May 2017

Photochromic dyes for plastic lenses: Part 2

David Cash, Toronto ON

In part 1 published in the April 2017 issue, the molecular structures of the photochromic dyes used in eyeglass lenses were discussed. In part 2, the engineering and chemical design of the lenses will be highlighted. The student questions will focus on kinetics and experimental design.

As noted in part 1, reference 1 is an excellent, brief introduction to the chemistry and application of photochromic dyes up to 1996. The authors describe the fade time as an important measurement used to design the lenses. The fade time is measured with a tungsten-lamp visible-region scanning spectrophotometer with the addition of a xenon-lamp beam passing UVA radiation through the temperature controlled sample compartment.¹ Each sample solution was irradiated with UVA light to produce the maximum possible absorbance at λ_max and then scanned to produce a complete visible spectrum from 400 to 700 nm.

The methodology used allowed the measurement of the fade half-life (t_½) of a dye by turning off the xenon lamp and timing the fall from an initial absorbance value to one-half that absorbance value (Question 2). The half-life times for the dyes produced from structures A and C (Fig. 1) were measured as, respectively, 45 s and >1800 s at 24 °C.¹ In order to produce a successful commercial lens, the combination of dyes used must be matched not only for complementary colours absorbed, but also for visible absorptivity, photochemical absorptivity and fade kinetics. The dyes must also be photochemically and thermally stable and resistant to oxidative degradation.¹

Lens manufacturers continually strive to improve the properties of their products. The Transitions® lenses now on sale are the seventh generation to have been commercialized. It is difficult to specify how much light a photochromic lens blocks, referred to as the percent tint, since this varies with the light conditions and the temperature of the lens. According to information on the Transitions trade web pages,^2a in typical daylight conditions a gray lens blocks about 75% of light at
77 °F (25 °C), which is a 15% increase over the previous sixth-generation lens. Also, it darkens more rapidly in daylight, clears more rapidly out of sunlight, and has less difference between low and high temperature darkening than the previous generation (Question 3). The plot (next page) was generated using data from Reference 2a.

Tint layer thickness and concentration. For either a glass lens or a plastic lens, the tint layer must be of uniform thickness across the entire lens to ensure that the darkening is uniform despite varying lens thickness. For most polymer lens types, the tint layer is a separate layer or layers, bonded to the outer surface of the lens. The concentration of the photochromic dyes and the thickness of the absorbing layer will be determined in accordance with Beer’s law with regard to the desired level of maximum visible light absorbance.

three dye molecules structure

Fig. 1 Molecular structures of two pyran family photochromic dyes (A and C). The rearrangement product of A after UVA absorption is structure B.

Tint layer material. Photochromic substances such as those described here can only be active in solution or in a medium that allows rapid, free molecular motion. Otherwise, the molecules cannot change shape as required. The tint layers of Transitions^2b lenses are composed of polyurethane.³ These layers are coated onto the surface of the polyallyl diglycol carbonate lens material. This latter material, known as CR-39®,⁴ was

first produced in 1940 for military purposes. Although the reference sources do not use the term, the photochromic layer material must be above its glass transition⁴ temperature during use. This is a factor to be considered when engineering lenses for cold weather uses such as snow sports and mountaineering.

Photochromic driving glasses?

Photochromic lenses that are activated by UVA light are not useful for driving. Modern windshield safety glass is a laminate containing a layer of polyvinyl butyral⁴ polymer that holds the glass fragments of the windshield in place if it shatters. The polymer is formulated with UV protectants that absorb the UVA light that would activate the photochromic dyes in eyeglass lenses.³ Specialty photochromic lenses are available for driving glasses in which the dyes are activated by blue light.^2b These lenses will not be clear even in low light outdoors or indoors, so they would not be suitable for everyday use. A good alternative for driving in bright sunlight is a lens such as Nupolar Brown-3 polarized prescription sunglass lenses⁵, which transmit a constant 19% of incident visible light that is 99.9% polarized (data from the Nupolar website in 2005 — current website FAQ data is less specific).

Discussion

In Part 1, it was noted that photochromic lenses become considerably darker in winter than in summer and take much longer to return to colourless indoors when they are colder. Because they become so dark, they are not suitable for some outdoor uses in very cold conditions, such as skiing or using a snow blower. These observations can now be seen to result from the chemical properties of the dyes:

Lenses are darker at lower temperatures. Consider the reversible reaction of structure A transforming to structure B and vice-versa. The forward process is dependant only on the amount of UVA light falling on the lens, and is not temperature dependant. The reverse process is thermal, follows first-order kinetics, has constant activation energy, and becomes slower as temperature decreases. Since the percent tint of the lens depends on the equilibrium between the forward and reverse processes, then under the same light conditions the lens is darker at lower temperature. This is confirmed by the data in Reference 2a that is displayed in the graphic (Fig. 2). This analysis applies to the interconversion of any similarly related chemical structures.

Fade half-life time is longer when the lens is colder. As noted above, the fade reaction is thermal, follows first-order kinetics, has constant activation energy and becomes slower as temperature decreases.

Questions for students

Given a photochromic plastic lens for your experiments, describe briefly how you could experimentally determine:
1. How the maximum darkening of the lens varies with temperature?
2. How the fade half-life time of the lens varies with the lens temperature?

Assuming that the fading of a photochemical dye follows first-order chemical kinetics:⁶
1. Calculate the first-order rate constant for the dye produced from structure A given that the fade half-life time (t_½)is 45 s at 24 °C. The relevant equation is kt_½ = ln 2.
2. If the fade half-life time is 90 s at 14 °C, calculate the activation energy of the reaction.

(Reference 6, Equation 6. Answer: 49.1 kJ/mol)

How would each of the following changes in dye property or lens manufacture affect lens performance with respect to: darkening time; fade half-life time; maximum tint; temperature variability? Explain each answer briefly:
1. Increased absorptivity of UVA by the parent dyes.
2. Increased absorptivity of visible light by the rearranged dyes.
3. Increased tint layer thickness.
4. Increased tint layer concentrations.
5. Decreased activation energies of dye fading.

graph