How do Reversacol™ Photochromic Dyes Work?
Under the influence of UV light, a photochromic molecule will change shape, opening up from a twisted figure-8 type structure into a brightly coloured planar form; the open form is a very effective absorber of visible light. This colour change is a reversible equilibrium, when the source of radiation is removed the molecule will revert back to its unactivated or ‘resting’ state.
The rate at which the dyes fade back to their colourless form is dependent on ambient temperature and the chemical structure of the dye.
Spiroxazines & Naphthopyrans
Vivimed Reversacol™ dyes are generally based on two major families of photochromic molecules; Spiroxazines & Naphthopyrans, which allow for a wide variety of innovative derivatives to be prepared, leading to a myriad of dye colours, from vivid purples and oranges for plastics through to neutrally toned greys and browns for ophthalmic lens applications. Reversacol dyes represent the widest palette of commercial dye colours currently available in the market.
Incorporation of different functional groups onto the backbone of the molecules allow different characteristics of activation and fade speeds (we refer to this as dye kinetics) which can allow dyes to be ‘tuned’ for specific performance applications.
Because the photochromic effect relies on a physical twisting of the molecule’s ring structures, dyes only become fully coloured if incorporated into a suitably flexible matrix. The dyes will not exhibit any photochromic effect when in their normal, crystalline powder state as they will be unable to move.
Light Absorption – Click here to see table
In this absorbance graph for Reversacol Flame, it can be seen that the unactivated absorption profile (yellow line) is quite flat and low, which equates to its unactivated resting state. The high absorption at 380nm in the UV will ensure strong activation of the dye. Once activated, the red line, maximum absorption occurs at around 475nm in the blue region of the spectrum. Removal of the blue fraction of the spectrum by the dye results in the molecule taking on an overall red-orange colour.
Neutral Grey – Click here to see table
This absorption profile of Reversacol Graphite shows a very different response. The activated form, the upper grey line, shows absorption spread across the spectrum from 430 to 680nm. All visible wavelengths are extracted by the dye’s absorption, hence the molecule has no particular colour tone, and appears neutral grey when activated. The company was awarded the UK’s coveted Queen’s Award for invention of Reversacol Graphite and other similar molecules exhibiting this unique effect.
Some photochromic dyes will not change colour behind automotive windows because the glass absorbs the UV light necessary to produce a colour change. However, some Reversacol dyes, such as Palatinate Purple, Sea Green, Plum and Aqua Green, can be activated by light in the visible light spectrum alone, i.e. around 420nm, allowing the coloured state to develop behind glass or with interior artificial lighting. Of course this can also lead to some ‘residual colouration’, typically a light pastel shade in the resting state. Other dyes such as Reversacol Rush Yellow, and Oxford Blue will require higher energy, lower wavelength UV light 360 – 380nm to activate fully and will not convert to their coloured form in the presence of visible or artificial light alone.