Balanced Second-Order Nonlinear Optical Properties of Adducts CHI<sub>3</sub>·(S<sub>8</sub>)<sub>3</sub> and AsI<sub>3</sub>·(S<sub>8</sub>)<sub>3</sub>: A Systematic Survey

Two isostructural adducts CHI<sub>3</sub>·(S<sub>8</sub>)<sub>3</sub> (<b>1</b>) and AsI<sub>3</sub>·(S<sub>8</sub>)<sub>3</sub> (<b>2</b>) are synthesized by a simple solution method. Both of them crystallize in noncentrosymmetric <i>R</i>3<i>m</i>, featuring van der Waals interaction linked CHI<sub>3</sub> tetrahedra or SbI<sub>3</sub> trigonal pyramids with crown-like S<sub>8</sub> molecules. Both <b>1</b> and <b>2</b> show strong SHG responses and phase matchability under either 1.064 or 2.1 μm. The results of DFT calculations indicate that the electron transfer happens from S-3p to I-5p orbitals, and both of them demonstrate indirect band gaps. The NLO effects of <b>1</b> and <b>2</b> are almost fully contributed from the CHI<sub>3</sub> or AsI<sub>3</sub> units. Compared with AgGaS<sub>2</sub>, their powder laser damage thresholds are ca. 15 and 51 times higher.