Electrochemical, Photophysical, and Anion-Binding Properties of a Luminescent Rhenium(I) Polypyridine Anthraquinone Complex with a Thiourea Receptor

We report the synthesis, characterization, and electrochemical and photophysical properties
of a new luminescent rhenium(I) polypyridine-thiourea-anthraquinone complex, [Re(Me4-phen)(CO)3(Py-An)](CF3SO3) (1), and its anthraquinone-free analogue [Re(Me4-phen)(CO)3(Py-Ph)](CF3SO3) (2) (Me4-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline; Py-An = N-(1-anthraquinonyl)-N‘-(4-pyridinylmethyl)thiourea; Py-Ph = N-(1-phenyl)-N‘-(4-pyridinylmethyl)thiourea)). The crystal structure of complex 2 has been studied by X-ray crystallography.
Upon irradiation, both complexes 1 and 2 show intense and long-lived green luminescence
in fluid solutions at 298 K and in alcohol glass at 77 K. The emission is assigned to an
excited state of triplet metal-to-ligand charge-transfer 3MLCT (dπ(Re) → π*(Me4-phen))
character. In low-temperature glass, biexponential decays are observed for both complexes
1 and 2, and the longer- and shorter-lived components are assigned to 3IL (π → π*)(Me4-phen) and 3MLCT (dπ(Re) → π*(Me4-phen)) excited states, respectively. The binding of anions
to complexes 1 and 2 has been investigated by electronic absorption and emission titrations.
Of the six anions we have studied (fluoride, acetate, dihydrogen phosphate, iodide, hydrogen
sulfate, and nitrate), complexes 1 and 2 can recognize F-, OAc-, and H2PO4-, with log Ks
values ranging from ca. 3.53 to 4.94. While the anion-binding properties of complex 2 can
only be reflected by its emission spectral changes, both the absorption and emission spectra
of complex 1 display significant changes upon binding of anions, as a result of the additional
anthraquinone unit.