CENTROSYMMETRIC CHROMOPHORES - 2ND-HARMONIC GENERATION FROM LANGMUIR-BLODGETT AND SPUN-COATED FILMS OF HYDROXY-SUBSTITUTED SQUARAINES

The linear and nonlinear optical properties of methyl-N-alkylamino)-2, 6-dihydroxyphenyl]squaraine (I), bis[4-(N,N-dialkylamino)-2-hydroxyphe nyl]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 x 10(-5) mo l dm(-3)) with mit values which may be assigned to dimeric aggregates for dye II. Solid solutions of this dye in poly(vinyl acetate) show se cond-harmonic generation (SHG), albeit very weak, and therefore indica te 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 interact ions between the hydrophobic groups play a more significant role in de termining the structure at the air-water interface. The absorption max ima of the Langmuir and deposited Langmuir-Blodgett films vary from 53 0 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 s econd-order behavior is not an inherent property of the molecule itsel f but, instead, is a result of intermolecular effects and is dependent upon the noncentrosymmetry of the aggregate and the film.