PHOTOCHROMISM: SEEING THE LIGHT . . . SAFELY

PHOTOCHROMISM: SEEING THE LIGHT . . . SAFELY

Chemistry Thesis Submitted in partial fulfillment

of requirements for

Master of Chemistry Education

by

Douglas G. Balmer University of Pennsylvania

Philadelphia, PA 2009

Thesis Committee:

Adviser: Michael Topp, Ph.D. Professor of Chemistry Director: Connie Blasie Director of PennSTI

Douglas G. Balmer PHOTOCHROMISM: SEEING THE LIGHT . . . SAFELY

Abstract Visible light has been linked to an increased risk of age-related macular degeneration (AMD). The photolytic reactions of free-radicals and reactive oxygen species lead to the damage of photoreceptors on the retina and proteins and lipids in the retinal pigment epithelium (RPE). Light with a wavelength near 430nm is particularly damaging because it is high in energy and is absorbed by waste products found in the (RPE). The body naturally uses antioxidants such as vitamin C, vitamin E, and zeaxanthin to terminate free-radical reactions. Low levels of antioxidants have been linked to increased levels of AMD. Sunglasses can also provide protection by absorbing the damaging blue light. Spirooxazines and other photochromic dyes can be incorporated into eyeglass lenses to provide such protection when activated by UVA light. The photochromic mechanism of spirooxazines relies on the cleavage of the Cspiro-O bond in the oxazine ring. The activation and fade kinetics of photochromic dyes is greatly inhibited by the rigidity of the lenses’ polymer matrix. The kinetics of photochromic dyes can be greatly enhanced by attaching flexible oligomers to the dye. The oligomer provides localized flexibility without compromising the strength of the lens. The use of dimethyl siloxane oligomers has achieved near-solution kinetics.

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Introduction “Seeing the Light . . . Safely” implies there is a risk or danger associated with

light. Anyone that has ever spent too much time in the sun and gotten a sunburn or experienced temporary snow blindness can attest to the fact that natural light can be dangerous. Synthetic light can also be dangerous. Welding (Magnavita 2002) and even flash photography (Mauget-Fa, et al. 2001) have been linked to retinal injury. Light is a broad term. It can refer to the visible portion of the electromagnetic radiation spectrum, which ranges from violet (400nm) to red (700nm). Light can also refer to ultraviolet (UV) radiation, but UV radiation can be further broken into UVC (200-280nm), UVB (280-320nm) and UVA (320nm-400nm). This paper focuses on the risk the visible portion of the spectrum, specifically the blue region, poses to the eye, and the safety that can be achieved through the use of photochromic eyeglasses. The first half of the paper will address the chemical and physiological mechanisms that lead to the development of age-related macular degeneration (AMD). The second half of the paper will address a photochromic solution to seeing light safely. It will center on the structure, photochromic mechanism, physical properties, and kinetics of photochromic spirooxazines used in ocular lenses.

Light can be described using various properties, such as wavelength (

€

λ ), frequency (

€

ν ), and energy (E). These variables are related by the speed of light (c) in Equation 1 and Plank’s constant (h) in Equation 2.

c =

€

λ

€

ν (1) E = h

€

ν (2)

As the frequency of light increases, the wavelength decreases while the energy per photon increases. Equations 1 and 2 show that UV radiation has more energy per photon than visible light, because it has a smaller wavelength and higher frequency. As a result, UV radiation should pose a greater threat to the eye. This paper focuses on visible light, though, for several reasons. Most UV radiation is filtered by the cornea and lens in the anterior regions of the adult human eye as shown in Figure 1. The absorption of UV radiation may be associated with corneal cataracts and clouding of the lens but not AMD.

1 Newsome, Paula R, Madeline L Romeu, Mary Seguiti, Susan Stenson, and Wael Yasesein. "The Effects of Ultraviolet and Visible Light on the Eye." Transitions. 2004. http://us.transitions.com/NR/rdonlyres/E826A9DC-AB6F-4B97-B499C5968498856F/0/UV_visiblelight_newsome.pdf (accessed July 19, 2009).

Figure 1 The penetrating power of UV and visible light in the eye1


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In addition, the majority of plastics used to make modern lenses are effective filters of UV radiation. Figure 2 shows how allyldiglycol carbonate (B) absorbs 90% of UVA and 100% of UVB radiation, while polycarbonate (C) removes 100% of UVA and UVB radiation. Normal eyeglass wearers are not really at risk for UV damage to the eye. It is for these reasons that this paper focuses on the threat of visible light.

Age-Related Macular Degeneration (AMD)

Visible light has less energy per

photon than UV radiation, but because it is focused on the back of the eye, it is one of the destructive forces that affects the retina leading to AMD. Figure 3 (The Normal Eye 2007) shows the retina, macula, and choroid at the back of the eye. The macula is not a separate structure but is the central portion of the retina. The retinal pigment epithelium, RPE, (not pictured) is located in between the retinal photoreceptors and the choroidal capillaries. These structures, especially the RPE, are impaired with AMD.

AMD is worth discussion because it

is one of the most common causes of blindness globally. The 2002 National Health Interview Survey (NHIS) projected 1.75 million cases of AMD in adults over the age of 40 in the United States with 7 million more individuals identified as at risk (Friedman, et al. 2004). For comparison, the same NHIS estimated 20.48 million cases of cataracts

2 Ross, Denwood F, III. "Ophthalmic Lenses: Accurately Characterizing Transmittance of Photochromic and Other Common Lens Materials." Applied Optics 30, no. 25 (September 1991): 3673-3677. 3 The Effects of Ultraviolet and Visible Light on the Eye. The Normal Eye. Familial Exudative Vitreoretinopathy. 2007. http://www.fevr.net/THE%20EYE/normal_eye.htm (accessed July 19, 2009).

Figure 2 The transmittance curves of white crown glass (A), allyldiglycol carbonate

(B), and polycarbonate (C)2

Figure 3 Age-related macular degeneration affects the macula, retina, choroid, and RPE (not pictured) at the

back of the eye3


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(Condon, et al. 2004) and 2.22 million cases of glaucoma (Friedman, et al. 2004). Since AMD occurrences increase with age, this topic has great significance to the aging baby-boomer generation. By 2020, it is estimated that AMD cases will increase to 2.95 million (Friedman, et al. 2004). AMD is diagnosed as a loss of central vision caused by chemical and physiological changes in the retinal pigment epithelium, RPE, of the macula. The RPE is responsible for the transfer of oxygen and nutrients from the choroidal capillaries to the retinal photoreceptors. Phagocytes in the RPE are responsible for the breakdown of worn receptor membranes. The remains are transported from the retinal photoreceptors to the choroidal capillaries. Drusen are deposits of oxidized proteins and lipids, pyridinium bisretinoid (A2E), and β-amyloid proteins (Aβ) found in varying stages of AMD.

The high oxygen concentration found in the capillaries of the choroid and the visible light that reaches the macula favor the oxidation of proteins and lipids. This process is achieved through the formation of radicals and other reactive species. These reactive species go on to damage proteins and lipids in the macula until the reactions are terminated. Two common reactive oxygen species (singlet O2 is not a radical) found in the eye are singlet(Δ) O2 and the superoxide ion. Singlet O2 can be formed when ground-state oxygen absorbs energy from an adjacent excited molecule. Singlet oxygen is 93.6kJ/mol more energetic than ground-

state oxygen. There are plenty of molecules in the eye that are excited by visible light, A2E, being one of them. The super oxide ion (O2

-) is formed when ground state oxygen oxidizes another molecule and accepts an electron. The difference between ground-state O2, singlet O2, and O2

- is illustrated in Figure 4 (Halliwell and Gutteridge 2007). Singlet oxygen is much more reactive than ground-state oxygen for two reasons. First, it is at a higher energy level, and second it can accept a pair of electrons forming an epoxide. Ground-state oxygen is not able to accept a pair of electrons under normal circumstances, because the electrons would have opposite spins and oxygen can only accept two electrons that have the same spin.

Figure 4 Molecular orbital diagrams for O2, singlet O2, and O2

•-

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The A2E found in drusen is a byproduct of trans-retinal side reactions. Light is perceived when a photon interacts with 11-cis-retinal rhodopsin (11cRAL) in the rod cells to form all trans-retinal (atRAL). The all trans-retinal is recycled back to 11-cis-retinal in the RPE so that the process can happen all over again. The entire process is illustrated in Figure 5 (Halliwell and Gutteridge 2007).

When the eye is exposed to intense light, the equilibrium shifts to the atRAL side in Equation 3, a simplified version of Figure 5. This has consequences because atRAL can undergo side reactions so that it is not recycled back to 11cRAL.

11cRAL

€

⇔hν

atRAL

Equilibrium shifts to the right with intense light exposure (3)

One of those side reactions involves phosphatidylethanolamine. When the two react, the final product formed is A2E. This is not favorable because it has an absorbance maximum at 435nm, which facilitates the formation of singlet oxygen. When A2E accepts an electron it becomes an A2E radical (A2E•) with no charge because A2E’s pyridinium group is positive at physiological pH. The A2E• radical is relatively long-lived (hundreds of milliseconds) in the absence of electron-acceptors as shown by the pulse radiolysis spectrum in Figure 6 (Broniec, et al. 2005). The kinetics of A2E• change drastically in the presence of electron acceptors like O2 in air. Under these conditions the lifetime of A2E• shortens to the order of tens of microseconds as it reacts with O2 to form O2

- as shown by the pulse radiolysis spectrum in Figure 7 (Broniec, et al. 2005). The formation of the superoxide ion promotes free-radical reactions in the eye.

Figure 5 Cis and trans retinal reactions of the rods

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In addition, A2E can readily react with up to nine singlet oxygens to form toxic oxiranes. The formation of A2E as well as its reaction with singlet oxygen is shown in Figure 8 (Halliwell and Gutteridge 2007).

Figure 8 A2E formation and its reaction with singlet oxygen

Figure 6 Slower kinetics of A2E• in

saturated N2O

Figure 7 Faster kinetics of A2E• in saturated air due to interaction with

oxygen

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In the early stages of AMD, the drusen are classified as hard or dry as shown in Figure 9 (Ding, Patel and Chan 2009). As AMD progresses, new vessels start to develop from the choroid to

compensate for the disruption of blood flow between the choroid and the photoreceptor cells. These new capillaries are classified as soft or wet as shown in Figure 10 (Ding, Patel and Chan 2009). These new capillaries can actually force the retina to detach. Numerous studies (Young 1988) have correlated blue-light exposure with increased levels of AMD. Figure 11 (Taylor, et al. 1990) shows that blue light with a wavelength of 430nm is most damaging to the retina. This area of the spectrum is particularly damaging because pigments in the eye have absorbance maxima near 430nm, and most wavelengths of even higher energy are absorbed by the cornea and lens. The solid line in Figure 11 indicates that even though 430nm is most damaging to the retina, human eyes are most sensitive to light with a wavelength of 555nm. This wavelength is close to the absorbance maximum of rhodopsin in the rods and the green

cones. The consequence of this is that the portion of visible light that humans are least sensitive to is most likely to impair sight. If a source of light has a high amount of 430nm light but not as much 555nm light, the eyes may not stimulate defense mechanisms like squinting or contracting the pupils (Taylor, et al. 1990).

Figure 9 Hard drusen Figure 10 Soft drusen

Figure 11 The visual sensitivity and retinal damage spectra of the retina


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Text Box #1: Au Naturel

“We are Flintstones kids. . .10 million strong and growing.” If you grew up watching television in the 1990’s, you may be familiar with the opening jingle. If not, you may have seen one of the nine Centrum multi-vitamins advertised: Centrum, Ultra Women’s, Ultra Men’s, Silver, Silver Ultra Women’s, Silver Ultra Men’s, Performance, Cardio, and Kids. From the advertising, it seems that vitamins are important to human health. But why? Vitamins got their name in the early 20th century from the Latin root for life, vita, and the class of compounds known as amino acids (Vitamin 2007). A vitamin is now thought of as an organic compound that is necessary in small quantities for healthy diets (vitamin 2007). The human body uses vitamins for many things, and one function is to protect against the effects of free-radicals. A free radical is a reactive chemical substance with an unpaired electron. The hydrogen atom, written H•, and the hydroxyl radical, written OH•, are two examples. Radicals formed in the eye during photochemical reactions are believed to be a possible cause of age-related macular degeneration, AMD (Young 1988). There is a significant correlation between blue-light exposure and the incidence of AMD in individuals with low levels of antioxidants such as vitamin C, vitamin E, zeaxanthin, and dietary zinc (Fletcher, et al. 2008). Figure 12 (Fletcher, et al. 2008) shows that the risk of AMD is highest for individuals between the ages of 50-59 with the lowest levels of antioxidants.

Vitamins and other antioxidants act as radical scavengers. This prevents highly-reactive radicals in and near the macula from destroying proteins and lipids that would eventually accumulate as drusen (see page 3). Vitamin C, also known as ascorbic acid, is pictured in Figure 13 (May 1998). The pKa for of the first H+ (circled in red) is 4.25, so at biological pH, that H+ will dissociate leaving the ascorbate ion. Vitamin C is an effective antioxidant because the ascorbate ion can release H+ (circled in blue) and donate an electron to a reactive radical. This process forms an ascorbyl radical, but the ascorbyl radical is much less reactive than other radicals because of resonance with the ring. Equation 4 shows how 2 ascorbyl radicals can terminate forming ascorbate and

Figure 12 Increased AMD risk due to blue light exposure and low levels of

anti-oxidants Figure 13 Free-radical scavengering of

ascorbic acid (vitamin C)

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dehydroascorbate (DHAsc). Reduced glutathione (GSH) and reduced nicotinamide adenine dinucleotide (NADH) can regenerate ascorbate from DHAsc.

2 Ascorbyl Radicals Ascorbate + Dehydroascorbate (4)

Vitamin E refers to a class of 8 natural antioxidants rather than an individual molecule. RRR-α-tocopherol, shown in Figure 14, is the most effective form in animals.

From this point forward, vitamin E will be referred to as α-tocopherol to eliminate any confusion. The H atom circled in red can be donated to lipid peroxyl radicals (LO2

•) four times faster than LO2

• can react with new lipids (Halliwell and Gutteridge 2007). This effectively terminates radical reactions that could damage lipids in the eye resulting in

drusen formation and eventually AMD. The α-tocopheryl radical, like the ascorbyl radical, is relatively stable due to resonance with the benzene ring. The resulting α-tocopheryl radical can be regenerated back to α-tocopherol by accepting a hydrogen atom from ascorbate as discussed above. Singlet oxygen (see page 3) can also be quenched by α-tocopherol. Eventually the α-tocopherol product of lipid peroxyl and singlet oxygen quenching is tocopherylquinone. Both quenching reactions are illustrated in Figure 15 (Halliwell and Gutteridge 2007).

Zeaxanthin (Figure 16) is part of the larger class of colored compounds known as carotenoids. It can quench lipid peroxyl radicals

and singlet oxygen. The mechanism is similar to that of vitamin C and α-tocopherol. Zeaxanthin can be regenerated by oxidation of α-tocopherol. The three antioxidants presented function in similar fashions. An electron or hydrogen atom is donated to a free radical. The antioxidant radical is stabled by resonance with conjugated double bonds. The antioxidants work together to recycle each other. Without sufficient vitamins and other antioxidants to scavenge free radicals, free-radical chain reactions can quickly damage

lots of tissue. This leads in part to the damaged proteins and lipids found in the drusen of patients with AMD.

Figure 14 RRR-α-tocopherol structure (Vitamin E)

Figure 15 α-tocopherol antioxidant reactions

Figure 16 Zeaxanthin structure

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Photochromic Lenses Limiting ocular exposure to visible light, especially blue light, is one way to

reduce the risk of AMD. Eyeglasses that use photochromic dyes, like those produced by Transitions®, can reduce the intensity of visible light in bright environments. Photochromic dyes are chemicals that change color when exposed to one wavelength (

€

λ1) of light, and then revert back when the source of light is removed or changed to a different, usually longer, wavelength (

€

λ2) as shown in Equation 5. The reverse reaction to the left can also be driven thermally (

€

Δ ).

€

A(λ1)hν 2 ,Δ

hν 1

⇔B(λ2) (5)

Photochromism was first reported at the end of the 19th century by M. Fritsche, 1867, and E. ter Meer, 1876 (Crano and Guglielmetti 1999). The first photochromic eyeglasses were produced by Corning in 1966. This glass lens made use of a photochromic silver/copper halide (Crano, Flood, et al. 1996). The first photochromic spirooxazine compound was reported by Fox in 1961 (Malatesta, Millini and Montanari 1995). Americal Optical used an indolino spironaphthoxazine (NISO) dye with allyl

diglycol carbonate to create the first plastic photochromic lens in 1982 (Crano, Flood, et al. 1996). A generic NISO compound is pictured in Figure 19. The indoline group has been circled in green, the spiro carbon has been circled in black, and the oxazine group has been circled in red.

The structure and stability of photochromic molecules can be used to explain differences in photochromic activity. Computer programs are now employed to choose the best candidates for synthesis and even predict their absorption patterns (Crano and Guglielmetti, Organic Photochromic and Thermochromic Compounds 1999). The

structural formulas of a photochromic spirooxazine (Figure 18 and a spiropyran (Figure 19) are shown below. The two compounds are almost identical. The main difference that the spirooxazine ring (circled

Figure 17 A generic indolino spironaphthoxazine (NISO)

Figure 18 Structural formula of a spirooxazine

Figure 19 Structural formula of a spiropyran


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in green) has a nitrogen para to the oxygen, whereas the spiropyran ring (circled in black) has a CH group para to the oxygen.

That one substitution has drastic effects on the three-dimensional structure of the molecules and their photochromic properties. The Oak Ridge Thermal Ellipsoid Plots (ORTEP) of the spirooxazine (Figure 20) and the spiropyran (Figure 21) obtained from

x-ray crystallography are shown below (Osano, et al. 1991). The naphthalene group in the spirooxazine is angled up toward the geminal methyl groups (C27 and C28), whereas the naphthalene group in the spiropyran is angled down and away from the geminal methyl groups. This affects how the rest of the molecule interacts with the ring-opening O14. The ORTEP models can be used to calculate the non-bonding intramolecular short contacts (steric strain) around the cleaving Cspiro-O bond as shown in Table 1 (Osano, et al. 1991). In general, the short contacts of the spiropyran are

shorter than the spirooxazine. This means that the spiropyran ring is more likely to open because of the strain surrounding the O14. The Cspiro-O bond length is essentially the same in both the spirooxazine and spiropyran, 146.35pm and 146.42nm, respectively (Osano, et al. 1991). There is a major difference in the Cspiro-N bond length, though. The spirooxazine has a Cspiro-N bond length of 142.56pm, while the spiropyran has a longer Cspiro-N

Figure 20 ORTEP structure of a

spirooxazine Figure 21 ORTEP structure of a spiropyran

Table 1 Non-bonding short contact forces give

an indication of steric strain on the cleaving Cspiro-O bond for the (1)spirooxazine and

(3)spiropyran


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bond length of 145.92pm (Osano, et al. 1991). A difference of 3.36pm may not seem like a lot, but it affects the resonance stability of the activated open ring. The ring-opening mechanism for a spirooxazine (Figure 22) results in species (circled in red) that is similar to a quinone or merocyanine. The mechanism for a spiropyran is identical.

Figure 22 shows that the activated structure’s Cspiro-N bond (circled in green) has double bond character. The spiropyran product is less stable because its Cspiro-N bond is longer and less able to assume the double bond resonance structure. Even though the spiropyran has more strain, its product is less stable so it is less likely to form. This helps to explain why the spiropyran only exhibits photochromic behavior at -20ºC

while the spirooxazine is photochromic at room temperature (Osano, et al. 1991). NISO dyes have been chosen for their stability and fast ring-opening kinetics. The mechanism for ring opening in Figure 22 is a bit more complex than displayed. Picosecond time-resolved absorption and emission spectroscopy has been used to detect an intermediate, X, between the closed and open forms (Schneider, et al.

1987). It takes less than 2ps for this intermediate X to form and another 2-12ps for the activated planar structures to form (Schneider, et al. 1987). These picosecond work-up times are shown in Figure 23 for the absorption plots at 570nm and 616nm. For a photochromic dye to be considered for eyeglasses, it must absorb visible light (especially blue light) when activated, activate and thermally bleach quickly, and resist degradation. The opening of the ring, as illustrated in Figure 22, is accompanied by a change in absorption spectra. The activated absorption spectrum of an indolino

Figure 22 The ring-opening mechanism and resonance of a spirooxazine upon UV irradiation

Figure 23 Picosecond absorption spectra of

intermediate X and activated planar products (photomerocyanines) at 570nm

and 616nm


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pyridobenzoxazine with five methyl groups is shown in Figure 24 (Crano, Flood, et al. 1996). NISO compounds, and the closely related indolino pyridobenzoxazines, generally have their maximum absorption between 560 and 630nm (Crano, Flood, et al. 1996). This makes these activated compounds appear blue.

Blue lenses would not absorb the harmful 430nm blue light. This complicates the manufacturing process a little, because a second photochromic compound must be added to absorb in the 450-500nm range to neutralize the activated color. Indolino spirobenz-oxazines can partially neutralize the color of an activated lens because they have absorption maxima around 470nm and 570nm as illustrated in Figure 25 (Crano, Flood, et al. 1996). When mixed with the indolino pyridobenzoxazine shown in Figure 24, partial color neutralization was achieved as shown by the spectrum in Figure 26

(Crano, Flood, et al. 1996).

The partial color neutralization in Figure 26 would not be acceptable for consumer lenses. There is a serious threat if the absorbance near 555nm is higher than the absorbance near 430nm. A high absorbance near 555nm would stimulate the eyes to stop squinting and to partially dilate the pupils since they are no longer being bombarded with the light to which the retina is most sensitive. If the lenses did not

Figure 24 Absorption spectrum of an

indolino pyridobenzoxazine

Figure 25 Absorption spectrum of an

indolino benzoxazine Figure 26 Absorption spectrum of a

mixture of indolino pyridobenzoxazine and indolino benzoxazine


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block light with wavelengths near 430nm as effectively as 555nm, the retina would actually be exposed to more of the damaging light than without the lenses (Young 1988). The use of photochromic diphenyl napthopyrans is a possible solution to this problem. These compounds generally have absorption maxima between 420nm and 480nm (Crano, Flood, et al. 1996). The absorption spectrum of a diphenyl napthopyran is shown in Figure 27 (Crano, Flood, et al. 1996). When combined with the indolino pyridobenzoxazine in Figure 24, a netralized activated spectrum is achieved as shown in Figure 28 (Crano, Flood, et al. 1996).

The absorption spectrum for a Transitions VI brown lens is shown in Figure 29 (Transitions VI Tech Notes-Brown 2009). The bleached lens is virtually clear as shown

by the near 90% transmission across the visible spectrum. The harmful effects of UV radiation are not an issue with this lens because even the bleached lens transmits 0% of the wavelengths below 400nm. The maximum absorbance (minimum transmittance) for the activated lens occurs around 450nm and 570nm. The absorption of light results in the expression of the complementary color. Absorbing 450nm (blue) would make the lens

appear yellow-orange. Absorbing 570nm (yellow) would make the lens appear violet. Together the absorbance pattern yields a brown activated lens. This lens meets the requirements that the absorption near 430nm is at least as strong as the absorption near 555nm.

Figure 27 Absorption spectrum of a diphenyl napthopyran

Figure 28 Absorption spectrum of a mixture of indolino pyridobenzoxazine and diphenyl

napthopyran

Figure 29 Non-activated and activated UV/Vis

Spectra for Transitions VI Brown lenses


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Text Box #2: Look at All the Pretty Colors

Part A : Leaves Why are leaves green? A trip back to art class would be very helpful in answering this question. The colors of the rainbow have a wavelength between 400nm (violet) and 700nm (red) as shown in the electromagnetic spectrum in Figure 30. If this spectrum is wrapped in a circle the result is the color wheel (Figure 31) familiar to art classes. Using the wheel, complementary colors are defined as colors that are across from each other, i.e. red & green, violet & yellow, blue & orange.

When a molecule absorbs a certain wavelength of light, the reflected light appears as the complementary color. It is a little more complicated with plants though, because there are two types of chlorophyll, and chlorophyll absorbs in two regions of the color spectrum. Chlorophyll a absorbs violet and red light (Figure 32) resulting in a green color. Chlorophyll b

absorbs light in the blue and orange regions resulting in a yellow-green color. In the fall, the amount and angle of sunlight starts to decrease. Leaves lose their chlorophyll pigments and are left with carotenoids. These pigments absorb violet, blue, and green light. This explains why leaves turn red, orange, and yellow in the fall.

Figure 30 Electromagnetic Spectrum1 Figure 31 Color wheel2

Figure 32 UV/Vis Spectrum for the chlorophylls and

carotenoids3


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Part B: Color Mixing

Name the three colors primary colors. If you said red, blue, and yellow, you would only be partially correct. It depends if you are talking about art class, color printing, or light emission. A look under the cover of most color printers reveals three color inks: cyan, magenta, and yellow. Color printing works based on the principle of subtractive coloring. As inks are applied to the paper, the complementary colors are absorbed. The paper background is white because none of the colors in the spectrum are absorbed. Magenta and yellow ink must be applied to get red. Magenta absorbs green light and yellow absorbs blue light, so red is reflected. The subtractive color wheel is shown in Figure 33.

The three primary colors for LCDs, computer monitors, and television screens are red, blue, and green (Figure 34). These devises work based on the principle of additive coloring. When no light is emitted, the screen appears black. Mixing red and green light yields yellow light. Mixing red, green, and blue yields white light. Photochromic lenses absorb light (See page 12 for more information). Therefore, the dyes in photochromic lenses follow the same absorption and complementary color rules as the subtractive CMY color system (Figure 33).

1www.dnr.sc.gov/.../pjpb/lecture/lecture.html 2www.designedlykristi.com/.../color_theory2.html 3 http://www.uic.edu/classes/bios/bios100/lecturesf04am/absorption-spectrum.jpg 4 http://dba.med.sc.edu/price/irf/Adobe_tg/models/rgbcmy.html

Figure 33 The subtractive (CMY)

color model4

Figure 34 The additive (RGB) color

models4


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The equilibrium concentrations of activated and bleached photochromic dyes are dependent upon temperature. The mechanism in Figure 22 shows that heat fades the

lenses back to the non-activated, bleached form. The percent transmission spectra of Transitions VI brown lenses at 10ºC, 23ºC, and 35ºC is presented in Figure 35 (Transitions VI Tech Notes-Brown 2009). The thermal equilibrium presents a problem on a hot summer day, because the lenses will not darken as much and achieve their maximum absorbance of visible light. On a bright winter day, the lenses will reach maximum absorbance because there is not

a lot of heat driving the reverse bleaching reaction. This explains why the lenses absorb over 20% more visible light at 10ºC than at 35ºC.

The kinetics of photochromic consumer lenses is probably just as important as their absorption spectra. Lenses that take considerable time to activate once being exposed to UV radiation and to thermally fade once being removed from the UV source would be quite undesirable. Figure 36 (Transitions VI Tech Notes-Brown 2009) shows that the kinetics of activation is much faster than the kinetics of bleaching. The maximum absorbance for the Transitions

VI lenses is 0.82 (15% transmittance). The time (T1/2 activation) it takes to reach half the maximum absorbance of 0.41 (39% transmittance) is less than a minute. The time (T1/2

bleach) it takes to fade to half the maximum absorbance is about 2 minutes. The improved kinetics from Next Generation brown lenses to the Transitions® VI lenses is illustrated in Figure 36. Transitions VI lenses show a greater activation

Figure 35 Temperature dependence of %

transmission for Transitions VI brown lenses

Figure 36 Improved kinetics for Transitions VI

lenses


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absorption (lower % transmission) even though the kinetics are comparable. At 15 minutes the UV irradiation source was removed. The steeper slope for the Transitions VI lenses indicates a faster fade rate. The T1/2 bleach for the older Next Generation lenses is about 30 seconds slower than the Transitions VI lenses. T3/4 bleach is the time it takes to fade by 75% of the maximum absorbance. The T3/4 bleach for the Transitions VI lenses is about 4 minutes, while the T3/4 bleach for the Next Generation lenses is about 90% longer (7.5minutes).

One way to improve photochromic kinetics is to incorporate oligomers into the polymer matrix of the lens. An oligomer is like a polymer except that it has less monomer units. Oligomers protect the spirooxazine from the rigidity of the polymer matrix. Figure 37 (Evans, et al. 2005) shows that NISO compounds are have orthogonal rings before activation. The resonance shown in Figure 22 forces the activated molecule to be planar. Flexibility of the polymer matrix is needed for the orthogonal planes of the NISO

to open and become planar. Usually flexibility and strength are inversely proportional, but because oligomers are not bonded to the rest of the polymer matrix, they make the environment directly around the photochromic molecule flexible while keeping the rigidity of the lens. Figure 37 (Evans, et al. 2005) shows several oligomers that can be incorporated into the NISO.

Figure 37 Oligomer-spirooxazine combinations


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The siloxane monomers in oligomer 1 do not interact with the polar carbonate monomers of polycarbonate. This allows the polymer to keep its rigid properties. The

inset plot of Figure 38 (Evans, et al. 2005) shows the necessity for the oligomer to be chemically bonded to the NISO. When the polydimethylsiloxane oligomer is mixed into the lens in combination with substituent 2 being bonded to NISO (inset plot C), there is no kinetic improvement over solely using substituent 2 bound to NISO (inset plot B). Inset plot A shows improved fade kinetics because oligomer 1 has been chemically bonded to NISO.

It appears that the fade kinetics are faster when the oligomer is attached to the naphthalene ring rather than the indoline ring. Oligomer 1, attached to the naphthalene ring, had a T1/2 of 3 seconds and a T3/4 of 7 seconds. Oligomer 4, attached to the indoline ring, had a T1/2 of 3 seconds but a T3/4 of 12 seconds (Evans, et al. 2005). Oligomers 1, 3, and 6 improved the activation kinetics of their NISO compounds. When activated, these compounds quickly overshot their equilibrium absorbance as shown by plot A in Figure 38. Out of the three though, oligomer 1 experienced the fastest fade kinetics. The fade kinetics for all six oligomers are summarized in Table 2 (Evans, et al. 2005).

Table 2 Fade kinetics for oligomers 1-6

Figure 38 Effects of oligomers on NISO kinetics


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Oligomers have made the ever-elusive, fast solid-phase fade kinetics a reality. The plots in Figure 39 show the difference between the kinetics of oligomer 1 and

substituent 2 in solution and a polymer matrix with a glass transition (Tg) of 120ºC (Evans, et al. 2005). Oligomer 1 and substituent 2 have almost the same T1/2 and T3/4 in solution, but there is a great disparity in the solid phase. Oligomer 1 exhibits near-solution fade kinetics while substituent 2 lags behind. The reversible reactions of photochromic dyes are susceptible to side reactions. These side reactions render the dye inactive just as the side reactions of atRAL prohibit it from recycling back to

11cRAL. When the spirooxazine (SO) ring opens, it forms the merocyanine (MC) compounds as previously illustrated in Figure 22. The merocyanine forms can become activated by the absorption of light. The activated merocyanines can then react with oxygen to form a superoxide intermediate. The light-driven photodegradation mechanism is shown in pathway “a” of Figure 40 (Malatesta, Millini and Montanari 1995).

In the presence of electron acceptors such as Cu2+ and Fe3+, spirooxazines have also been found to degrade nonphotolytically (Malatesta, Millini and Montanari 1995). These electron acceptors may not be intentional components of the lens, but they may be left over from polymer catalysts. The product of the dark degradation reaction, shown in pathway “b” of Figure 40, is a close relative of the photodegradation product.

Figure 39 Addition of oligomer achieves

near-solution kinetics

Figure 40 Light (a) and dark (b) oxidative

degradation of a NISO compound

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Conclusion Increased exposure to blue light has been linked to an increased risk for age-related macular deneration (AMD). The macula is at risk for developing drusen as a result of free-radicals and cytotoxins. These reactive species are prevalent in the eye due to the presence of singlet oxygen, the superoxide ion, and A2E. Adequate levels of antioxidants, such as vitamin C, vitamin E, zeaxanthin, and dietary zinc (Fletcher, et al. 2008) have been shown to reduce the risk for AMD. Reducing ocular exposure to visible light, especially blue light, also reduces the risk for AMD. Photochromic lenses provide the necessary protection from visible light. Indolino naphthospirooxazines (NISO’s) are popular for their absorbance, activity, and resistance to degradation. NISO’s absorb strongly near 600nm when activated. When coupled with a diphenyl naphthopyran absorbance is added near 450nm. This adds blue-light protection and yields a more neutral activated color. Resonance of the Cspiro-N bond makes activated NISO’s stable. The slow thermal fade rates of photochromic lenses have been a downfall in years past. Oligomers may be the solution to near-solution kinetics. An oligomer that is bonded to the orthogonal NISO ring planes provides the flexibility needed for the ring to open and become planar. The oligomer is not bound to the polymer matrix, so the strength of the polymer is not compromised. Future studies will work to increase activation and fade kinetics, protect the photochromic dye from degradation, and incorporate photochromic dyes into more materials.

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