Centrosymmetric Chromophores: Second-Harmonic Generation from Langmuir−Blodgett and Spun-Coated Films of Hydroxy-Substituted Squaraines

The linear and nonlinear optical properties of 2,4-bis[4-( N -methyl- N -alkylamino)-2,6-dihydroxyphenyl]squaraine ( I ), 2,4-bis[4-( N, N -dialkylamino)-2-hydroxyphenyl]squaraine ( II ), and 2,4-bis(8-hydroxy-9-julolidinyl)squaraine ( III ) are reported. Electrospray ionization mass spectrometry (ESI-MS) has provided evidence of association in dilute solution (ca. 2 × 10 - 5 mol dm - 3 ) with m / z values which may be assigned to dimeric aggregates for dye II . Solid solutions of this dye in poly(vinyl acetate) show second-harmonic generation (SHG), albeit very weak, and therefore indicate that the dimeric aggregate is noncentrosymmetric. In contrast, dyes I and III form heptameric aggregates with their solid solutions being SHG-inactive. There is some correlation between the type of aggregate from ESI-MS and the Langmuir film spectra, but van der Waals interactions between the hydrophobic groups play a more significant role in determining the structure at the air−water interface. The absorption maxima of the Langmuir and deposited Langmuir−Blodgett films vary from 530 nm to the near-infrared, but only those films with a maximum at 650−700 nm, or a significant shoulder in this range, are SHG-active. The second-order behavior is not an inherent property of the molecule itself but, instead, is a result of intermolecular effects and is dependent upon the noncentrosymmetry of the aggregate and the film.