A fluorescent chemosensor for Zn(ii). Exciplex formation in solution and the solid state

Abstract

The macrocyclic phenanthrolinophane 2,9-[2,5,8-triaza-5-(N-anthracene-9-methylamino)ethyl]-[9]-1,10-phenanthrolinophane (L) bearing a pendant arm containing a coordinating amine and an anthracene group forms stable complexes with Zn(II), Cd(II) and Hg(II) in solution. Stability constants of these complexes were determined in 0.10 mol dm⁻³ NMe₄Cl H₂O–MeCN (1 ∶ 1, v/v) solution at 298.1 ± 0.1 K by means of potentiometric (pH metric) titration. The fluorescence emission properties of these complexes were studied in this solvent. For the Zn(II) complex, steady-state and time-resolved fluorescence studies were performed in ethanol solution and in the solid state. In solution, intramolecular π-stacking interaction between phenanthroline and anthracene in the ground state and exciplex emission in the excited state were observed. From the temperature dependence of the photostationary ratio (I_Exc/I_M), the activation energy for the exciplex formation (E_a) and the binding energy of the exciplex (−ΔH) were determined. The crystal structure of the [ZnLBr](ClO₄)·H₂O compound was resolved, showing that in the solid state both intra- and inter-molecular π-stacking interactions are present. Such interactions were also evidenced by UV-vis absorption and emission spectra in the solid state. The absorption spectrum of a thin film of the solid complex is red-shifted compared with the solution spectra, whereas its emission spectrum reveals the unique featureless exciplex band, blue shifted compared with the solution. In conjunction with X-ray data the solid-state data was interpreted as being due to a new exciplex where no π-stacking (full overlap of the π-electron cloud of the two chromophores – anthracene and phenanthroline) is observed. L is a fluorescent chemosensor able to signal Zn(II) in presence of Cd(II) and Hg(II), since the last two metal ions do not give rise either to the formation of π-stacking complexes or to exciplex emission in solution.