Photostationary State Research Articles

Photoresponsive Organic Cages─Computationally Inspired Discovery of Azobenzene-Derived Organic Cages.

The incorporation of photoresponsive groups into porous materials is attractive as it offers potential advantages in controlling the pore size and selectivity to guest molecules. A combination of computational modeling and experiment resulted in the synthesis of two azobenzene-derived organic cages based on building blocks identified in a computational screen. Both cages incorporate three azobenzene moieties, and are therefore capable of 3-fold isomerization, using either ditopic or tetratopic aldehydes containing diazene functionality. The ditopic aldehyde forms a Tri2Di3 cage via a 6-fold imine condensation and the tritopic aldehyde forms a Tet3Di6 cage via a 12-fold imine condensation. The relative energies and corresponding intrinsic cavities of each isomeric state were computed, and the photoswitching behavior of both cages was studied by UV-Vis and 1H NMR spectroscopy, including a detailed kinetic analysis of the thermal isomerization for each of the EEZ, EZZ and ZZZ metastable isomers of the Tet3Di6 cage. Both cages underwent photoisomerization, where a photostationary state of up to 77% of the cis-isomer and overall thermal half-life of 110 h was identified for the Tet3Di6 species. Overall, this work demonstrates the potential of computational modeling to inform the design of photoresponsive materials and highlights the contrasting effects on the photoswitching properties of the azobenzene moieties on incorporation into the different cage species.

Emergent Behavior of a Photoswitchable Solute in a Biphasic Solvent System.

Due to thermal E/Z isomerization, hydrazones in solution typically exist in thermodynamic equilibria between their isomers. Irradiation of such solutions leads to photostationary states that may differ from the equilibrium distribution. Operating such switchable hydrazones in a biphasic system of two immiscible solvents introduces three new degrees of freedom: the E/Z equilibrium in the second solvent and two equilibria for distribution of each of the isomers between the solvents. Irradiation of such a system can be performed in three different ways - the first solvent only, the second solvent only, and both solvents at once - all yielding distinct outcomes. Depending on the choice of materials and the mode of irradiation, such setup may lead to different emergent behaviors that are not immediately intuitive, including net cyclic transport or the accumulation of one photoswitched product in one of the phases, beyond what is reachable by irradiating a simple solution of the same photoswitch.

Synthesis and Development of Inverse-Type Nucleoside Diarylethene Photoswitches.

Nucleosidic diarylethenes (DAEs) have evolved from an emerging class of photochromes into a well-established option for integrating photochromic functionalities into biological systems. However, a comprehensive understanding of how chemical structure influences their photochromic properties remains essential. While structural features, such as an inverse connection between the aryl residues and the ethene bridge, are well-documented for classical DAEs, their application to nucleosidic DAEs has been underexplored. In this study, we address this gap by developing three distinct types of inverse nucleosidic DAEs-semi-inverse thiophenes, semi-inverse uridines and inverse uridines. We successfully synthesized these compounds and conducted comprehensive analyses of their photostationary states, thermal stability, reversibility, and reaction quantum yields. Additionally, we conducted an in-depth comparison of their photochromic properties with those of their normal-type counterparts. Among the synthesized compounds, seven semi-inverse thiophenes exhibited the most promising characteristics. Notably, these compounds demonstrated excellent fatigue resistance, with up to 96 % retention of photochromic activity over 40 switching cycles, surpassing the performance of all comparable nucleosidic DAEs reported to date. These findings hold significant promise for future applications in various fields.

2-(1,1-Dicyanomethylene)rhodanine-Functionalized Oligothiophenes: A Structure-Property Investigation of Z/E Photoisomerization Behavior.

A series of oligothiophenes singly and doubly functionalized with dicyanorhodanine (RCN) units have been investigated to understand their Z/E photoisomerization behavior upon structural modulation. Monotopic RCN target molecules (1-Z-9-Z) were designed to observe the consequences of π-conjugation, solubilizing group substitution, and formylation of the thiophene units. In all cases, the Z isomer is obtained from synthesis as the thermodynamically stable isomer, whereas the E isomer is achieved through selective irradiation (including red light, λirr = 628 nm) as a Z/E mixture in solution. For the quarterthiophene entries, photoisomerization is inhibited, with photoirradiation resulting only in degradation. The result comports with concentration-dependent studies, which show that increasing π-conjugation results in greater aggregation and muted Z/E photoisomerization. Ditopic RCN targets (10-ZZ-12-ZZ), mimicking acceptor-donor-acceptor (A-D-A) oligomers relevant to OPV materials, also show evidence of photoisomerization in solution, with formation of Z,Z/Z,E mixtures at the photostationary state (PSS). Complementary ground- and excited-state DFT calculations show excellent agreement with the experimental findings. This comprehensive structure-property analysis is expected to both guide and caution the functional materials community with respect to the usage of photoisomerizable RCN-oligothiophenes for optoelectronic applications.

Tuning the Thermal Stability of Tetra-o-chloroazobenzene Derivatives by Transforming Push-Pull to Push-Push Systems.

Molecular photoswitches provide interesting tools to reversibly control various biological functions with light. Thanks to its small size and easy introduction into the biomolecules, azobenzene derivatives have been widely employed in the field of photopharmacology. All visible-light switchable azobenzenes with controllable thermostability are highly demanded. Based on the reported tetra-o-chloroazobenzenes, we synthesized push-pull systems, by introducing dialkyl amine and nitro groups as strong electron-donating and electron-withdrawing groups on the para-positions, and then transformed to push-push systems by a simple reduction step. The developed push-pull and push-push tetra-o-chloroazobenzene derivatives displayed excellent photoswitching properties, as previously reported. The half-life of the Z-isomers can be tuned from milliseconds for the push-pull system to several hours for the push-push system. The n-π* and π-π* transitions have better resolution in the push-push molecules, and excitation at different wavelengths can tune the E/Z ratio at the photostationary state. For one push-pull molecule, structure and absorption spectra obtained from theoretical calculations are compared with experimental data, along with data on the push-push counterpart.

Proximal Photocleavage: Controlling Polymer Solubility through Pathway Dependent Wavelength‐Orthogonal Photoreactions

AbstractCurrent families of reversible photochemical reactions present challenges for light‐controlled polymers of either photostationary states, which are common in photoinduced cycloaddition/cycloreversion reactions, or exclusively intramolecular bond changes, which characterize most photochromic units. In response to these challenges, here the concept of “proximal photocleavage” is presented, which combines photochemical crosslinking with a photocleavable linker, enabling a one‐time bond formation/cleavage sequence. Proximal photocleavage methacrylate monomers comprising, in series along the pendant of the methacrylate, a coumarin unit for crosslinking and either a phenacyl or ortho‐nitrobenzyl photocleavable group for decrosslinking are reported. The photophysical properties of these monomers and their statistical copolymers with methyl methacrylate are described, and wavelength selective crosslinking and de‐crosslinking of thin polymer films are demonstrated.

A study on the photoisomerization of phenylpropanoids and the differences in their radical scavenging activity using in-situ NMR spectroscopy and on-line radical scavenging activity analysis

Phenylpropanoids are naturally occurring secondary metabolites that exhibit various biological activities such as ultra-violet (UV) light protection and reactive-oxygen species (ROS) scavenging. In this study, we utilized a light-emitting diode (LED) based in-situ UV irradiation nuclear magnetic resonance (NMR) technique to monitor the photoisomerization reactions of these phenylpropanoids under UV irradiation in real-time. Through this approach, we measured the photochemical reaction rates and photostationary state (PSS) ratios of these molecules and observed distinct reaction rate and PSS ratio information depending on the variation of substituent groups in each phenylpropanoid molecule. We also evaluated the radical scavenging activity (RSA) for each photochemical product through diphenyl-1-picrylhydrazyl radical (DPPH) assay and 2,2’-azino-bis(3-ethylbenzenthiazoline-6-sulphonic acid) (ABTS) assay. We found that the photoisomerization product of caffeic acid can increase both DPPH and ABTS radical scavenging activities, and confirmed the enhanced ABTS radical scavenging ability of caffeic acid cis-isomer based on the online high-pressure liquid chromatography (HPLC)-ABTS analysis and the PSS ratio information of each isomer.

Understanding X-ray-induced isomerisation in photoswitchable surfactant assemblies.

Dynamic, responsive materials can be built using photosurfactants (PS) that self-assemble into ordered nanostructures, such as micelles or liquid crystals. These PS contain photoswitchable groups, such as azobenzene (Azo) or, more recently, arylazopyrazoles (AAPs), which change shape and polarity on photoisomerisation between the E and Z states, thus changing the self-assembled structure. Small-angle X-ray scattering (SAXS) is a powerful technique to probe the morphology of PS and can be used to measure the mechanisms of structural changes using in-situ light irradiation with rapid, time-resolved data collection. However, X-ray irradiation has been shown previously to induce Z-to-E isomerisation of Azo-PS, which can lead to inaccuracies in the measured photostationary state. Here, we investigate the effect of light and X-ray irradiation on micelles formed from two different PS, containing either an Azo or AAP photoswitch using SAXS with in-situ light irradiation. The effect of X-ray irradiation on the Z isomer is shown to depend on the photoswitch, solvent, concentration and morphology. We use this to create guidelines for future X-ray experiments using photoswitchable molecules, which can aid more accurate understanding of these materials for application in solar energy storage, catalysis or controlled drug delivery.

Azobenzene derivatives with two aliphatic chains: mesogenic and photoisomerisation studies of (E)-4-[(4-heptyloxyphenyl)diazenyl]phenyl alkanoates (7OABOOCm)

ABSTRACT Compounds from the homologous series of (E)-4-[(4-heptyloxyphenyl)diazenyl]phenyl alkanoates (7OABOOCm) for m = 1–19, were synthesised. The phase behaviour was investigated by differential scanning calorimetry, polarised light (optical) microscopy, and thermo-optical analysis. The enantiotropic nematic, smectic C and crystal G phases were found for derivatives with m = 1–18, m = 7–17, and m = 4, respectively. In addition, during cooling, smectic F, crystal G, and unknown X phases were observed for derivatives with m = 7–9, m = 3,8,9, and m = 4, respectively. The X-ray diffraction results suggest that the X phase is presumably monotropic crystal H or a second crystal phase. The E–Z photoisomerisation process was observed during UV light irradiation. A photostationary state was reached after exposure time equal to approximately 80 s. The derived kinetic constant k of this process was of the order of 10−2 s−1.

Visible‐Light Responsive Ionic Columnar Self‐Assemblies Exhibiting Fast Photothermal Energy Conversion at Room‐Temperature

AbstractA visible‐light responsive ionic liquid crystalline (LC) system based on gallic ester and tetra‐ortho‐substituted azobenzene is designed and synthesized. All the prepared derivatives exhibit columnar hexagonal mesophases at room temperature either in their pristine form or on cooling from isotropic states. The strategic design of these molecules allows for rapid photoisomerization, which is further aided by the dynamic LC phase. These systems attains photostationary states (PSSs) within 5 min of charging and discharging. The maximum Z conversion on charging in the solid‐state at room‐temperature is ≈80% with a highest temperature rise of 9.1 °C on discharging. This new molecular design enables the efficient working of solar thermal fuels (STFs) even in low sunlight intensity conditions, potentially promoting widespread adoption for mitigating global energy demands.

A high-affinity, cis-on photoswitchable beta blocker to optically control β2-adrenergic receptors in vitro and in vivo

This study introduces (S)-Opto-prop-2, a second-generation photoswitchable ligand designed for precise modulation of β2-adrenoceptor (β2AR). Synthesised by incorporating an azobenzene moiety with propranolol, (S)-Opto-prop-2 exhibited a high PSScis (photostationary state for cis isomer) percentage (∼90 %) and a favourable half-life (>10 days), facilitating diverse bioassay measurements. In vitro, the cis-isomer displayed substantially higher β2AR binding affinity than the trans-isomer (1000-fold), making (S)-Opto-prop-2 one of the best photoswitchable GPCR (G protein-coupled receptor) ligands reported so far. Molecular docking of (S)-Opto-prop-2 in the X-ray structure of propranolol-bound β2AR followed by site-directed mutagenesis studies, identified D1133.32, N3127.39 and F2896.51 as crucial residues that contribute to ligand-receptor interactions at the molecular level. In vivo efficacy was assessed using a rabbit ocular hypertension model, revealing that the cis isomer mimicked propranolol’s effects in reducing intraocular pressure, while the trans isomer was inactive. Dynamic optical modulation of β2AR by (S)-Opto-prop-2 was demonstrated in two different cAMP bioassays and using live-cell confocal imaging, indicating reversible and dynamic control of β2AR activity using the new photopharmacology tool.In conclusion, (S)-Opto-prop-2 emerges as a promising photoswitchable ligand for precise and reversible β2AR modulation with light. The new tool shows superior cis-on binding affinity, one of the largest reported differences in affinity (1000-fold) between its two configurations, in vivo efficacy, and dynamic modulation. This study contributes valuable insights into the evolving field of photopharmacology, offering a potential avenue for targeted therapy in β2AR-associated pathologies.

High ratio of blue:red light reduces fruit set in sweet pepper, which is associated with low starch content and hormonal changes

In sweet pepper (Capsicum annuum L.), the fruit yield is often negatively affected by fruit abortion. Here we investigated whether fruit abortion is affected by the blue:red light ratio (B:R) and the possible underlying physiological mechanisms related to carbohydrates and hormones. Sweet pepper plants were grown at B:R of 1:10, 1:3, 1:1 or 9:1 with a total photosynthetic photon flux density of 200 µmol m−2 s−1, resulting in a phytochrome photostationary state (PSS) of 0.88, 0.88, 0.86 and 0.72, respectively. For fruit set observations, each plant was allowed to retain 12 flowers on 4 main stems. Sweet pepper plants grown at the highest B:R (9:1) showed a low fruit set (around 3 fruits per plant), whereas the other three treatments resulted in higher fruit set (6–7 fruits per plant). This response matched with the changes in PSS, suggesting the B:R effect on fruit set might be controlled by phytochrome signaling, which requires further investigation. Plant shoot biomass and leaf area were reduced at B:R of 1:1 and 9:1. The reduced fruit set was associated with a drop in starch content and sucrose synthases activity; and a low auxin, high salicylic acid and high cis-Zeatin type levels in flowers. Flowers in the low fruit set treatment also failed to reduce the abscisic acid and ethylene levels after anthesis. We concluded that both the reduced starch content and the hormonal changes in flowers play a role in triggering fruit abortion at the high B:R of 9:1.

Synthesis of Styrylbenzazole Photoswitches and Evaluation of their Photochemical Properties.

Styrylbenzazoles form a promising yet under-represented class of photoswitches that can perform a light-driven E-Z isomerization of the central alkene double bond without undergoing irreversible photocyclization, typical of the parent stilbene. In this work, we report the synthesis and photochemical study of 23 styrylbenzazole photoswitches. Their thermal stabilities, quantum yields, maximum absorption wavelengths and photostationary state (PSS) distributions can be tuned by changing the benzazole heterocycle and the substitution pattern on the aryl ring. In particular, we found that push-pull systems show large redshifts of the maximum absorption wavelengths and the highest quantum yields, whereas ortho-substituted styrylbenzazole photoswitches exhibit the most favorable PSS ratios. Taking advantage of both design principles, we produced 2,6-dimethyl-4-(dimethylamino)-styrylbenzothiazole, a thermally stable and efficient P-type photoswitch which displays negative photochromism upon irradiation with visible light up to 470 nm to obtain a near-quantitative isomerization with a very high quantum yield of 59 %. Furthermore, 4-hydroxystyrylbenzoxazole was demonstrated to be a pH-sensitive switch which exhibits a 100 nm redshift upon deprotonation. Ortho-methylation of its benzothiazole analogue improved the obtained PSS ratio in its deprotonated state from E : Z=53 : 47 to E : Z=18 : 82. We anticipate that this relatively unexplored class of photoswitches will form a valuable expansion of the current family of photoswitches.

Photoresponse of new azo pyridine functionalized poly(2-hydroxyethyl methacrylate-co-methyl methacrylate)

A new azo polymer containing photoisomerizable azo pyridine functionalities was synthesized via Mitsunobu reaction of 4-(4-hydroxyphenylazo)pyridine with poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (p(HEMA-co-MMA)) for creating new photochromic materials. The resulting polymer with azo pyridine side groups was characterized for structural, thermal, and optical properties. UV–vis, 1H NMR and IR spectroscopies confirmed that all hydroxyl groups in p(HEMA-co-MMA) were substituted with azo dye. The obtained azo copolymer exhibited high thermal stability (around 240 °C) and a glass transition temperature (113 °C), promising for applications. The trans-to-cis isomerization upon UV irradiation and the thermal back reaction of the azo chromophore in the copolymer in the solid state was studied. A photostationary state with 50% content of cis-isomers upon 6 min of UV irradiation was reached, and during 48 h dark relaxation at ambient temperature, all cis-isomers converted to the trans form. Additionally, the possibility of efficient photogeneration of surface relief gratings with high amplitude of azo copolymer surface modulation was demonstrated.

Dicyanorhodanine-Pyrrole Conjugates for Visible Light-Driven Quantitative Photoswitching in Solution and the Solid State.

Small molecule photoswitches capable of toggling between two distinct molecular states in response to light are versatile tools to monitor biological processes, control photochemistry, and design smart materials. In this work, six novel dicyanorhodanine-based pyrrole-containing photoswitches are reported. The molecular design avails both the Z and E isomers from synthesis, where each can be isolated using chromatographic techniques. Inter- and intramolecular hydrogen bonding (H-bonding) interactions available to the E and Z isomers, respectively, uniquely impart thermal stability to each isomer over long time periods. Photoisomerization could be assessed by solution NMR and UV-vis spectroscopic techniques along with complementary ground- and excited-state computational studies, which show good agreement. Quantitative E → Z isomerization occurs upon 523 nm irradiation of the parent compound (where R = H) in solution, whereas Z → E isomerization using 404 nm irradiation offers a photostationary state (PSS) ratio of 84/16 (E/Z). Extending the π-conjugation of the pyrrole unit (where R = p-C6H4-OMe) pushes the maximum absorption to the yellow-orange region of the visible spectrum and allows bidirectional quantitative isomerization with 404 and 595 nm excitation. Comparator molecules have been prepared to report how the presence or absence of H-bonding affects the photoswitching behavior. Finally, studies of the photoswitches in neat films and photoinactive polymer matrices reveal distinctive structural and optical properties of the Z and E isomers and ultimately afford reversible photoswitching to spectrally unique PSSs using visible light sources including the Sun.

A Photoswitchable Metallocycle Based on Azobenzene: Synthesis, Characterization, and Ultrafast Dynamics.

The novel photoswitchable ligand 3,3'-Azobenz(metPA)2 (1) is used to prepare a [Cu2(1)2](BF4)2 metallocycle (2), whose photoisomerization was characterized using static and time-resolved spectroscopic methods. Optical studies demonstrate the highly quantitative and reproducible photoinduced cyclic E/Z switching without decay of the complex. Accordingly and best to our knowledge, [Cu2(1)2](BF4)2 constitutes the first reversibly photoswitchable (3d)-metallocycle based on azobenzene. The photoinduced multiexponential dynamics in the sub-picosecond to few picosecond time domain of 1 and 2 have been assessed. These ultrafast dynamics as well as the yield of the respective photostationary state (PSSZ = 65 %) resemble the behavior of archetypical azobenzene. Also, the innovative pump-probe laser technique of gas phase transient photodissociation (τ-PD) in a mass spectrometric ion trap was used to determine the intrinsic relaxation dynamics for the isolated complex. These results are consistent with the results from femtosecond UV/Vis transient absorption (fs-TA) in solution, emphasizing the azobenzene-like dynamics of 2. This unique combination of fs-TA and τ-PD enables valuable insights into the prevailing interplay of dynamics and solvation. Both analyses (in solution and gas phase) and quantum chemical calculations reveal a negligible effect of the metal coordination on the switching mechanism and electronic pathway, which suggests a non-cooperative isomerization process.

Arylazopyrazoles for Conjugation by CuAAC Click Chemistry.

Molecular photoswitches are increasingly being implemented in bioactive compounds and responsive materials. For this purpose, photoswitches must be coupled to a wide variety of substrates and scaffolds. We present a library of "clickable" arylazopyrazoles (AAPs), which can be conjugated by Cu-catalyzed alkyne azide cycloaddition (CuAAC). All synthesized AAP alkynes show good photostationary states (at least 88%) and long half-life times of the Z-isomer (18 to 108 h). The AAP azides decompose upon exposure to ultraviolet (UV) irradiation, but after CuAAC, all AAPs exhibit good photophysical properties.

Tris-Azo Triangular Paraphenylenes: Synthesis and Reversible Interconversion into Radial π-Conjugated Macrocycles.

We report the synthesis of cycloparaphenylene derivatives featuring tris-azo groups. The smaller derivative, [3]cycloazobenzene, adopts a triangular all-cis form and exhibits thermally and photochemically stable characteristics due to significant ring strain as well as symmetric Kagome-patterned crystal packing. In contrast, the as-synthesized [3]cycloazobenzene with three biphenylene bridges adopts a triangular all-cis form, which undergoes photoinduced isomerization, leading to a photostationary state. Interestingly, the addition of an excess of acid selectively leads to the formation of an all-trans form. DFT calculations reveal that the interconversion from a triangular to a circular shape correlates with an increase in HOMO and a decrease in LUMO, characteristics intrinsic to radial π-conjugated systems.

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies.

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

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.