SYNTHESIS AND CHARACTERIZATION OF SOME PYRIDINIUM-N-PHENOXIDE BETAINE DYES FOR 2ND-HARMONIC GENERATION

The synthesis and characterization of some pyridinium-N-phenoxide betaine dyes (including Reichardt's dye) for second-harmonic generation (SHG) is described. These dyes are highly solvatochromic, which indicates that they can have large second-order molecular polarizabilities. The effects of substituents on the solvatochromic and optical properties of the betaines are investigated. Also the crystal structure of the betaines is discussed and the molecular geometry in the crystal is compared to that predicted by AM1 calculations. The crystallographic space group of the betaines is found to be centrosymmetric; therefore these dyes are incapable of SHG as crystals. To overcome this obstacle, we have synthesized and characterized a chiral betaine; because of its chirality this compounds must necessarily crystallize in a noncentrosymmetric space group, thereby making it capable of SHG as a crystal. Lastly, the utility of these betaines for SHG in poled polymer films is discussed.