For the purpose of describing the mechanism over the course of the colour change, the Reversacol photochromic molecule has been represented simplistically as two rings.

Upon the action of UV light or direct sunlight, the structure twists from a perpendicular (closed) form to a flat, planar (opened) structure. This allows the two halves to interact, resulting in the absorption of visible light.

There are effectively two changes occurring simultaneously; a chemical change arises when the molecule is exposed to UV light, that enables conjugation to take place throughout the molecule; a structural change also occurs to enable the overlap of ∏-orbitals. Therefore, spatially, the molecules must be able to flatten out to allow this conjugation to take place.

It is a fully reversible reaction so that when the light source is removed, the molecule returns to its uncoloured state. Heat can also help drive the reaction back to the uncoloured form, so in very hot conditions, there is always competition between light and heat to determine the given colour observed. In general, a colour change is still observed, albeit weaker than at room temperature. Similarly, in cold conditions in the presence of sunlight, an intense colour is observed as there is little or no competition from the back reaction.

Kinetics

This is a cyclic reaction and the number of cycles (or the activation and fade rates) varies greatly by product. The activation times are generally much shorter than fade times. On average, fade times are two or three times longer than activation times. The Reversacol product range offers a very large variation in kinetics characteristics. Some fade in several seconds, whilst others can take several minutes. If you require specific fade rates that are not obtained within our current range, it is likely that we have your requirements amongst our other 1000 products in our database. To contact us to discuss your requirements further, click HERE.

Systems

As well as the nature of the product influencing the colour and kinetics result, the system or matrix used with the dye has a strong influence on such properties. For example, in some systems, a colour shift of up to 20nm has been observed.