All-optical switching devices have the potential to dramatically improve the speed of long-distance data transmission. These devices generally require materials with large real parts of the molecular third-order polarizability [|Re(γ)|] and small imaginary parts [Im(γ)] corresponding to small two-photon absorption at telecommunications wavelengths, in conjunction with a large chromophore concentration. It has been shown recently that cyanines have the potential to meet these requirements; however, few cyanines have large enough |Re(γ)/Im(γ)| for use in devices, and aggregation often substantially reduces |Re(γ)/Im(γ)| at large concentrations.
Here, we use theoretical methods to show the effect of molecular structure and aggregation on the nonlinear optical properties of these cyanines. If the transition dipole μee’ between the cyanine first and second excited states is reduced, Im(γ) will decrease and |Re(γ)| will increase. The magnitude of μee’ can be understood in terms of the coefficients of three key transitions and the spatial overlap between each pair of orbitals. Cyanine aggregation can be controlled by adding bulky substituents on various parts of the cyanine structures to limit cyanine-counterion and cyanine-cyanine interactions. Understanding the relationships between chemical structure, molecular properties, and aggregation will enable the design of cyanines with optimized molecular properties for all-optical switching.
Prof. Jean-Luc Bredas (404-385-4985)