Syntheses, crystal structures and steady state and time-resolved fluorescence properties of a PET based macrocycle and its dinuclear Zn/Cd/Hg complexes.

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 HL (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 d metal ion (Zn, Cd and Hg) complexes having compositions [ZnLCl]·2CHOH (1), [CdL(μ-HO)(HO)](NO)·4HO (2) and [HgLCl] (3). HL is colorless and weakly fluorescent, Cd compound 2 is colorless and more weakly fluorescent, Zn compound 1 is colorless and highly fluorescent and Hg compound 3 is yellow and nonemissive. The order of the radiative rate constant (K), obtained from time-resolved studies, corroborates the order of quantum yields, obtained from steady state studies: 1 (φ = 0.12; K = 116.6 × 10 s) ≫ HL (φ = 0.005; K = 7.726 × 10 s) > Cd compound 2 (φ = 0.0014; K = 0.675 × 10 s). Spectrophotometric and steady state/time-resolved spectrofluorimetric titrations of HL with Zn, Cd and Hg have been performed. The relative fluorescence efficiencies of HL, HL + various single metal ions (Zn, Cd, Hg, Cu, Ni, Co, Fe, Mn) and HL + Zn + 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, HL and 1 that HL 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 HL-Hg solution in the presence of various diamagnetic metal ions has been explored. Significant aspects of structures and spectroscopic properties have been discussed.