ZIF‑8 as Nonlinear Optical Material: Influence of Structure and Synthesis

Metal–organic
framework ZIF-8, from the zeolitic imidazolate
framework family, shows a large intrinsic second-order nonlinear optical
(NLO) response. In addition, ZIF-8 is a stable, inexpensive material
that is transparent in the visible (vis) and near-infrared (NIR) window.
This is crucial for NLO applications. The second-order NLO activity
is due to the noncentrosymmetric octupolar symmetry of the material.
We found that fast syntheses lead to a lower second-order NLO response.
Consistent with polarized second-harmonic generation (SHG) microscopy
measurements, we ascribe this to defects that create local centers
of centrosymmetry but do not affect the orientation of the crystal
lattice. Syntheses with slow nucleation lead to quasi-perfect crystals
with a large average second-order NLO coefficient ⟨deff⟩ of 0.25 pm/V, which is explained
and supported by ab initio theoretical calculations.

隶属于沸石咪唑酯骨架家族的金属有机框架ZIF-8，展现出显著的本征二阶非线性光学（second-order nonlinear optical, NLO）响应。此外，ZIF-8是一种稳定且廉价的材料，在可见光（visible, vis）与近红外（near-infrared, NIR）窗口范围内具有优异透光性，这一特性对于其NLO应用至关重要。该材料的二阶NLO活性源于其非中心对称的八极对称性。研究发现，快速合成得到的ZIF-8其二阶NLO响应较弱。结合偏振二次谐波产生（second-harmonic generation, SHG）显微镜的测量结果，我们将这一现象归因于缺陷引入了局部中心对称中心，但并未改变晶体晶格的取向。采用缓慢成核的合成策略可得到准完美晶体，其平均二阶NLO系数⟨d_eff⟩可达0.25 pm/V，这一结果可通过从头算理论计算（ab initio theoretical calculations）进行解释与验证。