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 (μ_ee′) 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 μ_ee′ into the transition dipole components of the constituent electronic transitions and investigate how variations in chain length and substitution patterns alter μ_ee′. The second, two-photon-allowed, excited state is shown to be composed primarily of three excitations, two of which contribute to μ_ee′ in an additive fashion, while the third reduces the magnitude of μ_ee′. As the conjugation path length growths, the competition between two factors, (i) the increased wave function overlap in each constituent transition vs (ii) the increased influence of the electronic configuration with a negative contribution to μ_ee′, results in a weak dependence of μ_ee′ on length. Electron-donating and -withdrawing substituents are shown to affect μ_ee′ by influencing the energetic spacing of the first few frontier molecular orbitals, which offers a path for further tuning of μ_ee′; in particular, it is found that a large energetic spacing between the HOMO-1 and HOMO levels and between the LUMO and LUMO+1 levels is a critical feature to achieve small μ_ee′ values. (Graph Presented).