Second-Order Nonlinear Optical (NLO) Properties of a Multichromophoric System Based on an Ensemble of Four Organic NLO Chromophores Nanoorganized on a Cyclotetrasiloxane Architecture

In this paper we report the synthesis, the photophysical, conductometric, and second-order nonlinear optical (NLO) characterization of an ensamble of four NLO active organic tails nanoorganized on a cyclotetrasiloxane ring, to produce various macrocyclic NLO chromophores. The second-order NLO response of the macrocyclic NLO chromophores measured by the EFISH technique as m1.91, where m is the dipole moment and beta=1.91 the projection along the dipole moment axis of the vectorial component of the quadratic hyperpolarizability working with an incident wavelength of 1.907 µm, increases, compared to the reference monomeric NLO chromophores, from 2.8 up to 3.5 times for nonionic macrocyclic NLO chromophores. The increase is mainly due to an increase of the dipole moment when compared to reference monomeric NLO chromophores while the beta=1.91 values remain almost unchanged. These macrocyclic NLO chromophores can be also considered as a simple model of a monolayer of organic NLO chromophores on a chemically engineered silica surface. Since the factor controlling their second-order NLO response is the orientation toward the dipole moment axis of the single organic NLO active tails, this kind of model confirms that the second-order NLO response of a monolayer of organic NLO chromophores on a chemically engineered silica surface is controlled by the topology of the binding sites on the surface, as suggested by previous investigations on multilayers on chemically engineered silica surface. The macrocyclic NLO chromophore with a ionic organic NLO active tail shows a strong concentration dependence of its second-order NLO response to be ascribed to a larger increase of its ionic dissociation by dilution, when compared to that of the reference ionic monomeric NLO chromophore. This behavior is evidence of a cooperative effect due to the nanoorganization, to be ascribed to a more facile ionic dissociation of the single ionic organic tail originated by a local increase of the solvent polarity favored by the closeness of the positive charges of the four organic tails. Finally it was shown that the Si(OSiMe3)3 group behaves in a classical organic push-pull NLO chromophore as a pull group as strong as the nitro group.