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Periodic density functional theory calculations have been performed to investigate the structures, the linear and second order nonlinear optical (NLO) properties of Zr-based UiO-66 metal–organic frameworks, in which the 1,4-benzene-dicarboxylate (BDC) linkers are functionalized by a series of isovalent substituents including −NH2, −OH, −SH, and halogen atoms. Our results show that the mixed phase will be formed for the case of the incorporation of a single hydroxyl or halogen atom, while it is more likely to synthesize the pure phase for the homodifunctionalized compounds. The ligand functionalization results in the appearance of the band gap states as well as the narrowing of the band gap. For the linear optical response, besides the redshift of the absorption edge, the ligand modification tends to increase the dielectric constant and enhance the optical anisotropy of UiO-66, especially when two −SH groups are incorporated into the BDC ligand. With regard to the NLO activity, the second harmonic generation (SHG) intensity of the pristine UiO-66 is comparable to that of KDP. The SHG response of UiO-66 can be effectively improved by the single-site substitution, and the obvious enhancement of the SHG activity is observed after introducing the sulfydryl group or the iodine atom. However, it is noted that the difunctionalizations by two −OH and −SH have a relatively weak impact on the SHG intensity because of the nearly centrosymmetric organization of the homodifunctionalized organic linkers. Among all derivatives, UiO-66-(SH)2 is the most promising candidate for use as a near-infrared NLO material with a strong SHG effect (>20 pm/V) and good phase matchability. Furthermore, the primary origin of the SHG response for UiO-66 and its derivatives is determined by analyzing the band structure. This study reveals the possibility of designing NLO materials with outstanding performances based on the UiO-66-derived MOFs through rational functionalization of the organic linking unit.
Ni et al. (Wed,) studied this question.