Functionalized siloxane-based polymers and network materials for second-order nonlinear optics

We have developed a new chemical methodology, based on simple acid-base hydrolysis of aminosilanes with molecules containing terminal OH groups, to prepare robust siloxane based polymers and organic-inorganic hybrid network materials for second-order nonlinear optics. First, a variety of polymers containing NLO-active chromophores covalently bound to the siloxane backbones, [-R₂Si-(O-SiR₂)_n-O-(NLO-Chromophore)-O-] _n (R = CH₃ or CH₃/C₆H₄) and [-R₂Si-(O-SiR₂)_n-O-R′-O-(NLO-Chromophore)-O-] _n. (R′ = -C₆H₄-C ₆H₄-C₆H₄-), were prepared. Their solubility in common organic solvents, and high thermal stability, imparted ease of thin film preparation, and subsequent poling at high temperatures. These polymers exhibit good second-harmonic generation susceptibilities, and the temporal stabilities of the SHG signals were dependent on the polymer backbone and the molecular structures of the NLO chromophoms. A detailed analysis of their physical properties is reported. Then, a methodology of acid-base hydrolysis was used to produce copolymers containing dimethylsiloxane, imide linkages and NLO-active chromophores. These copolymers were soluble in polar solvents, and possessed high thermal stabilities and glass transition temperatures. Easily fabricated and poled thin films of these polymers exhibited good second-order nonlinear optical susceptibilities with long-term temporal stabilities of the second-harmonic generation signals at room temperature. Finally, an alternative approach, based on the same acid-base hydrolysis technique, to prepare organic-inorganic hybrid network material, was developed. In these hybrid materials, NLO-active chromophores such as Disperse Red 19, a pyridinium. salt based dye, and 1-amino-4-nitrobenzene, were covalently locked into silica networks. The hybrids were soluble, and offered ease of processibility in the preparation of good optical quality thin films. The network materials that are akin to the traditional sol-gel approach were also prepared from the monofunctional NLO chromophore DRI and as expected, were found to be insoluble in common organic solvents. Physical properties of NLO-chromophore incorporated hybrids were tailored by changing the amount of chromophores and water used in hydrolysis, and the type of chromophores. High thermal stability and glass transition temperatures of these hybrids provided effective electric-field poling at high temperatures, and long-term temporal stabilities of the second-harmonic generation signals at room temperature and 80°C.