Postsynthetic Framework Contraction Enhances the Two-Photon Absorption Properties of Pillar-Layered Metal-Organic Frameworks

Aggregation-induced emission (AIE) dyes have been shown to be a potential ligand class for multiphoton absorbing metal-organic frameworks (MPA-MOFs); however, the influence of framework flexibility on the local ligand conformation and its ramifications on the nonlinear absorption properties of this material class have sparsely been understood. In this study, we systematically investigate the two-photon absorption properties of two pillar-layered MOFs comprising tetraphenylethylene AIE ligands and compare the results to the organic ligand in crystal form, using a combination of linear and nonlinear optical characterization methods and electronic-structure calculations. We demonstrate that self-confining the AIE ligand is key to enhance the nonlinear optical absorption properties, as a structure transformation to contracted frameworks strongly increases the two-photon absorption response, which can be addressed by specific ligand substitution. Our results have important implications on the design of MPA-MOFs and provide synthetic guidelines not only from a fundamental point of view but also application-wise.