Dithienylethene-Based Photoswitchable Phosphines for the Palladium-Catalyzed Stille Coupling Reaction.

Homogeneous transition metal catalysis is a constantly developing field in chemical sciences. A growing interest in this area is photoswitchable catalysis, which pursues in situ modulation of catalyst activity through noninvasive light irradiation. Phosphorus ligands are excellent targets to accomplish this goal by introducing photoswitchable moieties; however, only a limited number of examples have been reported so far. In this work, we have developed a series of palladium complexes capable of catalyzing the Stille coupling reaction that contain photoisomerizable phosphine ligands based on dithienylethene switches. Incorporation of electron-withdrawing substituents into these dithienylethene moieties allows variation of the electron density on the phosphorus atom of the ligands upon light irradiation, which in turn leads to a modulation of the catalytic properties of the formed complexes and their activity in a model Stille coupling reaction. These results are supported by theoretical computations, which show that the energy barriers for the rate-determining steps of the catalytic cycle decrease when the photoswitchable phosphine ligands are converted to their closed state.

Three-Photon Infrared Stimulation of Endogenous Neuroreceptors in Vivo.

To interrogate neural circuits and crack their codes, in vivo brain activity imaging must be combined with spatiotemporally precise stimulation in three dimensions using genetic or pharmacological specificity. This challenge requires deep penetration and focusing as provided by infrared light and multiphoton excitation, and has promoted two-photon photopharmacology and optogenetics. However, three-photon brain stimulation in vivo remains to be demonstrated. We report the regulation of neuronal activity in zebrafish larvae by three-photon excitation of a photoswitchable muscarinic agonist at 50 pM, a billion-fold lower concentration than used for uncaging, and with mid-infrared light of 1560 nm, the longest reported photoswitch wavelength. Robust, physiologically relevant photoresponses allow modulating brain activity in wild-type animals with spatiotemporal and pharmacological precision. Computational calculations predict that azobenzene-based ligands have high three-photon absorption cross-section and can be used directly with pulsed infrared light. The expansion of three-photon pharmacology will deeply impact basic neurobiology and neuromodulation phototherapies.

Fast Photoswitchable Molecular Prosthetics Control Neuronal Activity in the Cochlea.

Artificial control of neuronal activity enables the study of neural circuits and restoration of neural functions. Direct, rapid, and sustained photocontrol of intact neurons could overcome the limitations of established electrical stimulation such as poor selectivity. We have developed fast photoswitchable ligands of glutamate receptors (GluRs) to enable neuronal control in the auditory system. The new photoswitchable ligands induced photocurrents in untransfected neurons upon covalently tethering to endogenous GluRs and activating them reversibly with visible light pulses of a few milliseconds. As a proof of concept of these molecular prostheses, we applied them to the ultrafast synapses of auditory neurons of the cochlea that encode sound and provide auditory input to the brain. This drug-based method afforded the optical stimulation of auditory neurons of adult gerbils at hundreds of hertz without genetic manipulation that would be required for their optogenetic control. This indicates that the new photoswitchable ligands are also applicable to the spatiotemporal control of fast spiking interneurons in the brain.

Synthesis and In Vitro Studies of Photoactivatable Semisquaraine-type Pt(II) Complexes.

The synthesis, full characterization, photochemical properties, and cytotoxic activity toward cisplatin-resistant cancer cell lines of new semisquaraine-type Pt(II) complexes are presented. The synthesis of eight semisquaraine-type ligands has been carried out by means of an innovative, straightforward methodology. A thorough structural NMR and X-ray diffraction analysis of the new ligands and complexes has been done. Density functional theory calculations have allowed to assign the trans configuration of the platinum center. Through the structural modification of the ligands, it has been possible to synthesize some complexes, which have turned out to be photoactive at wavelengths that allow their activation in cell cultures and, importantly, two of them show remarkable solubility in biological media. Photodegradation processes have been studied in depth, including the structural identification of photoproducts, thus justifying the changes observed after irradiation. From biological assessment, complexes C7 and C8 have been demonstrated to behave as promising photoactivatable compounds in the assayed cancer cell lines. Upon photoactivation, both complexes are capable of inducing a higher cytotoxic effect on the tested cells compared with nonphotoactivated compounds. Among the observed results, it is remarkable to note that C7 showed a PI > 50 in HeLa cells, and C8 showed a PI > 40 in A2780 cells, being also effective over cisplatin-resistant A2780cis cells (PI = 7 and PI = 4, respectively). The mechanism of action of these complexes has been studied, revealing that these photoactivated platinum complexes would actually present a combined mode of action, a therapeutically potential advantage.

Reversible Photocontrol of Dopaminergic Transmission in Wild-Type Animals.

Understanding the dopaminergic system is a priority in neurobiology and neuropharmacology. Dopamine receptors are involved in the modulation of fundamental physiological functions, and dysregulation of dopaminergic transmission is associated with major neurological disorders. However, the available tools to dissect the endogenous dopaminergic circuits have limited specificity, reversibility, resolution, or require genetic manipulation. Here, we introduce azodopa, a novel photoswitchable ligand that enables reversible spatiotemporal control of dopaminergic transmission. We demonstrate that azodopa activates D1-like receptors in vitro in a light-dependent manner. Moreover, it enables reversibly photocontrolling zebrafish motility on a timescale of seconds and allows separating the retinal component of dopaminergic neurotransmission. Azodopa increases the overall neural activity in the cortex of anesthetized mice and displays illumination-dependent activity in individual cells. Azodopa is the first photoswitchable dopamine agonist with demonstrated efficacy in wild-type animals and opens the way to remotely controlling dopaminergic neurotransmission for fundamental and therapeutic purposes.

Reversibly Switchable Fluorescent Molecular Systems Based on Metallacarborane-Perylenediimide Conjugates.

Icosahedral metallacarboranes are θ-shaped anionic molecules in which two icosahedra share one vertex that is a metal center. The most remarkable of these compounds is the anionic cobalt-based metallacarborane [Co(C2 B9 H11 )2 ]- , whose oxidation-reduction processes occur via an outer sphere electron process. This, along with its low density negative charge, makes [Co(C2 B9 H11 )2 ]- very appealing to participate in electron-transfer processes. In this work, [Co(C2 B9 H11 )2 ]- is tethered to a perylenediimide dye to produce the first examples of switchable luminescent molecules and materials based on metallacarboranes. In particular, the electronic communication of [Co(C2 B9 H11 )2 ]- with the appended chromophore unit in these compounds can be regulated upon application of redox stimuli, which allows the reversible modulation of the emitted fluorescence. As such, they behave as electrochemically-controlled fluorescent molecular switches in solution, which surpass the performance of previous systems based on conjugates of perylendiimides with ferrocene. Remarkably, they can form gels by treatment with appropriate mixtures of organic solvents, which result from the self-assembly of the cobaltabisdicarbollide-perylendiimide conjugates into 1D nanostructures. The interplay between dye π-stacking and metallacarborane electronic and steric interactions ultimately governs the supramolecular arrangement in these materials, which for one of the compounds prepared allows preserving the luminescent behavior in the gel state.

Optical control of adenosine A3 receptor function in psoriasis.

Psoriasis is a chronic and relapsing inflammatory skin disease lacking a cure that affects approximately 2% of the population. Defective keratinocyte proliferation and differentiation, and aberrant immune responses are major factors in its pathogenesis. Available treatments for moderate to severe psoriasis are directed to immune system causing systemic immunosuppression over time, and thus concomitant serious side effects (i.e. infections and cancer) may appear. In recent years, the Gi protein-coupled A3 receptor (A3R) for adenosine has been suggested as a novel and very promising therapeutic target for psoriasis. Accordingly, selective, and high affinity A3R agonists are known to induce robust anti-inflammatory effects in animal models of autoimmune inflammatory diseases. Here, we demonstrated the efficacy of a selective A3R agonist, namely MRS5698, in preventing the psoriatic-like phenotype in the IL-23 mouse model of psoriasis. Subsequently, we photocaged this molecule with a coumarin moiety to yield the first photosensitive A3R agonist, MRS7344, which in photopharmacological experiments prevented the psoriatic-like phenotype in the IL-23 animal model. Thus, we have demonstrated the feasibility of using a non-invasive, site-specific, light-directed approach to psoriasis treatment.

Adrenergic Modulation With Photochromic Ligands.

Adrenoceptors are ubiquitous and mediate important autonomic functions as well as modulating arousal, cognition, and pain on a central level. Understanding these physiological processes and their underlying neural circuits requires manipulating adrenergic neurotransmission with high spatio-temporal precision. Here we present a first generation of photochromic ligands (adrenoswitches) obtained via azologization of a class of cyclic amidines related to the known ligand clonidine. Their pharmacology, photochromism, bioavailability, and lack of toxicity allow for broad biological applications, as demonstrated by controlling locomotion in zebrafish and pupillary responses in mice.

Multistimuli-Responsive Fluorescent Switches Based on Spirocyclic Meisenheimer Compounds: Smart Molecules for the Design of Optical Probes and Electrochromic Materials.

Fluorescent switches based on spirocyclic zwitterionic Meisenheimer (SZMC) complexes are stimuli-responsive organic molecules with application in a variety of areas. To expand their functionality, novel switching mechanisms are herein reported for these systems: (a) acid- and redox-triggered formation of an additional protonation state with distinct optical properties, and (b) solvent-induced fluorescence modulation. We demonstrate that these new features, which enable both multistimuli and multistate operation of SZMC switches, can be exploited in the preparation of smart organic materials: wide-range pH optical probes, electrochromic and electrofluorochromic films, and polymer-based fluorescent detectors of organic liquids.

Squaramide-Based Pt(II) Complexes as Potential Oxygen-Regulated Light-Triggered Photocages.

Two new squaramide-based platinum(II) complexes C1 and C2 have been synthesized and fully characterized. Their photoresponse has been assessed and is discussed. A remarkable enhancement in the DNA binding activity has been observed for both complexes, up on irradiation. For C2, the release of Pt(II) provoked by its irradiation has been studied. The response of C2 has been found to be regulated by the presence of oxygen. In vitro cytotoxicity tests show an enhancement in the activity of complex C2 after selective irradiation under hypoxic conditions. Resulting Pt(II) species have been isolated and characterized by various analytical methods establishing this type of squaramido-based complexes as a proof of concept for new Pt(II) photocages.

Fluorescent "Turn-Off" Detection of Fluoride and Cyanide Ions Using Zwitterionic Spirocyclic Meisenheimer Compounds.

Stable zwitterionic spirocyclic Meisenheimer compounds were synthesized using a one-step reaction between picric acid and diisopropyl (ZW1) or dicyclohexyl (ZW3) carbodiimide. A solution of these compounds displays intense orange fluorescence upon UV or visible light excitation, which can be quenched or "turned-off" by adding a mole equivalent amount of F- or CN- ions in acetonitrile. Fluorescence is not quenched in the presence of other ions such as Cl-, Br-, I-, NO2-, NO3-, or H2PO4-. These compounds can therefore be utilized as practical colorimetric and fluorescent probes for monitoring the presence of F- or CN- anions.

Temperature-Controlled Switchable Photochromism in Solid Materials.

A novel strategy to achieve thermally switchable photochromism in solid materials is reported, which relies on the preparation of polymeric core-shell capsules containing solutions of photochromic dyes in acidic phase-change materials. Upon changing the phase (solid or liquid) of the encapsulated medium, one of the two photochromic states of the system is selectively stabilized on demand, allowing for reversible interconversion between direct and reverse photochromism when thermally scanning through the melting temperature of the phase-change material. This strategy, which does not require the addition of external agents or chemical modification of the dyes, proved to be general for different spiropyran photochromes and to be applicable to the fabrication of a variety of functional materials by simply embedding the capsules obtained into a solid matrix of choice.

An Optimized Glutamate Receptor Photoswitch with Sensitized Azobenzene Isomerization.

A new azobenzene-based photoswitch, 2, has been designed to enable optical control of ionotropic glutamate receptors in neurons via sensitized two-photon excitation with NIR light. In order to develop an efficient and versatile synthetic route for this molecule, a modular strategy is described which relies on the use of a new linear fully protected glutamate derivative stable in basic media. The resulting compound undergoes one-photon trans-cis photoisomerization via two different mechanisms: direct excitation of its azoaromatic unit and irradiation of the pyrene sensitizer, a well-known two-photon sensitive chromophore. Moreover, 2 presents large thermal stability of its cis isomer, in contrast to other two-photon responsive switches relying on the intrinsic nonlinear optical properties of push-pull substituted azobenzenes. As a result, the molecular system developed herein is a very promising candidate for evoking large photoinduced biological responses during the multiphoton operation of neuronal glutamate receptors with NIR light, which require accumulation of the protein-bound cis state of the switch upon repeated illumination.

Disaggregation-induced fluorescence enhancement of NIAD-4 for the optical imaging of amyloid-ß fibrils.

The main hallmark of Alzheimer's disease is the deposition of amyloid-ß (Aß) aggregates in the brain. An early diagnosis of the disease requires a fast and accurate detection of such aggregates in vivo. NIAD-4 is one of the most promising in vivo markers developed due to its high emission at λ > 600 nm and its ability to rapidly cross the blood-brain barrier (BBB) and target Aß deposits. Furthermore, it shows a dramatic fluorescence enhancement upon binding to amyloid fibrils, which is essential for attaining good imaging contrast. Aiming at establishing novel design concepts for the preparation of optimized optical probes, the current work rationalizes the excellent performance of NIAD-4 by using a pool of computational (TD-DFT and CASPT2 calculations, ab initio molecular dynamics and protein energy landscape exploration) and spectroscopic techniques. Unlike other markers operating as molecular rotors or polarity-sensitive dyes, we uncover herein that the high fluorescence imaging contrast observed upon NIAD-4 binding to amyloid fibrils results from reversible aggregation. NIAD-4 forms non-emissive assemblies in aqueous solution already at very low concentrations, which convert into the highly fluorescent monomeric species by diffusion into the hydrophobic voids of Aß deposits. This result paves the way to exploit aggregation-induced processes as a new strategy towards advanced fluorescence markers for amyloid detection.

Ultrafast dynamics of single molecules.

The detection of individual molecules has found widespread application in molecular biology, photochemistry, polymer chemistry, quantum optics and super-resolution microscopy. Tracking of an individual molecule in time has allowed identifying discrete molecular photodynamic steps, action of molecular motors, protein folding, diffusion, etc. down to the picosecond level. However, methods to study the ultrafast electronic and vibrational molecular dynamics at the level of individual molecules have emerged only recently. In this review we present several examples of femtosecond single molecule spectroscopy. Starting with basic pump-probe spectroscopy in a confocal detection scheme, we move towards deterministic coherent control approaches using pulse shapers and ultra-broad band laser systems. We present the detection of both electronic and vibrational femtosecond dynamics of individual fluorophores at room temperature, showing electronic (de)coherence, vibrational wavepacket interference and quantum control. Finally, two colour phase shaping applied to photosynthetic light-harvesting complexes is presented, which allows investigation of the persistent coherence in photosynthetic complexes under physiological conditions at the level of individual complexes.

Two-photon neuronal and astrocytic stimulation with azobenzene-based photoswitches.

Synthetic photochromic compounds can be designed to control a variety of proteins and their biochemical functions in living cells, but the high spatiotemporal precision and tissue penetration of two-photon stimulation have never been investigated in these molecules. Here we demonstrate two-photon excitation of azobenzene-based protein switches and versatile strategies to enhance their photochemical responses. This enables new applications to control the activation of neurons and astrocytes with cellular and subcellular resolution.

Photomodulation of G protein-coupled adenosine receptors by a novel light-switchable ligand.

The adenosinergic system operates through G protein-coupled adenosine receptors, which have become promising therapeutic targets for a wide range of pathological conditions. However, the ubiquity of adenosine receptors and the eventual lack of selectivity of adenosine-based drugs have frequently diminished their therapeutic potential. Accordingly, here we aimed to develop a new generation of light-switchable adenosine receptor ligands that change their intrinsic activity upon irradiation, thus allowing the spatiotemporal control of receptor functioning (i.e., receptor activation/inactivation dependent on location and timing). Therefore, we synthesized an orthosteric, photoisomerizable, and nonselective adenosine receptor agonist, nucleoside derivative MRS5543 containing an aryl diazo linkage on the N(6) substituent, which in the dark (relaxed isomer) behaved as a full adenosine A3 receptor (A3R) and partial adenosine A2A receptor (A2AR) agonist. Conversely, upon photoisomerization with blue light (460 nm), it remained a full A3R agonist but became an A2AR antagonist. Interestingly, molecular modeling suggested that structural differences encountered within the third extracellular loop of each receptor could modulate the intrinsic, receptor subtype-dependent, activity. Overall, the development of adenosine receptor ligands with photoswitchable activity expands the pharmacological toolbox in support of research and possibly opens new pharmacotherapeutic opportunities.

Amplified photomodulation of a bis(dithienylethene)-substituted phosphine.

Phosphine ligands play a crucial role in homogeneous catalysis, allowing fine-tuning of the catalytic activity of various metals by modifying their structure. An ultimate challenge in this field is to reach controlled modulation of catalysis in situ, for which the development of phosphines capable of photoswitching between states with differential electronic properties has been proposed. To magnify this light-induced behavior, in this work we describe a novel phosphine ligand incorporating two dithienylethene photoswitchable moieties tethered to the same phosphorus atom. Double photoisomerization was observed for this ligand, which remains unhindered upon gold(I) complexation. As a result, the preparation of a fully ring-closed phosphine isomer was accomplished, for which amplified variation of phosphorus electron density was verified both experimentally and by computational calculations. Accordingly, the presented molecular design based on multiphotochromic phosphines could open new ways for preparing enhanced photoswitchable catalytic systems.

Encapsulation and release mechanisms in coordination polymer nanoparticles.

The interplay of guest encapsulation and release mechanisms in nanoscale metal-organic vehicles and its effect on the drug-delivery kinetics of these materials were investigated through a new multidisciplinary approach. Two rationally-designed molecular guests were synthesized, which consist of a red-fluorescent benzophenoxazine dye covalently tethered to a coordinating catechol group and a protected, non-coordinating catechol moiety. This allowed loading of the guests into compositionally and structurally equivalent coordination polymer particles through distinct encapsulation mechanisms: coordination and mechanical entrapment. The two types of particles delivered their fluorescent cargo with remarkably different kinetic profiles, which could be satisfactorily modeled considering degradation- and diffusion-controlled release processes. This demonstrates that careful selection of the method of guest incorporation into coordination polymer nanoparticles allows selective tuning of the rate of drug delivery from these materials and, therefore, of the time window of action of the encapsulated therapeutic agents.

Spiropyran-based chromic hydrogels for CO2 absorption and detection.

By enabling rapid, cost-effective, user-friendly and in situ detection of carbon dioxide, colorimetric CO2 sensors are of relevance for a variety of fields. However, it still remains a challenge the development of optical chemosensors for CO2 that combine high sensitivity, selectivity and reusability with facile integration into solid materials. Herein we pursued this goal by preparing hydrogels functionalized with spiropyrans, a well-known class of molecular switches that undergo different color changes upon application of light and acid stimuli. By varying the nature of the substituents of the spiropyran core, different acidochromic responses are obtained in aqueous media that allow discriminating CO2 from other acid gases (e.g., HCl). Interestingly, this behavior can be transferred to functional solid materials by synthesizing polymerizable spiropyran derivatives, which are used to prepare hydrogels. These materials preserve the acidochromic properties of the incorporated spiropyrans, thus leading to selective, reversible and quantifiable color changes upon exposure to different CO2 amounts. In addition, CO2 desorption and, therefore, recovery of the initial state of the chemosensor is favored by irradiation with visible light. This makes spiropyran-based chromic hydrogels promising systems for the colorimetric monitorization of carbon dioxide in a diversity of applications.