Geometrical Evolution and Formation of the Photoproduct in the Cycloreversion Reaction of a Diarylethene Derivative Probed by Vibrational Spectroscopy

The geometrical evolution of the reactant and formation of the photoproduct in the cycloreversion reaction of a diarylethene derivative were probed using time-resolved absorption spectroscopies in the visible to near-infrared and mid-infrared regions. The time-domain vibrational data in the visible region show that the initially formed Franck-Condon state is geometrically relaxed into the minimum in the excited state potential energy surface, concomitantly with the low-frequency coherent vibrations. Theoretical calculations indicate that the nuclear displacement in this coherent vibration is nearly parallel to that in the geometrical relaxation. Time-resolved mid-infrared spectroscopy directly detected the formation of the open-ring isomer with the same time constant as the decrease of the closed-ring isomer in the excited state minimum. This observation reveals that no detectable intermediate, in which the population is accumulated, is present between the excited closed-ring isomer and the open-ring isomer in the ground state.     

Photoinduced Reversible Topographical Changes on Diarylethene Microcrystalline Surfaces with Biomimetic Wetting Properties

Reversible topographical changes were observed on a photochromic diarylethene microcrystalline film surface by alternate irradiation with UV and visible light. Two types of surfaces were prepared from this film: 1) Storage of the film at 30 °C for 24 hours in the dark after UV irradiation afforded a surface that was covered with needle‐shaped crystals, whose diameter and length were approximately 1 μm and 10 μm, respectively, and showed a superhydrophobic lotus effect. 2) Storage of the film at 70 °C for 3 hours in the dark caused the needle‐shaped crystals to be converted into larger rod‐like crystals (5∼8 μm wide and 20∼30 μm long) by Ostwald ripening and a disappearance of the lotus effect. The obtained activation energy of the formation of the needle‐ and rod‐shaped crystals was 143 and 162 kJ mol⁻¹, respectively. Subsequent UV irradiation to the surface, which was followed by storage at 50 °C for 1 hour in the dark, gave a doubly rough structure; small needle‐shaped crystals were formed between the larger rod‐shaped crystals. The surface showed both superhydrophobic properties and the pinned effect of the water droplet: the petal effect. Fractal analysis of both surfaces were carried out using a box‐counting method, and the lotus effect was observed in the presence of smaller‐sized crystals, whilst the petal effect was observed with larger sized crystals (ca. 100 μm). We demonstrated that the hydrophobic property was controlled by the distribution in crystal size of the closed‐ring isomer of the diarylethene. Visible‐light irradiation of both rough surfaces afforded surfaces with cubic‐shaped micro‐crystals of the open‐ring isomer.     

A highly selective fluorescent probe for Sn based on a new photochromic diarylethene with a phenol-containing Schiff base unit

A novel diarylethene with a phenol-containing Schiff base unit was successfully synthesized and its multi-responsive photoswitching property was investigated by the stimulation of base/acid and light. The diarylethene showed not only normal photochromism by photoirradiation, but also abnormal photochromism by base and light stimuli. By the stimulation of sodium hydroxide, the absorption maximum of its open-ring isomer showed extraordinarily large redshift of 60 nm, accompanied with a notable color change from colorless to yellow. Subsequent irradiation with 297 nm light further caused a dramatic change in its structure transformation from the deprotonated open-ring isomer to the original closed-ring isomer. Similarly, its deprotonated closed-ring isomer returned to its original open-ring isomer upon irradiation with appropriate visible light. Its deprotonated forms could be restored by the stimulation of hydrochloric acid. Moreover, the diarylethene could be served as a fluorescence probe for recognition of Sn²⁺ with high selectivity.     

Effective photoswitching of intramolecular magnetic interaction by diarylethene: Backgrounds and applications

Diarylethene is a photochromic molecule, which is potentially applicable to various optoelectronic devices. In diarylethenes the π-system of the two aryl rings is separated in the open-ring isomer, while the π-system is delocalized throughout the molecule in the closed-ring isomer. Based on this idea the exchange interaction between two nitronyl nitroxide radicals connected by a diarylethene unit was photocontrolled reversibly along with photochromism. The switching efficiency is more than 150-fold and thiophene spacer transmits the interaction more efficiently. When diarylethene dimer is used for the switching unit, the electric circuit-like behavior was observed. Moreover, reversed switching is achieved by reversing the thiophene ring. Magnetochemical and photochemical behavior of the radical-substituted diarylethene is described extensively.     

Theoretical study of the electronic and optical properties of photochromic dithienylethene derivatives connected to small gold clusters

In the course of developing electronic devices on a molecular scale, dithienylethenes photochromic molecules constitute promising candidates for optoelectronic applications such as memories and switches. There is thus a great interest to understand and control the switching behavior of photochromic compounds deposited on metallic surfaces or nanoparticles. Within the framework of the density functional theory, we studied the effect of small gold clusters (Au3 and Au9) on the electronic structure and absorption spectrum of a model dithienylethene molecule. The molecular orbital interactions between the photochromic molecule and the gold cluster made it possible to rationalize some experimental findings (Dulic, D.; van der Molen, S. J.; Kudernac, T.; Jonkman, H. T.; de Jong, J. J. D.; Bowden, T. N.; van Esch, J.; Feringa, B. L.; van Wees, B. J. Phys. Rev. Lett. 2003, 91, 207402). For the closed-ring isomer, grafting a photochromic molecule on a small gold cluster does not change the characteristics of the electronic transition involved in the ring-opening reaction. On the opposite, the absorption spectrum of the photochromic open-ring isomer is strongly modified by the inclusion of the metallic cluster. In agreement with experimental results, our study thus showed that the cycloreversion reaction which involves the closed-ring isomer should be still possible, whereas the ring-closure reaction which involves the open-ring isomer should be inhibited. Connecting a dithienylethene molecule to a small gold cluster hence provides a qualitative comprehension of the photochromic activities of dithienylethenes connected to a gold surface.     

Self-assembly of photochromic diarylethenes with amphiphilic side chains: reversible thermal and photochemical control

Diarylethene derivatives with hexaethylene glycol side chains were synthesized and their self-assembling and photochromic reactivity were investigated. The diarylethenes showed photochromism in organic solvents and even in water. The aqueous solution of the compounds turned turbid quickly upon heating. The clouding behavior was investigated using 1H NMR spectroscopy, dynamic light scattering, and absorption spectroscopy. It was revealed that, in the aqueous solution, the compounds self-assembled into aggregates, and the aggregates were loosened by raising the temperature. The cloud-point temperature of the closed-ring isomer was 5-7 degrees C lower than that of the open-ring isomer. When asymmetric methyl groups were introduced in the amphiphilic side chains, induced circular dichroism (ICD) was observed upon irradiation with UV light in water. This ICD was explained by the difference in the self-assembling behavior between the open- and the closed-ring isomers. It was suggested that the closed-ring isomers assembled into a chiral nanostructure.     

Contact angles of surface nanobubbles on mixed self-assembled monolayers with systematically varied macroscopic wettability by atomic force microscopy

The dependence of the properties of so-called "surface nanobubbles" at the interface of binary self-assembled monolayers (SAMs) of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) on ultraflat template-stripped gold and water on the surface composition was studied systematically by in situ atomic force microscopy (AFM). The macroscopic water contact angle (θ(macro)) of the SAMs spanned the range between 107° ± 1° and 15° ± 3°. Surface nanobubbles were observed on all SAMs by intermittent contact-mode AFM; their size and contact angle were found to depend on the composition of the SAM. In particular, nanoscopic contact angles θ(nano) < 86° were observed for the first time for hydrophilic surfaces. From fits of the top of the bubble profile to a spherical cap in three dimensions, quantitative estimates of nanobubble height, width, and radius of curvature were obtained. Values of θ(nano) calculated from these data were found to change from 167° ± 3° to 33° ± 58°, when θ(macro) decreased from 107° ± 1° to 37° ± 3°. While the values for θ(nano) significantly exceeded those of θ(macro) for hydrophobic SAMs, which is fully in line with previous reports, this discrepancy became less pronounced and finally vanished for more hydrophilic surfaces.     

Tuning the surface potential of Ag surfaces by chemisorption of oppositely-oriented thiolated carborane dipoles

Two selected carboranethiol isomers were used to modify flat silver surfaces. Both isomers, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (a) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (b), are relatively strong dipoles with two SH groups per molecule. They are both anchored to the surface via two SH groups per molecule. Topography and surface potential changes of the modified silver surfaces were studied using Scanning Kelvin Probe Force Microscopy (SKPFM). These measurements proved that both isomers are oppositely oriented on the surface. The former isomer increases, and the latter one decreases the surface potential of a modified silver film. The relative changes of the surface potential correlate well with the dipole moments of the isomers. Competitive chemisorption from a 1:1 mixture of both isomers shows that the isomer (a) is found in a significantly higher concentration on the surface than the isomer (b). This has been proved by both SKPFM and X-ray photoelectron spectroscopy (XPS) techniques. Additionally, contact angle measurements were carried out to characterise the modified surfaces, and these and XPS results show the presence of hydrophobic hydrocarbon contaminants.     

Dynamic surface tension behavior in a photoresponsive surfactant system

The surface properties of a nonionic photoresponsive surfactant that incorporates the light-sensitive azobenzene group into its tail have been investigated. Cis-trans photoisomerization of this azobenzene group alters the ability of the surfactant to pack into adsorbed monolayers at an air/water interface or into aggregates in solution, thereby causing a significant variation in surface and bulk properties following a change in the illumination conditions. NMR studies indicate that a solution left in the dark for an extended period of time contains the trans isomer almost exclusively, whereas samples exposed to light of fixed wavelength eventually reach a photostationary equilibrium in which significant amounts of both isomers are present. At concentrations well above the cmc but under different illumination conditions (dark, UV light, visible light), freshly formed surfaces exhibit profoundly different surface tension trajectories as they approach essentially identical equilibrium states. This common equilibrium state corresponds to a surface saturated with the trans (more surface active) isomer. The dark sample shows a simple, single-step relaxation in surface tension after the creation of a fresh interface, whereas the UV and visible samples exhibit a more rapid initial decrease in tension, followed by a plateau of nearly constant tension, and finally end with a second relaxation to equilibrium. It is hypothesized that this behavior of the UV and visible samples is caused by competitive adsorption between the cis and trans isomers present in these mixtures. The cis surfactant reaches the interface more quickly, leading to an initially cis-dominated interface having a tension value corresponding to the intermediate plateau, but is ultimately displaced by the trans isomer. Fluorescence studies are used for cmc determination in the samples, and the results suggest that the two isomers segregate into distinct aggregate phases. The critical concentration associated with the formation of cis-rich aggregates is much larger than that of the trans-rich aggregates, which accounts for the faster diffusion of the cis isomer to a fresh interface. Models of the diffusion and adsorption of surfactant are developed. These consider the role of aggregates in the adsorption process by examining the limiting behavior of three aggregate properties: dissolution rate, mobility, and ability to incorporate into the interface. These models are used to analyze the surface tension relaxation of dark and UV samples, and the predictions are found to be in agreement with the observed characteristic relaxation time scales for these samples, though the results are inconclusive regarding the specific role of aggregates. High-intensity illumination focused on a surface saturated with surfactant is used to drive photoisomerization of the adsorbed surfactant, and rapid, substantial changes in surface tension result. These changes are consistent with proposed conformations of the adsorbed surfactant and with monolayer studies performed with a Langmuir film balance.     

Photoresponsive dithienylethene-urea-based organogels with "reversed" behavior

Dithienylperhydrocyclopentene-bisurea-based low molecular weight gelators are described that function as photoresponsive organogels that show a remarkable gel-to-liquid transition upon irradiation. The two series of derivatives, with and without alkyl spacers between the urea hydrogen bonding groups and the photochromic unit, show different gelation behavior. Upon UV irradiation of the gels, a gel liquified at only 1.4% conversion of the photochromic unit. Transmission electron microscopy (TEM) shows that the gel fibres consist of thin ribbons. Semi-empirical (PM3) calculations indicate that the hydrogen bonding between the open-ring isomer (o) molecules is weak, and that formation of the closed-ring isomer (c) destabilises the hydrogen bonding further. The results indicate that a small amount of the closed-ring isomer will disrupt the intermolecular hydrogen-bonding, leading to disintegration of the gel fibre ribbons and hence reversible liquification.     

Infrared spectra and ultraviolet-tunable laser induced photochemistry of matrix-isolated phenol and phenol-d5

Monomers of phenol and its ring-perdeuterated isotopologue phenol-d(5) were isolated in argon matrices at 15 K. The infrared (IR) spectra of these species were recorded and analyzed. In situ photochemical transformations of phenol and phenol-d(5) were induced by tunable UV laser light. The photoproducts have been characterized by IR spectroscopy supported by theoretical calculations of the infrared spectra. The primary product photogenerated from phenol was shown to be the phenoxyl radical. The analysis of the progress of the observed phototransformations led to identification of 2,5-cyclohexadienone as one of the secondary photoproducts. Spectral indications of other secondary products, such as the Dewar isomer and the open-ring ketene, were also detected. Identification of the photoproducts provided a guide for the interpretation of the mechanisms of the observed photoreactions.     

Photochromism of metal complexes composed of diarylethene ligands and Zn(II), Mn(II), and Cu(II) hexafluoroacetylacetonates

Metal complexes composed of bidentate 1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (1a) and monodentate 1-(2-methyl-5-phenyl-3-thienyl)-2-(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (2a) photochromic ligands and M(hfac)(2) (M = Zn(II), Mn(II), and Cu(II)) were prepared, and their photoinduced coordination structural changes were studied. X-ray crystallographic analyses showed the formation of coordination polymers and discrete 1:2 complexes for bidentate and monodentate ligands, respectively. The complexes underwent reversible photochromic reactions by alternate irradiation with UV and visible lights in solution as well as in the single-crystalline phase. Upon photoirradiation with UV and visible light, the ESR spectra of the copper complexes of 1a reversibly changed. While the open-ring isomer gave an axial-type spectrum, the photogenerated closed-ring isomer showed a rhombic-type spectrum. This indicates that the photoisomerization induced the change in the coordination structure.     

Temperature dependence of aggregation and dynamic surface tension in a photoresponsive surfactant system

The response of a nonionic photoresponsive surfactant system to changes in temperature is reported. This surfactant contains the light-sensitive azobenzene group, and when exposed to light, a solution of this surfactant contains a mixture of the cis and trans photoisomers of this group. The temperature of the surfactant solution has a strong impact on the time needed for the surfactant to diffuse and adsorb to a freshly formed interface. At surfactant concentrations that give rise to trans aggregates but not to cis aggregates, the transport of cis and of trans isomers to the surface of a pendant bubble have quite different temperature dependencies, owing largely to the difference in their aggregation states in bulk solution. Diffusion and adsorption of the cis isomer are described reasonably well by a simple diffusion model that accounts for the effect of temperature on the diffusion coefficient. The trans isomer, which was primarily bound in aggregates during these measurements, exhibits a stronger dependence of this adsorption time scale on the temperature of the solution. This temperature dependence of trans diffusion and adsorption is quantitatively consistent between samples containing only the trans isomer and samples containing a mixture of isomers. Fluorescence studies were done to determine the effect of temperature on the cmc of the surfactant. The critical concentration associated with the formation of cis-dominant aggregates increases modestly with increasing temperature. The cmc of the trans isomer also increases with increasing temperature, most significantly when the temperature exceeds about 35 degrees C. These trans cmc temperature-dependence data were incorporated into diffusion models that account for the potential roles of aggregates in the adsorption process. The observed temperature dependency of the trans adsorption time scale is consistent with a model that includes the effect of temperature on both the diffusivity and the supply of monomer via its effect on the cmc. Specifically, the results suggest that the dissolution of trans-dominant aggregates is important to the trans adsorption process. Further fluorescence studies were performed in which surfactant solutions containing aggregates were diluted rapidly, and the rate of dissolution of these aggregates was inferred from fluorescence decay. Aggregate breakup in colder trans samples is slower than in warmer samples, but these dissolution time scales are significantly shorter than those associated with the adsorption process. This is consistent with the assumption that aggregation kinetics do not contribute to the observed adsorption kinetics.     

Application of the UNIFAC model for prediction of surface tension and thickness of the surface layer in the binary mixtures

The specific sorption of Eu(III) and Y(III) on γ-alumina was investigated with solid-state (1)H NMR. Solution pH was shown to influence the recorded (1)H NMR spectra of γ-alumina, and thus, metal ion-containing samples were prepared under constant pH conditions, 8.00±0.05. The metal ion concentration in the samples was varied between 6.58×10(-7) M - 3.95×10(-4) M in case of Y(3+) and 6.58×10(-8) M - 1.32×10(-4) M in case of Eu(3+). The mineral concentration was kept constant at 4 g/l. After separation of the liquid phase, the samples were dried under vacuum to remove physisorbed water from the mineral surface. However, even after 48 h of drying at 150°C and 20 mTorr, water was still detected in the proton spectra as two distinct peaks with chemical shifts at 1.3 and 0.9 ppm. The europium addition to the γ-alumina samples induced significant spectral changes in comparison with yttrium-containing samples. These changes were attributed to the paramagnetism of europium rather than to complexation reactions occurring on the mineral surface. Proton spectra obtained for yttrium samples were therefore used to detect the spectral changes induced by the sorption reaction itself. The results revealed a large distribution of protons being removed from the mineral surface upon yttrium complexation. Removed protons were assigned to both bridging surface hydroxyls such as (Al(VI))(2)-OH as well as to terminal hydroxyls, e.g., of type Al(VI)-OH. Acidic protons belonging to (Al(VI))(3)-OH groups were not observed to participate in the surface reaction.     

New photoswitching unit for magnetic interaction: diarylethene with 2,5-bis(arylethynyl)-3-thienyl group

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene, and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have an extended pi-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity. Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum originating from the change in the magnetic interaction between two unpaired electrons. The open-ring isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic interaction between two radicals via the pi-conjugated chain was altered by photocyclization due to the change of the hybrid orbital at the 2-position of the thiophene ring from sp(2) to sp(3).     

A sensitive sensor for Cu(II) based on a novel diarylethene with a bipyridyl moiety

A novel diarylethene with a bipyridyl unit has been designed and synthesized for the first time. Its photochromic behaviors could be modulated by protonation and coordination with Cu(II). The absorption maximum of the closed-ring isomer shifted from 569 to 666 nm when trifluoroacetic acid was added. Furthermore, the closed-ring isomer behaved as a sensitive colorimetric sensor, exhibiting an open-ring reaction upon exposure to Cu(II). Its high selectivity toward Cu(II) over other competitive species makes the ‘naked-eye’ detection of Cu(II) possible.     

Synthesis, crystal structure and characterization of photochromic 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene

A novel photochromic diarylethene, 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene (BEDTP), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its photochromic and fluorescent properties were also investigated. The results showed that this compound exhibited reversible photochromism, changing from colorless to magenta after irradiation with UV light both in solution and in the crystalline phase. In hexane solution, the open-ring isomer of BEDTP exhibited relatively strong fluorescence at 400 nm when excited at 282 nm. The fluorescence intensity decreased along with the photochromism upon irradiation with 254 nm light and its closed-ring isomer showed almost no fluorescence.     

Electronic spectrum of a photochromic diarylethene derivative in a supersonic free jet. Internal conversion from S2(1B) to S1(2A)

The fluorescence excitation and dispersed fluorescence spectra of the open-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene have been measured in a supersonic free jet. No vibronic structure has been observed in the excitation spectrum. The intensity of fluorescence gradually increases with the excitation energy in the 25,500–28,700 cm⁻¹ region, indicating that the geometry of the molecule substantially changes upon photoexcitation. The dispersed fluorescence spectrum is anomaly Stokes-shifted with respect to the excitation energy, suggesting that the S₂(1B) state is initially excited followed by rapid internal conversion from the S₂(1B) to S₁(2A) state. The fluorescence is due to the S₁(2A)–S₀(1A) transition. Density functional theory calculations at the B3LYP/6-31G^** level have been carried out to investigate stable conformations responsible for the observed spectra.     

Synthesis, crystal structure and optoelectronic properties of a new unsymmetrical photochromic diarylethene

A new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (1a), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its optical and electrochemical properties, including photochromic reactivity both in solution and in the solid state (PMMA film and the single-crystalline phase), fluorescence and electrochemical properties were investigated in detail. The compound showed excellent photochromism even in the single-crystalline phase by photo-irradiation. In acetonitrile, the open-ring isomer of diarylethene 1 exhibited relatively strong fluorescence at 470nm when excited at 300nm, and its emission intensity decreased along with the photochromism upon irradiation with 313nm light. Its closed-ring isomer showed almost no fluorescence. The electrochemical properties of diarylethene were investigated by performing cyclic voltammetry experiment and its HOMO and LUMO energy level were calculated.     

Tracking surface evolution using ligand-assisted dissolution of cobalt oxyhydroxide

The use of surface-specific reactions to probe reactive surface area is a promising direction in materials research. The work presented herein examines how the kinetics of dissolution can be used to quantify particle growth as well as the evolution of site-specific reactive surface area. The dissolution of heterogenite (β-CoOOH) by IDA results in two geometric isomers of Co(IDA)(2)(-): the s-fac and u-fac isomers. The heterogenite particles studied here can generally be described as cylindrical plates, and the relative amount of s-fac isomer produced is found to increase as the height of the plates increases. The quantity of each isomer produced is shown to correlate with the relative number of two different types of surface sites, designated as edge and corner sites, while basal sites are seemingly unreactive. It is hypothesized that u-fac isomer results from the more accessible Co centers at the corner sites, while the s-fac isomer results from the less accessible edge sites. An empirical relationship is developed between the fraction of s-fac isomer produced and the height of the β-CoOOH particles, and this relationship is used to quantify particle growth by analysis of kinetic data. Finally, this new information is used to modify a previously proposed pH-dependent growth model, resulting in a significant improvement in the fit and physical relevance of the model.