Photostability of laser dyes in sol-gel-derived hosts

Improving the photostability of laser dyes within sol-gel derived hosts has been the focus of this study. To accomplish this, synthetic routes were established to incorporate laser dyes within various sol-gel matrices, the mechanisms of dye photodegradation were determined, and molecular engineering techniques were employed to improve the photostability of the dye doped sol-gel hosts. Various Coumarin (silylated and unsilylated) and Pyrromethene laser dyes were incorporated within sol-gel derived hosts ranging from SiO₂ xerogel films to SiO₂:PDMS Polyceram monoliths which were optically transparent, crack-free, and polishable. Processing parameters, such as the water content and the pre-hydrolysis of the silylated Coumarin dyes, greatly affected the degree of dye bonding. The chemical stability of the Pyrromethene laser dye was also greatly affected by processing parameters, such as the acid/base content. Both the Coumarin and Pyrromethene dyes were found to degrade by photo-oxidation processes. Therefore, it was expected that the photostability would improve by incorporating the dye molecules within molecular cages within the solid hosts, thereby preventing interactions of the dye with photo-reactive impurities such as oxygen. The photostability was found to improve using the following molecular engineering methods: (1) by covalently bonding the dye to the host matrix, where the photostability improvement was attributed to the greater probability of obtaining dye caging with the silylated dye; (2) by removing porosity within the host through control of sol-gel processing and composition, where the photostability improvement was attributed to the elimination of highly photo-reactive dyes located in the pores of the host; (3) by incorporating additives such as bases and hindered amine antioxidants which slowed the steps of the photodegradation process. The fluorescence photostability (fluorescence output intensity as a function of pump pulses) of the dye doped films and monoliths showed a characteristic behavior in the fluorescence output, signified by a rapid initial decay (attributed to dyes located within pores of the matrix) and then a slower long-term decay (attributed to photostable dye molecules located within SiO₂ cages). A model, which applied a Gaussian distribution of the photostabilities of the dye molecules, quantitatively described the observed photostability behavior of the dye doped samples