5 years ago

Tuning Two-Photon Absorption Cross Section in Metal Organic Frameworks

Tuning Two-Photon Absorption Cross Section in Metal Organic Frameworks
Xiaogang Liu, Venkatram Nalla, Jagadese J. Vittal, Chloe Aloycia Lee, Kezhi Zheng, Hong Sheng Quah
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.

Publisher URL: http://dx.doi.org/10.1021/acs.chemmater.7b02417

DOI: 10.1021/acs.chemmater.7b02417

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