Helquat Dyes: Helicene-like Push–Pull Systems with Large Second-Order Nonlinear Optical Responses

Helquat
dyes combine a cationic hemicyanine with a helicene-like
motif to form a new blueprint for chiral systems with large and tunable
nonlinear optical (NLO) properties. We report a series of such species
with characterization, including determination of static first hyperpolarizabilities
β0 via hyper-Rayleigh scattering and Stark spectroscopy.
The measured β0 values are similar to or substantially
larger than that of the commercial chromophore E-4′-(dimethylamino)-N-methyl-4-stilbazolium. Density functional theory (DFT)
and time-dependent DFT calculations on two of the new cations are
used to probe their molecular electronic structures and optical properties.
Related molecules are expected to show bulk second-order NLO effects
in even nonpolar media, overcoming a key challenge in developing useful
materials.