Impact of the Structural Modification of Diamondoid Cd(II) MOFs on the Nonlinear Optical Properties

A change in the degree of interpenetration (DOI) in metal-organic frameworks (MOFs) prompted by heat, pressure, or exchange of solvents is a fascinating phenomenon that can potentially impact the functional properties of MOFs. Structural transformation involving two noncentrosymmetric MOFs with different DOIs provides a rare opportunity to manipulate their optical properties. Herein, we report an unusual single-crystal-to-single-crystal (SCSC) transformation of a noncentrosymmetric 7-fold interpenetrated diamondoid (dia) Cd(II) MOF into another noncentrosymmetric but 8-fold interpenetrated dia MOF upon the removal of guest solvents. A hydrogen-bond network formed between the lattice solvents and linker trans-2-(4-pyridyl)-4-vinylbenzoate (pvb) in a 7-fold interpenetrated noncentrosymmetric MOF results in a significant increase in the two-photon absorption cross-section (11 times) as compared to that in the desolvated 8-fold interpenetrated MOF. Also, an increase in the DOI in the noncentrosymmetric crystals strengthened the π···π interaction between the individual diamondoid networks and enhanced the second-order nonlinear optical (NLO) coefficient (deff) by 4.5 times. These results provide a way to manipulate the optical properties of MOFs using a combined strategy of the formation of hydrogen bonds and interpenetration for access to tunable single-crystal NLO devices in an SCSC manner. By changing the experimental conditions, another dia Cd(II) MOF with 4-fold interpenetration can be isolated. In this centrosymmetric MOF, the olefin groups in the backbone of the ligand (pvb) undergo a [2 + 2] cycloaddition reaction quantitatively under UV light but in a non-SCSC fashion.