Tuning the Emission Properties of Fluorescent Ligands by Changing pH: The Unusual Case of an Acridine-Containing Polyamine Macrocycle

Synthesis and characterization of a new macrocyclic compound, composed by a triethylentetraamine chain linking the 4 and 5 positions of an acridine moiety, are reported. The molecule, devised as a fluorescent chemosensor for anions, has revealed an intriguing pH-dependent spectroscopic behavior, whose features are the specific object of this article. Ligand protonation in aqueous solution has been analyzed by means of potentiometric, 1H NMR, UV−vis, and fluorescence emission measurements. The molecule binds up to four protons in the pH range 2−11. Protonation takes place on the aliphatic tetraamine chain, while the acridine nitrogen does not participate to proton binding even at very low pH. Differently from acridine, the UV−vis spectra are almost unaffected by the pH. On the opposite, the emission spectra are strongly pH-dependent. In fact, at low pH values, the spectra show a blue-shifted emission, resembling that of unprotonated acridine, while at slightly acidic and alkaline pH the fluorescence features a red-shifted band similar to that of acridinium cation. This unusual behavior occurs in the mono-, bi-, and triprotonated forms of the compound and is interpreted as due to an excited state proton transfer from an aliphatic ammonium group adjacent to the acridine moiety to the acridine nitrogen. In the fully protonated state, this process is prevented owing to unfavorable molecular arrangements mainly determined by electrostatic repulsions. This interpretation is supported by quantum mechanical calculations as well as molecular dynamics simulations.