Preparation and photophysical properties of monomeric liquid-crystalline azo-dyes embedded in bulk and film SiO2-sonogel glasses
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SiO2-based bulk and film sol–gel hybrid materials were prepared with a family of novel liquid crystalline (LC) amphiphilic azo-dyes bearing oligo(ethylene glycol) spacers (named here RED-PEG-n, n = 2, 3, 4, 6). The catalyst-free-sonogel route was implemented to produce optically active hybrid monoliths and spin-coated films with these materials. Comprehensive morphological, thermal, photo-acoustic and spectroscopic sample characterizations were performed in order to elucidate the physical properties of these novel compounds within the sonogel environment. Film samples were also studied via the nonlinear optical (NLO) second harmonic generation (SHG)-Maker fringes technique. Results show that the chromophores were homogeneously embedded within the highly pure SiO2-sonogel network, showing a clear thermotropic mesogenic behavior. The push–pull structure of the implemented azo-dyes allowed effective electrically-induced monomeric alignment within the sonogel confinement; thus, stable quadratic NLO-SHG-activity in the organic–inorganic film samples was achieved despite the lack of glass transition temperature (T g ) of the guest LC-compounds.
KeywordsAzo-dyes Liquid crystals Thin films Sol–gel Sonogel Hybrid materials Optical materials Nonlinear optics
The authors are grateful to Dr. Neil Bruce for English revision of the manuscript. This work was supported by projects SEP-CONACyT (Grant: U-49846-F) and PAPIIT-DGAPA-UNAM (Grant: IN-115508). O G Morales-Saavedra also acknowledges financial support from the DAAD academic organization (Germany). E. Rivera thanks PAPIIT-DGAPA-UNAM (Grant: IN-105610).
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