Energy transfer from rhenium(i) complexes to covalently attached anthracenes and phenanthrenes

Article abstract of DOI:10.1039/b809494b

The synthesis and photophysical properties of a series of chromophore-quencher complexes are reported. They are all comprised of a luminescent rhenium(i) tricarbonyl diimine complex that is covalently attached to anthracene or phenanthrene moieties via rigid rod-like p-xylene bridges of variable lengths. Rhenium-to-anthracene energy transfer is strongly exergonic (-ΔG⁰≈ 0.9 eV) and causes very efficient rhenium MLCT luminescence quenching. By contrast, rhenium-to-phenanthrene energy transfer is only observed when complexes with sufficiently high MLCT energies are used because for these dyads, the driving force for energy transfer is low (-ΔG⁰≈ 0.1 eV). For a ～15 donor-acceptor distance, the rate constants of the weakly and the strongly exergonic energy transfer processes differ by more than 3 orders of magnitude. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b809494b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Energy transfer from rhenium(I) complexes to covalently attached anthracenes
and phenanthrenes
Mathieu E. Walther and Oliver S. Wenger*
Received 4th June 2008, Accepted 5th August 2008
First published as an Advance Article on the web 7th October 2008
DOI: 10.1039/b809494b
The synthesis and photophysical properties of a series of chromophore–quencher complexes are
reported. They are all comprised of a luminescent rhenium(I) tricarbonyl diimine complex that is
covalently attached to anthracene or phenanthrene moieties via rigid rod-like p-xylene bridges of
variable lengths. Rhenium-to-anthracene energy transfer is strongly exergonic (-DG0 ª 0.9 eV) and
causes very efficient rhenium MLCT luminescence quenching. By contrast, rhenium-to-phenanthrene
energy transfer is only observed when complexes with sufficiently high MLCT energies are used because
0
˚
for these dyads, the driving force for energy transfer is low (-DG ª 0.1 eV). For a ~15 A
donor–acceptor distance, the rate constants of the weakly and the strongly exergonic energy transfer
processes differ by more than 3 orders of magnitude.
differences in donor–bridge energy gaps,16 particularly in light of
Introduction
other recent theoretical and experimental studies.17 A systematic
Energy transfer is a key process in photosynthesis and has there-
investigation of this important issue asks for donor–bridge–
fore received much attention over the past few decades.1 A wealth
acceptor systems, in which (i) energy transfer is competitive with
of theoretical and experimental studies has provided remarkably
other excited-state deactivation processes, (ii) the donor–bridge
detailed insight into the subject of energy transfer.2 In inorganic
energy gap is independent upon the length of the bridge, and
photochemistry, ruthenium(II) tris-bipyridine (Ru(bpy)32+) type
complexes have been used frequently to investigate triplet–triplet
energy transfer.3 In a particularly insightful study, it was shown
(iii) donor–bridge energy gap effects can be isolated from driving
force (i.e. donor–acceptor energy gap) effects. The goal of the
present study was to identify simple molecular systems that fulfil
these requirements. Toward this end, we have synthesized and
investigated the six dyads shown in Scheme 1. They are comprised
of a rhenium(I) tricarbonyl diimine energy donor, p-xylene bridges,
and a phenanthrene or anthracene energy acceptor.
2+
2+
that energy transfer from a Ru(bpy)3 donor to an Os(bpy)3
acceptor through covalent oligo-p-phenylene spacers takes place
via a Dexter mechanism,4 thereby supporting prior studies
that have noted similarities between Dexter-type energy transfer
and super-exchange electron transfer.5 Rhenium(I) tricarbonyl
diimines have also received significant attention regarding both
electron and energy transfer processes.6,7 Investigations of such
rhenium complexes are among the few experimental studies that
provide direct evidence for the so-called inverted driving force
effect for energy transfer,8 a phenomenon that is now rather well
documented for electron transfer.9 These complexes posses the
following favourable properties: (i) their synthesis is facile,10 (ii)
they have relatively high lying MLCT excited states (>2.5 eV) that
emit light,10–12 (iii) the energies of these emissive states are tuneable
through ligand variations,11–13 and (iv) these MLCT states have
lifetimes of the order of 0.5–10 ms,12 thereby making long distance
energy (and electron) transfer investigations possible.
Despite the enormous advances made in energy transfer
research in recent years, there remain open questions, partic-
ularly regarding the factors that control energy transfer rates
˚
over long distances (>10 A). Recent independently performed
studies report on remarkably different distance dependencies for
energy transfer through covalent oligo-p-phenylene–ethynylene
bridges.14,15 It appears plausible that these discrepancies are due to
Department of Inorganic, Analytical and Applied Chemistry, University
of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva, Switzerland.
E-mail: oliver.wenger@unige.ch
Scheme 1
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refluxed over night. In some cases, refluxing had to be continued
for up to 3 d. Work-up included extraction with dichloromethane,
drying (MgSO4), and evaporation of the organic solvent phase.
The crude products were purified by column chromatography.
Experimental
General information
Unless stated otherwise, all chemicals are commercial reagents
and were used as supplied. All reactions and manipulations were
carried out under an inert nitrogen atmosphere using standard
(Schlenk) techniques. Solvents for synthesis were dried using
drying columns where appropriate. Column chromatography, for
product purification, was performed using silica gel from the
Procedure for the Stille-type C–C coupling
1 g of iodoarene and 1.2 equivalents of 5-(tri-n-butyltin)-2,2¢-
bipyridine (19) were dissolved or suspended together in 12 mL
m-xylene. After deoxygenating by bubbling with nitrogen gas for
15 min, 20 mol% of Pd(Ph3)4 was added, and the reaction mixture
was refluxed for 5 d. The solvent was subsequently evaporated
under reduced pressure, and the residue was subjected directly to
column chromatography. Yields were typically of the order of 20%;
the use of other solvents (THF, toluene) or catalysts (Pd(Ph3)2Cl2)
led to even poorer results.
1
Fluka chemical company (product no. 60745). H NMR spectra
were measured on a Bruker Avance 400 MHz spectrometer. All
chemical shifts were determined from their relative displacements
to partially deuterated solvent peaks and are reported relative
to the tetramethylsilane signal. High-resolution mass spectra
were measured on a QSTAR XL (AB/MDS Sciex) spectrometer
whereby the samples were dissolved in methanol or CH2Cl2.
Optical absorption spectra were measured on a Cary 5000
spectrophotometer from Varian, and steady-state luminescence
spectra were acquired using a Horiba Fluorolog-3 instrument.
Time-resolved luminescence and transient absorption experiments
were performed on the setup of Prof. A. Hauser, Department of
Physical Chemistry, University of Geneva. This setup has been
described recently in ref. 18. For all the optical spectroscopic
measurements, dichloromethane of spectrophotometric grade
(Sigma-Aldrich product no. 154792) was used. For the time-
resolved studies, samples were deoxygenated by three subsequent
freeze–pump–thaw cycles using appropriate home-built quartz
cuvettes.
General procedure for rhenium complexation
For dyads 1–4 the method described in ref. 10 was followed
(with the slight modification that silver triflate, rather than silver
perchlorate was used). For dyads 5 and 6, 300 mg of free
ligand 20 or 23 were reacted with 1 equivalent of Re(CO)5Cl in
10 mL toluene at reflux for 5 h.10,20 Solvent evaporation yielded
the chloro complexes in almost quantitative yields. These were
subsequently reacted with a 5-fold excess of pyridine in toluene
in the presence of silver triflate to yield the target complexes. The
triflate salts of all the final rhenium complexes were purified by
column chromatography whereby a dichloromethane–methanol
(98 : 2) eluent mixture was used. Complexation yields ranged from
25–85%.
General synthetic procedure for lithiation reactions
Product characterization data
The following procedure was applied for all borylation, stan-
nylation, and trimethylsilylation reactions.19 Typically, 1 g of
bromoarene was dissolved in 25 mL anhydrous tetrahydrofuran
(THF) and cooled to -78 ◦C in a dry ice–acetone bath. After
30 min, 1.1 equivalents of n-butyllithium solution (1.6 M in
hexanes) were added dropwise via a syringe. After stirring for 60–
120 min at -78 ◦C, 1.1 equivalents of the appropriate electrophile
were added dropwise: triisopropylborate for borylation, tri-n-
butyltin chloride for stannylation, and trimethylsilyl chloride for
trimethylsilylation. Then, the solutions/suspensions were allowed
to warm to room temperature over night. Work-ups usually
included addition of water to the THF solution (in the case of
borylation a 2 M aqueous hydrochloric acid solution to hydrolyze
the boronic esters), followed by extraction of the crude products
with several portions of dichloromethane. The crude products
were purified by column chromatography (or in the case of the
boronic acids by recrystallization). For the synthesis of 5-(tri-n-
butyltin)-2,2¢-bipyridine (19), the use of diethylether rather than
tetrahydrofuran turned out to be indispensable.
1
All H NMR data were recorded on the 400 MHz spectrometer
indicated above. The solvent was always CDCl3, and all chemical
shifts are given relative to the tetramethylsilane signal. Coupling
constants, J are reported in Hz. The following abbreviations
were used to designate the protons: An = anthracene, bpy =
2,2¢-bipyridine, Me = methyl, Pe = phenanthrene, phen = 1,10-
phenanthroline, py = pyridine, TMS = trimethylsilyl, xy = xylene.
Pe–xy–TMS 9. Obtained in 95% yield using the general
Suzuki coupling methodology. Purified on silica using pentane
as an eluent. dH/ppm: 0.47 (9H, s, TMS), 2.10 (3H, s, Me), 2.54
(3H, s, Me), 7.19 (1H, s, xy), 7.49 (1H, s, xy), 7.55 (2H, m, Pe),
7.62 (1H, d, Pe, J = 8.2 Hz), 7.67 (1H, s, Pe), 7.70 (2H, m, Pe),
7.92 (1H, d, Pe, J = 7.7 Hz), 8.79 (1H, d, Pe, J = 8.3 Hz), 8.82
(1H, d, Pe, J = 8.3 Hz).
Pe–xy–I 10. Obtained in quantitative yield from 9 using a
TMS–halogen exchange method.25 dH/ppm: 2.01 (3H, s, Me), 2.48
(3H, s, Me), 7.22 (1H, s, xy), 7.53 (2H, m, Pe), 7.60 (1H, s, Pe),
7.68 (3H, m, Pe), 7.85 (1H, s, xy), 7.90 (1H, d, Pe, J = 8.0 Hz),
8.76 (1H, d, Pe, J = 8.3 Hz), 8.79 (1H, d, Pe, J = 8.5 Hz).
General synthetic procedure for Suzuki-type C–C couplings
In a typical procedure, 1 g of bromo- or iodoarene was dissolved in
10 mL toluene or THF along with 1.1 equivalents of boronic acid.
After the addition of 10 mL of 2 M aqueous Na2CO3 solution,
the biphasic mixture was deoxygenated by bubbling nitrogen gas
through it for 15 min. Between 1 and 10 mol% of Pd(Ph3)4 (Ph =
phenyl) catalyst were then added, and the reaction mixture was
Pe–xy–py 12. Synthesized from 10 and 11 following the
general Suzuki coupling method. Purified on silica using a
dichloromethane–methanol eluent mixture. The isolated yield was
20%. dH/ppm 2.10 (3H, s, Me), 2.34 (3H, s, Me), 7.24 (1H, s, xy),
7.31 (1H, s, xy), 7.57 (2H, br, Pe), 7.57 (1H, dd, py, J = 1.6, 6.0 Hz),
7.63 (1H, dd, Pe, J = 12.0, 1.2 Hz), 7.68 (1H, s, Pe), 7.68 (1H, dd,
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Pe, J = 7.6, 1.2 Hz), 7.72 (1H, dd, py, J = 8.2, 1.6 Hz), 7.73 (1H,
dd, Pe, J = 12.0, 1.6 Hz), 7.93 (1H, dd, Pe, J = 7.6, 1.2 Hz), 8.69
(1H, dd, py, J = 5.0, 1.6 Hz), 8.78 (1H, d, Pe, J = 4.8 Hz), 8.79
(1H, d, Pe, J = 4.8 Hz), 8.82 (1H, d, py, J = 8.2 Hz).
[Re(phen)(CO)3(Pe–xy2–py)]+ 2. Synthesized from Re(phen)-
(CO)3Cl and free ligand 15 following the general rhenium com-
plexation method in 85% yield. dH/ppm: 2.01 (3H, m, Me), 2.07
(3H, s, Me), 2.15 (3H, s, Me), 2.19 (3H, m, Me), 6.83 (1H, m, xy),
7.09 (1H, d, xy, J = 5.6 Hz), 7.18 (1H, m, xy), 7.26 (1H, d, xy, J =
5.6 Hz), 7.50 (1H, dd, Pe, J = 7.6, 5.6 Hz), 7.59 (1H, dd, py, J =
7.2, 1.6 Hz), 7.63 (1H, m, Pe), 7.71 (3H, m, Pe), 7.71 (1H, s, Pe),
7.79 (1H, m, py, J = 7.2, 5.6 Hz), 7.93 (1H, dd, Pe, J = 7.6, 0.8 Hz),
8.10 (1H, d, py, J = 1.6 Hz), 8.22 (1H, d, phen, J = 5.2 Hz), 8.24
(1H, d, phen, J = 5.2 Hz), 8.31 (2H, s, phen), 8.46 (1H, s, py), 8.79
(1H, d, Pe, J = 8.6 Hz), 8.82 (1H, d, Pe, J = 8.6 Hz), 8.84 (2H, d,
phen, J = 8.2 Hz), 9.61 (2H, d, phen, J = 4.8 Hz). ESI-MS: m/z
observed 914.2390 (M+), expected 914.2387.
[Re(phen)(CO)3(Pe–xy–py)]+ 1. Synthesized in 25% yield from
Re(phen)(CO)3Cl and free ligand 12 following the general rhenium
complexation method. dH/ppm: 2.00 (3H, s, xy), 2.05 (3H, s, xy),
6.88 (1H, s, xy), 7.25 (1H, s, xy), 7.54 (1H, dd, Pe, J = 6.4, 1.6 Hz),
7.55 (1H, s, xy), 7.57 (2H, dd, Pe, J = 7.6, 1.6 Hz), 7.63 (1H, s, py),
7.68 (1H, dd, Pe, J = 7.6, 1.2), 7.72 (2H, dd, Pe, J = 8.2, 1.6 Hz),
7.82 (1H, d, py, J = 8.2 Hz), 7.91 (1H, dd, Pe, J = 7.6, 1.6 Hz),
8.13 (1H, dd, py, J = 5.6, 0.8 Hz), 8.23 (2H, m, phen), 8.29 (2H, s,
phen), 8.49 (1H, d, py, J = 1.9 Hz), 8.78 (1H, d, Pe, J = 8.2 Hz),
8.81 (1H, d, Pe, J = 8.2 Hz), 8.93 (2H, d, phen, J = 8.2 Hz), 9.62
(2H, d, phen, J = 5.2 Hz). ESI-MS: m/z observed 810.1751 (M+),
expected 810.1761.
[Re(Me4phen)(CO)3(Pe–xy2–py)]+
4. Synthesized
from
Re(Me4phen)(CO)3Cl and free ligand 15 following the general
rhenium complexation method in 25% yield. dH/ppm: 2.08 (3H, s,
Me–xy), 2.11 (3H, s, Me–xy), 2.18 (3H, d, Me–xy, J = 13.4 Hz),
2.21 (3H, d, Me–xy, J = 13.4 Hz), 2.76 (6H, s, Me–phen), 2.94
(6H, s, Me–phen), 6.90 (1H, m, xy), 7.12 (1H, d, xy, J = 5.6 Hz),
7.21 (1H, d, xy), 7.27 (1H, d, xy, J = 5.6 Hz), 7.56 (1H, dd, Pe,
J = 6.0, 3.6 Hz), 7.59 (1H, m, py, J = 7.2, 1.6 Hz), 7.65 (1H,
m, Pe), 7.70 (3H, m, Pe), 7.72 (1H, s, Pe), 7.79 (1H, m, py, J =
4.8, 1.6 Hz), 7.81 (1H, m, py, J = 7.2, 1.6 Hz), 7.94 (1H, dd, Pe,
J = 6.8, 0.8 Hz), 8.40 (2H, s, phen), 8.64 (1H, s, py), 8.79 (1H, d,
Pe, J = 8.4 Hz), 8.82 (1H, d, Pe, J = 8.4 Hz), 9.20 (2H, s, phen).
ESI-MS: m/z observed 970.2991 (M+), expected 970.3013.
[Re(Me4phen)(CO)3(Pe–xy–py)]+ 3. Synthesized in 45% yield
from Re(Me4phen)(CO)3Cl and free ligand 12 following the
general rhenium complexation method. dH/ppm: 2.07 (3H, s, Me–
xy), 2.11 (3H, s, Me–xy), 2.76 (6H, d, Me–phen, J = 2.8 Hz), 2.94
(6H, s, Me–phen), 6.95 (1H, s, xy), 7.58 (1H, dd, py, J = 4.0,
0.8 Hz), 7.60 (2H, d, Pe, J = 6.0 Hz), 7.65 (1H, s, xy), 7.68 (3H,
m, Pe), 7.72 (1H, dd, py, J = 8.8, 0.8 Hz), 7.73 (1H, dd, Pe, J =
8.0, 1.2 Hz), 7.79 (1H, d, py, J = 4.0 Hz), 7.84 (1H, dd, Pe, J =
8.0, 1.2 Hz), 8.39 (2H, s, phen), 8.68 (1H, d, py, J = 2.0 Hz),
8.78 (1H, d, Pe, J = 8.4 Hz), 8.82 (1H, d, Pe, J = 8.4 Hz), 9.20
(2H, d, phen, J = 1.6 Hz). ESI-MS: m/z observed 866.2356 (M+),
expected 866.2387.
An–xy–TMS 17. Synthesized in 60% yield from 9-bromo-
anthracene 16 and the asymmetric building block 8 using the gen-
eral Suzuki coupling method. Purified by column chromatography
on silica with pentane as an eluent. dH/ppm: 0.44 (9H, s, TMS),
1.54 (3H, s, Me), 2.48 (3H, s, Me), 7.04 (1H, s, xy), 7.34 (2H, ddd,
An, J = 8.8, 6.4, 1.2 Hz), 7.46 (2H, ddd, An, J = 8.4, 6.4, 1.2 Hz),
7.47 (1H, s, xy), 7.54 (2H, dd, An, J = 8.8, 1.2 Hz), 8.05 (2H, d,
An, J = 8.4 Hz), 8.48 (1H, s, An).
Pe–xy2–TMS 13. Obtained from reaction of 8 and 10 accord-
ing to the general Suzuki coupling procedure in 75% yield. Purified
on silica using pentane as an eluent. dH/ppm: 0.40 (9H, s, TMS),
2.50 (3H, s, Me), 2.13 (3H, s, Me), 2.20 (3H, m, Me), 2.05 (3H, m,
Me), 7.07 (1H, m, xy), 7.11 (1H, d, xy, J = 5.6 Hz), 7.23 (1H, d,
xy, J = 5.6 Hz), 7.40 (1H, d, xy, J = 4.2 Hz), 7.56 (1H, dd, Pe, J =
7.0, 1.2 Hz), 7.64 (1H, dd, Pe, J = 8.4, 1.2 Hz), 7.68 (2H, m, Pe),
7.71 (1H, s, Pe), 7.71 (1H, dd, Pe, J = 6.6, 1.2 Hz), 7.93 (1H, dd,
Pe, J = 8.0, 1.2 Hz), 8.78 (1H, d, Pe, J = 8.0 Hz), 8.81 (1H, d, Pe,
J = 8.4 Hz).
An–xy–I 18. Obtained from 17 using a TMS–halogen ex-
change procedure.25 The yield was 90%. dH/ppm: 1.80 (3H, s,
Me), 2.47 (3H, s, Me), 7.14 (1H, s, xy), 7.36 (2H, ddd, An, J = 8.4,
6.4, 1.2 Hz), 7.47 (2H, ddd, An, J = 8.8, 6.4, 1.2 Hz), 7.50 (2H,
dd, An, J = 8.8, 1.2 Hz), 7.92 (1H, s, xy), 8.06 (2H, d, An, J =
8.4 Hz), 8.51 (1H, s, An).
Pe–xy2–I 14. Obtained in quantitative yield from 13 following
a TMS–halogen exchange method.25 dH/ppm: 2.02 (3H, s, Me),
2.08 (3H, s, Me), 2.12 (3H, m, Me), 2.45 (3H, m, Me), 7.04 ( H, d,
xy, J = 5.6 Hz), 7.11 (1H, m, xy), 7.20 (1H, m, xy), 7.53 (1H, m,
Pe), 7.61 (2H, m, Pe), 7.67 (2H, m, Pe), 7.67 (1H, s, Pe), 7.77 (1H,
m, xy), 7.90 (1H, dd, Pe, J = 7.6, 0.8 Hz), 8.75 (1H, d, Pe, J =
8.4 Hz), 8.79 (1H, d, Pe, J = 8.4 Hz).
An–xy–bpy 20. Synthesized by coupling 18 to bipyridine 19
with the general Stille coupling method. Purification occurred on a
silica column using a 90 : 9 : 1 pentane–ethyl acetate–triethylamine
eluent mixture. The yield was 25%. dH/ppm: 1.90 (3H, s, Me), 2.37
(3H, s, Me), 7.23 (1H, s, xy), 7.36 (1H, s, xy), 7.36 (1H, ddd, bpy,
J = 7.6, 4.4, 1.2 Hz), 7.40 (2H, ddd, An, J = 8.4, 6.8, 1.2 Hz), 7.49
(2H, ddd, An, J = 8.8, 6.8, 1.2 Hz), 7.63 (2H, dd, An, J = 8.4,
0.8 Hz), 7.88 (1H, ddd, bpy, J = 8.4, 7.6, 1.6 Hz), 7.99 (1H, dd,
bpy, J = 8.0, 2.4 Hz), 8.08 (2H, d, An, J = 8.8 Hz), 8.50 (1H, ddd,
bpy, J = 8.4, 1.2, 0.8 Hz), 8.53 (1H, s, An), 8.54 (1H, dd, bpy, J =
8.0, 2.4 Hz), 8.75 (1H, ddd, bpy, J = 4.4, 1.6, 0.8 Hz), 8.86 (1H,
dd, bpy, J = 2.4, 0.8 Hz).
Pe–xy2–py 15. Synthesized from 11 and 14 according to the
general Suzuki coupling method. Product purification occurred
on silica with a dichloromethane–methanol mixture as an eluent.
The yield was 35%. dH/ppm: 2.08 (3H, s, Me), 2.18 (3H, s, Me),
2.24 (3H, m, Me), 2.34 (3H, m, Me), 7.16 (1H, d, xy, J 5.6 Hz),
7.20 (1H, m, xy), 7.22 (1H, m, xy), 7.26 (1H, d, xy, J = 5.6 Hz),
7.44 (1H, dd, Pe, J = 8.8, 4.8 Hz), 7.58 (1H, m, py, J = 6.8, 1.2 Hz),
7.65 (2H, m, Pe), 7.71 (2H, m, Pe), 7.73 (1H, s, Pe), 7.81 (1H, m,
py, J = 6.8, 1.6 Hz), 7.94 (1H, dd, Pe, J = 7.6, 1.2 Hz), 8.65 (1H,
dd, py, J = 4.8, 1.6 Hz), 8.73 (1H, d, py, J = 1.6 Hz), 8.79 (1H, d,
Pe, J = 8.4 Hz), 8.82 (1H, d, Pe, J = 8.4 Hz).
[Re(An–xy–bpy)(CO)3(py)]+ 5. Obtained from the free ligand
20 and Re(CO)5Cl following the general rhenium complexation
method. The overall yield was 80%. dHppm: 1.93 (3H, s, Me), 2.38
(3H, s, Me), 7.30 (1H, s, xy), 7.39 (1H, s, xy), 7.4–7.5 (4H, m, An),
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7.49 (1H, dd, bpy, J = 6.4, 5.0 Hz), 7.58 (2H, d, An, J = 8.4 Hz),
7.79 (1H, dd, bpy, J = 8.0, 6.4 Hz), 7.88 (1H, dd, bpy, J = 8.0,
7.6 Hz), 8.09 (2H, d, An, J = 8.4 Hz), 8.25 (1H, s, py), 8.26 (1H,
d, py, J = 4.4 Hz), 8.36 (1H, dd, bpy, J = 8.0, 7.6 Hz), 8.48 (1H,
d, bpy, J = 8.0 Hz), 8.54 (1H, s, An), 8.73 (1H, d, py, J = 4.4 Hz),
8.92 (1H, d, py, J = 8.8 Hz), 9.01 (1H, d, py, J = 8.8 Hz), 9.12
(1H, d, bpy, J = 5.0 Hz), 9.16 (1H, s, bpy). ESI-MS: m/z observed
786.1770 (M+), expected 786.1761.
(the synthesis of which has been described previously21) us-
ing a palladium(0) catalyzed (Suzuki) cross-coupling reaction.
Trimethylsilyl–halogen exchange was effected with ICl, following
a previously published protocol.21 The resulting iodo-compounds
10 and 18 were then either subjected to reaction with another
equivalent of the asymmetric p-xylene building block 8, in order
to lengthen the bridge, or they were coupled to pyridine 1122
and 2,2¢-bipyridine 19.23 Pyridine Suzuki coupling occurred under
identical conditions as the xylene couplings, i.e. in refluxing
aqueous tetrahydrofuran solution in presence of a carbonate base
and the Pd(Ph3)4 (Ph = phenyl) catalyst. 2,2¢-Bipyridine Stille
coupling required harsher conditions: 18/22 had to be reacted
with 19 in refluxing m-xylene in the presence of the same catalyst
for 5 d. Reaction yields were essentially quantitative for trimethyl–
halogen exchange, between 25–95% for all Suzuki couplings, and
only about 20% for the Stille couplings.
The organic ligands, 12 and 15, were coordinated to rhe-
nium by refluxing them with the [(Re(phen)(CO)3Cl] and
[Re(Me4phen)(CO)3Cl] precursors (phen = 1,10-phenanthroline,
Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline) in 1,2-di-
chloroethane in the presence of a silver triflate.10 Thereby, the
complexes 1, 2, 3, and 4 (Scheme 1) were obtained in variable
yields. Ligands 20 and 23, were first refluxed in toluene with
commercial Re(CO)5Cl. Then, the resulting Re(20)(CO)3Cl and
Re(23)(CO)3Cl complexes were reacted with an excess of pyridine
in toluene in the presence of a slight excess of silver triflate.10,20
Thus, the target complexes 5 and 6 were obtained.
An–xy2–TMS 21. Obtained following the general Suzuki
coupling method with reactants 8 and 18. Purified on a silica
column using pure pentane as an eluent. The isolated yield was
50%. dH/ppm: 0.40 (9H, s, TMS), 1.83 (3H, s, Me), 2.11 (3H, s,
Me), 2.23 (3H, s, Me), 2.51 (3H, s, Me), 7.12 (1H, s, xy), 7.14
(1H, s, xy), 7.18 (1H, s, xy), 7.37–7.41 (2H, m, An), 7.41 (1H, s,
xy), 7.49 (2H, dd, An, J = 7.6, 7.6 Hz), 7.60 (1H, dd, An, J = 8.4,
1.2 Hz), 7.66 (1H, dd, An, J = 8.4, 1.2 Hz), 8.07 (2H, d, An, J =
7.6 Hz), 8.51 (1H, s, An).
An–xy2–I 22. Obtained from 21 in essentially quantitative
yield following a TMS–halogen exchange procedure.25 dH/ppm:
1.84 (3H, s, Me), 2.18 (6H, s, Me), 2.48 (3H, s, Me), 7.14 (1H, s,
xy), 7.15 (1H, s, xy), 7.20 (1H, s, xy), 7.39 (2H, ddd, An, J = 8.8,
7.2, 0.8 Hz), 7.48 (2H, ddd, An, J = 8.0, 7.2, 0.8 Hz), 7.58 (1H,
dd, An, J = 8.0, 0.8 Hz), 7.63 (1H, dd, An, J = 8.0, 0.8 Hz), 7.81
(1H, s, xy), 8.07 (2H, d, An, J = 8.8 Hz), 8.51 (1 H, s, An).
An–xy2–bpy 23. Synthesized from 19 and 22 following the
general Stille coupling methodology. Purification was the same
as for 20. The yield was 20%. dH/ppm: 1.86 (3H, s, Me), 2.18
(3H, s, Me), 2.28 (3H, s, Me), 2.39 (3H, s, Me), 7.18 (1H, s, xy),
7.23 (1H, s, xy), 7.27 (1H, s, xy), 7.28 (1H, s, xy), 7.34 (1H, ddd,
bpy, J = 8.0, 4.8, 0.8 Hz), 7.40 (2H, ddd, An, J = 8.4, 7.6, 1.2 Hz),
7.49 (2H, ddd, An, J = 7.6, 5.6, 1.2 Hz), 7.61 (1H, d, An, J =
8.0 Hz), 7.67 (1H, d, An, J = 8.0 Hz), 7.86 (1H, ddd, bpy, J = 8.0,
7.6, 0.8 Hz), 7.91 (1H, dd, bpy, J = 8.0, 2.4 Hz), 8.08 (2H, d, An,
J = 8.4 Hz), 8.48 (1H, ddd, bpy, J = 8.8, 1.2, 1.2 Hz), 8.50 (1H,
dd, bpy, J = 8.0, 0.8 Hz), 8.51 (1H, s, An), 8.73 (1H, ddd, bpy, J =
4.8, 1.6, 0.8 Hz), 8.79 (1H, dd, bpy, J = 2.4, 0.8 Hz).
Optical spectroscopy
The solid lines in Fig. 1 are the optical absorption spectra of the
mono-p-xylene dyads 1, 3, and 5 in dichloromethane solution.
[Re(An–xy2–bpy)(CO)3(py)]+ 6. Obtained from the free ligand
23 and Re(CO)5Cl following the general rhenium complexation
method. The overall yield was 75%. dH/ppm: 1.86 (3H, s, Me),
2.17 (3H, s, Me), 2.33 (3H, s, Me), 2.42 (3H, s, Me), 7.19 (1H, s,
xy), 7.20 (1H, s, xy), 7.30 (1H, s, xy), 7.36 (1H, s, xy), 7.38–7.47
(4H, m, An), 7.49 (1H, m, bpy), 7.60 (1H, d, An, 8.8), 7.65 (1H,
d, An, 8.8), 7.78–7.92 (2H, m, bpy), 8.08 (2H, d, An, J = 8.4 Hz),
8.24 (1H, s, py), 8.25 (1H, d, py, J = 4.8 Hz), 8.34 (1H, dd, bpy, J =
8.0, 7.6 Hz), 8.39 (1H, d, bpy, J = 8.4 Hz), 8.52 (1H, s, An), 8.76
(1H, d py, J = 4.8 Hz), 8.86 (1H, d, py, J = 8.4 Hz), 8.94 (1H, d, py,
J = 8.4 Hz), 9.11 (1H, s, bpy), 9.15 (1H, dd, bpy, J = 4.8, 4.8 Hz).
ESI-MS: m/z observed 890.2411 (M+), expected 890.2387.
Results and discussions
Synthesis
Fig. 1 Solid traces: optical absorption spectra of dyads 1 (a), 3 (b),
and 5 (c) in dichloromethane solution. Dashed traces: luminescence
spectra for the same compounds measured after 380 nm excitation in
dichloromethane. Arrows mark the electronic origins of the emissive
3MLCT state.
The target molecules (Scheme 1) were obtained following the
synthetic strategy shown in Scheme 2. The commercially available
9-bromophenanthrene (7) and 9-bromoanthracene (16) molecules
were coupled to the asymmetric p-xylene building block 8
6314 | Dalton Trans., 2008, 6311–6318
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Scheme 2
The corresponding bi-p-xylene dyads 2, 4, and 6 have absorption
spectra that are superimposable onto those shown in Fig. 1a, 1b,
and 1c, respectively. This shows that xylene bridge lengthening
does not alter the p-conjugation between the individual parts of
the individual donor–bridge–acceptor molecules, in clear contrast
to what is commonly observed for unsubstituted phenylene
bridges. In oligo-p-phenylenes, there is a ~30◦ dihedral angle
between individual phenyl planes,24 whereas in oligo-p-xylenes
that dihedral angle must be significantly greater for steric reasons.
Indeed, recent work shows that molecular bridges, with up to
five p-xylene units, have essentially length-independent HOMO–
LUMO energy gaps.25 Additional support for this comes from
crystallographic studies, which found that in the solid state, the
two phenyl rings of an unsubstituted biphenyl molecule can be
almost co-planar,26 whereas the two xylyl rings in a bi-p-xylene
molecule were orthogonal to one another.27
and [Re(Me4phen)(CO)3(py)]+ reference complexes that lack the
phenanthrene unit.10,12 Indeed, only some of the fine structure
observed below 300 nm can be attributed to the phenanthrene, as
the rhenium complex provides the dominant absorption features
also between 250–300 nm; the extinction coefficients that are
observed essentially correspond to those reported for the above
mentioned reference complexes.10,12 By contrast, the absorption
spectrum of 5 (Fig. 1c) is dominated by anthracene bands rather
than absorptions that are due to the rhenium complex: The intense
(e ~ 140 000 M-1cm-1) band around 260 nm as well as the weaker
structured features observed between 340–390 nm are well-known
anthracene absorptions.29 The latter, commonly assigned to the S0
(1A1g) → S1 (1B1u) and S0 (1A1g) → S2 (1B1u) electronic transitions,
overlap spectrally with the Re → bpy MLCT excitations. The
bpy intra-ligand p–p* transitions are masked completely by the
anthracene absorptions.
Phenanthrene has no intense (>102 M-1cm-1) absorptions
at wavelengths longer than 300 nm.28 Consequently, the ab-
sorption spectra of 1 (Fig. 1a) and 3 (Fig. 1b) in the 300–
450 nm spectral range are identical to [Re(phen)(CO)3(py)]+
While in absorption striking similarities are observed between
all four phenanthrene-based donor–bridge–acceptor molecules,
steady-state luminescence experiments yield different results
(dashed traces in Fig. 1). Notably, for the Me4phen systems 3
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Table 1 Photophysical parameters for the six dyads
Dyad
E
MLCT/eVa
E
T1/eV
tX/ms
t
ref/ms
k
EnT/s-1
1
2
3
4
5
6
2.76
2.76
2.82
2.82
2.73
2.73
2.70b
2.70b
2.70b
2.70b
1.82c
1.82c
2.57
2.74
7.0
12.9
0.015
0.040
2.88
2.88
13.0
13.0
0.63
0.63
<3 ¥ 104
<3 ¥ 104
6.6 ¥ 104
<8 ¥ 103
7.0 ¥ 107
2.3 ¥ 107
a From this work and ref. 10 and 12. b From ref. 31. c From ref. 29.
and 4, the emission band maxima are blue-shifted relative to the
phen systems 1 and 2 by about 800 cm-1. This is due to the electron-
donating nature of the methyl substitutents of the phenanthroline
ligand. This raises the Re → phen MLCT energy and is an effect
that has been observed previously for the [Re(phen)(CO)3X]n+ and
[Re(Me4phen)(CO)3X]n+ (X = Cl, pyridine; n = 0, 1) reference
complexes.10,12,30 The reference molecules with X = pyridine have
emission spectra that are virtually identical to those shown in
Fig. 1a and 1b, which confirms the hypothesis that electronic
donor–bridge–acceptor coupling is weak in our molecules. Careful
comparison of the absorption tails beyond 350 nm for dyads 1
and 3 and reveals that the MLCT blue-shift is also observed in
3
absorption. The MLCT energies (EMLCT), estimated from these
absorption and emission data, are given in Table 1. They are also
marked by the arrows in Fig. 1.
The anthracene systems 5 and 6 have emission spectra
(Fig. 1c) that are essentially identical to that observed for the
[Re(bpy)(CO)3(py)]+ reference complex,10 i.e. electronic donor–
bridge–acceptor coupling is weak in these dyads, too. The emission
band maximum as well as the EMLCT are slightly red-shifted in the
bpy systems 5 and 6 relative to the phen systems 1 and 2. All EMLCT
values extracted from this analysis are summarized in the second
column of Table 1.
While the luminescence bandshapes are identical within a
homologous series comprised of a reference complex, the mono-
p-xylene, and the bi-p-xylene bridged dyad, the luminescence
intensities and lifetimes may still be expected to vary considerably
between the individual members of such a series. However, this
is only partly observed. Fig. 2a shows the luminescence decays
of dyads 1 (solid trace) and 2 (dashed trace) in deoxygenated
dichloromethane solution. Two virtually identical single exponen-
tial decays are observed. The MLCT lifetimes determined from
these data are 2.57 (1) and 2.74 ms (2), which are identical, within
15% error, to the MLCT lifetime of the [Re(phen)(CO)3(py)]+
reference complex, which is 2.88 ms under identical experimental
conditions. This indicates that the MLCT states in 1 and 2 are
essentially unperturbed by the presence of the p-xylene bridges
and the phenanthrene moiety. The emission properties of the
three systems are in fact essentially indistinguishable from one
another, and the conclusion is that in dyads 1 and 2, rhenium-to-
phenanthrene energy transfer is not competitive with other MLCT
deactivation pathways.
Fig. 2 MLCT luminescence decays of the six dyads in deoxygenated
dichloromethane. Excitation occurred at 355 nm or 410 nm with 10 ns
laser pulses, detection was at 560 nm. Note the three different timescales.
dyad 3 is clearly faster; it is a bi-exponential decay with a major
(90%) component that has a lifetime of 7.0 ms and a minor (10%)
component with a lifetime of 20.1 ms. The latter is attributed to
a luminescent impurity and is not considered further. The 7.0 ms
component is attributed to dyad 3; the MLCT population of this
molecule is thus found to decay almost twice as fast as that of
the [Re(Me4phen)(CO)3(py)]+ reference complex. This additional
excited state quenching is attributed to rhenium → phenanthrene
energy transfer, and its rate constant kEnT can be determined from
the relation7e,32
-1
-1
kEnT = tX - tref
(1)
where tX is the excited-state lifetime of the donor–bridge–acceptor
molecule X and tref is the lifetime of the appropriate reference
complex. Thus, one obtains kEnT = 6.6 ¥ 104 s-1 for the mono-p-
xylene bridged dyad 3 (Table 1) whereas for the bi-p-xylene bridged
dyad 4, kEnT must be less than a tenth of t-41.
The excited states of the anthracene dyads 5 and 6 decay on
a completely different timescale, see Fig. 2c. The luminescence
lifetimes t5 and t6 are ~15 and 40 ns, respectively. For comparison,
the [Re(bpy)(CO)3(py)]+ reference complex has an MLCT lifetime
of 630 ns. Thus, luminescence quenching is very efficient for the
A different result is obtained for the Me4phen systems 3 and 4,
see Fig. 2b. The decay trace of 4 is essentially identical to that of the
reference complex [Re(Me4phen)(CO)3(py)]+; the MLCT lifetime
is 12.9 ms in deoxygenated dichloromethane solution (Table 1). As
in 1 and 2, in dyad 4 rhenium-to-phenanthrene energy transfer is
found to be inefficient. However, the MLCT decay observed for
anthracene dyads 5 and 6. Based on eqn (1), one estimates kEnT
=
7 ¥ 107 s-1 for the mono-p-xylene dyad 5 and kEnT = 2.3 ¥ 107 s-1
for the bi-p-xylene dyad 6.
6316 | Dalton Trans., 2008, 6311–6318
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The interpretation of this excited state quenching, in terms of
rhenium-to-anthracene (triplet–triplet) energy transfer, is corrob-
orated by the transient absorption data shown in Fig. 3. The
upper part of this figure shows the transient absorption spectrum
measured within a 50 ms time window, 500 ns after excitation of
dyad 6 at 355 nm. This is the typical absorption spectrum of the
lowest energetic anthracene localized triplet excited state.3a,c,6a,33
The lower part of Fig. 3 shows the temporal evolution of the
transient absorption signal of dyad 6 at 425 nm. The negative signal
at t ª 0 is attributed to luminescence. Then, there is a rapid (<400
ns) rise, which reflects rhenium-to-anthracene energy transfer. The
anthracene triplet population decays subsequently with a lifetime
of 42 ms in deoxygenated dichloromethane solution, which is a
typical value for organic triplets.29a
Scheme 3
increase (~0.8 eV) in the driving force. It is possible, however,
that the nature of the MLCT excitation also has some influence
on kEnT. In 5 and 6, this excitation is localized right next to the
xylene bridges and the anthracene acceptor, whereas in the other
dyads, the MLCT excitation occurs on a molecular fragment that
is separate from the xylene/phenanthrene moieties.
Two important conclusions can be drawn from the bridge length
variations performed in this study: (i) p-xylene bridge lengthening
does not appear to influence the overall p-conjugation of the bridge
(i.e. the bridge levels stay at constant energy) and, (ii) only the
anthracene energy acceptor will be suitable for long-range energy
transfer investigations with rhenium(I) tricarbonyl diimine donors.
An additional point is noteworthy: in dyad 6, rhenium-to-
anthracene energy transfer occurs through a bi-p-xylene unit
Fig. 3 (a) Transient absorption spectrum measured after 355 nm excita-
tion of dyad 6 in deoxygenated dichloromethane solution. The spectrum
was recorded in a 50 ms time window starting 500 ns after the excitation
laser pulse at 355 nm. (b) Decay of the transient absorption signal from
(a) at 425 nm.
7
-1
˚
over a distance of ~15 A with a rate constant of 2.3 ¥ 10 s
(Table 1). This is a rather low value considering that in an oligo-p-
phenylene bridged Ru(II)–Os(II) dyad, energy transfer was found
4
˚
to proceed over 32.5 A with essentially the same rate constant.
A recent study even reports on an energy transfer that occurs
with a rate constant of 8.3 ¥1011 s-1 through a biphenyl bridge.34
There, energy transfer occurs via a hopping mechanism, which
we can rule out for our system based on the relative inefficiency
of the energy transfer process. However, even the super-exchange
mediated energy transfer in the above mentioned Ru(II)–Os(II)
dyad is orders of magnitude more efficient than in our rhenium–
bixylene–anthracene molecule. A key difference between these two
systems is the driving force for energy transfer. In our rhenium–
anthracene dyads, this process is almost three times more exergonic
than in the Ru(II)–Os(II) system. It is possible that inverted driving
force effects play a role in our dyads.35 Future investigations will
shed more light on this issue.
Conclusions
The steady-state and time-resolved absorption and emission data
presented in the prior sections allow us to establish the energy
level diagram shown in Scheme 3. For dyads 1 and 2, rhenium-to-
phenanthrene energy transfer is not observed. This is due to the
very low driving force (-DG0 = 0.06 eV) for this process. When
the MLCT energy is raised relative to the phenanthrene triplet
levels, triplet–triplet energy transfer becomes competitive with the
inherent MLCT excited state deactivation (-DG0 = 0.12 eV), at
least for dyad 3 with the short mono-p-xylene bridge. The bi-p-
xylene spacer in dyad 4 is already too long for intramolecular
energy transfer quenching to play a noticeable role. When the
phenanthrene acceptor is replaced by anthracene, the energy
transfer rate constants kEnT increase by more than 3 orders of
magnitude for both the mono-p-xylene and the bi-p-xylene bridged
systems. The main reason for this observation must be the dramatic
Acknowledgements
We acknowledge funding from the Swiss National Science Foun-
dation, the Fondation Ernst et Lucie Schmidheiny, and the Socie´te´
Acade´mique de Gene`ve. Professor Andreas Hauser is gratefully
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acknowledged for letting us use his nanosecond laser setup and
for helping us with some of the measurements. We thank N. Oudry
for the high resolution mass spectrometry measurements.
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6318 | Dalton Trans., 2008, 6311–6318
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