Topochemical Polymerization\nat Diacetylene Metal–Organic\nFramework Thin Films for Tuning Nonlinear Optics

Developing the topochemical polymerization of metal–organic\nframeworks (MOFs) is of pronounced significance for expanding their\nfunctionalities but is still a challenge on third-order nonlinear\noptics (NLO). Here, we report diacetylene MOF (CAS-1–3) films\nprepared using a stepwise deposition method and film structural transformation\napproach, featuring dynamic structural diversity. The MOF structures\nwere determined by the three-dimensional electron diffraction (3D\nED) method from nanocrystals collected from the films, which provides\na reliable strategy for determining the precise structure of unknown\nMOF films. We demonstrate the well-aligned diacetylene groups in the\nMOFs can promote topological polymerization to produce a highly conjugated\nsystem under thermal stimulation. As a result, the three MOF films\nhave distinct NLO properties: the CAS-1 film exhibits saturable absorption\n(SA) while CAS-2 and CAS-3 films exhibit reverse saturable absorption\n(RSA). Interestingly, due to the topochemical polymerization of the\nMOF films, a transition from SA to RSA response was observed with\nincreasing temperatures, and the optical limiting effect was significantly\nenhanced (∼46 times). This study provides a new strategy for\npreparing NLO materials and thermally regulation of nonlinear optics.