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 monolayer films of an anilmo squaraine, 2,4-bis(4-(N-methyl-N-octylamino)phenyl)squaraine, at the air-water interface. The thickness and area per molecule indicate that the chromophore aligns with its long axis parallel to the water surface in the low-oressure regime (l = 15.2 ± 0.5 Å and A = 87 ± 4 Ų molecule^-1 at π = 7 mN m^-1) but tilts upward when the layer is compressed (l = 17.5 ± 0.5 Å and A = 72 ± 3 Ų molecule^-1 at 18 mN m^-1). The absorption spectra of the Langmuir-Blodgett (LB) films are dependent upon the deposition pressure and indicate different aggregation phenomena: purple (λ_max = 545-555 nm and 660-670 nm) and blue-green (λ_max = 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 μm.. Spun-coated films are also SHG-active, and this is attributed to a partially aligned mosaic of acentric aggregates in films of the pure dye and a distribution of aggregates when dispersed in poly(vinyl acetate). Electrospray ionization mass spectrometry has provided evidence of aggregation in solution with masses which 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.