The Localized-to-Delocalized Transition in Mixed-Valence Chemistry

Demadis, Konstantinos D.; Hartshorn, Chris M.; Meyer, Thomas J.

doi:10.1021/cr990413m

Cited by 1,017 publications

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“…The distance r is taken to be 9.5 Å , 16d however this value corresponds to the centre-to-centre distance and does not 25 take into account the reduction in electron transfer distance due to strong mixing between metal orbitals and BL-based orbitals. 45 Hence the calculated values for a 2 and H ab represent the lower limit of the actual value. Furthermore, for 4, the value of Dn 1/2 determined by direct measurement of the IT band is larger than 30 the theoretical value (eqn (5)) confirming that the system is best described as type II (valence localised).…”

Section: Interaction Parameters In Mixed Valence Statesmentioning

confidence: 98%

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Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

et al. 2009

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A series of binuclear ruthenium and osmium complexes [(bipy) (5) and [(bipy) 2 Os(bpbt)Os(bipy) 2 ] 2+ (6) {bipy = 2,2¢-bipyridyl; qpy = 2,2¢:5¢,5¢¢:2¢¢,2¢¢¢-quaterpyridyl; pytr-bipy = 3-(2,2¢-bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato, and bpbt = 5,5¢-bis-(pyrid-2¢¢-yl)-3,3¢-bis-1,2,4-triazolato} are reported. Analysis of the electrochemical data focuses on structural factors and on determining the extent of electronic communication between the metal centres in the mixed valence oxidation state. Intervalence charge transfer (IT) bands could be identified in the spectra of the complexes 4 and 6 only. Analysis of their spectroelectrochemical data leads to the conclusion that the IT is superexchange mediated through the HOMO of the bridging ligand. IntroductionMetal complexes based on polypyridyl ligands constitute versatile components for the construction of multifunctional supramolecular systems in molecular photonics and molecular electronics, 1 sensors, 2 photocatalysis, 3 solar energy 5 conversion, 4 artificial photosynthesis, 5 non-linear optics, 6 and electrochemoluminescence 7 amongst others. 8, 9 The myriad of applications are founded on the well defined chemistry of these compounds and take advantage of the versatile photophysical, 10 photochemical 11 and electrochemical 12 properties that are associ-10 ated with the metal-to-ligand charge-transfer (MLCT) excitation in these metal complexes. 13 Amongst the many multinuclear polypyridyl complexes reported, 14 an interesting subclass is represented by those binuclear systems that display additional spectroelectrochemical features 15 due to the electronic interaction between the two metal centres in the mixed valence redox state(s). 15,16 "Communication" between metal centres where direct overlap of atomic orbitals is not possible is achieved when the bridging ligand (BL) has a system of polarisable electrons that are capable of mediating 20 the redistribution of the electronic charge between the two metal centres. Sciences, Dublin City University, Dublin 9, Analysis of these absorption bands 25 can provide detailed information as to the nature and extent of the interaction between the metals centres. 30 Recently, we reported the synthesis and characterisation of a series of homo-and hetero-binuclear ruthenium(II) and osmium(II) complexes of the general formula [M 1 (bipy) 2 -BL-M 2 (bipy) 2 ], where M = Ru or Os, bipy is 2,2¢-bipyridyl (bipy), and BL the bridging ligands 2,2¢:5¢,5¢¢:2¢¢,2¢¢¢-quaterpyridyl (qpy), 3-(2,2¢-35 bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato (pytr-bipy), and 5,5¢-bis-(pyrid-2¢¢-yl)-3,3¢-bis-1,2,4-triazolato (bpbt). 26 In the present contribution, we describe in detail the electrochemical and spectroelectrochemical properties of these compounds. The goal of the present study is to evaluate the extent 40 of electronic communication between the two metal centres as a function of the electronic properties of the bridging ligand and the positioning of the two metal centres. In particular, attention is directed towards the hom...

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Section: Interaction Parameters In Mixed Valence Statesmentioning

confidence: 98%

“…This interaction is small as indicated by the low intensity and broadness of the putative IT band. 45 The low intensity of the proposed IT band renders the evaluation of the 45 interaction parameters difficult (vide infra). 17,20-23…”

Section: Spectroelectrochemistrymentioning

confidence: 99%

Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

et al. 2009

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“…Molecular vibrations and solvent modes are coupled which have different time scales and coupling mechanisms. Different measurements can appear to give different results (Brown 1980;Prassides 1991;Demadis et al 2001;Brunschwig et al 2002;Crutchley 2005;D'Alessandro & Keene 2006a,b).…”

Section: Introductionmentioning

confidence: 99%

Probing the localized-to-delocalized transition

Concepcion

Dattelbaum

Meyer

et al. 2007

Phil. Trans. R. Soc. A.

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Detailed understanding of the transition between localized and delocalized behaviour in mixed valence compounds has been elusive as evidenced by many interpretations of the Creutz-Taube ion, [(NH 3 ) 5 Ru(pz)Ru(NH 3 ) 5 ] 5C . In a review in 2001, experimental protocols and a systematic model to probe this region were proposed and applied to examples in the literature. The model included: (i) multiple orbital interactions in ligand-bridged transition metal complexes, (ii) inclusion of spin-orbit coupling which, for dp 5 -dp 6 complexes, leads to five low-energy bands, two from interconfigurational (dp/dp) transitions at the dp 5 site and three from intervalence transfer transitions, (iii) differences in time scale between coupled vibrations and solvent modes which can result in solvent averaging with continued electronic asymmetry defining 'class II-III', an addition to the Robin-Day classification scheme, and (iv) delineation of coupled vibrations into barrier vibrations and 'spectator' vibrations. The latter provide direct insight into localization or delocalization and time scales for electron transfer. In this paper, the earlier model is applied to a series of mixed-valence molecules.

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“…As a starting point for the resolution of the apparent paradoxical behaviour of the Creutz-Taube ion, Meyer has highlighted three features that must play some role in the final resolution: the electronic structure must be neither strongly localized nor delocalized but rather fall into an intermediate class dominated by Born-Oppenheimer breakdown; the importance of time scale; and the influence of the molecular environment (Demadis et al 2001). While the Creutz-Taube ion and inorganic charge-transfer complexes, in general, are special in terms of the large distance through which the electron can move, problems of localization versus delocalization are ubiquitous in chemistry, with classic examples including delocalization in benzene versus localization in ammonia: benzene has a delocalized D 6h structure rather than a localized D 3h cyclohexatriene structure, while ammonia is pyramidal rather than planar.…”

Section: Introductionmentioning

confidence: 99%

Towards a comprehensive model for the electronic and vibrational structure of the Creutz–Taube ion

Reimers

Wallace

Hush

2007

Phil. Trans. R. Soc. A.

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Since the synthesis of the Creutz-Taube ion, the nature of its charge localization has been of immense scientific interest, this molecule providing a model system for the understanding of the operation of biological photosynthetic and electron-transfer processes. However, recent work has shown that its nature remains an open question. Many systems of this type, including photosynthetic reaction centres, are of current research interest, and thereby the Creutz-Taube ion provides an important chemical paradigm: the key point of interest is the details of how such molecules behave. We lay the groundwork for the construction of a comprehensive model for its chemical and spectroscopic properties. Advances are described in some of the required areas including: simulation of electronic absorption spectra; quantitative depiction of the large interaction of the ion's electronic description with solvent motions; and the physics of Ru-NH 3 spectator-mode vibrations. We show that details of the solvent electron-phonon coupling are critical in the interpretation of the spectator-mode vibrations, as these strongly mix with solvent motions when 0.75!2J/l!1. In this regime, a double-well potential exists which does not support localized zero-point vibration, and many observed properties of the Creutz-Taube ion are shown to be consistent with the hypothesis that the ion has this character.

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The Localized-to-Delocalized Transition in Mixed-Valence Chemistry

Cited by 1,017 publications

References 296 publications

Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

Probing the localized-to-delocalized transition

Towards a comprehensive model for the electronic and vibrational structure of the Creutz–Taube ion

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