title: Spectroscopic and structural investigations reveal the signaling mechanism of a luminescent molybdate sensor.
abstract: A heteroditopic ligand H(2)-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me(2)-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H(3)-L(+), H(2)-L, and H-L(-), showed distinct (1)H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re(bpy)(CO)(3)(H(2)-L)](+). It was found that upon deprotonation of the 2-hydroxy group of H(2)-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)→bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dπ)→bpy (3)MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO)(3)(H(2)-L)](+) with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO(2){Re(CO)(3)(bpy)(L)}(2)], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO(2) center.
authors.affiliation: Department of Chemistry, University of York, York, UK.
authors.forename: Vincent A
authors.initials: VA
authors.lastname: Corden
authors.forename: Anne-K
authors.initials: AK
authors.lastname: Duhme-Klair
authors.forename: Sarah
authors.initials: S
authors.lastname: Hostachy
authors.forename: Robin N
authors.initials: RN
authors.lastname: Perutz
authors.forename: Nicole
authors.initials: N
authors.lastname: Reddig
authors.forename: Hans-Christian
authors.initials: HC
authors.lastname: Becker
authors.forename: Leif
authors.initials: L
authors.lastname: Hammarström