Photoinduced electron transfer in ruthenium(II) trisbipyridine complexes connected to a naphthalenebisimide via an oligo(phenyleneethynylene) spacer

Article abstract of DOI:10.1039/b810856k

The preparation and the characterization of three new dyads composed of a ruthenium trisbipyridine complex linked to a naphthalene bisimide electron acceptor via a phenyleneethynylene spacer of different length (one or two units) are reported. The dyads a

Full text of DOI:10.1039/b810856k

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PAPER
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Photoinduced electron transfer in ruthenium(II) trisbipyridine complexes
connected to a naphthalenebisimide via an oligo(phenyleneethynylene)
spacerw
a
a
a
b
´
Frederique Chaignon, Fabien Buchet, Errol Blart, Magnus Falkenstrom,
Leif Hammarstrom* and Fabrice Odobel*
´
¨
b
a
¨
Received (in Montpellier, France) 26th June 2008, Accepted 31st October 2008
First published as an Advance Article on the web 16th December 2008
DOI: 10.1039/b810856k
The preparation and the characterization of three new dyads composed of a ruthenium
trisbipyridine complex linked to a naphthalene bisimide electron acceptor via a
phenyleneethynylene spacer of different length (one or two units) are reported. The dyads also
differ by the anchoring position of the spacer on the bipyridine, which is appended either at the
4-position or the 5-position. Cyclic voltammetry and the UV-Vis absorption spectroscopy
suggested that the spacer linked at the 5-position ensures a longer p-conjugation length but the
electron transfer rates indicate a lower electronic coupling, than in 4-position. Photoinduced
emission yields indicate a significant quenching of the MLCT excited-state of the ruthenium
complex in these dyads. Except for the dyad linked in 5 position with one phenyleneethynylene
unit, the transient absorption spectroscopy of all the other dyads evidences that the MLCT
excited-state decays almost exclusively by electron transfer to form the charge-separated state
RuIII–NBIꢀ. This state could not be observed, presumably because the subsequent recombination
to the ground state was much faster than its formation. In the dyad linked in 5 position with only
3
one phenyleneethynylene unit, at room temperature, the MLCT* state is in equilibrium with the
3NBI* state, and it also decays via electron transfer. The notable feature of these dyads is first the
occurrence of a relatively long-range electron transfer reaction via a bis(phenylethynylene) linking
unit anchored at the 5 position. Secondly, we show within these series of compounds that subtle
variations in the structure of the dyads (length of the spacer and anchoring position on bipy)
3
have a strong impact on the rates and in the mechanism of decay of the MLCT* state. The
photophysical properties of the dyads can be explained in terms of energy proximity of different
excited states and magnitude of the electronic coupling according to the anchoring position.
employed to produce long-lived charge separated-states and
Introduction
with high quantum yield.13–19 The two latter features are the
primary requirements that a system should fulfil in view of
approaching the function of the photosynthetic reaction
center. In this context, we have used oligophenylethynylene
to connect porphyrin or phthalocyanine sensitizers and we
showed that this type of bridge was particularly well-suited to
assist photoinduced charge separation between these dyes.20,21
The ruthenium tris(bipyridine) complex is a well-known
sensitizer that has been extensively used as photoactive elec-
tron donor in dyads and triads molecular systems for light
induced electron transfer.22–24 In our continuing goal to design
D–B–A systems performing bridge-mediated electron transfer,
we were interested to use the ruthenium tris(bipyridine)
complex as sensitizer, because its long-lived triplet MLCT
excited-state and its strong oxidizing power after electron
transfer (RuIII) make it particularly appealing for this applica-
tion. In a previous paper, we showed that the combination of a
ruthenium tris(bipyridine) complex with a fullerene unit could
lead to quantitative energy transfer upon excitation of the
ruthenium complex, instead of the desired electron transfer.25
This parasite reaction arises from the low lying level of the
fullerene triplet excited-state. Naphthalene bisimide (NBI) has
The rational design of donor–bridge–acceptor (D–B–A)
photomolecular system for photoinduced charge separation
is of both fundamental and practical importance for solar
energy conversion1–3 and molecular electronics.4–7 In D–B–A
systems, the bridge plays a determining role because its
electronic properties and its connectivity to A and to D control
the rate of the electron transfer between D and A.1,7–12 The
search of bridging units that can act as molecular wires and
promote electron transfer over very long distance with a
substantial rate is of high interest, because they can be
a
´
CNRS, UMR 6230, Universite de Nantes, CEISAM, Chimie Et
Interdisciplinarite, Synthese, Analyse, Modelisation, Faculte des
´
`
´
´
`
Sciences et des Techniques, 2, rue de la Houssiniere, BP 92208,
44322 NANTES Cedex 3, France.
E-mail: Fabrice.Odobel@univ-nantes.fr; Fax: +33 2 51 12 54 02;
Tel: +33 2 51 12 54 29
b Department of Photochemistry and Molecular Science, The
˚
Angstrom Laboratories, Uppsala University, Box 523, SE-751 20
¨
Uppsala, Sweden. E-mail: Leif.Hammarstrom@fotomol.uu.se;
Fax: +46-18-471 6844; Tel: +46-18-471 3648
w Dedicated to Prof. Jean-Pierre Sauvage on the occasion of his 65th
birthday.
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408 | New J. Chem., 2009, 33, 408–416

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been used with success as an electron acceptor in association
with porphyrin sensitizers26–32 but with ruthenium complexes
energy transfer sometimes occurs in conjunction with the
charge separation reaction.33–38 Moreover, in the ruthenium–
fullerene dyads, we found unexpectedly that the energy transfer
was essentially distance independent, giving the same rate
constant for the dyads with one, two and three phenylene-
ethylene groups in the bridge.25 Various spectroscopic data
showed that the nature of the ruthenium based excited state
changed for the longest bridge, presumably from the usual
metal-to-ligand charge transfer state to a state localized more
on the bridge, with more p–p* character. We thus rationalized
the similar energy transfer rate over distances from 1.1 to
2.3 nm by excitation energy hopping onto bridge states, which
were thermally accessible also for the dyads with longer bridges.
In this study we have prepared new dyads to investigate
if we could obtain a similarly distance-independent elec-
tron transfer via excitation energy hopping. To this end, the
fullerene was replaced by naphthalene bisimide. Moreover we
compare dyads with the bridge attached to either the 5-bpy or
4-bpy position. We finally varied the bridge length from one to
two phenylethylene units, to obtain the dyads D1p, D1m, D2p
and D2m (Chart 1). We show that the utilization of NBI
instead of C60 can restore the desired electron transfer from
the excited ruthenium tris(bipyridine) complex. In spite of the
very similar intrinsic properties of the ruthenium complexes
with different bridge attachment positions, the dyads show
very different behavior. We show that it can be explained by
the energy proximity of different excited states that makes the
resulting properties of the dyads very sensitive to small
variations in the structure.
Experimental
General
1H and 13C NMR spectra were recorded on a Bruker ARX 300
MHz or AMX 400 MHz Bruker spectrometer. Chemical shifts
for 1H NMR spectra are referenced relative to residual
protons in the deuterated solvent (CDCl3: d = 7.26 ppm,
MeOD: d = 3.31 ppm). Mass spectra were recorded on a
EI-MS HP 5989A spectrometer or on a JMS-700 (JEOL LTD,
Akishima, Tokyo, Japan) double focusing mass spectrometer of
reversed geometry equipped with electrospray ionization (ESI)
source. Fast atom bombardment mass spectroscopy (FAB-MS)
analyses were performed in m-nitrobenzyl alcohol matrix
(MBA) on a ZAB-HF-FAB spectrometer. MALDI-TOF
analyses were performed on an Applied Biosystems Voyager
DE-STR spectrometer in positive linear mode at 20 kV
acceleration voltage with a-cyano-4-hydroxycinnamic acid
(CHCA) as matrix.
Thin-layer chromatography (TLC) was performed on
aluminium sheets precoated with Merck 5735 Kieselgel
60F254. Column chromatography was carried out either with
Merck 5735 Kieselgel 60F (0.040–0.063 mm mesh) or with
SDS neutral alumina (0.05–0.2 mm mesh). Air-sensitive reac-
tions were carried out under argon in dry solvents and
glassware.
Chemicals were purchased from Aldrich and used as received.
Compounds
2,5-bis(dodecyloxy)-4-(20-trimethylsilylethynyl)-
iodobenzene 1,25 N-octyl-N0-(40-iodophenyl)naphthalene-1,4,5,8-
tetracarboxylic acid bisimide 4,39 ruthenium complexes 7,25 9,39
8,39 dyad D1p,39 and reference complexes R1p,39 R2m25 and
R1m25 were prepared according to literature methods.
Chart 1 Structures of the molecules studied in this work.
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The electrochemical measurements were performed with a
potentiostat-galvanostat MacLab model ML160 controlled by
resident software (Echem v1.5.2 for Windows) using a
conventional single-compartment three-electrode cell. The
working electrode was a 10 mm long Pt wire, the auxiliary
was a Pt wire and the reference electrode was the saturated
potassium chloride calomel electrode (SCE). The supported
electrolyte was 0.1 M Bu4NPF6 in DMF and the solutions
were purged with argon before the measurements. All poten-
tials are quoted relative to SCE. In all the experiments the scan
rate was 100 mV sꢀ1 for cyclic voltammetry and 15 Hz for
pulse voltammetry. The UV-Visible absorption spectra were
recorded on a UV-2401PC Shimadzu spectrophotometer.
Emission spectra were recorded on a SPEX Fluoromax
fluorimeter and were corrected for the wavelength dependent
response of the detector system (Hamamatsu R928).
N-Octyl-N0-(20,50-bis(dodecyloxy)-40-(200-triisopropylsilylethynyl)-
ethynylbenzene)naphthalene-1,4,5,8-tetracarboxylic acid bisimide 5.
A solution of compound 3 (157 mg, 0.24 mmol) and 4 (108 mg,
0.19 mmol) in dry triethylamine (6.5 mL) and tetrahydrofuran
(4.5 mL) was introduced in a sealed tube and degassed by
freeze–pump–thaw cycles. Then PPh3 (7.3 mg, 0.028 mmol),
Cu(OAc)2 (1.9 mg, 0.009 mmol) and PdCl2 (1.7 mg,
0.009 mmol) were added. The reaction mixture was heated at
70 1C for 15 h. The solvent was rotary evaporated and the crude
product was purified by flash chromatography (SiO2, petroleum
ether–dichloromethane 15 : 85). 5 was obtained as a red solid
1
(144 mg, 69%). H NMR (300 MHz, CDCl3): d 8.80 (s, 4H),
7.71 (d, 2H, J = 8.4 Hz), 7.31 (d, 2H, J = 8.4 Hz), 6.96 (s, 1H),
6.94 (s, 1H), 4.21 (m, 2H), 3.99 (m, 4H), 1.80 (m, 6H), 1.27
(m, 46H), 1.15 (m, 21H), 0.85 (m, 9H).13C NMR (75 MHz,
CDCl3): d 162.71, 162.56, 154.24, 153.45, 134.16, 132.47,
131.27, 130.89, 128.58, 126.88, 126.68, 126.46, 124.57, 117.63,
116.32, 114.30, 113.58, 102.90, 96.60, 93.68, 87.31, 69.66, 69.26,
40.98, 31.86, 31.75, 29.61, 29.31, 29.15, 28.02, 27.04, 26.15,
26.01, 22.62, 18.68, 14.04, 11.35. HRES+-MS: m/z: calc. for
C71H99N2O6 1103.7272; found 1103.7287 [M + H]+.
Time-resolved emission and transient absorption spectra
were recorded using an Applied Photophysics LKS60 laser-
flash photolysis system. Excitation was obtained with 5 ns,
460 nm pulses (ca. 20 mJ pulseꢀ1) from a Q-switched YAG
laser (Quantel Brilliant B) with an OPOTEK OPO. The
detection system of the LKS60 employed a 150 W pulsed
Xe-lamp for analyzing light at right angle configuration, a
monochromator after the sample, a P928 PMT, and a digital
oscilloscope (500 MHz, 2 Gs/s).
N-Octyl-N0-(20,50-bis(dodecyloxy)-40-(200-ethynyl)ethynyl-
benzene)naphtalene-1,4,5,8-tetracarboxylic acid bisimide 6.
Compound 5 (144 mg, 0.13 mmol) was dissolved in tetra-
hydrofuran (11 mL) under argon. Then tetrabutylammonium
fluoride (0.3 mL, 0.3 mmol) was added and the reaction
mixture was stirred at 30 1C for 2 h. The solvent was rotary
evaporated and water was added. The solution was extracted
with dichloromethane and the organic layer was washed with
water, dried over MgSO4 and rotary evaporated. 6 was
obtained as a red–brown solid (118 mg, 96%). 1H NMR
(300 MHz, CDCl3): d 8.79 (s, 4H), 7.71 (d, 2H, J = 8.4 Hz),
7.32 (d, 2H, J = 8.4 Hz), 6.96 (m, 2H), 4.21 (m, 2H), 3.99
(m, 4H), 3.35 (s, 1H), 1.80 (m, 6H), 1.27 (m, 46H), 0.85
(m, 9H).
Syntheses
2,5-Bis(dodecyloxy)-4-(20-trimethylsilylethynyl)triisopropyl-
silylacetylenylbenzene 2. A solution of compound 1 (0.4 g,
0.53 mmol) in dry triethylamine (2.7 mL) and tetrahydrofuran
(0.9 mL) was introduced in a sealed tube and degassed by
freeze–pump–thaw cycles. Then PPh3 (21 mg, 0.080 mmol),
Cu(OAc)2 (5.3 mg, 0.026 mmol), PdCl2 (9.5 mg, 0.053 mmol)
and triisopropylsilylacetylene (0.3 mL, 1.33 mmol) were
added. The reaction mixture was heated at 50 1C for 15 h. A
saturated aqueous solution of NaCl was added and the solu-
tion was extracted with ethyl acetate. The organic layer was
then washed with water, dried over MgSO4, and rotary
evaporated. The crude product was purified by flash chromato-
graphy (SiO2, petroleum ether–diethyl ether: 100 : 0 to 99 : 1).
2 was obtained as a yellow oil (327 mg, 85%). 1H NMR
(300 MHz, CDCl3): d 6.89 (s, 1H), 6.88 (s, 1H), 3.94 (m, 4H),
1.77 (m, 4H), 1.27 (m, 20H), 1.15 (s, 21H), 0.89 (m, 6H), 0.27
(s, 9H). 13C NMR (75 MHz, CDCl3): d 154.25, 153.99, 117.75,
116.73, 114.32, 113.89, 102.99, 101.31, 99.78, 96.43, 69.66,
69.28, 31.98, 29.69, 29.41, 26.21, 22.73, 18.74, 14.13, 11.43,
0.00. EI-MS+: 169.25 (10%), 722.60 (14%).
Dyad D2m. A solution of complex 7 (92 mg, 0.098 mmol)
and 6 (123 mg, 0.013 mmol) in dry triethylamine (0.96 mL)
and dimethylformamide (6.4 mL) was introduced in a
sealed tube and degassed by freeze–pump–thaw cycles.
Then Pd(dppf)Cl2 (15 mg, 0.020 mmol) and CuI (3.7 mg,
0.020 mmol) were added. The reaction mixture was heated at
45 1C for 15 h. Water was added and the solution was
extracted with dichloromethane. The organic layer was then
washed with water, dried over MgSO4, and rotary evaporated.
The crude product was purified by flash chromatography
(SiO2, dichloromethane–acetonitrile–KNO3: 100 : 0 : 0 to
78 : 20 : 2). The dyad D2m was obtained as a red solid
2,5-Bis(dodecyloxy)-4-(20-ethynyl)triisopropylsilylacetylenyl-
benzene 3. Compound 2 (175 mg, 0.24 mmol) was dissolved in
dichloromethane (3.3 mL) and methanol (6.6 mL). Then
potassium carbonate (0.33 g, 2.42 mmol) was added and the
reaction mixture was stirred at room temperature for 3 h.
Water was added and the solution was extracted with dichloro-
methane. The organic layer was washed with water, dried over
MgSO4 and rotary evaporated. 34 was obtained as a yellow oil
1
(60 mg, 33%). H NMR (300 MHz, CDCl3): d 8.80 (s, 4H),
8.45 (m, 6H), 8.01 (m, 6H), 7.71 (m, 8H), 7.48 (m, 5H), 7.31
(d, 2H, J = 8.4 Hz), 6.98 (s, 1H), 6.97 (s, 1H), 4.21 (m, 2H),
3.98 (m, 4H), 1.80 (m, 6H), 1.27 (m, 46H), 0.85 (m, 9H).
HRES+-MS: m/z: calc. for C92H100N8O6Ru 757.3405; found
757.3409 (M2+).
Dyad D2p. A solution of complex 8 (80 mg, 0.080 mmol)
and 6 (100 mg, 0.104 mmol) in dry triethylamine (0.8 mL) and
dimethylformamide (5.2 mL) was introduced in a sealed tube
and degassed by freeze–pump–thaw cycles. Then Pd(dppf)Cl2
1
(157 mg, quantitative). H NMR (300 MHz, CDCl3): d 6.92
(s, 1H), 6.91 (s, 1H), 3.94 (m, 4H), 3.31 (s, 1H), 2.53 (s, 24H),
1.77 (m, 4H), 1.28 (m, 20H), 1.15 (s, 21H), 0.89 (m, 6H).
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(12 mg, 0.016 mmol) and CuI (3 mg, 0.016 mmol) were added.
The reaction mixture was heated at 75 1C for 15 h. Water was
added and the solution was extracted with dichloromethane.
The organic layer was then washed with water, dried over
MgSO4, and rotary evaporated. The crude product was
purified by flash chromatography (SiO2, dichloromethane–
acetonitrile: 100 : 0 to 80 : 20). The dyad D2p was obtained
as a red solid (69 mg, 46%).1H NMR (300 MHz, CDCl3):
d 8.80 (s, 4H), 8.31–8.21 (m, 7H), 7.97 (m, 1H), 7.78–7.70
(m, 4H), 7.52–7.46 (m, 7H), 7.33 (d, 2H, J = 8.4 Hz), 7.07
(s, 1H), 7.03 (s, 1H), 4.21 (m, 2H), 3.98 (m, 4H), 2.53 (m, 12H),
1.80 (m, 6H), 1.27 (m, 46H), 0.85 (m, 9H). HRES+-MS: m/z:
calc. for C96H108N8O6Ru 785.3712; found 785.3698 (M2+).
compound 3 was connected to the iodonaphthalene bisimide
439 to give 5 in 69% isolated yield (Scheme 2). The last
step consists of a final Sonogashira cross-coupling reaction,
directly made on the complex (7 or 8) with the NBI derivative
6 or 9 (Scheme 1). The diphenylphosphinoferrocene (dppf)
was chosen as ligand of palladium in the catalytic system of
this last coupling because it proved to be particularly well-
suited in previously reported cross-coupling reactions carried
out on a ruthenium complex.25,41 The dyads D2m, D2p and
D1m were thus respectively obtained in 33, 46 and 55% yield
after purification on column chromatography.
Electronic absorption spectra
The absorption spectra of the dyads D1p, D1m, D2p, D2m are
shown in Fig. 1 and the spectroscopic data are collected in
Table 1. The most intense absorption in the UV, at around
290 nm, is assigned to a p–p* transition localized on the
bipyridine ligands. The two sharp absorptions at 360 and
380 nm are attributed to the NBI unit and are almost not
affected by comparison with the parent compound 4.25,42 This
is consistent with the presence of nodes of the frontier mole-
cular orbitals on the nitrogens of the imide groups, which lead
to weak electronic communication with any substituents
attached on these positions.39,43 Accordingly, the absorption
bands of the ruthenium complex and of those of the phenyl-
ethynylene spacer are not affected by the presence of the NBI,
as confirmed by the similar maximum absorption wavelengths
of these transitions in the dyads compared to those in the
parent reference compounds (Table 1). The p–p* transition of
the oligophenylethynylene spacer occurs at 390 nm in dyads
D2m–D2p and is naturally blue-shifted in dyad D1p and D1m
containing the shorter linker. The broad absorption band in
the visible region is attributed to the well-known spin-allowed
metal-to-ligand charge transfer transition (MLCT). Interest-
ingly, this absorption band is less intense and blue-shifted in
the dyads D2m and D1m in which the spacer is attached in the
5 position of the bipyridine compared to the other dyads
linked on the 4 position. The same effect has been observed in
other tris-bipyridine ruthenium complexes functionalized either
on the 4 position or on the 5 position of the bipyridine.44–48
Dyad D1m. NBI derivative 9 (57 mg, 0.11 mmol), ruthenium
complex 7 (85 mg, 0.09 mmol), Pd(dppf)Cl2 (14.5 mg,
0.02 mmol) and CuI (3.7 mg, 0.02 mmol) were introduced in
a sealed tube, then dry triethylamine (0.5 ml) and anhydrous
dimethylformamide (5 ml) were added and the resulting
mixture was degassed by three freeze–pump–thaw cycles.
The solution was heated at 80 1C for 17 h. The reaction
mixture was then concentrated on a rotary evaporator to
remove most of the solvent and the residue was diluted with
dichloromethane. The organic layer was washed with an
aqueous solution of NH4Cl, then water and finally with brine
and dried over MgSO4 and concentrated in vacuo to dryness.
The crude red solid was purified by flash column chromato-
graphy (SiO2, acetone–water–saturated aqueous KNO3 solu-
tion 9 : 1 : 0.002 to 9 : 1 : 0.004) to give the desired compound
as a dark red solid (66 mg, 55%). 1H NMR (300 MHz,
CD3CN): d 8.70 (dd, 4H, J = 12.3, 7.7 Hz), 8.48–8.54
(m, 6H), 8.19 (dd, 1H, J = 8.4, 1.8 Hz), 8.03–8.12 (m, 5H),
7.90 (br. d, 1H, J = 1.3 Hz), 7.85 (br. d, 1H, J = 5.7 Hz),
7.70–7.77 (m, 4H), 7.67 (d, 2H, J = 8.55 Hz), 7.44 (d, 2H, J =
8.55 Hz), 7.37–7.45 (m, 4H), 4.13 (dd, 2H, J = 9.0, 7.5 Hz),
1.68–1.77 (m, 2H), 1.27–1.42 (m, 10H), 0.88 (t, 3H, J =
6.8 Hz). 13C NMR (75 MHz, CD3CN): d 164.02, 163.78,
157.96, 157.91, 157.88, 157.84, 157.10, 154.28, 152.95,
152.76, 152.73, 152.70, 152.51, 140.73, 138.87, 138.78,
137.88, 133.43, 131.58, 131.34, 130.47, 128.62, 128.59,
128.52, 128.10, 127.82, 127.74, 127.60, 125.70, 125.35,
125.23, 124.87, 124.52, 122.73, 99.35, 85.75, 41.47, 32.47,
29.91, 29.86, 28.54, 27.75, 23.30, 14.32. MALDI-MS:
Electrochemical study
+
m/z: calc. for C60H48N8O4RuPF6 1192.2; found 1191.9
[M ꢀ PF6ꢀ]+.
The dyads were studied by cyclic voltammetry and differential
pulse voltammetry in DMF and the half-wave potentials
are collected in Table 2. The first oxidation reaction is a
ruthenium-centred process which involves the p(t2g) metal
orbital. In dyads D1p and D2p, the potential is cathodically
shifted by 100 mV compared to that in dyad D2m and D1m,
and to the unsubstituted [Ru(bpy)3]2+ (Table 2). This is
attributed mainly to the consequence of electron donating
methyl substituents, borne on the ancillary bipyridine of the
former complexes, which stabilize the Ru(III).49 The two first
reductions, taking place at about ꢀ0.45 and ꢀ1 V, respec-
tively, correspond to the consecutive one-electron reductions
of the NBI unit,42 while the third reduction, at around ꢀ1.2 V,
is attributed to the reduction of the bipyridine ligand that is
connected to the spacer. The third reduction is easier in the
dyads D1p, D1m, D2p, D2m with respect to the unsubstituted
Results and discussion
Synthesis of the compounds
The key step of the synthesis of the new dyads D1m, D2m and
D2p is the Sonogashira cross-coupling reaction between the
synthon 6 or 939and the bromobipyridine ruthenium complex
725and 825,40 (Scheme 1). The synthesis of the key intermediate
6 is depicted in Scheme 2 and it starts with the known
bisalkoxy para-(trimethylsilylethynyl)iodobenzene 125 which
was initially coupled with triisopropylsilylacetylene using the
classical conditions of Sonogashira reaction to afford 2 with
85% yield. The trimethylsilyl group was then cleaved with
potassium carbonate in a quantitative yield and the resulting
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Scheme 1 Reagents and conditions: (a) Pd(dppf)Cl2, CuI, Et3N, DMF (33% for D2m, 46% for D2p and 55% for D1m).
Scheme 2 Reagents and conditions: (a) Pd(PPh3)2Cl2, Cu(OAc)2, Et3N, THF, 50 1C (85%); (b) K2CO3, CH2Cl2–CH3OH, RT (100%);
(c) Pd(PPh3)2Cl2, Cu(OAc)2, Et3N, THF, 70 1C (69%); (d) Bu4NF, THF, RT (100%).
[Ru(bpy)3]2+; this indicates a stabilization of the p* orbital
upon the attachment of the spacer. It results from an increased
p-conjugation length between the bipyridine and the oligo-
ethynylenephenylene spacer as already observed when a
p-conjugated spacer is linked to a polypyridine ruthenium
complex.25,39,50–54 Interestingly, the stabilization of the
LUMO orbital of the bipyridine is less marked when the
spacer is connected on the 4 position (D1p or D2p) com-
pared to the 5 position of bipyridine (D1m or D2m). This
suggests a longer p-conjugation length when the bipyridine is
substituted on the 5 position, probably due to a higher spin
density of the LUMO orbital at this position and the larger
number of mesomer structures between the pyridines and the
oligo(phenyleneethynylene) spacer.55,56
Steady-state and time-resolved emission
The luminescence properties of the dyads D1p, D1m, D2p,
D2m were studied by steady-state and time-resolved spectro-
Fig. 1 Absorption spectrum of the dyads D1p, D1m, D2p and D2m
recorded in DMF.
scopy and were compared to the reference compounds
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Table 1 UV-Vis and emission data of the complexes
Complex
lAmbaxs/nm (e/Mꢀ1 cmꢀ1 a
)
l
em/nm (RT)a
l
em/nm (77 K)b
E00/eV (3MLCT)c
R1p
477 (14800), 372 (14200), 348 (14100), 291 (62000)
407 (35400), 310 (52600), 291 (75400)
360, 380
477 (21100), 381 (45600), 361 (42800), 325 (46350), 289 (94000)
410 (27800), 383 (34000), 361 (29400), 313 (47100), 292 (71000)
482 (27000), 382 (55200), 362 (45100), 291 (92500)
456 (9200) 380 (29100), 361 (38600), 342 (36600), 290 (60700)
655
671
368
674
668
676
670
626
617
608
636
618
636
618
1.98
2.01
2.04
1.94
2.01
1.94
2.01
R2m
NBI 4
D1p
D2m
D2p
D1m
a
b
c
Recorded in DMF. Recorded in ethanol–methanol glass. Calculated from the emission maximum at 77 K (E00 = 1240/lem(77 K)).
Table 2 Redox potentials of the dyads recorded in DMF with Bu4NPF6 (0.1 M) as supporting salt. The potentials are quoted vs. saturated
calomel electrode (SCE)
EOx(RuIII/RuII)/V
ERed1(NBI/NBIꢀ)/V
ERed2(NBIꢀ/NBI2ꢀ)/V
ERed3(bpy/bpyꢀ)/V
[Ru(bpy)3]2+
D1p
D2m
D2p
D1m
1.30
1.21
1.32
1.21
1.33
ꢀ1.24
ꢀ1.18
ꢀ1.04
ꢀ1.18
ꢀ1.11
ꢀ0.46
ꢀ0.47
ꢀ0.47
ꢀ0.47
ꢀ0.95
ꢀ0.97
ꢀ0.95
ꢀ0.94
(R1p, R2m and R1m) to determine the quenching efficiency of
the MLCT excited-state by the nearby NBI electron acceptor.z
Table 3 collects the emission lifetimes, the quenching percen-
tage and the quenching rate of the MLCT emission. The broad
emission band in the above compounds is attributed to the
phosphorescence of the triplet MLCT excited-state of the
ruthenium complex. At 77 K a vibronic structure typical for
3MLCT emission of Ru(II)polypyridyl complexes is observed
(see ref. 25 for spectra of R2m and R1m). In previous studies,25
our spectroscopic data suggested that with a tris(phenylethylene)
spacer in the 5-bpy position of the ruthenium complex, the
lowest excited-state of the system can be assigned to a
triplet p–p* of the spacer.25,51,57–59 In contrast, for the mono-
Table 3 Emission lifetimes (tem) and electron transfer rate constants
(kET) in degassed acetonitrile at room temperature
t
em/ns
1500
1400
1200
63
% quenchinga
kETb/sꢀ1
DG0 c/eV
ET
R1p
—
—
—
96
86
40
73
—
—
—
—
—
—
ꢀ0.27
ꢀ0.22
ꢀ0.26
ꢀ0.21
R2m
R1m
D1p
D2m
D2p
D1m
1.5 ꢂ 107
4.3 ꢂ 106
4.4 ꢂ 105
3 ꢂ 106
200
900
120, 420
a
Calculated as 100(t0 ꢀ tem)/t0, where t0 is tem of the corresponding
b
c
reference R1p or R2m. Calculated as kq = 1/tem ꢀ 1/t0. Calculated
as DG0ET = e(E0(RuIII/RuII) ꢀ E0(NBI/NBIꢀ)) ꢀ E00(3MLCT) taken
from Tables 1 and 2. Work terms and differences between redox
potentials in DMF (e = 38) and CH3CN (e = 37) were neglected.
3
and bis(phenylethylene) complexes the MLCT state was the
lowest excited state. Referring to previous work, the lowest
3
excited state is MLCT in all of the present dyads.25
The energy of the MLCT excited state was determined from
the emission spectrum recorded in ethanol glass at 77 K and it
shows that in the dyads D1p and D2p this level is lower lying
than that in D2m and D1m (Table 1). This can be explained
by the inductive effect of the methyl substituents on the
bipyridines of D1p and D2p that destabilize the p(t2g) HOMO
orbitals and decrease the energy gap with the p* LUMO
orbital on the bipyridine, whereas the longer p-conjugation
of the bpy with the bridge in D2m and D1m stabilizes the
LUMO orbitals, but to an overall lower extent.
the same time constant as the emission decay. Therefore, in the
dyads D1p, D2p and D2m the most plausible interpretation of
the results is an electron transfer process leading to
RuIII–NBIꢀ, which recombines with a faster rate than that
of its formation. This is in analogy to our previous study of
D1p and a related dyad, but where the electron transfer
products were sufficiently long-lived to be detected. In the
dyad D1m the decay of the emission and transient absorption
signals are instead biexponential. A global fit to the transients
at 360–700 nm gave time constants of 120 and 420 ns. The
initial spectrum is identical to that for the R1p reference, while
the spectrum that emerges after the faster phase can be
The emission lifetime measurements, made in degassed
acetonitrile solution, indicate a significant shortening of the
MLCT emission in the dyads D1p, D1m, D2p and D2m
compared to those in the reference complexes (Table 3). The
quenching rate of the ruthenium by the NBI decreases in the
following order D1p 4 D2m 4 D1m 4 D2p.
3
3
assigned to a mix of the original MLCT state and the NBI
state (Fig. 2). These two species decay simultaneously with a
420 ns time constant. The transient traces around 480 nm
3
show a clear rise-and-decay behavior as the NBI is formed
and then disappears.60 The results are consistent with electron
In dyads D1p, D2p and D2m, the transient absorption shows
only the features of the initial 3MLCT state, which decays with
3
transfer from the MLCT state to the NBI unit, in parallel to
excited state equilibration by triplet energy transfer between
z From comparison with previous studies, the longer spacer in D2p
than R1p would lead to an most 20% underestimation of the un-
quenched lifetime with the longer spacer and consequently at most a
10% underestimation of the electron transfer rate constant in D2p.25
3
3
the MCLT and NBI states. After equilibration the excited
states are depopulated by further electron transfer via the
3MLCT state (Scheme 3). From a kinetic analysis in the
ꢁc
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New J. Chem., 2009, 33, 408–416 | 413

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expect that its energy would also be lower for D1p and D2p,
and that therefore it would be just as thermally accessible.
Instead, if it is more a bridge-localized p–p* state, its energy
should be almost independent of anchoring position. Thus, it
would be somewhat higher lying relative to the energy of
3MLCT state in D1p and D2p and as a result they should not
be as easily populated as in D2m and D1m. Indeed, the
30ꢂ difference in rate for the former, with a distance difference
of 8 A, is consistent with a distance decay factor ‘‘b’’ of
0.4 Aꢀ1, which is normal for a super-exchange mediated
electron transfer with this chromophore-conjugated bridge
combination.20,61
Note added in proof: Harriman and co-workers62 have
pointed out the importance of bending flexibility of the
phenylethenyl bridges. They presented molecular modeling
simulations suggesting that the Ru(bpy)3 and C60 moieties in
a Ru–C60 dyad bridged by one phenylethenyl unit frequently
come into van der Waals contact, and suggested that electron
and energy transfer processes may occur through close
through-space contact and not through the bridge. While this
may be correct for the shorter bridge, a close contact is not
possible in the dyads with two or three bridge units, however.
The flexibility of the bridge can therefore not explain the
distance independent reaction rates in the series of 5-bpy
bridged Ru-C60 presented earlier25 or in the Ru–NBI dyads
presented here.
Fig. 2 Transient absorption spectra of D1m after excitation at
440 nm, after 30 ns (solid), 300 ns (dashed) and 1000 ns (dotted).
Inset: transient absorption traces of the same experiments, at (from
top to bottom) 450, 580, 480, 670 and 370 nm.
Appendix, the individual rate constants can be calculated to
give kET = 5 ꢂ 106 sꢀ1, while the charge recombination (kCR
)
is again much faster so that no charge separated state can be
observed, just as for the other dyads.
The ratio of the energy transfer rate constants kEnT and
k
in Scheme 3 is close to unity, which means that the
ꢀEnT
3MLCT and 3NBI states in D1m lie at very similar energy. For
the dyads D1p and D2p, linked in the 4-bpy position, the
3MLCT states are lower in energy. This can explain why
energy transfer to form 3NBI was not observed in D1p and
D2p. Also for D2m, which is linked in the 5-bpy position,
Conclusion
The preparation and characterizations of three new dyads
composed of a ruthenium trisbipyridine complex and a
naphthalene bisimide electron acceptor are described. In the
three dyads D1p, D2m and D2p the ruthenium trisbipyridine
sensitizer decays almost exclusively by an electron transfer
instead of an energy transfer process as observed in our
previous systems in which the NBI was replaced by the
fullerene.25 As a result, these new dyads constitute a significant
improvement over the previous ones. The present results show
the following effects of the structural variations of the dyads
discussed in the introduction. First, the replacement of C60
with NBI as acceptor makes energy transfer less probable and
opens for charge separation reactions. Second, the alternation
between 4- and 5-bpy substitutions shows different effects
that can be related both to the intrinsic difference in bridge-
3
the MLCT should be slightly lower than in D1m, due to the
longer p-delocalization on the bridge (as evidenced by the
lower reduction potential of bipy in D2m than in D1m,
Table 2) and we did not obtain clear evidence for energy
transfer in D2m. A minor transient component (5%) with a
longer, ca. 1000 ns lifetime, was indeed observed, but the
signal was small and the lifetime was similar to that for the
reference, so we could not determine if there was any 3NBI
contribution or if it can be attributed to a minor, unquenched
3MCLT population. In any case, the small magnitude means
that the possible effect on the calculated rate constant for
electron transfer in D2m is small, giving an error of at
most 10%.
The normal distance dependence of the electron transfer
rate in D1p and D2p is an interesting contrast to the results of
the other two dyads. If the putative, more bridge-localized
state responsible for the distance-independent electron transfer
3
chromophore coupling and to the proximity of the MLCT,
3NBI and bridge excited states: on the one hand, the electronic
coupling is stronger via the 4-bpy than via the 5-bpy position.
This is shown by comparison of the two dyads with the short
links, where D1p shows a faster electron transfer than D1m.
On the other hand, the dyads with the longer bridges show the
opposite behavior: electron transfer is much faster in the 5-bpy
substituted dyad D2m. This leads to the interesting observa-
tion that while the 4-bpy linked dyads show a 30-fold slower
electron transfer with addition of another phenyleneethylene
group, electron transfer is equally fast in D2m and D1m, in
spite of the 8 A difference in distance. The near-independence
on electron transfer distance in the 5-bpy linked dyads
parallels our previous results on triplet energy transfer in the
analogous 5-bpy linked Ru–C60 dyads.25 We concluded in that
3
in D2m and D1m were just another MLCT state, one could
Scheme 3 Reaction scheme for D1m after MLCT excitation of the
ruthenium complex.
ꢁc
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414 | New J. Chem., 2009, 33, 408–416

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paper, based on various spectroscopic data, that the lowest
excited-state of the Ru-bridge system was gradually shifted
more onto the bridge as the latter became longer, which could
explain the observed near-independence on bridge length on
the energy transfer rate. In the present study we seem to
observe an analogous effect for electron transfer. In summary,
we showed within these series of dyads that subtle structural
changes result in significant modifications of the rate and of
the mechanism of electron transfer from the ruthenium com-
the case (Fig. 2). This good agreement suggests that electron
transfer from the 3NBI state is negligible and gives a posteriori
support for Scheme 3. The accuracy of the value determined
for kET is obviously less than for a direct observation in the
absence of complicating parallel reactions. Nevertheless, the
corresponding time constant of ca. 300 ns, as reported in
Table 3, is clearly not shorter than for D2m with the longer
bridge.
3
plex to NBI. More precisely, the energy level of the MLCT*
Acknowledgements
and/or the electronic coupling, which can be tuned by
changing the length of the spacer, its anchoring position or
the presence of the methyl groups on the bpy, govern the
feasibility of the energy transfer to NBI. This information
can be useful for the future design of photomolecular systems
for photoinduced charge separation with ruthenium trisbi-
pyridine sensitizer. Interestingly, the dyad D2m containing
the bis(phenylethynylene) spacer appended in the 5-position
of the bipyridine represents a potentially useful molecular
basis to build multicomponent systems for long-range photo-
induced charge separation since long range electron transfer
occurs with a high quantum yield at room temperature in
acetonitrile (86%).
The French Research Ministry is gratefully acknowledged for
the financial support of these researches through the programs
‘‘ACI Jeune Chercheur’’ and the ANR blanc ‘‘PhotoCumElec’’
and the European Community for COST D35 program. L. H.
acknowledges support from the K&A Wallenberg Foundation,
the Swedish Energy Agency and the Swedish Foundation for
Strategic Research and thanks Dr Amanda Smeigh for fitting
and plotting the transient absorption data.
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+ (1 + NNBI/NMLCT)k
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