Second-harmonic generation studies of Langmuir-Blodgett monolayers of indodicarbocyanine and hemicyanine dyes and stilbazium salts

Second harmonic generation and UV-visible spectroscopic measurements have been performed on a series of Z-type Langmuir-Blodgett monolayer of indodicarbocyanine, hemicyanine and stilbazium salts. Optical spectra of these monolayer indicate the presence of hypsochromically shifted aggregates on the silica surface. The extent of aggregation is found to be largely dependent on the nature of the counter-ion as well as on the conditions of the monolayer preparation. The SHG intensity of dye aggregates are much smaller compared to that of monomeric dye molecules. In the case of amphiphillic counter-ion containing stilbazium salt compression speed of the monolayer has very little effect on the second harmonic signal and on the absorption spectrum, presumably due to the suppression of aggregation. It seems that the contribution from nonlinear optically active amphiphillic counter-ion to the SHG intensity is not that significant.