Significance Nonlinear optical materials have become increasingly essential to various fields, such as optoelectronics, communication, and information processing. Therefore, there is an urgent need to develop new and excellent nonlinear optical materials. Compared with traditional inorganic nonlinear optical materials, organic nonlinear optical materials have decisive advantages in damage threshold, response time, and nonlinear optical coefficient. Zeolitic imidazolate frameworks (ZIFs) are of special metal-organic frameworks with imidazole or its derivatives as ligands. Due to their structural diversity, high thermal and chemical stabilities, they have been repeatedly researched worldwide in recent years.
Progress Metal-organic materials having large π-electron conjugation systems or low charge-transfer excited states are used as nonlinear optical materials. Although organometallic materials exhibit considerable nonlinear optical properties over a wide wavelength range, their photostability and thermal stability properties limit their applications. Research has confirmed that metal-organic coordination polymers and frameworks effectively improved stability while maintaining several nonlinear optical properties. This review selected two ZIFs, ZIF-8 and ZIF-67, and discussed the recent progress in their synthesis, preparation, and nonlinear optical properties. ZIF-8 and ZIF-67 are commonly synthesized using the solvothermal method, room-temperature magnetic stirring method, and other ultrasound or microwave assistant methods. Recently, most micro-nano crystals with excellent physiochemical properties and stable structures are prepared in terms of ZIF-8 and ZIF-67. Alternatively, in the nanosecond framework structure of metals, the nonlinear optical properties are enhanced by careful design of molecular symmetry, π-electron conjugation length, intramolecular charge-transfer mechanism, and interaction among molecules. Phenomena such as multiphoton absorption, upconversion, and excited light are realized. Both ZIF-8 and ZIF-67 exhibited interesting nonlinear optical properties due to advanced structural designs on ZIFs. In 2016, it was proven that ZIF-8 possessed a large effective nonlinear coefficient of -0.25 pm/V, with link reorientation, the nanocrystal defects reduced the nonlinear second harmonic generation due to the induced inversion center, as in Fig. 3. With the cobalt (Co) ions induced in ZIF-8, the size of the synthesized ZIFs increased. Researchers have found that the absorption at 1125 nm was enhanced because of the transition between 4A2→ 4T1(F) in Co ions. Following these consequences, we investigated the nonlinear absorption coefficient, two-photon absorption cross-section, nonlinear refractive index, and third-order optical susceptibility of the ZIF-67 sample prepared using the solvothermal method. Results showed that the two-photon absorption cross-section was approximately 85×10 5 GM, the nonlinear refractive index was ~ -9.3×10 -4 cm 2/GW, and the third-order optical susceptibility was -8.2×10 -11 esu at 1 μm. Our results confirmed that ZIF-67 possessed excellent nonlinear optical properties, suitable for optical modulators, limiters, and detectors.
Conclusions and Prospects This review summarizes advances on the preparation and nonlinear optical properties of ZIF materials worldwide. ZIF-8 and ZIF-67 are examples of such organometallic nano framework material. ZIFs show good nonlinear optical properties, such as high modulation depth, large third-order nonlinear polarizability, nonlinear refractive index, low extinction coefficient, etc. It has important application value in electronics and optoelectronics.
However, the research of ZIF nanomaterials is concerned, there are still a series of problems to be solved.
1) The microscopic mechanism of photogenerated carriers is yet to be sufficiently explored. A large blank on how to generate, separate, combine, and manipulate the electron-hole pairs remains unresolved.
2) How the interaction between the metal ions and organic linkers affects the nonlinear optical properties? How to optimize the ratio and synthesis of metal ions and organic linkers? How to control and manipulate the synthesis conditions and solutions? The answers to these questions are key advancements in the preparation and investigation of ZIFs and its nonlinear optical properties, respectively.
3) The application of ultrafast photonics of ZIFs based on nanomaterials, especially the realization of ultrafast laser, is another work of urgent consideration.
