Tailoring the Linear and Second-Order Nonlinear Optical Responses of the Titanium-MIL-125 Metal–Organic Framework through Ligand Functionalization: A First Principles Study

Density functional theory calculations have been performed to investigate the linear and second-order nonlinear optical (NLO) properties of titanium-based MIL-125 metal-organic frameworks in crystalline form, in which the 1,4-benzenedicarboxylate (BDC) linkers are modified by introducing different functional groups or by extending the BDC ligand to contain two (MIL-126) and three (MIL-127) benzene rings. Our results reveal that the functionalization of the BDC linker tends to increase the dielectric constants and the magnitude of birefringence of MIL-125, especially for the aminated derivatives. Correspondingly, the incorporation of a substituent group will improve the phase matching performance of MIL-125. As for the second-harmonic generation (SHG) susceptibility, the SHG activity of the pristine MIL-125 is comparable to KDP, which can be attributed mostly to the contributions of TiO5(OH) octahedra. It is noted that after introducing the substituent group into the BDC linker, the organic part will have a remarkable influence on the SHG intensity. However, the specific effect on the NLO response is dependent on the type of functional group incorporated into the BDC ligand, and only the inclusion of the amine group that is strongly electron-donating can obviously enhance the SHG activity of MIL-125. In addition, MIL-126 and MIL-127 with longer aromatic linking units are not suitable to act as NLO materials due to their poor phase matching abilities, but they are promising candidates for the low dielectric constant materials. The present study can provide theoretical insights to design new second order NLO materials based on MIL-125.