Visible‐Light Photoswitching by Azobenzazoles

Azophotoswitches containing thiazole, isothiazole, thiadiazole, and isothiadiazole

We report a novel class of azophotoswitches incorporating various five-membered heteroaryl units such as thiazole, isothiazole, thiadiazole, and isothiadiazole. These azophotoswitches were developed through an initial screening of 24 compounds using DFT calculations to identify those with the wavelength of maximum absorption (λmax) at a long wavelength. Subsequently, eight selected azophotoswitches were synthesized. Compounds containing both thiazole and isothiazole moieties showed relatively long λmax compared to the other synthesized compounds. These azophotoswitches exhibited reversible isomerization under visible light irradiation at 430 nm, 450 nm, 470 nm (trans to cis) and 525 nm (cis to trans). Analysis of the X-ray crystal structures of the cis isomer of phenylazo[1,3,4-thiadiazole] exhibited a unique orthogonal geometry.

Large and long-term photon energy storage in diazetidines via [2+2] photocycloaddition

We report a series of p-functionalized phenylbenzoxazoles that offer remarkable energy storage, exceeding 300 J g-1, for the first time among intermolecular cycloaddition-based molecular solar thermal energy storage systems. The [2 + 2] photocycloaddition of phenylbenzoxazoles generates diazetidine cycloadducts that store energy for up to 23 years in the solid state and release energy upon triggered cycloreversion. The solid-state phase transition contributes to increasing overall energy storage densities, and the dearomative cycloaddition process is revealed to be critical for maximizing the intrinsic energy storage capacities. The solvent-assisted cycloreversion is also used to accelerate the energy release from the emerging molecular scaffold.

Visible-Light-Activated Heteroaryl Azoswitches: Toward a More Colorful Future

Azobenzenes (Ph-N═N-Ph) are known as the most widely studied molecular photoswitches, and the recent rise of azoheteroarenes (Het-N═N-Ph or Het-N═N-Het) offers great opportunities to advance this already mature field. A common limitation is that azo-switches generally require harmful UV light for activation, which hinders their application across various fields. Despite great efforts in developing visible-light azobenzenes over the past few decades, the potential of visible-light heteroaryl azoswitches remains largely unexplored. This Perspective summarizes the state-of-the-art advancements in visible-light heteroaryl azoswitches, covering molecular design strategies, the structure-property relationship, and potential applications. We highlight the distinctive advantages of azoheteroarenes over azobenzenes in the research and development of visible-light switches. Furthermore, we discuss the opportunities and challenges in this emerging field and propose potential solutions to address crucial issues such as spectral red-shift and thermal half-life. Through this Perspective paper, we aim to provide inspiration for further exploration in this field, in anticipation of the growing prosperity and bright future of visible-light azoheteroarene photoswitches.

Arylazobenzimidazoles: versatile visible-light photoswitches with tuneable Z-isomer stability

Benzimidazole heterocycles are of great importance in medicinal chemistry due to their applicability to a wide range of pharmacological targets, therefore representing a prototypical "privileged structure". In photopharmacology, azoheteroarene photoswitches have emerged as valuable tools for a variety of applications due to the high tuneability of their photophysical properties. Benzimidazole-based photoswitches could therefore enable the optically-controlled investigation of many pharmacological targets and find application in materials science. Here we report a combined experimental and computational investigation of such arylazobenzimidazoles, which allowed us to identify derivatives with near-quantitative bidirectional photoswitching using visible light and highly tuneable Z-isomer stability. We further demonstrate that arylazobenzimidazoles bearing a free benzimidazole N-H group not only exhibit efficient bidirectional photoswitching, but also excellent thermal Z-isomer stability, contrary to previously reported fast-relaxing Z-isomers of N-H azoheteroarenes. Finally, we describe derivatives which can be reversibly isomerized with cyan and red light, thereby enabling significantly "red-shifted" photocontrol over prior azoheteroarenes. The understanding gained in this study should enable future photopharmacological efforts by employing photoswitches based on the privileged benzimidazole structure.

Geometry-Induced Oligomerization of Fluorine-Substituted Phenylazothiazole Photoswitches

Photoswitches are molecules that can absorb light of specific wavelengths and undergo a reversible transformation between their trans and cis isomeric forms. In phenylazo photoswitches, it is common for the less stable cis (Z) isomer to convert back to the more stable trans (E) isomer either through photochemical or thermal means. In this research, we designed new derivatives of phenylazothiazole (PAT) photoswitches, PAT-Fn, which feature fluorine substituents on their phenyl component. These derivatives can reversibly isomerize under visible light exposure with the enrichment of E and Z isomers at photostationary state (PSS). Surprisingly, we observed an unconventional phenomenon when these PAT-Fn (n≧2) photoswitches were in their cis isomeric state in the absence of light. Instead of the anticipated transformation from cis to trans isomer, these compounds converted to an oligomeric compound. Our detailed experimental investigation and theoretical calculations, indicated the crucial role of fluorine substituents and the distinctive geometric arrangement of the cis isomer in driving the unexpected oligomerization process originating from the cis isomeric state.

Visible-Light Photoswitchable Benzimidazole Azo-Arenes as β-Arrestin2-Biased Selective Cannabinoid 2 Receptor Agonists

The cannabinoid 2 receptor (CB2 R) has high therapeutic potential for multiple pathogenic processes, such as neuroinflammation. Pathway-selective ligands are needed to overcome the lack of clinical success and to elucidate correlations between pathways and their respective therapeutic effects. Herein, we report the design and synthesis of a photoswitchable scaffold based on the privileged structure of benzimidazole and its application as a functionally selective CB2 R "efficacy-switch". Benzimidazole azo-arenes offer huge potential for the broad extension of photopharmacology to a wide range of optically addressable biological targets. We used this scaffold to develop compound 10 d, a "trans-on" agonist, which serves as a molecular probe to study the β-arrestin2 (βarr2) pathway at CB2 R. βΑrr2 bias was observed in CB2 R internalization and βarr2 recruitment, while no activation occurred when looking at Gα16 or mini-Gαi . Overall, compound 10 d is the first light-dependent functionally selective agonist to investigate the complex mechanisms of CB2 R-βarr2 dependent endocytosis.

Heterocyclic Hemipiperazines: Water-Compatible Peptide-Derived Photoswitches

Hemipiperazines are a recently discovered class of peptide-derived molecular photoswitches with high biocompatibility and therapeutic potential. Here, for the first time we describe photochromism of heterocyclic hemipiperazines. They demonstrate long thermal lifetimes, and enlarged band separation between photoisomers. Efficient photoisomerization occurs under aqueous conditions, although with a need for organic co-solvent. Bidirectional switching with visible light is observed for an extended aromatic system.

Photoresponsive Dissipative Macrocycles Using Visible-Light-Switchable Azobenzenes

Visible light can be used to shift dynamic covalent imine assemblies out of equilibrium. We studied a fluorinated azobenzene building block that reliably undergoes geometric isomerism upon irradiation. The building block was used in combination with two different amines, ethylenediamine and R,R-1,2-diaminocyclohexane, to create a library of imine macrocycles. Whereas the simple amine can be used to access a polymeric state and a defined bowl-shaped macrocycle, the chiral amine gives access to a rich network of macrocycles that undergo both isomerisation as well as interconversion between different macrocyclic species, thereby allowing for control over the number of monomers involved in the cyclo-oligomerization; 1 H- and 19 F-DOSY NMR, MALDI-MS measurements, and UV/Vis spectroscopy were used to study the processes.

Exploring Arylazo-3,5-Bis(trifluoromethyl)pyrazole Switches

Arylazopyrazoles stand out among the azoheteroarene photoswitches due to their excellent properties in terms of stability of the least stable isomer and conversion between isomers, leading to their use in several interesting applications. We report herein the synthesis of arylazo-trifluoromethyl-substituted pyrazoles and their switching behavior under light irradiation. UV-vis and NMR experiments showed that arylazo-1H-3,5-bis(trifluoromethyl)pyrazoles displayed very long half-lives in DMSO (days), along with reasonable values of other parameters that characterize a photoswitch. Inclusion of naphthyl moieties as aryl counterparts of the arylazopyrazoles is beneficial only in combination with trifluoromethyl groups, while extending the conjugation by grafting the pyrazole moiety with electron-donating or -withdrawing substituents positively affects the photoswitching behavior, in terms of isomerization yield and half-lives of the least stable isomer. The experimental values were correlated with theoretical calculations indicating the valuable influence of the trifluoromethyl groups onto the photoswitching behavior.

Visible-Light-Responsive Self-Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination

A photoswitchable ligand based on azobenzene is self-assembled with palladium(II) ions to form a [Pd2 (E-L)4 ]4+ cage. Irradiation with 470 nm light results in the near-quantitative switching to a monomeric species [Pd(Z-L)2 ]2+ , which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self-assembly, and the thermal half-life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Structure-Property Relationship for Visible Light Bidirectional Photoswitchable Azoheteroarenes and Thermal Stability of Z-Isomers

A modular approach has been adopted to synthesize a wide range of visible light-driven photoswitchable azoheteroarenes. In this regard, we considered ortho substitution of cyclic amines in the aryl ring and varied substitution patterns. Using detailed spectroscopic studies, we established a relationship between structure and photoswitching ability and also half-lives of the Z-isomers. Through this, we envision tunable and bidirectional longer wavelength photoswitches.

Molecular Solar Thermal Systems towards Phase Change and Visible Light Photon Energy Storage

Molecular solar thermal (MOST) systems have attracted tremendous attention for solar energy conversion and storage, which can generate high-energy metastable isomers upon capturing photon energy, and release the stored energy as heat on demand during back conversion. However, the pristine molecular photoswitches are limited by low storage energy density and UV light photon energy storage. Recently, numerous pioneering works have been focused on the development of MOST systems towards phase change (PC) and visible light photon energy storage to increase their properties. On the one hand, the strategy of simultaneously capturing isomerization enthalpy and PC energy between solid and liquid can not only offer high latent heat, but also promote the development of sustainable energy systems. On the other hand, the efficient photon energy storage in the visible light range opens a tremendously fascinating avenue to fabricate MOST systems powered under natural sunlight. Here, the recent advances of MOST systems towards PC and visible light photon energy storage are systematically summarized, the most promising advantages and current challenges are analyzed, and emerging strategies and future research directions are proposed.

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pKa shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO2 via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO2 .

Inner-sphere electron transfer at the ruthenium-azo interface

Metal complexes exhibiting multiple reversible redox states have drawn continuing research interest due to their electron reservoir features. In this context, the present article described ruthenium-acac complexes (acac=acetylacetonate) incorporating redox-active...

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors has been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc . The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.

Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

Push-Pull Stiff-Stilbene: Proton-Gated Visible-Light Photoswitching and Acid-Catalyzed Isomerization

Donor-acceptor substituted stiff-stilbene is shown to undergo isomerization induced by visible light avoiding the need for harmful UV light. This visible-light photoswitching is inhibited by protonation of the dimethylamino-donor unit, disrupting the push-pull character and thus, gating of the photochromic properties is allowed by acid/base addition. Remarkably, the addition of a mild acid also triggers fast thermal back-isomerization, which is unprecedented for stiff-stilbene photoswitches usually having a very high energy barrier for this process. These combined features offer unique orthogonal control over switching behavior by light and protonation, which is investigated in detail by 1 H NMR and UV/Vis spectroscopy. In addition, TD-DFT calculations are used to gain further insight into the absorption properties. Our results will help elevating the level of control over dynamic behavior in stiff-stilbene applications.

Advances in Application of Azobenzene as a Trigger in Biomedicine: Molecular Design and Spontaneous Assembly

Azobenzene is a well-known derivative of stimulus-responsive molecular switches and has shown superior performance as a functional material in biomedical applications. The results of multiple studies have led to the development of light/hypoxia-responsive azobenzene for biomedical use. In recent years, long-wavelength-responsive azobenzene has been developed. Matching the longer wavelength absorption and hypoxia-response characteristics of the azobenzene switch unit to the bio-optical window results in a large and effective stimulus response. In addition, azobenzene has been used as a hypoxia-sensitive connector via biological cleavage under appropriate stimulus conditions. This has resulted in on/off state switching of properties such as pharmacology and fluorescence activity. Herein, recent advances in the design and fabrication of azobenzene as a trigger in biomedicine are summarized.

Azobispyrazole Family as Photoswitches Combining (Near-) Quantitative Bidirectional Isomerization and Widely Tunable Thermal Half-Lives from Hours to Years*

Azobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono-heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis-heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near-)quantitative bidirectional photoconversion and widely tunable Z-isomer thermal half-lives from hours to years. The two five-membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z-isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z-isomer stability (favored by the orthogonal shape) encountered by mono-heteroaryl azo switches.

Tailoring the optical and dynamic properties of iminothioindoxyl photoswitches through acidochromism

Multi-responsive functional molecules are key for obtaining user-defined control of the properties and functions of chemical and biological systems. In this respect, pH-responsive photochromes, whose switching can be directed with light and acid-base equilibria, have emerged as highly attractive molecular units. The challenge in their design comes from the need to accommodate application-defined boundary conditions for both light- and protonation-responsivity. Here we combine time-resolved spectroscopic studies, on time scales ranging from femtoseconds to seconds, with density functional theory (DFT) calculations to elucidate and apply the acidochromism of a recently designed iminothioindoxyl (ITI) photoswitch. We show that protonation of the thermally stable Z isomer leads to a strong batochromically-shifted absorption band, allowing for fast isomerization to the metastable E isomer with light in the 500-600 nm region. Theoretical studies of the reaction mechanism reveal the crucial role of the acid-base equilibrium which controls the populations of the protonated and neutral forms of the E isomer. Since the former is thermally stable, while the latter re-isomerizes on a millisecond time scale, we are able to modulate the half-life of ITIs over three orders of magnitude by shifting this equilibrium. Finally, stable bidirectional switching of protonated ITI with green and red light is demonstrated with a half-life in the range of tens of seconds. Altogether, we designed a new type of multi-responsive molecular switch in which protonation red-shifts the activation wavelength by over 100 nm and enables efficient tuning of the half-life in the millisecond-second range.

Theoretical insights into a colorimetric azo-based probe to detect copper ions

In the present study, a colorimetric azobenzene-based probe (AZO 1) was reported that exhibits high selectivity toward Cu²⁺ and undergoes a red to yellow colour change upon its detection. Density functional theory (DFT) calculations were carried out to investigate the mechanism of the probe discoloration. The differences in the binding energies of complexes of 2 : 1 and 1 : 1 stoichiometry indicated that a two-step complexation process takes place as the Cu²⁺ content increases. However, the calculated absorption spectra suggested that a significant colour change would only be observed for the 1 : 1 AZO 1 : Cu²⁺ complex. A HOMO–LUMO electronic transition was a key factor for the blue shift of the absorption bands of the probe. Further studies indicated that solvent molecules participate in the complexation and that the presence of the o-methoxy group in AZO 1 led to formation of an octahedral complex because of the additional chelating site. A significant change in the conformation of AZO 1, namely the rotation of the N,N-di(carboxymethyl)amino group around the N–C_Ar bond by approximately 90°, resulted in a larger HOMO–LUMO energy gap, and the corresponding alteration of the intramolecular charge transfer (ICT) from the N,N-di(carboxymethyl)amino group to the phenyl ring led to the observed colour change.

DFT calculations indicated that the rotation of the N,N-di(carboxymethyl)amino group around the N–CAr bond by approximately 90°, resulted in a larger HOMO–LUMO energy gap, and led to the observed colour change.