Syntheses, crystal structures and steady state and time-resolved fluorescence properties of a PET based macrocycle and its dinuclear ZnII/CdII/HgII complexes

Abstract

The work in the present investigation reports the syntheses, crystal structures, ESI-MS (positive) and steady state and time resolved photophysical properties of a tetraaminodiphenol macrocyclic ligand H₂L (saturated analogue of a tetraiminodiphenol macrocycle obtained on 2 + 2 condensation of 4-ethyl-2,6-diformylphenol and 2,2-dimethyl-1,3-diaminopropane) and its three dinuclear d¹⁰ metal ion (Zn^II, Cd^II and Hg^II) complexes having compositions [Zn^II₂LCl₂]·2CH₃OH (1), [Cd^II₂L(μ-H₂O)(H₂O)₂](NO₃)₂·4H₂O (2) and [Hg^II₂LCl₂] (3). H₂L is colorless and weakly fluorescent, Cd^II₂ compound 2 is colorless and more weakly fluorescent, Zn^II₂ compound 1 is colorless and highly fluorescent and Hg^II₂ compound 3 is yellow and nonemissive. The order of the radiative rate constant (K_r), obtained from time-resolved studies, corroborates the order of quantum yields, obtained from steady state studies: 1 (φ = 0.12; K_r = 116.6 × 10⁶ s⁻¹) ≫ H₂L (φ = 0.005; K_r = 7.726 × 10⁶ s⁻¹) > Cd^II₂ compound 2 (φ = 0.0014; K_r = 0.675 × 10⁶ s⁻¹). Spectrophotometric and steady state/time-resolved spectrofluorimetric titrations of H₂L with Zn^II, Cd^II and Hg^II have been performed. The relative fluorescence efficiencies of H₂L, H₂L + various single metal ions (Zn^II, Cd^II, Hg^II, Cu^II, Ni^II, Co^II, Fe^II, Mn^II) and H₂L + Zn^II + another metal ion (metal ion competition study) have been studied. Relative fluorescence efficiency can be well rationalized in terms of photoinduced electron transfer (PET), perturbation of PET by metal coordination (chelation enhanced fluorescence, CHEF), heavy metal ion effect (spin–orbit coupling) and fluorescence quenching mechanism. It has been established from comparative fluorescence spectra of 4-ethylphenol, H₂L and 1 that H₂L can be reasonably considered as a PET species in which the fluorophore is 4-ethylphenol. The binding constants, as determined from titration experiments, qualitatively corroborate the results of metal ion competition studies. The modulation of the absorption spectra and color of the H₂L–Hg^II solution in the presence of various diamagnetic metal ions has been explored. Significant aspects of structures and spectroscopic properties have been discussed.