Efficiently Photocontrollable or Not? Biological Activity of Photoisomerizable Diarylethenes.

Diarylethene derivatives, the biological activity of which can be reversibly changed by irradiation with light of different wavelengths, have shown promise as scientific tools and as candidates for photocontrollable drugs. However, examples demonstrating efficient photocontrol of their biological activity are still relatively rare. This concept article discusses the possible reasons for this situation and presents a critical analysis of existing data and hypotheses in this field, in order to extract the design principles enabling the construction of efficient photocontrollable diarylethene-based molecules. Papers addressing biologically relevant interactions between diarylethenes and biomolecules are analyzed; however, in most published cases, the efficiency of photocontrol in living systems remains to be demonstrated. We hope that this article will encourage further discussion of design principles, primarily among pharmacologists, synthetic and medicinal chemists.

Efficient Synthesis and Photosensitizer Performance of Nonplanar Organic Donor-Acceptor Molecules.

Nonplanar organic donor-acceptor molecules bearing a carboxylic acid group were synthesized by the formal [2+2] cycloaddition-retroelectrocyclization reaction between aniline-substituted alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ). This reaction offers an atom-economic one-step approach to donor-acceptor chromophores in satisfactory high yields. The resulting donor-acceptor molecules were characterized by conventional analytical techniques. In addition, the nonplanarity and intermolecular interactions were investigated by X-ray crystallography. The energy levels and intramolecular charge-transfer (CT), evaluated by UV-Vis-near IR spectroscopy and electrochemistry, suggested that there is a linear correlation between the optical and electrochemical band gaps. Based on these structural and electronic analyses, the photosensitizer performances of the donor-acceptor molecules in dye-sensitized solar cells (DSSCs) were initially investigated using TiO2 or SnO2 electrodes. Although the power conversion efficiencies were limited, the incident-photon-to-current-conversion efficiency (IPCE) spectra indicated a better photocurrent generation for the devices on SnO2 as compared to those on TiO2.

Fluorescence and morphology modulation in a photochromic diarylethene self-assembly system.

Switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (1) were designed and fabricated. 1 can gelate several aprotic solvents with different morphologies. The gel turned into partial gel in ethyl acetate with the addition of DTE as a guest molecule. Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation. Meanwhile, the morphology of the coassembly of 1(2)·DTE changed to film from original pieces of gel 1 in ethyl acetate. When 1(2)·DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into fluorescence and morphology controlling, especially application in upscale smart responsive materials.

Synthesis and photochromic properties of imidazole-based diarylethenes.

A series of symmetrical imidazole- and N-methylated imidazole-based diarylethenes were synthesized, and the structures have been well characterized by NMR spectroscopy and mass spectrometry. Their photochromism were investigated upon UV/vis light irradiation in DMF solution. A significant red-shift of UV absorption was observed after irradiation with UV light when substituents on the benzene ring (such as methoxyl group) were introduced. But N-methylation on the two imidazole rings will result in a blue-shift of UV absorption. The photophysical properties can be adjusted by changing the substituents, which provides a new strategy for the design of diarylethenes with excellent properties.

Photochromism of diarylethene single molecules and single crystals.

Photochromism is characterized as a reversible colour change of a chemical species upon photoirradiation. Although vast numbers of photochromic molecules have been so far synthesized, molecules which exhibit thermally irreversible photochromic reactivity are limited within a few families of compounds. Among the thermally irreversible photochromic molecules, diarylethene derivatives are the most promising candidates for applications to opto-electronic devices because of their fatigue resistance, high sensitivity and rapid response. In this review, characteristic photochromic performances of the derivatives, detection and analysis of the photochromic reactions at a single-molecule level and application of the single-crystalline photochromism to molecular machinery will be described.

A photocontrolled molecular switch regulates paralysis in a living organism.

Using light to modulate biochemical agents in living organisms has a significant impact on photodynamic therapy and drug release. We demonstrate that a photoresponsive system can reversibly induce paralysis in nematodes as a model for living organisms when two different wavelengths of light are used to toggle the molecular switch between its two structural forms. This example illustrates how photoswitches offer great potential for advancing biomedical technologies.

Uni- and bi-directional light-induced switching of diarylethenes on gold nanoparticles.

Photochromic studies of diarylethenes with their switching unit linked to the surface of gold nanoparticles via a conjugated aromatic spacer show linker-dependent switching behavior.

A diarylethene as the SO2 gas generator upon UV irradiation.

A closed-ring isomer of a diarylethene having a sulfone group works as the reagent for SO2 gas generation with thermal stability even at 70 °C, and it rapidly reverts to the open-ring isomer and generates the SO2 gas to induce cell death upon UV irradiation.

The radiolysis of aqueous solutions of ethylene.

The products of the radiolysis of deaerated aqueous solutions of ethylene at atmospheric pressure are butane, hexane, octane, decane, acetaldehyde, butyraldehyde, ethanol, butanol, and hydrogen perioxide. Product yields, especially those of the aldehydes and hydrogen peroxide, increase with increasing ethylene concentration and decrease with increasing pH and radiation dose. Addition of efficient electron scavengers increases the yields of oxidized products and decreases the yields of reduced products. The results are compatible with a mechanism in which OH radicals add to rather than abstract from ethylene. Computer-aided analysis of the kinetics of competing reactions indicates that the low yields of aldehydes are related to the ability of these compounds to act as internal scavengers of solvated electrons.

The use of a calorimeter for neutron dosimetry.

The dose given to a polythene energy absorber in the radiation field from a 14 MeV neutron generator has been measured calorimetrically. The calorimeter was calibrated by giving the energy absorber a known dose of gamma-rays. The dose measured in this way was compared with that determined with a polythene-ethylene ionization chamber, and the two results agreed to better than 1%, though the estimated uncertainties on the calorimeter and ionization chamber results were each about 4%. This result can be used to determine a value of the ratio Wn/We, for 14 MeV neutrons in ethylene, where Wn is the W value for the charged particles generated in the neutron field in ethylene, and We is the W value for electrons in the same gas. The measured value of Wn/We was found to be 1.07 plus or minus 3.9%. The significance of the 'thermal defect' in polythene is discussed.

Comparison of the stopping cross-sections of ethylene and polyethylene using alpha particles in the energy range 1.5--4.2 MeV.

In order to design reliable gas-filled dosemeters it is necessary to have knowledge of the ratios of the stopping powers of various solids and their gaseous equivalents. This paper describes the measurement of the stopping cross-sections of ethylene and its polymer polyethylene using alpha particles, and compares the measurements with results published by other workers. The results indicate that there is no difference between the two sets of values, within the quoted errors. Reasons are suggested to account for part of the discrepancy between our results and those of others for thin solid sample measurements.

A tetraphenylethene-based caged compound: synthesis, properties and applications.

A tetraphenylethene-based caged compound (TPE-C) is designed and synthesized. TPE-C is non-fluorescent either in solution or in aggregated state, but its emission can be induced to emit strong cyan emission in the aggregated state by UV irradiation. This property enables TPE-C to be applied in photo-patterning and anti-counterfeiting related areas.

Carrier transport in the V[TCNE]x (TCNE = tetracyanoethylene; x ~ 2) organic-based magnet.

The carrier transport of chemical vapor deposition (CVD) prepared films of the room temperature organic-based magnet V[TCNE]x (TCNE = tetracyanoethylene; x ~ 2) over a broad temperature and magnetic field range is reported. Due to disorder the [TCNE](·-) sites are located in statistically different environments, and their energies vary from site-to-site, which leads to tailing the density of states into the energy gap, creating electronic traps and suppressing the electron mobility. Conversely, these variations have little effect on the valence band derived from the octahedral V(II)3d(t(2g)) levels, and, hence, on the hole mobility. Presuming a Gaussian distribution of the energies of the localized states in the gap, a model that adequately describes the experimental data is proposed. In this model the T(-1) temperature dependent term was added to the Arrhenius activation energy, Ea, which effectively describes its decrease on cooling. The linear increase of positive magnetoresistance with magnetic field, as well as its weak temperature dependence [ is proportional to (1-T/Tc)(-1/2)] is discussed in terms of a small contribution to Ea associated with the change of magnetic energy.

Photoswitchable fluorescent diarylethene in a turn-on mode for live cell imaging.

A new diarylethene-derived photochromic compound, with little cytotoxicity, plasma membrane-permeability, and efficient photoswitchability in cells upon alternative UV and visible light irradiation, was synthesized and applied to live cell imaging.

Interactions between circadian and hormonal signalling in plants.

Growth and development of plants is controlled by external and internal signals. Key internal signals are those generated by hormones and the circadian clock. We highlight interactions between the circadian clock and hormonal signalling networks in regulating the physiology and growth of plants. Microarray analysis has shown that a significant proportion of transcripts involved in hormonal metabolism, catabolism, perception and signalling are also regulated by the circadian clock. In particular, there are interactions between the clock and abscisic acid, auxin, cytokinin and ethylene signalling. We discuss the role of circadian modulation ('gating') of hormonal signals in preventing temporally inappropriate responses. A consideration of the daily changes in physiology provides evidence that circadian gating of hormonal signalling couples the rhythmic regulation of carbon and water utilisation to rhythmic patterns of growth.