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Experimental and Computational Exploration of Ground and Excited State Properties of Highly Strained Ruthenium Terpyridine Complexes
journal contribution
posted on 2013-08-01, 00:00 authored by Paul J. Vallett, Niels H. DamrauerDissociative electron transfer reactions
are prevalent in one-electron
reduced aryl halides; however, calculations applied to charge-transfer
excited states of metal complexes suggest that this reaction would
be strongly endergonic unless attention is paid to specific structural
details. In this current study, we explore the effect of introducing
intramolecular strain into a series of halogenated ruthenium(II) polypyridyls.
Parent [Ru(tpy)2]2+ (1) (tpy =
2,2′:6′,2″-terpyridine) is compared with two
complexes, [Ru(6,6″-Br2-tpy)(tpy)]2+ (2) and [Ru(6,6″-Br2-tpy)2]2+ (3) (6,6″-Br2-tpy = 6,6″-dibromo-tpy)
that incorporate interligand van der Waals strain derived from the
large halogen substituents. DFT calculations and the crystal structure
of 3 show how this strain distorts the geometry of 3 as compared to 1. Time-dependent DFT calculations
are used to explain the effect of this strain on electronic absorption
spectra where, in particular, a transition observed in 3 is attenuated in 2 and absent in 1 and
heralds interligand charge transfer mediated by the halogen substituent.
Ultrafast transient absorption spectroscopy reveals coherent vibrational
dynamics particularly in 3 but also in 2 that is interpreted as reflecting heavy-atom motion. Surprisingly,
in spite of the additional strain, the excited-state lifetime of 3 is observed to be approximately a factor of 6 longer than 2. Constrained-DFT calculations show that while the excited
behavior of 2 is similar to 1, the strain-induced
geometric distortions in 3 cause a nesting of excited
state triplet surfaces resulting in a longer excited state lifetime.
This may afford the additional time needed to engage in photochemistry,
and kinetic evidence is observed for the breaking of a C–Br
bond in 3 and formation of a contact ion pair state.
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metal complexescontact ion pair statestate triplet surfacesaryl halides3 causestate lifetimeabsorption spectroscopyExcited State Propertiesintramolecular strainheralds interligand charge transferabsorption spectravibrational dynamicshalogen substituents3 showcrystal structureinterligand van der Waals strainhalogen substituentDFT calculationsComputational ExplorationStrained Ruthenium Terpyridine ComplexesDissociative electron transfer reactions