Photochromic Process Research Articles

Aging of the nanosized photochromic WO3 films and the role of adsorbed water in the photochromism

Here it has been reported on aging of the nanosized WO3 film, which is revealed is continuous reduction of the photochromic sensitivity over time. Water molecules physically adsorbed on the film surface from ambient air form donor–acceptor and hydrogen bonds, changing gradually the adsorption state to chemisorption which prevents an access of organic molecules that serve as hydrogen donors by the photochromism. The mechanism of the process has been investigated and discussed. The role of water in the photochromism has been highlighted. The difference in the efficiency for being of a hydrogen donor in the photochromic process between water and organic molecules is discussed.

Multistate and Multicolor Photochromism through Selective Cycloreversion in Asymmetric Platinum(II) Complexes with Two Different Dithienylethene-Acetylides.

Four asymmetric bis(dithienylethene-acetylide) platinum(II) complexes trans-Pt(PEt3)2(L1o)(L5o) (1oo), trans-Pt(PEt3)2(L2o)(L5o) (2oo), trans-Pt(PEt3)2(L3o)(L5o) (3oo), and trans-Pt(PEt3)2(L4o)(L5o) (4oo) with two different dithienylethene-acetylides (L1o-L5o) were designed to modulate stepwise, multistate, and multicolor photochromism by modifying ring-closure absorption wavelengths. Upon irradiation under UV light, 1oo converts only to 1oc without the observation of 1co and dually ring-closed species 1cc. In contrast, both mixed ring-open/closed species oc and co as well as dually ring-closed species cc are observed upon UV light irradiation of 2oo-4oo, implying that a substantial stepwise photochromic process occurs following 2oo-4oo → 2oc-4oc/2co-4co → 2cc-4cc. The conversion percentage of dually ring-closed species at the photostationary state (PSS) is progressively increased following 1cc (0%) → 2cc (40%) → 3cc (86%) → 4cc (>95%), coinciding with the progressive red-shift of ring-closure absorption bands in free L1c (441 nm) → L2c (510 nm) → L3c (556 nm) → L4c (591 nm). Particularly, compound 2 affords four states (2oo, 2co, 2oc, and 2cc) with different colors (colorless, purple, blue, and dark blue, respectively) through a selective photochemical cycloreversion process upon irradiation with appropriate wavelengths of light. Although stepwise photocyclization reactions 3oo → 3co/3oc → 3cc and 4oo → 4co/4oc → 4cc are observed, multicolor photochromism of 3oo and 4oo could not be achieved because ring-closure absorption bands between L3c/L4c and L5c are significantly overlapped. The stepwise photochemical processes are well demonstrated by NMR, UV-vis, and infrared (IR) spectroscopy and time-dependent density functional theory (TD-DFT) computational studies.

Single particle SERS signal on gold nanorods: comparative study of diarylethene photochromic isomers

In this paper, gold nanorods (GNRs) were synthesized and characterized by absorption spectroscopy, scanning electronic microscopy and dark-field scattering spectroscopy. The density of nanorods on the glass substrate has been controlled in order to select locations where a single particle can be illuminated by the excitation focal spot. Diarylethene photochromic molecules were deposited on the GNRs and surface enhanced Raman scattering experiments were performed in single particle condition to study both isomer’s forms. In addition, calculations were performed to precisely resolve each isomer’s vibrational modes. By combining both calculation and experiments, the open and closed forms of the photochromic molecule have been characterized. Moreover, the presence of conformers in the case of the open form has been pointed out, which is an important parameter to take into account of the photochromic process due to different photo-activity with respect to the antiparallel or parallel configuration. Thanks to the results obtained, we propose a method that could be used in further experiments to follow photochromic reactions based on the selection of a specific fingerprint, which allows us to unambiguously assign the detected signal to one isomer form of the photochromic molecule.

Preparation and photochromic properties of PEG-400 assisted WO3–TiO2–ZnO composite films

WO3–TiO2–ZnO composite films were successfully prepared on glass substrates via a sol–gel method using polyethylene glycol 400 (PEG-400) as an organic additive. The influence of PEG-400 on microstructure and photochromic properties of the composite films was studied by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM) and UV–vis–NIR spectrophotometer. The addition of PEG-400 could increase the amount of the adsorbed water in the WO3–TiO2–ZnO composite films, which plays a vital role in the photochromic process. SEM observation indicated that the aggregation of the particles in the films became serious when the addition amount of PEG-400 was above 0.5g, for which a molecular ratio of WO3 and PEG-400 was 117:1. The photochromic property of the composite film firstly increased and then decreased with increasing amount of PEG-400. The WO3–TiO2–ZnO composite film with 0.5g PEG-400 addition presented a best photochromism, which showed saturated photochromism in 60s and bleached back to initial state in 10h.

Importance of Gold Nanorods’ Aggregation in Surface Plasmon Coupling with a Photochromic Film in Hybrid Structures

In this paper, hybrid samples composed of photochromic film deposited on glass substrates functionalized with gold nanorods were synthesized. Three samples of gold nanorods with different aspect ratios were produced and characterized by electron microscopy and extinction spectroscopy. The rod populations were chosen to have more or less overlap between the two surface plasmon resonance bands and the absorption band of the colored form of the photochromic compounds which is commercially available. Deposition of the nanoparticles on glass substrate exhibits a new surface plasmon resonance coupling band coming from their side-by-side organization. Spectroscopic studies of the photoisomerization process of photochromic film spin-coated on the gold nanorods show reversible interaction between photochromism and plasmonic properties. Despite such coupling has been previously reported emphasizing the main role of the bands overlapping, in this paper, we clearly show that the surface resonance coupling band is the key parameter to monitor the photochromic process. This result emphasizes the importance of coupling nanoparticles to increase their sensing potential and interaction with photochromic molecules.

Synthesis and Properties of Photochromic Diarylethenes Bearing Thiazole Moiety

Four new diarylethene derivatives based on thiophene or thiazole moieties were designed and synthesized, and the structures of compounds 4a and 5a were determined by single‐X‐ray diffraction analysis. All of these compounds showed reversible photochromic reactions and notable fluorescence photo‐switches in solution. Furthermore, diarylethenes 4a and 5a also showed good photochromism in single crystal phase. The electron‐donating and electron‐withdrawing substituents play the same role in the photochromic process: red‐shifted absorption and fluorescence (1a and 2a compared with 3a). The fluorescent modulation efficiencies of asymmetrical diarylethenes 4 and 5 were significantly enhanced compared with the similar dithienylethenes, and the fatigue resistance of 5 was much better than 4, which showed that the diarylethene bearing electron‐withdrawing group could improve its fatigue resistance.

Regulation of charge delocalization in a heteronuclear Fe2 ru system by a stepwise photochromic process.

Heteronuclear complexes FeCp2 -DTE-C≡C-Ru(dppe)2 Cl (1 o; dppe=1,2-bis(diphenylphosphino)ethane, Cp=cyclopentadienyl, DTE=dithienylethene) and FeCp2-DTE-C≡C-Ru(dppe)2-C≡C-DTE-FeCp2 (2 oo), with redox-active ferrocenyl and ruthenium centers separated by a photochromic DTE moiety, were prepared to achieve photoswitchable charge delocalization and Fe⋅⋅⋅Ru electronic communication. Upon UV-light irradiation of 2 oo, the Fe⋅⋅⋅Ru heterometallic electronic interaction is increasingly facilitated with stepwise photocyclization, 2 oo→2 co→2 cc; this is ascribed to the gradual increase in π-conjugated systems. The near-infrared absorptions in mixed-valence species [2 oo](+) /[2 co](+) /[2 cc](+) are gradually intensified following the conversion of [2 oo](+) →[2 co](+) →[2 cc](+) , which demonstrates that the extent of charge delocalization shows progressive enhancement with stepwise photocyclization. As revealed by electrochemical, spectroscopic, and theoretical studies, complex 2 exhibits nine switchable states through stepwise photochromic and reversible redox processes.

Hydrothermal synthesis and studies on photochromic properties of Al doped WO3 powder

A series of pure and aluminum (Al) doped WO3 powders were synthesized at low temperature. The as synthesized materials have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV–VIS) spectroscopy and color difference meter, respectively. A relation was found between the color difference and the UV–VIS photochromic activity. Moreover, the optical band gap of the as-prepared powder narrowed due to the appearance of impurity levels as the Al3+ ions migrated into the WO3 crystal lattice. Some rod-like particles have been found to decrease with the increase of Al concentration. An optimum Al concentration was found to be 0.25% to present the best UV-light induced photochromic activity. The mechanism of photochromic process of the Al-doped WO3 powders has been discussed in detail based on the characterization results.

Nondestructive readout fluorescence memory based on a gallium(III) corrole complex and photochromic cis-1,2-dithienylethene

Photochromic switching of fluorescence emission provides a viable principle to creation of all optical molecular memory. Successful operation of the fluorescence memory requires deliberate control of the energetics between a fluorophore and a photochrome. One essential requirement is that photoexcitation for fluorescence emission does not interfere with the photochromic processes. Gallium(III) corrole complexes outfit the condition because their fluorescence emissions display large Stokes shifts, permitting photoexcitation at the optical window where the photochromism of cis-1,2-dithienylethene is not executed. To demonstrate the capability for fluorescence memory, we prepared molecularly dispersed poly(methyl methacrylate) (PMMA) films of a gallium corrole complex and cis-1,2-dithienylethene. The memory cycle comprising fluorescence readout and reversible photochromic switching of the fluorescence emission is fully reversible without suffering from fatigue during repeated operation. The corresponding fluorescence on/off ratio is greater than those of previous memory based on porphyrins. Fluorescence lifetime measurements employing time-correlated single photon counting techniques reveal occurrence of fast energy transfer (~ 109 s-1) which is effectively gated by the photochromism.

Kelvin probe imaging of photo-injected electrons in metal oxide nanosheets from metal sulfide quantum dots under remote photochromic coloration.

Metal oxide and quantum dot (QD) heterostructures have attracted considerable recent attention as materials for developing efficient solar cells, photocatalysts, and display devices, thus nanoscale imaging of trapped electrons in these heterostructures provides important insight for developing efficient devices. In the present study, Kelvin probe force microscopy (KPFM) of CdS quantum dot (QD)-grafted Cs4W11O36(2-) nanosheets was performed before and after visible-light irradiation. After visible-light excitation of the CdS QDs, the Cs4W11O36(2-) nanosheet surface exhibited a decreased work function in the vicinity of the junction with CdS QDs, even though the Cs4W11O36(2-) nanosheet did not absorb visible light. X-ray photoelectron spectroscopy revealed that W(5+) species were formed in the nanosheet after visible-light irradiation. These results demonstrated that excited electrons in the CdS QDs were injected and trapped in the Cs4W11O36(2-) nanosheet to form color centers. Further, the CdS QDs and Cs4W11O36(2-) nanosheet composite films exhibited efficient remote photochromic coloration, which was attributed to the quantum nanostructure of the film. Notably, the responsive wavelength of the material is tunable by adjusting the size of QDs, and the decoloration rate is highly efficient, as the required length for trapped electrons to diffuse into the nanosheet surface is very short owing to its nanoscale thickness. The unique properties of this photochromic device make it suitable for display or memory applications. In addition, the methodology described in the present study for nanoscale imaging is expected to aid in the understanding of electron transport and trapping processes in metal oxide and metal chalcogenide heterostructure, which are crucial phenomena in QD-based solar cells and/or photocatalytic water-splitting systems.

Solid-State NMR Studies of the Photochromic Effects of Thin Films of Oxygen-Containing Yttrium Hydride

Oxygen-containing yttrium hydride thin films show a unique reversible change in optical properties as a result of illumination. In this article, different solid-state NMR methods are used to follow the changes during such a photochromic process. The novel stripline probe technology provides a high sensitivity for NMR experiments on thin film samples with a thickness of approximately 1 μm. A key observation is that these photochromic films show spectral evidence of a highly mobile hydrogen species, which disappears upon illumination with white light. A relaxation back to the transparent phase is observed after a few days, accompanied by the reappearance of the mobile proton NMR signal. These results are compared with NMR experiments on pure yttrium hydride thin films. Magic-angle spinning 1H and 89Y experiments are performed to obtain detailed information in polycrystalline samples and flexible thin films. In particular, the 89Y MAS experiments indicate changes in the local environment of the Y nucleus upon optical illumination. A detailed structural model for the photochromic change is not yet clear, but the present NMR observations may give some hints toward such a model.

Visible light photochromism of polyoxometalates-based composite film with deposition of ZnFe2O4 nanoparticles

By introducing ZnFe2O4 nanoparticles as light sensitizer into the silica sol–gel matrix, a novel phosphomolybdic acid (PMoA)-based composite film with high visible light photochromism was successfully prepared. The resulted thin film was denoted as PMoA/SiO2/ZnFe2O4. The microstructure, visible-light photochromic behaviors and mechanism were fully investigated. Characterizations revealed that the Keggin geometry of PMoA particles and structure of tetrahedral zinc ions were well retained in the film. With visible light irradiation, the colorless transparent PMoA/SiO2/ZnFe2O4 composite film turned to blue and exhibited high photochromic responsiveness as well as reversible photochromism. According to the XPS analysis, the appearance of Mo5+ species indicated the photo-reduction reaction occurred between PMoA naoparticles and ZnFe2O4 sensitizers suggesting the photo-generated electron transfer mechanism during the photochromic process.

Back Cover: New Hydrogen-Evolution Heteronanostructured Photocatalysts: Pt-Nb3O7(OH) and Cu-Nb3O7(OH) (ChemSusChem 8/2014)

The Back Cover picture illustrates the performance of Nb3O7(OH), a new hydrogen evolution photocatalyst: Under solar illumination, photogenerated holes are scavenged by methanol while the reduction of protons to hydrogen is kinetically hindered. Thus, the electrons remain in the conduction band and/or trap states, turning the photocatalyst deep blue. The deposition of metal particles on the surface introduces a new reaction side, which now is able to overcome the kinetic barrier. Subsequently, hydrogen is formed and no photochromic process is observed. More details can be found in the Communication by Hmadeh et al. on page 2104 (DOI: 10.1002/cssc.201402173).

UV-light and visible-light photochromism of inorganic–organic multilayer films based on polyoxometalate and poly(acrylamide)

Novel UV-light and visible-light photochromic inorganic–organic multilayers composed of polyoxometalates (phosphomolybdic acid (PMoA)) and poly(acrylamide) (PAM) were prepared by the layer-by-layer (LbL) self-assembly method. The grown process, composition, surface topography, and photochromic properties and mechanism of the multilayer films were investigated by ultraviolet–visible (UV–vis) spectra, Fourier transform infrared spectra (FT-IR), atomic force microscopy (AFM), electron resonance spectra (ESR), and X-ray photoelectron spectra (XPS). Irradiation with UV-light or visible-light, the transparent films changed from colorless to blue and showed reversible photochromism. PMoA/PAM LbL films had higher photochromic efficiency under UV-light irradiation than visible-light irradiation. The bleaching process occurred when the films were in contact with O2 in the dark or heated in air. The photochromic process of PMoA/PAM LbL film was in accordance with radical mechanism.

Structural rearrangement accompanying the ultrafast electrocyclization reaction of a photochromic molecular switch.

Probing the structural rearrangement of a model photochromic molecular switch provides a window on the fundamental dynamics of electrocyclization reactions. Taking advantage of resonance-enhanced femtosecond stimulated Raman scattering (FSRS) with a broadly tunable Raman excitation wavelength, we selectively probe the competing dynamics of both the reactive and nonreactive conformers of a diarylethene (DAE) derivative that are simultaneously present in solution. Measurements that preferentially probe the electrocyclization (ring-closing) reaction of the reactive species reveal an unexpectedly slow nuclear rearrangement, stretching to tens of picoseconds, in striking contrast with the prompt electronic dynamics in the formation of the closed-ring isomer. The different results from transient electronic and vibrational spectroscopies reflect the different aspects of the reaction that are probed by each technique, depending on whether one considers the electronic state of the molecule or the structural rearrangement of the nuclei. Using a different Raman excitation wavelength selectively probes the picosecond-scale intersystem crossing dynamics of the nonreactive conformer, revealing the vibrational spectra of the singlet and triplet excited states for the first time. The present study paves the way to a more complete understanding of the structural mechanisms accompanying the reversible photochromic switching process.

Zn2+ doped ormosil–phosphotungstate hybrid films with enhanced photochromic response

Owing to the wide use of photochromic materials in UV sensors and dosimeters, considerable efforts have been made to increase the UV-response and sensitivity of the existing classes of photochromic materials. In this study, we report a simple sol–gel route for the preparation of highly photochromic transparent films based on ormosil–phosphotungstate hybrid materials. The effect of addition of Zn2+ ions on the photochromic response of these hybrid films and the possible mechanism involved is discussed. Compared to the undoped samples, the photochromic response of the Zn2+-doped hybrid films increases by 59–237 % depending on the concentration of Zn2+ ions added to the sol formulation. No structural or electronic change in the phosphotungstate dye was observed by vibrational spectroscopy or UV spectroscopy, though micro X-ray fluorescence (μ-XRF) analysis showed that the addition of Zn2+ in the sol–gel preparations leads to an increase in the amount of phosphotungstate (HPW) incorporated in the dip-coated films. Furthermore, TEM and nano-energy-dispersive X-ray showed formation of nano-agglomerates consisting of Zn and HPW in the Zn2+-doped samples. Zn K-edge X-ray absorption near edge structure analysis also confirmed the formation of the salt Hx[Zn(OH2)6] 2−xPW12O40. It is suggested that these Zn-phosphotungstate nano-agglomerates get trapped into the ormosil network during films preparation leading to increased concentration of the phosphotungstate anions in the films, in accordance with results from μ-XRF analysis. Raman spectroscopy confirmed that the Keggin structure of HPW is preserved in the hybrid films. FTIR spectra of the matrix part of the samples are identical before and after UV-irradiation, which suggests that the photochromic process does not involve oxidation of the organic functionalities. These highly photochromic hybrid films are promising candidates for the design of practical UV-sensing devices and dosimeters.

Photochromic hybrid containing in situ-generated benzyl viologen and novel trinuclear [Bi3Cl14]5-: improved photoresponsive behavior by the π···π interactions and size effect of inorganic oligomer.

Two new member of (V)((2n+2)/2)[Bi(2n)Cl(8n+2)] series hybrids, (BzV)2[Bi2Cl10] (1) and (BzV)5[Bi3Cl14]2·(C6H5CH2)2O (2) (where BzV(2+) = N,N'-dibenzyl-4,4'-bipyridinium and (C6H5CH2)2O = dibenzyl ether) have been obtained, and compound 2 contains an unprecedented discrete trimer [Bi3Cl14](5-) counterion. The novel in situ-synthesized symmetric viologen cation with aromatic groups on both sides of 4,4'-bipy would provide more opportunities to create π···π interactions to optimize the photochromic property of the hybrid, and different bismuthated-halide oligomers enable us to discuss the size effect in this series of compounds. Both 1 and 2 are photochromic, and their photoresponsive rate is faster than that of reported viologen-metal halide hybrids. Experimental and theoretical data illustrated that the size of the inorganic oligomer can significantly influence the photoresponsive rate of the viologen dication, and the π···π interaction behaves as not only a powerful factor to stabilize the viologen monocation radical but also the second electron-transfer pathway, from a π-conjugated substituent to a viologen cation, for the photochromic process.

Photochromic behavior of silver nanoparticle incorporated titanosilicate ETS-10 films

The partially reversible photochromic behavior of Ag0 nanoparticle incorporated titanosilicate ETS-10 films was achieved for the first time. Ag+ ions were incorporated into ETS-10 matrix by ion-exchange of extra framework cations (i.e., Na+ and K+) to form Ag+ ion-exchanged ETS-10. The colored (i.e., activated state) and colorless (i.e., bleached state) forms of the photochromic system of Ag0 nanoparticle incorporated ETS-10 films were achieved through thermal reduction and exposure to visible laser at 532nm wavelength, respectively. The resulting Ag0-ETS-10 films and also Ag+-ETS-10 crystals have been characterized by ICP-OES, XRD, XPS, FE-SEM, HR-TEM, UV–vis spectroscopies. Partial restoration of color was achieved after second thermal treatment, which implies reversibility of the photochromic process. Furthermore, Density Functional Theory (DFT) results showed that ETS-10 had robust geometry and incorporation of the silver nanoparticles had minimal effect on the structure. The relationship of ETS-10’s unique structural formation along with the integration of Ag0 nanoparticles within its matrix and its subsequent photochromic property was shown for the first time.

Detailed Molecular Dynamics of the Photochromic Reaction of Spiropyran: A Semiclassical Dynamics Study

A realistic semiclassical dynamics simulation study is reported for the photoinduced ring-opening reaction of spiropyran. The main simulation results show that one pathway involves hydrogen out-of-plane (HOOP) torsion of phenyl ring nearby N atom in 254 fs on the excited state and the isomerization from cis- to trans-SP that is complete in about 10 ps on the ground state after the electron transitionπσ*; the other dominate pathway corresponds to the ring-opening reaction of trans-SP to form the most stable merocyanine (MC) product. Unlike the previous theoretical finding, one C−C bond cleavage on the real molecule rather than the C−N dissociation of the model one is more probable than the ring-opening reaction after the photoexcitation of SP. The simulation findings provide more important complementarity for interpreting experimental observations, confirming the previously theoretical studies of photochromic ring-opening process and even supplying other possible reaction mechanisms.

Auxiliary ligand-controlled photochromism and decolourization of two bipyridinium-based metal–organic hybrid materials with various water clusters

Two bipyridinium-based metal–organic hybrid materials, {[Cd(Bpyen)0.5(p-BDC)(H2O)Br]·5H2O} (1) and {[Cd(Bpyen)(o-BDC)Br2]·8H2O} (2), have been synthesized. 1 is a 2D coordination polymer while 2 shows 1D chain-like structure. Both compounds exhibit rapid color change under light irradiation. However, the decolourization of the photochromic products is very different. The influence of the auxiliary ligands on the photochromism and decolourization process of the bipyridinium-based hybrid materials has been explored. Interestingly, water molecules are encapsulated in these two compounds and form novel water aggregates consisting of novel water building units, such as the (H2O)10 water cluster in 1 and the (H2O)12 water cages in 2 through hydrogen-bond interactions. The effect of these water aggregates on the stability of the two compounds has been studied.