Syntheses, Structures, and Steady State and Time Resolved Photophysical Properties of a Tetraiminodiphenol Macrocyclic Ligand and Its Dinuclear Zinc(II)/Cadmium(II) Complexes with Coordinating and Noncoordinating Anions

The work in the present investigation reports the syntheses,
structures,
steady state, and time-resolved photophysical properties of a tetraiminodiphenol
macrocyclic ligand H2L and its eight dinuclear zinc­(II)
complexes and one cadmium­(II) complex having composition [Zn2L­(H2O)2]­(ClO4)2·2CH3CN (1), [Zn2L­(H2O)2]­(ClO4)2·2dmf (2),
[Zn2L­(H2O)2]­(NO3)2·2dmf (3), [Zn2LCl2] (4), [Zn2L­(N3)2]
(5), [Zn2L­(NCS)2] (6), [Zn2L­(NCO)2] (7), [Zn2L­(NCSe)2]2·dmf (8), and [Cd2L­(OAc)2] (9) with various coordinating
and noncoordinating anions. The structures of all the complexes 1–9 have been determined by single-crystal
X-ray diffraction. The noncovalent interactions in the complexes result
in the generation of the following topologies: two-dimensional network
in 1, 2, 4, 6, 7, 8, and 9; three-dimensional network
in 5. Spectrophotometric and spectrofluorometric titrations
of the diprotonated salt [H4L]­(ClO4)2 with triethylamine as well as with zinc­(II) acetate and cadmium­(II)
acetate have been carried out, revealing fluorescence enhancement
of the macrocyclic system by the base and the metal ions. Steady state
fluorescence properties of [H4L]­(ClO4)2 and 1–9 have been studied and their
quantum yields have been determined. Time resolved fluorescence behavior
of [H4L]­(ClO4)2 and the dizinc­(II)
and dicadmium­(II) complexes 1–9 have
also been studied, and their lifetimes and radiative and nonradiative
rate constants have been determined. The induced fluorescence enhancement
of the macrocycle by zinc­(II) and cadmium­(II) is in line with the
greater rate of increase of the radiative rate constants in comparison
to the smaller rate of increase of nonradiative rate constants for
the metal complexes. The fluorescence decay profiles of all the systems,
being investigated here, that is, [H4L]­(ClO4)2 and 1–9, follow triexponential
patterns, revealing that at least three conformers/components are
responsible to exhibit the fluorescence decay behavior. The systems
and studies in this report have been compared with those in the reports
of the previously published similar systems, revealing some interesting
aspects.