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.

Helquat染料将阳离子半花菁与类螺烯（helicene-like）结构基元相结合，为具备大尺寸且可调控非线性光学（nonlinear optical, NLO）特性的手性体系构建了全新的设计蓝图。我们报道了一系列此类化合物并完成了相关表征，包括通过超瑞利散射（hyper-Rayleigh scattering）与斯塔克光谱（Stark spectroscopy）测定其静态一阶超极化率β0。测得的β0数值与商用生色团E-4′-(二甲氨基)-N-甲基-4-苯并唑鎓（E-4′-(dimethylamino)-N-methyl-4-stilbazolium）相当，甚至显著更高。我们针对两种新型阳离子化合物开展了密度泛函理论（density functional theory, DFT）及时域密度泛函理论（time-dependent DFT, TD-DFT）计算，以探究其分子电子结构与光学性能。相关分子即便在非极性介质中也可展现体相二阶非线性光学效应，攻克了实用型非线性光学材料开发过程中的一项核心挑战。