Protection and Labelling of Thymidine by a Fluorescent Photolabile Group

A fluorescent photolabile group including coumarin and MeNPOC moieties was synthesized to protect 5′‐OH terminal function of thymidine (T). Its photochemical and photophysical properties were studied, in particular the photocleavage (photodeprotection under a 365‐nm irradiation) is only lowered by a factor of two by addition of the fluorophore. Fluorescence properties of the coumarin probe are not changed upon irradiation, which is satisfactory for the application required, i.e., in situ synthesis of DNA microarrays.     

Photochemical stability of collagen/poly(ethylene oxide) blends

The photochemical stability of the blends of collagen and poly(ethylene oxide) PEO has been studied by Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy and viscosimetry. Surface properties before and after UV irradiation were observed using an optical microscope.Collagen and PEO were immiscible in diluted solution and only small interactions between the two components in the solid state were observed. New materials based on the blending of collagen and PEO that we obtained have a different photochemical stability than those of single components. In general, collagen/PEO blends are less stable under UV irradiation than pure collagen. The influence of PEO on the photochemical stability of collagen depends on the concentration of this polymer in the blend. Microscopic photographs show that the surface characteristics of thin films of collagen/PEO blends are not drastically altered after UV irradiation.     

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical,Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid‐trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid‐cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD‐DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.     

Design, synthesis, and photochemical validation of peptide linchpins containing the S,S-tetrazine phototrigger

The design, solid-phase synthesis, and photochemical validation of diverse peptide linchpins, containing the S,S-tetrazine phototrigger, have been achieved. Steady state irradiation or femtosecond laser pulses confirm their rapid photofragmentation. Attachment of peptides to the C- and N-termini will provide access to diverse constrained peptide constructs that hold the promise of providing information about early peptide/protein conformational dynamics upon photochemical release.     

UV-Vis-NIR Light-deformable Shape-memory Polyurethane Doped with Liquid-crystal Mixture and GO towards Biomimetic Applications

Nature has been inspiring material researchers to fabricate biomimetic functional devices for various applications, and shape-memory polymer materials(SMPMs) have received tremendous attention since the promising intelligent materials possess more advantages over others for the fabrication of biomimetic functional devices. As is well-known, SMPMs can be stimulated by heat, electricity, magnetism, pH, solvent and light. From the viewpoint of practical applications, ultraviolet(UV)-visible(Vis)-near infrared(NIR) light-responsive SMPMs are undoubtedly more advantageous. However, up to now, UV-Vis-NIR light-deformable SMPMs by combining photothermal and photochemical effects are still rarely reported. Here we designed a UV-Vis-NIR light-deformable SMP composite film via incorporating a liquid crystal(LC) mixture and graphene oxide(GO) into a shape-memory polyurethane matrix. The elongated composite films exhibited interesting photomechanical bending deformations with different light-triggered mechanisms,(1) photochemically induced LC phase transition upon UV exposure,(2) photochemically and photothermally induced LC phase transition upon visible-light irradiation,(3) photothermally triggered LC phase transition and partial stress relaxation upon low-intensity NIR exposure. All the deformed objects could recover to their original shapes by high-intensity NIR irradiation.Moreover, the biomimetic circadian rhythms of acacia leaves and the biomimetic bending/spreading of fingers were successfully achieved, which could blaze a way in the field of biomimetic functional devices due to the excellent light-deformable and shape-memory properties of the SMP composite films.     

Recent Progress in Azopyridine-Containing Supramolecular Assembly: From Photoresponsive Liquid Crystals to Light-Driven Devices

Azobenzene derivatives have become one of the most famous photoresponsive chromophores in the past few decades for their reversible molecular switches upon the irradiation of actinic light. To meet the ever-increasing requirements for applications in materials science, biomedicine, and light-driven devices, it is usually necessary to adjust their photochemical property from the molecular level by changing the substituents on the benzene rings of azobenzene groups. Among the diverse azobenzene derivatives, azopyridine combines the photoresponsive feature of azobenzene groups and the supramolecular function of pyridyl moieties in one molecule. This unique feature provides pH-responsiveness and hydrogen/halogen/coordination binding sites in the same chromophore, paving a new way to prepare multi-functional responsive materials through non-covalent interactions and reversible chemical reactions. This review summarizes the photochemical and photophysical properties of azopyridine derivatives in supramolecular states (e.g., hydrogen/halogen bonding, coordination interactions, and quaternization reactions) and illustrates their applications from photoresponsive liquid crystals to light-driven devices. We hope this review can highlight azopyridine as one more versatile candidate molecule for designing novel photoresponsive materials towards light-driven applications.     

Stratospheric ozone dynamics according to the Chapman mechanism

The photochemical mechanisms behind the ozone dynamics in the stratosphere have been studied for a long time. Though the differential equations describing the kinetics associated with them are well known, their mathematical properties have not been fully explored. In this work some of these mathematical properties as well as their physical implications are investigated in the simplest photochemical mechanism, namely, Chapman’s one.     

Photoactivatable V-Shaped Bifunctional Quinone Methide Precursors as a New Class of Selective G-quadruplex Alkylating Agents

Combining the selectivity of G-quadruplex (G4) ligands with the spatial and temporal control of photochemistry is an emerging strategy to elucidate the biological relevance of these structures. In this work, we developed six novel V-shaped G4 ligands that can, upon irradiation, form stable covalent adducts with G4 structures via the reactive intermediate, quinone methide (QM). We thoroughly investigated the photochemical properties of the ligands and their ability to generate QMs. Subsequently, we analyzed their specificity for various topologies of G4 and discovered a preferential binding towards the human telomeric sequence. Finally, we tested the ligand ability to act as photochemical alkylating agents, identifying the covalent adducts with G4 structures. This work introduces a novel molecular tool in the chemical biology toolkit for G4s.     

BACTERIORHODOPSIN RECONSTITUTED FROM TWO INDIVIDUAL HELICES AND THE COMPLEMENTARY FIVE-HELIX FRAGMENT IS PHOTOACTIVE

Bacteriorhodopsin (bR), a light-driven proton pump, consists of a bundle of seven membrane-spanning alpha-helices connected to each other by short extramembranous loops. Previously it has been shown that bR can be reconstituted from three fragments corresponding to the first helix, the second helix, and the remaining five helices, and that this reconstituted material reforms the native structure of bR. In this study, it is shown that the native function is also recovered. Low-temperature spectroscopy was used to examine the photochemical properties of bR reconstituted from three fragments. At room temperature at pH 6, the reconstituted material shows essentially the same absorption spectrum as native bR, while upon raising the pH at room temperature or cooling the sample in glycerol, a second, blue-shifted peak appears. The pH and temperature dependence of the absorption spectrum indicates that the reconstituted bR is in an equilibrium between two pigments, which we call P560 and P480. Both pigments convert to their own K intermediates, which differ in absorption maxima, upon illumination with green light at -180 degrees C. Each K intermediate can be reverted to its initial state by light. Similarly, both pigments convert to their own M intermediates upon irradiation with yellow light at -77 degrees C. The M intermediate of both species can be reverted only to P560 by light. Both pigments are therefore photoactive. These unique photochemical properties of bR reconstituted from three fragments may be attributable to the lack of a covalent linkage in the loop connecting the A and B helices, and thus possibly to a change in the orientation of the B helix.     

Structure and solid-state photochromic properties of two new compounds with pyrazolone-ring

Two new organic photochromic compounds based on pyrazolone were synthesized and further characterized by elemental analysis, MS, IR spectra, and ¹H NMR spectra. The photochromic properties and photochemical kinetics of them have been studied by UV absorption spectra under irradiation of 365 nm light. The photochromism of pyrazolone derivatives in the solid state has been so far explained only in terms of the light-induced change of absorption and their crystal structures, which is caused by the shift of the tautomeric equilibrium between the enol and keto forms. In this work, their IR spectra have been measured before and after irradiation. Based on the IR spectra and crystal structure analysis, a reasonable mechanism was proposed, the photochromic phenomenon may be due to the photoisomerization from enol-form to keto-form through proton transfer.     

Extended mesoionic systems: synthesis and characterization of monocyclic, polycyclic and macrocyclic pyrimidinium-olate derivatives and their photochemical behavior

Mesoionic pyrimidinium-olate derivatives have been known to undergo photoreactions upon irradiation with UV light to form bis(beta-lactame) structures for about two decades. Here, new mono-, poly- and macrocyclic mesoions were prepared and their photochemical rearrangement behavior was investigated. The synthesized compounds were characterized by NMR, IR, UV/Vis spectroscopy, elemental analysis and mass spectrometry. The extension of the aromatic core leads to a significant red-shift of the absorption maximum from ∼380 to ∼430 nm, indicating a strong electronic coupling of the mesoionic base chromophore with the annellated aromatic subunit. The rigidity of the extended aromatic system prevents polycyclic mesoions from shifting to their bis(beta-lactame) isomers, while the alkyl bridge of the 16-membered macrocycle in an ansa-mesoion does not inhibit photochemical rearrangement.     

The excited-state interaction of resazurin and resorufin with amines in aqueous solutions. Photophysics and photochemical reactions

The photophysics and photochemical behavior of the phenoxazin-3-one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N-oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (phiF = 0.75). At lower pH the fluorescence is strongly reduced. The N-oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N-oxide in the presence of amines occurs from the triplet state.     

Binding, aggregation and photochemical properties of methylene blue in mitochondrial suspensions

Methylene Blue (MB) has well-established photochemical properties and has been used in a variety of photochemical applications including photodynamic therapy. Despite the fact that most of MB's cytotoxic effects in cells are attributed to mitochondrial damage, the interactions of this dye with mitochondria and the consequent effects on photochemical properties have not yet been fully determined. We monitored MB binding, aggregation and its ability to release singlet oxygen (1O2) on irradiation when interacting with mitochondrial suspensions. MB actively binds to mitochondria and enters the matrix in a manner stimulated by the mitochondrial proton potential and by the increase in mitochondrial concentrations. The greater accumulation of MB in mitochondria with elevated proton potentials or those treated with high concentrations of MB results in the formation of MB dimers, previously shown to be less effective generators of 1O2. Accumulation of MB within mitochondria with high membrane potentials also results in the reduction of MB to the photochemically inactive leuco-MB. Indeed, irradiation of mitochondria with high proton potentials in the presence of MB results in the generation of approximately half the quantity of 1O2 compared with 1O2 generated in mitochondria with low proton potentials. These differences in photochemical properties should influence the cytotoxic effects of photodynamic treatment in the presence of MB.     

The influence of UV light on collagen/poly(ethylene glycol) blends

The photodegradation behaviour of the collagen and poly(ethylene glycol) PEG blends has been studied by Fourier transform infrared spectroscopy (FTIR), UV-Vis spectroscopy and viscometry. Surface properties before and after UV irradiation were observed using optical microscope.Collagen and PEG were immiscible and the films obtained from the mixture were fragile with poor mechanical properties. The photochemical stability of the collagen and PEG blend was different from that of the single components. In general collagen/PEG blends are less stable under UV irradiation than pure collagen. The influence of PEG on the photochemical stability of collagen depends on its concentration in the blend. Microscope photographs showed that the surface characteristics of collagen and collagen/PEG blends in film form are not drastically altered after UV irradiation.     

Spiropyran-based advanced photoswitchable materials: A fascinating pathway to the future stimuli-responsive devices

Recent years have witnessed tremendous progress and developments of the photoswitchable spiropyran-based polymers, owing to distinctive and particular physicochemical properties of their isomers upon a variety of triggers, and especially light illumination. Light is a fascinating and green stimulus because of its remote control, micron- or submicron-sized focusing area with controllable wavelength and energy, non-invasiveness and non-destructive nature, precisely controlled direction, and availability. In this review, we have emphasized on and summarized the most recent observations and efforts in the progress of photoswitchable spiropyran-based materials and their applications as sensors for heavy metal cations, anions, pH, acid and base vapors, wettability and humidity. Other items include data recording and anticounterfeiting devices, photorheological fluids, optically reversible switching membranes, photoregulating surface plasmon resonance, photomodulation of ion conductivity and mechanoresponsive polymers. The bio-based field is another interesting subject that is discussed here and consists of reversible cell sheet engineering, photodynamic therapy, switchable fluorescence labeling, controlled drug delivery and biological ion channels. On the other hand, limited light penetration inside the living tissues and hazards of high-energy ultraviolet irradiation for initiating photochemical transformations have limited the use of such light-controlled systems in medicinal and therapeutic means. Those spiropyran-based materials which are susceptible to being triggered by low energy near IR (NIR) two-photon light irradiation and upconversion nanoparticles are recently under serious explorations and have been reviewed as a new perspective for their advanced applications.     

Photochemical and photophysical properties of three carbon-bridged fullerene dimers: C121 (I, II, III)

The photochemical and photophysical properties of the three C121 isomers (I, II, III) were investigated with MADLI-TOF-MS, UV-vis spectra, fluorescence spectra, absorption spectra of their DMA complexes, and theoretical calculations. The three isomers of C121 (I, II, III) have different stabilities under laser irradiation, but isomer I and isomer II show good stability against the heat-induced conversion between different isomers: No conversion between the isomers was found after heating the mixture of isomer I and isomer II at 353 K for 12 h in Ar atmosphere. The results of UV-vis absorption and fluorescence spectra indicate that interactions between two C60 moieties of C60=C=C60 in the ground and singlet states are not significant, C121 (I, II, III) behaves as an electron-acceptor similar to C60. These indicate that the formation of the fullerene chain structure (e.g., C60=C=C60) does not disturb the photochemical and photophysical properties of the C60 monomer itself, even that the properties were enhanced by the formation of the polymer. This is significant for the C60 polymer in photochemical or photoelectronic applications in which C60=C=C60 can be an excellent basic unit of polymers.     

Visible light excitation of CdSe nanocrystals triggers the release of coumarin from cinnamate surface ligands

The photochemical properties of organic ligands on the surface of nanocrystalline CdSe particles were examined. A number of thiols carrying a substituted cinnamate tail was synthesized. In solution, these cinnamate compounds undergo light-induced (374 nm) E-Z isomerization, followed by a nonphotolytic lactonization to give highly fluorescent coumarin. The cinnamate-thiols were successfully exchanged onto the CdSe nanocrystal, and the photochemical behavior of these conjugates was studied. Upon aerobic photolysis at 374 nm, the surface cinnamates released coumarin accompanied by rapid nanocrystal degradation. This degradation was not observed under similar anaerobic conditions or when the organic ligands did not contain the cinnamate group. Surprisingly, very similar results were obtained upon irradiation at visible wavelengths at which the cinnamate has no absorption. With the aid of UV-visible absorption spectroscopy, fluorescence spectroscopy, and electrochemistry, a unified theory for both the increased photoinstability of the nanocrystal as well as the coumarin release was proposed. It involves cinnamate radical anions on the CdSe surface, formed upon electron transfer from the excited nanocrystal to the surface cinnamate, undergoing E-Z isomerization. Practically, this results in the remarkable ability to release coumarin from nanocrystal ligands simply by exciting the nanocrystal with visible light. This new photorelease protocol not only aids in the understanding of fundamental nanocrystal-ligand interactions but may also offer new opportunities in the areas of drug delivery and imaging.     

On the Photochemistry of 12,13-Didehydrosqualene

The photochemical formation of bicyclooctenes b from 12,13-didehydrosqualene ( 1 ) upon irradiation at wavelengths longer than 290 nm is reported. The same photoreaction is also observed with trienes 7 and 8 which like 1 have a remote double bond two C-atoms removed from the triene system. A configurational assignment for bicyclooctene 10 obtained from 8 and a mechanism for the observed photochemical reaction are proposed.     

Photochemical stability of poly(vinyl pyrrolidone) in the presence of collagen

The photochemical stability of poly(vinyl pyrrolidone) (PVP) in the presence of 1%, 3% and 5% of collagen has been studied by UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), thermogravimetry analysis (TG) and derivative thermogravimetry (DTG). Surface properties have been studied by contact angle measurements. PVP samples and samples containing 1%, 3% and 5% of collagen were irradiated with UV light of wavelength λ = 254 nm in air for up to 24 h. The amount of gel created during UV irradiation was estimated.PVP in the presence of 1%, 3% and 5% of collagen is less stable both thermally and photochemically. Collagen enhances photochemical processes leading to crosslinking of PVP. The contact angle measurements and values of surface free energy showed that the wettability of PVP films was changed by the addition of collagen and by UV irradiation. The increase of polarity of samples indicates an efficient photooxidation on the surface upon UV irradiation.     

Reversible alteration of CO2 adsorption upon photochemical or thermal treatment in a metal-organic framework

A metal-organic framework (MOF) for reversible alteration of guest molecule adsorption, here carbon dioxide, upon photochemical or thermal treatment has been discovered. An azobenzene functional group, which can switch its conformation upon light irradiation or heat treatment, has been introduced to the organic linker of a MOF. The resulting MOF adsorbs different amount of CO(2) after UV or heat treatment. This remarkable stimuli-responsive adsorption effect has been demonstrated through experiments.