Molecular orientation of oxacyanine dye in an LB film determined by second harmonic generation and polarized absorption techniques

The molecular orientation of an oxacyanine dye (DOOC) in an LB film has been determined based on the second harmonic generation (SHG) using a fs-pulsed Ti-sapphire laser and the polarization absorption spectrometry. Both polarization and wavelength dependence of SHG varied with surface concentration of the dye. The dye was either in a monomer, a dimer or a J-aggregate form depending on the surface concentration. The tilt angle to the surface normal was 34° for the monomer, 26–29° for the dimer and 16–23° for the J-aggregate. The J-aggregate stayed more flat on surface and showed larger SHG intensity than the monomer did.