Tuning the excited-state energy of the organic chromophore in bichromophoric systems based on the RuII complexes of tridentate ligands

Article abstract of DOI:10.1002/chem.200601376

A series of new heteroleptic and homoleptic Ru^II complexes containing variously substituted bis(pyridyl)triazine ligands has been prepared and their absorption spectra, redox behaviour and luminescence properties (both in fluid solution at room

Full text of DOI:10.1002/chem.200601376

FULL PAPER
DOI: 10.1002/chem.200601376
Tuning the Excited-State Energy ofthe Organic Chromophore in
II
Bichromophoric Systems Based on the Ru Complexes ofTridentate Ligands
[
a]
[a]
[b]
Elaine A. Medlycott, Garry S. Hanan,* FrØdØrique Loiseau, and
[
b]
Sebastiano Campagna*
Abstract: A series of new heteroleptic
and homoleptic Ru complexes con-
taining variously substituted bis-
features belonging to specific subunits,
thus suggesting that these species can
be regarded as multicomponent, supra-
molecular assemblies from an electron-
ic coupling point of view. Whereas
most of the complexes exhibit lumines-
cence properties that can be attributed
For example, 2d (the anthryl-contain-
ing heteroleptic metal compound) ex-
hibits MLCT emission at room temper-
ature and emission from the anthra-
cene triplet at 77 K; 2e (the bromo-
substituted anthryl-containing hetero-
leptic metal compound) exhibits an-
thryl-based emission at 77 K and
MLCT emission at room temperature,
but with a prolonged lifetime, thus sug-
gesting equilibration between two trip-
let states that belong to different chro-
mophores. The equilibration regime
between MLCT and aromatic hydrocar-
bon triplet states is therefore reached
by suitable substitution on the organic
chromophore.
II
A
C
H
T
R
E
U
N
G
(pyridyl)triazine ligands has been pre-
pared and their absorption spectra,
redox behaviour and luminescence
properties (both in fluid solution at
room temperature and in a rigid matrix
at 77 K) have been investigated. For
some compounds, X-ray structures
have also been determined. The new
to
metal-to-ligand
charge-transfer
(MLCT) states involving the metal-
based subunit(s), the species containing
the anthryl and, even more, the bromi-
nated anthryl chromophores exhibit
complicated luminescence behaviour.
bis(pyridyl)triazines incorporate addi-
A
C
H
T
R
E
U
N
G
tional chromophores, such as biphenyl,
phenanthrene, anthracene and bro-
II
moanthracene derivatives, so the Ru
Keywords: cyclic voltammetry
emission spectroscopy multi-
chromophoric effect · ruthenium ·
triazine ligands
·
species can be considered as multichro-
mophoric species. The absorption spec-
tra and redox properties of all the
metal complexes have been assigned to
·
AHCTREUNG
AHCTREUNG
2
+
Introduction
The most studied complexes are based on [Ru
bpy=2,2’-bipyridine), as they have been shown to have
long-lived excited states at room temperature that originate
A
C
H
T
R
E
U
N
G
(bpy) ]
3
(
II
Ru complexes of polypyridine ligands have been intensely
researched as a result of their excellent photophysical prop-
erties and promising applications as photoactive devices.
[
3–5]
from their metal-to-ligand charge-transfer (MLCT) state.
[1,2]
However, bpy-based complexes induce chirality at the metal
centre, and separation of the diastereomers in polymetallic
complexes, although possible, can be complicated, thus at-
[6]
[
a] E. A. Medlycott, Prof. Dr. G. S. Hanan
DØpartement de Chimie
UniversitØ de MontrØal
tention has been turned to achiral tridentate 2,2’:6’,2’’-terpyr-
[7]
idine (tpy)-type ligands. Although tpy is a synthetically ap-
pealing target, on coordination to the metal ion, the triden-
tate ligand imposes a small bite angle and consequently its
2
900 Edouard Montpetit, MontrØal, QC, H3T1J4 (Canada)
Fax : (+1)514-343-2468
E-mail: garry.hanan@umontreal.ca
[8]
ligand-field strength is lower than that of bpy. This behav-
iour allows thermal access to low-lying non-emissive metal-
[
b] Dr. F. Loiseau, Prof. Dr. S. Campagna
Dipartimento di Chimica Inorganica
Chimica Analitica e Chimica Fisica, Università di Messina
Via Sperone 31, 98166 Messina (Italy)
Fax : (+39)090-393-756
centred (MC) states, and the MLCT excited-state is short
2+ [9]
lived (0.25 ns for [Ru(tpy) ] ). Many strategies have been
A
H
R
U
G
2
used to prolong excited-state lifetimes of complexes with tri-
dentate ligands, with most focusing on the manipulation of
the energies of non-emissive MC states relative to emissive
E-mail: campagna@unime.it
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[10,11]
MLCT states.
One such strategy makes use of electron-
Chem. Eur. J. 2007, 13, 2837 – 2846
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2837

deficient triazine ligands, which function to lower the energy
which the MLCT level is the lowest-energy state of the two
states involved in the equilibration process.
3
[18]
of the MLCT state relative to the metal-centred state
[12,13]
(
Scheme 1).
For this type of complex, excited-state life-
The structural formulae for the studied compounds are
shown in Scheme 2. A bromo substituent in the 4-position
of the pendant phenyl ring allows a “chemistry-on-the-com-
plex” approach to introduce additional organic components
times were extended to 8 ns at room temperature, thirty-
times greater than that of the parent complex.
[16,19–23]
with varying triplet-state energies.
Results and Discussion
Synthesis: Two approaches were applied to the synthesis of
complexes 2a–e and 3a–e. We previously reported a classi-
cal synthetic procedure for the preparation of 3c, the
“
ligand-synthesis” approach, in which the phenanthryl group
was introduced onto the triazine ligand prior to complexa-
[31]
tion. Alternatively, the brominated ligand L1 can be coor-
II
dinated to the Ru centre, thus affording an ideal precursor
for the “chemistry-on-the-complex” approach and enabling
the introduction of organic components using cross-coupling
[16,19,20,23,32–35]
reactions.
To evaluate the effectiveness of the “chemistry-on-the-
complex” approach, we set out to synthesise complexes 2b–
e and 3b–e in this fashion (Scheme 3). Thus, complex 2a
was synthesised by the addition of L1, prepared according
Scheme 1. Heteroleptic complex that incorporates a central triazine ring
2
+ [13]
A
H
R
U
G
2
relative to the parent complex [Ru(tpy) ] .
[27]
An alternative strategy has made use of the “multichro-
to a previously reported procedure,
to a suspension of
[14]
[28]
mophoric approach”, which relies on the presence of ad-
ditional chromophore(s) with a non-emissive, long-lived
one equivalent of [RuCl (tpy)]
A
H
R
U
G
and three equivalents of
3
AgNO in ethanol. After heating at reflux for four hours,
3
3
[14,15]
triplet state of comparable energy to the MLCT state.
the resulting dark solution was cooled, filtered to remove
AgCl and dried under reduced pressure. The residue was
solubilised and a precipitate formed on addition to aqueous
NH PF . The precipitate was collected and purified by
Reversible population of the triplet excited state localised
on the additional chromophore(s) can occur, thereby delay-
ing emission from the complex and prolonging the MLCT
excited-state lifetime, with the organic triplet behaving as an
excited-state storage element for MLCT emission. Initially,
4
6
column chromatography using acetonitrile/KNO (saturated)
3
(7:1) as the eluent. The second intense red band was collect-
II
ꢀ
this approach was used for Ru complexes of bidentate bi-
ed and was precipitated as its PF6 salt.
[15]
pyridine-type ligands and was later applied to tridentate
It was found that a one-pot synthesis of 3a was more suc-
cessful than the two-step approach, which would involve the
[
16,17]
tpy-type systems as well.
phore triplet is lower in energy than the triplet MLCT level,
Usually, the organic chromo-
formation of a [RuCl (L1)] intermediate. Two equivalents of
3
[17]
however, in rare examples the reverse is obtained,
al-
the ligand were added to one equivalent of [RuCl ]·3H O
3
2
though this makes the multichromophoric effect less effi-
cient.
and three equivalents of AgNO in N,N-dimethylformamide
3
(DMF). The reaction mixture was stirred in DMF at room
temperature for 20 min, heated to reflux for 1 h, cooled and
purified as for 2a. Complexes 2b–d and 3b–d were synthes-
ised by using the palladium-catalysed Suzuki–Miyaura reac-
tion in degassed DMF by adapting known procedures
In all the multichromophoric species investigated so far, it
is the MLCT state energy that has been modified, by poly-
pyridine ligand substitution or selection, to fit the suitable
organic chromophore triplet energy. Herein, we exploit the
possibility of using substituents on the organic chromophore
to activate the multichromophoric behaviour. As suitable
[16,29]
(Scheme 3).
cipitation in aqueous NH PF and then isolated by column
The complexes could be worked up by pre-
4
6
II
systems, we selected a family of Ru triazine-based com-
chromatography on silica gel with MeCN/KNO (saturated)
3
plexes coupled with variously substituted anthracene or
phenanthrene subunits. In such cases, the Ru-to-triazine CT
level can be lower in energy than either the phenanthrene
or anthracene triplets, so that the multichromophoric behav-
iour is not active in the simplest compounds. However, in-
troduction of the bromo substituent on anthracene lowers
the energy of the anthracene components and the multichro-
mophoric behaviour can take place. Therefore, the systems
investigated herein belong to the much less explored case in
(9:1) as the eluant. Counterion exchange to form the PF6
salts afforded 2b–d and 3b–d.
The synthesis of 2e was attempted by using the “chemis-
try-on-the-complex” approach; however, the reaction pro-
ceeded slowly and the purification of 2e was difficult. An al-
ternative approach introduced the bromoanthracene sub-
stituent into the ligand prior to complexation in a similar
manner to the approach previously employed for the synthe-
[31]
sis of 3c. A Suzuki–Miyaura reaction afforded p-(10-bro-
2838
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 2837 – 2846

Multichromophoric Systems in Bis(pyridyl)triazine–Ru Complexes
A
H
R
U
G
FULL PAPER
Scheme 2. Heteroleptic complexes 2a–e and homoleptic complexes 3a–e.
moanthryl)benzonitrile, thus modifying a previously report-
ed procedure to monosubstitute 9,10-dibromoanthracene
X-ray crystallography: The solid-state structures of L2 and
complexes 2a, 2b and 3a were studied by X-ray diffraction
(Table 1 and Figures 2 and 3). Y-ray quality crystals of L2
were obtained by slow diffusion of hexane into a concentrat-
ed solution of L2 in CHCl3.
[30,36]
using 4-cyanophenylboronic acid.
p-(10-Bromoanthryl)-
benzonitrile could then be used to synthesise the triazine
[
27,31]
ligand by employing the same procedure as used for L1.
1
The H NMR spectra for 3d and 3e in CD CN at
The ligand crystallises in the orthorhombic space group
P2 2 2 with four molecules in the unit cell. The two pyridyl
3
400 MHz are shown in Figure 1. The bromo substituent has
1
1 1
little effect on the chemical shift of the pyridyl protons (la-
belled in Figure 1), but the signals for the anthracene-based
protons in 3e are significantly deshielded with respect to
those in 3d. The disappearance of the anthracene-based sin-
glet at the 10-position probes the bromo substitution of the
anthracene moiety.
and phenyl rings twist away from the triazine ring by
15.6(2), 14.7(2) and 7.0(2)8, respectively. The pyridyl rings
are twisted twice as much as the phenyl ring as a result of a
combination of p-stacking effects between neighbouring
molecules and the NꢀN lone-pair repulsions between the
pyridyl N atoms and triazine-based N atoms. The ligand
molecules aggregate through continuous p-stacking interac-
tions in the extended lattice between the anthryl tail of one
Chem. Eur. J. 2007, 13, 2837 – 2846
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2839

G. S. Hanan, S. Campagna et al.
Scheme 3. Synthesis of complexes 2a–e and 3a–e. solv.=solvent.
molecule and the pyridyl head of the neighbouring molecule
plexes in acetonitrile. Complex 2a crystallised in the triclinic
ꢀ
¯
(
Figure 2). The centroid–centroid distances between the cen-
space group P1 with two cations, four PF ions and four
6
tral anthryl ring and the pyridyl rings above and below the
plane are 3.30 and 4.48 Š, respectively.
Crystals of 2a and 3a were obtained by slow diffusion of
isopropyl ether into a concentrated solution of the com-
molecules of acetonitrile in the asymmetric unit. The pend-
ant phenyl rings are twisted by 7.6(2) and 9.5(2)8 for both
independent cations (Table 1). The near coplanar configura-
tions are due to intramolecular hydrogen-bonding interac-
2840
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 2837 – 2846

Multichromophoric Systems in Bis(pyridyl)triazine–Ru Complexes
A
H
R
U
G
FULL PAPER
1
3
Figure 1. H NMR spectra of complexes 3d (top) and 3e (bottom) at 400 MHz in CD CN.
It can be noted that the tridentate bite angles N-M-N in
the triazine-based ligands are smaller (155.5–155.68) than
those in the tpy-based ligands (157.4–158.28). The additional
N atoms in the triazine ring distort the ring angles away
from the ideal 1208 in benzene significantly more than in
the central pyridyl ring of tpy. This behaviour will have a
negative effect on the ligand-field strength, thereby facilitat-
ing thermal access of metal-centred states (see below).
Complex 3a crystallises in the monoclinic space group
II
II
II
C2/c and is isostructural to the reported Fe , Co and Cu
complexes of L1.
[27,39]
Short bromine–bromine contacts are
observed in the solid state, and these interactions are small-
er than the sum of the Van der Waals radii (3.48 Š). In the
absence of stabilizing p-stacking interactions, these contacts
influence the cationic arrangements in the extended lattice.
Figure 2. X-ray crystal structure of L2 (left) as an ORTEP representation.
Thermal ellipsoids are set at the 50% probability level. The p-stacking,
face-to-face interactions are shown (right).
ꢀ
Efforts to obtain crystals of 2b as the PF6 salt were un-
successful under a variety of solvent conditions. However,
the addition of an excess of ammonium tetraphenylborate
to a solution of 2b in acetonitrile allowed salt exchange, and
the complex was crystallised as a mixed salt containing one
ꢀ
ꢀ
tions between the lone pairs on the triazine atoms N4 and
PF6 and one BPh4 ion per cation. The RuꢀN bond lengths
N5 and the CꢀH bond on the phenyl substituent. The slight
and N-Ru-N bond angles are very similar to those of 2a,
thus indicating that the substitution on the periphery has
little effect on the metal centre. The phenyl ring closest to
the triazine ring is twisted by 4.6(2)8 in a near coplanar in-
teraction. The second phenyl ring is twisted by 26.4(2)8
about the linking CꢀC bond, which is consistent with the
twist is imposed by edge-to-face interactions of the phenyl
ring with the pyridyl ring of a neighbouring cation (H···cent-
roid: 2.86 and 3.19 Š).
The RuꢀN bond lengths and N-Ru-N bond angles are re-
ported in Table 1. Longer RuꢀN bond lengths to the pyridyl
II
rings for both the triazine and tpy ligands are observed and
are a result of the constrained bite angle imposed by the ter-
twist reported for Ru complexes of biphenyl–tpy-type
[37]
systems.
Edge-to-face interactions between the biphenyl
[
37]
dentate ligands.
The RuꢀN
A
H
R
U
G
tails promote the twists about the interannular CꢀC bond.
The inner phenyl-ring face has a short contact with the
outer phenyl-ring edge of a neighbouring cation (H···cent-
roid: 2.71 Š). An additional edge-to-face interaction is
observed with a short contact of the face of the outer
phenyl ring with a neighbouring pyridyl ring (H···centroid:
3.10 Š).
N10) is shorter than the RuꢀN7
AHCTREUNG
This finding is consistent with that previously reported for
triazine-based complexes and is presumably a result of the
improved p-acceptor properties of the electron-deficient tri-
[13]
azine ligand. The RuꢀN
with those previously reported for [Ru(tpy) ] .
2
A
H
R
U
G
2
+ [38]
Chem. Eur. J. 2007, 13, 2837 – 2846
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2841

G. S. Hanan, S. Campagna et al.
ference between the cathodic
and anodic peak potentials in
Table 2, none of the processes
are truly reversible, a fact con-
firmed in most cases by the I /I
c
a
[
41]
ratios.
e all have a single one-elec-
tron oxidation assigned to the
Complexes 2a–c and
2
III
II
Ru /Ru couple as noted for
[13]
similar complexes.
Complex
2
d has two oxidative processes,
the first is assigned to an an-
thracene-based oxidation and
III
II
the second to
a
Ru /Ru
couple (Figure 4). The irreversi-
bility of both processes is con-
firmed by the difference be-
tween the cathodic and anodic
peak potentials and the I /I
c
a
III
II
ratios. The Ru /Ru couple is
at approximately the same po-
tential as in the other com-
plexes, thus indicating that the
anthracene component is elec-
tronically separated from the
Figure 3. X-ray crystal structure of 2a (left), 2b (central) and 3a (right) as ORTEP representations. Thermal
ellipsoids are set at the 50% probability level. The anions and H atoms have been omitted for clarity.
Table 1. Selected bond lengths and angles from the X-ray structural analysis of 2a, 2b and 3a.
2
+ [a]
2
a
2b
3a
A
H
R
U
G
2
[Ru(tpy) ]
bond lengths [Š]
II
Ru metal centre. Introduction
MꢀN1
2.089 (4)
1.957 (4)
2.101 (4)
2.065 (4)
1.986 (4)
2.075 (4)
2.106 (4)
1.966 (4)
2.083 (4)
2.081 (4)
1.985 (4)
2.067 (4)
2.084 (4)
1.955 (3)
2.081 (4)
2.084 (4)
1.985 (3)
2.054 (4)
2.096 (5)
1.972 (5)
2.085 (5)
2.07 (1)
1.99 (1)
2.05 (1)
2.09 (1)
1.96 (1)
2.07 (1)
of the bromo substituent signifi-
cantly affects the oxidation po-
tential of the anthracene
moiety. The first irreversible ox-
idation of 2e corresponds to
MꢀN2
A
C
H
T
R
E
U
N
G
(trz)
M
ꢀ
N3
MꢀN6
MꢀN7
MꢀN8
bond angles [8]
N1-M-N3
III
II
[
[
b]
c]
[b]
[c]
[b]
[c]
the Ru /Ru couple, and no
subsequent anthracene oxida-
tions are observed up to
155.60 (15)
157.34 (16)
7.60 (24)
155.7 (2)
157.6 (2)
9.47 (24)
155.60 (16)
158.08 (16)
4.60 (22)
155.45 (19)
14.71 (47)
158.1 (4)
158.2 (4)
N6-M-N8
trz–phenyl twist
phenyl–phenyl twist
26.35 (24)
+
2.0 V.
[
a] Data from reference [38]. [b] N-M-N tridentate bite angle for the triazine ligand. [c] N-M-N tridentate bite
All the heteroleptic com-
angle for the tpy ligand.
plexes 2a–e have three one-
electron reductions, which are
assignable by comparison with
[
13]
Electrochemistry: Cyclic voltammetric studies of the com-
plexes were performed (Table 2). As can be seen by the dif-
previously reported results. The first reduction is triazine-
based and is observed at a less negative potential than in
[
a]
Table 2. Half-wave potentials for 2a–d and 3a–d.
E
1/2 (ox.)
E
1/2 (red.)
ꢀ1.62 (107, 0.7)
2
2
2
2
2
3
3
3
3
3
a
b
c
1.43 (79, 0.9)
1.40 (71, 0.9)
1.45 (80, 1.1)
1.29 (64, 1.2)
1.43 (91, 0.8)
1.53 (76, 0.6)
1.53 (82, 1.0)
1.53 (70, 1.1)
1.32 (42, 0.9)
1.38 (80, 1.2)
1.31 (60)
ꢀ0.75 (64, 1.1)
ꢀ0.78 (53, 1.3)
ꢀ0.81 (82, 0.9)
ꢀ0.77 (48, 1.8)
ꢀ0.77 (74, 1.2)
ꢀ0.71 (50, 1.2)
ꢀ0.78 (84, 0.8)
ꢀ0.73 (53, 1.0)
ꢀ0.74 (48, 0.9)
ꢀ0.71 (82, 0.9)
ꢀ1.23 (70)
ꢀ1.37 (68, 1.0)
ꢀ1.39 (66, 1.1)
ꢀ1.45 (90, 0.8)
ꢀ1.39 (62, 1.0)
ꢀ1.40 (79, 1.2)
ꢀ0.87 (55, 1.3)
ꢀ0.97 (50, 0.7)
ꢀ0.89 (45, 1.0)
ꢀ0.91 (50, 1.2)
ꢀ0.86 (73, 1.2)
ꢀ1.47 (69)
ꢀ1.65 (82, 1.0)
ꢀ1.71 (90, 0.9)
ꢀ1.64 (70, 1.6)
ꢀ1.66 (80, 1.1)
ꢀ1.51 (70, 1.2)
ꢀ1.58 (50, 0.9)
ꢀ1.55 (77, 1.2)
ꢀ1.55 (64, 1.0)
ꢀ1.43 (irr, 0.06)
d
e
a
b
1.43 (98, 0.9)
ꢀ1.71 (128, 0.7)
ꢀ1.79 (138, 1.2)
ꢀ1.78 (72, 1.1)
ꢀ1.80 (61, 0.9)
[
b]
c
d
e
1.61 (173, 1.1)
1.57 (87, 1.2)
2
+ [c]
[
A
C
H
T
R
E
U
N
G
2
Ru(tpy) ]
[
a] Potentials are given in volts and are versus the saturated calomel electrode (SCE) for solutions in acetonitrile, 0.1m in tetra-n-butylammonium hexa-
ꢀ
1
fluorophosphate (TBAP), recorded at 25ꢁ18C at a sweep rate of 200 mVs ; the difference between the cathodic and anodic peak potentials (millivolts)
and I /I ratios are given in parentheses. ox.=oxidation, red.=reduction. [b] Data from reference [31]. [c] Data from reference [40].
c
a
2842
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 2837 – 2846

Multichromophoric Systems in Bis(pyridyl)triazine–Ru Complexes
A
H
R
U
G
FULL PAPER
6
Figure 4. Cyclic voltammogram of 2d in acetonitrile with 0.1m TBAPF , with magnified view of the first oxidation process on the right.
2
+
the parent complex [Ru
due to the ease of reducing the
electron-deficient bis
triazine ligands relative to
A
T
E
N
(tpy) ]
Table 3. Spectroscopic and photophysical data of the complexes in solutions of deaerated acetonitrile (298 K)
or in a butyronitrile rigid matrix (77 K).
2
[
24]
A
H
R
U
G
Luminescence at
98 K
Luminescence
at 77 K
t [ns] lmax [nm] t [ms]
2
ꢀ
1
ꢀ1
l
max [nm] (e [m cm ])
l
max [mm]
l
[
13]
tpy.
The second reduction is
tpy-based and compares well
with the reduction of the parent
ꢀ
4
4
2
2
2
a
b
c
282 (53.5); 301 (59.6) 476 (21.7)
739
733
734
1.2”10
1.0”10
0.9”10
12
10
13
693
695
694
1.2
1.9
2.0
ꢀ
273 (59.0); 301 (52.2) 332 (48.6); 478 (27.7)
254 (83.1); 273 (79.9) 299 (73.2); 331 (34.3) 477
(26.3)
ꢀ4
2
+
complex [Ru
A
C
H
T
R
E
U
N
G
(tpy) ] . The third
2
ꢀ
4
4
reduction is assigned to
a
2d
254 (121.4); 282 (55.7) 299 (56.5); 364 (14.4) 383
735
738
1.0”10
0.5”10
13
54
694
695
3400
3400
(
12.3); 477 (25.7)
258 (59.3); 272 (35.7) 281 (36.1); 298 (34.6) 330
sh); 357 (9.2) 377 (9.1); 397 (8.3) 478 (15.4)
second one-electron reduction
on the triazine ligand.
Homoleptic 3a and 3b have
ꢀ
2
3
e
a
(
ꢀ5
279 (50.9); 295 (50.6) 491 (27.5)
276 (64.7); 339 (49.6) 501 (26.5)
206 (86.1); 253 (85.6) 274 (74.7); 328 (sh) 491
714
–
717
<10
2
–
6
676
682
678
2.0
2.2
2.2
III
II
Ru /Ru couples at a more 3b
–
<10
ꢀ
5
3
3
c
positive potential than the het-
eroleptic complexes as a result
of the electron-deficient bis-
(pyridyl)triazine ligands draw- 3e
ing electron density away from
(
21.1)
254 (145.1); 280 (46.7) 346 (16.4); 364 (14.9) 383
12.9); 494 (20.7)
d
–
–
–
–
–
694
694
3500
4200
(
A
C
H
T
R
E
U
N
G
258 (155.6); 279 (60.7) 357 (15.6); 377 (18.2) 397
(17.3); 493 (26.8)
–
II
the Ru metal centres. Anthra-
cene-substituted 3d has an ad-
ditional oxidation consistent
with an anthracene-based oxidation as in 2d, also confirmed
as irreversible as a result of the difference between its
cathodic and anodic peak potentials and its I /I ratio.
c
a
Complexes 3 have several irreversible ligand-based reduc-
tions. There are two one-electron reductions on each tria-
zine unit to give a total of four reductions. The separation
between the second and third reduction processes is in the
range 610–660 mV for all of the complexes (leaving aside
3
e, whose irreversible third process makes the comparison
impossible), in good agreement with the electron-pairing
[42]
energy in tpy-type ligands.
Absorption spectra and photophysical properties: The elec-
tronic absorption and photophysical data of the new com-
plexes are shown in Table 3. Representative absorption and
emission spectra are shown in Figures 5–8. The absorption
Figure 5. Absorption spectra of 2a (straight line), 2d (grey line) and 2e
(
bold line).
*
spectra are dominated by ligand-based spin-allowed p!p
*
1
and n!p transitions in the UV region of the spectra and
MLCT absorption band, with the lmax value corresponding
1
by MLCT absorption bands in the visible region (Table 3,
to the transition involving the tpy ligand. A low-energy
shoulder corresponds to the singlet MLCT transition involv-
Figures 5 and 6). Complexes 2a–e have a relatively sharp
Chem. Eur. J. 2007, 13, 2837 – 2846
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2843

G. S. Hanan, S. Campagna et al.
Figure 8. Uncorrected emission spectra of 2b (straight line), 2d (grey
line), 2e (bold grey line) and 3a (bold line) in a butyronitrile rigid matrix
at 77 K.
Figure 6. Absorption spectra of 3a (straight line), 3d (grey line) and 3e
bold line).
(
[
13]
ing the bis
A
C
H
T
R
E
U
N
G
(pyridyl)triazine ligands. Complexes 2d and 2e
pounds relative to the heteroleptic species, as supported by
the comparison of crystallographic data of 2a and 3a. Such
a lowering of the MC state makes this latter level closer in
energy to the MLCT state, so thermally activated surface
crossing to the MC state is very efficient and finally leads to
very fast radiationless decay, which prevents luminescence.
In the whole series of complexes, varying the substituent
at the para position of the phenyl ring has little effect on
the energy of the MLCT absorption and emission bands,
thus indicating that the aryl groups are largely electronically
have additional characteristic sharp high-energy transitions
*
that correspond to the p!p transitions associated with the
[
43,44]
anthracene component (Figure 5).
The same absorption
features are observed in the homoleptic complexes 3d and
e. The MLCT bands in the homoleptic complexes are
broader, and the visible maxima are red-shifted relative to
the spectra of 2a--e, as a consequence of missing the higher-
energy MLCT transitions involving the tpy ligand.
The room-temperature emission of the heteroleptic com-
plexes 2a–e can be assigned to triplet MLCT states involv-
1
3
[13]
II
separated from the Ru centre and the associated MLCT
ing the bis
A
C
H
T
R
E
U
N
G
(pyridyl)triazine ligands on the basis of the ener-
states. However, the excited-state lifetimes are influenced to
a greater extent; in fact, whereas 2a–d have excited-state
lifetimes between 10 and 13 ns, approximately fifty-times
[13]
gies and shapes (Table 3 and Figures 7 and 8). As far as
the homoleptic 3a–e species are concerned, only 3a and 3c
exhibit a sizeable room-temperature emission, assigned to
the lowest-lying MLCT triplet state, whereas 3b, 3d and 3e
do not emit at room temperature. This behaviour is proba-
bly due to the lowering of the energy of the metal-centred
2
+
longer lived than the parent complex [Ru
A
H
R
U
G
II
temperature and well comparable with those of other Ru –
[13]
triazine-based compounds, the excited-state lifetime of 2e
is significantly longer (Table 3). Indeed, hydrocarbon aro-
matic chromophores appended to metal complexes can pro-
(
MC) state, as a consequence of increased distortion in the
[16,17]
octahedral geometry exhibited by the homoleptic com-
long MLCT emission lifetimes.
However, biphenyl,
phenanthryl and anthryl groups, which are present in 2b, 2c
and 2d, respectively, have no sizeable effect on the MLCT
lifetime at room temperature. The reason is that the lowest-
energy MLCT state in these complexes is so low lying that
thermal population of the relevant aromatic organic triplets
(
whose lowest-energy triplet is that of the 9-aryl anthracene
[16,42,43]
chromophore, at about 685 nm
) is ineffective; further-
more, the intrinsic decay rates of aromatic hydrocarbons
and MLCT triplets should be taken into consideration. As
far as 2e is concerned, this species bears a brominated an-
thracene and the electron-withdrawing effect of the bromo
substituent lowers the energy of the anthracene-based triplet
state. Actually, comparison between the triplet state of
ꢀ
1 [43,45]
anthracene (14870 cm )
and 9-bromoanthracene
ꢀ
1 [44,46]
(
14400 cm )
suggests that a bromo substituent stabilis-
es the anthracene-based triplet state by about 500 cm . As-
suming the same effect in the present systems and consider-
ꢀ
1
Figure 7. Uncorrected emission spectra of 2a (bold line) and 2e (straight
line) in solution with acetonitrile at room temperature. See Table 3 for
the corrected maxima values.
II
ing that the triplet state of the aryl anthracene/Ru com-
plexes is at about 685 nm (see above), as also evidenced by
2844
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 2837 – 2846

Multichromophoric Systems in Bis(pyridyl)triazine–Ru Complexes
A
H
R
U
G
FULL PAPER
[16]
the low-temperature emission of similar complexes,
the
Conclusion
triplet state of the bromoanthryl subunit in 2e should lie at
about 705 nm at room temperature. As a consequence, the
lowest-energy triplet state of the aromatic anthryl-based
chromophore in 2e becomes low enough to make equilibra-
tion between the MLCT and anthryl-based triplets effective
so that the multichromophoric effect can take place.
II
We prepared a series of new hetero- and homoleptic Ru
complexes based on bis(pyridyl)triazine ligands and studied
AHCTREUNG
their absorption spectra, redox behaviour and luminescence
properties, both at room temperature in fluid solution and
at 77 K in a rigid matrix. In some complexes, the anthryl
groups are linked to the triazine-based ligands.
The excited-state lifetimes of the heteroleptic complexes
2a–e are significantly longer than that of the parent complex
On cooling the samples to 77 K in a butyronitrile rigid
matrix, the excited-state lifetimes of 2a–e, as expected, are
prolonged relative to room temperature. This effect is much
larger for 2d and 2e, whose luminescence lifetimes under
these conditions are three orders of magnitude longer than
those of 2a–c (Table 3). The shape of the emission is also
different for 2d and 2e (in particular for the 2d species, for
which the higher-energy band is quite narrow) relative to
those of 2a–c (Figure 8). The lifetime and shape of the emis-
sions clearly indicate that emission of 2a–c are still from
MLCT states, whereas for 2d and 2e emission has to be as-
signed to phosphorescence from the anthracene-based trip-
let states. In fact, CT states are destabilized on passing from
a fluid solution at room temperature to a rigid matrix at
2
+
[Ru(tpy) ] and the homoleptic triazine complexes 3a–e at
A
H
R
U
G
2
room temperature. The presence of the triazine ligand
lowers the energy of the ligand-based orbitals and, there-
fore, the MLCT states, whilst the metal-centred states are
maintained at higher energy in 2a–e by the presence of an
orthogonal tpy ligand. In homoleptic triazine complexes 3a–
e, the MC states are lower in energy as a result of the small
tridentate N-M-N bite angle, as observed in the solid state.
As a consequence, a minimal improvement is observed in
the excited-state properties of 3a–e relative to the parent
2
+
complex [Ru(tpy) ] .
A
H
R
U
G
2
[3,47,48]
77 K,
whereas the energy of the organic triplets is es-
The introduction of additional chromophores can have an
impact on the excited-state lifetime when the energy of the
organic component is comparable to the energy of the
MLCT state. This result is obtained by tuning the excited-
state energy of the anthracene subunit with the introduction
of a bromo substituent. The lowering in energy of the
sentially unperturbed by media conditions. Since the MLCT
and anthryl-based triplet states are relatively close to one
another in 2d and 2e, the energy order of the two states is
inverted on passing to 77 K, an already reported behaviour
3
II
[49]
for Ru complexes that contain anthracene derivatives.
3
The emissions of 2d and 2e at 77 K both maximize at
95 nm. Surprisingly, 2d and 2e have the same emission
bromoanthracene excited state, which can then interact
with the emissive MLCT state in 2e, extends the excited-
3
6
maximum in this condition, thus suggesting that the pres-
ence of the bromo substituent is less important in the rigid
matrix, as opposed to what happens at room temperature
state lifetime to 54 ns at room temperature as compared to
an excited-state lifetime of 13 ns for 2d, which bears an un-
substituted anthracene unit. The use of stronger electron-
withdrawing (cyano) and electron-delocalising (acetylene)
groups to further tune the excited-state energy of the an-
thryl chromophore, and therefore the MLCT luminescence
(
see above). Indeed, it was expected that the emission
energy of 2e would be slightly lower than that of 2d. We
have no simple explanation for this behaviour, we could
only infer that some intermolecular interactions involving
the bromine atoms, as evidenced in the solid state of 3a (see
the X-ray crystallography section), can also occur in the
rigid matrix for 2e (and 3e), thus making the presence of
the bromo substituent on the anthracene unit less effective.
As far as the homoleptic complexes 3a–e are concerned,
all exhibit long-lived luminescence at 77 K, similar to the
behaviour of their heteroleptic counterparts (Table 3). The
same line of reasoning used for the emission of 2a–e at 77 K
allows us to assign the emission of 3a–c to MLCT states and
the emission of 3d and 3e to anthracene-based phosphores-
cence. Thermal population of the upper-lying MC states is
not effective at 77 K, thus allowing luminescence from the
homoleptic species. Interestingly, whereas the emission ener-
gies of 3d and 3e are identical to those of 2d and 2e, thus
confirming their common anthryl-based origin and the con-
stant energy level of the relevant aromatic hydrocarbon on
passing from hetero- to homoleptic compounds, the emission
energies of 3a–c are slightly higher than those of the corre-
sponding 2a–c species, in agreement with the redox poten-
tials and supporting once more their MLCT origin.
[50]
lifetime, will be investigated in due course.
Acknowledgements
The authors thank the Natural Sciences and Engineering Research Coun-
cil of Canada, UniversitØ de MontrØal, the Centre for Self-Assembled
Chemical Systems, and the University of Messina for financial support,
the ICCS for funding (E.A.M.), F. BØlanger-GariØpy for assistance with
3 2
the X-ray analysis and Johnson Matthey plc for a loan of [RuCl ]·3H O.
[
1] V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem.
Rev. 1996, 96, 759.
[
2] K. Kalyanasundaram, Photochemistry of Polypyridine and Porphyrin
Complexes, 1st ed., Academic Press, San Diego, 1992.
[3] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von
Zelewsky, Coord. Chem. Rev. 1988, 84, 85.
[
4] V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni, M. Venturi,
Acc. Chem. Res. 1998, 31, 26.
[
[
5] J. K. McCusker, Acc. Chem. Res. 2003, 36, 876.
6] T. J. Rutherford, D. A. Reitsma, F. R. Keene, J. Chem. Soc. Dalton
Trans. 1994, 3659.
Chem. Eur. J. 2007, 13, 2837 – 2846
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2845

G. S. Hanan, S. Campagna et al.
[
[
[
7] J. P. Sauvage, J. P. Collin, J. C. Chambron, S. Guillerez, C. Coudret,
V. Balzani, F. Barigelletti, L. De Cola, L. Flamigni, Chem. Rev. 1994,
[26] S. Campagna, G. Denti, S. Serroni, A. Juris, M. Venturi, V. Ricevuto,
V. Balzani, Chem. Eur. J. 1995, 1, 211.
[27] E. A. Medlycott, I. Theobald, G. S. Hanan, Eur. J. Inorg. Chem.
2005, 1223.
[28] B. P. Sullivan, J. M. Calvert, T. J. Meyer, Inorg. Chem. 1980, 19,
1404.
[29] M. Baumgarten, L. Gherghel, J. Friedrich, M. Jurczok, W. Rettig, J.
Phys. Chem. A 2000, 104, 1130.
9
4, 993.
8] M. Abrahamsson, M. Jaeger, T. Oesterman, L. Eriksson, P. Persson,
H.-C. Becker, O. Johansson, L. Hammarstroem, J. Am. Chem. Soc.
2
006, 128, 12616.
9] J. R. Winkler, T. L. Netzel, C. Creutz, N. Sutin, J. Am. Chem. Soc.
987, 109, 2381.
1
[
[
[
10] E. A. Medlycott, G. S. Hanan, Chem. Soc. Rev. 2005, 34, 133.
11] E. A. Medlycott, G. S. Hanan, Coord. Chem. Rev. 2006, 250, 1763.
12] M. I. J. Polson, N. J. Taylor, G. S. Hanan, Chem. Commun. 2002,
[30] S. Kotha, A. K. Ghosh, K. D. Deodhar, Synthesis 2004, 4, 594.
[31] E. A. Medlycott, G. S. Hanan, Inorg. Chem. Commun. 2006, DOI:
10.1016/j.inoche.2006.10.023.
1
356.
[32] K. O. Johansson, J. A. Lotoski, C. C. Tong, G. S. Hanan, Chem.
Commun. 2000, 819.
[
13] M. I. J. Polson, E. A. Medlycott, G. S. Hanan, L. Mikelsons, N. J.
Taylor, M. Watanabe, Y. Tanaka, F. Loiseau, R. Passalacqua, S.
Campagna, Chem. Eur. J. 2004, 10, 3640.
14] N. D. McClenaghan, Y. Leydet, B. Maubert, M. T. Indelli, S. Campa-
gna, Coord. Chem. Rev. 2005, 249, 1336.
15] a) W. E. Ford, M. A. J. Rodgers, J. Phys. Chem. 1992, 96, 2917;
b) G. J. Wilson, A. Launikonis, W. H. F. Sasse, A. W.-H. Mau, J.
Phys. Chem. A 1997, 101, 4860; c) J. A. Simon, S. L. Curry, R. H.
Schmehl, T. R. Schatz, P. Piotrowiak, X. Jin, R. P. Thummel, J. Am.
Chem. Soc. 1997, 119, 11012; d) A. Harriman, M. Hissler, A.
Khatyr, R. Ziessel, Chem. Eur. J. 1999, 5, 3366; e) D. S. Tyson, C. R.
Luman, X. Zhou, F. N. Castellano, Inorg. Chem. 2001, 40, 4063;
f) N. D. McClenaghan, F. Barigelletti, B. Maubert, S. Campagna,
Chem. Commun. 2002, 6, 602; g) S. Leroy-Lhez, C. Belin, A.
D’Aleo, R. Williams, L. De Cola, F. Fages, Supramol. Chem. 2003,
[33] F. Loiseau, R. Passalacqua, S. Campagna, M. I. J. Polson, Y.-Q. Fang,
G. S. Hanan, Photochem. Photobiol. Sci. 2002, 1, 982.
[34] K. J. Arm, J. A. G. Williams, Chem. Commun. 2005, 230.
[35] K. J. Arm, J. A. G. Williams, Dalton Trans. 2006, 2172.
[36] G. J. Pernꢁa, J. D. Kilburn, J. W. Essex, R. J. Mortishire-Smith, M.
Rowley, J. Am. Chem. Soc. 1996, 118, 10220.
[37] N. W. Alcock, P. R. Barker, J. M. Haider, M. J. Hannon, C. L. Paint-
ing, Z. Pikramenou, E. A. Plummer, K. Rissanen, P. Saarenketo,
Dalton Trans. 2000, 1447.
[38] S. Pyo, E. Perez-Cordero, S. G. Bott, L. Echegoyen, Inorg. Chem.
1999, 38, 3337.
[39] E. A. Medlycott, K. A. Udachin, G. S. Hanan, Dalton Trans. 2007, in
press.
[
[
[40] A. Harriman, A. Mayeux, A. De Nicola, R. Ziessel, M. Beley, Phys.
Chem. Chem. Phys. 2002, 4, 2229.
1
5, 627; h) X. Y. Wang, A. Del Guerzo, R. H. Schmehl, J. Photo-
chem. Photobiol. C 2004, 5, 55; i) J. Wang, Y.-Q. Fang, G. S. Hanan,
F. Loiseau, S. Campagna, Inorg. Chem. 2005, 44, 5.
16] a) R. Passalacqua, F. Loiseau, S. Campagna, Y.-Q. Fang, G. S.
Hanan, Angew. Chem. 2003, 115, 1646; Angew. Chem. Int. Ed. 2003,
[41] Although a theoretical value of 59 mV is expected for the difference
between the cathodic and anodic waves for a reversible one-electron
process, small deviations from this value have previously been re-
ported as “reversible”; see, for example: reference [26] and A. Har-
riman, M. Hissler, A. Khatyr, R. Ziessel, Chem. Eur. J. 1999, 5,
3366, and references therein. We thank a reviewer for suggesting the
[
4
2, 1608; b) J. Wang, Y.-Q. Fang, L. Bourget-Merle, M. I. J. Polson,
G. S. Hanan, A. Juris, F. Loiseau, S. Campagna, Chem. Eur. J. 2006,
2, 8539.
1
c a
inclusion of I /I ratios to Table 2 as another measure of the irrever-
[
[
17] B. Maubert, N. D. McClenaghan, M. T. Indelli, S. Campagna, J.
Phys. Chem. A 2003, 107, 447.
sibility of the processes described therein.
[42] A. Mamo, I. Stefio, M. F. Parisi, A. Credi, M. Venturi, C. Di Pietro,
S. Campagna, Inorg. Chem. 1997, 36, 5947.
[43] S. L. Murov, I. Carmichael, G. L. Hug, in Handbook of Photochem-
istry, Marcel Dekker, New York, 1993.
[44] Handbook of Photochemistry, 3rd Edition, Revised and Expanded
(Eds.: M. Montalti, A. Credi, L. Prodi, M. T. Gandolfi), CRC Press,
2006.
18] Strictly speaking, the states involved in the equilibration process can
II
be much more than two; in Ru – polypyridine complexes, such as
2
+
3
[Ru
A
C
H
T
R
E
U
N
G
3
(bpy) ] , for example, the lowest-energy MLCT manifold is
[
3,46]
made of three closely spaced levels.
However, these levels are
equilibrated at room temperature and 77 K and can be considered
as a “single” state.
[
[
19] C. J. Aspley, J. A. G. Williams, New J. Chem. 2001, 25, 1136.
20] C. Patoux, J.-P. Launay, M. Beley, S. Chodorowski-Kimmes, J.-P.
Collin, S. James, J.-P. Sauvage, J. Am. Chem. Soc. 1998, 120, 3717.
21] S. J. Dunne, E. C. Constable, Inorg. Chem. Commun. 1998, 1, 167.
22] E. C. Constable, A. M. W. Cargill Thompson, S. Greulich, J. Chem.
Soc. Chem. Commun. 1993, 1444.
[45] D. F. Evans, J. Chem. Soc. 1957, 1351.
[46] K. Hamanoue, S. Tai, T. Hikada, T. Nakayama, M. Kimoto, H. Tera-
nashi, J. Phys. Chem. 1984, 88.
[47] T. J. Meyer, Pure Appl. Chem. 1986, 58, 1193, and references there-
in.
[48] P. Chen, T. J. Meyer, Chem. Rev. 1998, 98, 1439.
[49] a) S. Serroni, S. Campagna, R. Pistone Nascone, G. S. Hanan, G. J.
Davidson, J.-M. Lehn, Chem. Eur. J. 1999, 5, 3523; b) J. Wang, E. A.
Medlycott, G. S. Hanan, F. Loiseau, S. Campagna, Inorg. Chim.
Acta, 2006, in press.
[50] CCDC-619166–619169 contain the supplementary crystallographic
data. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data
request/cif.
[
[
[
[
23] M. Beley, J. P. Collin, R. Louis, B. Metz, J. P. Sauvage, J. Am. Chem.
Soc. 1991, 113, 8521.
24] Emission quantum yields were measured at room temperature using
the optically dilute methods, see: G. A. Crosby, J. N. Demas, J. Phys.
2
+
Chem. 1971, 75, 991. [Ru
tion and [{(bpy) Ru(m-2,3-dpp)}
pyrazine) in deaerated acetonitrile solution were used as quantum
A
H
R
U
G
3
]
in air-equilibrated aqueous solu-
8
+
2
3
Ru] (2,3-dpp=2,3-bis(2’-pyridyl)-
[
25]
[26]
yield standards, assuming values of 0.028 and 0.005, respective-
ly.
Received: September 25, 2006
[
25] N. Nakamaru, Bull. Chem. Soc. Jpn. 1982, 55, 2697.
Published online: December 21, 2006
2846
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 2837 – 2846