Excited-state acid–base chemistry of coordination complexes

Hicks, Charles H.; Ye, Guozhong; Levi, Chaim; Gonzales, Marlyn; Rutenburg, Irina; Fan, Jainwei; Helmy, Roy; Kassis, Abe; Gafney, Harry D.

doi:10.1016/s0010-8545(00)00279-4

Cited by 44 publications

(46 citation statements)

References 47 publications

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“…40 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. 1,2,27,[29][30][31][32][33][34]40,48,96 Kinetics. When excited states localized on the dcb or mcb ligands were photogenerated at pH values where acid−base equilibria was relevant, time dependent PL spectral shifts were observed indicating that acid−base chemistry was occurring on the nanosecond time scale.…”

Section: Journal Of the American Chemical Societymentioning

confidence: 99%

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

O’Donnell

Sampaio

et al. 2016

J. Am. Chem. Soc.

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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 [Ru(II)(btfmb)2(LL)](2+), [Ru(II)(dtb)2(LL)](2+), and [Ru(II)(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 [Ru(III)(btfmb(-))L2](2+)* localized excited state and a [Ru(III)(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).

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Section: Journal Of the American Chemical Societymentioning

confidence: 99%

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

O’Donnell

Sampaio

et al. 2016

J. Am. Chem. Soc.

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“…A comparison of ground state and excited state acid dissociation constants is useful because the relative change in the pK a provides qualitative information on the localization of charge in MLCT states. [34][35][36][37][38][39][40][41] The results indicate that both pK a1 * and pK a2 * values of the complexes are higher compared to their ground-state pK a values. The higher excited-state pK a values than the ground state for the same process indicate that the MLCT state is principally localized on the phen-Hbzim-tpy moiety (the excited state has increased negative charge and is therefore less acidic).…”

Section: Background Informationmentioning

confidence: 88%

“…The higher excited-state pK a values than the ground state for the same process indicate that the MLCT state is principally localized on the phen-Hbzim-tpy moiety (the excited state has increased negative charge and is therefore less acidic). [34][35][36][37][38][39][40][41] It is quite logical to assume that the two phenanthroline imidazole containing MLCT states are electronically isolated and creation of one of them raises the pK a value and, therefore, the first deprotonation should always be for the other chromophore in its ground state. Moreover, it is quite possible that if one of the two phenanthroline imidazole chromophores is deprotonated, intramolecular energy transfer should populate the other chromophore (with protonated phen-imidazole).…”

Section: Background Informationmentioning

confidence: 99%

pH-Induced processes in wire-like multichromophoric homo- and heterotrimetallic complexes of Fe(ii), Ru(ii), and Os(ii)

et al. 2015

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In this work we studied the influence of pH on the absorption, steady state and time-resolved emission spectroscopic behaviors of recently reported multichromophoric trimetallic complexes of the forms [(bpy)2M(phen-Hbzim-tpy)M'(tpy-Hbzim-phen)M(bpy)2](6+) (M = Ru(II) or Os(II), and M' = Fe(II), Ru(II), and Os(II)) derived from a heteroditopic phenanthroline-terpyridine bridge, 2-(4-(2,6-di(pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5-f][1,10]phenanthroline (tpy-Hbzim-phen) and 2,2'-bipyridine (bpy) as the auxiliary ligand. For purposes of comparison, the UV-vis absorption and emission titrations of three monometallic model compounds [(bpy)2Ru(phen-Hbzim-tpy)](ClO4)2 (1), [(bpy)2Os(phen-Hbzim-tpy)] (ClO4)2 (2) and [(tpy-PhCH3)Ru(tpy-Hbzim-phen)](ClO4)2 (3), where tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine) were studied under the same experimental conditions. The absorption titration data were used to determine the ground state pKa values, whereas the luminescence and lifetime data were utilized for the determination of excited state pKa* values of the complexes. The evolving factor analyses of the set of absorption spectra of the complexes obtained by varying the pH of the solution confirm that only three absorbing species exist in the pH window of 2-12. Moreover, the modulation of the rate of the intramolecular energy transfer among the components in the homo- and heterotrimetallic complexes as a function of pH of the solution was also demonstrated.

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“…14 The redistribution of electron density in MLCT excited states often modulates the pKa values of Ru-based transition metal complexes leading to proton-coupled processes. 15,16 In general, acid/base groups will become either more acidic due to the electronic density depletion at the metal center in the MLCT states, or more basic due to the accumulation of the electronic density at the accepting ligand orbital of the MLCT state. 17 As a consequence, the acid-base groups located on the ligand that is not directly involved in the MLCT state will become more acidic in the excited state.…”

Section: Introductionmentioning

confidence: 99%

Proton-coupled Electron Transfer in a Ruthenium (II) Bipyrimidine Complex in its Ground and Excited Electronic States

Drummer

Weerasooriya

Gupta

et al. 2022

Preprint

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Proton-coupled electron transfer (PCET) was studied for the ground and excited electronic states of a [Ru(terpy)(bpm)(OH2)(PF6)] complex, Ru-bpm. Cyclic voltammetry measurements show that the Ru(II)-aqua moiety undergoes PCET to form Ru(IV)-oxo moiety in the anodic region, while the bpm ligand undergoes PCET to form bpmH2 in the anodic region. The photophysical behavior of Ru-bpm was studied using steady-state and femtosecond transient UV-vis absorption spectroscopy, coupled with density functional theory (DFT) calculations. The lowest-lying excited state of Ru-bpm is described as a (Ru bpm) metal-to-ligand charge transfer (MLCT) state, while the metal-centered (MC) excited state was found computationally to be close in energy to the lowest-energy bright MLCT state (MC state was 0.16 eV above the MLCT state). The excited state kinetics of Ru-bpm were found via transient absorption spectroscopy to be short-lived and were fit well to a biexponential function with lifetimes 1=4 ps and 2=65 ps in aqueous solution. Kinetic isotope effect of 1.75 was observed for both decay components, indicating that the solvent plays an important role in the excited-state dynamics of Ru-bpm. Based on the pH-dependent studies and the results from prior studies of similar Ru-complexes, we hypothesize that the 3MLCT state forms an excited-state hydrogen-bond adduct with the solvent molecules and that this process occurs with the 4 ps lifetime. The formation of such hydrogen-bond complex is consistent with the electronic density accumulation at the peripheral N atoms of the bpm moiety in the 3MLCT state. The hydrogen-bonded state 3MLCT’ decays to the ground state with a 65 ps lifetime. Such short lifetime is likely associated with the efficient vibrational energy transfer from 3MLCT state to the solvent.

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Excited-state acid–base chemistry of coordination complexes

Cited by 44 publications

References 47 publications

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

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

pH-Induced processes in wire-like multichromophoric homo- and heterotrimetallic complexes of Fe(ii), Ru(ii), and Os(ii)

Proton-coupled Electron Transfer in a Ruthenium (II) Bipyrimidine Complex in its Ground and Excited Electronic States

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