Photochromism and molecular motion

A number of molecules have the ability to interact with light, changing from colourless to coloured or dark when irradiated with light. These changes are reversible and form the basis of darkening sunglasses, familiar to most people. 

However, when the dye is dispersed in a polymer matrix, the decay of the colour follows more complex kinetics. Initially, the decay appears to be first order and is essentially the same as that observed in the liquid. This is because some of the molecules are located in parts of the matrix where there is sufficient free volume for the rotation process to occur, which is why first-order kinetics, as in a liquid, are observed. After a very short period of time, the decay slows down and becomes non-exponential. This slower decay is associated with molecules which are constrained in the matrix and do not have sufficient free volume to move. So at this stage the colour change depends on the rate at which the molecule can gain enough volume to move. In other words, the decay process depends on the rate at which the matrix can create the necessary free volume. This creation of free volume is essentially the same process as that involved in the glass transition temperature discussed in Chapter 4. 

Increasing the temperature will increase the number of holes with a large volume. So the decay becomes faster as the molecules in the excited state, B, convert back to the A form more easily. This phenomenon illustrates the dynamic nature of free volume in a solid polymer holes will continually grow and disappear as the polymer chains move. Thus the critical parameter for the matrix controlling the rate of decay of the colour is the glass transition temperature. 

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