Distance-independent photoinduced energy transfer over 1.1 to 2.3 nm in ruthenium trisbipyridine-fullerene assemblies

Article abstract of DOI:10.1039/b506837a

Ruthenium trisbipyridine C₆₀ dyads linked via para-phenyleneethynylene units have been prepared. They displayed a rapid energy transfer from Ru to C₆₀ with a rate that was independent of distance, from 1.1 to 2.3 nm. The results are

Full text of DOI:10.1039/b506837a

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P A P E R
Distance-independent photoinduced energy transfer over 1.1 to
2.3 nm in ruthenium trisbipyridine–fullerene assembliesw
a
a
a
b
´
Frederique Chaignon, Javier Torroba, Errol Blart, Magnus Borgstrom,
Leif Hammarstrom* and Fabrice Odobel*
´
¨
b
a
¨
a
`
Laboratoire de Synthese Organique, UMR 6513 CNRS, Faculte des Sciences et des
Techniques de Nantes, BP 92208, 2, rue de la Houssiniere, 44322 Nantes Cedex 03, France.
´
`
E-mail: Fabrice.Odobel@chimie.univ-nantes.fr; Fax: þ33 2 51 12 54 02;
Tel: þ33 2 51 12 54 29
b Department of Physical Chemistry, Uppsala University, Box 579, SE-751 23 Uppsala, Sweden.
E-mail: Leifh@fki.uu.se; Fax: þ46 18 471 3654
Received (in Montpellier, France) 16th May 2005, Accepted 26th July 2005
First published as an Advance Article on the web 26th August 2005
Ruthenium trisbipyridine C60 dyads linked via para-phenyleneethynylene units have been prepared. They
displayed a rapid energy transfer from Ru to C60 with a rate that was independent of distance, from 1.1 to
2.3 nm. The results are explained by a hopping mechanism involving a bridge-localized excited-state. In fact,
for the longest bridge this state was lower in energy than the Ru-based MLCT state, as evidenced by the
spectroscopic data.
for 1H NMR spectra are referenced relative to residual protium
Introduction
in the deuterated solvent (CDCl3 d ¼ 7.26 ppm, MeOD d ¼
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 geome-
try 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 per-
formed on an Applied Biosystems Voyager DE-STR spectro-
meter 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 alu-
There is a strong current interest in the preparation and study
of molecular assemblies performing efficient long-range elec-
tron- or energy-transfer, mediated via an intervening bridge
that links the reactant chromophoric and/or redox units to-
gether.1 If the excited states or charge transfer states involving
the bridge are not much higher in energy than the reactant
state, the electron- or energy-transfer may proceed via a
transient population of these bridge states.2
This mechanism is frequently referred to as ‘‘hopping’’, and
as the resulting transfer process can be very efficient over long
distances the molecular structure is often called a molecular
electronic or photonic wire.3
minium sheets precoated with Merck 5735 Kieselgel 60F254
.
In the present paper, we present a series of complexes based
on a RuII–polypyridine chromophore and a fullerene acceptor,
linked via a highly conjugated phenyl-ethynyl bridge of varying
length (1.1–2.3 nm center to center distance was estimated from
X-ray crystallographic data on similar building blocks from
refs 4 and 5). This chromophore–acceptor couple has been
employed in studies of electron- and energy-transfer.6–8 The
phenyl-ethynyl bridge motif has previously been shown to
mediate rapid intra-molecular electron transfer.9–13 The alkoxy
substituents on the phenyl groups of the present study make
electron transfer via reduced states of the bridge more difficult.
Instead, they lower the excited state energy of the bridge and
facilitate triplet energy transfer via a hopping mechanism.4 We
present the synthesis and triplet energy transfer properties of
these complexes (Scheme 1).
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 reactions
were carried out under argon in dry solvents and glassware.
Chemicals were purchased from Aldrich and used as re-
ceived. Compounds 4-trimethylsilylethynylbenzaldehyde, 5-
bromobipyridine,14 bischloro bisbipyridine ruthenium(II),15
bischloro(diphenylphosphino) ferrocene palladium(II)16 and
tetrakis(triphenylphosphine) palladium17 were prepared ac-
cording to literature methods.
Electrochemistry
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 Pt wire of 10 mm long, the auxiliary
was a Pt wire and the reference electrode was the saturated
potassium chloride calomel electrode (SCE). The supported
electrolyte was 0.15 N Bu4NPF6 in dichloromethane and the
solutions were purged with argon before the measurements. All
potentials 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.
Experimental
General methods
1H and 13C NMR spectra were recorded on a Bruker ARX 300
MHz or AMX 400 MHz Bruker spectrometer. Chemical shifts
w Electronic supplementary information (ESI) available: Transient
absorption spectra and traces for D1–D3 and R1, and a Table with
kinetic data from a global fit. See http://dx.doi.org/10.1039/b506837a
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
1272
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 7 2 – 1 2 8 4

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Scheme 1 Structures of the compounds studied in this work.
The reported values are averages of 5000–10 000 individual
measurements. All optical experiments were performed in
acetonitrile at 298 K.
Spectroscopy
UV–Visible absorption spectra were recorded on a UV-
2401PC Shimadzu spectrophotometer. Fluorescence spectra
were recorded on a SPEX Fluoromax fluorimeter and were
corrected for the wavelength dependent response of the detec-
tor system.
Syntheses of the compounds
2,5-Bis(dodecyloxy)bromobenzene 2. Bromohydroquinone 1
(5 g, 26 mmol) was dissolved in DMF (25 mL). NaOH (2.6 g,
66 mmol) and 1-bromododecane (14.1 mL, 57 mmol) were
added and the resulting solution was stirred at reflux for two
days. Water (100 mL) was added and the solution was ex-
tracted with dichloromethane. The organic layer was washed
with a saturated aqueous solution of NH4Cl, dried over
MgSO4 and evaporated. The crude product was purified by
flash chromatography (SiO2, petroleum ether 100%, then
petroleum ether–dichloromethane 95 : 5). Compound 2 was
obtained as a white solid (8.5 g, 62%).
1H NMR (300 MHz, CDCl3): d ¼ 7.10 (d, J ¼ 2.4 Hz, 1H),
6.81 (s, 1H), 6.79 (d, J ¼ 2.4 Hz, 1H), 3.94 (t, J ¼ 6.4 Hz, 2H),
3.87 (t, J ¼ 6.4 Hz, 2H), 1.77 (m, 4H), 1.26 (m, 36H), 0.88 (m,
6H). 13C NMR (300 MHz, CDCl3): d ¼ 153.54, 149.73, 119.43,
114.65, 114.34, 112.74, 70.18, 68.79, 31.93, 29.66, 29.60, 29.37,
29.27, 26.00, 22.71, 14.14. EI-MS: M1d: 524.50 (37%), 356.20
(21%), 187.95 (87%), 110.10 (11%), 43.15 (100%). Mp: 49 1C.
Time-resolved correlated single photon measurements were
performed on a previously described system.18 The emission
was collected at magic angle polarization (55.41) relative to the
excitation light and the instrument had an response function
with a FWHM of B60 ps. Time-resolved emission and tran-
sient absorption data on the ms timescale were collected with a
frequency tripled Q-switched Nd:YAG laser from Quantel.
The output light was manipulated in an optical parametric
oscillator (OPO) delivering o10 ns flashes at 460 nm with an
energy B15 mJ. The concentration of the samples in the
emission experiments was controlled so the absorption at the
pump wavelength was held around 0.1.
The laser pulses generated for the transient absorption
pump–probe experiments have been previously described.19
The 800 nm output was converted to 485 nm in an optical
parametric amplifier (TOPAS). A mechanical chopper blocked
every second pump beam and the energy of the beam was set
below 2 mJ. The probe light was passed through a movable
delay line allowing for a delay between pump and probe up to
10 ns. A vertically moving CaF2 crystal in the probe beam
produced a continuous white light and a l/2-plate adjusted the
polarization of the light so that the difference in polarization
between pump and probe was fixed at magic angle conditions.
The pump and probe beam were then focused in a vertically
moving 1 ꢁ 10 mm cell. The transmitted probe light was
divided spatially on an optical diffraction grating and further
detected on a diode array. To adjust for differences in laser
intensity part of the probe light was passed to the detector
without passing the sample and the transient absorption was
calculated as:
2,5-Bis(dodecyloxy)-4-bromo-1-nitrobenzene 3. Compound 2
(4 g, 7.6 mmol) was dissolved in chloroform (12 mL) and acetic
acid (13 mL) then the solution was stirred at room temperature
for 10 min. The solution was cooled to 5 1C and a mixture of
acetic acid (3.6 mL) and HNO3 (1.6 mL, 34 mmol) was added
dropwise for a whole hour. Chloroform (8 mL) was further
added and the solution was stirred at room temperature for 18
h. Water (100 mL) was added and this solution was extracted
with dichloromethane. The organic layer was dried over
MgSO4 and evaporated to yield 3 as a yellow solid (4.2 g,
97%).
1H NMR (300 MHz, CDCl3): d ¼ 7.41 (s, 1H), 7.30 (s, 1H),
Dabs ¼ ꢀlog [(Iprobe,t¼t/Ireference,t¼t)/(Iprobe,t¼0/Ireference,t¼0)].
4.02 (m, 4H), 1.81 (m, 4H), 1.26 (m, 36H), 0.88 (m, 6H).
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 7 2 – 1 2 8 4
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13C NMR (300 MHz, CDCl3): d ¼ 153.56, 149.75, 119.45,
114.69, 114.38, 112.76, 70.22, 68.82, 31.93, 29.65, 29.37, 29.27,
26.00, 22.70, 14.14. EI-MS: M1d: 569.15 (3%), 400.95 (6%),
232.85 (100%). Mp: 63 1C.
2,5-Bis(dodecyloxy)-4-(20-trimethylsilylethynyl)iodobenzene 8.
Compound 6 (954 mg, 1.49 mmol) and CH3I (12.5 mL) were
introduced into a sealed tube. The mixture was heated at
120 1C for 18 h. Then the solvent was removed by rotary
evaporation. The crude product was purified by flash chroma-
tography (SiO2, petroleum ether–Et2O 95 : 5) and 88 mg of 8 as
a yellow solid were obtained (yield ¼ 88%).
2,5-Bis(dodecyloxy)-4-bromo-1-aminobenzene 4. Compound
3 (3.8 g, 6.6 mmol) was dissolved in THF (30 mL) and HCl (8
mL) was added. Tin powder (4 g, 34 mmol) was divided into 8
fractions that were slowly added. Then THF (20 mL) was
added and the solution was stirred at room temperature for
18 h. The mixture was diluted with water (40 mL) and
dichloromethane (80 mL) and K2CO3 were added to bring
the pH of the solution to neutrality. The solution was filtered,
the organic layer separated and rotary evaporated. The residue
was dissolved in dichloromethane and the solution was washed
with a saturated aqueous NaCl solution. The organic layer was
dried over MgSO4 and evaporated. Compound 4 was obtained
as a beige solid (3.3 g, 92%).
1H NMR (300 MHz, CDCl3): d ¼ 6.91 (s, 1H), 6.37 (s, 1H),
3.91 (t, 4H), 3.80 (s, 2H); 1.77 (m, 4H), 1.26 (m, 36H), 0.88 (m,
6H). 13C NMR (300 MHz, CDCl3): d ¼ 149.94, 141.34, 137.37,
116.61, 102.70, 98.88, 70.22, 69.28, 31.93, 29.62, 29.37, 26.00,
22.70, 14.14. EI-MS: M1d: 541.50 (68%), 461.50 (32%), 292.30
(9%), 124.10 (29%) 43.15 (100%). Mp: 74 1C.
1H NMR (300 MHz, CDCl3): d ¼ 7.25 (s, 1H), 6.83 (s, 1H),
3.93 (t, J ¼ 6.6 Hz, 4H), 1.78 (m, 4H), 1.26 (m, 36H), 0.88 (m,
6H), 0.25 (s, 9H). 13C NMR (300 MHz, CDCl3): d ¼ 154.89,
151.69, 123.79, 116.26, 113.39, 100.82, 99.44, 87.94, 70.09,
69.77, 31.98, 29.70, 29.42, 29.22, 26.06, 22.76, 14.19, 0.00. EI-
MS: M1d: 668.45 (41%), 543.50 (8%), 404.15 (40%), 331.95
(68%), 316.95 (72%), 235.95 (100%). Mp: 34 1C.
4-{[20-(200,500-Bis(dodecyloxy)-400-(20-trimethylsilylethynyl)phenyl)
]ethynyl}-2,5-bis(dodecyloxy)-(30,30-diethyltriazeno)benzene 9. In a
sealed tube, a solution of compound 8 (714 mg, 1.1 mmol) and
compound 7 (912 mg; 1.6 mmol) in Et3N (15 mL) and THF (10
mL) was degassed with argon. Then PdCl2 (10 mg, 0.053 mmol),
PPh3 (42 mg, 0.16 mmol) and Cu(OAc)2 (11 mg; 0.053 mmol)
were added. The solution was further degassed before closing the
sealed tube. The reaction was heated at 50 1C for 18 h. The
solvent was evaporated by rotary evaporation. The crude product
was purified by flash chromatography (Al2O3; petroleum ether–
Et3N 99 : 1 and then petroleum ether–dichloromethane–Et3N 69 :
30 : 1) to give 9 as a yellow oil (529 mg, 44%).
1H NMR (300 MHz, CDCl3): d ¼ 7.07 (s, 1H), 6.95 (s, 1H),
6.94 (s, 1H), 6.92 (s, 1H), 4.01 (m, 8H), 3.79 (q, J ¼ 7.2 Hz,
4H), 1.81 (m, 8H), 1.26 (m, 78H), 0.88 (m, 12H), 0.26 (s, 9H).
ES1-MS: m/z: calcd for C71H124N3O4Si 1110.9361; found
1110.9382 [M þ H]1.
2,5-Bis(dodecyloxy)-4-bromo-(30,30-diethyltriazeno)benzene 5.
Compound 4 (1.2 g, 2.2 mmol) was dissolved in THF (25 mL)
under nitrogen. The solution was degassed and cooled to ꢀ8 1C.
BF3 ꢂ Et2O (0.42 mL, 3.3 mmol) diluted with THF (0.6 mL) and
was injected. After 10 min, tBuNO2 (0.32 mL, 2.7 mmol) diluted
with THF (0.7 mL) was slowly added dropwise for 1 h. The
reaction was stirred at ꢀ8 1C for 3 h and then cooled to ꢀ15 1C.
A mixture of K2CO3 (2.46 g, 18 mmol), H2O (10 mL), Et2NH
(2.4 mL) and acetonitrile (10 mL) was added slowly. This
solution was stirred for 15 min and then the solvent was
removed by rotary evaporation. A saturated aqueous solution
of NaCl was added and this solution was extracted with Et2O.
The organic layer was dried over MgSO4 and evaporated.
Compound 5 was obtained as a red oil (1.32 g, 95%).
1H NMR (300 MHz, CDCl3): d ¼ 7.12 (s, 1H), 6.96 (s, 1H),
3.97 (m, 4H), 3.77 (q, 4H), 1.76 (m, 4H), 1.26 (m, 42H), 0.88
(m, 6H). 13C NMR (300 MHz, CDCl3): d ¼ 150.50, 147.36,
120.80, 111.78, 108.03, 103.73, 71.30, 69.98, 32.07, 29.79, 29.51,
26.17, 22.84, 14.27. EI-MS: M1d: 625.55 (18%), 545.55 (7%),
444.50 (100%), 276.30 (54%).
4-{[20-(200-500-Bis(dodecyloxy)-400-(20-trimethylsilylethynyl)phenyl)
]ethynyl}-2,5-bis(dodecyloxy)iodobenzene 10. Compound 9 (529
mg, 0.48 mmol) and CH3I (7.4 mL) were introduced into a sealed
tube. The mixture was heated at 120 1C for 18 h. Then the solvent
was removed by rotary evaporation. The crude product was
purified by flash chromatography (SiO2, petroleum ether–Et2O
95 : 5). Compound 10 was obtained as a yellow solid (500 mg,
91%).
1H NMR (300 MHz, CDCl3): d ¼ 7.29 (s, 1H), 6.94 (s, 1H),
6.93 (s, 1H), 6.89 (s, 1H), 3.96 (m, 8H), 1.81 (m, 8H), 1.26 (m,
72H), 0.88 (m, 12H), 0.26 (s, 9H). 13C NMR (300 MHz,
CDCl3): d ¼ 154.34, 154.18, 153.21, 151.84, 124.05, 117.90,
116.41, 116.07, 114.25, 113.96, 103.00, 96.48, 90.86, 87.49,
70.01, 69.82, 69.21, 31.95, 29.69, 29.39, 26.24, 26.12, 25.99,
22.72, 18.73, 14.15, 0.00. ES1-MS: m/z: calcd. for C67H113O4I-
NaSi 1159.7351; found 1159.7341 [M þ Na]1. Mp: 54 1C.
2,5-Bis(dodecyloxy)-4-(20-trimethylsilylethynyl)-(30,30-diethyl-
triazeno)benzene 6. A solution of compound 5 (1.5 g, 2.4 mmol)
in Et3N (20 mL) was degassed with argon in a sealed tube. Then
PdCl2 (42 mg; 0.24 mmol), PPh3 (94 mg, 0.36 mmol) and
Cu(OAc)2 (24 mg, 0.12 mmol) were added. The solution was
further degassed and trimethylsilylacetylene (0.86 mL, 6 mmol)
was finally added. The reaction mixture was heated at 100 1C for
18 h. The solution was filtered, the precipitate was washed with
ethyl acetate and the filtrate was rotary evaporated. The residue
was dissolved with ethyl acetate and was washed with a satu-
rated aqueous solution of NaCl, dried over MgSO4 and evapo-
rated. The crude product was purified by flash chromatography
(SiO2, petroleum ether–Et2O–Et3N 98 : 1 : 1) to give 6 as a
yellow oil (954 mg, 62%).
1H NMR (300 MHz, CDCl3): d ¼ 6.99 (s, 1H), 6.90 (s, 1H),
3.97 (m, 4H), 3.78 (q, 4H), 1.77 (m, 4H), 1.25 (m, 42H), 0.88
(m, 6H), 0.24 (s, 9H). 13C NMR (300 MHz, CDCl3): d ¼
155.73, 146.62, 142.77, 120.58, 109.31, 102.55, 102.23, 97.86,
70.94, 69.36, 32.05, 29.78, 29.61, 29.49, 29.19, 26.22, 22.82,
14.24, 11.74, 0.24. EI-MS: M1d: 641.65 (19%), 542.55 (10%),
373.25 (38%), 73.10 (100%).
4-{[20-(200,500-Bis(dodecyloxy)-400-(20-trimethylsilylethynyl)phenyl)
]ethynyl}benzaldehyde 12. A solution of compound 8 (300 mg, 0.45
mmol) and compound 11 (170 mg, 1.3 mmol) in 15 mL of dry
Et3N and 10 mL of dry THF in a sealed tube under argon was
degassed. Then PdCl2 (8 mg, 0.045 mmol), PPh3 (18 mg, 0.067
mmol) and Cu(OAc)2 (4.5 mg, 0.022 mmol) were added. The
solution was degassed before closing the sealed tube. The reaction
was heated at 50 1C for 18 h. Then the solvents were removed by
rotary evaporation. The crude product was purified by flash
chromatography (SiO2; petroleum ether–Et2O 98 : 2) and 220
mg of 12 as a yellow oil were obtained (yield ¼ 73%).
1H NMR (300 MHz, CDCl3): d ¼ 10.01 (s, 1H), 7.85 (d, J ¼
8.1 Hz, 2H), 7.65 (d, J ¼ 8.1 Hz, 2H), 6.97 (s, 1H), 6.95 (s, 1H),
4.01 (m, 4H), 1.81 (m, 4H), 1.26 (m, 36H), 0.88 (m, 6H); 0.27 (s,
9H). 13C NMR (300 MHz, CDCl3): d ¼ 191.40, 154.18, 153.80,
135.40, 132.05, 129.84, 129.61, 117.18, 116.91, 114.68, 113.27,
101.00, 100.72, 93.91, 90.22, 69.56, 31.98, 29.71, 29.42, 26.11,
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22.75, 14.18, 0.00. ES1-MS: m/z: calcd for C44H66O3NaSi
693.4679; found 693.4675 [M þ Na]1.
[4-{2-[20,50-Bis(dodecyloxy)-40-(200-(trimethylsilyl)ethynyl)ph-
enyl]ethynyl}-2,5-bis(dodecyloxy)phenyl]ethynylbenzene 25. A
solution of compound 10 (70 mg, 0.057 mmol) and phenyla-
cetylene 23 (0.019 mL, 0.17 mmol) in 1.6 mL of dry Et3N and
1.1 mL of dry THF in a sealed tube under argon was degassed.
Then PdCl2 (1 mg, 0.0057 mmol), PPh3 (2.2 mg, 0.0086 mmol)
and Cu(OAc)2 (0.05 mg, 0.0029 mmol) were added. The
solution was degassed before closing the sealed tube. The
reaction was heated at 50 1C for 18 h. Then the solvent was
evaporated by rotary evaporation. The crude product was
purified by flash chromatography (SiO2, petroleum ether–
dichloromethane 90 : 10) and 53 mg of 25 as a yellow solid
were obtained (yield ¼ 78%).
4-[(4-{[20-(200,500-Bis(dodecyloxy)-400-(20-trimethylsilylethynyl)phe-
nyl)]ethynyl}-2,5-bis(dodecyloxy)phenyl)ethynyl]benzaldehyde 13. A
solution of compound 10 (500 mg, 0.44 mmol) and compound 11
(172 mg, 1.3 mmol) in 15 mL of dry Et3N and 10 mL of dry THF
in a sealed tube under argon was degassed. Then PdCl2 (8 mg,
0.045 mmol), PPh3 (18 mg, 0.067 mmol) and Cu(OAc)2 (4.5 mg,
0.022 mmol) were added. The solution was degassed before closing
the sealed tube. The reaction was heated at 50 1C for 18 hours.
Then the solvent was removed by rotary evaporation. The crude
product was purified by flash chromatography (SiO2, petroleum
ether–Et2O 95 : 5) and 443 mg of 13 as a yellow solid were
obtained (yield ¼ 88%).
1H NMR (300 MHz, CDCl3): d ¼ 7.52 (m, 2H), 7.34 (m,
3H), 7.00 (s, 2H), 6.94 (s, 1H), 6.93 (s, 1H), 3.99 (m, 4H), 1.81
(m, 4H), 1.26 (m, 36H), 1.14 (s, 21H), 0.88 (m, 6H). 13C NMR
(300 MHz, CDCl3): d ¼ 154.70, 153.51, 154.49, 154.48, 131.62,
131.57, 129.30, 128,55, 123.48, 118.56, 117.45, 116.94, 115.26,
114.34, 113.73, 112.30, 101.22, 100.14, 99.18, 99.02, 94.88,
85.86, 69.15, 69.12, 31.63, 29.87, 29.58, 26.27, 22.66, 14.05,
0.00. ES1-MS: m/z: calcd for C81H131O4Si 1195.9817; found
1195.9812 [M þ H]1. Mp: 53 1C.
1H NMR (300 MHz, CDCl3): d ¼ 10.02 (s, 1H), 7.86 (d, J ¼
8.4 Hz, 2H), 7.66 (d, J ¼ 8.4 Hz, 2H), 7.01 (s, 2H), 6.96 (s, 1H),
6.94 (s, 1H), 4.01 (m, 8H), 1.85 (m, 8H), 1.24 (m, 72H), 0.88 (m,
12H), 0.26 (s, 9H). 13C NMR (300 MHz, CDCl3): d ¼ 191.91,
154.77, 154.51, 154.03, 153.97, 135.91, 132.57, 130.40, 130.13,
118.04, 117.76, 117.61, 117.52, 115.85, 114.98, 114.47, 113.56,
101.75, 100.76, 94.46, 91.88, 91.12, 90.93, 70.27, 70.06, 32.50,
30.25, 29.94, 26.67, 23.26, 14.68, 2.22. ES1-MS: m/z: calcd for
C76H118O5NaSi 1161.8646; found 1161.8624 [M þ Na]1. Mp:
52 1C.
General procedure for Prato reaction. A mixture of the
appropriate aldehyde (0.15 mmol), N-methylglycine (132 mg,
1.5 mmol) and fullerene (215 mg, 0.3 mmol) in dry toluene (250
mL) was stirred at reflux for 18 h under argon atmosphere. The
solvent was removed by rotary evaporation. The crude product
was purified by flash chromatography (SiO2, from petroleum
ether–toluene 7 : 3 to pure toluene) to give a brown solid.
2,5-Bis(dodecyloxy)-(20-trimethylsilylethynyl)benzene 21. Co-
mpound 2 (1.5 g, 2.9 mmol) was dissolved in 15 mL of dry
Et3N and 5 mL of dry THF in a sealed tube under argon. The
solution was degassed, then PdCl2 (50 mg, 0.29 mmol), PPh3
(112 mg, 0.43 mmol) and Cu(OAc)2 (29 mg, 0.14 mmol) were
added. The solution was degassed again and the trimethylsilyl-
acetylene (1.01 mL, 7.1 mmol) was added before closing the
sealed tube. The reaction was heated at 100 1C for 18 h. The
solution was filtered with ethyl acetate and evaporated. The
residue was dissolved in ethyl acetate and washed with a
saturated aqueous solution of NaCl, dried over MgSO4 and
evaporated. The crude product was purified by flash chroma-
tography (SiO2, petroleum ether–Et2O 99 : 1) and 1.23 g of 21
as a yellow solid were obtained (yield ¼ 78%).
Compound 15. The procedure described above was applied
using aldehyde 14 to give 15 (110 mg, 77%).
1H NMR (300 MHz, CDCl3): d ¼ 7.54 (d, J ¼ 8.4 Hz, 2H),
7.17 (d, J ¼ 8.4, 2H), 4.98 (d, J ¼ 9.6 Hz, 1H), 4.92 (s, 1H), 4.26
(d, J ¼ 9.6 Hz, 1H), 2.79 (s, 3H), 0.23 (s, 9H). ES1-MS: m/z:
calcd for C74H20NSi 950.1365; found 950.1354 [M þ H]1. Mp
4200 1C.
1H NMR (300 MHz, CDCl3): d ¼ 6.92 (d, J ¼ 2.7 Hz, 1H),
6.79 (d, J ¼ 2.7 Hz, 1H), 6.77 (s, 1H), 3.94 (t, J ¼ 6.5 Hz, 2H),
3.86 (t, J ¼ 6.5 Hz, 2H), 1.75 (m, 4H), 1.25 (m, 36H), 0.88 (m,
6H), 0.25 (s, 9H). 13C NMR (300 MHz, CDCl3): d ¼ 154.62,
152.62, 118.56, 116.94, 113.96, 113.29, 101.37, 98.15, 31.91,
29.63, 29.35, 26.01, 22.68, 14.12, 0.00. EI-MS: M1: 542.60
(69%), 374.35 (33%), 206.20 (76%), 191.20 (100%). Mp:
35 1C.
Compound 16. The procedure described above was applied
using aldehyde 12 to give 16 (131 mg, 62%).
1H NMR (300 MHz, CDCl3): d ¼ 7.79 (m, 2H), 7.59 (d, J ¼
8.1 Hz, 2H), 6.92 (s, 2H), 4.99 (d, J ¼ 9.3 Hz, 1H), 4.95 (s, 1H),
4.27 (d, J ¼ 9.3 Hz, 1H), 3.96 (m, 4H), 2.82 (s, 3H), 1.80 (m,
4H), 1.25 (m, 36H), 0.88 (m, 6H), 0.25 (s, 9H). ES1-MS: m/z:
calcd for C106H72NO2Si 1418.5332; found 1418.5344 [M þ
H]1. Mp 4200 1C.
2-[20,50-Bis(dodecyloxy)-40-(200-(trimethylsilyl)ethynyl)phenyl]
ethynylbenzene 24. A solution of compound 8 (90 mg, 0.13
mmol) and phenylacetylene 23 (0.044 mL, 0.4 mmol) in 3.8 mL
of dry Et3N and 2.6 mL of dry THF in a sealed tube under
argon was degassed. Then PdCl2 (2.3 mg, 0.013 mmol), PPh3
(5.3 mg, 0.02 mmol) and Cu(OAc)2 (1.3 mg, 0.0067 mmol) were
added. The solution was degassed before closing the sealed
tube. The reaction was heated at 50 1C for 18 h. Then the
solvent was evaporated by rotary evaporation. The crude
product was purified by flash chromatography (SiO2, petro-
leum ether–dichloromethane 90 : 10) and 63 mg of 24 as a
yellow oil were obtained (yield ¼ 73%).
Compound 17. The procedure described above was applied
using aldehyde 13 to give 17 (223 mg, 76%).
1H NMR (300 MHz, CDCl3): d ¼ 7.80 (m, 2H), 7.60 (d, J ¼
8.40 Hz, 2H), 6.98 (s, 1H), 6.97 (s, 1H), 6.95 (s, 1H), 6.93 (s,
1H), 4.99 (d, J ¼ 9.30 Hz, 1H), 4.95 (s, 1H), 4.27 (d, J ¼ 9.30
Hz, 1H), 3.97 (m, 8H), 2.82 (s, 3H), 1.81 (m, 8H), 1.26 (m,
72H), 0.88 (m, 12H), 0.26 (s, 9H). ES1-MS: m/z: calcd for
C138H124NO4Si 1886.9300; found 1886.9331 [M þ H]1. Mp
4200 1C.
Compound 29. (bpy)2RuCl2 ꢂ 2H2O (200 mg, 0.38 mmol) and
5-bromo-2,20-bipyridine 28 (100 mg, 0.42 mmol) were dis-
solved in a mixture of water (32 mL) and ethanol (16 mL).
The solution was stirred at reflux for 3 h under argon atmo-
sphere. The solvents were removed by rotary evaporation. The
product was dissolved in a minimum of methanol and was
precipitated by addition of a saturated aqueous solution of
KPF6 (25 mL). The orange solid 29 was collected by filtration,
washed with water and dried under vacuum (340 mg, 94%).
1H NMR (300 MHz, CDCl3): d ¼ 7.51 (m, 2H), 7.33 (m,
3H), 6.96 (s, 1H), 6.94 (s, 1H), 3.99 (m, 4H), 1.81 (m, 4H), 1.25
(m, 36H), 0.88 (m, 6H), 0.26 (s, 9H). 13C NMR (300 MHz,
CDCl3): d ¼ 154.22, 153.52, 131.59, 128.30, 123.48, 118.45,
117.37, 116.94, 114.34, 113.73, 101.20, 100.04, 94.88, 85.92,
69.65, 69.54, 31.94, 29.67, 29.38, 26.08, 22.71, 14.13, 0.04. ES1-
MS: m/z: calcd for C43H66O2NaSi 665.4730; found 665.4725
[M þ Na]1. Mp: 31 1C.
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1H NMR (300 MHz, CDCl3): d ¼ 8.50 (m, 5H), 8.39 (d, J ¼
8.7 Hz, 1H), 8.23 (dd, J ¼ 2.1 Hz, J ¼ 8.7 Hz, 1H), 8.06 (m,
5H), 7.80 (d, J ¼ 5.4 Hz, 1H), 7.75 (d, J ¼ 2.1Hz, 1H), 7.72 (m,
4H), 7.41 (m, 5H). Mp 4200 1C.
36H), 0.88 (m, 6H). EI-MS: M1: 470.45 (22%), 302.35 (24%),
134.20 (100%).
2-[20,50-Bis(dodecyloxy)-40-ethynylphenyl]ethynylbenzene 26.
The procedure described above was applied using 24 and 26
was obtained as a yellow oil (quantitative).
1H NMR (300 MHz, CDCl3): d ¼ 7.53 (m, 2H), 7.49 (m,
3H), 7.00 (s, 1H), 6.98 (s, 1H), 4.00 (m, 4H), 3.34 (s, 1H), 1.82
(m, 4H), 1.27 (m, 36H), 0.89 (m, 6H).
General procedure for the cleavage of the TMS protecting
group. The appropriate protected acetylenic derivative (1.4
mmol) was dissolved in dichloromethane (20 mL). K2CO3
(1.93 mg, 14 mmol) and methanol (40 mL) were added. The
solution was stirred at room temperature for 2 h under argon.
Water was added and the mixture was extracted with dichlo-
romethane. The organic layer was dried over MgSO4 and
evaporated to dryness leading to the free acetylenic derivative
sufficiently pure to be used in the next step as such.
4-{2-[20,50-Bis(dodecyloxy)-40-ethynylphenyl]ethynyl}-2,5-bis(do-
decyloxy)phenylethynylbenzene 27. The procedure described
above was applied using 25 and 27 was obtained as a yellow-
brown solid (quantitative).
1H NMR (300 MHz, CDCl3): d ¼ 7.52 (m, 2H), 7.34 (m,
3H), 7.01 (s, 1H), 7.00 (s, 1H), 6.98 (s, 1H), 6.97 (s, 1H), 3.99
(m, 4H), 3.34 (s, 1H), 1.81 (m, 4H), 1.26 (m, 36H), 0.88
(m, 6H).
2,5-Bis(dodecyloxy)-4-ethynyl-(30,30-diethyltriazeno)benzene 7.
The procedure described above was applied using 6. Compound
7 was obtained as a yellow oil (quantitative).
1H NMR (300 MHz, CDCl3): d ¼ 6.95 (s, 1H), 6.86 (s, 1H),
3.96 (t, J ¼ 6.6 Hz, 2H), 3.91 (t, J ¼ 6.6 Hz, 2H), 3.72 (q, J ¼
6.9 Hz, 4H), 3.23 (s, 1H), 1.76 (m, 4H), 1.26 (m, 42H), 0.88 (m,
6H). EI-MS: M1: 569.65 (26%), 470.55 (4%), 301.35 (25%),
133.15 (12%), 43.25 (100%).
General procedure for Sonogashira cross-coupling with me-
tallo-synthon 29. A solution of compound 29 (50 mg, 0.053
mmol) and the appropriate spacer (1.5 eq., 0.080 mmol) in
Et3N (0.6 mL) and DMF (4 mL) were introduced into a sealed
tube and degassed under argon. Then Pd(dppf)Cl2 (8 mg, 0.01
mmol) and CuI (2 mg, 0.01 mmol) were added. The solution
was further degassed before closing the sealed tube by pump–
freeze cycles. The reaction mixture was heated then water (50
mL) was added and the solution was extracted with dichlor-
omethane. The organic layer was then washed with water,
dried over MgSO4 and rotary evaporated. The crude product
was first purified by flash chromatography on silica gel and
then by size exclusion chromatography (Sephadex LH-20)
eluted with THF.
4-Ethynylbenzaldehyde 11. The procedure described above
was applied using 14. 11 was obtained as a white solid (93%).
1H NMR (300 MHz, CDCl3): d ¼ 10.02 (s, 1H), 7.85 (d, J ¼
8.1 Hz, 2H), 7.64 (d, J ¼ 8.1 Hz, 2H), 3.29 (s, 1H).
Compound 18. The procedure described above was applied
using 15 (100 mg, 0.1 mmol), but the reaction was conducted in
the following mixture of solvents dichloromethane–toluene–
methanol (4 mL : 2 mL : 7.5 mL) and was heated to reflux for
48 h. Compound 18 was obtained as a brown solid (107 mg,
98%).
1H NMR (300 MHz, CDCl3): d ¼ 7.77 (m, 2H), 7.56 (d, J ¼
8.1 Hz, 2H), 4.99 (d, J ¼ 9.6 Hz, 1H), 4.94 (s, 1H), 4.27 (d, J ¼
9.6 Hz, 1H), 3.09 (s, 1H), 2.80 (s, 3H).
Complex R1. The procedure described above was applied
using spacer 22 and heating at 30 1C for 3 h. Flash column
chromatography on SiO2 using a solvent gradient from pure
CH3CN to CH3CN–H2O–aqueous saturated KNO3 80 : 20 : 5
drops. R1 was obtained as a red solid (82%).
1H NMR (300 MHz, CD3OD): d ¼ 8.68 (m, 6H), 8.11 (m,
6H), 7.85 (m, 6H), 7.51 (m, 5H), 6.92 (s, 2H), 3.96 (t, J ¼ 6.6
Hz, 2H), 3.94 (t, J ¼ 6.6 Hz, 2H), 1.78 (m, 4H), 1.25 (m, 36H),
0.88 (m, 6H). ES1-MS: m/z: calcd for C62H76N6O2Ru
519.2537; found 519.2547 C21. Mp: 127 1C.
Compound 19. The procedure described above was applied
using 16 (110 mg, 0.077 mmol), but the reaction was conducted
in the following mixture of solvents dichloromethane–toluene–
methanol (4 mL : 2 mL : 8 mL) and was heated to reflux for
24 h. 19 was obtained as a brown solid (quantitative).
1H NMR (300 MHz, CDCl3): d ¼ 7.80 (m, 2H), 7.59 (d, J ¼
7.9 Hz, 2H), 6.96 (s, 2H), 4.99 (d, J ¼ 9.1 Hz, 1H), 4.95 (s, 1H),
4.28 (d, J ¼ 9.1 Hz, 1H), 3.97 (m, 4H), 3.33 (s, 1H), 2.82 (s,
3H), 1.81 (m, 4H), 1.25 (m, 36H), 0.88 (m, 6H).
Complex R2. The procedure described above was applied
using spacer 26 and heating at 45 1C for 18 h. Flash column
chromatography on SiO2 using a solvent gradient from pure
dichloromethane to dichloromethane–acetonitrile 80–20. R2
was obtained as a red solid (45%).
1H NMR (300 MHz, CDCl3): d ¼ 8.43 (m, 6H), 7.97 (m,6H),
7.70 (m, 6H), 7.48 (m, 7H), 7.33 (m, 3H), 6.97 (s, 1H), 6.96 (s,
1H), 3.94 (m, 4H), 1.74 (m, 4H), 1.25 (m, 36H), 0.88 (m, 6H).
ES1-MS: m/z: calcd for C70H80N6O2Ru 569.2693; found
569.2712 C11. Mp: 126 1C.
Compound 20. The procedure described above was applied
using 17 (110 mg, 0.058 mmol), but the reaction was conducted
in the following mixture of solvents dichloromethane–toluene–
methanol (4 mL : 2 mL : 3 mL) and was heated to reflux for
24 h. 20 was obtained as a brown solid (106 mg, 82%).
1H NMR (300 MHz, CDCl3): d ¼ 7.80 (m, 2H), 7.60 (d, J ¼
8.40 Hz, 2H), 6.99 (s, 1H), 6.98 (s, 1H), 6.97 (s, 1H), 6.96 (s,
1H), 5.00 (d, J ¼ 9.60 Hz, 1H), 4.95 (s, 1H), 4.27 (d, J ¼ 9.60
Hz, 1H), 3.96 (m, 8H), 3.33 (s, 1H), 2.82 (s, 3H), 1.80 (m, 8H),
1.25 (m, 72H), 0.88 (m, 12H).
Complex R3. The procedure described above was applied
using spacer 27 and heating at 45 1C for 18 h. Flash column
chromatography on SiO2 using a solvent gradient from pure
dichloromethane to dichloromethane–acetonitrile 80 : 20. R3
was obtained as a red solid (35%).
1H NMR (300 MHz, CDCl3): d ¼ 8.45 (m, 6H), 8.01 (m,
6H), 7.72 (m, 6H), 7.44 (m, 7H), 7.35 (m, 3H), 7.02 (s, 1H), 6.99
(s, 1H), 6.98 (s, 2H), 3.99 (m, 8H), 1.79 (m, 8H), 1.24 (m, 72H),
0.86 (m, 12H). ES1-MS: m/z: calcd. for C102H132N6O4Ru
803.4677; found 803.4682 C11. Mp: 120 1C.
2,5-Bis(dodecyloxy)ethynylbenzene 22. The procedure de-
scribed above was applied using 21 and 22 was obtained as a
yellow solid (quantitative).
1H NMR (300 MHz, CDCl3): d ¼ 6.97 (d, J¼2.7 Hz, 1H),
6.82 (d, J ¼ 2.7 Hz, 1H), 6.79 (s, 1H), 3.97 (t, J ¼ 6.6 Hz, 2H),
3.87 (t, J ¼ 6.6 Hz, 2H), 3.23 (s, 1H), 1.77 (m, 4H), 1.26 (m,
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Williamson reaction in 62% yield. Nitration of 2 carried out in
a mixture of acetic acid and chloroform afforded the nitro
derivative in quantitative yield. The nitro group was reduced
with tin in hydrochloric acid and the subsequent diazotization
with tert-butyl nitrite and boron trifluoride etherate gave the
triazene derivative 5 in almost quantitative yield.25 It is note-
worthy that triazene 5 was not purified at this stage, because
the crude product was particularly clean and we observed a
significant drop in the yield after column chromatography,
which was ascribed to the decomposition of triazene on the
acidic silica medium. In the next step, the bromo monomer 5
was coupled with trimethylsilylacetylene using the catalytic
mixture: PdCl2, P(Ph)3 and Cu(OAc)2. One part of the result-
ing coupled material 6 was reacted with methyl iodide to
transform the triazene into iodide 8 with a 65% yield after
purification. The other part was subjected to potassium carbo-
nate to cleave the trimethylsilyl-protecting group of acetylene
and gave 7. Molecules 7 and 8 were then coupled by Sonoga-
shira cross-coupling reaction with the same catalytic conditions
as for 6 and resulted in the formation of dimer 9 in 85% yield.
Heating 9 in methyl iodide afforded 10 in 80% yield. Finally,
iodoarenes 8 and 10 were coupled to the aldehyde 11 to form
the precursor for the Prato reaction (Scheme 3).
Dyad D1. The procedure described above was applied using
spacer 18 and heating at 90 1C for 18 h. Flash column
chromatography on SiO2 using a solvent gradient from pure
THF to THF–saturated KPF6 in methanol 90 : 10. D1 was
obtained as a red solid (24%).
1H NMR (300 MHz, CDCl3): d ¼ 8.46 (m, 6H), 8.05 (m,
6H), 7.63 (m, 6H), 7.45 (m, 5H), 7.30 (d, J ¼ 9 Hz, 2H), 6.82 (d,
J ¼ 9 Hz, 2H), 3.95 (m, 2H), 3.26 (s, 1H), 2.97 (s, 3H). ES1-
MS: m/z: calcd. for C101H33N7Ru 722.5920; found 722.5936
C11. Mp 4200 1C.
Dyad D2. The procedure described above was applied using
spacer 19 and heating at 90 1C for 18 h. Flash column
chromatography on SiO2 using a solvent gradient from pure
THF to THF–saturated KPF6 in methanol 90 : 10. D2 was
obtained as a red solid (30%).
1H NMR (300 MHz, CDCl3): d ¼ 8.39 (m, 6H), 7.99 (m,
6H), 7.68 (m, 8H), 7.58 (d, J ¼ 8.4 Hz, 2H), 7.48 (m, 5H), 6.97
(s, 1H), 6.94 (s, 1H), 4.99 (d, J ¼ 9.0 Hz, 1H), 4.95 (s, 1H), 4.27
(d, J ¼ 9.0 Hz, 1H), 3.94 (m, 4H), 2.82 (s, 3H), 1.85 (m, 4H),
1.25 (m, 36H), 0.88 (m, 6H). ES1-MS: m/z: calcd for
C
133H85N7O2Ru 956.7904; found 956.7923 C11. Mp 4200 1C.
The attachment of fullerene to the spacers 12–14 was
achieved through a 1,3-dipolar cycloaddition reaction of full-
erene with azomethine ylide, resulting from the in situ reaction
of aldehyde with N-methylglycine.26 The deprotection of the
trimethylacetylene with potassium carbonate was performed at
room temperature and afforded compounds 18–20 in almost
quantitative yield. It is important to mention here that the
utilization of a triisopropylsilyl group instead of trimethylsilyl
for the protection of the ethynyl group in compounds 15–17
did not allow us to obtain 18–20 by desilylation with tetra-
butylammonium fluoride. The treatment of the triisopropylsi-
lylacetylene derivatives of 15–17 with tetrabutylammonium
fluoride induced a total decomposition of the materials within
few minutes probably due to the nucleophilic attack of fluoride
anion on the pyrrolidino moiety. The spacers lacking the C60
unit, naming 22, 26 and 27, for the reference compounds, were
synthesized with a similar procedure, but using 23 instead of 11
(Scheme 4).
Dyad D3. The procedure described above was applied using
spacer 20 and heating at 30 1C for 18 h. Flash column
chromatography on SiO2 using a solvent gradient from pure
THF to THF–saturated KPF6 in methanol 90 : 10. D3 was
obtained as a red solid (32%).
1H NMR (400 MHz, CDCl3): d ¼ 8.47 (m, 6H), 8.04 (m,
6H), 7.79 (m, 8H), 7.61 (d, J ¼ 8.20 Hz, 2H), 7.53 (m, 5H), 6.99
(s, 1H), 6.81 (s, 1H), 6.57 (s, 1H), 5.52 (s, 1H), 5.03 (d, J ¼ 9.55
Hz, 1H), 5.00 (s, 1H), 4.27 (d, J ¼ 9.55 Hz, 1H), 3.97 (m, 8H),
2.83 (s, 3H), 1.87 (m, 8H), 1.25 (m, 72H), 0.88 (m, 12H).
MALDI-TOF-MS: m/z: calcd for C165H137N7O4Ru 2383.0;
found 2382.8. Mp 4200 1C.
Results
Synthesis of the compounds
In the study, the alkoxy substituents on the spacers have been
introduced only for solubility purposes. However, it is known
that electron donating substituents, such as alkoxy groups,
increase the p-electron conjugation and decrease the HOMO–
LUMO gap in oligo(phenylethynylene) systems.4 The strategy
adopted here for the synthesis of the dyads D1–D3 relies on a
convergent modular approach in which the three units, namely
the ruthenium trisbipyridine complex, the spacer and the full-
erene units, are joined to one another by successive cross-
coupling reactions. Recently, there have been considerable
efforts in the synthesis of monodisperse phenylethynylene
oligomers and several strategies were reported towards this
goal.11,20,21 We adopted a similar methodology to that de-
scribed by Tour and coworkers, which is a divergent/conver-
gent strategy based on the use of the diethyltriazene functional
group.22 The major advantage of the triazene group comes
from its high polarity which makes it possible to cleanly
separate by column chromatography the Glaser by-product,
which is systematically formed during this type of Sonogashira
cross-coupling reaction, from the desired Sonogashira pro-
duct.23 The Glaser by-product contains two triazene groups
against only one in the Sonogashira product which results in a
large difference in the mobility of these two compounds on
column chromatography. The other advantage of triazene is its
straightforward transformation into an iodo group in high
yield by simply heating with methyl iodide.24
The preparation of the ruthenium trisbipyridine synthon 29
was obtained by the classical method for preparing ruthenium
complexes, which consists in reacting the bromo bipyridine 28
with bisbipyridine dichloro ruthenium complex followed by
anion metathesis (Scheme 5).27
Cross-coupling Sonogashira reactions are less developed
with metallo-synthon substrates than with purely organic
halides. However, Tor and coworkers28 have shown that this
type of coupling proceeds in good yield using the catalytic
mixture Pd(dppf)Cl2 and CuI in DMF (dppf ¼ diphenylphos-
phinoferrocene). More recently Ziessel and coworkers10,29,30
reported direct coupling of acetylenic derivatives on polypyri-
dine ruthenium complexes, but using classical Sonogashira
conditions. These conditions applied to complex 29 and the
spacers 22, 26–27 or 18–20 led to the expected coupled
products with yields varying from 24 to 84%. It is noteworthy
that other conditions gave much lower yields for this transfor-
mation (Pd(PPh)3–CuI or Fu conditions31 gave respectively
50% and 0% yields for the formation of R1 from 22 and 29).
This shows the importance of the ligand diphenylphosphino-
ferrocene for the success of this coupling.
Electronic absorption spectra
The spectroscopic absorption data of the dyads D1–D3 and the
reference complexes R1–R3 are collected in Table 1 and the
spectra of the dyads are shown respectively in Fig. 1 and 2.
The spectra of the reference ruthenium complexes R1–R3
exhibit the absorption bands of the two chromophoric units,
The preparation of the spacer 10, required for this work, is
shown on Scheme 2. The commercially available bromohydro-
quinone 1 is etherified with n-bromododecane according to a
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Scheme 2 Synthesis of the spacer 10. Reagents and conditions: i, NaOH, 1-bromododecane in DMF at reflux (62%); ii, acetic acid, HNO3 in
chloroform at rt (97%); iii, tin powder, HCl in THF at rt (92%); iv, BF3 ꢂ Et2O, tBuNO2, in THF at ꢀ10 1C (65%); v, trimethylsilylacetylene, PdCl2,
PPh3, Cu(OAc)2 in Et3N at 100 1C (62%); vi, K2CO3, methanol, dichloromethane at rt (100%); vii, CH3I at 120 1C (88%).
Scheme 3 Synthetic route for the preparation of the spacers 18–20. Reagents and conditions: i, PdCl2, PPh3, Cu(OAc)2 in Et3N and THF at 50 1C
(73%); ii, N-methylglycine and fullerene in dry toluene at reflux (62 to 77%); iii, K2CO3, methanol, dichloromethane, toluene at reflux (82 to 100%).
Scheme 4 Synthetic route for the preparation of spacers 22, 26 and 27. Reagents and conditions: i, trimethylsilylacetylene, PdCl2, PPh3, Cu(OAc)2
in Et3N and THF at 100 1C (73 to 78%); ii, K2CO3, methanol, dichloromethane at rt (100%).
namely the ruthenium trisbipyridine complex and the oligo
(phenylethynylene) spacer. In the UV region, the band at
290 nm is a p–p* transition mostly localized on the bipyridine
ligands, whereas the band around 310 nm and which is growing
in intensity as the spacer length increases is most certainly
attributed to p–p* transitions of the oligo(phenylethynylene)
chromophore.
In the visible range, we can see at approximately 450 nm the
well-known MLCT transition of the ruthenium complex.32 In
the 350–430 nm region, there is a broad absorption which is
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Scheme 5 Preparation of the reference complexes R1–R3 and the dyads D1–D3. Reagents and conditions: i, Pd(dppf)Cl2, CuI in Et3N and DMF at
30, 45 or 90 1C (24 to 82%).
characteristic of the conjugated oligo(phenylethynylene)
spacer.21 In agreement with other studied examples of this
type of molecule, this band is shifted to longer wavelength with
a concomitant rise of the molar absorption coefficient as the
p-electron system expands.21,22 This transition is assigned as
p–p* transition which is essentially parallel to the long mole-
cular axis of the spacer. Although the p–p* transition of the
spacer overlaps significantly with the MLCT transition, this
latter band seems to be slightly bathochromically shifted upon
attachment of the spacer to the 5 position of the bipyridine
(Fig. 2). This indicates that the ground state electronic inter-
actions of the ruthenium complex with the spacer are not very
strong, but noticeable. Klemm and coworkers observed that
the conjugation length was longer in bipyridine substituted in
the 4,40 positions than those substituted at 5,50.33
fulleropyrrolidine derivatives can be clearly observed in the
spectra of dyads D1 at 430 nm, but this band is not discernible
in dyads D2–D3 because it is covered by the intense p–p*
transitions of the spacer.
Electrochemical study
The redox potentials of these novel dyads were studied by
cyclic voltammetry at room temperature in dichloromethane
and the data are collected in Table 2.
The measured redox potentials are in agreement with the
results published on molecules containing similar moieties.6,30
A common feature of these dyads is the quasi-reversibility of
all electrochemical processes. In the cathodic region, the first
and the second reduction took place on the fullerene at around
ꢀ0.6 V and ꢀ1.06 V. The third process is attributed to the
reduction of the bipyridine unit which is connected to the
spacer. This process is shifted to lower potential (Bꢀ1.12 V)
compared to a regular ruthenium trisbipyridine complex
(ꢀ1.32 V).35 This is most probably a consequence of the
stabilization of the LUMO orbitals of the bipyridine which is
induced by the p-electron delocalization with the conjugated
spacer. In the anodic region, the oxidation of the ruthenium
complex took place around 1.36 V and is not significantly
modified compared to the unsubstituted ruthenium trisbipyri-
dine complex.
In the dyads D1–D3, the supplementary chromophore is the
fullerene unit. Its spectroscopic signature can be distinguished
by the enhanced p–p* band around 310 nm in the UV region of
the spectra of the dyads D1–D3.34 The characteristic band of
Table 1 Absorption characteristic of the molecules recorded at room
temperature in DMF
Compounds
l
max/nm (e/Mꢀ1 ꢁ cmꢀ1
)
R1
R2
R3
D1
D2
D3
450 (11 100); 378 (17 300); 291 (63 800)
407 (35 400); 310 (52 600); 291 (75 400)
421 (52 400); 318 (55 400); 291 (81 300)
452 (7500); 423 (11 700); 286 (59 400)
405 (42 100); 318 (82 300); 289 (108 000)
429 (63 300); 312 (88 300); 287 (119 600)
Steady state and time resolved luminescence
The steady state emission spectra were recorded in deaerated
acetonitrile solution at room temperature and at 77 K. These
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 7 2 – 1 2 8 4
1279

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Table 2 Electrochemical data of the molecules recorded in dichloro-
methane with 0.15 M Bu4NPF6 as supporting electrolyte, potential in V
vs. SCE. Peak separation were in the range of 60 mV to 100 mV
Oxidation
Reduction
EOx(RuIII
RuII)/V
/
ERed(C60
C60ꢀ)/V
/
ERed(C60 /
ERed(RuII/
RuI)/V
ꢀ
C602ꢀ)/V
Compounds
Rubpy3
R1
R2
1.38
1.35
1.36
1.42
1.39
1.40
1.38
ꢀ1.32
ꢀ1.14
ꢀ1.13
ꢀ1.12
ꢀ1.17
ꢀ1.12
ꢀ1.12
R3
D1
ꢀ0.67
ꢀ0.66
ꢀ0.63
ꢀ1.06
ꢀ1.02
ꢀ1.01
D2
D3
the model we assumed, following Meyer and co-workers, that
one medium and one low frequency vibrational mode is
enough to simulate the experimental data. The wave number
for the medium frequency mode was obtained as ca. 1450
cmꢀ1, and can be attributed to C–C stretching modes in the
ligand. The results presented in Table 4 show that the E00
energy, the Huang–Rhys factors and the FWHM of the
individual gaussians decrease with increasing length of the
phenylethynylene bridge.
Fig. 1 Overlay of the absorption spectrum of ruthenium trisbipyri-
dine along those of the dyads R1–R3 recorded in DMF.
The room temperature emission maximum showed a pro-
nounced solvent dependence in R1. When the polarity of the
solvent changed, going from acetonitrile, via ethanol, to di-
chloromethane, the emission maxima changed by almost 20 nm
(Table 3). A similar dependence is seen for [Ru(bpy)3]21 and is
3
expected for the MLCT transition. This solvent effect is less
pronounced in R2 and even absent in R3. This indicates that the
nature of the excited state changes as the length of the
phenylethylene spacer increase, as will be discussed further
below. The idea of a different origin of the excited state was
further supported by time resolved emission measurements of
R1–R3. Thus for R1: t ¼ 1.2 ms, for R2: t¼1.4 ms and for R3:
t ¼2.4 ms. The lifetimes of R1 and R2 are very similar while
the lifetime of R3 is considerably longer.
The luminescence in dyads D1–D3 is almost completely
quenched at room temperature, showing only 4% intensity
relative to the corresponding reference complex R1–R3 (Table
3). The strong quenching of the excited state in D1–D3 is
confirmed by time-correlated single photon counting measure-
ments in acetonitrile showing a 1000 fold decrease in the
lifetime of the luminescence in dyads D1–D3 compared to the
reference complexes R1–R3 (Table 3). Notably the observed
lifetime is very similar in D1–D3 (t ¼ 0.6–0.8 ps). The time
resolved emission also showed minor components of B3 ns
and B100 ns contributing to most of the steady-state emission.
Fig. 2 Overlay of the absorption spectra of dyads D1–D3 recorded
in DMF.
measurements enabled us to determine both the energy level of
the excited state and the percentage of luminescence quenching
in dyads D1–D2 due to the presence of C60. The pertinent
spectroscopic data are collected in Table 3.
The data showed that the emission maximum of the refer-
ence complexes R1–R3 is shifted to longer wavelength as the
spacer length increases (Fig. 3). The bipyridine connected to
the spacer is therefore most certainly the ligand where the
excited state is localized and this assumption is effective both in
the dyad D1 as well as in the reference R1. To obtain more
details from the 77 K measurements a spectral fit to the data
was done using the relation below.36
Transient spectroscopy study
To follow the fast kinetics of the photo induced processes in
R1–R3 and D1–D3, femtosecond pump–probe experiments were
performed on all six molecules in acetonitrile. The molecules
were excited with B100 fs laser pulses at 485 nm, in the red
edge of the ruthenium MLCT-band. In R1–R3 the transient
absorption spectra were constant during the first 5 ns after
excitation, except for a t E 100 ps component, in R1 and R2,
responsible for a minor absorption rise in the blue part of the
spectra contributing to B10% of the total transient absorption
intensity. We tentatively attribute this spectral dynamics to
some kind of excited state relaxation.
ꢀ
ꢁ ꢀ
ꢁꢀ
ꢁ
3
M
L
X X
nꢀ00 ꢀ uMnꢀM ꢀ uLnꢀL
nꢀ00
SMu
SLu
IðnꢀÞ ¼
uM!
uL!
uM uL
(
!)
ꢀ
ꢁ
2
nꢀ ꢀ nꢀ00 þ uMnꢀM þ uLnꢀL
nꢀ1=2
ꢁ
exp ꢀ4 ln 2
In the expression n00 is the wavenumber of the zero–zero
electronic transition, nꢀM and nꢀL are the wave numbers of
vibrational transitions, uM and uL are vibrational quantum
numbers, nꢀ1/2 is the full width half maximum (FWHM) of the
gaussians and SM and SL are the Huang–Rhys factors relating
the nuclear distortion between the ground and excited state. In
For R1, a bleaching of the MLCT ground state band is
observed around 480 nm, somewhat overlapping with an
excited state absorption band around 440 nm. The latter can
be attributed to a LC transition on the substituted bpy that is
formally reduced in the MLCT state. In the unsubstituted
[Ru(bpy)3]21 the corresponding band appears around 360
1280
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 7 2 – 1 2 8 4

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Table 3 Spectroscopic characteristics of compounds studied
Compounds
lem/nm RT (CH3CN/EtOH/CH2Cl2)
lem/nm 77 K (C3H7CN)
tem/ns RT
fem (%)a
tabs/ns RT
R1
R2
R3
D1
D2
D3
a
666/656/648
669/660/658
669/667/669
610, 664
616, 674
637, 704
—
1200
1400
2400
100
100
100
4
0.13
0.11
b
—
—
—
0.8
0.18, 0.89
0.12, 0.57
0.17, 0.72
—
—
0.6
0.8
4
4
b
Determined from steady emission spectra and compared to the corresponding reference complex. The amplitude of a possible rise was too small
to detect.
nm.37 In the red part of the spectrum bands are typically
attributed to lower transitions of the formally reduced bpy
and MLCT from the unreduced ligands to the formally
oxidized ruthenium.38 In R2 the excited state absorption bands
around 440 and 560 nm are red-shifted to around 480 and 680
nm, consistent with an MLCT state delocalized over a larger
bridge. The apparent bleach maximum occurs below 420 nm,
which is below the ground state absorption maximum, pre-
sumably because of extensive overlap with the excited state
absorption. Finally, for R3 the apparent ground state bleaching
maximizes around 420 nm. The excited state absorption is
strong, very broad and relatively featureless, with a weak
maximum around 580 nm. This is qualitatively different from
R1 and R2.
component originates from excited state dynamics in a subset
of complexes with a lower reactivity, possibly an impurity,
since the spectral features of this species were very similar to
the initially excited spectra. The major component (t2) agreed
well with the emission lifetimes obtained from the time corre-
lated single photon counting experiments and the global fit
gave the following results: for D1 (t ¼ 0.9 ns), for D2 (t ¼ 0.6
ns) and for D3 (t ¼ 0.7 ns). This is attributed to energy transfer
to the C60 unit. The 150 ps component is instead attributed to
excited state dynamics, since this lifetime was observed also in
the reference complexes R1–R3. Indeed, a closer look at the
transient absorption spectra of D1 around 690 nm where the
3C60* absorption peaks clearly shows that the 0.8 ns compo-
nent gives an increase in the fullerene triplet absorption while
the shorter time constant does not (see ESIw).
For the complexes D1–D3 the initial spectra formed upon
excitation were identical to those of the references (Fig. 4).
During the first few nanoseconds these initial excited state
spectra changed. The resulting spectra after 4 ns were very
similar for the three dyads, with a broad absorption peak
around 690 nm (Fig. 4). At the same time the features of the
initial, excited state disappeared. The fulleropyrrolidine has a
moderate absorption peak around 1010 nm (e ¼ 8000 Mꢀ1
To follow the slow decay of the formed 3C60* we used a flash
3
photolysis setup. In all three complexes D1–D3 the C60*
spectra decayed to the ground state single exponentially with
a lifetime, t E 20 ms and lmax E 690 nm, which is in good
agreement with the reported lifetime and lmax of the full-
eropyrrolidine triplet reported by Armaroli40 (t ¼ 31 ms,
lmax ¼ 690 nm); see Fig. S5.w
cmꢀ1 34,39
)
and in a separate experiment we tried to detect the
radical by a 1000 nm probe. However, the transient
ꢀ
d
C60
Discussion
absorption data at this wavelength showed nothing but excited
state decay. From these observations we conclude that the new
species formed must be the triplet excited state of the fullerene
(3C60*) and the resulting spectrum at 4 ns is in good agreement
The synthetic molecular approach used to synthesize the dyads
D1–D3 relies on the utilization of the performed metallo-
synthon 29 that was used as a substrate for the Sonogashira
cross-coupling reaction. This approach differs from the general
strategy that consists in first preparing the bipyridine ligand
substituted by the spacer and subsequently performing the
coordination chemistry. The synthetic strategy described here
could be valuable if the substituent to graft on the bipyridine is
either a fragile or a strongly coordinating moiety. In these
cases, it can be either damaged or it can compete with the
bipyridine ligand during the coordination chemistry step.
According to the redox potentials determined by cyclic
voltammetry and the energies of the different excited states
calculated from the luminescence study; the strong quenching
of the MLCT excited state in dyads D1–D3 could be rationa-
lized by three possible mechanisms.
with literature spectra (e E 10 000 cmꢀ1
M
ꢀ1).34,39
The kinetics of the decay of the excited state and build up of
3C60* in D1–D3 were followed at 4–5 different wavelengths and
a global fit to a sum of exponentials was performed. The
analysis of the transient absorption data gave two lifetime
components of similar magnitude in all three complexes (t1 E
150 ps and t2 E 0.6–0.9 ns). For R1 and R3 a third, smaller
component with t3 E 5 ns was necessary. We believe that this
3Ru*–S–C60 - RuIII–S–C60ꢀ - Ru–S–C60
3Ru*–S–C60 - RuIII–S–C60ꢀ - Ru–S–3C60*
3Ru*–S–C60 - Ru–S–3C60*
(1)
(2)
(3)
Table 4 Spectral fit data from the 77 K emission experiments. In the
table the Huang–Rhys factors from the low frequency mode is omitted
but they follow the same trend
Compounds
E00 /cmꢀ1(eV)
vm/cmꢀ1
SM
v1/2/cmꢀ1
R1
R2
R3
16 470 (2.04)
16 190 (2.01)
15 690 (1.95)
1484
1466
1475
0.88
0.76
0.66
800
750
560
Fig. 3 Overlay of the emission spectrum of the reference complexes R1
(solid line), R2 (dashed line) and R3 (dotted line) recorded at 77 K in
butyronitrile.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 7 2 – 1 2 8 4
1281

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ꢀDG0CS ¼ E0–0(3C60*) þ DG0CR
ꢀDG0CS ¼ E0–0(3Ru*) þ DGC0 R
ꢀDG0CR ¼ E0Ru ꢀ E0C þ DG0S
60
DG0S ¼ (e2/8pe0)[(1/eAN ꢀ 1/eDCM)((9–4)/rRu þ 1/rC )]
60
E0–0(3Ru*), E0ꢀ0(3C60*), E0Ru, EC0 , rRu, rC refer, respectively
60
to the energy of the MLCT triplet excited 6s0tate, the energy of
the fullerene triplet excited state (1.45 eV), the Ru31/21 poten-
tial, the C600/ꢀ potential, the radius of ruthenium trisbipyridine
(7.0 A) and the radius of the fullerene (4.4 A).
A similar mechanism to 2 was indeed reported by Armaroli
and coworkers to explain the formation of the triplet excited
state of fullerene in ruthenium and rhenium bipyridine com-
plexes connected to a fullerene unit.7 The third deactivation
mechanism is a direct energy transfer leading to the fullerene
triplet excited state but without passing through the intermedi-
ate charge separated state (eqn (3)). It is important to note that
in these dyads, the photoinduced energy transfer process is
much more exergonic (DGEnT ¼ ꢀ0.56 eV for dyad D2 for
example) than the photoinduced charge separation (DGCS
¼
ꢀ0.25 eV as calculated from the Rehm–Weller equation in-
dicated above for dyad D2 for example).
DGE0 nT ¼ E0–0(3Ru*), ꢀ E0–0(3C60*)
The transient absorption spectra recorded in acetonitrile did
not feature at any time any signal around 1000 nm, which is the
well-known specific region diagnostic for the fullerene radical
anion fingerprint. The formation of the charge-separated state
is therefore ruled out, at least as a long-lived intermediate state.
In the light of these observations, we conclude that the forma-
tion of an intermediate charge separated state in dyads D1–D3
is unlikely, which suggests that mechanisms 1 and 2 are not
important in the deactivation of the dyads. Instead energy
transfer to the C60 unit quenches the MLCT excited state.
For the energy transfer in D1–D3 a Forster type dipole–
¨
dipole mechanism is excluded due to the negligible spectral
overlap between the C60-absorption and the ruthenium-spacer
emission.42 The rate of emission quenching in dyads D1–D3 is
almost independent of the distance between the ruthenium
complex and the fullerene unit (Table 3) and this excludes also
a Dexter type energy transfer via a simple superexchange type
mechanism, since this rate should fall off exponentially with
distance as the orbital overlap decreases. The participation of
the excited state of the spacer itself in the energy transfer is
therefore likely, and the bridge-localized excited state is indeed
expected to be close in energy to the 3MLCT state. The role of
the spacer could be either to substantially delocalize the
ruthenium 3MLCT excited state out on the phenylethylene
bridge forming a ‘‘super-donor state’’,9,43 thus decreasing the
effective energy transfer distance. An alternative mechanism is
that a triplet state of the spacer itself becomes populated. In
other words, the energy is first transferred to the spacer before
arriving at the fullerene (eqn (4)).
Fig. 4 Transient absorption spectra of D1 (a), D2 (b) and D3 (c)
recorded at different times after excitation with a 485 nm laser pulse; 10
ps (solid line), 200 ps (dashed line), 800 ps (dashed-dotted line) and 5 ns
(dotted line). The experiments were done in 298 K acetonitrile.
3Ru*–S–C60 - Ru–3S*–C60 - Ru–S–3C60*
(4)
The first is a photoinduced electron transfer that would lead to
the intermediate charge separated state composed of the
oxidized ruthenium and the reduced fullerene and that would
finally recombine to the ground state (eqn (1)). In the second
mechanism, the charge separated state can decay to the triplet
spin-state of fullerene, since the driving force for this process is
substantial (DG0CS ¼ ꢀ0.31 eV for dyad D2 for example) (eqn
(2)). The free energy change for charge separation, DG0CS and
Assessment of the energy of the triplet excited state of phenyl
ethynylene derivatives has been reported by Schanze and
coworkers13,44 and by Kohler and coworkers.45 From these
¨
studies, we estimate that the energy of the spacer triplet excited
state is certainly in the range of 1.85–2.1 eV in dyads D2 and D3
with the lowest value for D3. This means that the triplet p - p*
state of the spacer probably becomes populated in D3 and that
the corresponding state is thermally accessible from the
3MLCT excited state of the ruthenium complex in D2 and
possibly also D1.
The solvent independence of the R3 emission clearly sup-
ports the idea of a stepwise energy transfer since the p–p*
emission from the aromatic spacer should be fairly indepen-
3
DG0CS, from the 3Ru* and C60* respectively, and charge
recombination DG0CR can be estimated from the Rehm–Weller
equation 41 and based on the electrochemical and luminescence
data. DG0S takes into account the solvation energy due to the
solvent change between electrochemistry (CH2Cl2, eR 9.0) and
photochemistry (CH3CN, eS ¼ 38.8). The Coulombic term was
omitted because of the large distance between the charges.
1282
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 7 2 – 1 2 8 4

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dent of the solvent polarity. It strongly suggests that the lowest
excited state of the system is that localized on the spacer. On
the other hand, the solvent dependence in R1 is explained by
the charge transfer character of the ruthenium 3MLCT excited
state, which should be stabilized by more polar solvents. Also
the relatively long emission lifetime in R3 and the qualitatively
different transient absorption spectrum favor a spacer based
excited state. Possibly the relatively fast rate of the forbidden
deactivation in R3, and the significant emission, is due to spin–
orbit coupling with the heavy ruthenium center.
Further support for the p–p* character of the lowest excited
state of R3 was obtained from the 77 K emission spectra (Fig.
3). The trend of the excited state energy for R1–R3 obtained
from the spectral fit to the steady-state emission data shows a
substantial decrease when going form R2 to R3: E00 ¼ 2.04,
2.01 and 1.95 eV in the series R1–R3. The same trend is seen for
Griep-Raming, D. Haase and W. Saak, Angew. Chem., Int. Ed.,
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N. Martin, L. Sanchez, B. Illescas and I. Perez, Chem. Rev., 1998,
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N. Armaroli, G. Accorsi, D. Felder and J. -F. Nierengarten,
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(a) D. Armspach, E. C. Constable, F. Diederich, C. E. Housecroft
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Meijer, G. P. M. van Klink and G. van Koten, Coord. Chem. Rev.,
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Scorrano, P. Ceroni, M. Marcaccio, F. Paolucci and S. Roffia,
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Mondini, G. Scorrano, F. Paolucci, P. Ceroni and S. Roffia,
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6
7
8
9
N. P. Redmore, I. V. Rubtsov and M. J. Therien, J. Am. Chem.
Soc., 2003, 125, 8769.
10 C. Goze, D. V. Kozlov, F. N. Castellano, J. Suffert and R. Ziessel,
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11 (a) A. Khatyr and R. Ziessel, J. Org. Chem., 2000, 65, 3126; (b) J.
M. Tour, Chem. Rev., 1996, 96, 537; (c) J. M. Tour, Acc. Chem.
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Diederich, Angew. Chem., Int. Ed., 1999, 38, 1351; (c) Y. Liu, S.
Jiang and K. S. Schanze, Chem. Commun., 2003, 650; (d) D.
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the Huang–Rhys factors of the medium frequency mode: SM
¼
0.88, 0.76, and 0.66 in the same series. The Huang–Rhys factor
and the band-width (n1/2) for R3 is extremely low if it were to
originate from a ruthenium-to-bipyridine 3MLCT state and
reflects a small nuclear displacement when going from the
ground to the excited state. This is most probable if the excited
state of R3 is actually the p - p* state of the phenylethylene
spacer. Note that the expression for the spectral composition
used is based on a general theory for radiative decay and its use
is not restricted to MLCT emission. As the medium frequency
modes are dominating, these are likely to be similar in fre-
quency for the pyridine and phenyl breathing modes.
15 I. P. Evans, A. Spencer and G. Wilkinson, J. Chem. Soc., Dalton
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In conclusion the p–p* state of the spacer is the lowest
excited state of the R3 chromophore. The initial transient
absorption spectrum for D3 is identical to that for the reference
complex R3. This shows that the excited ruthenium MLCT
state rapidly forms the p–p* state of the spacer, which subse-
quently transfers energy to the C60 unit. In D1 and D2 the
corresponding p–p* state is higher in energy but most likely
accessible by thermal population so that energy transfer to the
C60 unit may be facilitated by a hopping mechanism. Our
experiments suggest an interesting alternative for long-range
energy transfer where the excited state is efficiently transferred
via near-isoenergetic bridge states. The intermediate popula-
tion of the bridge states, which decrease in energy as the spacer
length increases, may rationalize the surprisingly constant
energy transfer rates over edge-to-edge distances between 1.1
and 2.3 nm for D1–D3.
16 T. Hayashi, M. Konishi, Y. Kobori, M. Kumada, T. Higuchi and
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Acknowledgements
The authors acknowledge financial support from the French
Ministry of Research with the ACI ‘‘jeunes chercheurs’’ 4057
(salary of FC), the Swedish Foundation for Strategic Research,
The Wallenberg Foundation, The Royal Swedish Academy of
Sciences, The Swedish Research Council and The Swedish
Energy Agency.
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and R. Ziessel, Inorg. Chem., 2004, 43, 7359.
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