Photophysical properties of binuclear ruthenium(II) bis(2,2′: 6′,2″-terpyridine) complexes built around a central 2,2′-bipyrimidine receptor

Article abstract of DOI:10.1039/b506818e

A binuclear complex has been synthesized having ruthenium(II) bis(2,2′:6′,2″-terpyridine) terminals attached to a central 2,2′-bipyrimidine unit via ethynylene groups. Cyclic voltammetry indicates that the substituted terpyridine is the most easily reduced subunit and the main chromophore involves charge transfer from the metal centre to this ligand. The resultant metal-to-ligand, charge-transfer (MLCT) triplet state is weakly emissive and has a lifetime of 60 ns in deoxygenated solution at room temperature. The luminescence yield and lifetime increase with decreasing temperature in a manner that indicates the lowest-energy MLCT triplet couples to at least two higher-energy triplets. Cations can bind to the central bipyrimidine unit, forming both 1 : 1 and 1 : 2 (ligand : metal) complexes as confirmed by electrospray MS analysis. The photophysical properties depend on the number of bound cations and on the nature of the cation. In the specific case of binding zinc(II) cations, the 1 : 1 complex has a triplet lifetime of 8.0 ns while that of the 1 : 2 complex is 1.8 ns. The 1 : 1 complexes formed with Ba²⁺ and Mg²⁺ are more luminescent than is the parent compound while the 1 : 2 complexes are much less luminescent. It is shown that the coordinated cations raise the reduction potential of the central bipyrimidine unit and thereby increase the activation energy for coupling with the metal-centred state. Complexation also introduces a non-emissive intramolecular charge-transfer (ICT) state that couples to the lowest-energy MLCT triplet and provides an additional non-radiative decay route. The triplet state of the 1 : 2 complex formed with added Zn²⁺ cations decays preferentially via this ICT state. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506818e

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F U L L P A P E R
Photophysical properties of binuclear ruthenium(II) bis(2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridine) complexes built around a central 2,2ꢀ-bipyrimidine
receptor
Anthony Harriman,*a Annabelle Mayeux,a Christophe Strohb and Raymond Ziessel*b
a Molecular Photonics Laboratory, School of Natural Sciences-Chemistry, Bedson Building,
University of Newcastle, Newcastle-upon-Tyne, UK NE1 7RU.
E-mail: Anthony.harriman@ncl.ac.uk; Fax: 44 191 222 8660; Tel: 44 191 222 8660
b Laboratoire de Chimie Mole´culaire, Ecole de Chimie, Polyme`res et Mate´riaux,
Universite´ Louis Pasteur, 25 rue Becquerel, F-67087, Strasbourg Cedex 02, France
Received 13th May 2005, Accepted 6th July 2005
First published as an Advance Article on the web 25th July 2005
A binuclear complex has been synthesized having ruthenium(II) bis(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine) terminals attached to a
central 2,2ꢀ-bipyrimidine unit via ethynylene groups. Cyclic voltammetry indicates that the substituted terpyridine is
the most easily reduced subunit and the main chromophore involves charge transfer from the metal centre to this
ligand. The resultant metal-to-ligand, charge-transfer (MLCT) triplet state is weakly emissive and has a lifetime of
60 ns in deoxygenated solution at room temperature. The luminescence yield and lifetime increase with decreasing
temperature in a manner that indicates the lowest-energy MLCT triplet couples to at least two higher-energy triplets.
Cations can bind to the central bipyrimidine unit, forming both 1 : 1 and 1 : 2 (ligand : metal) complexes as
confirmed by electrospray MS analysis. The photophysical properties depend on the number of bound cations and on
the nature of the cation. In the specific case of binding zinc(II) cations, the 1 : 1 complex has a triplet lifetime of 8.0 ns
while that of the 1 : 2 complex is 1.8 ns. The 1 : 1 complexes formed with Ba2+ and Mg2+ are more luminescent than is
the parent compound while the 1 : 2 complexes are much less luminescent. It is shown that the coordinated cations
raise the reduction potential of the central bipyrimidine unit and thereby increase the activation energy for coupling
with the metal-centred state. Complexation also introduces a non-emissive intramolecular charge-transfer (ICT) state
that couples to the lowest-energy MLCT triplet and provides an additional non-radiative decay route. The triplet
state of the 1 : 2 complex formed with added Zn2+ cations decays preferentially via this ICT state.
ligand. 2,2ꢀ-Bipyrimidine, which has often been used as a ligand
Introduction
for mono- and binuclear RuII complexes,24 was selected for this
The photophysical properties of numerous binuclear ruthe-
nium(II) bis(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine) complexes have been re-
ported in recent years.1 Interest in these systems stems, in
part, from the realization that whereas the parent complex,
[Ru(terpy)2]2+ (terpy = 2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine), is essentially non-
luminescent in fluid solution at ambient temperature2 cer-
tain derivatives emit with reasonable efficiency under these
conditions.3 It has been recognised4 that the emission lifetime
of the binuclear complex is often substantially longer than
that of the corresponding mononuclear species when the
bridging ligand is highly conjugated. Examples of bridging
ligands range from simple hydrocarbon chains, both saturated5
and unsaturated,6 to aromatic polycycles such as benzene,7
naphthalene,8 anthracene,9 pyrene,10 thiophene,11 biphenyl,12
9,9-dimethylxanthene,13 quinoxaline,14 azobenzene15 and 2,6-
bis(benzimidazol-2-yl)pyridine.16 Extended ligands have been
reported17 for which the metal–metal separation distance is
increased progressively whilst a variety of heterotopic bridging
ligands has been described.18 In addition, the luminescence
properties can be modified by replacing one of the RuII centres
with other cations, such as OsII, so that intramolecular triplet
energy transfer becomes viable.1 An interesting variant has
been to include chelating functions within the bridging ligand
that can coordinate added cations.19 This strategy allows fine-
tuning of the electronic properties of the bridging ligand by
attachment of different analytes from solution. Examples of
such secondary chelating groups include 1,10-phenanthroline,20
2,2ꢀ-bipyridine,21 polyethers22 and calixarenes.23 We now seek
to extend the range of such multifunctional supermolecules by
describing the properties of a binuclear Ru-terpy complex built
around a central 5,5ꢀ-diethynylene-2,2ꢀ-bipyrimidine bridging
work because it can bind two cations.25
The photophysical properties of mononuclear Ru-terpy
derivatives are often dominated by the lowest-energy metal-
to-ligand, charge-transfer (MLCT) triplet state. This species is
short-lived in the parent complex (sLUM = 250 ps)2 but can be
much longer lived in certain substituted complexes (e.g., sLUM
=
1.2 ls).6 Even for the parent complex, a proper description of
the emissive state requires consideration of both an upper-lying
MLCT state and a high-energy metal-centred (MC) state.26 The
situation is more intricate for many binuclear complexes where
the effects of ligand-localised (LL) and intramolecular charge-
transfer (ICT) states have to be taken into account.27 Evidence
has been presented to the effect that the triplet quantum yields
for certain binuclear Ru-terpy complexes can be less than
unity.28 Furthermore, with mixed-metal binuclear complexes it is
possible to observe intervalence charge transfer intermediates.29
It is clear, therefore, that the triplet manifold is likely to
be highly congested for those binuclear Ru-terpy complexes
assembled around conjugated bridging ligands, especially those
containing ancillary binding sites. In such cases, the lumines-
cence properties might be sensitive to minor changes in the
surrounding environment. For example, it has been reported that
coordination of cations to the vacant bipyridine-based bridging
ligand increases the emission quantum yield for appropriate
binuclear Ru-terpy complexes.21 Likewise, the solution pH
has a marked influence on the various reduction potentials
for certain binuclear Ru-terpy derivatives assembled around
benzimidazole-based bridging ligands16 whereas coordination
of cations curtails electron transfer in some calixarene-bridged
complexes.23 Here, we examine in more detail the effects of cation
binding to the bridging ligand and show that the emission yield
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 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2
2 9 2 5
©

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can either increase or decrease according to the nature and
number of bound cations.
temperature for 6 days before KPF6 (0.050 g, 5 eq.) in water
(2 ml) was added and the solvent removed. The crude product
was chromatographed on alumina, eluting with a mixture of
CH2Cl2/CH3CN 1/9 containing a gradient of CH3CN/H2O
(10%). The analytically pure compound was obtained by
Experimental
1
Nuclear magnetic resonance spectra were recorded at room
temperature on a Bruker AC-200 instrument using 200.1 MHz
recrystallization from CH3CN/diethylether (0.016 g, 32%). H
NMR (CD3CN) d = 9.39 (s, 4H), 8.96 (s, 4H), 8.78 (d, 4H, 3J =
1
for H NMR and 50.3 MHz for 13C NMR. Chemical shifts
8.0 Hz), 8.49 (m, 10H), 7.95 (m, 8H), 7.39 (m, 8H), 7.19 (m, 8H).
1
13C{ H} NMR (CD3CN) d = 161.3, 160.6, 158.3, 157.8, 156.1,
are reported in parts per million (ppm) relative to residual
protiated solvent (1.93 ppm d3-acetonitrile) and the carbon
resonance of the solvent. High-resolution mass spectral analysis
(HRMS) was performed using a Mariner ESI-ToF instrument
from Applied Bio-Systems/Perkin Elmer. All relevant patterns
have the expected isotopic profiles, as compared with simulated
profiles. Routine absorption spectra were measured in CH3CN
solution at room temperature with a Kontron Uvikon 941
spectrophotometer. FT-IR spectra were recorded in KBr pellets
with an IFS 25 Bruker spectrometer. The identity of the final
compounds, and all important precursors, was made on the
basis of 1H NMR, FAB+-MS and elemental composition.
Solvents were spectroscopic grade and were freshly distilled
155.5, 153.0, 138.7, 136.8, 128.7, 128.2, 127.9, 125.8, 125.1,
125.0, 124.2, 118.8, 94.0, 90.1, 72.6, 61.4. MALDI-TOF m/z
(nature of the peak, rel. inten.%) 494.4 ([M − 3PF6]3+, 23), 334.4
([M − 4PF6]4+, 100). IR (KBr, cm−1): 3421, 2924, 821, 558. UV-
vis (CH3CN) kmax, nm (e, M−1 cm−1) 498 (68,600), 309 (141,700),
272 (92,600). Anal. calc. for C72H46N16P4F24Ru2 (M = 1917.27):
C, 45.11; H, 2.42; N, 11.69. Found: C, 44.97; H, 2.33; N, 11.71.
Absorption spectra were recorded with a Hitachi U3310
spectrophotometer and luminescence studies were made with
an Yvon-Jobin Fluorolog tau-3 spectrometer equipped with
a cryogenic Ge photodetector. Emission spectra were fully
corrected using a standard lamp. Luminescence spectra were
recorded for optically dilute solutions after purging with dried
N2. The solution was sealed in the sample cell and cooled to
the required temperature with an Oxford Instruments Optistat
DN cryostat. Samples were equilibrated for at least 15 min at
each temperature. Emission quantum yields were recorded with
respect to ruthenium(II) tris(2,2ꢀ-bipyridine) in deoxygenated
solution at room temperature.31 Quantum yields were repro-
ducible to within 8% and temperatures were accurate to
within 0.2 K. Emission lifetimes were recorded after excitation
of the sample with pulses delivered from either a frequency-
doubled, Q-switched Nd-YAG laser (FWHM = 4 ns) or a
frequency-doubled, mode-locked Nd-YAG laser (FWHM =
30 ps). Luminescence was isolated from scattered laser light
with glass cut-off filters and focused onto the entrance slits of
a high-radiance monochromator. The output was directed to a
fast response PMT and then to a transient recorder for signal
averaging. Laser flash photolysis studies were made with the Q-
switched laser and with a pulsed Xe arc lamp as the monitoring
source. Transient spectra were compiled point-by-point, with 5
individual laser shots being averaged at each wavelength. Kinetic
measurements were made by averaging 25 individual records.
Decay of the triplet excited state of 1 in the presence of excess
Zn(ClO4)2 was measured after excitation with the mode-locked
Nd-YAG laser and using a fast-response diode as detector. A
total of 125 laser shots were averaged to obtain the required
triplet lifetime.
21a
before use. The precursors [Ru(terpy)(terpy-Br)](PF6)2 and
5,5ꢀ-dibromo-2,2ꢀ-bipyrimidine30 were prepared according to
literature procedures.
Synthesis of 5,5ꢀ-di(trimethylsilylethynyl)-2,2ꢀ-bipyrimidine
5,5ꢀ-Dibromo-2,2ꢀ-bipyrimidine (0.108 g, 0.34 mmol),
trimethylsilylacetylene (110 ll, 1.12 mmol), [PdCl2(PPh3)2]
(0.024 g, 10 mol%), and CuI (0.013 g, 20 mol%) were added
i
to an argon-degassed solution of Pr2NH (1 mL) and THF
(10 mL). After 20 h, the solvent was evaporated under vacuum
and the residue quenched with water (100 ml). The residual
organic matter was extracted with CH2Cl2 (3 × 100 ml). The
organic phase was dried over MgSO4 and the target compound
purified by column chromatography using flash silica as support
and a mixture of CH2Cl2–1% MeOH as eluent. The pure
compound was recovered as a white powder (0.097 g, 81%).
1
1H NMR (CDCl3) d 9.0 ppm (s, 4H), 0,3 (s, 18H). 13C{ H}
NMR (CDCl3) d 160.1, 159.3, 119.9, 105.1 (C≡C), 97.7 (C≡C),
−0.3 ppm (TMS). FAB+ (m-NBA) m/z (%) 351.3 ([M + H]+,
100). IR (KBr, cm−1) 3415, 2960, 2160 (mC≡C), 1567, 1507, 1419,
1245, 863, 836, 757. UV-vis (CH2Cl2) k (nm) (e, M−1 cm−1) 297
(45,900), 313 (46,000), 328 (sh, 33,600). Elemental analysis for
C18H22N4Si2 (M = 350.57): C, 61.67; H, 6.33; N, 15.98; found:
C, 61.44; H, 6.19; N, 15.65.
Synthesis of 5,5ꢀ-diethynyl-2,2ꢀ-bipyrimidine
Electrochemical studies employed cyclic voltammetry with a
conventional 3-electrode system using a BAS CV-50 W voltam-
metric analyzer equipped with a Pt microdisk (2 mm2) working
electrode and a silver wire counter-electrode. Ferrocene was
used as internal standard and was calibrated against a saturated
calomel reference electrode (SSCE) separated from the electrol-
ysis cell by a glass frit pre-soaked with electrolyte solution. So-
lutions contained the electrode-active substrate in deoxygenated
anhydrous acetonitrile containing twice-recrystallized tetra-N-
butylammonium hexafluorophosphate (0.1 M) as supporting
electrolyte. The concentration of the solute was ca. 1.2 mM. The
quotedhalf-wavepotentials werereproducibletowithin 15 mV.
Spectrophotometric titrations were made by adding small
amounts of the required salt to a known solution of 1 in
acetonitrile containing 0.1 M tetra-N-butylammonium hex-
afluorophosphate. Corrections were made for small changes
in volume. The salts used were zinc(II) perchlorate, barium
perchlorate, magnesium perchlorate, lithium perchlorate, potas-
sium perchlorate or perchloric acid. The concentration of 1 was
varied and at least 20 different concentrations of salt were used
for each titration. Data analysis was made with SPECFIT after
performing a statistical vector analysis to determine the number
of significant components.
5,5ꢀ-Bis-[2-(trimethylsilyl)-1-ethynyl]-2,2ꢀ-bipyrimidine (0.148 g,
0.42 mmol) and KF (0.060 g, 1.00 mmol) were added to a
mixture of MeOH/THF (14 mL, 1/1) and reaction was allowed
to proceed for 14 h. Afterwards the solvent was evaporated to
dryness and the crude product purified by chromatography on
flash silica with a mixture of CH2Cl2 and MeOH (0.5%) as eluent,
1
affording a white powder (0.087 g, 99%). H NMR(CDCl3) d
1
9.0 ppm (s, 4H), 3.5 (s, 2H). 13C{ H} NMR (CDCl3) d 160.4,
159.8, 119.0, 86.4. FAB+ (m-NBA) m/z (%) 207.4 ([M + H]+,
100%), 181.2 ([M–C≡C–H]+, 20). IR (KBr, cm−1) 3453, 3278
(mC≡C–H), 2360, 1637, 1569, 1514, 1421, 1370, 1244, 1111, 939,
765. UV-vis (CH2Cl2) k (nm) (e, M−1 cm−1) 291 (5920). Elemental
analysis calculated for C12H6N4 (M = 206.21): C, 69.90; H, 2.93;
N, 27.17; found: C, 69.72; H, 2.75; N, 27.01.
Synthesis of the dinuclear complex 1
A Schlenk flask was charged with 5,5ꢀ-diethynyl-2,2ꢀ-bipyrimi-
dine (0.005 g, 0.025 mmol), [Ru(terpy)(terpy-Br)](PF6)2 (0.046 g,
0.05 mmol), 10 ml degassed CH3CN, [Pd(PPh3)2Cl2] (0.0011 g,
6 mol%) and CuI (0.0005 g, 10 mol%). After purging with argon,
iPr2NH (0.5 mL) was added. The solution was stirred at room
2 9 2 6
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2

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Fig. 1 Protocol used for the preparation of 1.
Results and discussion
Synthesis
Preparation of the key 5,5ꢀ-diethynyl-2,2ꢀ-bipyrimidine plat-
form was inspired by our previous synthesis of oligopyridine-
based building blocks.32 A Sonogashira protocol33 was used
to functionalise 5,5ꢀ-diethynyl-2,2ꢀ-bipyrimidine with trimethyl-
silylacetylene (Fig. 1). Quantitative deprotection was afforded
with KF in protic media. A cross-coupling reaction was used to
connect two [Ru(terpy)(terpy-Br)]2+ fragments to the bipyrimi-
dine core. This smooth reaction is promoted by palladium(0)
generated in situ from palladium(II) and CuI. A secondary
amine is required to quench the nascent acid. It is worth
noting that the presence of CH3CN, needed to dissolve the
starting [Ru(terpy)(terpy-Br)](PF6)2 complex, does not signifi-
cantly perturb the catalytic reaction. Similar observations were
reported previously during the stepwise construction of ordered
networks34 and for certain copper-free aryl bromide cross-
coupling reactions.35 This strategy provides access to the target
binuclear complex 1 without complications from side reactions
or scrambling of the ruthenium centre.
Fig. 2 Absorption and emission spectra recorded for 1 in deoxygenated
acetonitrile at 20 ◦C.
lifetime (sLUM ) was measured as 60 5 ns. Decay profiles were
found to be mono-exponential at all monitoring wavelengths
and independent of excitation wavelength. Both ULUM and sLUM
were highly reproducible among three different batches of 1.
Further purification by TLC immediately prior to making the
photophysical measurements had no effect on either ULUM or
sLUM
.
Photophysical properties of the binuclear complex 1
Laser flash photolysis studies, made following excitation of 1
in deoxygenated acetonitrile at 532 nm, gave rise to a transient
species that decayed with a lifetime of 60 7 ns. The transient
differential absorption spectrum shows strong bleaching of the
MLCT band at 500 nm and of the absorption bands attributed to
the bridging ligand around 300 nm (Fig. 3). There is significant
absorption in the far-red region, as has been noted previously
for alkynylene-substituted Ru-terpy complexes.4 The measured
decay kinetics were independent of excitation wavelength and of
laser intensity. Addition of molecular oxygen caused a modest
reduction in the lifetime of the transient.
The absorption spectrum recorded for 1 in acetonitrile solution
shows a prominent metal-to-ligand, charge-transfer (MLCT)
transition centred at 498 nm (Fig. 2). Weaker absorption stretch-
ing towards 630 nm can be attributed to the spin-forbidden
MLCT transition while the capping terpy ligands absorb
strongly around 280 nm. The bridging ligand displays several
absorption bands within the 300–400 nm region. Overall, the
spectrum is comparable to those noted for related alkynylene-
substituted binuclear Ru-terpy complexes.4 Luminescence could
be observed in deoxygenated acetonitrile with a maximum at
705 nm (Fig. 2). The corrected excitation spectrum agrees well
with the absorption spectrum across the entire visible and near-
UV regions. However, the emission quantum yield was found
In common with numerous other Ru-terpy complexes,26,36
the emission yield and lifetime recorded for 1 in deoxygenated
butyronitrile increased upon cooling the solution (Fig. 4). Fol-
lowing from earlier work,37 the rate constant for non-radiative
to be very low (ULUM = 0.0005
0.0001) while the emission
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2
2 9 2 7

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this particular state is almost 2000-fold higher (kA = 4.5 ×
107 s−1) than k0. This second triplet can be attributed to an
MLCT state that possesses more singlet character.37 Both these
MLCT triplets couple to a higher-energy state that is reached
by crossing a more substantive barrier (EB = 2,780 cm−1).
The overall rate constant (k2 = 1.1 × 1013 s−1) associated with
reaching this uppermost triplet is relatively high. Again, based
on prior work,26,38 the uppermost triplet is probably a metal-
centred (MC) state. The derived values are in good accord
with results observed for related Ru-terpy derivatives and are
consistent with the emitting state being primarily of MLCT
character. The radiative rate constant (kRAD = ULUM /sLUM
≈
104 s−1) is low but not inconsistent with values determined for
other RuII poly(pyridine) complexes.26
Fig. 3 Differential triplet absorption spectrum recorded after laser
excitation of 1 in deoxygenated acetonitrile solution. The insert shows a
decay kinetic trace recorded at 700 nm.
Cyclic voltammetry was carried out with 1 (ca. 1.2 mM)
in deoxygenated acetonitrile containing 0.1
M tetra-N-
butylammonium hexafluorophosphate. Upon anodic scans, a
reversible, two-electron oxidation process was observed with a
half-wave potential (E1/2) of +1.27 V vs. SSCE. This electrode
reaction corresponds to the one-electron oxidation of each metal
centre. Analysis of the peak profile indicates that the two centres
are oxidised at the same potential, thereby suggesting that they
are not in strong electronic communication. No other oxidative
process could be discerned in the cyclic voltammograms.
Fig. 4 Effect of temperature on the rate constant for nonradiative
decay of 1 (ꢀ) and the 1 : 1 complex (᭹) formed with Mg(ClO4)2 in
deoxygenated butyronitrile.
Three separate reductive steps could be discerned for 1 on
cathodic scans, each process being quasi-reversible and having a
well-defined half-wave potential (Table 1). To aid assignment,
the measured reduction potentials are compared with those
recorded for the parent complex,3 [Ru(terpy)2]2+, and with the
corresponding binuclear complex having 2,2ꢀ-bipyridine in place
of bipyrimidine, 2.21a The reductive pattern deduced for 2 has
the first step corresponding to the one-electron reduction of
each of the two ethynylated terpy ligands, followed by the one-
electron reduction of each capping terpy ligand.21a The final step
is due to the one-electron reduction of the central bipyridine unit
(Table 1). On this basis, it seems likely that the least negative
process found for 1 (E1/2 = −1.11 V vs. SSCE) can be assigned to
the one-electron reduction of the ethynylated terpy ligands. This
assignment takes into account the realisation that this would
be a two-electron step and that the presence of an ethynylene
group at the 4ꢀ-position renders 2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine much easier
decay of the triplet state (kNR = (1 − ULUM )/sLUM ) was found
to follow a modified Arrhenius-type behaviour (eqn (1)) over a
reasonable temperature range.
ꢀ
ꢁ
ꢀ
ꢁ
EA
kBT
EB
kBT
kNR = k0 + k1 exp
−
+ k2 exp
−
(1)
According to this expression, the lowest-energy triplet state
decays via an activationless process with rate constant k0 (=
2.7 × 104 s−1) at low temperature to reform the ground state.
This triplet state also couples to two higher-energy triplets, each
of which is itself coupled to the ground state. The lower of these
two upper-lying triplets is accessed by passing over a modest
barrier (EA = 950 cm−1), while the rate constant for decay of
Table 1 Comparison of the electrochemical properties of 1, with and without added cation, with 2 and the parent complex. The value given in
parentheses refers to the splitting between anodic and cathodic peaks (in mV) and the number of electrons accompanying the electrode process is
reported as ne
OX
RED
Cmpd
Cation
E1/2 c/V vs. SSCEa
E1/2
d/V vs. SSCE
E1/2RED/V vs. SSCE
E1/2RED/V vs. SSCE
1
—
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.27 (60) 2e
1.31 (60) 2e
1.31 (60) 2e
1.27 (60) 1e
−1.11 (60) 2e
−0.94 (60) 1e
−0.90 (60) 1e
−1.00 (60) 2e
−0.97 (60) 2e
−0.97 (60) 2e
−0.94 (60) 2e
−1.05 (70) 2e
−1.04 (70) 2e
−1.06 (70) 2e
−1.04 (70) 2e
−1.06 (60) 2e
−0.99 (60) 2e
−1.27 (60) 1e
−1.33 (70) 1e
−1.28 (65) 2e
−1.26 (65) 2e
−1.27 (65) 1e
−1.09 (65) 1e
−1.25 (65) 1e
−1.06 (65) 1e
−1.26 (65) 1e
−1.14 (65) 1e
−1.27 (65) 1e
−1.12 (65) 1e
−1.42 (65) 2e
−1.23 (70) 1e
−1.51 (70) 1e
−1.44 (84) 2e
−1.50 (90) 2e
−1.52 (90) 2e
−1.41 (90) 2e
−1.41 (90) 2e
−1.40 (80) 2e
−1.37 (80) 2e
−1.44 (85) 2e
−1.44 (85) 2e
−1.48 (90) 2e
−1.48 (90) 2e
−1.57 (85) 1e
−1.43 (75) 2e
—
Zn2+a
Zn2+b
Ba2+a
Ba2+b
Mg2+a
Mg2+b
Li+
a
Li+
b
K+
a
K+
b
2
—
Zn2+
a
[Ru(terpy)2]2+
—
a Perchlorate salt, molar ratio 1 : 2. b Excess salt. c Refers to oxidation of the metal center. d Refers to reduction of the ligand. e All potentials are
reported in V vs. SSCE and using ferrocene as internal standard, +0.38 V vs. SSCE (DEp = 60 mV).
2 9 2 8
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2

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to reduce.39 There is a second two-electron reduction with E1/2
= −1.44 V vs. SSCE that can be assigned to the simultaneous
reduction of the capping terpy ligands. The difference between
these two E1/2 values (DE1/2 = 0.33 eV) is comparable to that
found for 2 (DE1/2 = 0.36 eV). A one-electron peak is situated
between the terpy-based reductions and corresponds to E1/2
=
−1.33 V vs. SSCE. This process is attributed to one-electron
reduction of the central bipyrimidine fragment. This assignment
allows for the fact that (i) bipyrimidine is easier to reduce than
bipyridine, (ii) reduction would be a one-electron step, (iii) the
ethynylene substituents would make reduction easier and (iv) no
other reductive site is obvious.
On the basis of this interpretation, it becomes clear that the
lowest-energy MLCT triplet state should be formed by selective
charge injection from the RuII cation into the ethynylated terpy
ligand. This would account for the red-shifted emission spectrum
relative to the parent complex. It is possible that extensive
electron delocalisation takes place at the triplet level since the
reduction potentials of the terpy and bipyrimidine groups are
not too dissimilar. Spectral analysis40 of the emission spectrum
recorded in fluid solution indicates that the reorganisation
energy accompanying decay of the lowest-energy MLCT triplet
state is ca. 600 cm−1 and that the triplet energy is ca. 14 800 cm−1.
Fig. 5 Spectroluminescence titrations carried out for addition of
zinc(II) perchlorate (᭹) and barium perchlorate (D) to 1 in acetonitrile
containing background electrolyte (0.1 M). The solid line drawn through
each data set corresponds to the best non-linear, least-squares fit to the
model described by Table 2.
to that of 1. Comparable stability constants were derived from
analysis of the corresponding absorption spectral titrations.
The results of a time-resolved emission spectroscopic study
were consistent with those derived from the above steady-
state measurements. Thus, addition of low concentrations of
zinc(II) perchlorate led to a gradual shortening of the lifetime
of the emitting species from 60 to 8 ns. Once the shorter
lifetime had been attained, the size of the signal decreased
with increasing concentration of added cation but the lifetime
remained constant. At high (>2 mM) concentrations of added
cation, emission could not be separated from scattered laser
light. Such behaviour indicates that the 1 : 1 complex has a
luminescence lifetime of ca. 8 ns whilst no emission is apparent
for the 1 : 2 (ligand : metal) complex. The precision of our set-up
is insufficient to resolve individual lifetimes of 60 and 8 ns and
we measure only the average value. The triplet lifetime of the 1 :
2 complex, measured by transient absorption spectroscopy with
detection at 632 nm in the presence of 5 mM Zn(ClO4)2, was
found to be 1.8 0.2 ns at room temperature.
Complexation of zinc(II)
[ML]
b11
b12
=
=
[M][L]
[M2L]
[M]2[L]
(2)
Addition of a two-fold molar excess of zinc(II) perchlorate
to a solution of 1 in acetonitrile results in a red shift of
4 nm and a small increase in absorptivity. Upon addition of
excess Zn(ClO4)2, there was a further red shift of 4 nm and a
modest loss of absorptivity. There is a corresponding decrease
in the luminescence intensity. Spectroluminescence titrations
carried out at constant ionic strength showed that the emission
decreased non-linearly with increasing cation concentration
(Fig. 5). A vector analysis41 indicated that three species were
needed to fit the entire data set. Taking into account the
absorption spectral data, and allowing for the known complexes
that form between bipyrimidine and zinc(II) cations,42 it can be
concluded that these species refer to 1 and the resultant 1 : 1 and
1 : 2 (ligand : metal) complexes.25 Least-squares fitting of the
titration data gave overall stability constants of log b11 = 3.41
and log b12 = 4.88 (Table 2). Further analysis of the luminescence
data indicated that the relative ULUM for the 1 : 1 complex was
15% that of 1 but the 1 : 2 species did not emit over the relevant
spectral range. Emission from the 1 : 1 complex was red shifted by
ca. 6 nm but otherwise retained similar spectral characteristics
The effect of added Zn2+ cations on the cyclic voltammograms
recorded for 1 in acetonitrile was also examined (Table 1). It was
observed that, whilst the presence of Zn2+ did not affect E1/2 for
oxidation of the RuII centres, the first reduction process occurred
at a less negative potential. In fact, the first step observed in
the presence of a two-fold molar excess of Zn(ClO4)2 displayed
the characteristic features43 of a quasi-reversible, one-electron
reduction process with E1/2 = −0.94 V vs. SSCE. There are
two further reduction steps, each corresponding to a quasi-
reversible two-electron reduction. On the basis of our earlier
assignment, it is suggested that the first reduction process
corresponds to addition of one electron to the coordinated
bipyrimidine group. The second step can then be attributed to
simultaneous reduction of the two ethynylated terpyridines with
Table 2 Parameters derived from analysis of the spectroluminescence titrations carried out with 1 in acetonitrile containing tetra-N-butylammonium
hexafluorophosphate (0.1 M) as ionic strength mediator
a
b
Cation
log b11
log b12
UREL11 (%) c
s11/ns d
UREL12 (%) e
s12/ns f
Zn2+
Mg2+
Ba2+
Li+
3.41
2.10
1.74
1.66
1.55
1.91
4.88
3.75
3.34
3.00
2.85
3.66
15
140
135
125
115
110
8
160
140
NA
NA
NA
<2
10
10
35
40
45
1.8
5
8
20
30
32
K+
H+
a Overall stability constant for formation of the 1 : 1 complex, 7%. b Overall stability constant for formation of the 1 : 2 complex, 18%.
c Luminescence yield for the 1 : 1 complex measured relative to 1 in the absence of added cation, 10%. d Luminescence lifetime of the 1 : 1 complex
measured in deoxygenated acetonitrile, 10%. e Luminescence yield for the 1 : 2 complex measured relative to 1 in the absence of added cation, 25%.
f Luminescence lifetime of the 1 : 2 complex measured in deoxygenated acetonitrile, 10%.
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2
2 9 2 9

View Article Online
E1/2 = −1.28 V vs. SSCE while the final step corresponds to
reduction of the capping ligands with E1/2 = −1.50 V vs. SSCE.
This interpretation requires that coordination of Zn2+ to
the vacant bipyrimidine group renders E1/2 some 0.4 eV more
positive. A similar change was found earlier for bipyridine21
(Table 1) and it is well established that reduction potentials of
poly(pyridines) depend markedly on the nature of any bound
cations. Prior reduction of the bipyrimidine ligand decreases
the electron affinity for the ethynylated terpy groups and causes
E1/2 to shift towards more negative potentials. It is likely that
reduction of these latter ligands will localise the added electron
and this would push E1/2 for the capping terpy ligands to a
slightly more negative value (Table 1). In the presence of excess
Zn(ClO4)2, the first reduction potential becomes slightly more
negative, E1/2 = −0.90 V vs. SSCE, but the other processes
remain essentially unaffected.
reaction is assigned to the simultaneous reduction of both
ethynylated terpyridine ligands. It is seen that this step is
made easier (DE1/2 = 0.11 eV) by attachment of Ba2+ to the
central bipyrimidine unit. The second reduction involves a quasi-
reversible one-electron process for which E1/2 = −1.27 V vs.
SSCE. This step can be assigned to reduction of the coordinated
bipyrimidine group and, again, it is seen that this process is
slightly assisted by cation binding (DE1/2 = 0.06 eV). It is likely
that the full effect of cation binding is not seen because of
prior reduction of the appended terpyridine ligand. The capping
terpy ligands are reduced with E1/2 = −1.41 V vs. SSCE. In the
presence of excess Ba(ClO4)2, the reduction potential attributed
to the coordinated bipyrimidine group moves to a less negative
potential. The limiting value corresponds to E1/2 = −1.09 V vs.
SSCE. We assign this latter process to the one-electron reduction
of the corresponding 1 : 2 complex.
Complexation of other cations
Electrospray mass spectral analysis
Several other cations, namely Ba2+, Mg2+, Li+, K+ and H+, were
found to coordinate to 1 in acetonitrile solution. As noted
above, 1 binds up to two cations in the form of 1 : 1 and 1 :
2 complexes and the derived stability constants are collected in
Table 2. Binding is relatively weak but both steps are clearly
resolved in the spectrophotometric titrations. Coordination of
a cation causes a small (e.g., 2 nm) red shift to both absorption
and emission spectra. It was notable that for these cations
the 1 : 1 complex is somewhat more luminescent than is 1
(Table 2). This effect is most evident for the addition of barium
or magnesium perchlorate (Fig. 5). In all cases, the 1 : 2
species is less luminescent than the non-coordinated species.
Attempts to follow the binding of Ba2+ cations to 1 by isothermal
calorimetry and 13C NMR were unsuccessful. Small shifts were
observed for certain NMR peaks but these were insufficient
for quantitative measurements. The calorimetry results were
made irreproducible by the solvent attacking the rubber seals.
It was noted, however, that 1 facilitates the entry of potassium
picrate into CH2Cl2 from water. No such extraction occurs in
the absence of 1.
Electrospray mass spectra (ES-MS) were recorded in acetonitrile
at low extraction cone voltage (Vc = 40V). This analysis indicates
that the dinuclear Ru(II) complex 1 is weakly associated with
−
its PF6 counteranion, exhibiting a major molecular peak at
334.4 corresponding to the [1–4PF6]4+ cation and a minor peak
at 495.4 due to the [1–3PF6]3+ species. Upon decreasing the
concentration from 0.15 mM to 0.015 mM, this latter peak
disappeared, indicating dissolution of the ion pair. Addition of
sub-stoichiometric amounts of Zn(ClO4)2·6H2O to 1 in CH3CN
(0.15 mM; 1/Zn < 1) does not perturb significantly the spectra.
Increasing the molar ratio of Zn forms a variety of polynuclear
complexes that can be attributed to coordination of added cation
to 1. At high concentrations of cation (Zn/1 > 5), four additional
peaks are clearly identified at 291.1, 308.5, 400.0, and 465.5
corresponding respectively to the [1–4PF6 + 2Zn + 2ClO4 +
2CH3CN]6+, [1–4PF6 + Zn + ClO4 + CH3CN]5+, [1–4PF6 +
Zn + 2ClO4]4+, [1–4PF6 + 2Zn + 4ClO4]4+ species. Upon ten-
fold dilution of the same solution, the situation becomes more
complicated due to association of Zn with various anions present
in the solution. In trying to identify the most likely assemblies,
we note that addition of 10 equivalents of tetrabutylammonium
chloride to an acetonitrile solution containing 1 and 5 equiv.
of Zn(ClO4)2 at a concentration of 0.015 mM, the spectra
gave a major peak at 368.1 corresponding probably to the [1–
4PF6 + Zn + 2Cl]4+ cation. This interesting observation can
be understood in terms of the strong affinity of Zn cations
for chloride under these conditions (high dilution and absence
of supporting electrolyte) that stabilizes the complex in the
form of a neutral [ZnCl2(bipyrimidine)] fragment. Under these
conditions the di-zinc species is not observed.
Time-resolved luminescence studies made in the presence
of excess cation showed that complexation causes a modest
decrease in the lifetime of the emitting state (Table 2). Under
these conditions it is likely that the 1 : 2 species predominates.
For both K+ and Li+, the triplet lifetime is essentially halved
whereas a much shorter lifetime results from coordination of
Ba2+ or Mg2+ cations. With the divalent cations, the time-resolved
emission studies showed that the presence of low concentrations
of cation causes a pronounced increase in sLUM (Table 2). At
intermediate concentration of added cation, the time-resolved
decay records are best analysed in terms of dual-exponential
kinetics:
For Ba2+ and Mg2+, qualitative ES-MS studies also confirm the
existence of complex 1 associated with these cations. However,
the situation is complicated by kinetic problems and by hydrol-
ꢂ
ꢃ
ꢂ
ꢃ
t
s11
t
s12
I(t) = A11 exp
−
+ A12 exp
−
(3)
−
ysis of the PF6 anion imported by the starting Ru complex.
At high concentration and high ratio of the salts (typically in
CH3CN/H2O 4/1 v/v, 0.2 mM; Ba or Mg/1 >10) the following
peaks are clearly identified at 264.4 and 370.1, corresponding
respectively to the [1–4PF6 + 2Mg + ClO4 + F + 2CH3CN]6+
and [1–4PF6 + Mg + ClO4 + F]4+ cations and at 323.4 and
418.2, corresponding respectively to the [1–4PF6 + Ba + ClO4 +
CH3CN]5+, [1–4PF6 + Ba + 2ClO4]4+ cations. We notice that
the di-Ba complex is not observed under these experimental
conditions and that upon dilution by a factor of ten-fold the
peaks corresponding to the supramolecular Ba complex vanish
in favour of the starting complex 1. These qualitative ES-MS
studies are in keeping with the stability constants measured by
spectrophotometric titrations (Table 2) which clearly highlight
the higher affinity of complex 1 for Zn versus Mg and Ba.
Calculated values corresponding to the monoisotopic m/z peak
of the isotopic distribution and containing only the higher
isotopes of each element are given in ref. 44.
Here, s11 and s12 refer, respectively, to the emission lifetimes of
the 1 : 1 and 1 : 2 complexes whilst A11 and A12 are their respective
amplitudes. The averaged lifetimes derived for the 1 : 1 and 1 :
2 species are collected in Table 2. With increasing concentration
of Ba2+ or Mg2+, the significance of the shorter-lived component
(A12) increased but without affecting the measured lifetimes. This
behaviour is a clear indication for the formation of complexes
of different stoichiometry. It was not possible to make the same
analysis for the univalent cations but, even so, the lifetimes of
the 1 : 2 complexes could be obtained in the presence of excess
salt (Table 2).
The presence of a two-fold molar excess of Ba(ClO4)2 affects
the reductive processes seen in the cyclic voltammograms with-
out perturbing E1/2 for the RuII centres (Table 1). In particular,
the first reduction step corresponds to a quasi-reversible two-
electron process43 with E1/2 = −1.00 V vs. SSCE. This electrode
2 9 3 0
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2

View Article Online
Effect of the bound cation
bridging ligands and for the 1 : 1 complexes formed between 1
and cations such as Ba2+, Mg2+, Li+, K+ or H+.
This work has shown that cation binding affects the photophys-
ical and electrochemical properties of the luminophore. Similar
behaviour has been observed previously, notably with bipyridine
as the bridging ligand where coordination leads to an increase
in emission from the Ru-terpy based terminals.21 The origin of
this effect has not been explained in any detail. It is notable
that whereas the 1 : 1 complex formed with Zn2+ is somewhat
less luminescent than 1, the 1 : 1 complexes formed with other
cations are slightly more emissive. In all cases, the 1 : 2 complexes
are less emissive than 1. It is assumed that the added cations bind
at the bipyrimidine N atoms, as is known from X-ray diffraction
studies with simpler molecular fragments.25 Clear experimental
evidence for the stoichiometry of the resultant complexes was
established by electrospray mass spectroscopy. Certain cations,
such as Zn2+, are likely to bind across the bipyrimidine group but
other, notably Li+, will bind to a single N atom. Cation binding
raises the reduction potential for the bipyrimidine unit to a less
negative value, according to the nature of the added cation.
There are additional perturbations of the reduction potentials
for the ethynylated terpy ligands.
Except for complexation with zinc perchlorate, there is no
real change in the absorption spectrum upon coordination of
the cation such that the radiative rate constant is unlikely to
change significantly. Likewise, there are no obvious changes
in the luminescence spectrum, other than slight spectral shifts
of <10 nm. We can conclude, therefore, that cation binding
does not induce a change in the nature of the emitting species,
which is the lowest-energy MLCT triplet state associated with
a Ru-terpy terminal. The modified luminescence yields and
In an effort to justify this hypothesis, the triplet lifetime of
the 1 : 1 complex formed between 1 and Mg2+ in butyronitrile
was studied as a function of temperature. It was observed
that kNR decreased progressively with decreasing temperature
in accordance with eqn (1). A non-linear least-squares fit of the
experimental data (Fig. 4) gave k0 = 2.4 × 104 s−1; which is
within the experimental error of the value found for 1. Likewise,
the barrier to reaching the upper-lying MLCT triplet (EA
=
1,180 cm−1) and the rate constant for decay of this state (kA
=
5.2 × 107 s−1) remain comparable to those determined for 1
under the same conditions. There is a more significant effect on
the activation energy for reaching the MC state, where EB
=
2,935 cm−1, although the combined rate constant for population
of the MC state from the two MLCT triplets (k2 = 1.0 ×
1013 s−1) is not greatly affected by cation binding. The difference
in activation energies between 1 and its Mg2+ complex (DEB =
155 cm−1) is sufficient to account for the change in triplet lifetime.
However, since neither k nor the actual energy gap between MC
and MLCT triplets is known, it is not possible to attribute the
alteration of EB solely to a change in DG◦.
In terms of the above explanation, attachment of Zn2+ cations
to 1, or formation of the corresponding 1 : 2 complexes, should
cause a substantial prolongation of the triplet lifetime at room
temperature. This is contrary to the experimental findings,
where sLUM decreases in these cases. Under these conditions,
cation binding must introduce a new quenching route. Since
coordination raises the reduction potential for the bipyrimidine
ligand to a less negative value, we suggest that this additional
quenching process is associated with this phenomenon. The
most likely possibility is that cation coordination lowers the
energy of the intramolecular charge-transfer (ICT) state formed
by charge injection from the RuII centre to the bipyrimidine unit.
This ICT state lies at higher energy than the emitting MLCT
triplet for 1 and is unlikely to figure in the overall photophysics.
However, attaching a single Zn2+ cation reduces the energy of
the ICT such that it lies slightly above that of the emitting
triplet state. These two states will be in thermal equilibrium at
lifetimes, therefore, must reflect cation-induced changes in kNR
.
Unfortunately, as is apparent from eqn (1), the magnitude of kNR
is set by a variety of kinetic and thermodynamic factors whilst k0
is most likely determined by the energy-gap law.45 In attempting
to unravel the effects of bound cations on the photophysics
of 1 we make three basic assumptions: (1) The main effect of
the cation cannot be related to changes in k0, since the triplet
lifetimes vary at room temperature where the activated processes
dominate.46 (2) Cation binding induces an electronic effect, as
observed for the change in reduction potential. (3) No single
effect can be responsible for all the observations made across
the range of complexes.
First, we consider the increase in ULUM (and sLUM ) found
for most of the 1 : 1 complexes: Identical behaviour has been
observed with bipyridine-based bridging ligands and seems to be
the norm.21 The general effect can easily be explained in terms of
eqn (1), simply by raising one (or both) of the activation energies
and/or decreasing one (or both) of the activated rate constants.
Since cation binding has little obvious effect on the properties
of the lowest-energy MLCT triplet state it is unreasonable to
suppose that there is a significant perturbation of the upper-
lying MLCT triplet. As such, we might expect both EA and k1 to
remain fairly independent of the state of coordination around
the bipyrimidine group. However, population of the MC state
involves electron transfer from the ethynylated terpy p-radical
anion to the eg orbital of the RuIII centre.47 The activation energy
for this process, EB, can be expressed in terms of eqn (4)
room temperature such that the measured triplet lifetime (sLUM
=
8 ns) refers to decay of the equilibrated mixture.49 The ICT state
appears to be non-emissive and, in order to affect the lifetime
of the MLCT triplet, it must couple directly to the ground state.
For the other 1 : 1 complexes, the energy gap between ICT
and MLCT triplets lies between those evaluated for 1 and the
1 : 1 Zn2+ complex so that population of the ICT state will be
relatively unimportant.
Formation of the ICT state can be considered in terms of an
intramolecular charge-shift reaction and this process becomes
highly favourable for the 1 : 2 complex formed with Zn2+. The
equilibrium mixture at room temperature will be dominated by
the ICT state and the observed triplet lifetime of 1.8 ns can
be attributed to this transient species. Decay of the ICT state
must involve charge recombination to reform the ground state.
The decay rate constant is insensitive to changes in temperature
over a reasonable range (Fig. 4), in marked contrast to that
of 1. For the 1 : 2 complexes formed with other cations
the MLCT and ICT states are expected to reside in thermal
equilibrium at ambient temperature48 and the measured triplet
lifetime, therefore, corresponds to decay of the mixture. The
actual distribution of triplet states, in any given case, depends
on the stoichiometry, geometry and electronic properties of the
complex.
ꢄ
ꢅ
(k + DG0)2
4k
EB =
(4)
where k is the reorganization energy and DG◦ is the change in
Gibbs free energy.48 Because cation binding affects the reduction
potential of the ethynylated terpy, there will be a decrease in DG◦
for intramolecular charge transfer. Provided DG◦ is positive, as
seems most likely,26 and ignoring any off-setting change in k,
cation binding will raise EB and thereby decrease kNR at ambient
temperature. This situation will hold also for bipyridine-based
Acknowledgements
We thank the EPSRC, the CNRS, the University of Newcastle
and the Universite´ Louis Pasteur de Strasbourg for financial
support.
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2
2 9 3 1

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2 9 3 2
D a l t o n T r a n s . , 2 0 0 5 , 2 9 2 5 – 2 9 3 2