Aligned chromophores in a host–guest MOF crystal for switchable polarized nonlinear optical response

Abstract

Switching between second-harmonic generation (SHG) and two-photon-pumped (TPP) lasing is scientifically important but hard to realize. Herein, a host–guest metal–organic framework (MOF) material is prepared, in order to elaborately arrange electron donor (D)–acceptor (A) structural ligand and dye, which are defined as nonlinear optical (NLO) building units. Such a hybrid MOF is an intriguing example in which SHG and TPP lasing can be switched in a single crystal via orthogonally polarized excitation. The acentric host MOF is sythesized using metal nodes, and the octupolar D–A ligand has a desymmetrized conformation, with an orientation roughly vertical to the one-dimensional (1D) channels of the MOF, leading to the polarized SHG response. The dipolar D–A dye of matched size is then introduced into the well-faceted host MOF as a guest and aligns along the 1D channels, resulting in polarized TPP lasing. Due to the specific alignment of these NLO building blocks, the excitation polarization dependence of SHG and TPP lasing are mutually perpendicular in the single host–guest MOF crystal, with a high degree of polarization, with values of about 81.2% and 96.1%, respectively. The two NLO signals can be alternated through simply manipulating the polarization direction of the pump laser. These results facilitate fundamental understanding of the generation of polarized NLO responses in MOFs, and provide an interesting perspective to develop versatile switching NLO materials.