Temperature dependence of excited-state electron-transfer reactions. Quenching of RuL32+ emission by copper(II) and europium(III) in aqueous solution

Abstract

The mechanism of electron-transfer reactions of charge-transfer excited states of ruthenium(I1) complexes containing 2,2'-bipyridine, 1,lO-phenanthroline, and their Substituted derivatives with aquo complexes of copper(I1) and europium(II1) has been investigated by using temperature studies in the range 290-350 K. The europium reactions show the expected increase in rate constant as the overall free-energy change for the reaction (-AGO) increases. The copper sequence shows behavior different from that predicted by Marcus' theory, with rate constants well below the diffusion-controlled limit yet only weakly dependent on AGO. Indeed, increasing (-AGO) is shown to result in increases in the activation energy of the reactions. Several alternative explanations for this behavior are available, many of which can be dismissed in the light of this work. It is suggested that the rate-determining step in the Cu(1I) sequence involves a configuration change in the encounter pair. A similar step may be present in the Eu(II1) series but it must not be rate determining. One model suggests that this step is associated with the necessary distorted-octahedral/tetrahedral geometry change in the reduction of Cu(I1).The temperature dependence of the electron-transfer quenching of *Ru(4,7-Me2phen)t+ by copper(I1) in DzO solvent is also included in the present work in order to test one of the proposed models.

Analysis and improvement of rate constant determination of reactions involving charged reactants

“…As was observed for copper(II) nitrate and perchlorate, the application of the Debye-Hu¨ckel extended law did not result in a straight line. Our quenching rate constant is lower than any previously published value, [21][22][23] and the applicable kinetic rationale was previously discussed by Hoffman et al 1 Namely, the equation…”

Fractional dependence of the free energy of activation on the driving force of charge transfer in the quenching of the excited states of substituted phenanthroline homoleptic ruthenium(ii) complexes in aqueous medium

“…The homoleptic set of ruthenium complexes under investigations were synthesized and characterized as in literature. [74][75][76] Structure of the ligands are as shown in Scheme 1.…”

Preparation and characterisation of 2,2′-bipyridine-4,4′-disulphonic and -5-sulphonic acids and their ruthenium(II) complexes. Excited-state properties and excited-state electron-transfer reactions of ruthenium(II) complexes containing 2,2′-bipyridine-4,4′-disulphonic acid or 2,2′-bipyridine-4,4′ dicarboxylic acid

“…Calc. for C,,H,,N,S,: C, 60.85; H, 5.85; N, 10.15%), mass spectrum m/z (relative intensity) 278 (9), 277 (23), 276 ( M , 85), 261 (15), 248 (loo), 243 (30), 219 (1 I), 216 (36); 'H n.m.r., 6 (CDCI,, 60 MHz) 1.40 (6 H, t, J = 8), 3.00 (4 H, 9, J = 8), 6.95 (2 H, dd, J = 1.5,5.0), 8.05 (2 H, d, J = 1.5), 8.25 (2 H, d, J = 5 Hz). Disodium 2,2'-Bipyridine-4,4'-disulphonate.--This is a fivestep synthesis from bipy, giving a yield of cu.…”