Two Carboxylate-Cyanurates with Strong Optical Anisotropy and Large Band Gaps.

The first metal carboxylate-cyanurates, namely, K(H 3 C 3 N 3 O 3 )(HCO 2 ) ( I ) and Ba 2 (H 2 C 3 N 3 O 3 )(CH 3 CO 2 ) 3 (H 2 O) ( II ), which contain π-conjugated carboxylate and cyanurate groups, have been synthesized by hydrothermal methods. They crystallize in centrosymmetric space groups of P 1̅ and P 2 1 / n , respectively. Compound I exhibits a novel three-dimensional (3D) structure based on a [K(H 3 C 3 N 3 O 3 )] + cationic framework with 12-membered ring (12-MR) channels, and the (HCO 2 ) - anions are located within the 12-MR channels. The [K(H 3 C 3 N 3 O 3 )] + cationic framework is composed of K + ions interconnected by H 3 C 3 N 3 O 3 ligands. Compound II features a 3D network formed by [Ba 2 (CH 3 CO 2 ) 3 ] + cationic double chains bridged by (H 2 C 3 N 3 O 3 ) - anions. The [Ba 2 (CH 3 CO 2 ) 3 ] + cationic double chain is composed of (CH 3 CO 2 ) - anions and Ba 2+ ions. Optical property measurements show that both compounds exhibit short ultraviolet cutoff edges ( I , 208 nm; II , 218 nm) and wide band gaps ( I , 5.43 eV; II , 5.20 eV). Importantly, K(H 3 C 3 N 3 O 3 )(HCO 2 ) ( I ) features a large birefringence of 0.285@532 nm due to the parallel alignment of π-conjugated H 3 C 3 N 3 O 3 and (HCO 2 ) - groups, indicating that K(H 3 C 3 N 3 O 3 )(HCO 2 ) ( I ) is a promising short-wave ultraviolet birefringent material. Detailed theoretical calculations elucidate that their excellent optical properties originate from the synergetic effect of both types of π-conjugated groups.