Indole-based fluorescence sensors for both cations and anions

The fluorescence properties of the ligand idpa (1H-Indole-3-ethanamine, N,N-bis(2-pyridinylmethyl)amine) complexed with metal ions along with their crystal structures were investigated to further understand the role of pi contacts in quenching fluorescence. In particular, the pi contacting ability of indole fluorophore of idpa and its impact on the fluorescence properties are compared to those of anthracene (in adpa) and coumarin fluorophores (in cdpa) investigated previously by us. Unlike adpa and cdpa, the fluorescence of free idpa increases with rising pH, which is attributed via TD-DFT studies to quenching at lower pH values by a proton transfer in the excited state from the protonated tertiary amine to the pydridyl nitrogen. Structural studies show that the Ag-I complex of idpa is a dimer held together by very short inter-complex eta(1) Ag...C pi contacts of 2.382 angstrom, indicating powerful pi contacting ability of indole as expected from the electrostatic potential map. However, structural studies of the Zn-II, Ni-II, Pb-II, and Cd-II complexes of idpa show no metal-fluorophore pi contacts present in the solid state, which was ascribed to a plethora of other pi contacts and hydrogen bondings possible only in solid state. All metal ions on complex-formation cause quenching of fluorescence of idpa relative to that of free idpa at pH 7.45, suggesting the existence of pi contacts in solution. DFT/TD-DFT studies showed that O-H...C pi contacts with the solvent stabilizes the excited state and makes the S-1 -> S-0 emission to be a charge transfer transition, quenching the fluorescence of Zn(II)idpa, and possibly Cd(II)idpa as well. The existence of pi contacts in solution was also reflected in the fact that the Zn-II and Cd-II complexes show increased fluorescence intensity when titrated with Cl-, which could be due to the disruption of fluorescence-quenching O-H...C pi contacts with solvent molecules. The observed fluorescence responses of Zn-II and Cd-II complexes to Cl- render these complexes to be used as a possible sensor for Cl-.