Photochromic Glass Michaela McDowell Mr. Spangler Whitefish High School Advanced Chemistry

How They Work Transitions lenses contain special photochromic dyes or ions that cause the lens to activate, or darken, when exposed to ultraviolet rays from sunlight. When the UV light diminishes, the lenses fade back. As light conditions change, the level of tint adjusts, offering the right tint at the right time. However, temperature affects the time taken for the dyes or ions to return to their resting phase.

Chemistry Behind Photochromic Glass The reduction-oxidation reaction includes two half reactions, usually an ion gaining electrons from another ion. An example of this would be Silver Chloride and Copper(I) Chloride: Ag + + e > Ag Cu +2 + Ag > Cu +1 + Ag + The first half of the reaction on the left (the silver chloride) is the reaction causing the glass to darken while the second reaction on the right (the copper chloride) fades the glass. This reaction is nonspontaneous, meaning a energy source must be provided for the reaction to occur. In photochromic eyewear, the ultraviolet rays given off by the sun provide the energy needed to start the reaction. When the ultraviolet rays are no longer present the reaction reverts back to its original form. Cl + Cu > Cu +2 + Cl - Cu +2 + Ag > Cu +1 + Ag +

Chemistry Behind Photochromic Glass Cont The chloride ions are oxidized to produce chlorine atoms and an electron. The electron is then transferred to silver ions to produce silver atoms. These atoms cluster together and block the transmittance of light, causing the lenses to darken. This process occurs almost instantaneously. The presence of copper (I) chloride reverses the darkening process by dispersing the clusters of silver atoms. The chlorine atoms formed by the exposure to light are reduced by the copper ions, preventing their escape as gaseous atoms from the matrix. The copper (+1) ion is oxidized to produce copper (+2) ions, which then reacts with the silver atoms.The effect of these reactions is that the lenses become transparent again as the silver and chloride atoms are converted to their original oxidized and reduced states.

Hypothesis and Purpose ● The purpose of this experiment to investigate the effect temperature has on photochromic glass as it fades after exposure to ultraviolet light. In other words, how temperature affects the quantum yield in photochromic glass ● It was hypothesized that the cooler the temperature the longer it will take for the lenses to fade back because the cooler temperature slows the movement of energy in the dyes or ions in the glass. Quantum yield reaction

Procedure I ran four temperature controlled tests, repeating each test two times for validity. I’ve ran all tests on 2 pairs of prescription transition glasses. For each test I exposed the glasses to a black light for ten minutes before placing them in a temperature controlled area and timing how long the lenses take to fade using the second lens as a comparison. I’ve tested at -5.3 degrees celsius (a freezer), 2.2 degrees celsius (a refrigerator), 20.6 degrees celsius (room temperature), and 80 degrees celsius (a drying oven).

Results My data has come to support my hypothesis as the lenses took an average of 2 minutes to fade in the drying oven, 17 minutes to fade at room temperature, 5 days to fade back in the refrigerator, and 20 days to fade in the freezer.

Conclusions My research has come to the conclusion that photochromic glass takes longer to fade back to its clear state in cooler temperatures. This is due to the slowing of energy in the ions and dyes used in photochromic glass. The redox reaction taking place in the glassware slows, resulting in a longer fade back time. The quantum yield of the photochemical reaction, the efficiency of the photochromic change with respect to the amount of light absorbed, is strongly dependent on the conditions surrounding the glass. If the quantum yield can be manipulated into being more efficient with less dependence on its surroundings, we can create a photochromic material that would be less susceptible to temperature.

Improvements Some improvements for this project would include: ● A more accurate timer ● Knowing more about the lenses I tested; including who manufactured them, what ions or dyes are in the glass etc. ● Testing at more temperatures ● Having more than two lenses to test

References Woodford, Chris. "Photochromic Lenses." How Do Transitions® Photochromic Lenses Work? N.p., 24 Nov Web. 17 May "Photochromic Technology | Photochromic Lenses | Transitions Optical." Transitions® Lenses. N.p., Web. 20 May "How Can My Glasses Change from Transparent, When I'm Inside, to Dark When I Go Outdoors?" HowStuffWorks. HowStuffWorks.com, Web. 22 May Transitions. "Photochromic and Polarized Technology | Transitions Optical." Transitions® Lenses. Transitions, Web. 25 May 2015.

Acknowledgements I would like to thank the opthamologist’s office for donating the two pairs of prescription glasses I used in my testing. I would also like to thank Mr. Spangler for allowing me to use the resources I needed to continue my testing in class.