Magnetokinetic Probing of Extremely Fast Electron Spin Relaxation in Paramagnetic Ruthenium Complexes*

Abstract

Magnetic field effects I Paramagnetic relaxation I Magnetokinetics I Photo electron transfer I Spin chemistryA regularity previously observed in the ligand dependence of the magnetokinetic effect on the photoelectron transfer efficiency between photoexcited bileptic Ru-trisdiimine complexes and methylviologen has been substantiated by investigating, in addition, the bipyridine/phenanthroline series of ligands. Our theoretical model of the mechanism has been improved by taking into account non-rotational mechanisms of spin relaxation in the photooxidized Ru 3 + -complex. The field dependences observed for the two bileptic series of complexes can be theoretically reproduced in a very consistent way. Paramagnetic relaxation times found to range around 10 -11 s, reveal a striking regularity in the course of ligand variation through the bileptic series.Eine kiirzlich beobachtete RegelmiiJ3igkeit in der Ligandenabhiingigkeit des magnetokinetischen Effekts auf die Effizienz des Photoelektronentransfers zwischen photoangeregten bileptischen Ru-trisdiimin Komplexen und Methylviologen wurde an einem weiteren Beispiel, Komplexen mit der Bipyridin/Phenanthrolin Ligandenreihe, bestiitigt. Unser Modell zur theoretischen Beschreibung der Magnetfeldabhiingigkeit wurde verbessert, indem es nun auch rotationsunabhiingige Mechanismen der Spinrelaxation im photooxidierten Ru 3 + -Komplex beriicksichtigt. Die fUr die beiden bileptischen Komplexserien beobachteten Feldabhiingigkeiten k6nnen theoretisch in sehr konsistenter Weise reproduziert werden. Die paramagnetischen Relaxationszeiten, die sich dabei ergeben, liegen urn 10 -11 S und weisen einen sehr regelmiiJ3igen Gang mit der Ligandenvariation innerhalb der bileptischen Reihen auf.

Ligand dependence of magnetic spin effects on photooxidation of [Ru(bpy)3−n(CN)2n](+2−2n) type complexes

“…The full kinetics of the geminate RP is properly described by a Stochastic Liouville Equation, from the numerical solution of which the MFE on h ce can be predicted [29,31,33]. The parameters of the kinetic model are the g tensor values of the radical species and the three rate parameters k ce , k bet and t S .…”

Temperature-Dependent Spin Relaxation: A Major Factor in Electron Backward Transfer Following the Quenching of *Ru(bpy)32+ by Methyl Viologen

“…After the first experimental effects of this kind were discovered 8,9 in the photoinduced electron transfer reaction of [Ru(bpy) 3 ] 2+ with methyl viologen (MV 2+ ), our laboratory has systematically explored the effect experimentally [10][11][12][13] and theoretically. 14,15 The connection between spin processes and chemical kinetics in the Ru(bpy) 3 2+ /MV 2+ system is represented by Scheme 1.…”

Spin-chemical effects on intramolecular photoinduced charge transfer reactions in bisphenanthroline copper(i)-viologen dyad assemblies

“…It is well known that such a situation causes very short, magnetic-eld independent, spin relaxation times, due to the so-called Orbach mechanism, operating through thermal excitation of the lowest electronically excited state. 29,92,93 Furthermore, the g-tensor components deviate strongly from the free electron value. For the Cu(II) complexes investigated here, a lowest excitation energy of ca.…”

“…5,23,25,27 The authors le open the question of whether spin restriction has an effect in this regard. On the other hand, the signicance of spin effects has been demonstrated and extensively studied in our laboratories for unlinked [Ru(bpy) 3 ] 2+ /MV 2+ dyad systems, [28][29][30][31][32] as well as for linked phenazine/[Ru(phen) 3 ] 2+ /MV 2+ triads, where an electron donor is appended to achieve multistep electron transfer. [33][34][35] In both, Ru-and Cu-based dyads, the primary events aer photoexcitation may be represented by Scheme 1.…”

“…The major factors determining the magnetic eld dependence of T/S conversion kinetics in Ru-based systems are (1) fast electron spin-relaxation and (2) different Zeeman interactions of the Ru(III) complex and MV + radical resulting from their distinct g-factors. Theoretical analysis of the magnetic eld dependence on the CT lifetime in unlinked C-A dyads of [RuL 3 ] 2+ /MV 2+ yielded specic kinetic parameters of the C + .A À state; [28][29][30][31][32] particularly, the rate constant of spin-allowed backward electron transfer and spin relaxation. Although these studies dealt with unlinked C-A dyads, the magnetic eld dependence of the spin-conversion process should, in principle, also apply to the linked systems where the rate constant of dissociation is zero.…”