Design Strategy of Highly Efficient Nonlinear Optical Orange‐Colored Crystals with Two Electron‐Withdrawing Groups

A new class of highly efficient nonlinear optical organic salt crystals is reported. In nonlinear optics based on organic materials, it is well known that using two electron‐withdrawing groups (EWGs) onto cationic electron acceptors instead of conventional one EWG remarkably enhances microscopic optical nonlinearity for chromophores. However, the corresponding organic crystals possessing enhanced large macroscopic optical nonlinearity have not been reported yet. Herein, a design strategy is proposed for obtaining highly efficient nonlinear optical crystals based on two EWGs in cationic electron acceptors. Introducing a phenolic electron donor, promoting a head‐to‐tail interionic assembly, along with a two‐EWG N‐pyrimidinyl pyridinium electron acceptor in cationic chromophores results in a preferred non‐centrosymmetric, perfectly parallel alignment of chromophores in crystal. Newly designed OPR (4‐(4‐hydroxystyryl)‐1‐(pyrimidin‐2‐yl)pyridinium) crystals exhibit approximately two times larger effective first hyperpolarizability than that of analogous N‐alkyl OHP (4‐(4‐hydroxystyryl)‐1‐methylpyridinium) crystals based on only one EWG. OPR crystals exhibit comparable second‐order optical nonlinearity to benchmark red‐colored DAST (4‐(4‐(dimethylamino)styryl)‐1‐methylpyridinium 4‐methylbenzenesulfonate) crystals, but a significant blue‐shifted absorption resulting in orange‐color crystals. Therefore, phenolic organic salt crystals using two EWGs are highly promising materials for various nonlinear optical applications.