Metal induced enhancement of fluorescence and modulation of two-photon absorption cross-section with a donor-acceptor-acceptor-donor receptor

A metal ion sensing fluorophore L that exhibits a large two-photon absorption cross-section has been synthesized in good yields. The influences of different metal ion inputs, on the one- and two-photon spectroscopic properties of L, have been investigated. The ligand itself does not show any fluorescence although in presence of a metal ion like Zn(II), Cd(II), Mg(II) or Ca(II), a ∼25 time enhancement of fluorescence is observed. The ligand with symmetrical “donor-acceptor-acceptor-donor” characteristics exhibits a large two-photon absorption cross-section measured by femtosecond open-aperture Z-scan technique at 880 nm. However, presence of any of the above metal ions lowers its two-photon absorption cross-section (δ) to different extents at 880 nm. Theoretical calculation carried out in DFT formalism on the ligand and its Zn(II) complex corroborate experimental results.