Photoresponsive hybrid materials: Synthesis and characterization of coumarin-dimer-bridged polysilsesquioxanes

A chemically stable photodimer of 7-allyloxycoumarin (1) has been identified. An efficient synthesis of the syn-ht photodimer (2) was developed, and the compound was subsequently elaborated to the bis(triethoxysilyl) derivative (7). Sol-gel polymerization produced a robust hybrid glass (7X) that could be fabricated as monoliths or cast as transparent colorless thin films. Photopatterning of thin films with UV light (254 nm) produces fluorescent and refractive index patterns which are revealed by fluorescence microscopy (λ_ex = 330 nm, λ_em = 392 nm) and imaging ellipsometry. At short irradiation times the photoinduced cleavage produces a slight collapse of the silsesquioxane network (∼3%) that is attributed to rupture of the coumarin photodimer cross-links. The topological patterns are also observed by imaging ellipsometry. This material provides a new robust matrix for producing "hidden" fluorescent images, refractive index gratings, and topographical features in dense, hybrid glassy materials. Prolonged irradiation at short wavelengths results in a hard → soft transformation that deforms and "melts" the hard brittle thermoset.