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General Mechanism of Photochromism

General Mechanism of Photochromism

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General Mechanism of Photochromism

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  1. General Mechanism of Photochromism • mechanism over the course of the colour change, the photochromic molecule represented simplistically as two rings. • Upon the action of UV light or direct sunlight, the structure twists from a perpendicular (closed) form to a flat, planar (opened) structure. This allows the two halves to interact, resulting in the absorption of visible light. • There are effectively two changes occurring simultaneously; a chemical change arises when the molecule is exposed to UV light, that enables conjugation to take place throughout the molecule; a structural change also occurs to enable the overlap of ∏-orbitals. Therefore, spatially, the molecules must be able to flatten out to allow this conjugation to take place. • It is a fully reversible reaction so that when the light source is removed, the molecule returns to its uncoloured state. Heat can also help drive the reaction back to the uncoloured form, so in very hot conditions, there is always competition between light and heat to determine the given colour observed. In general, a colour change is still observed, albeit weaker than at room temperature. Similarly, in cold conditions in the presence of sunlight, an intense colour is observed as there is little or no competition from the back reaction. • Kinetics • This is a cyclic reaction and the number of cycles (or the activation and fade rates) varies greatly by product. The activation times are generally much shorter than fade times. On average, fade times are two or three times longer than activation times. The Reversacol product range offers a very large variation in kinetics characteristics. Some fade in several seconds, whilst others can take several minutes. • Systems • As well as the nature of the product influencing the colour and kinetics result, the system or matrix used with the dye has a strong influence on such properties. For example, in some systems, a colour shift of up to 20nm has been observed. James Robinson Reversacol Photochromic Dyes

  2. US Patent 5458815Photochromic naphthopyran compounds   The photochromic activity of these compounds is caused by the reversible light-induced cleavage of the bond between the heterocyclic oxygen atom and the quaternary carbon: the closed colorless forms undergo ring-opening to become the corresponding open forms, the photomerocyanines, which are responsible for coloration.

  3. Benzopyrans • Benzopyrans-United States Patent 4446113 • describes compounds having blood pressure lowering activity

  4. Benzopyrans • “Optical Control of Divalent Metal Ion Binding to a Photochromic Catechol: Photoreversal of Tightly Bound Zn2+” • Required the Benzopyran to attach to a metal such as Pb or Zn • This could lead to other complications

  5. Benzopyrans - References , United States Patent 446113) •

  6. Spiropyran • The spiropyran compounds exhibit heliochromic properties. • They color in sunlight and fade rapidly at ambient temperatures in the absence of U.V. light, making them good candidates for use in the manufacture of sunglasses. • Past inventions with this includes lenses which darken in sunlight and incorporate the novel spiropyrans and a process for the preparation of the spiropyran compounds.

  7. Spiropyran • United States Patent 4826977 Photochromic spiropyran compounds • The only concern with this is that the observation by Becker was restricted to temperatures below about -40*C and Becker reported that the color change was reversed when the temperature was raised to a temperature in the range of -10*C to 0*C. Therefore we believe that this compound could be temperature dependent.

  8. Spiropyran • The stability of the photomerocyanine is related to the high degree of conjugation allowed by a nearly planar conformation. • The conversion of SP to MC occurs via a photochemical route involving ultraviolet photons. • Once irradiated with UV light, the ringopened, and colored MC form slowly rearranges back to the SP form. • The color of the MC form as well as the rate of rearrangement back to the SP form are both dependent • on the solvent polarity. • The dependence of the rate of the back reaction on the polarity of the solvent arises from the zwitterionic MC form which is stabilized in polar solvents. • The stabilization of the MC form in polar solvents leads to a larger energy of activation and a slower MC->SP transition as compared to non-polar solvents. The color dependence of the MC form (known as solvatochromism) arises from the difference in polarity between the photo-excited MC form and the zwitterionic ground state MC form. • For the case of 6-NO2-BIPS the excited state of the MC form is less polar than the zwitterionic ground state.

  9. Spiropyran • In polar solvents the ground state of the MC form is stabilized relative to the excited state of the MC form leading to a blue shift in the absorption maximum as shown in figure 1 below. Thus a solution of 6-NO2-BIPS appears pink in acetone after irradiation with UV light as the MC form has an absorption maximum at 568 nm. In comparison a solution of 6-NO2-BIPS in cyclohexane appears blue after UV irradiation because the MC form has an absorption maximum at a longer wavelength (~600 nm).

  10. Inorganic Photochromic Pigments:Tungsten Oxide “Figure 5 shows the optical absorbance spectrum of the mesoporous WO3 nanocrystals. An abrupt increase in absorbance is observed at wavelengths below 360 nm corresponding to the band gap of the WO3. From the UV region, one can determine the optical band gap from the fundamental absorption edge or coefficient. The optical band gap value is determined by considering an indirect transition between the 2p electrons from the valence band of the oxygen and the 5d conduction bands of tungsten. The optical band gap is formally defined as the intercept of the plot of (αhυ)1/2 against hυ, where α and hυ denote the absorption coefficient and photon energy, respectively.” - Teoha et al

  11. Inorganic Photochromic Pigments:Titanium Doped Tungsten Oxide “A 1:1 [W(OPh)6]:[Ti(OiPr)4] precursor mix resulted in a 92:8 W:Ti atom% film composition, while a 1:2 [W(OPh)6]:[Ti(OiPr)4] precursor mix resulted in a 82:18 W:Ti film composition. The band gaps of these films were determined as 2.8 eV for the 92:8 film and 2.9 eV for the 82:18 film. All films showed photochromism, that is, a reversible colour change from yellow to blue, after UV irradiation. UV/visable spectra showed an increase in absorption in the 600–900 nm region corresponding to this colour change.” -Palgrave et al

  12. Silica Hydro-Gel & PMMA(polymethacylate)

  13. PMMA • PMMA or polymethylmethacrylate is a clear plastic vinyl polymer that is more transparent than glass. • PMMA has been used safely for almost 50 years for contact lenses.

  14. PMMA • Advantages • Studies with spiropyrans and spiro-oxazines show that it does not slow down UV response times of Photochromic pigments. • It is also water soluble • It is able to readily bind with the pigment • Disadvantages • not very gas permeable( very low oxygen penetration) • low elasticity

  15. Silicone Hydro Gel • An inorganic matrix of silicon dioxide. • Used in creating current soft contact lenses.

  16. Silicone Hydrogel • Advantages • It allows the contact lens to have greater flexibility • Allows for greater oxygen penetration • Disadvantages • Is not very compatible with the photochromic pigment • Slows down transition time • The binding process is complicated

  17. Next Step • Picking the top two photochromic pigments based on our ratings and performing additional tests. • Construct a hybrid material which allows us to combine the best aspects of the two most commonly used contact materials.

  18. Spiropyran - References •'Spiropyrans' •'Spiropyrans' • spiropyran compounds , United States Patent 4826977) • •

  19. References • Lay Gaik Teoha, Jiann Shiehb, Wei Hao Laia, I Ming Hunga, Min Hsiung Hon. “Structure and optical properties of mesoporous tungsten oxide.” Journal of Alloys and Compounds. (2005) 251–254 • Robert G. Palgrave and Ivan P. Parkin. “Aerosol assisted chemical vapour deposition of photochromic tungsten oxide and doped tungsten oxide thin films.” Journal of Materials Chemistry. 2004, 14, 2864 – 2867 • • S. Delbaere,a, J.-C. Micheau,b Y. Teral,c C. Bochu,a M. Campredon, c and G. Vermeerscha. “NMR Structural and Kinetic Assignment of Fluoro-3H-naphthopyran Photomerocyanines.”

  20. References • www. • Andersson, Nina, Alberius, Peter, Ortegren, Jonas, Lindgren, Mikael, and Bergstrom, Lennart. “Photochromic Mesostructured Silica Pigments Dispersed in Latex Films”