KC 9 H 5 O 6 (H 2 O): A Promising UV Nonlinear-Optical Material with Large Birefringence Based on a π-Conjugated (C 9 H 5 O 6 ) - Group.

In order to meet the growing needs for the laser technology and optics industries, the goal is to find suitable fundamental building blocks with large nonlinear-optical (NLO) coefficients and birefringence for an excellent-performance NLO or birefringent system. Via preliminary investigations and calculations, it has been found that the planar π-conjugated group (C 9 H 5 O 6 ) - possesses large polarizability anisotropy (δ) and hyperpolarizability (β max ), comparable to well-known groups such as (B 3 O 6 ) 3- , (C 3 N 3 O 3 ) 3- , etc. Herein, we report a new alkali-metal 3,5-dicarboxybenzoate, KC 9 H 5 O 6 (H 2 O) (KH 2 BTC), which crystallized in the acentric space group Pna 2 1 . Second-harmonic-generation (SHG) measurements of KH 2 BTC under 1064 nm laser radiation show that the SHG response of KH 2 BTC is 1.2 times that of KDP with type I phase-matching behavior. Birefringence measurements show that KH 2 BTC owns a large birefringence of about 0.372 at 550 nm. The band gap of KH 2 BTC obtained by ultraviolet (UV) diffuse-reflectance spectroscopy is 3.91 eV, indicating that KH 2 BTC has potential applications as UV NLO or birefringent materials. Theoretical calculation further confirmed that the impressive optical properties of KH 2 BTC are derived from the large polarizability anisotropy of the (C 9 H 5 O 6 ) - anions.