Abstract
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 which contribute to μee′ in an additive fashion, while the third reduces the magnitude of μee′. 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 μee′, results in a weak dependence of μee′ on length. Electron-donating and -withdrawing substituents are shown to affect μee′ by influencing the energetic spacing of the first few frontier molecular orbitals, which offers a path for further tuning of μee′; 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 μee′ values.