Na 2 (C 4 O 4 )(H 3 BO 3 )(H 2 O) 4 ·H 3 BO 3 : the first borate-squarate with strong SHG response obtained by superior synergy.

We herein report the first sodium borate-squarate, namely, Na 2 (C 4 O 4 )(H 3 BO 3 )(H 2 O) 4 ·H 3 BO 3 (1). It has been successfully synthesized via introducing the planar π-conjugated (C 4 O 4 ) 2- into a borate system. Compound 1 exhibits a strong second-harmonic-generation (SHG) response (2.1 × KDP), wide transparency in the ultraviolet (UV) region and a suitable birefringence of 0.26 at 1064 nm. Theoretical calculations indicate that the synergy effect between inorganic planar π-conjugated species H 3 BO 3 and organic moiety (C 4 O 4 ) 2- dominates the SHG process.