A flexible functional module to regulate ultraviolet optical nonlinearity for achieving a balance between a second-harmonic generation response and birefringence.

Rigid planar π-conjugated groups are adopted for designing ultraviolet (UV) nonlinear optical (NLO) materials extensively. However, for these UV NLO crystals, the realization of a strong second harmonic generation (SHG) response is commonly accompanied by undesired overlarge birefringence. Herein, we propose a new functional gene, the flexible π-conjugated (C 3 H 2 O 4 ) 2- group, for designing a UV NLO crystal with a balance between the SHG response and birefringence. Furthermore, the combination of low-coordinated and high-coordinated alkali cations with the flexible (C 3 H 2 O 4 ) 2- group results in finding a new mixed alkali malonate, KLi(C 3 H 2 O 4 )·H 2 O (KLMW). As expected, KLMW exhibits a strong SHG efficiency (3 × KDP) and moderate birefringence (0.103 @ 1064 nm). In addition, it has a short UV cut-off edge of 231 nm and can be conveniently grown from solution. More importantly, it realized fourth harmonic generation with type-I phase-matching. Therefore, these excellent properties make KLMW a potential practical UV NLO material.