Photochromic dyes for plastic lenses: Part 1

Introduction

The Merriam-Webster dictionary defines photochromic¹ as: “capable of changing color on exposure to radiant energy (as light), e.g., photochromic glass”. Photochromic eyeglass lenses were introduced in 1966, and photochromic plastic lenses in 1982.² In the US, plastic lenses were about 5% of the market in 1971, and 85% by 1994. Glass lenses are scratch resistant but much heavier than polymer lenses and prone to shatter, and have been largely succeeded by polymer lenses.

Everyday users of photochromic plastic lenses will have noticed that the lenses become considerably darker in winter than in summer and take much longer to return to clear 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. What chemistry allows plastic eyeglass lenses to reversibly darken outdoors in high-light conditions and clear outdoors in low-light conditions, or indoors? Does the chemistry account for the observations of the temperature variability of the lens properties?

In this two-part series I hope to answers these questions and relate the topic to the high school material with student questions. In Part 1, the structures of the molecules will be discussed. Although these structures are beyond the scope of high school chemistry, students will be asked to look for the change of shape of the molecules. There is a change of absorption in each case when a tetrahedral carbon atom with four single 2-electron bonds changes to a trigonal planar carbon atom with two single bonds and one double bond, resulting in both a shape change and a colour change. Part 2 will deal with the design of the lenses and therefore is an opportunity to discuss first-order kinetics and activation energy.

Photochromic glass lenses

Glass photochromic lenses are based on the silver chloride chemistry of classical photography. Particles of silver chloride are dispersed in the glass. Near UV light (320 – 400 nm wavelength) causes electrons from the glass to reduce silver ions to silver metal particles, which absorb or scatter visible light, darkening the lens.^3,4 The reaction is reversible, and is extremely resistant to degradation over many cycles.

Photochromic polymer lenses

Photochromic polymer lenses are made possible by applying organic photochemistry.⁵ A photochromic dye that is useful for eyeglass lenses does not absorb in the visible spectrum⁵ (390 - 700 nm) but does absorb in the near ultraviolet (UVA).⁵ Absorption of a UVA photon by the non-coloured form of the dye produces a photochemical reaction, converting the molecule to an isomeric structure that absorbs in the visible region, causing the lens to darken and become tinted with colour. To be useful, the absorbing species must revert to the original structure either by a photochemical or thermal rearrangement, and both forms must be resistant to both photochemical and oxidative degradation over many cycles of use.

The first dyes used in commercial polymer lenses in 1982 were the family of indolino⁵ spironaphthoxazines⁵ (structure A). “Oxazines are heterocyclic compounds containing one oxygen and one nitrogen atom in a doubly unsaturated six-membered ring.”⁵ Spiro compounds contain two rings linked by a single common atom.⁵Spiro-oxazine dye compounds absorb in the UVA region and rearrange photochemically. The oxazine ring opens, allowing the molecule to form a single, large, planar π-bonded system (structure B). The larger a delocalized molecular orbital system, the more energy levels that are available to the electrons and the lower the energy of the transition to the lowest excited state from the ground state. The large delocalized system of B absorbs in the visible red region; the absorption band is fairly narrow and a lens produced using this dye has a pronounced blue colour tint.

Two other families of indolino spiro-oxazines can also be used as photochromic dyes: indolino spirobenzoxazines (structure C) and indolino spiropyridobenzoxazines (structure D). However, it is difficult to obtain a neutral tint using only members of any of these three families of dyes, so a further family of photochromic dyes was developed, the diaryl naphthopyrans⁵ (structure E). Like the spiro-oxazines, the pyran families of photochromic dyes isomerize photochemically. The pyran ring opens, allowing the molecule to form a single, large, planar π-bonded system (structure F). But this family of dyes absorbs in the blue region of the visible. Combining dyes that absorb in the red and the blue regions after   photochemical isomerization produces a lens with a more acceptable neutral tint when darkened. More recently, the diaryl indenonaphthopyran⁵ family of dyes (structure H) have been developed, which are also blue region absorbers but more resistant to oxidative degradation.