Indole-based fluorescence sensors for both cations and anions

Abstract

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 π contacts in quenching fluorescence. In particular, the π 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 AgI complex of idpa is a dimer held together by very short inter-complex η1 Ag···C π contacts of 2.382 Å, indicating powerful π contacting ability of indole as expected from the electrostatic potential map. However, structural studies of the ZnII, NiII, PbII, and CdII complexes of idpa show no metal-fluorophore π contacts present in the solid state, which was ascribed to a plethora of other π 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 π contacts in solution. DFT/TD-DFT studies showed that O-H···C π contacts with the solvent stabilizes the excited state and makes the S1 → S0 emission to be a charge transfer transition, quenching the fluorescence of ZnIIidpa, and possibly CdIIidpa as well. The existence of π contacts in solution was also reflected in the fact that the ZnII and CdII complexes show increased fluorescence intensity when titrated with Cl−, which could be due to the disruption of fluorescence-quenching O-H···C π contacts with solvent molecules. The observed fluorescence responses of ZnII and CdII complexes to Cl− render these complexes to be used as a possible sensor for Cl−.