Tunable Nonlinear Optical Properties Based on Metal–Organic Framework Single Crystals

Solid-state hybrid materials with designability in topology and tunable photo-responsiveness hold great promise for multitudinous applications from optoelectronics to information storage/communication. However, the judicious design and synthesis of metal–organic frameworks (MOFs) with intrinsically custom-made nonlinear optical (NLO) properties including multiple photon absorption and harmonic generation effects suitable for miniaturized devices remain highly challenging. Herein, the design and synthesis of a novel pillared framework named after Zn-TCPE-DPNI MOF coordinated by zinc ions (Zn²⁺), aggregation induced emission (AIE) featured [1,1′-biphenyl]-4-carboxylic acid (H₄TCPE), and N,N′-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNI) are reported. The highly ordered Zn-TCPE-DPNI MOF single crystal exhibits tunable NLO responses by switching incident excitation wavelengths of a femtosecond laser from 900 to 1500 nm. The noticeable transformation from two- or three-photon excited photoluminescence (2PL or 3PL) to the concurrent occurrence of 3PL and third harmonic generation (THG), and eventually to the mere emergence of THG with a high effective susceptibility χ⁽³⁾ (3ω) of 2.9 × 10⁻¹² esu is observed. This study paves a novel avenue for the design and synthesis of photoluminescent MOF crystals with tunable NLO responses toward the fabrication of multifunctional NLO devices targeted for optoelectronics and information communication.