Impact of the Nature of the Excited-State Transition Dipole Moments on the Third-Order Nonlinear Optical Response of Polymethine Dyes for All-Optical Switching Applications

Designing molecular materials with the figures-of-merit needed for all-optical switching applications requires that, at the wavelengths of interest, the molecules have large real components |Re(γ)| of the third-order polarizability (γ) while at the same time maintaining small imaginary components Im(γ). Polymethines have the potential to meet these conditions, though to date only a few polymethines exhibit large enough |Re(γ)/Im(γ)| for device applications. From the sum-over-states expression for γ, it can be deduced that when the transition dipole moment (μee′) between the polymethine first and second excited states is minimized, Im(γ) decreases and |Re(γ)| increases. Here, focusing on a series of streptocyanines, we decompose μee′ into the transition dipole components of the constituent electronic transitions and investigate how variations in chain length and substitution patterns alter μee′. The second, two-photon-allowed, excited state is shown to be composed primarily of three excitations, two of whi...