Structural and nonlinear optical properties of Crystal-Violet octupolar dyes dispersed in bulk SiO2-sonogel optical-glasses

The catalyst-free (CF) sonogel (SG) route has been implemented to fabricate highly pure porous SiO2 glasses as host materials for Crystal-Violet (CV) octupolar chromophores. Developed CV/SG-based organic–inorganic bulk hybrid composites exhibited rigid monolithic structure with controllable geometrical shapes and dopant concentrations, allowing the control of the optical properties in the solid-state. Comprehensive photophysical and structural sample characterizations were performed according to absorption and photoluminescent spectroscopies; the linear refractive indices of several highly and slightly CV-doped samples were also estimated according to the Brewster angle technique. Morphology, homogeneity and structural features of the hybrid composites were also evaluated via reflection microscopy, AFM and XRD-measurements. Results show that the CV-chromophores were homogeneously embedded within the SiO2-sonogel matrix with only small guest–host molecular interactions. SG-glasses are amorphous in nature but the developed hybrids exhibited ordered crystalline structures at high CV-dopant concentrations leading to stable quadratic nonlinear optical (NLO) activity as confirmed via transmission second harmonic generation (SHG) measurements. It is concluded that the relatively strong macroscopic SHG-signals (in the order of 10−11 esu) observed in massive bulk SG-samples arise from both the multipolar CV-nature and convenient supramolecular self-assembling organization of the guest CV-chromophores within the sonogel environment.