Die physikalische Bedeutung dieser Beobachtungen wird im Rah- men eines ... First publ. in: Zeitschrift für Physikalische Chemie, N. F.169 (1990), pp. 147-158.
First publ. in: Zeitschrift für Physikalische Chemie, N. F.169 (1990), pp. 147-158
Aspects of Ligand and Electron-Acceptor Dependence of Magnetic Field Effects 011 Net Electron Transfer Efficiencies in Photooxidation of Ru(II)-trisbipyridyl Type Complexes By Hans-Joachim Wolff and Ulrich E. Steiner Fakultiit fUr Chemie, Universitiit Konstanz, D-7750 Konstanz, Federal Republic of Germany (Received October 31,1990)
Photo electron transfer / Magnetic field effects / Spin-orbit coupling The heteroleptic series of Ru(bpY)n(dce)~:::n, with bpy = 2,2'-bipyridine, and dce = 4,4'diethoxycarbonyl-2,2'-bipyridine, has been investigated for the kinetic parameters of photo electron transfer, and for the magnetic field dependence of the net electron transfer efficiency (rIce) in photooxidations with methylviologen and propylviologen sulfonate (the latter only in the case n = 3) using ns-laser-flash spectroscopy and stationary illumination methods. Whereas the electric charge of the electron acceptor had no influence on the magnetic field effect on !Jceo the variation of ligands was found to result in a strictly linear (with n) increase of the magnetic field effect on kbet , the effective rate constant of backward electron transfer in the primary redox pair. The physical implications of these observations with respect to a previously developed magnetokinetic model are discussed. Die heteroleptische Reihe der Komplexe Ru(bpY)n(dceH:::n mit bpy = 2,2'-bipyridin. und dce = 4,4'-diethoxycarbonyl-2.2'-bipyridin, wurde im Hinblick auf die kinetischen Parameter der photochemischen Elektroneniibertragung und auf die Magnetfeldabhiingigkeit der Nettoausbeute der Elektroneniibertragung (tTce) fUr die Photoreaktion mit Methylviologen und Propylviologensulfonat (letzteres nur fUr n = 3) mittels ns-Laserblitzspektroskopie und kontinuierlicher Belichtungsmethoden untersucht. Wiihrend die Ladung des Elektronenakzeptors keinen EinfluJ3 auf den Magnetfeldeffekt auf !Jce hatte. bewirkte die Variation der Liganden eine streng lineare Zunahme (beziiglich n) des Magnetfeldeffektes auf kbelo die effektive Geschwindigkeitskonstante der Elektronen-Riickiibertragung im primiiren Redox-Paar. Die physikalische Bedeutung dieser Beobachtungen wird im Rahmen eines kiirzlich entwickelten magnetokinetischen Modells diskutiert.
1. Introduction The field of magnetokinetics, dealing with external and internal magnetic field effects (MFEs) on the kinetics of chemical reactions, has received
Konstanzer Online-Publikations-System (KOPS) URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-73868 URL: http://kops.ub.uni-konstanz.de/volltexte/2009/7386
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growing interest in recent years [1- 3]. Magnetokinetic effects, such as ClDNP, ClDEP, RYDMR, MARY, can provide powerful methods for gaining information on reaction mechanisms as a whole (only reactions in which unpaired electron spins are involved show the mentioned effects) as well as for the evaluation of rate constants of processes that cannot be observed directly, e.g. because they are too fast for usual detection methods. A particularly interesting field of magnetokinetic applications are photoinduced electron transfer reactions. Usually there are several reaction channels available for the decay of a primary photochemical electron transfer product, one of them being fast backward electron transfer to the singlet ground state educts, a process that is spin-forbidden if the forward reaction occurs between a triplet excited donor or acceptor and a diamagnetic substrate. The magnetokinetic effects that will be dealt with in this paper involve spin-orbit coupling (SOC) as the mechanism that is responsible for, in our case, triplet to singlet transitions in the primary redox pair of photochemically generated Ru(IIl)-trisbipyridyl type complexes and methylviologen radicals (MY+;), systems well known from model studies of photochemical water cleavage [4 - 9]. 15C
The reaction mechanism for oxidative quenching of Ru(II) complexes is represented in Scheme 1. The excited Ru(bpYH- (or derivative) complex undergoes fast intersystem crossing (ISC) into the lowest excited triplet state that is quenched by My2+ (or derivative) with the rate constant k q , yielding the primary redox pair 3FRu(bpy)~ + ... lMY+;] furtheron denoted RP. This pair can escape from the solvent cage (rate constant k ee ) to form free radicals, or can undergo backward electron transfer (K bet ) to the ground state educts, the latter reaction being, in principle. spin-forbidden. Due to the strong SOC at the Ru center neither the excited triplet charge transfer state nor the RP (both with d 5 configuration at the Ru-center) are of pure triplet character, a fact that is indicated by the symbol 3'. The singlet contamination and a pronounced SOC-induced spin relaxation process providing triplet -+ singlet transitions are responsible for quite efficient backward electron transfer in the RP, lowering the efficiency of net electron transfer Y/ee (to be consistent with the literature, the subscript 'ce' will be used, denoting this quantity as an efficiency of cage escape from the primary
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Magnetic Field Effects on Net Electron Transfer Efficiencies
RP through which all the quenching reaction is assumed to proceed) to about 25% in the case of the Ru(bpyH +/My 2 + system. As we reported previously [10] application of an external magnetic field decreases the yield of free radicals, but not the values of k o (for the monomolecular deactivation process) and k q , from which one has to conclude that it is IJce that is decreased, and the MFE must be clearly due to changes in the spin and reaction dynamics of the primary RP. According to the sign of the MFE, the effective rate of backward electron transfer (Kbet) must be concluded to increase in a magnetic field. For a detailed theoretical treatment of the mechanisms of spin relaxation and MFE the reader is referred to the paper by Steiner and BiirBner in this issue [11]. Here we will report some experimental results which characterize the way how chemical modifications of the species constituting the RP influence IJce and its magnetic field dependence. These experiments have been carried out as part of a program set up to provide a broad empirical basis for this novel magneto kinetic situation, involving strongly SOC coupled transition metal complexes.
\' 0-0
H3C-
_ \
'I
\@
_ N-CH 3
bpy
dee In the series of complexes investigated the bpy ligands of the Ru complex were replaced in a stepwise fashion by 4,4'-diethoxycarbonyl substituted bpy (dee), affording the complete series Ru(bpyMdceH~n with n = 0-3. On the acceptor side methylviologen (My2+) has been used in general. In the reaction with the parent Ru(bpyH+ complex propylviologen sulfonate (PYS) has been also used, in order to study the effect of charge variation. Although the present results have not yet been fully rationalized on the basis of our mechanistic model [11], they are remarkable in that they exhibit some regular empirical features of the chemical structure dependence of the magnetokinetic effect very clearly.
2. Experimental 4,4'-diethoxycarbonyl-2,2'-bipyridine (dce) was prepared in a 3-step synthesis: the 4,4'dimethyl-2,2'-bipyridine according to Sprintschnik et al. [12], the oxidation to the
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Hans-Joachim Wolff and Ulrich E. Steiner
dicarboxylic acid according to Elliott et at. [13], and the esterification according to Sprintschnik et at. [12]. The product was recrystallized from ethylacetate. The homoleptic Ru complexes Ru(bpyh(PF 6 h and Ru(dceh(PF 6 h were synthesized following a procedure by G. Furrer [14]. 2,2'-bipyridine (bpy) was purchased from Fluka (>98%). The heteroleptic complexes Ru(bpyh(dce)(PF 6 h and Ru(bpy)(dceh(PF 6 h were prepared according to Ref. [12] via Ru(bpYhCl 2 and Ru(dcehCI 2 , respectively. Propylviologen sulfonate (PVS) was obtained following the procedure of Nagamura et at. [15]. The compounds were characterized by elemental analysis, UV/VIS and NMR spectroscopy. Methylviologen dichloride trihydrate (MV 2 +) was purchased from Aldrich and used without further purification, but the purity was checked by elemental analysis and UV spectroscopy. The disodium salt of ethylene-diamine tetraacetic acid (EDTA, dihydrate, Merck, p.a.) was recrystallized from methanol/water as described by Blaedel et at. [16]. All measurements were carried out in water (distilled)/acetonitrile (Merck, p.a.) 1/1 (v/v) at room temperature; the solutions were deaerated by purging with solvent saturated supra pure N 2. The concentration of the Ru complexes was 2 x 10- 5 or 4 x 10 - 5 M, of the acceptor 5 x 10 - 3 to 0.1 M, depending on the k q value. The ionic strength, j1., of the solutions was adjusted to 0.2 M by adding suitable amounts of NaCI. except in the case of Ru(dce)~ + were j1. was 0.3 M, because of the high MV" + concentration required. For time-resolved experiments (determination of k o , k q , I)eo, MFE on I)eo) the ns-Iaserflash equipment described in Ref. [17] has been used. With Coumarin 2 as laser dye the Ru complexes were excited at 455 nm. The efficiency of net electron transfer, 1)«0 was determined by the saturation method. detecting the generated MV +. absorption at 600 nm as a function of the laser energy and extrapolating to saturation (LJA~a~~ra'ion) [18]. 'lee
=
II)] x 8MV 600 'I q x [R· U\ +•
(1)
0
The value of 'Ice was obtained according to Eq. (1) with '1q the quenching efficiency, kq[QJI + kq[QJ), [Ru(II)]o the total concentration of the Ru complex, and 8~
