Microsecond charge recombination in a linear triarylamine-Ru(bpy) 32+-anthraquinone triad

Article abstract of DOI:10.1039/c1cc13831f

Linear triads with ruthenium photosensitizers are frequently based on the Ru(terpyridine)₂²⁺ unit. We report on vectorial photoinduced electron transfer in a linear triad based on the Ru(bipyridine)₃²⁺ photosensitizer. Electron-hole separation over a 22 A-distance is established with a quantum yield greater than 64% and persists for 1.3 μs in acetonitrile. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc13831f

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COMMUNICATION
Microsecond charge recombination in a linear
triarylamine–Ru(bpy)32+–anthraquinone triadwz
Jihane Hankache and Oliver S. Wenger*
Received 27th June 2011, Accepted 25th July 2011
DOI: 10.1039/c1cc13831f
Linear triads with ruthenium photosensitizers are frequently
2+
based on the Ru(terpyridine)2
unit. We report on vectorial
photoinduced electron transfer in a linear triad based on the
2+
Ru(bipyridine)3
photosensitizer. Electron–hole separation
˚
over a 22 A-distance is established with a quantum yield greater
than 64% and persists for 1.3 ls in acetonitrile.
Photogeneration of long-lived charge-separated states is one
of the key goals of research on artificial photosynthesis. Aside
from porphyrins,1 d6 metal diimine complexes are popular
photosensitizers in artificial dyads and triads.2 There are many
examples of linear donor–photosensitizer–acceptor triads with
Scheme 1 Possibilities for incorporation of ruthenium(II) photo-
sensitizers in donor–photosensitizer–acceptor triads. (a)–(c) are
scenarios which have been previously explored, (d) represents the
newly explored case of the present study.
porphyrins,1,3 but so far this has not been achieved with the
2+
Ru(bpy)3
(bpy = 2,20-bipyridine) photosensitizer. When
electron donor (D) and acceptor (A) units are attached to
different bipyridine ligands, a mixture of isomers results
(Scheme 1a),4 and the kinetics of the photoinduced reactions
become difficult to elucidate in detail.5
is structurally similar to tpy (Scheme 1c).15 Due to an increased
2+
bite angle, the Ru(dqp)2
motif exhibits an excited-state
In order to avoid this problem, much research has focused
2+
lifetime in the microsecond regime,16 thereby allowing efficient
photoinduced electron transfer chemistry. Here, we report on a
Ru(bpy)32+-based system that is comprised of a rigid rod-like
donor–bipyridine–acceptor moiety and two ancillary bpy ligands
(Scheme 1d), which exhibits very favorable light-induced
photochemistry.
on triads incorporating the Ru(tpy)2
(tpy = 2,20;60,
20 0-terpyridine) motif with electron donor and acceptor units
attached to the central pyridine ring of the two tridentate
ligands (Scheme 1b).6–9
However, Ru(tpy)22+ has an excited state lifetime that is about
2+ 10
,
three orders of magnitude shorter than that of Ru(bpy)3
Scheme 2 illustrates the synthetic pathway to the rigid rod-like
donor–bipyridine–acceptor molecule used for the triad in this
work. The starting point is 5,50-dibromo-2,20-bipyridine (1) to
which p-xylene units (2) are attached by Suzuki coupling.17
Following trimethylsilyl (TMS)–halogen exchange, the resulting
dibromo compound (4) is reacted with a boronic acid pinacol
ester of 9,10-anthraquinone (5). The final step is a palladium
catalyzed N–C coupling between molecule 6 and dianisylamine
(7). The overall yield for the four reaction steps from bipyridine 1
to the final molecular rod 8 is 33%. Reaction with Ru(bpy)2Cl2
affords the triad of central interest to this study, hereafter
referred to as TAA–Ru2+–AQ (TAA = triarylamine; AQ =
9,10-anthraquinone). Detailed synthetic protocols and product
characterization data are given in the ESI.z
and consequently the excited-state population decays too rapidly
in order for electron transfer to be efficient.11,12 One possibility to
2+
circumvent this problem is to replace the Ru(tpy)2 sensitizer
by the isostructural Ir(tpy)23+ motif.13 Triads with this sensitizer
have yielded spectacularly long-lived charge-separated states
which are formed with high quantum yields.13,14 However,
photoexcitation has to occur at relatively short wavelengths,
and the reaction conditions to effect iridium(III) coordination
by two tpy ligands are very harsh. A recently explored possibility
involves the use of a bis(diquinolinyl)pyridine (dqp) ligand which
Georg-August-Universitat Gottingen, Institut fur Anorganische
¨
Chemie, Tammannstrasse 4, D-37077 Gottingen, Germany.
¨
¨
¨
E-mail: oliver.wenger@chemie.uni-goettingen.de;
In acetonitrile solution, the optical absorption spectrum of
the triad essentially corresponds to a superposition of its indivi-
dual molecular components (see Fig. S1 in ESIz), confirming
our expectation that the electronic coupling between donor,
photosensitizer, and acceptor is weak. Following pulsed laser
Fax: +49 (0)551 39 3373; Tel: +49 (0)551 39 19424
w This article is part of the ChemComm ‘Artificial photosynthesis’ web
themed issue.
z Electronic supplementary information (ESI) available: Synthetic
protocols and product characterization data, additional spectroscopic
and electrochemical data. See DOI: 10.1039/c1cc13831f
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10145–10147 10145

Fig. 1 (a) Transient absorption spectrum measured in a 200 ns time
window after excitation of TAA–Ru2+–AQ in deoxygenated aceto-
nitrile solution at 450 nm with 8 ns laser pulses; (b) optical absorption
spectra measured after different time periods of electrochemical oxida-
tion of a TAA reference molecule in acetonitrile solution; (c) optical
absorption spectra measured after different time periods of electro-
chemical reduction of AQ in dichloromethane solution.
Scheme 2 Synthesis of the molecular triad: (a) Pd(PPh3)4, Na2CO3,
THF/water 1 : 1 at reflux; (b) Br2, NaOAc, THF, 0 1C; (c) PdCl2(dppf),
t
KOAc, DMSO, 80 1C; (d) BuOK, Pd(dba)2, P(tBu)3, toluene, 60 1C;
(e) Ru(bpy)2Cl2, CHCl3/C2H5OH, reflux. For further details see
the ESI.z
3
excitation, the MLCT population of the Ru(bpy)3
2+
unit in
the triad decays with a time constant of 8 ns in deoxygenated
acetonitrile solution, which is an instrumentally limited value in
the setup used for the time-resolved experiments reported
Fig. 2 Decays of the transient absorption signals from Fig. 1a at 565
here (see ESIz). Given a 3MLCT excited state lifetime of
nm (a) and 770 nm (b).
2+
855 ns of unbound Ru(bpy)3
under identical conditions,18
this indicates that more than 99% of the 3MLCT population of
the TAA–Ru2+–AQ triad is fed into a nonradiative process.
The transient absorption data in Fig. 1 provide direct
evidence that this nonradiative process involves two consecu-
tive electron transfer steps. The spectrum shown in Fig. 1a was
obtained in a 200 ns time gate starting immediately after
ruthenium excitation at 450 nm. It shows three prominent
absorption bands centered at 770 nm, 565 nm, and 378 nm,
which can be explained by the simultaneous presence of
TAAꢀ radical cations and AQꢀ radical anions. This inter-
pretation is corroborated by the spectroelectrochemistry data
shown in Fig. 1b and c: upon electrochemical oxidation of a
TAA reference molecule (see ESIz for the exact molecular
structure) in acetonitrile solution, an absorption band centered
around 762 nm becomes observable (Fig. 1b). Following
electrochemical reduction of 9,10-anthraquinone, absorption
bands centered at 540 nm and 392 nm become detectable
(Fig. 1c). On this basis (and in agreement with literature
spectra)5,14 we assign the three absorption bands in the
instrumentally limited 3MLCT luminescence lifetime (see above).
The decays of the transient absorption intensities at 770 nm and
565 nm are shown in Fig. 2.
Single exponential fits yield essentially identical decay con-
stants of 1.3 ms in both cases, as would be expected for an
intramolecular thermaꢁl charge-recombination process between
+
the TAAꢀ and AQꢀ radical ion species. Importantly, the
rate constants for photoinduced charge-separation (kCS) and
thermal charge-recombination (kCR) differ by more than a
factor of 160. Such high kCS/kCR ratios are not without
precedent in triads, and they are usually explained by the fact
that forward electron transfer can happen in two consecutive
steps occurring over relatively short distances, while the
reverse reaction can only take place in a single step over a
substantially longer distance.13,19
+
ꢁ
In the present case, the two forward electron transfers likely
involve first TAA to Ru(bpy)32+ electron transfer, followed by
+
electron transfer from reduced Ru(bpy)3 to AQ (Scheme 3).
The opposite sequence is unlikely on energetic grounds: based
+
transient absorption spectrum shown in Fig. 1a to TAAꢀ
on cyclic voltammetry data (Table 1 and Fig. S2 in ESIz),
ꢁ
2+
(770 nm) and AQꢀ (565 nm, 378 nm), and we conclude that
450 nm excitation of TAA–Ru2+–AQ leads to a fully charge-
oxidative quenching of the Ru(bpy)3
3MLCT state by
anthraquinone is endergonic by 0.1 eV, while reductive
quenching by TAA is exergonic by 0.4 eV.
separated state of the typꢁe TAAꢀ+–Ru2+–AQꢀ
.
ꢁ
+
The TAAꢀ and AQꢀ absorption bands at 770 nm and
565 nm in Fig. 1a build up within the 8-ns laser pulse used
for these experiments, in line with the observation of an
Indeed, experiments on reference dyad TAA–Ru2+, comprised
2+
of a triarylamine unit connected to a Ru(bpy)3
complex
(Scheme 4, left), confirms that electron transfer from TAA to
c
10146 Chem. Commun., 2011, 47, 10145–10147
This journal is The Royal Society of Chemistry 2011

constant for thermal charge recombination between TAAꢀ+ and
Ru+ in the triad can be estimated to 6.7 ꢂ 107 sꢁ1. This in turn
permits the conclusion that in the triad the doubly charge
separated state is formed with a yield greater than 64% from
the initially excited Ru(bpy)32+ MLCT state.
+
Subsequent thermal charge recombination between TAAꢀ
and AQꢀꢁ (kET = 7.7 ꢂ 105 sꢁ1) may be slow not only because
of the long distance between the reaction partners (B22 A
center-to-center), but inverted driving force effect and spin
selection rule may play a non-negligible role as well.20
In summary, we have demonstrated vectorial electron trans-
2+
fer in a linear triad incorporating the Ru(bpy)3
complex.
Due in large part to the use of this particular photosensitizer,
a long-lived charge separated state is formed with high quan-
tum yield. An interesting aspect will be to elucidate how the
rate constants and quantum yields for the individual charge
separation and charge recombination processes depend on
distance.21,22,23
Scheme 3 Energy level scheme showing the relevant charge separated
2+
states which can be formed after photoexcitation of the Ru(bpy)3
photosensitizer. Rate constants (k) for the individual electron transfer
steps were estimated as described in the text.
Table
1 Reduction potentials of the individual redox-active
This research was funded by the Swiss National Science
Foundation (grant number 200021-117578) and the Deutsche
Forschungsgemeinschaft (INST 186/872-1).
components of the TAA–Ru2+–AQ triad. Cyclic voltammetry was
performed in acetonitrile solution with 0.1 M tetrabutylammonium
hexafluorophosphate as the electrolyte. Excited-state potentials were
calculated from ground-state potentials using the common
approximations10
Notes and references
Redox couple
V vs. Fc+/Fc
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2+
97% from the initially excited Ru(bpy)3 MLCT state. In the
triad, the singly charge separated state is undetectable with
nanosecond time resolution, because the doubly charge separated
state (TAAꢀ+–Ru2+–AQꢀꢁ) forms with a rate constant of
Z1.3 ꢂ 108 sꢁ1 (see above). In the TAA–Ru2+ reference dyad,
the TAAꢀ+–Ru+ charge-separated state is observable and
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10145–10147 10147