Hydrogen Bond Effect in Nonlinear Optical Properties of Azobenzene Derivatives

Three kinds of 4-hydroxyl azobenzene compouds with different substituents varied as -NO₂, -H, -OCH₃ in the para-position were synthesized and characterized. Their nonlinear optical properties in tetrahydrofuran (THF) and chloroform (CHCl₃) solution were determined using Z-scan technique. The results revealed that the nonlinear absorption (β) and nonlinear refraction (γ) values of three azobenzene compounds in THF solution were larger than those in CHCl₃ solution. It was mainly due to the regular arrangement and effective π-conjugation of azobenzene molecules caused by the formation of hydrogen bonds between the hydroxyl groups of azobenzene molecules and the oxygen atom of the THF molecules. Among three kinds of azobenzene compounds, the 4-nitro-4'-hydroxyazobenzene (NAzoOH) had the largest coefficients of βand γ values in both THF and CHCl₃ solution. It was mainly because that a push-pull (D-π-A) electron system was formed by the electron withdrawing nitro-substituent with electron donating hydroxyl-group in both extremities of azobenzene, resulting in a higher electron delocalization.