Photochromic Response Research Articles

Photochromic Behavior of Spiropyran and Fulgide in Thin Films of Blends of PMMA and SBS

Two types of photochromic colorants of either spiropyran or fulgide were dispersed in thin films of blends of polymethyl methacrylate (PMMA) and styrene-butadiene-styrene copolymer (SBS) with various ratios. The thin films were illuminated with ultraviolet light (365 nm) under various temperatures. The photochromic response was monitored with a multichannel photodetector. The results show that the color form of spiropyran is more stable in polar PMMA and caused a blue shift in the absorption spectrum of merocyanine.

Fabrication of photochromic phosphomolybdic acid monolayer film

A photochromic monolayer film of phosphomolybdic acid (denoted as PMo12) was fabricated by self-assembly approach. UV-visible spectrum and AFM observation show that the monolayer film is composed of aggregated PMo12 molecules. The monolayer film shows good photochromic properties, with enough stability and reversibility. The color change of the monolayer after UV-irradiation can be captured by a microscope equipped with a color CCD camera. Photochromic response of the monolayer film can be doubled after being modified by an amine monolayer.

Langmuir–Blodgett monolayers from lower generation amphiphilic monodendrons

Amphiphilic monodendrons of lower generations, AD12-N, containing a benzyl-15-crown-5 polar focal point, photochromic spacer and different number of dodecyl tails as peripheral groups ( n=1–8) have been investigated for their ability to form uniform monolayers at solid surfaces. The surface pressure–area behavior, photomechanical behavior and the morphology of the monomolecular films were investigated. We observed that all compounds studied are capable of forming stable Langmuir and Langmuir Blodgett monolayers, with virtually flat packing of molecules. Higher generation dendrimers form very uniform monolayers, without the usual domain microstructure. For AD12-4 monolayer on solid support, we observed stripped microstructure with several layers (3–6) bundled together. The periodicity of this structure of 8 nm was close to layered spacing, obtained from X-ray data for bulk material. For this compound, we proposed the model of double-layered packing of the molecules, with partial overlapping of the central segments and suggested that deposition on a solid substrate resulted in changing orientation of molecular fragments. Fast reversible photochromic response was observed for all monolayers with a conversion level of 50%.

Modified mesoporous MCM-41 as hosts for photochromic spirobenzopyrans.

The influence of the chemical composition and silylation of mesoporous MCM-41 materials on the photochromic behaviour of adsorbed spiropyran (BIPS) and 6-nitrospiropyran was studied. Upon incorporation, the spiropyrans underwent ring opening to form either zwitterionic merocyanine or its corresponding O-protonated form. In all silica MCM-41 or in the MCM-41 containing aluminium, the O-protonated merocyanine was predominantly formed. In the case of MCM-41 modified by silylation of the OH groups, a mixture of zwitterionic merocyanine and spiropyran was present. The photochromic response was studied by means of steady-state irradiation and by laser flash photolysis. Steady-state irradiation (lambda > 450 nm) of the solid samples gives rise in all cases to an intensity decrease of the absorption bands corresponding to either the protonated or the unprotonated merocyanine form (reverse photochromism). In contrast, laser flash photolysis at 308 nm of spiropyrans supported on silylated MCM-41 allows observation of the photochemical ring opening of residual spiropyran to the corresponding zwitterionic form (normal photochromism).

A novel photoluminescent and photochromic europium com-plex

A ternary europium complex of 4-aminobutyric acid (ABA) with 1,10-phenanthroline (phen) [Eu2(ABA)4 (phen)4](phen)4(CIO4)6 was synthesized and characterized by X-ray single crystal diffraction. The result shows that 4-aminobutyric acid exists in zwitterion form in the binuclear complex and that the carboxylates coordinate with Eu3+ ion in bidentate bridging and tridentate chelating-bridging modes. There are two types of phen molecules, one is coordinated and the other is uncoordinated. When excited by YAG: Nd laser with 355 nm light, the title complex can emit strong red fluorescence, and its high-resolution emission spectrum was recorded at 77 K. The Eu 3+ion site is in low symmetry, which is in agreement with the result of X-ray single crystal diffraction analysis. When irradiated with a mercury lamp, the aqueous solution of the title complex can perform photochromism with the color change from colorless to green and the green color can fade away in the dark. The photochromic response time is related to the concentration and pH of the solution, the temperature and the light intensity.

Photochromic response of bismuth germanium oxide doped with chromium

Doping the photorefractive material bismuth germanium oxide Bi12GeO20(BGO) with chromium significantly affects its optical properties. An absorption spectrum taken at 300 K shows that the introduction of chromium produces a strong unresolved band which overlaps the absorption cutoff of BGO and a series of smaller overlapping bands between 650 and 1100 nm. Cooling BGO:Cr to 10 K sharpens the small bands and resolves the strong band into a peak near 510 nm. This peak is responsible for the reddish-brown color of BGO:Cr. Excitation at room temperature with visible light increases the bands in the 650–1100 nm range. The production of this additional room temperature photochromic absorption is most efficient with excitation near 490 nm but extends throughout the visible. Chromium occupies the tetrahedral germanium site in BGO; and the observed spectra are consistent with a Cr4+ tetrahedral state. During crystal growth some of the chromium gives up an electron to the antisite bismuth native defect and becomes Cr5+. Exposing the sample to light returns the electron to the chromium and increases the Cr4+ absorption. The photoinduced absorption is thermally stable for temperatures up to about 425 K. This stability suggests that doping BGO with Cr should lead to room temperature persistent photorefractive gratings. © 2001 American Institute of Physics.

Remarkable Improvements in the Photochromic Reversibilities of Fulgides in Solid Films

Oxazolyl fulgide with p-dimethylaniline group (fulgide 1) revealed favorable absorption spectra, high photochromic responses and high thermostabilities in a polystyrene or a poly(methyl methacrylate) film spin-coated on a silica glass, but it suffered from extensive photochemical fatigues while phenyl-substituted oxazolyl fulgide (fulgide 2) revealed much less fatigues. The photochemical fatigues of fulgide 1 could be substantially suppressed by a simple treatment of the spin-coated films to keep air-tight. Similar improvement in the photochemical fatigue resistance was obtained with films of the well-known furylfulgide. Further, thermal stabilities were significantly improved towards heating at 80°C. These effects are ascribable to the effective shielding of the air (molecular oxygen).

User controllable photochromic (UCPC) devices

A novel class of photochromic devices is described which allows the user control over the photochromic response of the device. The ability to limit the photochromic response of the device under illumination potentially allows the use of these devices in applications where traditional photochromics are excluded today. User controllable photochromic devices are described in terms of their general materials, properties, construction and performance.

On the Thermal and Photochemical Stabilities of Photochromic Spirooxazine Dyes Encapsulated in Ormocer Matrices Derived by Sol-Gel Processing

Spiroindolinonaphthoxazine (abbreviated as SO), a most promising class of photochromic dyes due to their better photochromic response to UV irradiation (e.g. sunlight), higher fading rate of the coloured form and higher photofatigue resistance, is incorporated into Ormocer matrices synthesized by the sol-gel route aiming to achieve some application prospects. As for all technical applications the stability of the martials as a function of time and /or under certain harsh circumstances must be considered and improved if necessary. In the present work the thermal and photochemical stabilities of our photochromic SO-Ormocer gels and coatings are investigated by room temperature (RT) holding, heat treatment and UV irradiation experiments and compared with the SO-PMMA coatings. It is shown by our results that both the thermal and photochemical stabilities of our SO-Ormocer coatings are similar to those of the SO-PMMA coatings. Considering that both the photochromic response and the fading rate of our SO-Ormocer coatings are much better than those of the SO-PMMA coatings, it is reasonable to conclude that the overall performance of our SO-Ormocer coatings is acceptable for certain application purposes.

Comparison of the photochromic properties of fulgides and fulgimides

Photochromic properties of some arylfulgimides such as furyl-, thienyl-, and thiazolylfulgimides were compared with corresponding fulgides in detail. Photochemical reversibility and thermal stability as well as the photochromic response of fulgimides were substantially comparable with those of the fulgides, both in solution and in solid polymer film. Fulgimides revealed high resistance to solvolytic degradation in hydroxylic media while corresponding fulgides underwent rapid degradations. Thus, N-(4-carboxyphenyl)fulgimide, being soluble in aqueous media, revealed moderate photochromic performance in aqueous ethanol solution without extensive solvolytic degradation, as well as in ionic polymer film of allylamine hydrochloride.

Spirooxazine- and spiropyran-doped hybrid organic–inorganic matrices with very fast photochromic responses

Both spirooxazine and spiropyran dyes have been embedded into two different hybrid matrices, which were formed from hydrolysis and cocondensation between diethoxydimethylsilane and zirconium propoxide and between methyldiethoxysilane (DH) and triethoxysilane (TH) respectively. The nature and the kinetics of the photochromic response depend strongly on the hydrophobic/hydrophilic balance (HHB) of the hybrid material. The HHB controls the competition between direct and reverse photochromism. The photochromic behaviour of the strongly hydrophobic spirooxazine-doped DH/TH coatings is direct, highly efficient (ΔA>1), reversible and extremely fast (thermal bleaching time constant, k=0.2 s -1 ). The photochromic kinetics of this hybrid material are, to the best of our knowledge, much faster than those reported for spirooxazine in any other solid matrix.

Photochromic molecular recognition of β-cyclodextrin bearing spiropyran moiety for organic guests

The photochromic behaviour and guest binding properties of β-cyclodextrin modified with 1′,3′,3′-trimethyl-6-nitro-8-methylspiro[2H-1-benzopyran-2,2′-indoline](1) has been investigated. Compound 1 exhibits a unique photochromic response in comparison to those of the 1′-(3-oxopropyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline] modified β- and γ-cyclodextrins (2 and 3, respectively). Compound 1 shows reverse photochromism in aqueous or ethylene glycol solutions, however, in less polar solvent such as dimethyl sulfoxide, 1 exhibits a mixed type of normal and reverse photochromism. The closed form of 1 in aqueous solution is converted to the opened form by keeping it in the dark. The halflives of the closed form in the dark are affected by the presence of a guest. The highest value is obtained when 1-borneol was used. The magnitude of the half-lives and the binding constant for a guest examined are comparable, and is dependent on the guest molecular size.

Low-temperature photochromic response of phosphorus-doped bismuth silicon oxide

Phosphorus is one of several dopants that electronically compensate the native deep donor responsible for the yellow coloration observed in bismuth silicon oxide (BSO). Low-temperature optical absorption measurements of a series of Czochralski-grown P-doped BSO crystals show that ∼0.1–0.15 at. % P is needed in the sample to fully remove the yellow coloration. The absorption cutoff in the fully compensated P-doped sample was at 3.2 eV while compensated Al- and Ga-doped samples cutoff at 3.35 eV. Excitation at 10–15 K with near band-edge light produces photochromic absorption bands. In the lightly-doped (partially bleached) samples these bands were identical to those observed in undoped BSO. In the fully bleached sample a new spectrum was observed. Its major contribution was a band centered near 1.8 eV with a weaker absorption in the blue-green. By comparison with the spectra observed in undoped and in Al-doped material before and after photoexcitation it is believed that the 1.8 eV band is due to the [PO4]− center and that the broad 2.45 eV band observed in Al- and Ga-doped BSO is due to the [BiO4]0 center.

Study of the Fatigue Process and the Yellowing of Polymeric Films Containing Spirooxazine Photochromic Compounds

Abstract Photochromic polyurethane films containing 5-substituted 1,3-dihydro-3,3-dimethyl-1-alkylspiro[2H-indole-2,3′-[3H]pyrido[3,2-f][1,4]benzoxazine] and 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3′-[3H]naphth[2,1-b][1,4]oxazine] were degraded under xenon light exposure. The loss of the photochromic response was monitored by spectrophotometry as a function of the irradiation time; meanwhile, the photoproduct formation and the amount of photochromic material still available were analyzed by chromatography after liquid–solid extraction. At the same time, the fade rates were monitored after flash-excitation photolysis or after the photostationary state during degradation. It was clearly demonstrated that the loss of photochromism was due exclusively to a photooxidation process of the initial dye, and not to an acceleration of the bleaching or a potential screen effect caused by the photoproducts.

Optically synaptic films using spiropyran J‐aggregates

AbstractSpiropyran SP1822 J‐aggregate LB films show a non‐linear photochromic response with a threshold against the incident light intensity of a CW dye laser. Using two laser beams operating below the threshold level intensity, the logic ‘AND’ function was found to be optically stored as an overlapping region of two incident patterns. We proposed the adoption of an LB film of spiropyran J‐aggregates to store the memory matrix Tijkl as an overlapping region of learning input pattern Vij and multi‐image input pattern Vkl in order to construct an optical neural network system based on Hopfield model.

TWO DIFFERENT TYPES OF PHOTOCHEMISTRY IN PHYCOERYTHROCYANIN α‐SUBUNIT*

AbstractThe photochemical activities of phycoerythrocyanin α‐subunits from Mastigocladus laminosus separated by isoelectric focusing were tested by irradiating at 500, 550, 577 and 600 nm. Two types of photoreversible photochromic responses have been characterized by absorption and absorption difference spectroscopy. Type I is the well‐known absorption shift from 571 to 506 nm. Type II is a new response characterized by a line‐broadening of the 570 nm absorption.

Low-temperature photochromic response of gallium-doped bismuth germanium oxide

Gallium is one of several dopants that electronically compensate the deep donor responsible for the yellow coloration observed in bismuth germanium oxide (BGO). A series of Ga-doped BGO crystals were grown using the Czochralski method to confirm the formation of a trapped hole center following exposure to near band edge light at low temperatures. The deep donor absorption shoulder disappeared for Ga concentrations in the 3%–4% (melt) range. Excitation at 10 K with 3.3 eV light produces photochromic absorption bands. For low gallium concentrations the photochromic bands were the same as the ones observed in undoped BGO. For concentrations of 4% or more the photoinduced absorption consisted of overlapping bands at 1.1, 1.55, and 2.45 eV. These bands grew uniformly with exposure time and annealed together in the 100–120 K temperature range. By comparison with earlier results on Al-doped BGO and bismuth silicon oxide (BSO), we believe that the 1.1 eV band in BGO:Ga and the 1.0 eV band in BGO:Al (and BSO:Al) are due to trapped hole centers ([GaO4]0 and [AlO4]0, respectively).

Reversible photochemistry in the ?-subunit of phycoerythrocyanin: Characterization of chromophore and protein by electrostatic calculations

We used a macroscopic dielectric model to study the effects of solvation and interaction between titratable and permanent partial charges on the protein conformational energy and the acid–base equilibria in the cyanobacterial photoreceptor phycoerythrocyanin, whose photoreversible photochromic response is attributed to a Z/E isomerization of the covalently bound tetrapyrrole chromophore. The calculations revealed the stabilization of the charged protonation state of the chromophore by a small set of strong local interactions. Although the protein is globular and water-soluble, the complex counterion structure has a striking similarity to the arrangement found for the photochemical active transmembrane protein bacteriorhodopsin. This could be attributed to the fact that the protonation site in the α-subunit of phycoerythrocyanin is buried in the interior of the protein. Due to the strong shielding from solvent, the interaction pattern is conserved upon a ground-state isomerization of the chromophore. The partial solvent exposure of the isomerization site resulted in a drastic influence of the chromophore configuration on the aqueous solvation energy of the protein. Implications for the sensitivity of the photochemistry to environmental factors and molecular binding are discussed. © 1993 John Wiley & Sons, Inc.

The low-temperature photochromic response of bismuth silicon oxide

Exposing the photorefractive material bismuth silicon oxide (BSO) at low temperatures to 2.4–3.3 eV light produces photochromic absorption bands. In undoped and Fe-doped BSO these bands appear to consist of a series of overlapping bands ranging from around 1.5 eV in the infrared to near the band edge. The infrared component is always weaker than the visible range contributions. The infrared portion anneals just above 100 K; in some samples this anneal is accompanied by the appearance of additional structure in the visible region. In undoped BSO the major anneal of the photochromic bands takes place above 200 K. If iron is present the photochromic bands are weaker and an anneal stage in the 120–150 K range appears. Bleaching with either 1.51 or 2.28 eV laser light uniformly lowered the photochromic bands in both undoped and Fe-doped BSO. In BSO:Al the aluminum electronically compensates the deep donor centers responsible for the yellow coloration observed in undoped crystals. At low temperatures, photoexcitation using near band-edge light produces the same overlapping bands at 1, 1.38, and 2.45 eV that were observed in BGO:Al. These bands anneal together between 80 and 100 K. The [AlO4]0 center which causes the coloration observed in smoky quartz is a plausible model for the photochromic bands in BSO:Al.

Photochromic Response of Vacuum‐Evaporated Nb2O5 Thin Films in Ethanol Vapor

AbstractAmorphous Nb2O5 thin films are obtained by vacuum evaporation of high‐purity Nb2O5 powders onto NESA glass substrates. These films were found to exhibit both photochromism and electrochromism. Reasonably good photochromic responses were observed in ethanol and in the N2 atmosphere, but no photochromism was observed in the air or in a pure O2 atmosphere.