = −20 KJ mol⁻¹ (K_a = 3000 M⁻¹). When the same receptor solution was exposed to the UV light, the chloride affinity was reduced to ΔG = −15 kJ mol⁻¹ (K_a = 380 M⁻¹). Furthermore, exposure to 436 nm light restored the dominance of the trans-trans isomer in solution and the stronger chloride affinity. Thus, the chloride levels can be regulated with light. In order to verify this, a conductivity experiment was carried out using equimolar concentrations of 13 and tetrabutylammonium chloride (TBACl) (1 mM, CH₃CN, 298 K). It was discovered that the visible polystyrene sulfonate (PSS) will have a free chloride concentration of 0.23 mM, which will rise to 0.56 mM when exposed to UV light (Figure 10) [29].

Figure 10.

Representations of the helical-to-random coil equilibria for 13 _trans-trans and the UV-driven PSS (66% 13_cis-trans + 33% 13_cis-cis).

Russell et al. designed and synthesized a novel (pyridylazo) phenol derivative 8-[4-(2 pyridylazo)phenoxy]octyl disulfide 14 and used this for the formation of a monolayer film on gold-coated optical waveguides through self-assembly. The 14-self-assembled monolayer (SAM) in the trans isomeric form provides a bidentate ligand that chelates with metal ions, such as Ni²⁺ and Co²⁺, and results in a strong red-shifted UV-visible absorption at 433 nm. Further photoirradiation with waveguided UV light at 365 nm via the evanescent field, the 14-SAM underwent photoisomerization to the cis form. But, cis − 14-SAM was not able to chelate with metal ions. However, the above process was reversible, thus irradiation at visible light at 439 nm, cis − 14-SAM converted back to trans-14-SAM. Thus, through this process, the trans-14-SAM was used to detect Ni²⁺ in the range 9.6 × 10⁻⁵–2.44 × 10⁻³ M (Figure 11) [30].

Figure 11.

Diagrammatic summary of the photoswitching and metal-ion chelation of the 14-SAM.

Jurczak et al. developed a system where the lifetime of cis isomer could be decreased by the introduction of anions. Thus, azobenzene urea derivatives 15–16 were synthesized. Photoirradiation of compounds 15 and 16 revealed the decrease in the cis lifetime of 16 due to the presence of urea substituent (cis −15 = 13.1 h, cis −16 = 1.8 h). Interestingly, the addition of different anions decreases the lifetime of the cis −16 isomer (with F⁻ = 15 min, MeCO₂⁻ = 8 min, PhCO₂⁻ = 25 min, HSO₄⁻ = 8 min, and H₂PO₄⁻ = 28 min). Thus, acetate ions have a high-rate constant for cis -trans isomerization in both cases (for cis − 15 = 31 × 10⁻⁴ s, for cis − 16 = 14 × 10⁻⁴ s), which was thought to be due to its high basicity and geometry being complementary to the urea group. Thus, varying the nature and concentration of anions can be used for precise control of the thermal cis →trans isomerization rates. Furthermore, these changes could be easily stopped by the addition of an acid and restored again by the addition of the base (Figure 12) [31].

Figure 12.

Trans-cis isomerization of the urea derivative of azobenzenes.

Manabe et al. reported a new synthetic azobenzene ionophore attached to a crown ether ring and ammonium ion tail, 19, which was capable of facilitating intramolecular complexation. Thus, photoirradiation of compound trans-19 photoisomerizes to cis −19, which was found to show that thermal cis-trans isomerization was slower as compared to free amine analog, and the rate was accelerated selectively by the addition of K⁺ ion (Figure 13) [32].

Figure 13.

Trans-cis isomerization of the ammonium ion attached to the azobenzene derivative.

3. Conclusion

This chapter aims to highlight the recent literature on the emerging sensing application through multifunctional azobenzene photoswitching. The colorimetric sensing could be easily detected through the trans-cis isomerization due to a change in the inherent UV-vis absorbance pattern of the trans-cis compounds. Characteristically, the simple and tiny -N〓N- azo functional group could be incorporated into any molecule and have more chances in this category of compound to modulate and get better and better probes for sensing and other applications. This diverse nature including acyclic, cyclic, and macrocyclic azobenzenes has been developed, which have their own characteristic photophysical properties. Also, a good number of works have been done to stabilize the cis -isomer by modulating the substituent nearer to the azo functional group. In this chapter, we summarized the latest few and selected examples to highlight the sensing application of photoswitchable azobenzene, which could be easily monitored by UV-visible spectroscopy. The progress in this field is continuing and we expect that we will see a few reports in the near future where fluorescence emission change of azobenzene is used when interacting with ions through photoswitching.

Acknowledgments

The authors, Chandra, Rani, B. Mahto, and Mahato, are grateful to the CUSB-Gaya for providing the research infrastructure. The author Rani also thanks the CUSB-Gaya for the research fellowship from the CUSB and the author Mahto thanks the UGC New Delhi for the research fellowship.

Conflict of interest

The authors declare no conflict of interest.