Dimerization of merocyanine dyes. Structural and energetic characterization of dipolar dye aggregates and implications for nonlinear optical materials.

Aggregation of polar merocyanine dyes has been identified as an important problem in the fabrication of organic materials for photonic applications. In this work, a series of merocyanine dyes is synthesized, and their aggregation is investigated by a combination of several experimental techniques to reveal structure-property relationships. These studies provide clear evidence for the formation of centrosymmetric dimers for all investigated merocyanines in concentrated solution and in the solid state. The thermodynamics of dimerization in liquid solution is studied by concentration-dependent permittivity measurements, UV-vis spectroscopy, and electrooptical absorption experiments. A centrosymmetric dimer structure with antiparallel ordering of the dipole moments is observed in solution by 2D NMR spectroscopy as well as in the solid state by X-ray crystallography and interpreted in terms of dipolar and pi-pi interactions. The optical properties of the dimer aggregates are satisfactorily explained by an excitonic coupling model. The effect of an external electric field on the dimerization equilibrium is considered and quantitatively determined by electrooptical absorption measurements. Implications of the observed findings on the design of nonlinear optical and photorefractive materials are discussed.