Intramolecular Electron Transfer from Mn or Ligand Phenolate to Photochemically Generated RuIII in Multinuclear Ru/Mn Complexes. Laser Flash Photolysis and EPR Studies on Photosystem II Models

Article abstract of DOI:10.1021/ja991402d

In a mononuclear Mn^IV and a trinuclear Mn^II complex, the ligands of which contain electron-rich phenols (coordinated to the Mn('s)) and covalently attached ruthenium(II) 2,2′-trisbipyridyl(=bpy)-type groups, intramolecular electron transfer (ET) from the phenolate ligand (in the mononuclear Mn^IV complex) or from a Mn^II (in the trinuclear Mn^II complex) to the photochemically (λ_exc= 455 nm) generated Ru^III takes place with k ≥ 5 × 10⁷ s^-1, giving rise to the corresponding phenoxyl radical (complexed to Mn^IV) or to Mn^III, respectively. Thus, in the trinuclear Mn^II complex, the source of the electron that reduces the photogenerated Ru^III(bpy^?-) moiety is a Mn^II, in contrast to the situation with the mononuclear Mn^IV complex, where the electron stems from a phenolate. The half-life of the coordinated phenoxyl-type Ru(bpy)/Mn complex (as produced in the presence of [Co^III(NH₃)₅Cl]²⁺) is of the order 0.5-1 ms. The Ru(bpy) compound containing three (phenolate-ligated) Mn^II atoms is the first example of a photochemically induced intramolecular ET from a multinuclear Mn cluster to an attached sensitizer , and the Ru complex containing one (phenolate-ligated) Mn^IV is the first case of an ET from a synthetic Mn^IV-coordinated phenolate to a photochemically produced oxidant (Ru^III).

Full text of DOI:10.1021/ja991402d

J. Am. Chem. Soc. 1999, 121, 10781-10787
10781
Intramolecular Electron Transfer from Mn or Ligand Phenolate to
Photochemically Generated RuIII in Multinuclear Ru/Mn Complexes.
Laser Flash Photolysis and EPR Studies on Photosystem II Models
Dirk Burdinski, Karl Wieghardt,* and Steen Steenken*
Contribution from the Max-Planck-Institut fu¨r Strahlenchemie, D-45413 Mu¨lheim, Germany
ReceiVed April 28, 1999. ReVised Manuscript ReceiVed August 26, 1999
Abstract: In a mononuclear MnIV and a trinuclear MnII complex, the ligands of which contain electron-rich
phenols (coordinated to the Mn(’s)) and covalently attached ruthenium(II) 2,2′-trisbipyridyl()bpy)-type groups,
intramolecular electron transfer (ET) from the phenolate ligand (in the mononuclear MnIV complex) or from
a MnII (in the trinuclear MnII complex) to the photochemically (λexc) 455 nm) generated RuIII takes place
with k g 5 × 107 s-1, giving rise to the corresponding phenoxyl radical (complexed to MnIV) or to MnIII,
respectively. Thus, in the trinuclear MnII complex, the source of the electron that reduces the photogenerated
RuIII(bpy•-) moiety is a MnII, in contrast to the situation with the mononuclear MnIV complex, where the
electron stems from a phenolate. The half-life of the coordinated phenoxyl-type Ru(bpy)/Mn complex (as
produced in the presence of [CoIII(NH3)5Cl]2+) is of the order 0.5-1 ms. The Ru(bpy) compound containing
three (phenolate-ligated) MnII atoms is the first example of a photochemically induced intramolecular ET
from a multinuclear Mn cluster to an attached “sensitizer”, and the Ru complex containing one (phenolate-
ligated) MnIV is the first case of an ET from a synthetic MnIV-coordinated phenolate to a photochemically
produced oxidant (RuIII).
Introduction
complex.2-5,12-14 After four consecutive (light-driven) electron
abstractions from the Mn cluster, which stores the accumulated
oxidizing equivalents,15-18 oxidation of two water molecules
takes place with the release of one oxygen molecule.
In photosynthesis, water oxidation is accomplished by
photosystem II (PSII), which is a large membrane-bound protein
complex.1-4 To the central core proteins D1 and D2 are attached
different cofactors, including a redox-active tyrosyl residue,
tyrosine Z (YZ),1-5 which is associated with a tetranuclear
manganese complex,6 these components constituting the so-
called water oxidizing complex (WOC), the site in which the
oxidation of water to molecular oxygen occurs.2,3,7
Concerning the function19-22 of the tyrosyl radical, which is
located ≈5 Å away from the manganese cluster and 10-15 Å
23
•
from P680
,
it has been proposed that YZ abstracts hydrogen
atoms from water coordinated to the manganese cluster,11,24 in
analogy with known metal-radical enzyme mechanisms25 and
in agreement with the results of quantum chemical calculations,26
On excitation of the primary electron donor chlorophylls, P680
,
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with a light quantum, an electron is transferred to the primary
electron acceptor pheophytin, and subsequently to the quinones
QA and QB.2,5 The oxidized P680 has a reduction potential of
1.12 V/NHE and is reduced ()repaired) within nanoseconds
by electron donation from YZ, which thereby is converted into
a neutral radical, YZ . 1,2,8,9 YZ• is reduced back1,10 by extraction
•
of an electron from the Mn complex, which takes from 50 µs
to 1.3 ms,11 depending on the oxidation state of the Mn
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10.1021/ja991402d CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/09/1999

10782 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Burdinski et al.
which indicate that the O-H-bond strength in water coordinated
to manganese complexes is lowered by about 30 kcal/mol,
allowing direct hydrogen abstraction by a tyrosyl radical. This
(new) concept involves the idea of the WOC being of metal-
coordinated radical nature rather than a metal redox center.
With the aim of mimicking, on a basic level, the photoinduced
electron transfer (ET) process from WOC to P680+ in the reaction
center of PSII, ruthenium polypyridyl complexes have been
used27-30 as photosensitizers, these compounds being particu-
larly suitable since their photophysical and photochemical
properties are very well known31-33 and since the reduction
potential, E(RuIII(bpy)3/RuII(bpy)3) ) 1.26 V/NHE34,35 (bpy )
bipyridyl), is sufficiently positive to effect oxidation of phenols
such as tyrosine (E ) 0.93 V/NHE).36 As traps of the
photochemically “mobilized” electron, viologens or a CoIII
complex (as the respective chloride salts) were used.27-29 It was
found that, when the RuIIbpy moiety connected to a phenol
ligand (as with 1) or complexed with a MnII (as with 2) was
excited in the presence of an external electron acceptor,
intramolecular ET from the phenol or the MnII, respectively, to
the photogenerated RuIII took place.
Figure 1. Spectra of transients measured 0.1, 0.3, 1, and 50 µs after
455-nm photolysis of a 70 µM solution of 4 in WAN 4:1. Inset: Decay
of the (negative) signal at 630 nm due to fluorescence and the recovery
of absorption at 445 nm.
synthesized41 a model system which contains a ruthenium tris-
(bipyridyl)-type complex as photosensitizer covalently connected
to phenolates, which in turn coordinate Mn ion(s), thus taking
the modeling of PSII one step further. Here we report the results
of photophysicochemical studies on two such systems (8 (with
one MnIV) and 9 (with three MnII’s)) and their building blocks.
Results and Discussion
All compounds described42 (in the following, {tbpy} repre-
sents a generalized tris-2,2′-bipyridyl unit, which includes ring-
substituted ones; the unsubstituted 2,2′-bipyridyl moiety will
be abbreviated as bpy) were dissolved in water-acetonitrile
(≡WAN) mixtures 4:1 (v/v) and photolyzed with the 20-ns, 455-
nm laser pulses (≈5 mJ) from a Lambda Physik FL105M dye
laser pumped with the 308-nm pulses from a Lambda-Physik
EMG-MSC excimer laser. The 455-nm excitation light is close
to the λmax of the Ru{tbpy} metal-to-ligand charge transfer
(MLCT) band. The time-resolved light-induced changes in the
transmission of the solutions were detected spectrophotometri-
cally and on-line converted into ∆A (absorption) values.37,43
1. Tris-bpy-Type Ru Complexes Containing No Phenol-
(ate)s. The complexes synthesized41 and studied are shown
below.
After studying the formation and properties of metal com-
plexes in which phenoxyl radical is a ligand,37-40 we have now
(25) Babcock, G. T.; Espe, M.; Hoganson, C. W.; Lydiakis-Simantiris,
N.; McCracken, J.; Shi, J.; Styring, S.; Tommos, C.; Warncke, K. Acta
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T.; Akermark, B.; Korall, P. J. Am. Chem. Soc. 1997, 119, 8285.
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L.; Korall, P.; Bo¨rje, A.; Philouze, C.; Berg, K.; Tran, A.; Andersson, M.;
Stenhagen, G.; Martensson, J.; Almgren, M.; Styring, S.; Akermark, B. J.
Am. Chem. Soc. 1997, 119, 6996.
(28) Sun, L.; Hammarstro¨m, L.; Norrby, T.; Berglund, H.; Davydov, R.;
Andersson, M.; Bo¨rje, A.; Korall, P.; Philouze, C.; Almgren, M.; Styring,
S.; Akermark, B. J. Chem. Soc., Chem. Commun. 1997, 607.
(29) Magnuson, A.; Berglund, H.; Korall, P.; Hammarstro¨m, L.; Aker-
mark, B.; Styring, S.; Sun, L. J. J. Am. Chem. Soc. 1997, 119, 10720.
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B.; Sun, L.; Girerd, J. J.; Hammarstro¨m, L.; Styring, S. J. Am. Chem. Soc.
1999, 121, 89.
In Figure 1 are presented the time-dependent spectral changes
(∆A ) f(t)) observed on 455-nm photolysis (20-ns pulse) of a
deoxygenated solution of compound 4 in WAN 4:1.
(31) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193.
Clearly visible is the pulse-induced depletion of the 450-nm
MLCT band and of the 290-nm ligand-centered (LC) transition
of the parent, 4. The positiVe ∆A’s at 310 and 370 nm are
characteristic of the MLCT state of the ruthenium complex
[RuIII(bpy•-)]2+, formed by absorption of the 455-nm light,
whereby the 370-nm band has been assigned to an LC transition
of bpy•-.44,45 The negatiVe signal (-∆A) at 600-700 nm
(maximum at 620 nm) represents46-48 the light emission
(32) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85.
(33) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphy-
rine Complexes; Academic: London, 1992.
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D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815.
(35) The higher value, 1.29 V/NHE, has been measured by Creutz and
Sutin: Creutz, C.; Sutin, N. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 2858.
(36) Harriman, A. J. Phys. Chem. 1987, 91, 6102.
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Chem. Soc. 1993, 115, 11222.
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brand, K.; Sokolowski, A.; Steenken, S.; Weyhermu¨ller, T.; Wieghardt, K.
Chem. Eur. J. 1997, 3, 308.
(39) Hockertz, J. M.; Steenken, S.; Stockheim, C.; Wieghardt, K. In Free
Radicals in Biology and EnVironment; Minisci, F., Ed.; Kluwer Academic:
Dordrecht, 1997; Vol. 3/27, p 133.
(40) Chaudhuri, P.; Hess, M.; Flo¨rke, U.; Wieghardt, K. Angew. Chem.,
Int. Ed. 1998, 37, 2217.
(41) Burdinski, D.; Bothe, E.; Wieghardt, K. Inorg. Chem., in press.
(42) In all cases, the hexafluorophosphate salts were used.
(43) Steenken, S.; McClelland, R. A. J. Am. Chem. Soc. 1990, 112, 9648.
(44) Valenty, S. J.; Behnken, D. E.; Gaines, G. L. Inorg. Chem. 1979,
18, 2160.

LFP and EPR Studies on Photosystem II Models
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10783
resulting from the relaxation of [RuIII{tbpy}•-)]2+ into the
ground state, [RuII{tbpy}]2+. As shown in the inset of Figure
1, this process is first-order, with k ) 1.8 × 106 s-1, which is
exactly the same rate as that of the concomitant recovery of
the 445-nm MLCT band and in excellent agreement with
literature values47,49 for similar compounds in aqueous solution.
On addition of an electron scavenger such as methyl viologen
(MV2+) to solutions of 3 or 4 under photolysis, an electron
transfer takes place from the MLCT state of 3 or 4 to MV2+
(eq 1a), with k ) (5-6) × 108 M-1 s-1 (which is in very good
[RuIII{tbpy}•-]2+ + MV2+ f [RuIII{tbpy}]3+ + MV•+ (1a)
agreement with reference data49-51), to yield [RuIII{tbpy}]3+
(λmax ≈ 420 nm) and MV•+, with its characteristic absorption
bands at 606 (ꢀ ) 13 700 M-1 cm-1) and 396 nm (ꢀ ) 42 100
M-1 cm-1).52On a longer time scale, the electron transfer is
reversible; i.e., RuII is regenerated (eq 1b), for which the rate
constants measured (7-8) × 109 M-1 s-1) are again in excellent
agreement with literature data for similar systems.29,53,54
in analogy to results described for 1.27
RuIII{tbpy}•-ArOH N RuII{tbpy}•-ArO• + H+ (3)
However, if the excess electron on the bpy ligand is removed
by an external electron acceptor such as MV2+ or [CoIII(NH3)5-
[RuIII{tbpy}]3+ + MV•+ f [RuII{tbpy}]2+ + MV2+ (1b)
Cl]2+
,
In the case of the oxidant [CoIII(NH3)5Cl]2+, the electron
transfer from [RuIII{tbpy}•-]2+ (k ) 2.2 × 108 M-1 s-1, in
agreement with comparable cases55) is irreversible due to
hydrolysis of the CoII complex thus formed:
RuIII{tbpy}•-ArOH + MV2+ f RuIII{tbpy}ArOH + MV•+
(4)
k ) (3-6) × 108 M-1 s-1
the resulting RuIII, whose “vicinal electron density” is thus
decreased compared to the case when the radical anion {bpy}•-
is the ligand, is reduced back to the RuII state (eq 5).57
[CoII(NH3)5Cl]+ + 6H2O f
[CoII(H2O)6]2+ + 5(NH3)aq + Cl- (2)
[(ArOH){tbpy}RuIII]3+ f [(ArO•){tbpy}RuII]2+ + H+ (5)
k g 4 × 106 s-1
Thus, in the presence of [CoIII(NH3)5Cl]2+, [RuIII{tbpy}]3+ is
the stable end product of the photolysis of 3 or 4.
2. Phenol(ate)-Containing Ligands. 2.1. Laser Flash Pho-
tolysis (LFP) Results. In the phenol-containing compounds 5
(which is equivalent to 1)29 and 6 (a derivative of 1,4,7-
triazacyclononane (tacn), which is a powerful ligand with respect
to transition metals),41,56 the phenol moieties are expected (and
have, in similar cases, already been observed29) to participate
in electron-transfer processes induced by photochemical produc-
tion of RuIII. On photolysis of these compounds in WAN 4:1
with the 455-nm light of the dye laser, emission at ≈620 nm
and transmission changes were observed of exactly the same
type as described in section 1 for the systems containing no
phenols. Thus, the conclusion from this is that the photochemi-
cally produced RuIII in the [RuIII{tbpy}•-] moiety is not reduced
by adjacent phenol groups; i.e., reaction 3 does not take place,
This is concluded from the observation that the absorption at
≈450 nm (due to RuII), which decreased after the laser flash,
increased again to exactly the level before the laser flash (i.e.,
quantitative regeneration of RuII), whereby the rate of regenera-
tion (for 5, k ) 9 × 106 s-1; for 6, k ) 4 × 106 s-1) was the
same as that for oxidation of the {tbpy}•- moiety (eq 4), as
monitored by the formation of MV•+ at 630 nm (see Figure 2).
This means that the intramolecular electron transfer from the
phenol to the RuIII{tbpy} moiety is faster than (4-9) × 106
s-1; i.e., rate limiting under these conditions is the bimolecular
reaction 4 and not the unimolecular reaction 5. Since the
Bro¨nsted acidity of phenol-type radical cations is g10 orders
of magnitude higher than that of the parents,58,59 it is likely that
the phenol group deprotonates upon electron transfer to RuIII,
as shown in eq 5 for compound 6.
(45) McClanahan, S. F.; Dallinger, R. F.; Holler, F. J.; Kincaid, J. R. J.
Am. Chem. Soc. 1985, 107, 4853.
On a longer time scale, all the absorption changes induced
by the laser pulse as monitored at 396 and 630 nm, the λmax
values of MV•+, disappear by second-order kinetics. Since there
are no absorption changes at 445 nm (which means that the
RuII concentration remains the same on this (long) time scale),
the reaction is interpreted in terms of regeneration of the parent
complex by reduction of the phenoxyl-containing one-electron-
(46) Mabrouk, P. A.; Wrighton, M. S. Inorg. Chem. 1986, 25, 526.
(47) Bensasson, R.; Salet, C.; Balzani, V. J. Am. Chem. Soc. 1976, 98,
3722.
(48) Yoshimura, A.; Hoffman, M. Z.; Sun, H. J. Photochem. Photobiol.,
A 1993, 70, 29.
(49) Gaines, G. L. J. Phys. Chem. 1979, 83, 3088.
(50) Rau, H.; Frank, R.; Greiner, G. J. Phys. Chem. 1986, 90, 2476.
(51) Ochiai, E.-I.; Schaffer, D. I.; Wampler, D. L.; Schettler, P. D. Trans.
Met. Chem. 1986, 11, 241.
(52) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617.
(53) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159.
(54) Young, R. C.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. Soc. 1976,
98, 286.
(55) Hoffman, M. Z.; Bolletta, B.; Moggi, L.; Hug, G. L. J. Phys. Chem.
Ref. Data 1989, 18, 219.
(57) The difference in potential between Ru(III)(tbpy) and Ru(III)-
(tbpy)•- is ca. 0.4 V (see ref 32).
(58) Dixon, W. T.; Murphy, D. J. Chem. Soc., Faraday Trans. 2 1974,
72, 1221.
(59) For a review, see, e.g.: Steenken, S. Free Rad. Res. Commun. 1992,
16, 349.
(56) Wainwright, K. P. Coord. Chem. ReV. 1997, 166, 35.

10784 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Burdinski et al.
Figure 2. Absorption changes (∆A) monitored at 630 (due to formation
of MV•+, eq 4) and 445 nm (due to recovery of RuII{tbpy}, eq 5)
resulting from 455-nm photolysis of a 52 µM solution in WAN 4:1 of
Figure 4. Spectrum measured 60 (for 5) or 10 µs (for 6) after flashing
the solution described in Figure 3. The spectra are assigned to the
phenoxyl-type radicals from 5 or 6.
5 or 6, respectively, in the presence of 15 mM MV2+
.
Figure 3. Spectra of transients recorded after 0.1, 0.3, 1, and 50 µs
on photolyzing 52 µM 5 in the presence of 15 mM [CoIII(NH3)5Cl]2+
.
Inset: Characteristic kinetic traces.
deficient complex by MV•+:
Figure 5. EPR spectrum recorded during photolysis of a solution in
WAN 4:1 of 1.5 mM 6 in the presence of 10 mM [CoIII(NH3)5Cl]2+
using a modulation amplitude of 4 G and 1 mW microwave power.
The dotted line represents the spectrum simulated using the splittings
indicated in Table 1.
[(ArO•){tbpy}RuII]2+ + MV•+ + H+ f
[(ArOH){tbpy}RuII]2+ + MV2+ (6)
k ) 1 × 109 M-1 s-1 or 2 ×
109 M-1 s-1 for 5 or 6, respectively
from the ArOH to the RuIII moiety. Since the rate of this process
could not be time-resolved, it must be g3 × 106 s-1 ()kobserved
for oxidation of RuIII{tbpy}•- by 15 mM [Co(NH3)Cl]2+, see
At 1 × 109 M-1 s-1, the rate constant for this intermolecular
“back electron transfer” is similar to the analogous one
observed29 in the case of 1, and it is smaller than that ((7-8) ×
109 M-1 s-1) measured for the corresponding back electron
transfer to [RuIII{tbpy}]3+ (see eq 1b). This is not unreasonable
since, in the latter (eq 1b), the electron acceptor is (the stronger
oxidant) RuIII compared to ArO• in eq 6.
above), which is in line with the number (k g 9 × 106 s-1
)
obtained in the experiment with MV2+. The spectrum of the
resulting phenoxyl-radical-containing complex is shown in
magnified version in Figure 4. The spectrum with the bands at
306 and 380 nm is similar to those of 2-MeO-phenoxyl60 and
of 2,6-di-tert-butyl-4-methoxyphenoxyl radical,61 and the extinc-
tion coefficients62 in the range (0.3-1) × 104 M-1 cm-1 are
also similar.
To support the formation of ArO• on electronic excitation of
the RuII center, MV2+ was replaced by the “irreversible electron
acceptor” [CoIII(NH3)5Cl]2+, in analogy to the studies on 129
and on 3 and 4 (section 1). Taking 5 as an example, the results
are shown in Figure 3. Evident immediately after the pulse are
the (negative signal of the) fluorescence at ≈630 nm, the
depletion of RuII at ≈450 nm, and bands at 312 and 370 nm,
due45 to formation of RuIII{tbpy}•-. The rates of the “regenera-
tion” processes as monitored at 312, 445, and 630 nm (see inset
to Figure 3) increase with [[CoIII(NH3)5Cl]2+], and from this
(linear) dependence (not shown), the rate constant obtained for
both 5 and 6 according to
2.2. EPR Detection. Solutions of the same compositions as
those described in the LFP experiments were subjected to
continuous (“in situ”) photolysis in the EPR cavity of a
modified63 Varian E9 spectrometer using the focused and filtered
light (λ g 395 nm) of a xenon lamp while the EPR spectrum
was recorded. In the case of the “non-phenolic” reference
compounds 3 and 4, absolutely no EPR signals were detected.
In contrast, with the phenol(ate)-containing compounds, rela-
tively strong 1:2:1 triplet signals were seen. As an example, in
Figure 5 is presented the spectrum recorded on in situ photolysis
of a solution containing 6.
[(ArOH){tbpy}•-RuIII]2+ + [CoIII(NH3)5Cl]2+
f
The 1:2:1 signal from 6 with a splitting of 6.2 G and g )
2.0046 disappears with a half-life of ≈20 s when the light is
turned off. In the case of 5, after the exciting light focused into
the EPR cavity was turned off, the triplet signal decayed,
[(ArOH){tbpy}RuIII]3+ + [CoII(NH3)5Cl]+ (7)
is 2 × 108 M-1 s-1, a value equal to that measured for the case
of the reference compounds 3 and 4 (see section 1). As can be
seen in the inset of Figure 3, with 5 the absorption loss at 445
nm is quantitatively recovered relative to the situation before
the laser pulse. This means that here the RuII{tbpy} unit is
regenerated, which is in contrast to the situation for 3 or 4, where
the RuIII complex is the (final) product (see section 1). The
explanation is in terms of rapid intramolecular electron transfer
(60) O’Neill, P.; Steenken, S. Ber. Bunsen-Ges. Phys. Chem. 1977, 81,
550.
(61) Sokolowski, A. Ph.D. Dissertation, Ruhr-Universita¨t, Bochum,
Germany, 1996.
(62) Estimated from the measured pulse-induced absorption changes at
450 nm in comparison with those for the reference compound 1 and
assuming a quantitative electron transfer from ArOH to RuIII, see: Burdinski,
D. Ph.D. Dissertation, Ruhr-Universita¨t, Bochum, Germany, 1998.
(63) See: Hildenbrand, K. J. Chem. Soc., Perkin Trans. 1995, 2153.

LFP and EPR Studies on Photosystem II Models
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10785
Table 1. EPR Parameters of Phenoxyl-Type Radicals in Solution
splitting (G) due to
methyl(ene) H’s
parent phenol
4-methylphenol
2,4-dimethylphenol
phenoxyl radical
4-methylphenoxyl
2,4-dimethylphenoxyl
ring protons
g
ref(s)
65
66
12 (para)
6.0 (ortho)
6.5 (ortho)
6.5 (ortho)
11.2 (para)
6.0 (ortho)
9.1 (para)
2,6-dimethyl-4-ethylphenol
2-methoxyphenol
2,6-dimethyl-4-ethylphenoxyl
2-methoxyphenoxyl
65
1.8 (o-OCH3)
∼4.3 (ortho)
∼8.5 (para)
4.1 (ortho)
2.00439
2.0043
48, 65, 67
4-allyl-2-methoxyphenol
4-carboxymethyl-2-methoxyphenol
4-allyl-2-methoxyphenoxyl
4-carboxymethyl-2-methoxyphenoxyl
8.0 (p-CH2)
1.6 (o-OCH3)
7.1 (p-CH2)
1.3 (o-OCH3)
6.0 (para)
68
69
6.1 (ortho)
5
6
a
a
b
b
2.0044
2.0046
c
c
6.3 (para)
a See text. b The splittings due to ortho or meta ring protons are e the line width. c This work.
independent of field position, with the shorter half-life of ≈1
s, which decreased on increasing the exciting light intensity,
indicating contributions of second-order decay kinetics.
As mentioned, in the case of 6, the half-life is ≈20 s. In 6,
a molecule containing three phenolate ligands connected to the
central tacn ring, several intramolecular H-bonds are possible
which probably protect the phenoxyl radical from rapid inter-
molecular (radical-radical) decay.38,64
a potential of ≈0.85 V can be estimated.41 The obvious question
is whether the oxidized species can be produced alternatively,
e.g., by photochemical excitation of the RuII{tbpy} moiety.
In Table 1 are collected the EPR parameters of the phenoxyl-
type radicals obtained from 5 and 6, together with those of
model phenoxyls from the literature.
It is evident from the splittings of the methylene group (apara
≈ 6 G) that the distribution of spin in the phenoxyl ligand is
similar to that in “simple” phenoxyls.70 Also, the g-factors of
the “complex” phenoxyls are comparable with those (≈2.004)
65,71,72 of the “simple” ones. In this connection, it is interesting
that the g-factor of the tyrosine Z radical of the WOC has the
similar value, 2.0045.73
3. Mn Complexes 7 and 8. In compound 7, three phenolate
ligands, connected by a tacn ring, are coordinated to the central
MnIV. The compound can be reversibly oxidized electrochemi-
cally (E ) 0.56 V vs Fc+/Fc).74 The one-electron-oxidized form
was characterized spectroscopically to consist of a phenoxyl
radical coordinated to the MnIV ion.38,61 The corresponding
complex (having, however, the MeO group in the ortho position)
containing [RuII{tbpy}] units is 8. Unfortunately, its electro-
chemical oxidation, by which the coordinated phenoxyl radical
is expected to be formed, turned out to be irreversible, although
(64) In the WOC, upon oxidation of YZ, the phenolic proton is transferred
to an adjacent histidine via a preexisting intramolecular H-bond (see: Hays,
A.-M. A.; et al. Biochemistry 1998, 37, 11352. Diner, B. A.; et al.
Biochemistry 1998, 37, 17931. Mamedov, F.; et al. Biochemistry 1998, 37,
14245. O’Malley, P. J. J. Am. Chem. Soc. 1998, 120, 11732).
(65) Stone, T. J.; Waters, W. A. J. Chem. Soc. 1964, 213.
(66) Huysmans, W. G. B.; Waters, W. A. J. Chem. Soc. (B) 1966, 1047.
(67) Dixon, W. T.; Moghimi, M.; Murphy, D. J. Chem. Soc., Faraday
Trans. 2 1974, 1713.
(68) Thompson, D.; Norbeck, K.; Olsson, L. I.; Constantin-Teodosiu,
D.; Van der Zee, J.; Moldeus, P. J. Biol. Chem. 1989, 264, 1016.
(69) Valavanidis, A.; Gilbert, B. C.; Whitewood, A. C. Chim. Chron.,
New Ser. 1995, 24, 217.
(70) The smaller ring splittings seen for the “simple” phenoxyls are buried
in the large line-width characteristic of the “complex” () intermediate
molecular weight) phenoxyls. As is evident from Table 1, the methoxy
substituent decreases the spin density such that the splittings at the ortho
and para positions are decreased.
(71) Davies, A. G.; Howard, J. A.; Lehnig, M.; Roberts, B. P.; Stegmann,
H. B.; Uber, W.; In Landolt-Bo¨rnstein; Fischer, H., Hellwege, K.-H., Eds.;
Springer: Berlin, 1979; Vol. 9c, p 2.
In Figure 6 is shown the spectrum observed on 445-nm
photolysis of a 20 µM solution of 8 in WAN 4:1.
From inspection of the figure, it is obvious that even in the
short time of 200 ns after the 20-ns pulse, there is no
fluorescence visible; thus, the MLCT state is quenched in e200
ns.75 This interpretation is supported by the fact that the
absorption at 445 nm, the MLCT band of RuII{tbpy}, returns
to the prepulse level in a similarly short time. These observations
indicate the occurrence of a fast intramolecular electron
transfer,76 which can, in principle, proceed by eq 8a or b, these
processes differing by the nature of the intramolecular electron
(75) Note the long lifetime of the MLCT state in the model compound
1.
(76) Quenching by energy transfer (Hammarstro¨m, L.; et al. Biochim.
Biophys. Acta 1998, 1365, 193) can be excluded on the basis of the
observations described below.
(72) Barry, B. A.; Babcock, G. T. Chem. Scr. 1988, 28A, 117.
(73) Babcock, G. T.; Sauer, K. Biochim. Biophys. Acta 1975, 376, 315.
(74) All potentials refer to the Fc+/Fc couple in acetonitrile, unless
specified otherwise.

10786 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Burdinski et al.
Figure 6. Spectra recorded 0.05, 0.2, 2, and 20 µs after flashing a 20
µM solution of 8 in WAN 4:1. Insets: Kinetic traces at the indicated
wavelengths.
Figure 7. Absorption spectrum measured 10 µs after flashing a 23
µM solution of 8 in the presence of 15 mM [CoIII(NH3)5Cl]2+
.
donor and acceptor:
followed by the intramolecular ET,
(11)
(8)
At present, no attempt was made to differentiate between these
alternatives.81
The oxidation of {tbpy}Ru(ArO)MnIV according to eq 9 or
10 is reversible; i.e., the photooxidized complex which contains
ArO• is back-reduced:
The following arguments serve to differentiate between paths
a and b in eq 8. The positive ∆A’s at λ g 500 nm can be
explained only in terms of production of the RuII{tbpy}•- unit
(ꢀ values (1-1.5) × 104 M-1 cm-1).77-80 In line with this are
{tbpy}RuII(ArO•)MnIV + MV•+
f
the positive ∆A’s at ≈350 nm, where the RuII-coordinated bpy•-
77-80
moiety also absorbs (ꢀ350 ≈ (1-2) × 104 M-1 cm-1),
all
{tbpy}RuII(ArO-)MnIV + MV2+ (12)
this supporting path b. In addition, the alternative, path a, can
be discarded on the basis of the fact that the RuIIIMnIII unit that
would result from reaction a does not absorb at λ g 450 nm,
The rate constant for this process, as measured from the
absorption change at 610 nm, is of the order 109-1010 M-1 s-1
.
the MLCT absorption band of the RuII{tbpy} unit. Thus, if
53
An analogous experiment was performed using [Co(NH3)5-
Cl]2+ as oxidant in place of MV2+. In <50 ns after the laser
flash, the concentration of the RuII{tbpy} moiety was back to
its prepulse level, as monitored at 370 and 450 nm (see Figure
7, inset a).
path a were right, at 445 nm, the initial light-induced depletion
would persist; i.e., at this wavelength there would be no recoVery
to the prepulse level, which is in contrast to observation (Figure
6).
As can be seen in the insets of Figure 6, the “repair” of
{tbpy•-}RuII to yield {tbpy}RuII consists kinetically of two
components, the first (the major one) occurring on the ≈100-
ns time scale and the second within several microseconds. At
present, we have no explanation for this phenomenon.
Also, in the presence of MV2+, the decay of the MLCT state
is very fast, giving rise to long-lived absorptions at 396 and
g600 nm. These are due to MV•+.52 This proves that an electron
transfer has occurred. Concomitant with the formation of MV•+,
the absorption at 440 nm, which is due to the RuII{tbpy} moiety,
returns to the prepulse level. These observations are explained
in terms of eq 8b, followed by eq 9.
The spectrum recorded at the time (10 µs) after completion
of the reactions described above is shown in Figure 7. This
spectrum with bands at 310-315 and 380 nm is similar to those
of (the uncomplexed) phenoxyls shown in Figure 4. It decays
slowly within 1-2 ms (see inset b in Figure 7). On this basis,
it is identified in terms of a phenoxyl radical which is suggested
to be coordinated to the central MnIV. The strongly reduced
lifetime as compared to that of the uncoordinated complex
(1-20 s, see above) is ascribed to the decreased electron density
of the phenoxyl moiety due to coordination to the Lewis-acidic
MnIV.
4. Complex 9. 9 was photolyzed the same way and under
the same conditions as described in the preceding. After
{tbpy}•-RuII(ArO•)MnIV + MV2+
f
{tbpy}RuII(ArO•)MnIV + MV•+ (9)
Rather than {tbpy}•-RuIII(ArO-)MnIV undergoing the intramo-
lecular ET according to eq 8b, followed by the intermolecular
ET via 9, at sufficiently high concentration of MV2+ the order
may be reversed, i.e., intermolecular ET to MV2+
,
{tbpy}•-RuIII(ArO-)MnIV + MV2+
f
{tbpy}RuIII(ArO-)MnIV + MV•+ (10)
excitation, the only transient seen was the MLCT state, which
decayed with ≈5 × 107 s-1, as concluded from the rapid
(79) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. J. Chem. Soc.,
Chem. Commun. 1981, 287.
(77) Anderson, C. P.; Salmon, D. J.; Meyer, T. J.; Young, R. C. J. Am.
Chem. Soc. 1977, 99, 1980.
(78) Mulazzani, Q. G.; Emmi, S.; Fuochi, P. G.; Hoffman, M. Z.; Venturi,
M. J. Am. Chem. Soc. 1978, 100, 981.
(80) Bugnon, P.; Hester, R. E. Chem. Phys. Lett. 1983, 102, 537.

LFP and EPR Studies on Photosystem II Models
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10787
Scheme 1
Figure 8. Spectra of transients from the 455-nm photolysis (20 ns) of
12 µM 9 in the presence of 15 mM MV2+ measured 0.02, 0.04, 0.06,
and 2 µs after the pulse.
decrease of the fluorescence at 620 nm and the absorption
recovery at 450 nm. Since there was no spectroscopic evidence
at any wavelength and any time (>20 ns) for the formation of
a phenoxyl-type species, the MnII cluster must be the ultimate
intramolecular electron source (see eq 13). This is in accord
(13)
(methoxylated) phenol as compared to the situation in 1 and (b)
the shorter length of the “spacer” between the phenol and RuIII.75
In the trinuclear MnII complex 9, the source of the electron
that reduces the photochemically generated RuIII{tbpy}•- moiety
is not a phenolate but a MnII. The ET situation may be visualized
as in Scheme 1, whereby the electron transfer from MnII to RuIII
may or may not proceed through the connecting phenolate
moiety. To check on this, a time resolution better than 20 ns
would be necessary.
with the lower first oxidation potential of the MnII cluster
(≈-0.4 V vs Fc+/Fc) compared to that of a phenolate moiety,
which is anticipated to be ≈+0.8 V (see corresponding
potentials of 5 (+0.77 V) and 8 (+0.85 V)).41
The product of the process described in eq 13, i.e., the moiety
{tbpy}•-RuII, can again be scavenged by oxidants such as MV2+
or [CoIII(NH3)5Cl]2+. The resulting situation is shown in Figure
8. Visible are the fast decay of the fluorescence at g600 nm,
the equally rapid decay of the characteristic absorption at 370
nm of the MLCT state, and the recovery of the absorption at
440 nm of the RuII MLCT transition.82 The corresponding
formation of MV•+ is seen at 396 and 610 nm. The observations
are explained in terms of reaction 13 followed by reaction
14. The spectroscopic consequences of the change of oxidation
In this complex, with respect to RuIII, MnII is apparently a
better reductant than phenolate. In contrast, in the MnIV complex
8, the electron is extracted from a phenolate. This is not
surprising since in this case the Mn is in a higher oxidation
state. A further point of interest is the large difference in lifetime
between the phenoxyl radical from 1 and those described in
this work: Whereas in the case of 1 the half-life of the phenoxyl
radical (as produced in the presence of [CoIII(NH3)5Cl]2+) is
50 ms, it is g1 s-1 for those from 5 and 6. This difference is
probably related to the larger electron density84 (due to the
electron-donating OCH3 substituent) and, in addition, to the
steric inaccessibility of the phenoxyl radical which is hidden in
the H-bonding network,85 thus preventing a rapid radical-radical
decay in the latter case.86
{tbpy}•-RuIIbpy(ArO-)MnIIIMnIIMnII + MV2+
f
{tbpy}RuIIbpy(ArO-)MnIIIMnIIMnII + MV•+ (14)
state from MnII to MnIII are expected to be small and
uncharacteristic.83 In line with this expectation, no further
spectroscopic changes were observed as reaction 14 proceeded.
The compound 9 (MnIIMnIIMnII) is the first example of a
photochemically induced intramolecular electron transfer from
a multinuclear Mn cluster to a covalently attached “sensitizer”,
and the complex 8 is the first example of an electron transfer
from a MnIV-coordinated phenolate to a photochemically
produced oxidant (RuIII). It is hoped that the understanding of
the photochemistry of these complexes will be helpful in
elucidating the mechanism(s) by which the water oxidizing
complex operates.
Summary and Conclusions
It has been found that 455-nm photolysis of RuII{tbpy}-type
complexes in the presence of external electron scavengers such
as MV2+ or [Co(NH3)5Cl]2+ leads to the production of the
strongly oxidizing RuIII{tbpy} moiety, in agreement with earlier
studies.29 In Ru-MnIV complexes with bpy-type ligands func-
tionalized with electron-rich phenols (as in 8), intramolecular
electron transfer to RuIII from the phenolate ligand takes place
with k g 5 × 107 s-1, giving rise to the corresponding
(complexed) phenoxyl radical. This rate constant is g3 orders
of magnitude higher than that (5 × 104 s-1) for the similar
complex 1 studied29 previously. The reasons for this difference
are possibly (a) the higher electron-donating power of the
Acknowledgment. We are grateful to Marion Toubartz for
her highly qualified help with the LFP experiments and to Dr.
K. Hildenbrand for performing and analyzing the EPR
measurements.
JA991402D
(81) This would be possible by a variation of the concentration of MV2+
.
However, we are planning to measure the rate of eq 8b or 11 directly using
a time resolution better than the presently available 20 ns.
(82) As in the case of 8, there seem to be two kinetic components in
these processes; see inset to Figure 8.
(83) See: Burdinski, D. Ph.D. Dissertation, Ruhr-Universita¨t, Bochum,
Germany, 1998.
(84) The influence of electron density can be seen from the much shorter
half-life of the Mn(IV)-coordinated radical as compared to that of the “free”
one.
(85) As concluded from crystal data on similar compounds; see ref 38.
(86) Nature may have developed an analogous strategy to prevent
•
undesired side reactions of YZ in the WOC.