Conformational effects on long-range electron transfer: Comparison of oligo-p-phenylene and oligo-p-xylene bridges

Article abstract of DOI:10.1002/ejic.200900396

Phototriggered intramolecular electron transfer across variable-length oligo-p-phenylene and oligo-p-xylene bridges was investigated in seven molecules. For both, types of bridges, charge transfer rates decrease exponentially with increasing number of spa

Full text of DOI:10.1002/ejic.200900396

FULL PAPER
DOI: 10.1002/ejic.200900396
Conformational Effects on Long-Range Electron Transfer: Comparison of
Oligo-p-phenylene and Oligo-p-xylene Bridges
David Hanss[a] and Oliver S. Wenger*[a]
Keywords: Charge transfer / Donor-acceptor systems / Molecular wires / Photochemistry / Ruthenium
Phototriggered intramolecular electron transfer across vari- experimental data, phenyl-phenyl coupling is found to be
able-length oligo-p-phenylene and oligo-p-xylene bridges
was investigated in seven molecules. For both types of brid-
ges, charge transfer rates decrease exponentially with in-
creasing number of spacer units. The distance decay param-
eter is 0.21 Å–1 for the phenylenes and 0.77 Å–1 for the xy-
lenes. A simple analysis based on superexchange theory in-
dicates that this difference is due to unequal electronic cou-
pling between adjacent bridging units. On the basis of the
roughly 7 times stronger than xylyl-xylyl coupling. This dif-
ference in electronic coupling strengths can be explained
satisfactorily on the sole basis of conformational effects. It is
consistent with equilibrium torsion angles of 35–40° between
two phenyls and 65–70° between two xylyls.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
omenological parameter with little actual physical meaning
because the overall donorǞacceptor electron transfer is a
combination of several individual electron transfer steps be-
tween adjacent bridging units.
Introduction
“Flatter is better” is the current credo when it comes to
finding molecular wires that are capable of mediating
charge transfer over several nanometers. As a matter of
fact, the most efficient organic molecular wires known to
date are highly π-conjugated (“flat”) systems such as oligo-
p-phenylene vinylenes,[1] oligo-p-phenylene ethynylenes,[2]
oligo-fluorenes,[3] and oligo-p-phenylenes.[4] These materials
mediate long-range charge transfer far better than al-
kanes,[5] peptide bridges,[6] or ordinary protein backbone.[7]
A commonly used measure to describe the efficiency of
long-range electron transfer is its distance decay parameter
(β). The smaller the β value, the longer is the distance over
which charge can be transferred in a given amount of time.
Typical values for β are 0.01–0.1 Å–1 for some of the above-
mentioned π-conjugated wires,[1a,1c,1e,3a,3b] 0.9–1.4 Å–1 for
non-conjugated covalent bridges,[5,6,8] and 1.2–1.7 Å–1 for
charge transfer across frozen solvent matrices.[9] However,
these β values are governed by several factors that some-
times all vary at the same time when one is to compare
different molecular wires with one another. One complica-
tion emerges from the fact that matching of the energy
levels of the wire and the donor may affect strongly the β
values.[1a,10] In practice it can therefore be difficult to isolate
the influence of conformational effects on the long-range
charge transfer efficiency. Another point to consider is that
for some molecular wires, the β value is reduced to a phen-
There have been several prior studies on the influence of
molecular conformation on electronic coupling across bi-
phenyl bridging units.[11–13] In the current work we have
sought to explore conformational effects on charge transfer
across longer bridges. In view of the abovementioned chal-
lenges, suitable model systems were selected according to
the following criteria: (i) the molecular bridge should medi-
ate long-range charge transfer by a single-step tunneling
process, (ii) the overall wire should be comprised of a
changeable number of identical units, (iii) a significant al-
teration in equilibrium conformation between the individ-
ual bridging units should be easily attainable, and (iv) the
donors and acceptors should always be the same. On this
basis we identified oligo-p-phenylene and oligo-p-xylene
molecules as promising bridges. The equilibrium torsion an-
gle between adjacent p-xylene units is substantially greater
than that between two neighboring phenyl moieties due to
steric hindrance.[4f,14] Length variation is synthetically via-
ble for both types of bridges,[15] and they can both be at-
tached relatively easily to common donors and ac-
ceptors.[14,16] Our choice fell upon a ruthenium(II) tris(2,2Ј-
bipyridine) complex [Ru(bpy)32+] as a photosensitizer and
a phenothiazine (PTZ) as an electron donor, because pho-
totriggered charge transfer between these two redox part-
ners produces clear-cut spectroscopic signatures.[17–21] In
prior work we have already investigated phototriggered
charge tunneling across oligo-p-xylene bridges,[14,22]
amongst others in the Ru-xyn-PTZ molecules from
Scheme 1.[23] Here, we report on a similar investigation of
[a] University of Geneva, Department of Inorganic, Analytical and
Applied Chemistry,
30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
Fax: +41-22-3796069
E-mail: oliver.wenger@unige.ch
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Conformational Effects on Long-Range Electron Transfer
oligo-p-phenylene bridged systems (Ru-phn-PTZ). Com-
parison of the phenylene and xylene charge transfer data
provides direct insight into the importance of conforma-
tional effects on long-range charge transfer.
Scheme 1. The molecules investigated in this work (bpy = 2,2Ј-bi-
pyridine).
Scheme 2. Synthesis of the phenothiazine-(oligo-p-phenylene)-bi-
pyridine ligands needed for the Ru-phn-PTZ molecules: (a)
Pd(dba)2, P(tBu)3, tBuOK, toluene (60 °C); (b) ICl, CH3CN/
CH2Cl2 (3:1, 25 °C); (c) Pd(PPh3)4, m-xylene (reflux); (d)
Pd(PPh3)4, Na2CO3, toluene/ethanol/water (85:10:5, reflux).
Results and Discussion
Synthesis of Phenothiazine-Phenylene-Bipyridine Ligands
to bipyridine 5 and subsequent (column chromatography)
purification of the desired coupling product 14 from unde-
sired byproducts is still possible, but for the tetra-p-phenyl-
ene congener this is not the case any more. We are thus
limited to a bridge length comprised of three phenyl units,
whereas for the p-xylene bridges solubility was no issue up
to n = 4. It is plausible that intermolecular stacking interac-
tions are responsible for the lower solubility of the phenyl-
ene molecules: oligo-p-phenylenes adopt more planar equi-
librium conformations than oligo-p-xylenes (see below),
and this may favor formation of aggregates through π-π in-
teractions.
Complexation of the ligands from Scheme 2 to ruthe-
nium is a synthetically trivial matter: Ru(bpy)2Cl2 precursor
is reacted with 6, 10, or 14 in ethanol/chloroform mixtures
at reflux,[28] whereby the target molecules (Ru-phn-PTZ) are
obtained.
The most important part in the synthesis of the mole-
cules from Scheme 1 is the synthesis of phenothiazine-
bridge-bipyridine ligands as illustrated in Scheme 2 for the
oligo-p-phenylene systems. Five different molecular build-
ing blocks are necessary for this: Phenothiazine (1) (com-
mercially available), (4-bromophenyl)trimethylsilane 2 (ac-
cessible in one step from buyable 1,4-dibromoben-
zene),[14,15,24]
5-(tri-n-butylstannyl)-2,2Ј-bipyridine
(5)
(made in 3 steps from the commercial chemicals 2-bromo-
pyridine and 2,5-dibromopyridine),[25] 4-bromo-4Ј-(trimeth-
ylsilyl)biphenyl (7) (available in one step from the buyable
4,4Ј-dibromo compound),[14] and 4-trimethylsilylphenylbo-
ronic acid 11 (synthesized in two steps from 1,4-dibro-
mobenzene).[14,15,24] With these building blocks at hand, the
desired ligands are accessible in acceptable yields using only
four different reaction types: (a) a palladium(0)-catalyzed
N–C coupling to attach the phenothiazine to the phenylene
bridges,[26] (b) a trimethylsilyl (TMS)–iodo exchange to de-
protect and activate the coupling product for further cross-
coupling reactions,[15] (c) coupling of the iodo species to the
2,2Ј-bipyridine (bpy) ligand using a Stille methodology, and
(d) lengthening of the bridge in a Suzuki-type C–C coupling
π-Conjugation Effects
Figure 1 shows the optical absorption spectra of the Ru-
reaction. Reactions (a), (b), (d) all proceed with satisfactory phn-PTZ and Ru-xyn-PTZ molecules from Scheme 1 in ace-
yields [typically superior to 60 percent for (a) and (b), and tonitrile solutions at room temperature. Three absorption
80% for (d)]. Substantially lower yields (23% to 54%) result bands are easily noticeable in all seven spectra, notably the
for the Stille couplings (d). We have sought to replace stan- ruthenium(II)-to-bipyridine metal-to-ligand charge transfer
nane 5 by the corresponding boronic acid or its pinacol (MLCT) band at ca. 460 nm, a bpy-localized π-π* band at
ester, but our own attempts to synthesize any of these build- ca. 290 nm, and a band around 260 nm that is due to a
ing blocks following an existing synthetic protocol have so PTZ-localized electronic transition.[23] A fourth absorption
far been unsuccessful.[27] Global yields for the final ligands band is noticeable as a detached band only in the Ru-ph2-
from Scheme 2 are 48% (6), 20% (10), and 7% (14).
PTZ and Ru-ph3-PTZ molecules with maxima at 320 nm
The synthesis of the analogous p-xylene-bridged pheno- and 330 nm, respectively. In all other spectra, notably in
thiazine-bipyridine ligands has been previously reported by those of all Ru-xyn-PTZ molecules, it merely appears as a
us.[23] Briefly, it follows an identical synthetic strategy, only shoulder to the above-mentioned bpy π-π* band. For both
with methylated analogs of building blocks 2, 7, and 11. An types of molecular bridges, this absorption red-shifts with
important difference between the phenylene- and xylene- increasing number of bridging units, and it does so more
bridged molecules concerns their solubility. Tri-p-phenylene than any other absorption band. Thus it appears plausible
molecule 13 is already rather poorly soluble in all common to assign these absorption bands to predominantly bridge-
organic solvents even at elevated temperatures. Its coupling localized electronic transitions.[29]
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D. Hanss, O. S. Wenger
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thaw deoxygenated acetonitrile solution, all Ru-xyn-PTZ lu-
minescence lifetimes are between 828 and 861 ns (Table 1),
whereas that of the free Ru(bpy)32+ complex is 865 ns under
identical experimental conditions. The Ru-phn-PTZ emis-
sion lifetimes are 139 ns (n = 3), 75 ns (n = 2), 30 ns (n = 1).
Thus it is obvious that an excited-state quenching process is
at work in the Ru-phn-PTZ molecules that is inactive in
the Ru-xyn-PTZ systems. There exist numerous reports on
2+
reductive quenching of the Ru(bpy)3 3MLCT state by
phenothiazine.[17,19,20,31] However, the driving-force associ-
2+
ated with PTZǞ*Ru(bpy)3 electron transfer is usually
very low, and therefore in certain instances this process is
inefficient and little to no luminescence quenching is ob-
servable.[32] We have previously used this argument to ex-
plain the absence of quenching in the Ru-xyn-PTZ mole-
cules.[23a] Now, for the Ru-phn-PTZ systems, such quench-
ing is observed, suggesting that PTZǞ*Ru(bpy)32+ excited-
state electron transfer is more exergonic in these new mole-
cules.
Figure 1. Optical absorption spectra of the molecules from
Scheme 1 in acetonitrile solution at room temperature.
The observed red-shifts are consistent with increasing π-
conjugation with increasing length – an effect which is ex-
pected to be more important for the oligo-p-phenylenes
than for the oligo-p-xylenes due to steric reasons.[4f] Inter-
phenyl torsion angles are of the order of 35° (calculated
gas-phase structures) which implies substantial π-conjuga-
tion across adjacent phenyls.[4c,30] Analogous calculations
on oligo-p-xylene molecules (see below) indicate that inter-
xylyl equilibrium torsion angles are closer to 70°, angle at
which the overlap between pz-orbitals from neighboring xy-
lyls is substantially smaller. This data interpretation is in
line with that from recent work on similarly substituted
oligo-p-phenylene wires.[4f] Analogous experimental obser-
vations have also been made in our prior work on p-xylene
and p-phenylene bridged rhenium(I)-phenothiazine sys-
tems.[14,22]
Figure 2. Luminescence spectra of the molecules from Scheme 1 in
acetonitrile solution at room temperature. Excitation occurred at
450 nm; the optical density at this wavelength was 0.10 for all sam-
ples.
Excited-State Quenching Due to Electron Transfer in the
Ru-phn-PTZ Molecules
This driving-force issue can be addressed with electro-
When acetonitrile solutions of the molecules from chemical experiments. The cyclic voltammograms in Fig-
1
Scheme 1 are excited into the RuIIǞbpy MLCT band at ure 3 were obtained from acetonitrile solutions of Ru-ph1-
450 nm, the emission spectra in Figure 2 are observed. The PTZ and Ru-xy1-PTZ molecules in presence of a ferrocene
shapes and the energetic positions of the luminescence (Fc) reference and tetrabutylammonium hexafluorophos-
2+
bands are virtually identical to those of the free Ru(bpy)3
phate electrolyte. One observes the typical waves associated
complex. On this basis we assign the luminescence in our with bpy reduction (ca. –1.2 V vs. SCE), PTZ oxidation (ca.
Ru-phn-PTZ and Ru-xyn-PTZ molecules to the same +0.75 V vs. SCE), and RuII oxidation (ca. 1.25 V vs. SCE).
3MLCT emission as observed for the reference complex. The values given here and in Table 2 are half-wave poten-
Significant differences in emission intensities are observed tials (E1/2), but in Figure 3 we show four vertical dashed
between Ru-phn-PTZ molecules of different bridge length: lines that mark the positions of oxidation wave maxima.
The shorter the bridge, the weaker the luminescence signal This makes the following interesting observation more obvi-
is. The Ru-xyn-PTZ molecules, by contrast, exhibit bridge- ous: PTZ oxidation occurs at a less positive potential in Ru-
length independent luminescence intensities. Time-resolved ph1-PTZ than in Ru-xy1-PTZ, the difference in E1/2 values
luminescence experiments are consistent with the results is 70 mV. It appears tempting to attribute the discrepancy
from steady-state emission spectroscopy: in freeze-pump- in redox potentials to differences in π-conjugation, after all
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Conformational Effects on Long-Range Electron Transfer
Table 1. Donor-acceptor distances (rDA), luminescence quantum
yields (φlum) relative to Ru(bpy)32+, MLCT luminescence lifetimes
(τMLCT), rate constants for intramolecular (thermal) charge recom-
bination (kCR) between Ru(bpy)3 and PTZ·+ in the Ru-phn-PTZ
+
molecules, and rate constants for intramolecular PTZ-to-RuIII
(bpy)3 ground-state electron transfer (kET,g.s.) in the Ru-xyn-PTZ
-
3+
molecules. All measurements were made in acetonitrile solution.
τMLCT [ns][b] kCR [s–1][b] kET, g.s. [s–1][b]
[a]
rDA [Å] φlum
Ru-ph1-PTZ
Ru-ph2-PTZ
Ru-ph3-PTZ
Ru-xy1-PTZ
Ru-xy2-PTZ
Ru-xy3-PTZ
Ru-xy4-PTZ
[Ru(bpy)3]2+
10.6
14.9
19.2
10.6
14.9
19.2
23.5
0.01
0.04
0.10
0.92
0.87
0.85
0.95
1.00
30
75
6.9ϫ107
2.3ϫ107
1.1ϫ107
139
842
828
861
855
865
Ն 108
3.8ϫ107
7.1ϫ105
4.9ϫ104
[a] Relative to [Ru(bpy)3]2+. [b] In freeze-pump-thaw deoxygenated
acetonitrile.
Figure 3. Cyclic voltammograms for the mono-p-phenylene and
mono-p-xylene bridged dyads in acetonitrile solution using tetrabu-
tylammonium hexafluorophosphate as supporting electrolyte. The
dashed vertical lines mark oxidation peak potentials, but the values
given in Table 1 are half-wave potentials.
the PTZ-ph torsion angle is expected to be significantly
lower than the PTZ-xy dihedral angle for steric reasons.
However, we also note that the uncertainty associated with
our experimental redox potentials is on the order
ofϮ0.05 V. The absorption (Figure 1) and luminescence
Table 2. Half-wave redox potentials (E) of the dyads and two refer-
spectra (Figure 2) indicate that the energy of the emissive ence molecules, measured in acetonitrile solution at room-tempera-
3MLCT state is the same in all seven molecules from
ture using an Ag/AgCl quasi-reference electrode, but reported vs.
SCE.
Scheme 1. From the similarity of these spectra to those of
2+
2+/+
3+/2+
E(Ru{bpy}3
)
E(Ru{bpy}3
)
E(PTZ+/0
)
the Ru(bpy)3 reference complex we assume an identical
3MLCT energy of E00 = 2.12 eV.[33] This leads to an esti-
[V vs. SCE]
[V vs. SCE]
[V vs. SCE]
mated potential of roughly 0.9 V vs. SCE for one-electron Ru-ph1-PTZ
–1.22
–1.25
–1.24
–1.22
–1.22
–1.17
–1.27
–1.28
1.24
1.23
1.23
1.22
1.26
1.21
1.17
1.26
0.72
0.70
0.72
0.79
0.75
0.80
0.73
2+
Ru-ph2-PTZ
reduction of photoexcited Ru(bpy)3 for all Ru-phn-PTZ
Ru-ph3-PTZ
and Ru-xyn-PTZ molecules. In first approximation,[34] this
Ru-xy1-PTZ
yields the following reaction free energies: –∆GET,xy
≈
Ru-xy2-PTZ
Ru-xy3-PTZ
Ru-xy4-PTZ
[Ru(bpy)3]2+
10-(p-xylyl)-PTZ
0.11 eV for Ru-xyn-PTZ and –∆GET,ph ≈ 0.18 eV for Ru-phn-
PTZ based on the E1/2 values in Table 2. Given the approxi-
mate nature of these calculated –∆GET values and the exper-
imental uncertainty associated with the redox potentials ob-
tained from cyclic voltammetry, it would be daring to draw
more quantitative conclusions than to state that both ex-
cited-state electron transfers have very small driving forces.
We are thus forced to accept it as a simple fact that (for
kinetic reasons) excited-state electron transfer is observed
for the Ru-phn-PTZ molecules but not for the Ru-xyn-PTZ
dyads.
0.79
Direct experimental evidence for excited-state electron
transfer in the Ru-phn-PTZ molecules comes from transient
absorption spectroscopy. The spectrum shown in Figure 4
was measured on a 2ϫ10–5  sample of Ru-ph1-PTZ in
deoxygenated acetonitrile; the data was acquired in a 100-
ns time window starting 30 ns after the 457.9-nm excitation
pulse. The positive signal around 380 nm can be attributed
to a one-electron reduced bpy ligand (bpy·–),[32,35] whereas
the bleach between 400 and 480 nm is due to the disappear-
ance of RuII.[23,32] The positive signal between 480 and
550 nm can be assigned to overlapping absorptions from
[Ru(bpy)3]+ and phenothiazine radical cation (PTZ·+).[18–20]
From prior work it is known that the molar extinction coef-
Figure 4. Transient absorption spectrum measured on a 2ϫ10–5

solution of Ru-ph1-PTZ in freeze-pump-thaw deoxygenated aceto-
nitrile. Excitation occurred at 457.9 nm with 10-ns laser pulses
(λexc), and the signal was detected during a 100-ns time window
starting 30 ns after the pulse.
+
ficient of Ru(bpy)3 at 510 nm is a factor of 2 greater than
that of PTZ·+ at the same wavelength (εRu+,510
=
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D. Hanss, O. S. Wenger
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10Ј000 –1 cm–1, εPTZ·+,510 = 5Ј000 –1 cm–1).[18,32,36,37] This account for this behavior.[32] Due to the close energetic
explains why the Ru-ph1-PTZ transient absorption band be- proximity of these two states in our Ru-phn-PTZ molecules
tween 480 and 550 nm reflects more the symmetrical band- (see above), the possibility of a quasi-equilibrium must be
+
shape of the Ru(bpy)3 signal than that of the more asym- considered. Direct experimental evidence for this phenome-
metrical PTZ·+ absorption.[18–20] Analogous transient ab- non can be obtained from the transient absorption data,
sorption spectra were obtained for the longer congeners of since the two electronic states in question have different
2+
the Ru-phn-PTZ molecules (n = 2 or 3), but for the Ru-xyn- spectra: at 510 nm, 3MLCT-excited Ru(bpy)3
absorbs
PTZ molecules no such transient absorption bands can be only very weakly (εMLCT Ͻ 500 –1 cm–1),[35] but the
observed. In short, PTZ-to-RuII excited-state electron Ru(bpy)3+-PTZ·+ charge-separated state produces a strong
transfer occurs in the Ru-phn-PTZ molecules but not in the absorption at this wavelength (εRu+,510 + εPTZ·+,510
=
Ru-xyn-PTZ systems.
15Ј000 –1 cm–1; see above).[18,36,37] At 370 nm, both states
There is no evidence for triplet-triplet energy transfer have the same extinction (εbpy,370 = 20Ј000 –1 cm–1) because
from the photoexcited metal center to the phenylene or xy- the absorption at this wavelength is due to a one-electron
lene bridges. This is not surprising since the lowest-energetic reduced bpy ligand that is formed in both cases: MLCT
triplet state in terphenyl is at Ն 2.5 eV,[29] that is energeti- excitation produces a RuIII(bpy)2(bpy·–) state, whereas the
2+
cally well above the 3MLCT state of Ru(bpy)3 (E00
=
charge-separated state can be described as RuII(bpy)2-
2.12 eV). The shorter phenylene bridges have even higher (bpy·–)–PTZ·+.[35] Consequently, for the charge-separated
triplet energies.
state, one expects an intensity ratio of 4:3 for the transient
absorption signals at 370 nm and 510 nm, respectively. Our
experimental observation is that these ratios are about 8:3
for all Ru-phn-PTZ molecules (Figure 4), suggesting that
Kinetics of Charge Transfer in the Ru-phn-PTZ Molecules
the Ru MLCT and the Ru(bpy)3+-PTZ·+ charge-separated
3
state are populated each to about 50%. This in turn implies
that PTZǞ*Ru(bpy)32+ excited-state electron transfer is ne-
arly isoergonic in the Ru-phn-PTZ molecules, as suspected
above.
The experimentally observed emission and transient ab-
sorption decays (Figure 5) are single exponential and yield
The right column of Figure 5 shows the temporal evol-
ution of the transient absorption signals at 510 nm of the
three Ru-phn-PTZ molecules in deoxygenated acetonitrile
solution. For a given dyad, the kinetic trace obtained is
practically identical to that measured for the luminescence
signal at 650 nm (left column in Figure 5).
The same observation has been made previously for a lifetimes of 30 ns (n = 1), 75 ns (n = 2), and 139 ns (n =
covalently linked Ru(bpy)32+-PTZ donor-acceptor couple. 3) as summarized in Table 1. In the quasi-equilibrium, the
A recent study demonstrated that the establishment of a experimentally accessible decay rate constant kobs is a func-
3
quasi-equilibrium between the Ru 3MLCT state and the tion of the intrinsic MLCT excited-state decay rate con-
Ru(bpy)32+-PTZ·+ charge-separated state (Scheme 3) can stant kMLCT (which itself contains a radiative and a nonra-
Figure 5. Left: decays of the luminescence signals in the Ru-phn-PTZ molecules after pulsed excitation of 2ϫ10–5  (deoxygenated)
acetonitrile solutions at 457.9 nm with laser pulses of about 10 ns duration. Right: temporal evolution of the Ru-phn-PTZ transient
absorption signals from Figure 4 with a detection wavelength of 510 nm.
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Conformational Effects on Long-Range Electron Transfer
cules. For n = 2, φexp = 0.04 and φcalc = 0.045, whereas for
n = 1, φexp = 0.01 and φcalc = 0.018 (Table 1).
The quasi-equilibrium scenario is also consistent with the
absolute magnitudes of the experimentally observed ∆OD
values in the transient absorption experiments (Figure 4
and Figure 5): the extinction of the charge-separated state
at 510 nm is 15Ј000 –1 cm–1 (see above),[18,36,37] the concen-
trations of our solutions were 2ϫ10–5 . If 20% of the Ru-
phn-PTZ molecules are excited and 50% of them populate
the charge-separated state, this yields ∆OD510 = 0.03, which
compares favorably to our experimental values that are of
the order of ∆OD510 = 0.02 to 0.07. This point is note-
worthy because it rules out a hypothetical scenario in which
kCR ϾϾ kf. Numerical simulations based on a set of cou-
pled differential equations for a simple three-level scheme
Scheme 3. Energy level scheme for the Ru-phn-PTZ molecules. The
close energetic proximity of the Ru 3MLCT (|A›) and the Ru-
(bpy)3+-PTZ·+ charge-separated state (|B›) leads to a quasi-equilib- demonstrate that the decays of the transients in Figure 5
rium situation (kf, kb) between these two states.
can also be reproduced when kCR values exceed those for kf
by roughly two orders of magnitude. This finding is cap-
tured also by an analytical expression describing the pop-
diative part) and the rate constant kCR for decay of the
ulation of intermediate B in a reaction sequence A Ǟ B Ǟ
Ru(bpy)3+-PTZ·+ charge-separated state to the ground
C:[38] Under the condition that the rate constant for trans-
state:
formation of B to C (kbc) is much greater than that for
transformation of A to B (kab), the concentration of B is
NMLCT
NCS
given by:[38]
kobs
=
·kMLCT
+
·kCR
(1)
NMLCT + NCS
NMLCT + NCS
kab
where NMLCT and NCS represent the population numbers of
the MLCT and charge-separated states, respectively. The
[B] =
·[A]0·exp(–kab·t)
(2)
3
kbc
numerical value of kMLCT for all three Ru-phn-PTZ mole-
where [A]0 represents the initial concentration of starting
material A. As seen from Equation (2), the decay is charac-
terized by kab, suggesting that the transient absorption de-
cay in Figure 5 could be associated with kf, that is with the
charge-separation process. However, the first factor in the
analytical Equation (2) anticipates a result that also
emerges from the abovementioned numerical simulations,
namely that under these circumstances the maximum pop-
ulation of B (or in our case of the charge-separated state)
is about 1/100 of the initial population of A. One would
thus expect ∆OD values smaller than 0.001, which cannot
be conciliated with experiment.
cules can be set equal to the inverse of the luminescence
2+
lifetime of free Ru(bpy)3 in reasonable approximation.
Thus, with kMLCT = 1.2ϫ106 s–1 (Table 1) and NCS
=
NMLCT = 0.5 (see above), one obtains the charge-recombi-
nation rate constants listed in the fourth column of Table 1:
kCR = 6.9ϫ107 s–1 for n = 1, kCR = 2.3ϫ107 s–1 for n = 2,
and kCR = 1.1ϫ107 s–1 for n = 3. As pointed out earlier,[32]
there exist analytical expressions that put the rise and decay
of the transients into relation with the rate constants kf and
kb (Scheme 3) governing the kinetics of the equilibration
process. This analysis yields kf = 4.9ϫ107 s–1 for each of
the three Ru-phn-PTZ molecules, due to the fact that our
rise kinetics are limited by the experimental setup used for
these experiments. Thus, it is impossible to extract meaning-
ful values for kf (and kb) from our data, but it is clear that
kf = 4.9ϫ107 s–1 represents a lower limit for the rate of
*Ru(bpy)32+ǞPTZ excited-state electron transfer; the equil-
Distance Dependence of Charge Transfer in Ru-phn-PTZ
and Ru-xyn-PTZ
When the natural logarithms of the rate constants for
ibration process must be rapid with respect to the decays of Ru(bpy)3+ǞPTZ·+ electron transfer (kCR) in the Ru-phn-
3
the MLCT state and the charge-separated state.
PTZ molecules (Table 1) are plotted as a function of donor-
Steady state luminescence experiments show that the acceptor (center-to-center) distance rDA, the three data
emission intensity of the Ru-ph3-PTZ molecule is ca. 10% points fall onto a single line (open circles in Figure 6). Such
2+
of that emitted by the free Ru(bpy)3 reference complex exponential distance dependences of charge transfer rates
(Table 1). This is in good agreement with estimates based are typical of the superexchange mechanism in which the
on the above kinetic data: the amount of the quasi-equilib- charge carrier, either an electron or a hole, must tunnel
3
rium population decaying from the MLCT state is repre- through the barrier imposed by the bridging medium.[7a]
sented by the first summand in Equation (1). Its divison The rate constant for charge tunneling is therefore a func-
through kobs gives a calculated quantum yield φcalc,ph3
=
tion of only two parameters, namely the rate constant for
0.083 relative to Ru(bpy)32+. Similarly good agreement be- charge transfer for a situation in which the donor and the
tween experimental and calculated relative emission quan- acceptor are in van der Waals contact (k0) and the distance
tum yields is obtained for the shorter Ru-phn-PTZ mole- decay parameter β:[7a]
Eur. J. Inorg. Chem. 2009, 3778–3790
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D. Hanss, O. S. Wenger
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ktunnel(rDA) = k0·exp(–β·rDA
)
(3)
Ru-phn-PTZ molecules. Instead, charge tunneling across the
oligo-p-xylene bridges was studied using a bimolecular
flash-quench technique with methylviologen, whereby a
3+
highly oxidizing Ru(bpy)3 species can be generated.[23] It
is then possible to investigate intramolecular PTZǞRuIII
electron transfer.[32,42] The distance dependence of this pro-
cess is shown by the gray-filled squares in Figure 6, and the
numerical values for the electron transfer rate constants are
given in the last column of Table 1. From a linear regression
fit to these data we obtained β = 0.77 Å–1.[23a] Attempts
to investigate the kinetics of the same PTZǞRuIII electron
transfer in the Ru-phn-PTZ molecules were unsuccessful
due to the rapidity of this process in these systems. There
are both inherent physical and experimental limits on the
time resolution for such investigations: ca. 1 ns for genera-
tion of RuIII via bimolecular electron transfer with methyl-
viologen,[42] and ca. 10 ns for the laser equipment used in
this work. The ca. 10 ns experimental limit precluded mea-
Figure 6. Distance dependence of hole tunneling from PTZ·+ to
+
Ru(bpy)3 across oligo-p-phenylene bridges (open circles) and hole
3+
tunneling from photogenerated Ru(bpy)3 to PTZ across oligo-p-
xylenes (gray filled squares). The lines are linear regression fits,
their slopes correspond to the distance decay parameters (β values) surement of the PTZǞRuIII charge transfer kinetics even
for the Ru-ph3-PTZ molecule.[43] The significance of this is
for the phenylene and xylene systems.
that despite the close chemical resemblance of the two dyad
In applying Equation (3) to analysis of the electron trans- series in Scheme 1, two different charge transfer processes
fer rate distance dependence, one neglects the distance de- are actually observed and compared to one another.
pendence of all nuclear factors contributing to electron
For both bridges, a hole tunneling mechanism is expected
transfer rates, which corresponds to the standard approxi- to prevail since the one-electron oxidized bridge levels are
mation in the field.[1–11] Estimates for k0 and β for the Ru- energetically much closer to the donor/acceptor levels than
phn-PTZ system can be obtained from a linear regression the one-electron reduced bridge states.[33] As illustrated in
fit to the three data points in Figure 6. Assuming rDA = 6 Å Scheme 4, in the Ru-phn-PTZ molecules there is hole tun-
2+
for free Ru(bpy)3 and PTZ molecules in van der Waals neling from PTZ·+ to Ru(bpy)3+, whereas in the Ru-xyn-
contact, one obtains k0 = 1.8ϫ108 s–1. For driving-force- PTZ systems there is RuIII to PTZ hole tunneling. The en-
optimized charge transfer between two reactants that are in ergy levels in Scheme 4 were estimated from the redox po-
direct contact, rate constants can be as large as 1013 s–1.[7a] tentials of all relevant species (Table 2); the bridge potential
The comparatively small value found here for the Ru(bpy) comes from a prior electrochemical study of the 4,4Ј-di-
+/PTZ·+ donor-acceptor couple is a manifestation of the methyldiphenyl (dmdp) molecule.[44] It has not been pos-
3
fact that this thermal charge recombination process is far sible to mesure the oligo-p-phenylene and oligo-p-xylene
from driving-force-optimized.[32] Roughly 2 eV must be lib- bridge redox potentials directly by cyclic voltammetry, but
erated during this process, and it is therefore plausible that dmdp represents a reasonable approximation to the real
it is associated with a significant activation barrier such as bridges in our dyads. The dmdp molecule is oxidized at
the case for charge-recombination in rhenium(I)/PTZ sys- +1.67 V vs. SCE and reduced at –2.60 eV,[44] hence the pref-
tems for which the occurrence of an inverted driving-force erence for hole over electron tunneling.
effect has long been reported.[39] More significant for the
present work is the slope of the linear regression fit which
yields β = 0.21 Å–1. This value is at the lower end of what
has been reported previously for charge transfer across
oligo-p-phenylene bridges: prior investigations have pro-
duced β values ranging from 0.35 Å–1 to 0.67 Å–1 for tun-
neling through such bridges.[4,40] For tetra- and penta-p-
phenylene spacers, hopping processes with much weaker
distance dependences were observed.[4c] However, from re-
cent work it is clear that β is not solely dependent on the
bridge.[10] Instead, it is a function of the entire donor-
bridge-acceptor system,[41] hence our interest in a compari-
son of oligo-p-phenylene and oligo-p-xylene bridges at-
tached to the same donor and acceptor.
In the Ru-xyn-PTZ molecules there is no Ru(bpy)32+Ǟ
Scheme 4. Energy level diagram for hole tunneling from PTZ·+ to
PTZ excited-state electron transfer (see above),[23a] and it is
+
Ru(bpy)3 through oligo-p-phenylene bridges (left) and for hole
3+
therefore impossible to study the distance dependence of
tunneling from photogenerated Ru(bpy)3 to PTZ across oligo-p-
the same charge recombination reaction as observed for the xylenes (right).
3784
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Eur. J. Inorg. Chem. 2009, 3778–3790

Conformational Effects on Long-Range Electron Transfer
The barriers and driving-forces associated with the two estimated barrier heights (Scheme 4) it is now possible to
hole tunneling processes in Scheme 4 differ quite signifi- estimate the relative magnitudes of phenyl-phenyl (hbb,ph
cantly. According to superexchange theory, bridge-mediated and xylyl–xylyl coupling (hbb,xy):
donor-acceptor couplings HDA are a sensitive function of
)
the barrier height ∆ε:[7a,41,45]
hbb,ph ∆εph
=
α
·exp[ ·(βxy – βph)]
(7)
hbb,xy ∆εxy
2
hDb hbb
HDA
=
·(
)
n–1·hbA
(4)
With the numerical values from above (∆εph = 1.0 eV,
∆εxy = 0.45 eV, α = 4.3 Å, βxy = 0.77 Å–1, βph = 0.21 Å–1),
∆ε ∆ε
Equation (7) yields hbb,ph ≈ 7·hbb,xy
.
In the strict sense of this model, ∆ε is defined as the
energy difference between the donor-acceptor system at its
transition state configuration and the energy levels of the
bridge. Since this quantity cannot be extracted easily from
experiment, it has become common to approximate ∆ε as
the difference in donor and bridge redox potentials, as done
in Scheme 4. The other parameters that affect HDA in this
model are the electronic couplings between adjacent molec-
ular units, specifically: donor-bridge coupling (hDb), bridge-
bridge (hbb), and bridge-acceptor (hbA) coupling. The dis-
tance dependence of HDA shows up in the exponent n – 1,
wherein n represents the number of (identical) bridging
The absorption data in Figure 1 indicate that there are
significant differences in π-conjugation between the oligo-
p-phenylene and oligo-p-xylene bridges (see above). Thus it
appears reasonable to attribute the differences in hbb,ph and
hbb,xy primarily to differences in π-conjugation. This π-con-
jugation is governed by the overlap between pz-orbitals on
adjacent bridging units, which in turn is a function of the
dihedral angle φ between them. The angular dependence of
hbb is then given by:[12]
hbb(φ) = hbb(φ = 0)·[cos(φ)2]
(8)
units. According to semiclassical theory, electron transfer
A plot of this function is given in the upper panel of
Figure 7. The lower panel in Figure 7 shows curves that
satisfy the relation hbb,ph(φph–ph) = x·hbb,xy(φxy–xy), among
which the x = 7 curve (solid line) represents the most rel-
evant case for our Ru-phn-PTZ and Ru-xyn-PTZ systems.
There have been several computational studies on the inter-
phenyl torsion angle φph–ph in oligo-p-phenylenes, reporting
values that range from 35° to 38° for gas-phase equilibrium
structures.[4c,30] Using the Jaguar 3.5 software package we
have been able to reproduce the results from these prior
calculations (density functional theory using Becke’s three-
parameter hybrid functional with Lee, Yang, Parr corre-
lation function B3LYP and a 6-31G* basis set).[47] Our re-
sult obtained for the tri-p-phenylene molecule is shown at
the lower right bottom of Figure 7. The two phenyl-phenyl
torsion angles in this calculated (gas-phase) equilibrium
structure are 38°. When applying the exactly same compu-
tational method to a tri-p-xylene molecule, one obtains the
equilibrium structure shown in the upper left corner of the
lower panel in Figure 7. This structure displays xylyl–xylyl
torsion angles of 69°. The black circle in the lower panel of
Figure 7 marks the point at which the two equilibrium tor-
sion angles φph–ph and φxy–xy are at the above calculated val-
ues (38° and 69°, respectively). We note that this point falls
onto the x = 5 line. In other words, based on the gas-phase
equilibrium structure calculations and Equation (8), one
would expect phenyl-phenyl coupling to be 5 times stronger
than xylyl–xylyl coupling. The close agreement to the factor
of 7 found from experiment supports our analysis of the
experimental data in terms of distinct phenyl-phenyl and
xylyl-xylyl equilibrium torsion angles. In other words, the
experimentally observed differences in long-range charge
transfer efficiencies for oligo-p-phenylene and oligo-p-xy-
lene bridges can be explained satisfactorily on the sole basis
2 [46]
rate constants kET are proportional to HDA
,
and their
distance dependence is essentially governed by that of
[7a,46]
HDA
.
This allows us to set the experimentally deter-
mined distance decay constant β into relation with the
superexchange parameters from Equation (4).
Scheme 5. Graphical illustration of the definition of the distance
decay parameter β and its relation to electron transfer rate con-
stants kET and the length (α) of a phenylene or xylene molecular
unit.
From Scheme 5, it follows that
β = [ln(kET,n) – ln(kET,n+1)]/α
(5)
where α is the length of a phenylene or xylene bridging unit
2
(4.3 Å). With kET ϱ HDA and Equation (4), Equation (5)
simplifies to Equation (6).
2
β = ·ln(
α
∆ε
)
(6)
hbb
Thus, β is a function of only three parameters (α, ∆ε, hbb). of conformational effects. Other effects, such as possible
Notably, it is independent of the reaction free energy variations in the tunneling barriers (∆ε) as a function of
–∆GET. From the experimental β values (Figure 6) and the bridge length,[4c,4d] appear to play a subordinate role. This
Eur. J. Inorg. Chem. 2009, 3778–3790
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3785

D. Hanss, O. S. Wenger
FULL PAPER
is remarkable because in principle the energies of the one- conformational effects. Our analysis is based on the evalu-
electron oxidized state of oligo-p-phenylenes are strongly ation of distance decay constants for long-range charge
length-dependent.[4d]
transfer rates, and our result is in line with those from prior
investigations of conformational effects on electronic cou-
pling across substituted and unsubstituted biphenyls.[12]
Experimental Section
1
Instrumentation: H-NMR spectra were acquired on a Bruker Av-
ance 400 MHz spectrometer. All chemical shifts are reported in
ppm relative to the tetramethylsilane signal. Deuterated solvents
were purchased from Sigma Aldrich. The different types of mul-
tiplets observed are denoted by the following abbreviations: s (sing-
let), d (doublet), t (triplet), q (quadruplet), dd (doublet of dou-
blets), ddd (doublet of doublets of doublets), m (multiplet), dm
(doublet of multiplets). High resolution mass spectra were recorded
on a QSTAR XL (AB/MDS Sciex) spectrometer. Methanol (HPLC
grade, VWR) was used to solubilize the samples. Optical absorp-
tion spectra were measured on a Cary 5000 UV/Vis/NIR spectro-
photometer (Varian), and luminescence experiments were measured
on a Horiba Fluorolog-3 instrument. The solvent used for these
measurements was acetonitrile of spectrophotometric grade. For
luminescence lifetime and transient absorption spectroscopy, 10–4
to 2ϫ10–5  acetonitrile solutions of the Ru-phn-PTZ molecules
were deoxygenated via three subsequent freeze-pump-thaw cycles
in home-built quartz cuvettes. Sample excitation occurred using a
Quantel Brilliant Nd:YAG laser with an integrated Magic Prism
OPO. The detection system was comprised of a Spex 270M mono-
chromator, a Hamamatsu photomultiplier, and a Tektronix TDS
540B oscilloscope. The probe beam used for transient absorption
spectroscopy came from a 900-W tungsten lamp. Cyclic voltamme-
try was performed using a Versastat3–100 Potentiostat equipped
with the K0264 Micro-Cell kit from Princeton Applied Research. A
silver wire was used as a quasi-reference electrode. The supporting
electrolyte was a 0.1  solution of tetrabutylammonium hexafluo-
rophosphate (Fluka, product No. 86879) in dry acetonitrile (HPLC
grade, VWR). The solution was deoxygenated prior to voltamme-
try sweeps by bubbling nitrogen gas.
Figure 7. Upper panel: effect of torsion angle (φ) on the electronic
bridge-bridge coupling (hbb) via pz-orbital overlap between adjacent
bridge units. Lower panel: functions that satisfy the equation
hbb,ph(φph–ph) = x·hbb,xy(φxy–xy), i.e., curves for which phenyl-phenyl
coupling is calculated [Equation (8)] to be a factor of x stronger
than xylyl-xylyl coupling.
Conclusions
There have been several prior studies that investigated
the distance dependence of electron transfer across oligo-p-
phenylene wires,[4b–4d,4g] and we have recently reported on
the distance dependence of hole tunneling across oligo-p-
xylene bridges.[14,23] Since the distance decay constant (β) is
a system-specific rather than bridge-specific parameter,[10,23]
we have sought to compare the two types of molecular wires
directly by attaching them to the same donor and acceptor.
Synthetic Protocols: Compound 3 (PTZ-ph1-TMS): To a double-
neck flask containing phenothiazine (1) (5 g, 25.2 mmol), (4-bro-
mophenyl)trimethylsilane (2) (5.78 g, 25.2 mmol), potassium tert-
butoxide (4.24 g, 37.8 mmol) and Pd(dba)2 (dba: dibenzylidene ace-
tone) (290 mg, 504 µmol) were added freshly distilled toluene
(125 mL) and a 1  solution of P(tBu)3 (tBu: tert-butylphosphane)
in toluene (0.5 mL, 504 µmol) under a nitrogen atmosphere. The
yellow suspension was heated at 60 °C and the reaction followed
by TLC until total consumption of the phenothiazine (ca. 2 h).
Then the solvent was evaporated under reduced pressure. The crude
brown product was then purified by column chromatography on
silica gel using a pentane/CH2Cl2 (98:2) mixture to elute the N–C
coupled product as a slightly yellow solid in 89% yield (7.79 g). 1H
NMR (400 MHz, CDCl3, 25 °C): δ = 0.39 (s, 9 H, SiMe3), 6.30
(dd, J = 8.0, 0.9 Hz, 2 H, PTZ), 6.86 (m, 4 H, PTZ), 7.05 (dd, J =
7.2, 1.5 Hz, 2 H, PTZ), 7.12 (d, J = 8.4 Hz, 2 H, Ph), 7.89 (d, J =
8.4 Hz, 2 H, Ph) ppm.
2+
In retrospect, the choice of a phenothiazine/Ru(bpy)3 re-
actant couple turned out to be less than optimal because
only two quite different phototriggered charge transfer pro-
cesses were ultimately amenable to experimental investiga-
tion in the oligo-p-phenylene and oligo-p-xylene bridged
+
systems. That is, PTZ·+ǞRu(bpy)3 hole tunneling in the
Ru-phn-PTZ molecules and Ru(bpy)33+ǞPTZ hole tunnel-
ing in the Ru-xyn-PTZ systems. The barrier heights and
driving-forces associated with these two tunneling processes
are quite different, but using superexchange theory it is nev-
ertheless possible to extract quantitative information on the
relative electronic coupling strengths provided by the oligo-
p-phenylene and oligo-p-xylene bridges.
Compound 4 (PTZ-ph1-I): To a double-neck flask containing a yel-
low suspension of the trimethylsilyl-protected compound 3 (1 g,
2.9 mmol) in a mixture of CH2Cl2/CH3CN (1:3) at 0 °C was added
a solution of ICl (934 mg, 5.8 mmol) in CH2Cl2 (25 mL) under a
The key finding is that two adjacent phenyl units are cou-
pled ca. 7 times stronger than two neighboring xylyl units,
and this observation can be explained on the sole basis of nitrogen atmosphere. The resulting dark orange suspension was
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Conformational Effects on Long-Range Electron Transfer
warmed to ca. 20 °C and stirred during 20 h at this temperature.
The reaction was followed by NMR until disappearance of the
TMS singlet. Then, the mixture was hydrolyzed using aqueous
Na2S2O3 solution and extracted by CH2Cl2. The combined organic
phases were concentrated under reduced pressure. The crude brown
product was then purified by column chromatography on silica and
eluted by a pentane/CH2Cl2 (98:2) mixture to give the desired prod-
uct as a yellow solid in 99% yield (1.15 g). 1H NMR (400 MHz,
CDCl3, 25 °C): δ = 6.30 (dd, J = 8.0, 1.50 Hz, 2 H, 2 H, PTZ),
6.86 (m, 4 H, PTZ), 7.05 (dd, J = 7.2, 2.0 Hz, 2 H, PTZ), 7.11 (d,
J = 8.4 Hz, 2 H, Ph), 7.89 (d, J = 8.4 Hz, 2 H, Ph) ppm.
The combined organic phases were dried, and the solvent was evap-
orated under reduced pressure. The remaining yellow oil was puri-
fied subsequently by column chromatography on silica gel using
pentane as the eluent. The product yield was 88% (8.65 g). 1H
NMR (400 MHz, CDCl3, 25 °C): δ = 0.32 (s, 9 H, SiMe3), 7.41–
7.48 (m, 2 H, Ph), 7.55–7.63 (m, 6 H, Ph) ppm.
PTZ-ph2-TMS 8: To a two-neck flask containing phenothiazine (1)
(5 g, 25.2 mmol), 4-bromo-4Ј-trimethylsilyl-1,1Ј-biphenyl (7)
(7.70 g, 25.2 mmol), potassium tertiobutanolate (4.24 g,
37.8 mmol) and Pd(dba)2 (290 mg, 504 µmol) were added freshly
distilled toluene (125 mL), and a 1  solution of P(tBu)3 in toluene
(0.5 mL, 504 µmol) under a nitrogen atmosphere. The yellow sus-
pension was heated at 60 °C, and the reaction was followed by TLC
until complete disappearance of 1. Then the solvent was evaporated
under reduced pressure. Purification of the crude brown product
occurred by column chromatography on silica gel using a pentane/
CH2Cl2 (98:2) eluent mixture. The product was obtained in 62%
yield in the form of a slightly yellow solid. 1H NMR (400 MHz,
CDCl3, 25 °C): δ = 0.34 (s, 9 H, SiMe3), 6.34 (dd, J = 8.0, 1.2 Hz,
2 H, PTZ), 6.86 (m, 4 H, PTZ), 7.05 (dd, J = 7.2, 1.6 Hz, 2 H,
PTZ), 7.45 (d, J = 8.4 Hz, 2 H, Ph), 7.66 (s, 4 H, Ph), 7.81 (d, J =
8.4 Hz, 2 H, Ph) ppm.
Ligand 6 (PTZ-ph1-bpy): To a stirred and deoxygenated suspension
of iodo compound 4 (1.00 g, 2.5 mmol), 5-(tri-n-butylstannyl)-2,2Ј-
bipyridine (5) (1.44 g, 3.2 mmol) in m-xylene under nitrogen atmo-
sphere was added Pd(PPh3)4 (Ph: phenyl) (144 mg, 125 µmol). The
yellow suspension was deoxygenated for an additional 10 min by
bubbling nitrogen gas and then heated to 140 °C during 48 h. After
cooling to room temperature, the solvent was evaporated under
reduced pressure. The dark brown remaining solid was then puri-
fied by two successive column chromatographies on silica gel, first
using a CH2Cl2/CH3OH (98:2) eluent mixture and then a pentane/
ethyl acetate eluent mixture (80:20). This gave the desired coupling
product in the form of an orange oil in 54% yield (0.58 g). 1H
NMR (400 MHz, CDCl3, 25 °C): δ = 6.39 (dd, J = 8.0, 1.2 Hz, 2
H, PTZ), 6.86 (ddd, J = 7.6, 7.6, 1.2 Hz, 2 H, PTZ), 6.93 (ddd, J
= 7.6, 7.6, 2.0 Hz, 2 H, PTZ), 7.08 (dd, J = 7.6, 1.6 Hz, 2 H, PTZ),
7.34 (ddd, J = 7.6, 4.8, 1.2 Hz, 1 H, bpy), 7.50 (d, J = 8.4 Hz, 1 H,
Ph), 7.85 (m, 1 H, bpy), 7.86 (d, J = 8.4 Hz, 2 H, Ph), 8.09 (dd, J
= 8.4, 2.4 Hz, 1 H, bpy), 8.47 (ddd, J = 7.6, 1.2, 1.2 Hz, 1 H, bpy),
8.53 (dd, J = 8.0, 0.8 Hz, 1 H, bpy), 8.72 (dm, J = 4.8 Hz, 1 H,
bpy), 9.00 (d, J = 2.4 Hz, 1 H, bpy) ppm. ESI-MS: m/z calcd. for
protonated PTZ-ph1-bpy: 430.5; found 430.3.
PTZ-ph2-I 9: To
a suspension of PTZ-ph2-TMS (8) (2.0 g,
4.7 mmol) in a mixture of CH2Cl2/CH3CN (1:3) at 0 °C was added
a solution of ICl (1.53 g, 9.4 mmol) in CH2Cl2 (25 mL) under a
nitrogen atmosphere. The resulting dark orange suspension was
warmed to ca. 20 °C and stirred at this temperature. The reaction
was followed by 1H-NMR until disappearance of the characteristic
TMS singlet (ca. 20 h). Then the mixture was hydrolyzed using
aqueous Na2S2O3 solution and extracted by CH2Cl2. After solvent
evaporation, the crude brown product was purified by column
chromatography on silica gel using a pentane/CH2Cl2 (98:2) mix-
ture as an eluent. This afforded the product in 80% yield as a yel-
low solid (1.83 g). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 6.34
(dd, J = 8.0, 1.2 Hz, 2 H, PTZ), 6.83–6.92 (m, 4 H, PTZ), 7.05 (dd,
J = 7.2, 1.6 Hz, 2 H, PTZ), 7.40 (d, J = 8.4 Hz, 2 H, Ph), 7.44 (d,
J = 8.4 Hz, 2 H, Ph), 7.75 (d, J = 8.4 Hz, 2 H, Ph), 7.81 (d, J =
8.4 Hz, 2 H, Ph) ppm.
Ru-ph1-PTZ:
A solution of PTZ-ph1-bpy ligand (100 mg,
233 µmol) and Ru(bpy)2Cl2·2H2O (128 mg, 233 µmol) in a mixture
of CHCl3/C2H5OH (2:8) was heated to reflux under nitrogen atmo-
sphere overnight. After solvent evaporation under reduced pres-
sure, the remaining dark solid was purified twice by column
chromatography on silica: first using a CH2Cl2/CH3OH eluent mix-
ture (98:2), then using an eluent mixture comprised of CH3CN and
saturated aqueous KNO3 (99:1) to afford the nitrate salt of the
desired complex. Nitrate to hexafluorophosphate anion exchange
was achieved by precipitation of the orange complex from an aque-
ous solution using saturated aqueous KPF6. The yield was 68%
Ligand 10 (PTZ-ph2-bpy): To a deoxygenated suspension of PTZ-
ph2-TMS (9) (1.00 g, 2.1 mmol), 5-(tri-n-butylstannyl)-2,2Ј-bipyr-
idine 5 (1.21 g, 2.7 mmol) in 20 mL m-xylene under nitrogen atmo-
sphere was added Pd(PPh3)4 (121 mg, 105 µmol). The reaction mix-
ture was degassed for an additional 10 min by bubbling nitrogen
gas prior to heating to 140 °C during 48 h. After cooling to room
temperature and solvent evaporation under reduced pressure, the
remaining dark brown solid was purified by two successive column
chromatographies on a silica gel stationary phase: first with a
CH2Cl2/MeOH (98:2) mixture, then with a pentane/ethyl acetate
(80:20) eluent mixture. This afforded the desired coupling product
in 40% yield as an orange oil (425 mg). 1H NMR (400 MHz,
CDCl3, 25 °C): δ = 6.37 (dd, J = 8.0, 1.2 Hz, 2 H, PTZ), 6.85 (ddd,
J = 7.6, 7.6, 1.2 Hz, 2 H, PTZ), 6.91 (ddd, J = 7.6, 7.6, 2.0 Hz, 2
H, PTZ), 7.06 (dd, J = 7.6, 1.6 Hz, 2 H, PTZ), 7.34 (ddd, J = 7.6,
4.8, 1.2 Hz, 1 H, bpy), 7.49 (dm, J = 8.4 Hz, 2 H, Ph), 7.80 (s, 4
H, Ph); 7.85 (dm, J = 8.4 Hz, 2 H, Ph), 7.87 (m, 1 H, bpy), 8.10
(dd, J = 8.4, 2.4 Hz, 1 H, bpy), 8.52 (ddd, J = 7.6, 1.2, 1.2 Hz, 1
H, bpy), 8.72 (dd, J = 8.0, 0.8 Hz, 1 H, bpy), 9.01 (dm, J = 2.4 Hz,
1 H, bpy) ppm. ESI-MS: m/z calcd. for protonated PTZ-ph2-bpy:
506.6; found 506.5.
1
(153 mg). H NMR (400 MHz, CD3CN, 25 °C): δ = 6.66 (dd, J =
7.6, 1.2 Hz, 2 H, PTZ), 7.02 (ddd, J = 7.2, 7.2, 1.2 Hz, 2 H, PTZ),
7.09 (ddd, J = 7.6, 7.2, 2.0 Hz, 2 H, PTZ), 7.23 (dd, J = 7.6, 1.6 Hz,
2 H, PTZ), 7.26 (dm, J = 8.8 Hz, 2 H, Ph), 7.41 (m, 5 H), 7.50
(dm, J = 8.4 Hz, 1 H, bpy), 7.74 (m, 2 H), 7.77 (dm, J = 5.6 Hz, 1
H, bpy), 7.80 (dm, J = 5.6 Hz, 1 H, bpy), 7.85 (dm, J = 5.6 Hz, 1
H, bpy), 7.88 (dm, J = 5.6 Hz, 1 H, bpy), 8.07 (m, 5 H), 8.32 (dd,
J = 8.8, 2.4 Hz, 1 H, bpy), 8.56 (m, 5 H), 8.60 (dm, J = 8.8 Hz, 1
H, bpy) ppm. ESI-MS: m/z calcd. for the Ru-ph1-PTZ dication:
421.5853; found 421.5852. C48H35F12N7P2RuS·0.2CHCl3: calcd. C
49.80, H 3.05, N 8.42; found C 49.84, H 3.21, N 8.18.
4-Bromo-4Ј-trimethylsilyl-1,1Ј-biphenyl (7): To a stirred solution of
4,4Ј-dibromo-1,1Ј-biphenyl (10.0 g, 32.1 mmol) in dry ethyl ether
cooled to –78 °C under nitrogen atmosphere, 1.6  solution of n-
butyllithium in hexane (41.7 mmol, 26.0 mL) was added dropwise
via syringe. After 1 h, chlorotrimethylsilane (41.7 mmol, 5.3 mL)
was added. After stirring for an additional 30 min at –78 °C, the
reaction mixture was warmed to room temperature. Following ad-
Ru-ph2-PTZ: A solution of PTZ-ph2-bpy (10) (100 mg, 198 µmol)
and Ru(bpy)2Cl2·2H2O (109 mg, 198 µmol) in a mixture of CHCl3/
dition of water, the mixture was extracted with dichloromethane. C2H5OH (2:8) was heated to reflux under nitrogen atmosphere
Eur. J. Inorg. Chem. 2009, 3778–3790
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3787

D. Hanss, O. S. Wenger
FULL PAPER
overnight. After solvent evaporation, the remaining dark solid was
purified by two successive column chromatographies on silica gel.
The first used a CH2Cl2/CH3OH mixture (98:2), the second a mix-
ture of CH3CN and saturated aqueous KNO3 (99:1) to elute the
product. NO3– to PF6– anion exchange occurred as described above
2 H, PTZ), 6.85 (ddd, J = 7.2, 7.2, 1.2 Hz, 2 H, PTZ), 6.90 (ddd,
J = 7.6, 7.2, 2.0 Hz, 2 H, PTZ), 7.05 (dd, J = 7.6, 1.6 Hz, 2 H,
PTZ), 7.34 (ddd, J = 7.6, 4.8, 1.2 Hz, 1 H, bpy), 7.48 (dm, J =
8.8 Hz, 2 H, Ph), 7.79 (s, 8 H, Ph), 7.87 (dm, J = 8.8 Hz, 2 H, Ph),
7.87 (m, 1 H, bpy), 8.10 (dd, J = 8.0, 2.0 Hz, 1 H, bpy), 8.47 (dm,
J = 8.0 Hz, 1 H, 8.47), 8.52 (dm, J = 8.4 Hz, 1 H, bpy), 8.73 (d, J
for Ru-ph1-PTZ. Ru-ph2-PTZ was isolated as a deep orange solid
1
in 65% yield (134 mg). H NMR (400 MHz, CD3CN, 25 °C): δ = = 4.0 Hz, 1 H, bpy), 9.00 (dm, J = 2.4 Hz, 1 H, bpy) ppm. ESI-
6.66 (dd, J = 7.6, 1.2 Hz, 2 H, PTZ), 7.02 (ddd, J = 7.2, 7.2, 1.2 Hz,
2 H, PTZ), 7.09 (ddd, J = 7.6, 7.2 Hz, 2 H, PTZ), 7.23 (dd, J =
7.6, 1.6 Hz, 2 H, PTZ), 7.26 (dm, J = 8.8 Hz, 2 H, Ph), 7.43 (m, 6
H), 7.54 (dd, J = 8.8, 2.0 Hz, 2 H, Ph), 7.77 (m, 5 H), 7.84 (dm, J
= 4.8 Hz, 1 H, bpy), 7.87 (dm, J = 2.0 Hz, 1 H, bpy), 7.90 (dm, J
= 6.0 Hz, 1 H, bpy), 8.08 (m, 5 H), 8.37 (dm, J = 8.4 Hz, 1 H,
bpy), 8.57 (m, 5 H), 8.63 (dm, J = 8.0 Hz, 1 H, bpy) ppm. ESI-MS:
m/z calcd. for the Ru-ph2-PTZ dication: 459.6010; found 459.6012.
C54H39F12N7P2RuS·2.0H2O: calcd. C 52.09, H 3.48, N 7.87; found
C 52.33, H 3.43, N 7.73.
MS: m/z calcd. for protonated PTZ-ph3-bpy: 582.7; found 582.5.
Ru-ph3-PTZ: A solution of ligand 14 (100 mg, 233 µmol) and
Ru(bpy)2Cl2·2H2O (128 mg, 233 µmol) in a mixture of CHCl3/
C2H5OH (2:8) was heated to reflux under nitrogen atmosphere
overnight. The resulting orange solution was evaporated to dryness
under reduced pressure, and the remaining dark red solid was puri-
fied by two subsequent column chromatographies on a silica gel
stationary phase. The eluent for the first column was a CH2Cl2/
CH3OH mixture (98:2), whereas for the second column a saturated
aqueous CH3CN/KNO3 mixture (99:1) was employed. Anion ex-
change from nitrate to hexafluorophosphate was accomplished as
described above for Ru-ph1-PTZ. The Ru-ph3-PTZ product was
obtained in 65% yield (84 mg). 1H NMR (400 MHz, CD3CN,
25 °C): δ = 6.39 (dd, J = 8.0, 1.2 Hz, 2 H, PTZ), 6.89 (ddd, J =
7.2, 7.2, 1.2 Hz, 2 H, PTZ), 6.96 (ddd, J = 7.6, 7.2, 2.0 Hz, 2 H,
PTZ), 7.09 (dd, J = 7.6, 1.6 Hz, 2 H, PTZ), 7.44 (m, 8 H), 7.53
(dm, J = 8.4 Hz, 2 H, Ph), 7.80 (m, 10 H), 7.87 (d, J = 1.6 Hz, 1
H, bpy), 7.92 (m, 4 H), 8.08 (m, 6 H), 8.37 (dm, J = 8.4 Hz, 1 H,
bpy), 8.58 (m, 6 H), 8.62 (dm, J = 8.4 Hz, 1 H, bpy) ppm. ESI-MS:
m/z calcd. for the Ru-ph3-PTZ dication: 497.6166; found 497.6164.
C60H43F12N7P2RuS·2C4H8O2: calcd. C 55.89, H 4.07, N 6.71;
found C 55.61, H 3.74, N 6.53.
PTZ-ph3-TMS 12: To a stirred and deoxygenated suspension of
PTZ-ph2-I (9) (1.00 g, 2.1 mmol), 4-(trimethylsilyl)phenylboronic
acid 11 (447 mg, 2.3 mmol), and Na2CO3 (666 mg, 6.3 mmol) in a
mixture of toluene/ethanol/water (85:10:5) under nitrogen atmo-
sphere was added Pd(PPh3)4 (48 mg, 42 µmol). The yellow suspen-
sion was degassed for an additional 10 min and then heated to
reflux overnight. After cooling to room temperature, the mixture
was extracted with CH2Cl2, and the combined organic phases were
evaporated to dryness under reduced pressure. The resulting dark
brown solid was purified by column chromatography on silica gel
using a CH2Cl2/CH3OH (98:2) mixture to elute the product. The
yield was 80% (0.84 g). 1H NMR (400 MHz, CDCl3, 25 °C): δ =
0.32 (s, 9 H, SiMe3), 6.34 (dd, J = 8.0, 1.2 Hz, 2 H, PTZ), 6.82–
6.92 (m, 4 H, PTZ), 7.05 (dd, J = 7.2, 1.2 Hz, 2 H, PTZ), 7.47 (d,
J = 8.4 Hz, 2 H, Ph), 7.65 (m, 4 H, Ph), 7.74 (m, 4 H, Ph), 7.81
(d, J = 8.8 Hz, 2 H, Ph) ppm.
Acknowledgments
PTZ-ph3-I 13: To a suspension of PTZ-ph2-TMS (12) (2.0 g,
4.0 mmol) in a mixture of CH2Cl2/CH3CN (1:3) at 0 °C was added
a solution of ICl (1.30 g, 8.0 mmol) in CH2Cl2 (25 mL) under a
nitrogen atmosphere. The dark orange suspension was warmed to
room temperature and stirred during 20 h at this temperature.
Then, aqueous Na2S2O3 solution was added, and the biphasic mix-
ture was extracted with CH2Cl2. The combined organic phases were
dried prior to solvent evaporation. The resulting crude brown prod-
uct was purified by column chromatography on silica gel, using a
pentane/CH2Cl2 (98:2) mixture to elute the product in 73% yield
(1.62 g). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 6.35 (dd, J = 8.0,
1.20 Hz, 2 H, PTZ), 6.82–6.92 (m, 4 H, PTZ), 7.06 (dd, J = 7.2,
1.6 Hz, 2 H, PTZ), 7.41 (d, J = 8.4 Hz, 2 H, Ph), 7.47 (d, J =
8.4 Hz, 2 H, Ph), 7.68 (d, J = 8.4 Hz, 2 H, Ph), 7.75 (d, J = 8.8 Hz,
2 H, Ph), 7.80 (d, J = 8.4 Hz, 2 H, Ph), 7.84 (d, J = 6.8 Hz, 2 H,
Ph) ppm.
This research was sponsored by the Swiss National Science Foun-
dation (grant number PP002-110611). The Société Académique de
Genève and the Fondation Ernst & Lucie Schmidheiny are ac-
knowledged for additional financial support.
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We note that even faster laser equipment and a diffusion-lim-
ited time resolution of ca. 1 ns is likely to be insufficient to
investigate the PTZǞRuIII charge transfer kinetics in the Ru-
Eur. J. Inorg. Chem. 2009, 3778–3790
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. Hanss, O. S. Wenger
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FULL PAPER
ph1-PTZ molecule. From a fit to the Ru-xyn-PTZ data we de-
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for rDA = 19.2 Å (Ru-ph3-PTZ) ktunneling Ն 108 s–1, and thus β
Յ 0.43 Å–1 for this process. Consequently, for the Ru-ph1-PTZ
system the tunneling rate is expected to be Ն 4.9ϫ109 s–1.
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Received: May 1, 2009
Published Online: July 23, 2009
3790
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 3778–3790