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

Excited
state proton transfer studies of six Ru polypyridyl compounds
with carboxylic acid/carboxylate group(s) revealed that some were
photoacids and some were photobases. The compounds [RuII(btfmb)2(LL)]2+, [RuII(dtb)2(LL)]2+, and [RuII(bpy)2(LL)]2+, where bpy is 2,2′-bipyridine, btfmb is 4,4′-(CF3)2-bpy, and dtb is 4,4′-((CH3)3C)2-bpy, and LL is either dcb = 4,4′-(CO2H)2-bpy or mcb = 4-(CO2H),4′-(CO2Et)-2,2′-bpy, were synthesized and characterized. The
compounds exhibited intense metal-to-ligand charge-transfer (MLCT)
absorption bands in the visible region and room temperature photoluminescence
(PL) with long τ > 100 ns excited state lifetimes. The mcb
compounds
had very similar ground state pKa’s
of 2.31 ± 0.07, and their characterization enabled accurate determination
of the two pKa values for the commonly
utilized dcb ligand, pKa1 = 2.1 ±
0.1 and pKa2 = 3.0 ± 0.2. Compounds
with the btfmb ligand were photoacidic, and the other compounds were
photobasic. Transient absorption spectra indicated that btfmb compounds
displayed a [RuIII(btfmb–)­L2]2+* localized excited state and a [RuIII(dcb–)­L2]2+* formulation for all the
other excited states. Time dependent PL spectral shifts provided the
first kinetic data for excited state proton transfer in a transition
metal compound. PL titrations, thermochemical cycles, and kinetic
analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable
group photobasic or photoacidic through ligand design
was unprecedented and was understood based on the orientation of the
lowest-lying MLCT excited state dipole relative to the ligand that
contained the carboxylic acid group(s).