Distinct nonlinear optical responses in three pairs of 2D homochiral Ag(I) enantiomers modulated by dicarboxylic acid ligands

Three pairs of homochiral Ag(I) enantiomers formulated as [Ag₂(L_R)(pa)]_n/[Ag₂(L_s)(pa)]_n (R1/S1, H₂pa = phthalic acid), [Ag₂(L_R)(tda)]_n·H₂O/[Ag₂(L_s)(tda)]_n·H₂O (R2/S2, H₂tda = 3,4-thiophene dicarboxylic acid), and [Ag₄(L_R)₂(3-npa)₂]_n/[Ag₄(L_s)₂(3-npa)₂]_n (R3/S3, 3-H₂npa = 3-nitrophthalic acid) were prepared by using different dicarboxylic acid coligands via the reactions of AgNO₃ with enantiopure monobidentate N-donor heterocyclic ligands named (+)/(−)-4,5-pinenepyridyl-2-pyrazine (L_S/L_R), respectively. Investigations on the nonlinear optical (NLO) response suggest R1/S1 and R2/S2 enantiomers present very strong third-harmonic generation (THG) responses, and the lowest THG intensities of R1/S1 and R2/S2 are 244 and 142 times that of α-SiO₂, respectively. However, R3/S3 enantiomers only exhibit mild second-harmonic generation (SHG) response (0.36 × KDP). A comparative study on the molecular structures of these Ag(I) enantiomeric pairs reveals that both the symmetry and degree of π-conjugation of the dicarboxylate ligands are mainly responsible for their different NLO responses. Our research result not only provides a feasible method of constructing coordination polymers (CPs) with strong THG response but also points out an effective strategy for switching NLO behaviors from THG to SHG response.