Solution and Solid-State Emission Toggling of a Photochromic Hydrazone

E- or Z-Isomers Arising from the Geometries of Ligands in the Mercury Complex of 2-(Anthracen-9-ylmethylene)-N-phenylhydrazine Carbothioamide

An anionic mercury(II) complex of 2-(anthracen-9-ylmethylene)-N-phenylhydrazine carbothioamide (HATU) and two isomers of a neutral mercury(II) complex of the anion of the same ligand (ATU) were reported. The anionic complex [Hg(HATU)2Cl2]·CH2Cl2 had a monodentate HATU ligand (a neutral form of the ligand) and chloride ligands. The two conformational isomers were of the neutral mercury(II) complex Hg(ATU)2·2DMF. The two isomers were from the E or Z geometry of the ligands across the conjugated C=N-N=C-N scaffold of the coordinated ligand. The two isomers of the complex were independently prepared and characterized. The spectroscopic properties of the isomers in solution were studied by 1H NMR as well as fluorescence spectroscopy. Facile conversion of the E-isomer to the Z-isomer in solution was observed. Density functional theory (DFT) calculations revealed that the Z-isomer of the complex was stable compared to the E-isomer by an energy of 14.35 kJ/mol; whereas, E isomer of the ligand was more stable than Z isomer by 8.37 KJ/mol. The activation barrier for the conversion of the E-isomer to the Z-isomer of the ligand was 167.37 kJ/mol. The role of the mercury ion in the conversion of the E-form to the Z-form was discussed. The mercury complex [Hg(HATU)2Cl2]·CH2Cl2 had the E-form of the ligand. Distinct photophysical features of these mercury complexes were presented.

Dynamic Metal-ligand Coordination for Fluorescence Color Regulation of Hydrazone-based Bistable Photoswitches

Achieving effective manipulation of emission color in photoresponsive materials is crucial for various advanced photonic applications. In this study, we designed and synthesized a hydrazone compound 1, ethyl (Z)-2-(2-([2,2':6',2''-terpyridin]-4'-yl)hydrazineylidene)-2-(4-(diphenylamino)phenyl)acetate, which possesses a push-pull structure incorporating triphenylamine and terpyridine. The emission intensity of compound 1 can be repeatedly switched "off" and "on" by irradiation with visible light and UV light, which induces the isomerization transition between the Z and E forms. In addition, compound 1 is capable of changing its emission wavelength from 540 nm to 607 nm through coordination with different zinc salts in toluene/CH2 Cl2 mixture (v : v=1 : 1). Importantly, we have successfully achieved dynamic manipulation of fluorescence color and intensity by altering the counterions of zinc complexes and switching the isomer from Z to E. Moreover, both compound 1 and its zinc complexes demonstrate remarkable photoswitchable properties with different fluorescence colors in the thin films. Finally, these films with various fluorescence colors were used for the production of luminescent tags.

Photoresponsive Supramolecular Polymers: From Light-Controlled Small Molecules to Smart Materials

Photoresponsive supramolecular polymers are well-organized assemblies based on highly oriented and reversible noncovalent interactions containing photosensitive molecules as (co-)monomers. They have attracted increasing interest in smart materials and dynamic systems with precisely controllable functions, such as light-driven soft actuators, photoresponsive fluorescent anticounterfeiting and light-triggered electronic devices. The present review discusses light-activated molecules used in photoresponsive supramolecular polymers with their main photo-induced changes, e.g., geometry, dipole moment, and chirality. Based on these distinct changes, supramolecular polymers formed by light-activated molecules exhibit photoresponsive disassembly and reassembly. As a consequence, photo-induced supramolecular polymerization, "depolymerization," and regulation of the lengths and topologies are observed. Moreover, the light-controlled functions of supramolecular polymers, such as actuation, emission, and chirality transfer along length scales, are highlighted. Furthermore, a perspective on challenges and future opportunities is presented. Besides the challenge of moving from harmful UV light to visible/near IR light avoiding fatigue, and enabling biomedical applications, future opportunities include light-controlled supramolecular actuators with helical motion, light-modulated information transmission, optically recyclable materials, and multi-stimuli-responsive supramolecular systems.

Solvent-Controlled Photoswitching of Azobenzene: An Excited State Shuttle

The light-controlled excited state trans-cis isomerization process is a key to the development of conversion of light energy to mechanical motion at the molecular level. Considerable efforts have been made in tuning the isomerization process with electron donor and acceptor substituents by altering the excited state reaction coordinate. Here, we report a two novel push-pull series of para-diethylamino (DEA) and diphenylamino (DPA) substituted (E)-4'-((4-(diethylamino)phenyl)diazenyl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (1) and (E)-4'-((4-(diphenylamino)phenyl)diazenyl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (2). Compounds 1 and 2 undergo both photochemical and photophysical excited state deactivation pathways which can be controlled by varying the solvent polarity. These structural motifs of 1 and 2 would undergo torsional motions upon excitation to exhibit either trans→cis photoisomerization or to form a twisted intramolecular charge transfer state and both the process originates from the same excited state and competes with each other. Thus, alternations in the surrounding environment such as solvent polarity, solution viscosity, and protonation were employed to understand the preferential excited state deactivation pathway and thereby these systems could be employed as a new class of azobenzene-based luminescent photochromic molecules. For instance, in nonpolar solvent, toluene photoisomerization is preferred over TICT, but polar solvent, ethanol preferentially stabilizes the TICT state by virtue of N-C rotation which renders the energy barrier unfavourable for photoisomerization. The photostationary state of 1 in toluene is predominantly the Z isomer, whereas in ethanol E isomer is nearly two-fold higher than the Z isomer. These feature sets up a new approach towards the construction of multinary molecular switches and subsequent development in diverse fields.

Confinement-Driven Photophysics in Hydrazone-Based Hierarchical Materials

Confinement-imposed photophysics was probed for novel stimuli-responsive hydrazone-based compounds demonstrating a conceptual difference in their behavior within 2D versus 3D porous matrices for the first time. The challenges associated with photoswitch isomerization arising from host interactions with photochromic compounds in 2D scaffolds could be overcome in 3D materials. Solution-like photoisomerization rate constants were realized for sterically demanding hydrazone derivatives in the solid state through their coordinative immobilization in 3D scaffolds. According to steady-state and time-resolved photophysical measurements and theoretical modeling, this approach provides access to hydrazone-based materials with fast photoisomerization kinetics in the solid state. Fast isomerization of integrated hydrazone derivatives allows for probing and tailoring resonance energy transfer (ET) processes as a function of excitation wavelength, providing a novel pathway for ET modulation.

Ultrafast Evaluation of Two-Photon Absorption with Simplified Time-Dependent Density Functional Theory

This work presents the theoretical background to evaluate two-photon absorption (2PA) cross-sections in the framework of simplified time-dependent density functional theory (sTD-DFT). Our new implementation allows the ultrafast evaluation of 2PA cross-sections for large molecules based on a regular DFT ground-state determinant as well as a variant employing our tight-binding sTD-DFT-xTX flavor for very large systems. The method is benchmarked against higher-level calculations for trans-stilbene and typical fluorescent protein chromophores. For eGFP, a quadrupolar chromophore and its branched version, the flavine mono-nucleotide, and the iLOV protein, we compare sTD-DFT 2PA spectra to experimental ones. This includes extension and testing of our all-atom quantum chemistry methodology for the evaluation of 2PA for a system of ∼2000 atoms, providing striking agreement with the experimental spectrum.

Photon Energy Storage in Strained Cyclic Hydrazones: Emerging Molecular Solar Thermal Energy Storage Compounds

The generally small Gibbs free energy difference between the Z and E isomers of hydrazone photoswitches has so far precluded their use in photon energy storing applications. Here, we report on a series of cyclic and acyclic hydrazones, which possess varied degrees of ring strain and, hence, stability of E isomers. The photoinduced isomerization and concurrent phase transition of the cyclic hydrazones from a crystalline to a liquid phase result in the storage of a large quantity of energy, comparable to that of azobenzene derivatives. We demonstrate that the macrocyclic photochrome design in combination with phase transition is a promising strategy for molecular solar thermal energy storage applications.

Fabricating and Modulating Robust Multi-Photoaddressable Systems with the Derivatives of Diarylethylene and Donor-Acceptor Stenhouse Adducts

Multi-photoaddressable systems (MPSs) belong to complex systems, which are comprised of more than one photoswitching molecule and can respond to different wavelengths of light simultaneously. While MPSs have been extensively applied in various fields, there are also some challenges, such as the deficiency of the wavelength-selective control and the interference from the poor thermodynamic stability of used photoswitching molecules. Herein, we reported two robust MPSs (MPS1/2) consisting of diarylethylene derivative (DAE) and different donor-acceptor Stenhouse adducts (DASAs), in which both opened and closed forms of DAE and opened forms of DASAs are thermodynamically stable. MPS1/2 enable fully reversible cyclic photoswitching with improved thermal interference resistance. Moreover, MPS2 also shows a favorable property in PMMA films and has been applied in multicolor display. It is expected that the prepared MPSs could be used in more fields such as information storage and reading and encoding light.

Hydrazone Photoswitches for Structural Modulation of Short Peptides

Molecules that undergo light-driven structural transformations constitute the core components in photoswitchable molecular systems and materials. Among various families of photoswitches, photochromic hydrazones have recently emerged as a novel class of photoswitches with superb properties, such as high photochemical conversion, spectral tunability, thermal stability, and fatigue resistance. Hydrazone photoswitches have been adopted in various adaptive materials at different length scales, however, their utilization for modulating biomolecules still has not been explored. Herein we present new hydrazone switches that can photomodulate the structures of short peptides. Systematic investigation on a set of hydrazone derivatives revealed that installation of the amide group does not significantly alter the photoswitching behaviors. Importantly, a hydrazone switch comprising an upper phenyl ring and a lower quinolinyl ring was effective for structural control of peptides. We anticipate that this work, as a new milestone in the research of hydrazone switches, will open a new avenue for structural and functional control of biomolecules.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

3D Hydrazone-Functionalized Covalent Organic Frameworks as pH-Triggered Rotary Switches

The property expansion of 3D functionalized covalent organic frameworks (COFs) is important for developing their potential applications. Herein, the first case of 3D hydrazone-decorated COFs as pH-triggered molecular switches is reported, and their application in the stimuli-responsive drug delivery system is explored. These functionalized COFs with hydrazone groups on the channel walls are obtained via a multi-component bottom-up synthesis strategy. They exhibit a reversible E/Z isomerization at various pH values, confirmed by UV-vis absorption spectroscopy and proton conduction. Remarkably, after loading cytarabine (Ara-C) as a model drug molecule, these pH-responsive COFs show an excellent and intelligent sustained-release effect with an almost fourfold increase in the Ara-C release at pH = 4.8 than at pH = 7.4, which will effectively improve drug-targeting. Thus, these results open a way toward designing 3D stimuli-responsive functionalized COF materials and promote their potential application as drug carriers in the field of disease treatment.

DNA Functionality with Photoswitchable Hydrazone Cytidine*

A new family of hydrazone modified cytidine phosphoramidite building block was synthesized and incorporated into oligodeoxynucleotides to construct photoswitchable DNA strands. The E-Z isomerization triggered by the irradiation of blue light with a wavelength of 450 nm was investigated and confirmed by ¹ H NMR spectroscopy and HPLC in the contexts of both nucleoside and oligodeoxynucleotide. The light activated Z form isomer of this hydrazone-cytidine with a six-member intramolecular hydrogen bond was found to inhibit DNA synthesis in the primer extension model by using Bst DNA polymerase. In addition, the hydrazone modification caused the misincorporation of dATP together with dGTP into the growing DNA strand with similar selectivity, highlighting a potential G to A mutation. This work provides a novel functional DNA building block and an additional molecular tool that has potential chemical biology and biomedicinal applications to control DNA synthesis and DNA-enzyme interactions using the cell friendly blue light irradiation.

Turning On Solid-State Luminescence by Phototriggered Subtle Molecular Conformation Variations

The development of solid-state intelligent materials, in particular those showing photoresponsive luminescence (PRL), is highly desirable for their cutting-edge applications in sensors, displays, data-storage, and anti-counterfeiting, but is challenging. Few PRL materials are constructed by tethering the classic photochromic systems with newly-emerged solid-state emitters. Selective solid-state photoreactions are demanded to precisely manipulate the luminescent behavior of these emitters, which require dramatic structural change and enough free space, thus limiting the scope of the PRL family. Here, a new PRL material, TPE-4N, that features sensitive and reversible fluorescence switching is reported. The interesting on-off luminescent property of TPE-4N can be facilely tuned through fast phototriggering and thermal annealing. Experimental and theoretical investigations reveal that subtle molecular conformation variation induces the corresponding PRL behavior. The crystalline and amorphous state endows an efficient and weak ISC process, respectively, to turn on and off the emission. The readily fabricated thin-film of TPE-4N exhibits non-destructive PRL behavior with high contrast (>10² ), good light transmittance (>72.3%), and great durability and reversibility under room light for months. Remarkably, a uniform thin-film with such fascinating PRL properties allows high-tech applications in invisible anti-counterfeiting and dynamic optical data storage with micro-resolution.

Studies of novel trifluoroacetylated diaryl hydrazone molecular photoswitches in solution and in the solid state

Photoisomerization of trifluoroacetylated diaryl hydrazones induces a change in color due to the formation of a new hydrogen bond in solution and in the solid state.

Solid state emissive azo-Schiff base ligands and their Zn(ii) complexes: acidochromism and photoswitching behaviour

Two novel azo Schiff base ligands L1 and L2 and Zn(ii) complexes C1 and C2 exhibiting reversible acidochromic behavior and photoswitching properties have been described.

Photophysical Properties of 4-Dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran Revisited: Fluorescence versus Photoisomerization

Although 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM) has been known for many decades as a bright and photostable fluorophore, used for a wide variety of applications in chemistry, biology and physics, only little attention has been paid so far to the presence of multiple isomers and conformers, namely s-trans-(E), s-cis-(E), s-trans-(Z), and s-cis-(Z). In particular, light-induced E-Z isomerization plays a great role on the overall photophysical properties of DCM. Herein, we give a full description of a photoswitchable DCM derivative by a combination of structural, theoretical and spectroscopic methods. The main s-trans-(E) isomer is responsible for most of the fluorescence features, whereas the s-cis-(E) conformer only contributes marginally. The non-emitting Z isomers are generated in large conversion yields upon illumination with visible light (e.g., 485 or 514 nm) and converted back to the E forms by UV irradiation (e.g., 365 nm). Such photoswitching is efficient and reversible, with high fatigue resistance. The E→Z and Z→E photoisomerization quantum yields were determined in different solvents and at different irradiation wavelengths. Interestingly, the fluorescence and photoisomerization properties are strongly influenced by the solvent polarity: the fluorescence is predominant at higher polarity, whereas photoisomerization becomes more efficient at lower polarity. Intermediate medium (THF) represents an optimized situation with a good balance between these two features.

Two-photon excited deep-red and near-infrared emissive organic co-crystals

Two-photon excited near-infrared fluorescence materials have garnered considerable attention because of their superior optical penetration, higher spatial resolution, and lower optical scattering compared with other optical materials. Herein, a convenient and efficient supramolecular approach is used to synthesize a two-photon excited near-infrared emissive co-crystalline material. A naphthalenediimide-based triangular macrocycle and coronene form selectively two co-crystals. The triangle-shaped co-crystal emits deep-red fluorescence, while the quadrangle-shaped co-crystal displays deep-red and near-infrared emission centered on 668 nm, which represents a 162 nm red-shift compared with its precursors. Benefiting from intermolecular charge transfer interactions, the two co-crystals possess higher calculated two-photon absorption cross-sections than those of their individual constituents. Their two-photon absorption bands reach into the NIR-II region of the electromagnetic spectrum. The quadrangle-shaped co-crystal constitutes a unique material that exhibits two-photon absorption and near-infrared emission simultaneously. This co-crystallization strategy holds considerable promise for the future design and synthesis of more advanced optical materials.

New Donor-Acceptor Stenhouse Adducts as Visible and Near Infrared Light Polymerization Photoinitiators

Polymerization photoinitiators that can be activated under low light intensity and in the visible range are being pursued by both the academic and industrial communities. To efficiently harvest light and initiate a polymerization process, dyes with high molar extinction coefficients in the visible range are ideal candidates. In this field, Donor-acceptor Stenhouse Adducts (DASA) which belong to a class of recently discovered organic photochromic molecules still lack practical applications. In this work, a series of DASA-based dyes are proposed as photoinitiators for the free radical polymerization of (meth)acrylates upon exposure to a near infrared light (laser diode at 785 nm).

The Many Ways to Assemble Nanoparticles Using Light

The ability to reversibly assemble nanoparticles using light is both fundamentally interesting and important for applications ranging from reversible data storage to controlled drug delivery. Here, the diverse approaches that have so far been developed to control the self-assembly of nanoparticles using light are reviewed and compared. These approaches include functionalizing nanoparticles with monolayers of photoresponsive molecules, placing them in photoresponsive media capable of reversibly protonating the particles under light, and decorating plasmonic nanoparticles with thermoresponsive polymers, to name just a few. The applicability of these methods to larger, micrometer-sized particles is also discussed. Finally, several perspectives on further developments in the field are offered.

Visualizing intracellular particles and precise control of drug release using an emissive hydrazone photochrome

The spatiotemporal control over the structure of nanoparticles while monitoring their localization in tumor cells can improve the precision of controlled drug release, thus enhancing the efficiency of drug delivery. Here, we report on a photochromic nanoparticle system (LSNP), assembled from fluorescent bistable hydrazone photoswitch-modified amphiphilic copolymers. The intrinsic emission of the hydrazone switch allows for the visualization of particle uptake, as well as their intracellular distribution. The Z → E photoswitching of the hydrazone switch within the nanoparticle leads to the expansion of the nanoparticles (i.e., drug release) accompanied by emission quenching, the degree of which can function as an internal indicator for the amount of drug released. The bistability of the switch enables the kinetic trapping of particles of different sizes as a function of irradiation time, and allows for the exhibition of light-dependent cell cytotoxicity in MDA-MB-231 cells using LSNP loaded with doxorubicin.

Planarization-Induced Activation Wavelength Red-Shift and Thermal Half-Life Acceleration in Hydrazone Photoswitches

The optimization and modulation of the properties of photochromic compounds, such as their activation wavelengths and thermal relaxation half-lives (τ1/2), are essential for their adaptation in various applications. In this work, we studied the effect of co-planarization of the rotary fragment of two photochromic hydrazones with the core of the molecule on their switching properties. The Z and E isomers of both compounds exhibit red-shifted absorption bands relative to their twisted versions, allowing for their photoswitching using longer wavelengths of light. Additionally, the thermal half-lives of both hydrazones are drastically shortened from hundreds of years to days.

Synergistic Approach of Ultrafast Spectroscopy and Molecular Simulations in the Characterization of Intramolecular Charge Transfer in Push-Pull Molecules

The comprehensive characterization of Intramolecular Charge Transfer (ICT) stemming in push-pull molecules with a delocalized π-system of electrons is noteworthy for a bespoke design of organic materials, spanning widespread applications from photovoltaics to nanomedicine imaging devices. Photo-induced ICT is characterized by structural reorganizations, which allows the molecule to adapt to the new electronic density distribution. Herein, we discuss recent photophysical advances combined with recent progresses in the computational chemistry of photoactive molecular ensembles. We focus the discussion on femtosecond Transient Absorption Spectroscopy (TAS) enabling us to follow the transition from a Locally Excited (LE) state to the ICT and to understand how the environment polarity influences radiative and non-radiative decay mechanisms. In many cases, the charge transfer transition is accompanied by structural rearrangements, such as the twisting or molecule planarization. The possibility of an accurate prediction of the charge-transfer occurring in complex molecules and molecular materials represents an enormous advantage in guiding new molecular and materials design. We briefly report on recent advances in ultrafast multidimensional spectroscopy, in particular, Two-Dimensional Electronic Spectroscopy (2DES), in unraveling the ICT nature of push-pull molecular systems. A theoretical description at the atomistic level of photo-induced molecular transitions can predict with reasonable accuracy the properties of photoactive molecules. In this framework, the review includes a discussion on the advances from simulation and modeling, which have provided, over the years, significant information on photoexcitation, emission, charge-transport, and decay pathways. Density Functional Theory (DFT) coupled with the Time-Dependent (TD) framework can describe electronic properties and dynamics for a limited system size. More recently, Machine Learning (ML) or deep learning approaches, as well as free-energy simulations containing excited state potentials, can speed up the calculations with transferable accuracy to more complex molecules with extended system size. A perspective on combining ultrafast spectroscopy with molecular simulations is foreseen for optimizing the design of photoactive compounds with tunable properties.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Tunable Aggregation-Induced Emission of Imidazole Hydrazones by pH and Anions

Aggregation-induced emission (AIE) materials have drawn great attention for applications as organic light-emitting diodes (OLED) and probes. The applications are, however, restricted by the complex syntheses and hydrophobic properties. Herein, a one-step synthesis of an AIE material based on imidazole hydrazone is assessed. Protonation of the imidazole-H leads to emission color change from yellow to green in the solid state. The emission color is recovered upon imidazole-H⁺ deprotonation. Moreover, the emission wavelength shifts from 532 to 572 nm by anion exchange. In addition, an enhanced emission (Φ_F up to 22.6 %) was obtained with the Br^- anion compared with NTf₂ ^- , SbCl₅ ^- , PF₆ ^- , and OTf^- anions. X-ray crystallography studies together with theoretical calculations show that the enhanced emission of hydrazone salts arises from strong hydrogen bonding between the hydrazone proton and the halide ion (Cl^- or Br^- ).

Visualization and Manipulation of Molecular Motion in the Solid State through Photoinduced Clusteroluminescence

The development of molecular machines has long been a dream of scientists and is expected to revolutionize many aspects of technology and medicine. As the prerequisite of a practicable molecular machine, studies on the solid-state molecular motion (SSMM) are not only of scientific importance but also practically useful. Herein, two nonconjugated molecules, 1,2-diphenylethane (s-DPE) and 1,2-bis(2,4,5-trimethylphenyl)ethane (s-DPE-TM), are synthesized, and their SSMM is investigated. Experimental and calculation results reveal that s-DPE and s-DPE-TM are capable of performing light-driven SSMM to form excited-state through-space complexes (ESTSC). The radiative decay of ESTSC generates an unexpected visible emission termed clusteroluminescence, which serves as a tool to visualize the process of SSMM. Meanwhile, the original packing structure can be recovered from ESTSC after the removal of light irradiation. This work provides a new strategy to manipulate and "see" the SSMM and gains new insights into clusteroluminescence.

Ultrafast processes triggered by one- and two-photon excitation of a photochromic and luminescent hydrazone

In this work we apply a combination of steady state and time resolved luminescence and absorption spectroscopies to investigate the excited-state dynamics of a recently developed molecular photoswitch, belonging to the hydrazone family. The outstanding properties of this molecule, involving fluorescence toggling, bistability, high isomerization quantum yield and non-negligible two-photon absorption cross section, make it very promising for numerous applications. Here we show that the light induced Z/E isomerization occurs on a fast <1 ps timescale in both toluene and acetonitrile, while the excited state lifetime of the Z-form depends on solvent polarity, suggesting a partial charge transfer nature of its low lying excited state. Time-resolved luminescence measurements evidence the presence of a main emission component in the 500-520 nm spectral range, attributed to the Z-isomer, and a very short living blue-shifted emission, attributed to the E-isomer. Finally, transient absorption measurements performed upon far-red excitation are employed as an alternative method to determine the two-photon absorption cross-section of the molecule.

Solvation-dependent switching of solid-state luminescence of a fluorinated aromatic tetrapyrazole

Creating stimuli-responsive materials with switchable solid-state luminescence remains a challenge. We report that the solvation of a novel organic fluorophore can be utilized to prepare such a material, which emits in the blue (442-446 nm) region when wet and in the green (497-503 nm) region when dry.

Strategies to maximize performance in STimulated Emission Depletion (STED) nanoscopy of biological specimens

Super-resolution fluorescence microscopy has become an important catalyst for discovery in the life sciences. In STimulated Emission Depletion (STED) microscopy, a pattern of light drives fluorophores from a signal-emitting on-state to a non-signalling off-state. Only emitters residing in a sub-diffraction volume around an intensity minimum are allowed to fluoresce, rendering them distinguishable from the nearby, but dark fluorophores. STED routinely achieves resolution in the few tens of nanometers range in biological samples and is suitable for live imaging. Here, we review the working principle of STED and provide general guidelines for successful STED imaging. The strive for ever higher resolution comes at the cost of increased light burden. We discuss techniques to reduce light exposure and mitigate its detrimental effects on the specimen. These include specialized illumination strategies as well as protecting fluorophores from photobleaching mediated by high-intensity STED light. This opens up the prospect of volumetric imaging in living cells and tissues with diffraction-unlimited resolution in all three spatial dimensions.

Shape-Persistent Actuators from Hydrazone Photoswitches

Interfacing molecular photoswitches with liquid crystal polymers enables the amplification of their nanoscale motion into macroscopic shape transformations. Typically, the mechanism responsible for actuation involves light-induced molecular disorder. Here, we demonstrate that bistable hydrazones can drive (chiral) shape transformations in liquid crystal polymer networks, with photogenerated polymer shapes displaying a long-term stability that mirrors that of the switches. The mechanism involves a photoinduced buildup of tension in the polymer, with a negligible influence on the liquid crystalline order. Hydrazone-doped liquid crystal systems thus diversify the toolbox available to the field of light-adaptive molecular actuators and hold promise in terms of soft robotics.

Boosting the performance of energetic materials through thermally-induced conformational transition

We presented an effective strategy to improve the performance of energetic materials through thermally-induced conformational transition.

White-light emission from a structurally simple hydrazone

Two hydrazones featuring a unique excitation wavelength-dependent dual fluorescence emission have been developed.