NEUTRON REFLECTION FROM 2,4-BIS(4-(N-METHYL-N-OCTYLAMINO)PHENYL)SQUARAINE AT THE AIR-WATER-INTERFACE AND THE LINEAR AND NONLINEAR-OPTICAL PROPERTIES OF ITS LANGMUIR-BLODGETT AND SPUN-COATED FILMS

Neutron specular reflection has been used to probe the structure of mo nolayer films of an anilino squaraine, 2,4-bis(4-(N-methyl-N-octylamin o)phenyl) at the air-water interface. The thickness and area per molec ule indicate that the chromophore aligns with its long axis parallel t o the water surface in the low-pressure regime (l = 15.2 +/- 0.5 Angst rom and A = 87 +/- 4 Angstrom(2) molecule(-1) at pi = 7 mN m(-1)) but tilts upward when the layer is compressed (l = 17.5 +/- 0.5 Angstrom a nd A = 72 +/- 3 Angstrom(2) molecule(-1) at 18 mN m(-1)). The absorpti on spectra of the Langmuir-Blodgett (LB) films are dependent upon the deposition pressure and indicate different aggregation phenomena: purp le (lambda(max) = 545-555 nm and 660-670 nm) and blue-green (lambda(ma x) = 660-690 nm) phases exhibit strong second-harmonic generation (SHG ) with effective second-order susceptibilities of 20-40 and 30-60 pm V -1 respectively at 1.064 mu m. Spun-coated films are also SHG-active, and this is attributed to a partially aligned mosaic of acentric aggre gates in films of the pure dye and a distribution of aggregates when d ispersed in poly(vinyl acetate). Electrospray ionization mass spectrom etry has provided evidence of aggregation in solution with masses whic h correspond to the monomer (517, [M + H](+)) and dimer (1033, [2M + H ](+)). The aggregate, rather than the centric monomer, is responsible for the anomalous second-order nonlinear optical properties.