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 silyl ated coumarin dye films have been determined and related to the sol-ge l processing conditions and host composition. FTIR spectroscopy was us ed 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 an d the silylated dye was accomplished by prehydrolysis of the silylated dye. The photostability of the dyes in the xerogels was measured by m onitoring the drop in fluorescence intensity upon pumping with a N-2 l aser and also by monitoring the drop in dye absorption upon irradiatin g the films with a UV lamp. At optimized prehydrolysis times, a silyla ted coumarin dye (derCoum) within a SiO2 xerogel host demonstrated a 3 -fold improvement in long-term photostability compared to its unsilyla ted counterpart. The use of a silylated dye results in covalently bond ing the dye to the host matrix, increasing the probability that the dy e will be caged and inhibiting dynamic processes which can lead to pho todegradation. CP-MAS Si-29 NMR data for a derCoum xerogel confirmed t hat a large degree of dye bonding occurred. The addition of PDMS to th e basic SiO2 xerogel host composition had little effect on the photost ability of the dye. The neat dye films showed improved fluorescence ph otostability with increase in hydrolysis time. These films showed poor photostability upon exposure to UV lamp degradation due to lower ther mal degradation resistance and/or thermal conductivity compared to the SiO2 xerogel films.