Vectorial electron transfer in donor-photosensitizer-acceptor triads based on novel bis-tridentate ruthenium polypyridyl complexes

Article abstract of DOI:10.1002/chem.200902716

The first examples of rodlike donor-photosensitizer-acceptor arrays based on bis-2,6-di(quinolin-8-yl)pyridine Ru^II complexes 1a and 3a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time-resolved absorption spectroscopy reveals long-lived, photoinduced charge-separated states (τ_CSS (1a)=140 ns, τ_CSS (3a)=200 ns) formed by stepwise electron transfer. The overall yields of charge separation (≥50% for complex 1a and ≥95% for complex 3a) are unprecedented for bis-tridentate Ru^II polypyridyl complexes. This is attributed to the long-lived excited state of the [Ru(dqp)₂]²⁺ complex combined with fast electron transfer from the donor moiety following the initial charge separation. The rodlike arrangement of donor and acceptor gives controlled, vectorial electron transfer, free from the complications of stereoisomeric diversity. Thus, such arrays provide an excellent system for the study of photoinduced electron transfer and, ultimately, the harvesting of solar energy.

Full text of DOI:10.1002/chem.200902716

DOI: 10.1002/chem.200902716
Vectorial Electron Transfer in Donor–Photosensitizer–Acceptor Triads Based
on Novel Bis-tridentate Ruthenium Polypyridyl Complexes
Rohan J. Kumar,[a] Susanne Karlsson,[a] Daniel Streich,[a] Alice Rolandini Jensen,[a]
Michael Jꢀger,[a] Hans-Christian Becker,[a] Jonas Bergquist,[b] Olof Johansson,*[a] and
Leif Hammarstrçm*[a]
Abstract: The first examples of rodlike
donor–photosensitizer–acceptor arrays
based on bis-2,6-di(quinolin-8-yl)pyri-
dine RuII complexes 1a and 3a for
photoinduced electron transfer have
been synthesized and investigated. The
complexes are synthesized in a conver-
gent manner and are isolated as linear,
single isomers. Time-resolved absorp-
tion spectroscopy reveals long-lived,
photoinduced charge-separated states
(tCSS (1a)=140 ns, tCSS (3a)=200 ns)
formed by stepwise electron transfer.
The overall yields of charge separation
(ꢀ50% for complex 1a and ꢀ95% for
complex 3a) are unprecedented for
bis-tridentate RuII polypyridyl com-
plexes. This is attributed to the long-
lived excited state of the [RuACHTUNGTRENNUNG
(dqp)2]2+
complex combined with fast electron
transfer from the donor moiety follow-
ing the initial charge separation. The
rodlike arrangement of donor and ac-
ceptor gives controlled, vectorial elec-
tron transfer, free from the complica-
tions of stereoisomeric diversity. Thus,
such arrays provide an excellent system
for the study of photoinduced electron
transfer and, ultimately, the harvesting
of solar energy.
Keywords: donor–acceptor
sys-
tems · electron transfer · photo-
chemistry · ruthenium · tridentate
ligands
Introduction
efficient build-up of redox equivalents at the oxygen-evolv-
ing complex for subsequent oxidation of water.
The complexity of all natural systems on earth is dependent
on energy provided by photons from the sun.[1] The energy
of these photons is stored in chemical bonds through the
action of photosynthetic organisms.[2–4] A key step in photo-
synthesis is the directional, multi-step electron-transfer pro-
cess that separates the charges across a membrane and
thereby prevents rapid recombination. The precise direc-
tionality of electron transfer in this process ensures a large
distance between the electron and the hole, and allows for
Ruthenium(II) polypyridyl complexes, covalently attached
to electron-donor (D) and electron-acceptor (A) units in
dyads and triads, have been frequently used as model sys-
tems for the charge-separation event in photosynthesis and
the study of photoinduced electron transfer.[5–9]Also, multi-
nuclear complexes have been proposed as the light-harvest-
ing component in a variety of solar energy generating pro-
cesses.[10,11] The RuII complexes that have been developed
for these applications are primarily based on two classes:
RuII tris-bipyridine ([Ru
([Ru ACHTGNUTRENNUNG
(tpy)2]2+). Although [Ru
ACHTUNGTRENNUNG
G
[a] Dr. R. J. Kumar, S. Karlsson, D. Streich, A. Rolandini Jensen,
Dr. M. Jꢀger, Dr. H.-C. Becker, Dr. O. Johansson,
Prof. L. Hammarstrçm
physical properties with ~1 ms metal-to-ligand charge trans-
fer (MLCT) excited-state luminescence lifetimes, the com-
plex is intrinsically chiral (D3 symmetry, D and L configura-
tions), which leads to diastereomeric mixtures in polynu-
Department of Photochemistry and Molecular Science
Uppsala University
Box 523, 751 20 Uppsala (Sweden)
Fax : (+46)18-471-6844
AHCTUNGTRENNUNG
clear assemblies.[12,13] Moreover, in donor–photosensitizer–
E-mail: leif@fotomol.uu.se
acceptor (D-P-A) charge-separation assemblies, multiple
geometrical isomers with different D–A distances are nor-
mally obtained.[14–16]
olof.johansson@fotomol.uu.se
[b] Prof. J. Bergquist
Department of Physical and Analytical Chemistry
Uppsala University
Several complications can arise in [RuACTHNUTRGNEUNG
(bpy)3]2+-based D-
P-A assemblies because of this heterogeneity. One or sever-
al of the isomers may have short charge-separated lifetimes
due to the through-space proximity of the donor and accept-
Box 599, 751 24 Uppsala (Sweden)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902716.
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FULL PAPER
room temperature.[21] As an alternative approach the trans-
[RuACHTNUTGRNEUNG
(bpy)2(py)2]2+ (py=pyridine) motif has been used, but it
unfortunately also suffers from a short room-temperature
excited-state lifetime and, additionally, poor photostabili-
ty.[22]
Recently, our group has detailed the rational develop-
ment[23,24] and facile synthesis[25,26] of a new class of RuII bis-
tridentate photosensitizers, based on RuII bis-2,6-di(quino-
lin-8-yl)pyridine ([Ru
dral coordination sphere, with short and equal Ru–N dis-
tances, and consequently a strong ligand field is created
through the use of six-membered chelates leading to photo-
physical properties similar to those of [Ru
(bpy)3]2+. The
dqp ligands adopt a helical conformation when coordinated
to the metal centre and [Ru
(dqp)2]2+ is best described by
the symmetry label D2 with a C2 axis running through the
pyridyl 4-positions. Therefore, [Ru
(dqp)2]2+ combines both
(dqp)2]2+). An almost perfect octahe-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
or. Also, the directionality of electron transfer in three-di-
mensional space will be different in the different isomers,
which complicates the coordinated use of electron-transfer
products from several triads. Finally, our understanding of
D-P-A systems is sometimes complicated by the non-expo-
nential decays arising from the different decay rates of iso-
meric charge-separated states. Although there are successful
routes to the isolation of single isomers of D-P-A assemblies
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
the desirable properties of bis-tridentate complexes (elimi-
nation of geometrical isomers) with those of tris-bidentate
complexes (favourable photophysical properties).
Herein, we demonstrate for the first time the synthesis
and full characterization of D-P-A triads based on [Ru-
based on the [Ru
A
(dqp)2]2+ sensitizers (1a and 3a), designed for vectorial pho-
ACHTUNGTRENNUNG
only rodlike arrangements of the donor and the acceptor
around the photosensitizer would be a more straightforward
and less demanding route to vectorial electron transfer.
toinduced charge separation. Benzoquinone (Q) and pheno-
thiazine (PTZ) are used as the electron acceptor and elec-
tron donor, respectively. The corresponding P–A dyads (2a
and 4a) were also prepared as model systems for their cor-
responding triads. Although there are many examples in
which quinones are used as the acceptor in molecular as-
semblies for photoinduced electron transfer,[27,28] the combi-
nation of RuII polypyridyl complexes and benzoquinones is
not frequently found in the literature.[29–34] However, benzo-
quinones have many attractive properties, such as the high
energies of their excited states,[35] inaccessible to energy
transfer from RuII polypyridyl excited states. Benzoquinones
can also be seen as mimics of the plastoquinones in photo-
Conversely, [RuACHTUNGTRENNUNG
(tpy)2]2+ is achiral (D2d symmetry) and
free of geometrical isomers in D-P-A assemblies when sub-
stituted along the C2 axis. However, the MLCT excited-state
lifetime at room temperature is approximately three orders
of magnitude shorter than that for [Ru
to inefficient electron-transfer quenching.[18–20] The short
MLCT excited-state lifetime in [Ru
(tpy)2]2+ has been attrib-
(bpy)3]2+, which leads
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
uted to the poor bite angle of the tpy ligands (1588), which
leads to a weak ligand field, and the complex therefore un-
dergoes rapid non-radiative decay via ligand-field states at
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L. Hammarstrçm, O. Johansson et al.
system II and the purple bacteria reaction centres.[3] Addi-
tionally, benzoquinones are attractive as they can accept two
electrons and two protons, to give the hydroquinone.[36,37]
This makes them ideal acceptors in systems in which multi-
ple, proton-coupled electron transfer is desired, although
this lies beyond the scope of the current study.
tion and purification than the amide-linked complexes. This
linkage was synthesized from 4-bromo-2,6-dimethylaniline
(8, Scheme 1), from which the corresponding nitrile 9 was
obtained under Sandmeyer conditions. Compound 9 was
subsequently reduced to the aldehyde 10 by using diisobut-
AHCTUNGTREGyNNUN laluminium hydride (DIBAL-H). The dqp motif (11) was
then introduced through reaction of 8-acetylquinoline and
ammonia under basic conditions, in a one-pot reaction as
described by Jꢀger et al.[25] The low yield in this step is pre-
sumably a result of the additional steric demands placed on
the intermediates by the 2,5-dimethyl substitution. However,
the enone intermediate was isolated from the reaction mix-
ture in high yield and reacted with further 8-acetylquinoline
to give the product. The DMB motif was installed by Suzuki
coupling in good yield to give the desired dqp ligand 12.
For the PTZ linker a methylene group was used, primarily
to break conjugation and minimize electronic coupling. The
PTZ unit was installed subsequent to the Suzuki coupling to
avoid catalyst poisoning by the sulfur atom. Thus, from 5,
esterification followed by NaBH4 reduction gives the alcohol
(13 and 14, Scheme 2), which is the substrate for the Suzuki
reaction giving 15. The alcohol was then brominated with
PBr3 (16) and the PTZ motif introduced through nucleophil-
ic attack of the deprotonated PTZ (17).
Results and Discussion
Design and synthesis: To install the quinone functionality, a
2,5-dimethoxybenzene (DMB) unit was utilized as a masked
quinone, to be deprotected and oxidized after coordination
to the RuII centre. The DMB precursors 1b–4b also acted as
reference complexes for the corresponding dyads and triads
1a–4a during electrochemical and photochemical characteri-
zation.
The initial choice for the photosensitizer–acceptor linkage
was an amide functionality, governed by the desire to break
the conjugation between the photosensitizer and acceptor in
order to reduce the electronic coupling between the two sys-
tems and decrease the rate of back electron transfer. Our
recent electrochemical studies of a variety of dqp-based
ruthenium complexes[24,26] showed that the substituent at the
pyridal 4-position may allow the excited state to be prefer-
entially located on one of the dqp ligands. Thus, it was in-
With the substituted dqp ligands in hand, synthesis of the
desired dyads and triads by complexation to the ruthenium
proceeded via tris-acetonitrile intermediates (18 and 19,
Scheme 3)[24] previously reported by our group for this class
of ligands.[26] The complexation of the second tridentate
ACHTUNGTRENNUNGferred that the electron-withdrawing amide should also aid
the localization of the excited state onto the dqp ligand clos-
est to the acceptor. An N,N-dicyclohexylcarbodiimide
(DCC)-promoted coupling of
dichloroACHTUNGTRENNUNGisonicotinic acid (5,
Scheme 1) with 2,5-dimethoxy-
benzylamine provided 2,6-di-
chloro-N-(2,5-dimethoxybenz-
ACHTUNGTRENNUNGyl)isonicotinamide (6), and the
quinoline groups were intro-
duced by a Suzuki coupling as
described by Jꢀger et al.[25] to
give the desired DMB ligand 7.
Although the amide linkage
may have many favourable
properties, it also introduced in-
stability to the system, which is
discussed in more detail below.
Thus, another linkage function-
ality was explored,
a 2,5-di-
ACHTUNGTRENNUNGmethylphenyl motif, which is
more linear than the amide
link. The methyl groups in the
2- and 5-positions serve to force
the ring systems of the dqp
unit, linker, and acceptor out of
plane with one another, which
reduces the conjugation of the
system and thus the electronic
coupling. This system showed
Scheme 1. Synthesis of acceptor ligands. Reagents and conditions: i) 2,5-dimethoxybenzylamine, DCC, N-hy-
droxysuccinimide; CH2Cl2, room temperature, 3 h; ii) 8-quinolinylboronic acid, [PdACHTUNTRGNEUNG(dba)2], SPhos, K2CO3;
MeCN, H2O, 1308C, microwave heating, 1 h; iii) NaNO2, HCl then NaHCO3, KCN, CuCN; H2O, 0 to 70 8C,
1 h; iv) DIBAL-H, CH2Cl2; 08C to room temperature, overnight; v) 8-acetylquinoline, KOH, NH4OH; 408C,
96 h; vi) 2,5-dimethoxyphenylboronic acid, [PdACTHNUGRTNENUG(dba)2], SPhos, K2CO3; MeCN, H2O, 1308C, microwave heating,
greater stability during prepara- 1 h. dba=dibenzylideneacetone, SPhos=2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl.
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Electron Transfer in Donor–Photosensitizer–Acceptor Triads
FULL PAPER
porphyrin amide quinone sys-
tems[38,39] was also excluded
given the similar redox poten-
tials of CAN and PbO. Thus, a
mild two-step deprotection
strategy by using BBr3 followed
by oxidation with DDQ, as re-
ported by Schanze and Sauer,[32]
gave the desired quinones in
41–52% yield over the two
steps. To ensure purity, the
complexes were purified by
preparative HPLC as the chlo-
ride salt prior to characteriza-
tion; however, this was not pos-
sible with the quinone-contain-
Scheme 2. Synthesis of donor ligand. Reagents and conditions: i) H2SO4, EtOH; 808C, 2 h; ii) NaBH4; EtOH,
08C to room temperature, 3 h; iii) 8-quinolinylboronic acid, [PdACHTUNGTRENGN(U dba)2], SPhos, K2CO3; MeCN, H2O, 1308C,
microwave heating, 1 h; iv) PBr3; CH2Cl2, 08C to room temperature; v) NaH, 10H-PTZ; THF, 08C to room
temperature, overnight.
ligand gave the DMB-containing complexes (1b–4b,
Scheme 4), which served as reference complexes in the pho-
tophysical studies. As discussed before, the use of tris-aceto-
nitrile precursors for the second complexation in nBuOH at
reflux only gives small amounts of RuII complexes in which
the dqp ligands bind in a facial mode.[26] For 1b–4b, yields
were generally good and the products isolated were free of
facial isomers, as detected by NMR spectroscopy and LC–
MS. Significant care was required in handling many of the
complexes; those containing an amide functionality were
found to be prone to hydrolysis under aqueous conditions
and on standing, even at neutral pH. This decomposition
pathway may be responsible for the inability to obtain fit-
ting elemental analysis for 3a; however, the data are report-
ed for completeness. Additionally, those compounds con-
taining a PTZ motif were oxidized by air to give the sulfox-
ide on a time frame of minutes to days. The use of conven-
tional Schlenk techniques and dry, degassed solvents was
thus employed to prevent decomposition.
ing amide complexes due to the aforementioned hydrolysis
of the amide bond. Here, preparative HPLC was undertak-
en on the DMB complex prior to deprotection and oxida-
tion.
The purification of quinone-containing RuII polypyridyl
complexes free from hydroquinone is a challenge in terms
of both isolation and characterization, as has been observed
previously.[29,32,33] Elemental analysis and HRMS fail to dis-
tinguish between the quinone and the hydroquinone. NMR
spectroscopy offers a useful tool to discriminate between
the two, but can only reliably ensure that the compound is
of 95% purity. Examination of the 1H proton signals in com-
pounds for which the quinone-containing region of the spec-
tra is not heavily obscured by other signals can indicate
purity, and in the case of the amide-linked complexes, one
can observe differences in the coupling of the protons alpha
to the amide bond depending on the aromatic or non-aro-
matic nature of the substituent. Figure 1 displays the
1H NMR spectra of complexes 4a and 4b which show a
simple doublet for the -CH2-NHCO- protons for 4b, where-
as a more complex allylic coupling is observed for 4a con-
taining the non-aromatic quinone unit; analysis of the spec-
tra indicates a purity of 90–95%. A similar analysis for 3a
indicated up to 30% of the hydroquinone impurity. At-
tempts to further purify 3a led to increased amounts of hy-
droquinone or hydrolysis products. In contrast, for com-
plexes 1a and 2a the 1H NMR spectra suggest >95%
purity. LC–MS analysis of the samples showed a single
The DMB motif was subsequently deprotected and oxi-
(bpy)3]2+
dized to give the target complexes 1a–4a. For [RuCAHTUNGTERNUNGN -
based complexes this is routinely achieved in a single step
by using ceric ammonium nitrate (CAN). However, the use
of this reagent led to decomposition of the complexes, and
also for 2b and 4b lacking the PTZ unit, which was attribut-
ed to the lower oxidation potential of [Ru
parison to [Ru
(bpy)3]2+. The use of PbO as reported by
Schmidt et al.[38] in the synthesis of the extensively studied
(dqp)2]2+ in com-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Scheme 3. Synthesis of RuII precursors. Reagents and conditions: i) RuCl3·xH2O; EtOH, 808C, overnight; ii) MeCN; EtOH, H2O; 808C, overnight.
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L. Hammarstrçm, O. Johansson et al.
Scheme 4. Synthesis of final complexes. Reagents and conditions: i) nBuOH, 1308C, overnight; ii) 1) BBr3; CH2Cl2, ꢁ788C to room temperature, over-
night; 2) 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); acetone, room temperature, 3 h.
peak, but the masses observed for the peak show both the
quinone and the hydroquinone. This may be an artefact of
the ionization procedure, but one cannot rule out co-elution
of the associated complexes. However, given the efforts
made to exclude other impurities from the samples, it can
be concluded that any observed long-lived components in
the emission and transient absorption spectra can be as-
signed to residual hydroquinone present in the sample.
plexes 1b–4b, are very similar and can be found in the Sup-
porting Information. Electronic absorption and emission
data for all complexes are summarized in Table 2, along
with the results from time-resolved emission and transient
absorption studies.
By comparison with the absorption spectrum of the
[23]
parent complex [Ru
(dqp)2]2+
,
the broad band at 495 nm
can be assigned to the lowest excited singlet MLCT state,
and the higher bands (at 290 and 340 nm) are attributed to
ligand-centred (LC) transitions. The small peak at 410 nm in
the absorption spectrum of 1a is found in all phenyl-linked
complexes studied here (1a, 1b, 2a and 2b). Complexes 3a,
3b, 4a and 4b, in which the dqp ligand is functionalized by
an amide group, display increased absorption at 430 and
Electrochemistry: The redox properties of the dyad and
triad complexes in dry MeCN were studied by cyclic voltam-
metry (CV) and differential pulse voltammetry (DPV). The
electrochemical data are collected in Table 1. Half-wave po-
1
tentials (E = ) were determined by CV as the average of the
2
1
anodic and cathodic peak potentials (E = =(Epa +Epc)/2).
550 nm, similar to that previously observed for ester-func-
2
[24]
Adsorption spikes were observed in the voltammograms
(dqp)2]2+
tionalized [RuACHTUNGRTNEGNU .
after reduction to neutral or negative overall charge. For
cases in which the CV waves were perturbed by these ef-
fects, half-wave potentials were determined from DPV peak
potentials. The assignments given in Table 1 are based on
The emission maximum shifts slightly with the different
substituents (ca. 690–710 nm) and is assigned to a triplet
MLCT (3MLCT) state origin, in analogy to [Ru
A
.
Emission quantum yields, determined with [RuCAHTUNGTRENNNUG
a standard, are given in Table 2. The amide-functionalized
complexes 3b and 4b display increased emission, in analogy
comparison with literature values for [Ru
plexes and similar triads.
A
to the documented observations for ester-functionalized
[24]
Photochemistry: The electronic absorption and emission
spectra of the triads 1a and 3a are shown in Figure 2. The
spectra of dyads 2a and 4a, and of the DMB reference com-
[Ru
(dqp)2]2+
.
The emission quantum yields of 3a and 4a
are evaluated versus their DMB references 3b and 4b, re-
spectively.
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Electron Transfer in Donor–Photosensitizer–Acceptor Triads
FULL PAPER
Figure 2. Electronic absorption (left scale) and emission (right scale)
spectra for a) 1a and b) 3a in MeCN at 298 K. Emission spectra were
collected in deaerated solutions and are normalized at maximum peak in-
tensity.
Figure 1. 1H NMR spectra of a) 4a and b) 4b. Shown is the aromatic
region that contains differing chemical shifts in the case of a quinone
(d=6.6–7.1 ppm) or DMB motif (d=6.8–7.1 ppm). Also shown is the
CH2 unit alpha to the amide bond (~4.5 ppm), which shows a doublet in
the case of an aromatic system but an allylic coupling for the quinone.
state lifetimes of 2.7–3.5 ms (Table 2), similar to the lifetime
of [RuACHTNUGRTNEUNG
(dqp)2]2+. On the other hand, the quinone-containing
complexes 1a–4a are significantly quenched, as observed in
both the emission yield and time-resolved emission mea-
AHCTUNGTRENNUNG
surements. Their 3MLCT excited-state lifetimes upon
400 nm excitation were determined by time-correlated
single-photon counting. The emission lifetimes reported in
Table 2 correspond to excited-state quenching rate constants
3
The MLCT excited-state decay upon excitation by visible
light was investigated by time-resolved emission and tran-
sient absorption. Complexes 1b–4b all have long excited-
of k
(1a)=1.1ꢂ109, k (2a)=1.1ꢂ109, k (3a)=8.3ꢂ108 and
qACHTUGNRTENUNGN qAHCUTNRTGENNUNG qACTHNUGTRENNGUN
G
kqACHTUGNTRENNNUG
(4a)=5.9ꢂ108 sꢁ1. All the complexes 1a–4a also show
long-lived unquenched emission
assigned to the corresponding
hydroquinone complexes, as
discussed above. These account
for most of the emission quan-
tum yields.
The transient absorption
spectra upon 480 nm excitation
of the dyads 2a and 4a (com-
plex 2a shown in Figure 3)
Table 1. Electrochemical data.
[c]
E = [b] (DEp
)
1
2
Complex[a]
[Ru(L)2]2+/+
BQꢁ/BQ2ꢁ
BQ/BQꢁ
PTZ+/PTZ
0.40 (100)
[Ru(L)2]3+/2+
0.71 (93)
DMB+/DMB
–
1a
ꢁ1.70[d,e]
ꢁ1.52 (87)[d]
,
ꢁ0.82 (184),[d]
ꢁ1.52[e]
ꢁ0.80[e]
1b
2a
–
–
–
0.42 (94)
–
0.73 (92)
0.71 (60)
0.89 (68)
–
ꢁ1.73 (59),[d]
ꢁ1.71[e]
ꢁ1.76 (58)
ꢁ1.74[d,e]
ꢁ1.41 (171),[d]
ꢁ1.45[e]
–
ꢁ0.84 (69)
2b
3a
–
–
0.67 (63)
0.73 (87)
0.84 (63)
–
ꢁ1.41[d,e]
ꢁ0.78 (165),[d]
0.38 (80)
show
a MLCT ground-state
ꢁ0.78[e]
bleach at about 500 nm and
broad excited-state absorption
bands at 400 and 600–650 nm,
in excellent agreement with the
3b
4a
ꢁ1.70 (61)
ꢁ1.80 (67),[d]
ꢁ1.69[e]
–
–
0.40 (60)
–
0.72 (64)
0.71 (68)
0.87 (90)
–
ꢁ1.52[d,e]
ꢁ0.84 (143),[d]
ꢁ0.89[e]
4b
ꢁ1.68 (57)
–
–
–
0.72 (59)
0.88 (83)
(dqp)2]2+ excited-state fea-
ACHTUNGTRENNUNG
ꢁ
[a] Ionic complexes as PF6 salts. [b] V versus ferrocenium/ferrocene (Fc+/0) in MeCN, 0.1m Bu4N
[c] mV, v=0.1 mVsꢁ1. [d] Irreversible or poorly defined reverse wave. [e] Potential from DPV peak.
AHCTUNTGREG(NNUN PF6).
[Ru
tures.[24] There are two isosbes-
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L. Hammarstrçm, O. Johansson et al.
Table 2. Photophysical and electron-transfer data at 298 K.
remaining excited-state fea-
tures.[40] The difference spectra
thus obtained clearly show the
Complex[a]
Absorption (MLCT)
lmax [nm]
Emission (MLCT)
Electron transfer
lmax[b] [nm]
Fem
tem [ns]
kCS[c] [sꢁ1
]
kCR [sꢁ1
]
[b]
1a
1b
2a
2b
3a
3b
4a
4b
497
497
495
495
496
495
492
494
490
ꢁ
708
708
714
703
698
701
688
689
700
0.002
0.02
0.005
0.02
0.02
0.05
0.006
0.05
0.02
0.91
2700[b]
0.88
1.0ꢂ109
5.0ꢂ106 signatures of the Qꢁ and PTZ+
–
–
radicals
around
440
and
1.1ꢂ109
@1ꢂ109
510 nm, respectively.[41–44] This
finding is interpreted as the sig-
nature of the fully charge-sepa-
rated state PTZ+RuIIQꢁ.
2900[b]
1.2
–
–
5.2ꢂ108
7.1ꢂ106
2700[b]
1.7
–
–
5.1ꢂ108
@5ꢂ108
3500[b]
3000
–
–
–
–
Transient absorption traces
for the excited-state decay (at
400 and 600 nm), ground-state
recovery/Qꢁ build-up (450 nm)
[RuACHTUNGTRENNUNG
(dqp)2]2+[d]
[a] Ionic complexes as PF6 salts in MeCN. [b] Deaerated MeCN. [c] From ultrafast transient absorption meas-
urements, 480 nm excitation. [d] Values from Abrahamsson et al.[24]
Figure 3. Transient absorption spectra at representative times for 2a in
MeCN at 298 K upon excitation by ~120 fs pulses at 480 nm. O.D.: opti-
cal density.
tic points at 427 and 564 nm, and thus it is likely that only
two states (excited state and ground state) contribute to the
transient spectra. Still, both 2a and 4a are significantly
quenched compared to their DMB analogues 2b and 4b.
The most likely quenching mechanism is that of electron
transfer from the excited [Ru
none, as discussed further below. If the rate of charge re-
combination (kCR) to the ground state is very fast (kCR @kCS
(dqp)2]2+ moiety to the qui-
ACHTUNGTRENNUNG
Figure 4. Transient absorption spectra for a) 1a and b) 3a in MeCN at
298 K upon excitation by ~120 fs pulses at 480 nm. Insets: the respective
difference spectra at 5 ns, in which the excited-state features have been
subtracted (see ref. [40]).
;
kCS is the rate of charge separation), the effective population
of the charge-separated state will be very small and it will
not be detected in the transient absorption spectra. The ex-
cited-state features decay with
kCSACHTUNGTRENNUNG
kCSACTHUNGETRNNUG
(2a)=1.1ꢂ109 and
(4a)=5.1ꢂ108 sꢁ1, which matches the quenching rates
from emission measurements well. The rate of charge sepa-
ration was observed to vary slightly with the water content
in the solvent, but we did not pursue systematic investiga-
tion of this effect.
In the transient absorption of the triads 1a and 3a
(Figure 4), the initial spectra are identical to the excited-
state spectra observed for 2a and 4a. At longer times, how-
ever, positive features grow at 450 and 510 nm simultane-
ously with the ground-state recovery. This state is still grow-
ing at 5 ns, which is the longest delay obtained in our ultra-
fast experiments. The insets in Figure 4 show the difference
between the almost fully developed spectra (at 5 ns) and the
and ground-state recovery/growth of PTZ+ (510 nm) of 1a
and 3a are shown in Figure 5. The concomitant growth of
the Qꢁ and PTZ+ signatures and decay of the [Ru
N
*
excited state was fitted to rate constants of k
CSACHTUGNTRENNNUG
109 and k
CHTUNGTRENNUNG
five to ten wavelengths. This is interpreted as the rate of for-
mation of the charge-separated state PTZ+RuIIQꢁ. These
rates correlate well with the quenching rates (kq) deter-
mined by time-resolved emission spectroscopy. An offset
(k !1.0ꢂ108 sꢁ1) was also needed to fit the data, which cor-
responded in part to the long-lived charge-separated state,
and which does not decay on the 10 ns timescale.
2836
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 2830 – 2842

Electron Transfer in Donor–Photosensitizer–Acceptor Triads
FULL PAPER
Figure 6. Charge recombination kinetics monitored by nanosecond transi-
ent absorption spectroscopy following the decay of Qꢁ at 440 nm and of
PTZ+ at 520/510 nm for a) 1a and b) 3a in deaerated MeCN at 298 K.
Global fits (solid lines) give charge-separated-state lifetimes of a) 140
and b) 200 ns.
Figure 5. Transient absorption decay for a) 1a and b) 3a in MeCN at
298 K upon excitation by ~120 fs pulses at 480 nm with global fits (solid
lines).
The lifetime of the charge-separated state PTZ+RuIIQꢁ in
1a and 3a was investigated by transient absorption spectros-
copy on the nanosecond to microsecond timescale by
480 nm excitation pulses of ~8 ns duration. The resulting
spectra at representative times are found in the Supporting
Information. The initial spectrum matches well the last spec-
trum in the ultrafast measurements (5 ns). As seen in
Figure 6, the Qꢁ (440 nm) and PTZ+ (510/520 nm) signa-
tures decay within <1 ms, and a global fit at five wavelengths
In principle, the PTZ+RuIIQꢁ charge-separated state
could be formed through two alternative routes, reductive
quenching [Eq. (1)] and oxidative quenching [Eq. (2)]:
II
þ
I
þ
II
ꢁ
*
ð1Þ
ð2Þ
PTZ- Ru -Q ! PTZ -Ru -Q ! PTZ -Ru -Q
II
III
ꢁ
PTZ- Ru -Q ! PTZ-Ru -Q ! PTZþ-RuII-Qꢁ
*
The reductive quenching [Eq. (1)] would involve popula-
tion of the PTZ+RuIQ charge-separated state, which is ther-
modynamically uphill from the 3MLCT excited state
gives charge recombination rate constants of k
CRACHTUNGTRNE(NUNG 1a)=5.0ꢂ
106 and k (3a)=7.1ꢂ106 sꢁ1. In line with the emission and
CRA
ultrafast transient absorption measurements, an additional
decay component attributed to the presence of unquenched
hydroquinone triads (tꢂ3 ms) also came out of the global
fit. The charge recombination as such is single exponential,
as can be expected for triads with only one geometrical
isomer.
(DG0
ACHTUGTNRENNUG ETHCATUNGTRNE(NUGN 1a,3a)=+0.3 eV, as calculated from the redox po-
tentials in Table 1;[46] see Figure 7). No quenching was ob-
served for the DMB-substituted triad complexes 1b and 3b
(see Table 2 and the Supporting Information). Instead, the
overall rates of charge separation in 1a and 3a are very sim-
ilar to the charge separation rate of the corresponding dyads
2a and 4a, respectively. Thus, it was concluded that the
mechanism is electron transfer through the PTZRuIIIQꢁ
charge-separated state [Eq. (2)]. This step is apparently the
rate-determining one, followed by a very fast hole transfer
to form the PTZ+RuIIQꢁ state. Assuming that the charge re-
combination for the first charge-separated state in 1a and
3a is as rapid as in dyads 2a and 4a, this means that the
yield of overall charge separation is strongly dependent on
the rate of hole transfer to PTZ, which has to out-compete
the charge recombination from the PTZRuIIIQꢁ state.
The energy of the lowest triplet excited state of benzoqui-
none in polar solvents is ꢀ2.35 eV according to the litera-
3
ture.[35] Thus, energy transfer from the MLCT excited state
(E00 =1.84 eV for [RuACTHNUTRGNEUNG
(dqp)2]2+ in a MeOH/EtOH glass)[23]
to form the quinone triplet is a considerably uphill process,
and not a likely quenching mechanism. From the electro-
chemistry data given in Table 1, the driving force for the
electron transfer is approximately DG0
(2a,4a)=ꢁ0.3 eV
in the dyads, and for complete charge separation in the
triads DG0
G
U
Chem. Eur. J. 2010, 16, 2830 – 2842
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2837

L. Hammarstrçm, O. Johansson et al.
has been observed before in other RuII polypyridyl-based
triads with comparable yields.[30,47–49]
Complexes 1a and 3a are the first examples known to us
in which D-P-A triads based on bis-tridentate RuII poly-
pyridyl motifs give such high yields of charge separation.
Perhaps more outstanding than the fast hole transfer from
PTZ is the high yield of formation of the primary charge
separation (almost unity). The [RuACTHNUTRGNEUNG
(dqp)2]2+ unit and its in-
trinsic long excited-state lifetime makes this possible, as the
yield of electron transfer no longer suffers from the same ki-
netic limitations as with the [RuACHTUNGTRNEUNG
(tpy)2]2+ or similar motifs.
Notably, the overall yield for 3a is among the highest re-
ported in the literature for RuII polypyridyl-based triads
Figure 7. Schematic energy diagram of relevant excited and charge-sepa-
rated states versus the ground state (GS) for the triads 1a and 3a. Triplet
excited-state energies are taken from the literature.[23,35] The charge-sepa-
rated-state energies are calculated from data in Table 1, as described in
the text. Solid arrows mark the observed charge-separation (CS) and
charge-recombination (CR) processes.
(quoted above), which makes the [Ru
potent competitor to [Ru
(bpy)3]2+ in D-P-A triad assem-
blies for photoinduced charge separation. Additionally, the
[Ru
(dqp)2]2+ unit enables a rodlike arrangement and does
(dqp)2]2+ motif a
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
not introduce the same stereoisomeric heterogeneity in D-
P-A assemblies as tris-bidentate complexes such as [Ru-
In multi-step electron transfer, there is competition be-
tween the charge recombination and further charge shift
after the primary charge separation, in which fast recombi-
nation can decrease the total yield of charge separation. Ex-
(bpy)3]2+. This point was illustrated by the single exponen-
ACHTUNGTRENNUNG
tial charge recombination in 1a and 3a. There is but one
donor–acceptor distance, and the electron moves vectorially
from one point in three-dimensional space to another. In all,
these combined properties make triads 1a and 3a remark-
able examples in the RuII polypyridyl family for vectorial
electron transfer.
amples of [RuACHTUNGTRENNUNG
(bpy)3]2+-based triads for which the overall
yield is high (ꢀ80%) have been reported.[30,47–49] In general,
however, the overall yields of charge separation in RuII
polypyridyl-based D-P-A assemblies given in the literature
are moderate (ꢃ40%).[14,16,50–52] For bis-tridentate motifs
such as [Ru
N
Conclusions
large extent due to the short room-temperature lifetime of
the chromophore.[18,53]
We have presented the very first examples of D-P-A triads
The yield of the fully charge-separated state PTZ+RuIIQꢁ
was evaluated from the spectroscopic signatures in the dif-
ference spectra at 5 ns (see Figure 4a and b, insets). The ex-
cited-state versus ground-state molar absorption coefficient
based on [RuACTHNGUTERNNUG
(dqp)2]2+, 1a and 3a. The complexes were syn-
thesized in good yield by adopting the convergent strategy
of stepwise coordination of functionalized dqp ligands via a
well-characterized [Ru
Upon visible excitation, the excited [RuACTHNGUTERNNUG
(MeCN)3]2+ intermediate.
ACHUTGTNRNEUG(N R’-dqp)ACHTUGNTRENNUGN
of [Ru
A
(dqp)2]2+ in MeCN was found to be De500
E
(dqp)2]2+ 3MLCT
(dqp)2*])=ꢁ1.1ꢂ104 mꢁ1 cmꢁ1.[54] Several sources give
state is formed and subsequently quenched by electron
transfer, which results in the PTZ+RuIIQꢁ charge-separated
state in high overall yields, ꢀ50% for complex 1a and
ꢀ95% for 3a. To our knowledge, these are the highest
yields of overall charge separation reported for bis-triden-
tate RuII polypyridyl assemblies. This is to a large extent as-
e430(Qꢁ)=7ꢂ103 mꢁ1 cmꢁ1 for the unprotonated benzoqui-
none radical.[30,43,44,55] On the other hand, the protonated
semiquinone radical has a slightly blue-shifted peak with
lower
extinction
coefficient
(e415(QH)=4ꢂ
103 mꢁ1 cmꢁ1).[43,44,55] We cannot rule out that the benzoqui-
none radical is protonated to some degree, considering the
indications that the rate of charge separation is sensitive to
the water content in the solvent (see above). At this point,
we did not investigate this aspect systematically, but rather
base our evaluation of the charge-separated-state yield on
the PTZ+ radical signature. The molar absorption coeffi-
cient of the oxidized PTZ in MeCN given in the literature is
cribed to the intrinsically long lifetime of the [RuACHTUNGTRENNUNG
(dqp)2]2+
MLCT excited state. The kinetics of charge recombination
are monoexponential, as a consequence of the well-defined
donor–acceptor distance stemming from the bis-tridentate
[Ru
(tCS(1a)=140, tCSACTHUNGETRNNU(G 3a)=200 ns) and the vectorial electron
(dqp)2]2+ motif. The long-lived charge-separated state
ACHTUNGTRENNUNG
T
transfer make these complexes very promising candidates
for further studies towards artificial photosynthesis and elec-
tron transfer in molecular arrays.
e515ACHTUNGTRENNUNG
(PTZ+)=5ꢂ103 mꢁ1 cmꢁ1.[51] By using this value, we esti-
mate the overall yield of charge separation to be ꢀ50% in
1a and ꢀ95% in 3a.[56] These are remarkably high yields
considering the very fast charge recombination (kCR @kCS)
of the first charge-separated state PTZRuIIIQꢁ. Apparently,
the electron transfer from PTZ to the oxidized RuIII moiety
is approximately as fast or up to nine times faster than the
charge recombination. Rapid electron transfer from PTZ
Experimental Section
1
NMR spectra were recorded on a JEOL spectrometer at 293 K. H NMR
spectra were recorded at 400 MHz and 13C NMR spectra were recorded
2838
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Chem. Eur. J. 2010, 16, 2830 – 2842

Electron Transfer in Donor–Photosensitizer–Acceptor Triads
FULL PAPER
at 101 Hz; chemical shifts are given in ppm and referenced internally to
the residual solvent signal, and all coupling constants are in hertz. Micro-
wave heating was performed in an Initiator single-mode microwave
cavity at 2450 MHz (Biotage). Flash chromatography was detected by
TLC (254 nm UV light) or visual inspection. Automated flash chromatog-
raphy was performed on a Biotage SP4 system and detected at 254 and
310 nm by using pre-packed 50–100 g silica columns. High-resolution
ESIMS was performed on a superconducting 9.4 T Fourier transform ion
cyclotron resonance (FTICR) mass spectrometer equipped with an in-
house-developed emitter. HPLC–MS data were obtained on a Dionex
Ultimate 3000 system on a Phenomenex Gemini C18 column (150ꢂ
3.0 mm, 5 mm) coupled to a Thermo LCQ Deca XP spectrometer with
electrospray ionization (ESI). Solvents used for HPLC: 0.05% HCO2H
in H2O and 0.05% HCO2H in MeCN. Preparative HPLC was performed
on a C18 column (150ꢂ21.2 mm, 10 mm) with the indicated eluents. Elec-
trochemical experiments were performed with a three-electrode set-up in
a three-compartment cell connected to an Autolab potentiostat with a
general-purpose electrochemical system (GPES) electrochemical inter-
face (Eco Chemie). The working electrode was a glassy carbon disc (di-
ameter 3 mm, freshly polished). Potentials were measured versus a non-
aqueous Ag/Ag+ reference electrode (CH Instruments, 10 mm AgNO3 in
MeCN) with a potential of ꢁ0.080 V versus the ferrocenium/ferrocene
(Fc+/0) couple in MeCN.
d=7.30 (s, 2H), 2.50 ppm (s, 6H); 13C NMR (CDCl3): d=143.8, 130.5,
127.1, 116.6, 112.4, 20.5 ppm.
4-Bromo-2,6-dimethylbenzaldehyde
(10):
DIBAL-H
(11.4 mL,
11.4 mmol) was added dropwise to a solution of 9 (2.0 g, 9.5 mmol) in
CH2Cl2 (20 mL) at 08C under an inert atmosphere. The solution was al-
lowed to warm to room temperature and stirred overnight. The resulting
solution was quenched with dilute HCl and heated at reflux for 30 min.
The solution was extracted with CH2Cl2 and the organic layers dried and
concentrated in vacuo. The crude residue was purified by flash chroma-
tography (SiO2, hexane, 10% EtOAc) to give 10 (1.6 g, 7.5 mmol, 79%
yield). 1H NMR (CDCl3): d=10.53 (s, 1H), 7.25 (s, 2H), 2.57 ppm (s,
6H); 13C NMR (CDCl3): d=192.5, 142.9, 132.5, 131.1, 127.7, 20.3 ppm.
4-(4-Bromo-2,6-dimethylphenyl)-2,6-di(quinolin-8-yl)pyridine (11): 8-
Acetylquinoline (1.28 g, 7.51 mmol) and 10 (0.8 g, 3.75 mmol) were dis-
solved in EtOH (4 mL). A solution of KOH (0.42 g, 7.51 mmol) in aque-
ous NH4 (4 mL, 28%) was added. The reaction mixture was heated to
408C and stirred for 96 h before it was allowed to cool to room tempera-
ture, filtered and the obtained solid washed with EtOH. The solid was
then purified by flash chromatography (SiO2, CH2Cl2, 4% MeOH) to
give 11 (0.110 g, 0.213 mmol, 6% yield) as an off-white solid. 1H NMR
(CDCl3): d=8.94 (dd, J=1.8, 4.1 Hz, 2H), 8.43 (dd, J=1.5, 7.2 Hz, 2H),
8.21 (dd, J=1.8, 8.3 Hz, 2H), 8.02 (s, 1H), 7.88 (dd, J=1.4, 8.1 Hz, 2H),
7.69 (m, 2H), 7.42 (dd, J=4.1, 8.3 Hz, 2H), 7.34 (s, 2H), 2.36 ppm (s,
6H); 13C NMR (CDCl3): d=156.6, 150.2, 146.1, 145.9, 139.2, 138.6, 136.4,
131.6, 130.4, 128.7, 126.8, 126.7, 121.3, 121.0, 21.1 ppm; LC–MS (ESI):
m/z: calcd for C31H23BrN3+: 516.1 [M+H]+; found: 516.7; r.t.: 8.04 min.
All starting materials were purchased from Aldrich (St. Louis, USA) and
used as received. [Ru
the literature procedure.[26]
2,6-Dichloro-N-(2,5-dimethoxybenzyl)isonicotinamide (6): 2,6-Dichloro-
isonicotinic acid (5, 1.3 g, 6.7 mmol), 2,5-dimethoxybenzylamine (1.2 g,
ACHTUNGTRENNUNG(dqp)ACHUTNRTGEN(GNNU MeCN)3]ACHTNGUTERN(NUGN PF6)2 was synthesized according to
4-(2’,5’-Dimethoxy-3,5-dimethylbiphenyl-4-yl)-2,6-di(quinolin-8-yl)pyri-
dine (12): 2,5-Dimethoxyphenylboronic acid (46.5 mg, 0.26 mmol), 11
(110 mg, 0.21 mmol), SPhos (4.4 mg, 10.6 mmol), [PdACTHNUGTRENUNG(dba)2] (6.1 mg,
ACHTUNGTRENNUNG
7.5 mmol), DCC (1.5 g, 7.5 mmol) and N-hydroxysuccinimide (0.78 g,
6.8 mmol) were stirred in CH2Cl2 (30 mL) at room temperature. The so-
lution was stirred for 3 h before being filtered, washed with H2O and con-
centrated in vacuo. The crude product was then purified by flash chroma-
tography (SiO2, CH2Cl2, 1% CH3OH) to give 6 (1.8 g, 5.3 mmol, 78%
yield) as a white solid. 1H NMR (CDCl3): d=7.54 (s, 2H), 6.90–6.77 (m,
4H), 4.57 (d, J=5.8 Hz, 2H), 3.85 (s, 3H), 3.76 ppm (s, 3H); 13C NMR
10.6 mmol) and K2CO3 (44.2 mg, 0.32 mmol) were suspended in H2O
(0.5 mL) and MeCN (0.5 mL). The solution was degassed for 5 min
before being heated to 1308C for 1 h by microwave heating. The crude
mixture was diluted with CH2Cl2 and the organic layer washed with H2O
before being concentrated in vacuo. The crude residue was purified by
flash chromatography (SiO2, CH2Cl2, 4% MeOH) to give 12 (75 mg,
0.13 mmol, 61% yield) as an off-white solid. 1H NMR (CDCl3): d=8.95
(dd, J=1.8, 4.2 Hz, 2H), 8.43 (dd, J=1.5, 7.2 Hz, 2H), 8.22 (dd, J=1.8,
8.3 Hz, 2H), 7.88 (dd, J=1.5, 8.1 Hz, 2H), 7.70 (dd, J=7.2, 8.1 Hz, 2H),
7.42 (dd, J=4.1, 8.3 Hz, 2H), 7.35 (s, 2H), 6.91 (ddd, J=3.0, 9.5, 11.9 Hz,
3H), 3.82 (s, 3H), 3.81 (s, 3H), 2.43 ppm (s, 6H); 13C NMR (CDCl3): d=
156.4, 153.7, 150.8, 150.1, 146.9, 146.0, 139.4, 138.9, 137.5, 136.2, 136.0,
(CDCl3): d=162.6, 153.6, 151.7, 151.4, 147.1, 125.8, 120.7, 116.2, 113.5,
+
111.5, 55.9, 55.7, 40.6 ppm; LC–MS (ESI): m/z: calcd for C15H15Cl2N2O3
341.0 [M+H]+; found: 341.0; retention time (r.t.): 10.09 min.
:
N-(2,5-Dimethoxybenzyl)-2,6-di(quinolin-8-yl)isonicotinamide (7): De-
gassed H2O (2 mL) and MeCN (2 mL) were added to a mixture of 6, 8-
quinolinylboronic acid (1.0 g, 5.8 mmol), K2CO3 (1.1 g, 7.9 mmol), SPhos
(0.054 g, 0.13 mmol) and [PdACTHNUTRGNEUNG(dba)2] (0.076 g, 0.13 mmol). The suspension
131.4, 128.6, 128.6, 128.5, 127.0, 126.6, 120.9, 116.7, 113.1, 112.3, 56.3,
was then stirred and degassed for a further 5 min before being heated to
1308C for 1 h by microwave heating. The solution was diluted with
CH2Cl2, filtered and washed with H2O before being concentrated in
vacuo. The product was purified by automated flash chromatography
(SiO2, hexane, EtOAc 10–90%) to give 7 (1.0 g, 1.9 mmol, 72% yield) as
an off-white solid. 1H NMR (CDCl3): d=8.97 (dd, J=1.7, 4.1 Hz, 2H),
8.46 (d, J=9.3 Hz, 2H), 8.30 (m, 2H), 8.24 (dd, J=1.7, 8.3 Hz, 2H), 7.90
(dd, J=1.2, 8.1 Hz, 2H), 7.67 (t, J=7.7 Hz, 2H), 7.46 (dd, J=4.2, 8.3 Hz,
2H), 7.00–6.75 (m, 3H), 4.68 (d, J=5.9 Hz, 2H), 3.83 (s, 3H), 3.76 ppm
(s, 3H); 13C NMR (CDCl3): d=166.5, 157.8, 153.8, 152.0, 150.5, 146.1,
141.2, 138.8, 136.7, 131.8, 129.1, 128.8, 127.1, 126.8, 123.2, 121.3, 116.1,
113.6, 111.5, 56.1, 56.0, 40.5 ppm; LC–MS (ESI): m/z: calcd for
C33H27N4O3+: 527.2 [M+H]+; found: 528.0; r.t.: 5.22 min.
+
55.8, 21.3 ppm; LC–MS (ESI): m/z: calcd for C39H32N3O2
[M+H]+; found: 574.4; r.t.: 8.06 min.
: 574.2
Ethyl 2,6-dichloroisonicotinate (13): Sulfuric acid (5.5 mL, 103 mmol)
and 5 (3.3 g, 17.1 mmol) were mixed in EtOH (50 mL) to give a brown
solution. The reaction vessel was heated to 808C for 2 h before the reac-
tion mixture was poured onto ice (10 g) and filtered through a sintered
glass funnel. The solid was partitioned between CH2Cl2 and H2O, basified
with NaHCO3 and the layers were separated. The aqueous layer was ex-
tracted with CH2Cl2 (3ꢂ25 mL) and the combined organic layers were
dried over Na2SO4, filtered and concentrated. The crude product was pu-
rified by recrystallization from MeOH to give 13 (3.2 g, 14.5 mmol, 85%
yield) as colourless needles. 1H NMR (CDCl3): d=7.80 (s, 2H), 4.42 (q,
J=7.1 Hz, 2H), 1.41 ppm (t, J=7.1 Hz, 3H); 13C NMR (CDCl3): d=
163.0, 151.7, 143.0, 122.9, 62.9, 14.3 ppm.
4-Bromo-2,6-dimethylbenzonitrile (9): 4-Bromo-2,6-dimethylaniline (8,
4.5 g, 22.4 mmol) was taken up in H2O (25 mL) and HCl (37%, 8 mL)
was added. The mixture was sonicated to form a fine suspension and
then cooled to 08C. NaNO2 (1.7 g, 24.2 mmol) in H2O (5 mL) was then
added dropwise and the temperature maintained at 08C. The resulting
solution was stirred at 08C for 30 min and then neutralized by the addi-
tion of solid NaHCO3. The resulting solution was added in portions to a
stirred solution of CuCN (2.4 g, 27.0 mmol) and KCN (3.7 mL,
56.1 mmol) in H2O at 708C. The mixture was then stirred for 30 min, al-
lowed to cool and extracted with toluene. The organic layer was washed
with H2O and brine, dried and concentrated in vacuo. The crude residue
was purified by flash chromatography (SiO2, hexane, 5% EtOAc) to give
(2,6-Dichloropyridin-4-yl)methanol (14): NaBH4 (2.7 g, 70.4 mmol) was
added in portions over 5 min to a solution of 13 (3.1 g, 14.1 mmol) in
EtOH (100 mL) at 08C. The resulting solution was heated to 808C for
3 h. The solution was then evaporated to dryness and taken up in H2O
(80 mL). The excess NaBH4 was then destroyed with 1m HCl (20 mL)
and the solution adjusted to pH 7 with aqueous saturated (sat.) Na2CO3.
This solution was then extracted with CH2Cl2 and the combined organic
fractions concentrated in vacuo. The crude residue was purified by re-
crystallization from MeOH to give 14 (2.3 g, 12.9 mmol, 92% yield) as a
white crystalline solid. Spectra were identical to those reported in the lit-
erature.[57]
1
9 (3.1 g, 14.76 mmol, 66% yield) as an off-white solid. H NMR (CDCl3):
Chem. Eur. J. 2010, 16, 2830 – 2842
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2839

L. Hammarstrçm, O. Johansson et al.
4-Hydroxymethyl-2,6-di(quinolin-8-yl)pyridine (15): Degassed H2O
(2 mL) and MeCN (2 mL) were added to a mixture of 14 (0.75 g,
4.2 mmol), 8-quinolinylboronic acid (1.6 g, 9.3 mmol), K2CO3 (1.7 g,
(ESI): m/z: calcd for C39H35N7O3Ru2+ ([M2+]): 375.6; found: 375.4; r.t.:
4.02 min.
[Ru(12)ACTHNUGTRENN(UG MeCN)3]ACHTUNGTREN(NUGN PF6)2 (19): Compound 19 was synthesized by the
12.6 mmol), SPhos (0.086 g, 0.21 mmol) and [PdACTHNURGTNEUNG(dba)2] (0.12 g,
method used for 18 from RuCl3·xH2O (45.6 mg, 0.174 mmol) and 12
(100 mg, 0.174 mmol) to give 19 as a yellow solid (111 mg, 0.102 mmol,
66% yield). 1H NMR (CD3CN): d=9.14 (dd, J=1.4, 5.1 Hz, 2H), 8.62
(dd, J=1.4, 8.3 Hz, 2H), 8.55 (dd, J=1.2, 7.5 Hz, 2H), 8.26 (dd, J=1.1,
8.2 Hz, 2H), 7.87 (m, 4H), 7.70 (dd, J=5.2, 8.3 Hz, 2H), 7.37 (m, 1H),
6.96 (m, 3H), 3.79 (s, 3H), 3.75 (s, 3H), 2.21 (s, 9H), 1.99 ppm (s, 6H);
13C NMR (CD3CN): d=159.6, 158.2, 154.8, 152.7, 151.7, 147.2, 139.9,
139.6, 137.1, 136.1, 135.2, 135.0, 132.8, 131.6, 129.8, 129.7, 129.6, 129.0,
127.3, 123.1, 117.3, 114.2, 113.8, 56.7, 56.3, 21.0, 4.4, 3.8 ppm; LC–MS
(ESI): m/z: calcd for C45H40N6O2Ru2+ ([M2+]): 399.1; found: 398.6; r.t.:
4.57 min.
0.21 mmol). The suspension was then stirred and degassed a further
5 min before being heated to 1308C for 1 h by microwave heating. The
solution was diluted with CH2Cl2, washed with H2O, dried with Na2SO4
and concentrated in vacuo. The crude product was purified by automated
flash chromatography (SiO2, hexane, EtOAc 10–90%) to give 15 (0.81 g,
2.215 mmol, 53% yield) as a white solid. 1H NMR (CDCl3): d=8.90 (dd,
J=1.8, 4.2 Hz, 2H), 8.18 (dd, J=1.8, 8.3 Hz, 2H), 8.13 (dd, J=1.5,
7.2 Hz, 2H), 7.91 (s, 2H), 7.82 (dd, J=1.5, 8.2 Hz, 2H), 7.59 (m, 2H),
7.38 (dd, J=4.2, 8.3 Hz, 2H), 4.78 ppm (s, 2H); 13C NMR (CDCl3): d=
156.9, 150.1, 149.3, 145.8, 139.4, 136.5, 131.4, 128.6, 128.4, 126.5, 123.0,
120.9, 63.8 ppm; LC–MS (ESI): m/z: calcd for C24H18N3O+: 364.1
[M+H]+; found: 364.4; r.t.: 5.12 min.
Complex 4b: A suspension of 7 (46.6 mg, 0.088 mmol) and [Ru
ACHTUNGTRENNUNG(dqp)-
A
ACHTUNGTRENNUNG
4-Bromomethyl-2,6-di(quinolin-8-yl)pyridine (16):
A solution of PBr3
nBuOH (2 mL) overnight. The red solution was poured into aqueous
NH4PF6 and extracted with CH2Cl2. The organic phase was concentrated
in vacuo and the crude residue purified by flash chromatography on
silica gel (40:4:1 MeCN/H2O/KNO3 (sat.)). Fractions containing the
product were collected, concentrated in vacuo and diluted with aqueous
NH4PF6 before being extracted with CH2Cl2. The PF6 salt was suspended
in degassed MeOH/H2O (1:1) and Dowex 1 anion-exchange resin (Clꢁ
form) was added. The suspension was then stirred for 3 h, filtered and
concentrated in vacuo to give the chloride salt. This salt was then purified
by preparative HPLC (isocratic elution with H2O, 45% MeOH, 50 mm
NaHPO4, pH 7) before being isolated as the PF6 salt as described above
to give 4 (61 mg, 0.049 mmol, 55% yield) as a red solid. 1H NMR
(CD3CN): d=8.23–8.13 (m, 3H), 8.09–8.02 (m, 8H), 7.94–7.87 (m, 3H),
7.81 (dd, J=7.5, 1.3 Hz, 2H), 7.76–7.65 (m, 4H), 7.52–7.42 (m, 4H),
7.09–7.00 (m, 4H), 6.95–6.87 (m, 2H), 6.86–6.78 (m, 1H), 4.55 (d, J=
5.9 Hz, 2H), 3.80 (s, 3H), 3.71 ppm (s, 3H); 13C NMR (CD3CN): d=
164.5, 159.5, 159.4, 158.6, 157.7, 154.5, 152.5, 147.4, 143.5, 139.3, 138.7,
134.3, 134.0, 132.7, 132.5, 131.8, 131.6, 129.0, 128.0, 127.8, 127.8, 127.6,
126.1, 123.1, 123.0, 116.7, 113.2, 112.6, 56.6, 56.2, 40.0 ppm; LC–MS
(ESI): m/z: calcd for C56H41N7O3Ru2+ ([M2+]): 480.6; found: 480.8; r.t.:
4.25 min.
(0.48 mL, 5.0 mmol) in CH2Cl2 (15 mL) was added dropwise to a stirred
solution of 15 (1.5 g, 4.2 mmol) in CH2Cl2 (15 mL) under an inert atmos-
phere at 08C. The solution was allowed to warm to room temperature
and stirred overnight. The solution was recooled to 08C, quenched with
H2O (30 mL), and the aqueous layer was basified with NaHCO3 and ex-
tracted with CHCl3. The combined organic layers were dried with
Na2SO4 and concentrated in vacuo. The residue was purified by automat-
ed flash chromatography (SiO2, hexane, EtOAc 10–90%) to give 16
(1.7 g, 4.0 mmol, 94% yield) as a white solid. 1H NMR (CDCl3): d=9.00
(dd, J=1.8, 4.2 Hz, 2H), 8.29 (dd, J=1.5, 7.2 Hz, 2H), 8.24 (dd, J=1.8,
8.3 Hz, 2H), 8.18 (s, 2H), 7.89 (dd, J=1.5, 8.1 Hz, 2H), 7.67 (m, 2H),
7.46 (dd, J=4.2, 8.3 Hz, 2H), 4.67 ppm (s, 2H); 13C NMR (CDCl3): d=
157.3, 150.3, 145.9, 144.5, 138.8, 136.4, 131.6, 128.7, 128.6, 126.6, 125.6,
121.0, 32.0 ppm; LC–MS (ESI): m/z: calcd for C24H17BrN3
[M+H]+; found: 426.3; r.t.: 6.2 min.
+
: 426.1
10-{[2,6-Di(quinolin-8-yl)pyridin-4-yl]methyl}-phenothiazine (17): 10H-
PTZ (1.6 g, 7.8 mmol) in THF (10 mL) was added dropwise over 2 min to
a suspension of NaH (60%, 0.3 g, 7.8 mmol) in THF (20 mL) at 08C
under an inert atmosphere. The solution was allowed to warm to room
temperature and stirred for an additional 1 h before being recooled to
08C. Then a solution of 14 (1.7 g, 3.9 mmol) in THF (5 mL) was added
dropwise. The crude product was purified by flash chromatography
(SiO2, hexane, EtOAc 50%) to give 17 (1.6 g, 2.9 mmol, 75% yield) as
an off-white solid. 1H NMR (CDCl3): d=8.81 (dd, J=1.8, 4.1 Hz, 2H),
8.32 (dd, J=1.5, 7.2 Hz, 2H), 8.18 (dd, J=1.8, 8.3 Hz, 2H), 8.09 (s, 2H),
7.85 (dd, J=1.5, 8.1 Hz, 2H), 7.65 (dd, J=7.3, 8.1 Hz, 2H), 7.39 (dd, J=
4.2, 8.3 Hz, 2H), 7.08 (m, 4H), 6.90 (m, 4H), 5.30 ppm (s, 2H); 13C NMR
(CDCl3): d=225.3, 157.0, 150.2, 146.0, 144.7, 144.3, 139.1, 136.4, 131.6,
128.7, 128.7, 127.5, 126.7, 126.6, 124.0, 122.9, 122.6, 121.0, 116.0,
53.2 ppm; LC–MS (ESI): m/z: calcd for C36H25N4S+: 545.2 [M+H]+;
found: 545.3; r.t.: 7.89 min.
Complex 1b: Prepared as for 4b from 17 (61.8 mg, 0.102 mmol) and 19
(111 mg, 0.102 mmol), and purified by preparative HPLC (isocratic elu-
tion with H2O, 65% MeOH, 0.1% trifluoroacetic acid (TFA)) to give 1b
(77 mg, 0.051 mmol, 50% yield) as a red solid. 1H NMR (CD3CN): d=
8.18 (dd, J=5.2, 1.4 Hz, 2H), 8.09–7.98 (m, 6H), 7.87 (s, 2H), 7.79 (s,
2H), 7.74 (dd, J=7.4, 1.3 Hz, 2H), 7.69–7.63 (m, 4H), 7.47–7.34 (m, 8H),
7.26 (dd, J=7.6, 1.5 Hz, 2H), 7.20–7.14 (m, 2H), 7.09–6.99 (m, 5H),
6.96–6.84 (m, 6H), 5.36 (q, J=17.5 Hz, 2H), 3.78 (s, 3H), 3.74 (s, 3H),
2.19 ppm (s, 6H); LC–MS (ESI): m/z: calcd for C75H55N7O2RuS2+
AHCTUNGTRENNUNG
A
E
ACHTUNGTRENNUNG
[Ru(7)ACHTUNGTRENNUNG(MeCN)3]ACHTUNGTRENNUNG(PF6)2 (18): A solution of RuCl3·xH2O (248 mg,
HPLC (isocratic elution with H2O, 55% MeOH, 0.1% TFA) to give 2b
(32 mg, 0.025 mmol, 28% yield) as a red solid. 1H NMR (CD3CN): d=
8.23–8.15 (m, 3H), 8.12–8.06 (m, 6H), 7.91 (d, J=8.1 Hz, 2H), 7.79 (s,
2H), 7.78–7.72 (m, 4H), 7.71–7.66 (m, 4H), 7.38–7.50 (m, 4H), 7.37 (s,
2H), 7.06 (ddd, J=6.9, 10.6, 18.1 Hz, 5H), 6.91 (td, J=2.9, 4.2 Hz, 2H),
3.79 (s, 3H), 3.75 (s, 3H), 2.20 ppm (s, 6H); 13C NMR (CD3CN): d=
159.5, 159.4, 157.9, 157.8, 154.7, 152.2, 151.6, 147.6, 139.8, 139.1, 138.6,
138.6, 137.1, 136.0, 134.4, 134.0, 132.8, 132.6, 131.6, 129.8, 129.6, 128.9,
127.8, 127.6, 127.5, 122.9, 117.4, 114.1, 113.7, 56.6, 56.2, 20.9 ppm; LC–MS
(ESI): m/z: calcd for C62H46N6O2Ru2+ ([M2+]): 504.1; found: 503.9; r.t.:
4.73 min.
0.95 mmol) and 7 (500 mg, 0.95 mmol) was heated at reflux in EtOH
(25 mL) overnight. The resulting inorganic solid was isolated by filtration
and washed with EtOH (3ꢂ10 mL) and Et2O (3ꢂ10 mL). The solid was
dried in vacuo to give [Ru(7)Cl3] (630 mg, 0.86 mmol, 90% yield) as a
green/brown solid. The crude product (630 mg, 0.86 mmol) was suspend-
ed in a mixture of H2O (5 mL), EtOH (5 mL) and MeCN (25 mL), and
AgNO3 (481 mg, 2.8 mmol) was added. The solution was stirred at 808C
for 5 h before being filtered through celite, washed with MeCN and con-
centrated in vacuo. The crude residue was purified by flash chromatogra-
ꢁ
phy (SiO2, 40:4:1 MeCN/H2O/KNO3 (sat.)) and isolated as the PF6 salt
to give 18 as a yellow solid (830 mg, 0.80 mmol, 84% yield over two
steps). 1H NMR (CD3CN): d=9.07 (dd, J=5.1, 1.4 Hz, 2H), 8.63–8.60
(m, 4H), 8.28 (dd, J=8.2, 1.2 Hz, 2H), 8.24 (s, 2H), 7.93 (dd, J=8.1,
7.6 Hz, 2H), 7.85 (t, J=5.8 Hz, 1H), 7.67 (dd, J=8.2, 5.2 Hz, 2H), 6.92–
6.79 (m, 3H), 4.55 (d, J=5.8 Hz, 2H), 3.79 (s, 3H), 3.70 (s, 3H), 2.44 (s,
6H), 2.15 ppm (s, 3H); 13C NMR (CD3CN): d=164.5, 159.8, 159.2, 154.6,
152.7, 147.2, 144.4, 139.8, 135.2, 135.2, 133.3, 129.7, 129.2, 128.1, 127.6,
126.2, 123.36, 116.7, 113.4, 112.7, 56.7, 56.3, 40.2, 4.6, 3.9 ppm; LC–MS
Complex 3b: Prepared as for 4b from 17 (209 mg, 0.384 mmol) and 18
(400 mg, 0.384 mmol), and purified by preparative HPLC (65% MeOH
in H2O) to give 3b (335 mg, 0.229 mmol, 60% yield) as a red solid.
1H NMR (CD3CN): d=8.15 (s, 2H), 8.07–7.99 (m, 6H), 7.94 (dd, J=5.2,
1.4 Hz, 2H), 7.85 (s, 2H), 7.84–7.76 (m, 3H), 7.69–7.62 (m, 4H), 7.49–
7.36 (m, 6H), 7.25 (dd, J=7.6, 1.5 Hz, 2H), 7.19–7.10 (m, 2H), 7.06–6.98
(m, 4H), 6.94–6.79 (m, 7H), 5.35 (q, J=17.4 Hz, 2H), 4.55 (d, J=5.9 Hz,
2840
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 2830 – 2842

Electron Transfer in Donor–Photosensitizer–Acceptor Triads
FULL PAPER
2H), 3.79 (s, 3H), 3.71 ppm (s, 3H); LC–MS (ESI): m/z: calcd for
C69H50N8O3RuS2+ ([M2+]): 586.14; found: 586.26; r.t.: 4.90 min.
with an optical density of ꢃ0.1 at the excitation wavelength. All samples
were Ar(g)-purged for >10 min additionally prior to meaACHTUNGTRENNUNGsurements,
which were performed under Ar(g) pressure. Emission quantum yields
were determined from the integrated emission peaks of the corrected
Complex 4a: BBr3 (1m in hexanes, 1.6 mL, 1.6 mmol) was added drop-
wise to a solution of 4b (40 mg, 0.032 mmol) in CH2Cl2 (4 mL) under an
inert atmosphere at ꢁ788C. The solution was allowed to warm slowly to
room temperature overnight. The solution was then diluted with CH2Cl2,
cooled to 08C and quenched with aqueous NH4PF6. The organic layer
was separated and the aqueous layer washed with CH2Cl2/10% MeOH.
The organic layers were combined and concentrated in vacuo before the
crude residue was purified by flash chromatography on silica gel (40:4:1
MeCN/H2O/KNO3 (sat.)). Fractions containing the product were collect-
ed, concentrated in vacuo and diluted with aqueous NH4PF6 before being
extracted with CH2Cl2/10% MeOH. The resulting solution was concen-
trated in vacuo to give the hydroquinone as a red solid. The product was
dissolved in acetone (5 mL) at room temperature under an inert atmos-
phere and DDQ (35 mg, 0.155 mmol) was added. The resulting solution
was stirred for 3 h. The solution was triturated with Et2O to precipitate
the product, which was collected by filtration and washed several times
with Et2O to remove DDQ. Complex 4a (20 mg, 0.016 mmol, 52%) was
obtained as a red solid and used without further purification. 1H NMR
(CD3CN): d=8.21–8.13 (m, 3H), 8.10–8.02 (m, 8H), 7.90 (d, J=8.1 Hz,
2H), 7.85–7.77 (m, 3H), 7.76–7.65 (m, 6H), 7.47 (dd, J=15.6, 7.8 Hz,
4H), 7.10–7.00 (m, 4H), 6.86–6.74 (m, 2H), 6.69–6.65 (m, 1H), 4.40 ppm
(t, J=4.1 Hz, 2H); HRMS (ESI-FTICR MS): calcd for C54H35N7O3Ru2+
([M2+]): 465.592245; found: 465.58743; calcd for C54H35F6N7O3PRu:
1076.14867 [M+PF6]+; found: 1076.10921; elemental analysis calcd (%)
for C56H39Cl4F12N7O3P2Ru (4a·2CH2Cl): C 48.36, H 2.83, N 7.05; found:
C 48.33, H 3.09, N 7.40.
spectra, normalized by the optical density at 520 nm. [Ru
(PF6ꢁ)2, in which Fem =0.02,[24] was measured under the same conditions
and used as a reference.
ACHTUNGTRENNUNG
(dqp)2]2+
AHCTUNGTRENNUNG
Time-correlated single-photon counting: Time-correlated single-photon
counting was set up with a regenerative amplified Ti:sapphire system
from Coherent, which produced 800 nm pulses of 150 fs duration at
200 kHz repetition rate. The 400 nm pump was obtained by doubling of
the Ti:sapphire fundamental. Samples were prepared in air-equilibrated
MeCN to an optical density of ꢃ0.1 at 400 nm in a 1ꢂ1 cm2 quartz opti-
cal cell. The sample chamber was optimized for minimal scattering and a
combination of a 455 nm cut-off filter and 751 nm bandpass filter was
used between the sample and detector to remove scattered pump light
and reduce filter emission. A polarizer set to magic angle (54.78) versus
the vertically polarized pump was used in front of the detector. The emis-
sion was collected perpendicular to the probe on a cooled Hamamatsu
R38094-5 microchannel-plate PMT with a response function of ~80 ps
FWHM. The data contain additional short-lived components, not report-
ed in the text, with no counterpart in the transient absorption measure-
ments, due to filter fluorescence or other experimental defects.
Nanosecond time-resolved emission and transient absorption spectrosco-
py: Time-resolved emission and transient absorption measurements on
the nanosecond to microsecond timescale were performed with a fre-
quency-tripled Q-switched Nd:YAG laser (Quantel) pumping an optical
parametric oscillator (Opotek) to obtain 520 nm pump light of ꢃ10 ns
pulse duration. Spectroscopic-grade MeCN purged with Ar(g) to remove
oxygen was used. Samples were prepared in 1ꢂ1 cm2 optical quartz cells
and all samples were Ar(g)-purged for >10 min additionally prior to
measurements, which were performed under Ar(g) pressure. The emis-
sion was detected at right angles to the incoming laser beam with a
Complex 1a: Prepared as for 4a from 1b (70 mg, 0.046 mmol) to give 1a
(31 mg, 0.021 mmol, 46%) as a red solid. 1H NMR (CD3CN): d=8.17
(dd, J=5.2, 1.4 Hz, 2H), 8.08–7.98 (m, 6H), 7.86 (s, 2H), 7.77 (s, 2H),
7.72 (dd, J=7.5, 1.3 Hz, 2H), 7.68–7.63 (m, 4H), 7.47–7.35 (m, 6H), 7.34
(s, 2H), 7.25 (dd, J=7.6, 1.5 Hz, 2H), 7.20–7.13 (m, 2H), 7.08–6.99 (m,
4H), 6.95–6.84 (m, 7H), 5.35 (q, J=17.4 Hz, 2H), 2.18 ppm (s, 6H);
HRMS (ESI-FTICR MS): calcd for C73H49N7O2RuS2+ ([M2+]):
594.63559; found: 594.61664; calcd for C73H49F6N7O2PRuS2+ ([M2+]):
1334.23537; found: 1334.18609; elemental analysis calcd (%) for
C73H49F12N7O2P2RuS: C 59.27, H 3.34, N 6.63; found: C 59.33, H 3.58, N
6.41.
monoACTHNUGRTENUNGchromator and an R928-type PMT. The same set-up and detector
were used for transient absorption spectroscopy, in which the probe light
was provided by a pulsed Xe arc lamp. The output was recorded on a
Hewlett–Packard digital oscilloscope and processed with Applied
PhotophysACTHNUTRGNEUiGN cs LKS60 software.
Ultrafast transient absorption spectroscopy: The Ti:sapphire laser system
and experimental set-up for transient absorption pump–probe measure-
ments on the femtosecond scale used here have been previously de-
scribed elsewhere.[58] Pump light of 480 nm was obtained by three-wave
mixing of the optical paramagnetic amplifier (TOPAS) output at 1440
and 720 nm. Filters were used to remove stray light and obtain a clean
pump. The white-light probe was generated from a fraction of the
800 nm Ti:sapphire output passed through an optical delay line (<10 ns
total delay) and focused onto a moving CaF2 plate. The polarization of
the pump was set at magic angle (54.78) relative to that of the probe by
using a polarizer and a l/2 plate. Samples were prepared in a 1ꢂ10 mm2
optical cell to an approximate optical density of 0.2 at the excitation
wavelength in spectroscopic-grade MeCN, purged with Ar(g) prior to
sample preparation to remove oxygen. The pump energy at the vertically
moving sample cell was reduced by filters to <1 mJ (diameter <400 mm)
to avoid sample degradation and non-linear effects. The reported spectra
are averages of 500–10000 individual measurements. The chirp was much
smaller than 5 ps, which is the earliest time at which spectra are shown.
Complex 2a: Prepared as for 4a from 2b (20 mg, 0.015 mmol) to give 2a
(9.4 mg, 7 mmol, 49%) as a red solid. 1H NMR (CD3CN): d=8.24–8.15
(m, 3H), 8.12–8.05 (m, 6H), 7.91 (d, J=8.1 Hz, 2H), 7.78 (s, 2H), 7.76–
7.71 (m, 4H), 7.71–7.65 (m, 4H), 7.49–7.37 (m, 4H), 7.37–7.33 (m, 2H),
7.12–7.04 (m, 4H), 6.92–6.83 (m, 3H), 2.21 ppm (s, 6H); HRMS (ESI-
FTICR MS): calcd for C60H40N6O2Ru2+ (M2+): 489.11281; found:
489.12072; calcd for C60H40F6N6O2PRu+: 1123.18980 [M+PF6]+; found:
1123.14751; elemental analysis calcd for C60H40F12N6O2P2Ru (2a):
C
56.83, H 3.18, N 6.63; found: C 56.69, H 3.32, N 6.59.
Complex 3a: Prepared as for 4a from 3b (150 mg, 0.103 mmol) to give
3a (61 mg, 0.042 mmol, 41%) as a red solid. 1H NMR (CD3CN): d=
8.19–8.14 (m, 2H), 8.08–7.98 (m, 6H), 7.97–7.91 (m, 3H), 7.87–7.77 (m,
4H), 7.70–7.62 (m, 4H), 7.49–7.36 (m, 6H), 7.27–7.21 (m, 2H), 7.18–7.12
(m, 2H), 7.08–6.98 (m, 4H), 6.95–6.88 (m, 2H), 6.87–6.82 (m, 2H), 6.81–
6.60 (m, 3H), 5.43–5.27 (m, 2H), 4.48 (d, J=6.1 Hz, 0.5H), 4.41–
4.35 ppm (m, 1.5H); HRMS (ESI-FTICR MS): calcd for
C67H44N8O3RuS2+ ([M2+]): 571.11503; found 571.09923; calcd for
C67H44F6N8O3PRuS+: 1287.19424 [M+PF6]+; found: 1287.15548.
General considerations and photostability: The compounds were stored
under dark and oxygen-free conditions, and exposed to light and air only
for short periods during sample preparation. In general, the solvent was
purged with Ar(g) to remove oxygen prior to sample preparation.
Steady-state emission and absorption spectroscopy were used in between
measurements and scan-to-scan degradation was carefully monitored in
the transient absorption measurements.
Steady-state absorption and emission spectroscopy; emission quantum
yields: UV/Vis absorption spectra were measured on a Varian Cary 5000
instrument in a 1ꢂ1 cm2 optical quartz cell with spectroscopic-grade
MeCN. Emission spectra were recorded on a Horiba Fluorolog (Jobin
Yvon) fluorimeter equipped with double monochromators for excitation
and emission. The emission upon 520 nm excitation was detected at 908
with an R928P photomultiplier tube (PMT) detector in single-photon-
counting mode with 5.0 nm resolution on the monochromators. The re-
ported spectra were corrected for varying detector sensitivity at different
wavelengths. Spectroscopic-grade MeCN purged with Ar(g) to remove
oxygen was used. Samples were prepared in 1ꢂ1 cm2 optical quartz cells
Chem. Eur. J. 2010, 16, 2830 – 2842
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2841

L. Hammarstrçm, O. Johansson et al.
[31] R. A. Berthon, S. B. Colbran, G. M. Moran, Inorg. Chim. Acta 1993,
204, 3–7.
Acknowledgements
[32] K. S. Schanze, K. Sauer, J. Am. Chem. Soc. 1988, 110, 1180–1186.
[33] V. Goulle, A. Harriman, J.-M. Lehn, J. Chem. Soc. Chem. Commun.
1993, 1034–1036.
This work was financially supported by the Swedish Energy Agency, the
Knut and Alice Wallenberg Foundation, and the EU/Energy Network
project SOLAR-H2 (FP7 contract 212508).
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(DGET)=E00 +e
ꢁ
G
[E
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1/2ACHTGNUTRENNUNG
ꢁ
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E
CHTUNGTRENNUNG
(PTZ+/0)], respectively. The Cou-
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ACHTUNGTRENNUNG
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N
U
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Received: October 2, 2009
Published online: January 19, 2010
2842
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