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.