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The rate constants for the quenching of the excited states of metal ions and complexes in homogeneous fluid solution are reported in this compilation. Values of k_q for dynamic, collisional processes between excited species and quenchers have been critically evaluated, and are presented with the following information, among others, from the original publications, when available: description of the solution medium, temperature at which k_q was determined, experimental method, range of quencher concentration used, lifetime of the excited state in the absence of quencher, activation parameters, quenching mechanism. Data collection is complete through the end of 1986, and covers the coordination compounds of twenty-six metals, including the ions and complexes of the inner- and outer-transition metals, and porphyrin complexes of non-transition metals. Data for 261 excited states quenched by more than 400 inorganic quenchers and 600 organic quenchers have been extracted from almost 500 publications. The introduction to the work contains a discussion of the conceptual background to quenching, including a general treatment of the kinetics, an explanation of the tables, and a list of recent review articles. Uncommon kinetics mechanisms and equations, used to obtain the reported values of k_q, are discussed in detail as part of the notes to the printed tables.

Arrangement of the Data

For the purpose of browsing, as in the published data tables, the data are divided into twenty-nine sets which are ordered alphabetically according to the name of the metal of the substrate. In the case of ruthenium, which constitutes a large fraction of the total, the entries are subdivided further to display separately Ru(bpy)₃²⁺ and inorganic quenchers, Ru(bpy)₃²⁺ and organic quenchers except methylviologen (N,N'-dimethyl-4,4'-bipyridinium dication; MV²⁺), Ru(bpy)₃²⁺ and MV²⁺, and all other complexes of ruthenium. Entries in the browsing index are linked to individual data pages by a number similar to the table numbers in the published compilation.

Excited States

Within each set, the various excited states with the same metal are ordered alphabetically according to the letters in the formula; some formulas denote the exact molecular composition while others incorporate names or abbreviations of the ligands. Singlet states and triplet states of the same molecule are also separated.

Quenchers

For any given excited state, the quenchers are divided into two groups: "inorganic" (including metal complexes) and "organic". Within each group, they are further ordered alphanumerically.

Data Pages

The data pages are headed with the name of the excited state and quencher which is repeated with each reported value for the rate constant along with the conditions of the measurement. For any given page, the order of presentation of the data is alphabetical according to the name, formula, or abbreviation of the solvent. For mixed solvent systems, the more abundant solvent is listed first; their ratio (v/v) is given in parentheses. When the ratio is 1:1 and H₂O is one of the solvents, it is presented first because of the importance of that substance.

Within any given solvent or solvent mixture, the ordering is by ascending values of the temperature, with the explicit value first and the approximate value second. The lowest level of ordering is given to those systems for which no value of the temperature is entered.

Within any temperature group, the ordering is determined by the absence or presence of additional solutes, and by the degree of detail to which the solution medium is known. Systems that consist of the substrate and the quencher as the only solutes are presented first with the "Solution Medium" column blank; thereafter, pH and ionic strength (in units of mol/L; salt used to control ionic strength in parentheses) conditions are given in ascending order, followed by buffers and other solutes in ascending values of their concentrations. Within this hierarchy of ordering, we have attempted to group data obtained in similar media close together.

When more than one additional solute is used in the experiments, we have adopted a specific protocol for the designation of the components, depending to the degree to which the information is specifed in the original paper. For example, if both LiCl and NaCl were present, the designation "LiCl and NaCl" is used. However, if independent measurements were made on solutions containing LiCl and NaCl separately which gave the same value of k_q, "LiCl, NaCl" is shown. If, on the other hand, experiments were performed with either LiCl or NaCl, but the actual contents are not specified, "LiCl or NaCl" is displayed.

Rate Constants

Each reported value of the rate constant for quenching, k_q, begins with the author names and reference to the source of the data followed by the solvent and the conditions and additives.

Values of k_q are given in the units of L mol^–1 s^–1 for the second-order reaction. Values of k_cor are reported here, and are labeled as "(corr)". For values of k_q calculated by us from K_SV and tau₀ values, or k₀, k_d, k_–d, and k_de values appearing in the same paper, the indicator "(calc)" is appended.

Comments

The Comments contain a wealth of information in essentially this order: the quenching mechanism (if uncertain, "?" is added); the experimental method used in the evaluation of k_q; the lifetime of *S in the absence of quencher (tau₀ in the absence of air, tau₀^air in air-equilibrated solutions, tau₀^und in solutions of unknown O₂ content); the experimental method used for the determination of the lifetime in the absence of quencher, or the literature reference for the value used; the range of [Q] used over which the value of k_q was determined, and, presumably, is valid; activation parameters; acid-base properties of *S; references to duplicate or alternative entries; other relevant information. Values of f, the fraction of the quenching acts that yield electron-transfer products in the bulk solution, are reported here; when eta* < 1, the value of eta* used in its calculation is also given.

Introduction to Rate Constants for the Quenching of Excited States of Metal Complexes in Fluid Solution.