The enhancement effect of hydrogen bond on the third-order nonlinear optical properties

Three conjugated molecules', 4-phenylethynyl-4′-[N,N-diethylamino]azobenzene (3a), 4-phenylethynyl-4′-{[N-ethyl-N-(2-hydroxyethyl)]amino}azobenzene (3b), and 4-phenylethynyl-4′-[N,N-di(2-hydroxyethyl) amino]azobenzene (3c), nonlinear optical properties were determined using Z-scan technique with 532 nm, 8 ns laser pulses. The results indicate that these compounds possess large nonlinear optical absorptions, which are attributed to the long π-conjugation structure owing to the substituent of phenylethynyl group. The nonlinearity increases with an increase in the number of hydroxyl group, due to the regular arrangement induced by the strong hydrogen bond interaction between hydroxyl groups of molecules. Their nonlinearity enhancement mechanism was studied by FT-IR spectroscopy.