Structure - Optical Relationships in Photochromic Materials

Abstract

Photochromic materials such as hackmanite have the remarkable ability to change colour under external irradiation and then return to their original colour. This reversible colour change is known as photochromism, or tenebrescence. Over many years, researchers have tried to understand this beautiful colour change and how it can be tuned. One of the most fascinating ideas in this field is that this perfect colour change comes from the imperfections of the material. These imperfections, or defects, work together with the crystal structure, sulfur related electron donors, anion vacancies, and local cage geometry to form the final colour-centre, known as the F-centre. Although the current literature links photochromism in sodalite type materials to these defect related processes, the relationships between structure, optical response, and kinetic behaviour remain difficult to separate experimentally. This thesis investigates these structure-optical-kinetic relationships in a set of photochromic sodalite based materials. A custom measurement setup was designed and optimized for repeated coloration and fading measurements. The setup enabled controlled UV induced coloration and halogen lamp induced fading experiments while collecting time/dose resolved intensity data. From a broader collection of 57 available samples, 18 samples showing measurable coloration and fading behaviour were selected for detailed analysis. Structural descriptors were obtained from X-ray diffraction, including the unit-cell parameter, Na fractional coordinate, and calculated Na–Cl distance, vacancy size width (dvac). Optical parameters included the post-UV reflectance valley position, coloration threshold, and thermal bleaching energy. Kinetic parameters were extracted by fitting coloration and fading curves using a two component ExpDec2 model. The results show that dvac is strongly related to the post-UV reflectance valley, supporting the F-centre confinement, or “box”, model discussed in the literature. The structure–energy comparison further showed that optical transition energy decreases as structural length scale increases, consistent with longer-wavelength colour center absorption in larger confinement environments. Kinetic analysis showed that fading is more closely associated with the stability of the formed F-centre population, whereas coloration depends on additional photoactivation factors. Multivariate analysis, including principal component analysis and hierarchical clustering, together with leave-one-out cross validation screening, supported the same interpretation. Overall, this thesis supports a coherent structure–optical–kinetic framework in which vacancy size influences the F-centre optical state, which is then linked to fading stability. The developed workflow provides a good foundation for future photochromic material design.