Metal ion sensing novel calix[4]crown fluoroionophore with a two-photon absorption property

1,3-Alternate calix[4]arene-based fluorescent chemosensors bearing two-photon absorbing chromophores have been synthesized, and their sensing behaviors toward metal ions were investigated via absorption band shifts as well as one- and two-photon fluorescence changes. Free ligands absorb the light at 461 nm and weakly emit their fluorescence at 600 nm when excited by UV-vis radiation at 461 nm, but no two-photon excited fluorescence is emitted by excitation at 780 nm. Addition of an Al³⁺ or Pb²⁺ ion to a solution of the ligand causes a blue-shifted absorption and enhanced fluorescence due to a declined resonance energy transfer (RET) upon excitation by one- and two-photon processes. Addition of a Pb²⁺ ion to a solution of 1·K⁺ results in a higher fluorescence intensity than the original 1·Pb1 complex regardless of one- or two-photon excitation, due to the allosteric effect induced by the complexation of K ⁺ with a crown loop.