Enhancement in Two-Photon Absorption and Photoluminescence in Single Crystals of Cd(II) Metal Organic Frameworks

The emergence of metal organic frameworks (MOFs) as advanced photonic materials has placed them at the forefront of exploration. Nonlinear optical (NLO) phenomena such as simultaneous two-photon absorption and consequent upconversion emission have been in demand for promising applications. A rational design approach based on the fundamental structure-property relationship is key for the fabrication of nonlinear optically active MOF materials. Here, we investigate two-photon-absorption (2PA)-induced photoluminescence of four new Cd(II) MOFs based on an acceptor-π-donor-π-acceptor trans, trans-9, 10-bis(4-pyridylethenyl)anthracene chromophore linker. The use of auxiliary carboxylate linkers resulted in the variation of crystal structures, leading to the modulation of NLO properties. On comparison with a standard Zn(II)-MOF, two MOFs showed enhancement in 2PA, while the other two showed a mild decrease. We tried to establish a structural correlation to explain the trend in NLO activity. The interplay of various factors such as chromophore density, degree of interpenetration, chromophore orientation, and π···π interactions between the individual networks affects the NLO activities. These results show the modulation of the optical properties of MOFs based on a combined strategy for the development of tunable single crystal NLO devices.