Neutron Reflection from 2,4-Bis(4-(<i>N</i>-methyl-<i>N</i>-octylamino)phenyl)squaraine at the Air−Water Interface and the Linear and Nonlinear Optical Properties of Its Langmuir−Blodgett and Spun-Coated Films

Abstract

Neutron specular reflection has been used to probe the structure of monolayer films of an anilino 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-pressure regime (l = 15.2 ± 0.5 Å and A = 87 ± 4 Å2 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 Å2 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 (λmax = 545−555 nm and 660−670 nm) and blue-green (λmax = 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.