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.