Photophysical Properties of Ruthenium Bipyridine (4-Carboxylic acid-4‘-methyl-2,2‘-bipyridine) Complexes and Their Acid−Base Chemistry

Abstract

Photophysical properties such as absorption and emission spectra, lifetimes, and redox potentials of eight ruthenium complexes, Ru(LL)2(MebpyCOOH)2+, where LL represents bpy, phen, Me2bpy, Me4bpy, (MeO)2bpy, (EtO)2bpy, Cl2bpy, and NO2phen, have been measured. The acid dissociation constants of ground and excited states have been determined. The ground-state pKa values were obtained from the pH dependence of the complex absorbance changes. The excited-state pKa* values were extracted from the emission titration curve and corrected for the excited-state lifetime of both protonated and deprotonated species. The largest DeltapKa, pKa*-pKa, found for Ru(Me2bpy)2(MebpyCOOH)2+ and Ru(Me4bpy)2(MebpyCOOH)2+ of 1.7 indicate that MebpyCOOH gains most of the MLCT excited-state electron. The big negative DeltapKa found for Ru(Cl2bpy)2(MebpyCOOH)2+, -4.2, clearly indicates the metal-to-ligand charge transfer to the Cl2bpy ligands.

Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s)

“…Compounds with additional carboxylic acid groups, such as [Ru II (btfmb) (dcb) 2 ] 2+ would be expected to give even larger blue shifts in the PL spectra after excited state proton transfer. Similar behavior might have been expected for 4,4′-(Cl) 2 -2,2′-bipyridine compounds that also possess low-lying π* orbitals, however no significant spectral shifts were observed and protonation led to an unexpected decrease in PL intensity suggesting some alternative photochemistry . The bathochromic shift expected for photobases was observed for the dtb and bpy compounds reported here and for a much larger number of previously reported dcb containing transition metal compounds. ,,,− ,,, …”

“…Many literature reports of excited state acid–base chemistry in transition metal compounds have appeared since the first reports by Demas and Peterson in 1976. ,,− All known ruthenium polypyridine compounds with pendant carboxylates − or amines − were found to be photobases. Photoacidic ruthenium compounds are relatively rare and until now appear to be limited to phenol-type or ambidentate cyanide ligands .…”

Improvement of the Dynamic Range of pH Sensing by Using a Luminescent Tricarbonylpolypyridylrhenium(I) Complex with Three Different Protonation Sites

“…The dynamic range in emission quantum yields and lifetimes is near 13 and 5, respectively, making complex 1 a promising luminescent pH sensor of the on - off - on type over a 0−8 pH range. The presence of three protonation sites has thus considerably improved the dynamic range of pH sensing compared to previously reported rhenium and ruthenium complexes. , …”

Luminescent Cyclometalated RhIII, IrIII, and (DIP)2RuII Complexes with Carboxylated Bipyridyl Ligands: Synthesis, X-ray Molecular Structure, and Photophysical Properties

“…For ruthenium(II) polypyridine complexes bearing mono- or polycarboxylate functionalities, results of titration experiments of absorption and emission spectra as performed in water are available that indicate what species are present in solution. , Thus, for the 4-carboxylic acid-4′-methyl-2,2′-bipyridine monocarboxylate species [Ru(phen) 2 (MebpyCOOH)](PF 6 ) 2 , for both the ground and excited states the actual cationic species present in water at pH > 5 is [Ru(phen) 2 (MebpyCOO − )] + given that the ground-state acid dissociation constant p K a is 2.6 and that the excited-state acid dissociation constant p K a * is 4.6 . The less acidic character of the excited state (i.e., the lower ability to dissociate) is a consequence of the MLCT character of the emitting level and of the fact that the involved ligand becomes more electron-rich with respect to the ground state .…”