A chemical approach to control the refractive index of sol–gel matrices for liquid-crystal dispersion devices

Devices based on gel-glass-dispersed liquid crystals (GDLC) were synthesized and assembled comprising 4-n-pentyl-4′-cyanobiphenyl (5CB) and hybrid organosilica–titania matrices obtained with sol–gel processes. The refractive index of the host matrices was modified by tailoring the chemical composition of the sol with appropriate mixtures of silicon and titanium alkoxide precursors. The chemical reactivity of the titanium–alkoxide precursor (titanium tetraisopropoxide) toward hydrolysis was controlled with a chelating agent (ethyl acetylacetonate) and a non-hydrolytic sol–gel process. Measurements of transmitted visible spectra of thin films deposited on glass substrates with spin coating yielded data on the refractive indices of the synthesized matrices. The operation of the GDLC devices was tested with a laser optical system; hybrid matrices having a refractive index similar to the ordinary refractive index of the LC produced the best results: up to 74 % transmittance applying an alternating voltage 30 V.