Tuning Two-Photon Absorption Cross Section in Metal Organic Frameworks

The development of alternative nonlinear optical metamaterials has attracted much attention recently due to technological demands. Upconversion emission via a simultaneous two-photon absorption process is a nonlinear process that is widely studied in synthetically challenging organic compounds. Hereby, we report 9 metal organic frameworks constructed with various combinations of the following ligands: trans, trans -9,10-bis(4-pyridylethenyl) anthracene, trans, trans -9,10-bis(4-pyridylethynyl) anthracene, 1,4-bis[2-(4′-pyridyl)ethenyl]benzene, 4,4′-stilbene dicarboxylate, 4,4′-biphenyl dicarboxylate, 4,4′-benzene dicarboxylate, and benzene-1,3,5-tricarboxylate. Altering the auxiliary carboxylate ligands not only changes the structure but also varies the two-photon excited fluorescence. The two-photon excited emission is enhanced when longer spacer ligands are used and when they are packed in more expanded structures in hms topology. Unusually, the emission becomes stronger when a pair of pyridyl type ligands are perfectly aligned in parallel which could be due to reduction in nonradiative decay caused by molecular rotation. The comparison of two-photon absorption cross sections with their action cross section counterpart revealed a dissimilar trend. High level of absorption in MOFs does not necessitate the formation of a highly excited emissive state. To the best of our knowledge, this is the first example of a systematic structural–property relationship study on the two-photon excited fluorescence in metal organic frameworks.