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(Me 4 -phen)(CO) 3 (Py-An)](CF 3 SO 3 ) ( 1 ), and its anthraquinone-free analogue [Re(Me 4 -phen)(CO) 3 (Py-Ph)](CF 3 SO 3 ) ( 2 ) (Me 4 -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 3 MLCT (dπ(Re) → π*(Me 4 -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 3 IL (π → π*)(Me 4 -phen) and 3 MLCT (dπ(Re) → π*(Me 4 -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 H 2 PO 4 -, with log K s 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.