Silylated Coumarin Dyes in Sol-Gel Hosts. 2. Photostability and Sol-Gel Processing

The photostability characteristics of numerous silylated coumarin dyes within SiO₂ xerogels and SiO₂:PDMS Polyceram films, and of neat silylated coumarin dye films have been determined and related to the sol-gel processing conditions and host composition. FTIR spectroscopy was used to monitor the hydrolysis reaction rates of the silylated dyes and other Si-alkoxide precursors. The silylated dyes had varying reaction rates depending on the degree of functionality and the linkage between the alkoxide and the dye. Matching the reaction rates between TMOS and the silylated dye was accomplished by prehydrolysis of the silylated dye. The photostability of the dyes in the xerogels was measured by monitoring the drop in fluorescence intensity upon pumping with a N₂ laser and also by monitoring the drop in dye absorption upon irradiating the films with a UV lamp. At optimized prehydrolysis times, a silylated coumarin dye (derCoum) within a SiO₂ xerogel host demonstrated a 3-fold improvement in long-term photostability compared to its unsilylated counterpart. The use of a silylated dye results in covalently bonding the dye to the host matrix, increasing the probability that the dye will be caged and inhibiting dynamic processes which can lead to photodegradation. CP-MAS ²⁹Si NMR data for a derCoum xerogel confirmed that a large degree of dye bonding occurred. The addition of PDMS to the basic SiO₂ xerogel host composition had little effect on the photostability of the dye. The neat dye films showed improved fluorescence photostability with increase in hydrolysis time. These films showed poor photostability upon exposure to UV lamp degradation due to lower thermal degradation resistance and/or thermal conductivity compared to the SiO₂ xerogel films.