Two-color luminescence from a tetranuclear Ir(III)/Ru(II) complex

Article abstract of DOI:10.1039/b508063k

A new tetranuclear compound containing RU(II) and Ir(III) polypyridine subunits exhibits two independent emissions at room temperature, as a consequence of weak interchromophoric coupling; in contrast, at 77 K energy transfer from Ir-based chromophores to

Full text of DOI:10.1039/b508063k

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Two-color luminescence from a tetranuclear Ir(III)/Ru(II) complex
Marco Cavazzini,a Paola Pastorelli,a Silvio Quici,*a Fre´de´rique Loiseau*b and Sebastiano Campagnab
Received (in Cambridge, UK) 6th June 2005, Accepted 7th September 2005
First published as an Advance Article on the web 23rd September 2005
DOI: 10.1039/b508063k
luminescence properties (both at room temperature in acetonitrile
solution and at 77 K in butyronitrile rigid matrix) of the new
complexes are reported here. Interestingly, 3 exhibits independent
Ru-based and Ir-based emissions at room temperature. In
particular, Ru-based emission can be univocally addressed by
long-wavelength excitation. At 77 K in a rigid matrix, efficient
energy transfer takes place, leading to complete quenching of the
Ir-based emission and sensitization of the Ru-based luminescence.
Ligand L was achieved using Pd-catalyzed cross-coupling
between 3,5-dibromotoluene and terpy-49-phenylboronic ester.8
Complexes 1 and 2 were obtained by reaction of L with
Ru(terpy)Cl3 and Ir(terpy)Cl3 respectively, with minor adjustments
to the literature procedures.5a,b,9 The preparation of 3 required a
stepwise synthetic route10 by refluxing an ethanolic solution of
[Cl3Ru(L)RuCl3] with two equivalents of L in the presence of
N-ethylmorpholine and subsequent complexation with two
equivalents of Ir(terpy)Cl3 in refluxing diethyleneglycol to
provide 3.{
A new tetranuclear compound containing Ru(II) and Ir(III)
polypyridine subunits exhibits two independent emissions at
room temperature, as a consequence of weak interchromo-
phoric coupling; in contrast, at 77 K energy transfer from Ir-
based chromophores to the Ru-based ones is quantitative.
Multicomponent (supramolecular) assemblies containing lumines-
cent metal complexes are extensively investigated because they can
exhibit intercomponent energy and/or electron transfer processes,
possibly leading to valuable functions such as charge separation
and/or energy migration.1 The study of such types of processes and
functions is quite useful for both fundamental knowledge and
applicative reasons, including the development of artificial
antennae and reaction centers for artificial photosynthesis.2
By contrast, covalently-linked multicomponent compounds
containing two types of luminescent metal complexes, properly-
designed to exhibit multiple, almost independent emissions are
rare. Such types of system can be extremely interesting for the
design of white light emitters,3 in particular for applications in
electroluminescent displays and for backlights.4 A (small) electro-
nic interaction between the chromophores is desired, since it could
allow for tunability.
Ru(II)5 and Ir(III)6 polypyridine complexes are well known
emitters, so they can be suitable components for multiple emission
assemblies. In order to allow multiple emission, the bridging
ligands used to build up the discrete multicomponent species
should be relatively rigid in structure and allow only weak
interactions between the chromophoric sites. Indeed, the literature
reports several examples of relatively-rigid briding ligands with
these properties, exhibiting slow intercomponent energy transfer at
room temperature and therefore featuring multiple luminescence,7
but in most of the cases the quantum yield of one of the two
chomophores is negligible compared to that of the other: a
noticeable exception is the recently-reported case of a mixed Ir(III)–
Eu(II) system.3
The absorption spectrum of 1 (see Table 1, Fig. 2) shows intense
bands both in the UV and visible regions. By comparison with
literature data,5a,6,7 the bands in the UV region are assigned to
spin-allowed ligand-centered (LC) transitions and those in the
visible to spin-allowed metal-to-ligand charge-transfer (MLCT)
transitions. Whereas a similar assignment holds for 2 as far as the
Based on the above arguments, we synthesized the bridging
ligand L, containing two coordinating terpy subunits (terpy 5
2,29:69,20-terpyridine), linked to each other by a relatively rigid
polyphenyl spacer in which a meta arrangement is used to reduce
electronic interactions, and prepared the multinuclear systems
[(terpy)Ru(L)Ru(terpy)](PF6)4 (1), [(terpy)Ir(L)Ir(terpy)](PF6)6 (2)
and [(terpy)Ir(L)Ru(L)Ru(L)Ir(terpy)](PF6)10 (3). Fig. 1 shows the
structural formulas of L and 1–3. The absorption spectra and the
aIstituto ISTM-CNR Dipartimento di Chimica Organica e Industriale
dell’Universita`, Via Golgi 19, I-20133 Milano, Italy.
E-mail: silvio.quici@istm.cnr.it
bDipartimento di Chimica Inorganica, Chimica Analitica e Chimica
Fisica, Universita` di Messina, Via Sperone 31, 98166 Messina, Italy.
E-mail: photochem@chem.unime.it
Fig. 1 Structural formulae of the compounds. Charges of the complexes
2
are omitted for clarity; counter-ions are PF6
.
5266 | Chem. Commun., 2005, 5266–5268
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Table 1 Absorption and photophysical data in acetonitrile deaerated solution at room temperature or in butyronitrile rigid matrix at 77 K
Luminescence 298 K
max/nm
Luminescence 77 K
Compound
—
Absorption
max/nm (e/M21 cm21
l
)
l
t/ns
W
lmax/nm
t/ms
1
2
3
a
309 (281000) 486 (104800)
252 (69100) 281 (74800) 378 (33200)
284 (200200) 313 (272200) 495 (124800)
b
683
578
83
2774
7.7 6 1025
6.7 6 1023
9.7 6 1025 a
641
524
649
13
205
13
685a, 572; 681c
82b, 2900d
c
d
Excitation wavelength 5 495 nm. Recorded at 680 nm. Excitation wavelength 5 350 nm. Recorded at 560 nm.
Fig. 2 Absorption spectra of 1 (red line), 2 (green line) and 3 (black line)
in acetonitrile solution.
UV bands are concerned,9 this latter species only exhibits poor
visible absorption (Fig. 2). Spin-allowed MLCT transitions are in
fact significantly blue shifted in Ir(III) polypyridine species
compared to Ru(II) polypyridine ones, so the lowest-energy
MLCT band is in the UV and should be responsible for the
absorption maximum at 378 nm. Even spin-forbidden MLCT/LC
transitions, particularly important in Ir(III) complexes, could
contribute to the visible absorption tail. The absorption spectrum
of 3 (Table 1, Fig. 2) contains contributions from both Ru-based
and Ir-based chromophores: the UV bands are mainly due to
overlapping spin-allowed LC and MLCT transitions originating
from the two types of metal-based subunits, and the visible
Fig. 3 Top: uncorrected emission spectra of 1 at room temperature (red
dashed line) and at 77 K (red full line) and 2 at room temperature (green
dashed line) and at 77 K (green full line). Bottom: uncorrected emission
spectra of 3 at room temperature, excitation wavelength 5 350 nm
(dashed line) or 495 nm (full line). In inset: emission spectrum of 3 at 77 K,
at any excitation wavelength. Corrected emission values in Table 1.
absorption bands are mainly due to spin-allowed MLCT
transitions involving the Ru(II)-terpy sites.
Luminescence of 1 (Table 1, Fig. 3), both at room temperature
and at 77 K, can be straightforwardly assigned to the lowest-lying
3MLCT state(s), on the basis of emission energies, lifetimes, and
temperature/matrix sensitivity.5 The low quantum yield at room
temperature is in line with those of similar Ru-terpy compounds
and is due to efficient radiationless decay via thermally-activated
surface crossing to a higher-energy metal-centered state.5a,b
Luminescence of 2 (Table 1, Fig. 3) is less straightforward. The
nature of the lowest emitting excited state for a series of related
3
agrees well with a pure LC assignement. To further support this
point, a solution of free L in the presence of zinc(II) salts (room
temperature, acetonitrile solution) gives an emission with a
maximum at 503 nm, which can be assigned to the zinc-perturbed
p–p* triplet state of L.
species, including Ir(terpy)2 has been assigned as 3LC.9,11
2+
On exciting at 350 nm, where both the Ir-based and Ru-based
chromophores are addressed, 3 exhibits two emission features
(Table 1, Fig. 3): the one at higher energy has a maximum at
572 nm (t 5 2.9 ms), and the one at lower energy has a maximum
at 681 nm (t 5 82 ns). From the data in Table 1, the two emissions
are assigned to Ir-based and Ru-based chromophores, respectively.
For excitation at 495 nm, only the lowest-lying, Ru-based emission
is found (Fig. 3), as expected since the Ir(III) chromophores do not
However, in substituted terpyridine Ir(III) complexes, as in bis-
tolylterpyridine or bis-di-tert-butylterpyridine derivatives, a mixed
3MLCT/3LC state appears to be more appropriate.9,11 On the
basis of emission energy, lifetime, and quantum yield of 2 and their
comparison with literature data,11 the mixed assignment appears
to be preferred also in this case, at least at room temperature. At
77 K, emission is much longer-lived and blue-shifted (Table 1), and
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consequence of bridging ligand design, which slows down
intercomponent energy transfer. By contrast, energy transfer is
effective at 77 K, because of the long intrinsic excited-state lifetime
of the donor chromophore(s) under these experimental conditions.
This work was supported by PRIN 2003. Project title:
‘‘Molecular devices for the harvesting, conversion and storage of
light energy’’, FISR, project title ‘‘Nanotecnologie molecolari per
l9immagazzinamento e la trasmissione delle informazioni’’ and
FIRB, project no. RBNE019H9K.
Notes and references
Fig. 4 Structural formulae of compounds 4 and 5.
{ Selected data for ligand L. Isolated yield: 70%. 1H-NMR (CDCl3,
300MHz) d 8.84 (s, 4H), 8.78 (m, 4H), 8.72 (d, 4H, J 5 12.0 Hz), 8.05 (d,
4H, J 5 12.5 Hz), 7.92 (ddd, 4H, J 5 2.8, 8.8, 12.0 Hz), 7.84 (d, 4H,
J 5 12.5 Hz), 7.80 (bs, 1H), 7.54 (bs, 2H), 7.36–7.41 (m, 4H), 2.55 (s, 3H).
13C-NMR (CDCl3, 75MHz) d 149.2, 136.8, 127.7, 127.3, 123.8, 123.5,
121.4, 118.7, 21.5. ES-MS (MeOH/CH3COOH): m/z (%) 5 707.0 (100).
Anal. Calcd. for C49H34N6: C, 83.26; H, 4.85; N, 11.89. Found: C, 83.20;
H, 4.88; N, 11.80%. Selected data for [(terpy)Ir(L)Ru(L)Ru(L)-
absorb at wavelength longer than 480 nm. On the basis of the
lifetimes of the 570 nm emission of 2 and 3, energy transfer from
3
the higher-lying MLCT/3LC emitting state of the Ir(III) subunits
3
to the lower-lying MLCT levels of the Ru(II) components is not
effective in 3 at room temperature. The inefficiency of the energy
transfer indicates that such a process should be significantly slower
than the room temperature intrinsic decay rate of 2, that is 3.7 6
105 s21, in spite of the non-negligible driving force (about 0.3 eV,
from emission data). Electronic factors have to be at the origin of
such a behavior. Energy transfer between Ru(II) and Os(II)
polypyridine subunits having three interposed phenyls
(compound 4 in Fig. 4) is reported to occur with a rate constant
of 6.7 6 108 s21, by a Dexter mechanism (superexchange),12 in
spite of the low electronic interaction energy Hen, which was
calculated to be lower than 1 cm21 (driving force is comparable to
that of 3). In a similar Ru(II)/Rh(III) species (5, Fig. 4),
photoinduced *Ru-to-Rh electron transfer takes place with a rate
1
Ir(terpy)](PF6)10 (3). Isolated yield: 15%. H-NMR (CD3CN, 300MHz) d
9.12 (s, 12H), 8.73 (bd, 14H, J 5 8.2 Hz), 8.40 (bd, 16H, J 5 8.2 Hz), 8.21
(bd, 18H, J 5 8.2 Hz), 8.10 (bs, 3H), 7.99 (m, 16H, J 5 7.8 Hz), 7.81 (s,
6H), 7.51 (m, 16H), 7.24 (m, 14H), 2.65 (s, 9H). 13C-NMR (CDCl3,
75MHz) d 159.1, 156.4, 153.3, 148.6, 143.6, 141.4, 140.7, 138.9, 136.8,
130.7, 130.0, 129.2, 128.6, 125.4, 124.0, 122.4, 21.6. Anal. Calcd. For
C177H124F60Ir2N24P10Ru2: C, 45.98; H, 2.70; N, 7.27. Found: C, 46.20; H,
2.90; N, 7.20%.
1 V. Balzani, A. Credi and M. Venturi, Molecular Devices and Machines,
Wiley-VCH, Weinheim, 2003, ch. 2–6 and references therein.
2 See, for example: (a) V. Balzani, S. Campagna, G. Denti, A. Juris,
S. Serroni and M. Venturi, Acc. Chem. Res., 1998, 31, 26; (b)
M. H. V. Huynha, D. M. Dattelbauma and T. J. Meyer, Coord. Chem.
Rev., 2005, 249, 457.
3 P. Coppo, M. Duati, V. N. Kozhevnikov, J. W. Hofstrat and L. De Cola,
Angew. Chem., Int. Ed., 2005, 44, 1806.
4 (a) B. W. D_ Andrade and S. R. Forrest, Adv. Mater., 2004, 16, 1585;
(b) J. Slinker, D. Bernerds, P. L. Houston, H. D. Abruna, S. Bernhard
and G. G. Malliaras, Chem. Commun., 2003, 2392.
5 (a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and
A. von Zelewsky, Coord. Chem. Rev., 1988, 84, 85; (b) T. J. Meyer, Pure
Appl. Chem., 1986, 58, 1193; (c) G. A. Crosby, J. Chem. Educ., 1983, 60,
791.
6 I. M. Dixon, J.-P. Collin, J.-P. Sauvage, L. Flamigni, S. Encinas and
F. Barigelletti, Chem. Soc. Rev., 2000, 29, 385 and references therein.
7 V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni, Chem.
Rev., 1996, 96, 759 and references therein.
8 (a) C. J. Aspley and J. A. G. Williams, New J. Chem., 2001, 25, 1136; (b)
W. Goodall, K. Wild, K. J. Arm and J. A. G. Williams, J. Chem. Soc.,
Perkin Trans. 2, 2002, 1669.
9 J.-P. Collin, I. M. Dixon, J.-P. Sauvage, J. A. G. Williams, F. Barigelletti
and L. Flamigni, J. Am. Chem. Soc., 1999, 121, 5009.
10 (a) G. R. Newkome, T. J. Cho, C. N. Moorefield, R. Cush, P.S. Russo,
L. A. God´ınez, M. J. Saunders and P. Mohapatra, Chem. Eur. J., 2002,
8, 2946; (b) G. R. Newkome, T. J. Cho, C. N. Moorefield, P. Mohapatra
and L. A. God´ınez, Chem. Eur. J., 2002, 10, 1493.
11 (a) E. Baranoff, I. M. Dixon, J.-P. Collin, J.-P. Sauvage, B. Ventura and
L. Flamigni, Inorg. Chem., 2004, 43, 3057; (b) L. Flamigni, B. Ventura,
F. Barigelletti, E. Baranoff, J.-P. Collin and J.-P. Sauvage, Eur. J. Inorg.
Chem, 2005, 1312.
12 B. Schlicke, P. Belser, L. De Cola, E. Sabbioni and V. Balzani, J. Am.
Chem. Soc., 1999, 121, 4207.
13 F. Scandola, C. Chiorboli, M. T. Indelli and M. A. Rampi, in Electron
Transfer in Chemistry, ed. V. Balzani, Wiley-VCH, Weinheim, 2001,
vol. 3, p. 337.
constant of 3.0 6 107 s21 13
However, the introduction of hexyl
.
substituents on the central phenyl ring of this latter species
decreases the electron transfer rate constant to 1.1 6 106 s21, as a
consequence of a decrease in the adjacent coupling terms of the
superexchange mechanism, due to variation in the twist angle
between the phenyl rings. The examples mentioned deal with para-
polyphenyls: in our system, a meta arrangement is present, and this
should affect the electronic interactions to a much larger extent
than the presence of ring substituents. Therefore, a significantly
lower Hen for the energy transfer in 3 compared to 4 is expected,
which would translate into an intercomponent energy transfer rate
slower than 105 s21, as suggested by the observed results.
At 77 K, energy transfer from Ir-based to Ru-based chromo-
phores is quantitative, as the only emission recorded is the
Ru-based 3MLCT emission regardless of the excitation wavelength
(Fig. 3, Table 1). Under these conditions, the excited-state lifetime
of the Ir(III) subunit(s) is quite long (205 ms) thus allowing the
energy transfer pathway to successfully compete with the intrinsic
decay of the Ir(III) centers. Indirectly, the room and low
temperature results tend to suggest that Ir-to-Ru energy transfer
in 3 (a process which is expected to be less sensitive to temperature)
would occur with a rate constant of the order of 104 s21
.
In summary, we have prepared a tetranuclear, mixed-metal
multichomophoric species capable of exhibiting two-colors, almost
independent emission at room temperature: this feature is a
5268 | Chem. Commun., 2005, 5266–5268
This journal is ß The Royal Society of Chemistry 2005