An iridium(iii)-caged complex with low oxygen quenching

Article abstract of DOI:10.1039/c0cc01178a

We here report the synthesis and structural characterization of the first iridium(iii) complex with a caged ligand structure, which shows a 80% decrease of oxygen quenching compared to the archetypical Ir(ppy)₃.

Full text of DOI:10.1039/c0cc01178a

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An iridium(III)-caged complex with low oxygen quenchingw
Albert Ruggi, Miguel Berenguel Alonso, David N. Reinhoudt and Aldrik H. Velders*
Received 29th April 2010, Accepted 28th July 2010
DOI: 10.1039/c0cc01178a
We here report the synthesis and structural characterization of
the first iridium(III) complex with a caged ligand structure, which
shows a 80% decrease of oxygen quenching compared to the
archetypical Ir(ppy)3.
Iridium(III) complexes have received much attention because
of their versatile photophysical properties.1,2 Cyclometallated
iridium(III) complexes,3 in particular, have extensively been
employed, e.g. as components for electrochemiluminescent
devices,4,5 as sensitizers for solar cells,6 as organic light emitting
devices (OLEDs)7,8 and, more recently, as luminescent probes
for biomedical diagnostic applications.2,9 Along with the
remarkable photophysical properties, like the tunability
of the emission wavelength,10 iridium(III) cyclometallated
complexes also suffer from a serious drawback, the strong
oxygen quenching of the luminescence.11 Metal-caged complexes
are an intriguing class of compounds which are notoriously
difficult to synthesize.12–15 A cage-type ligand can prevent the
luminescence quenching by solvent molecules, as shown for
lanthanide(III) complexes,15 or prevent photo-decomposition
as shown for a ruthenium(II) complex.16 We present here the
synthesis, characterization and photophysical characteristics
of the first caged Ir(III) complex. A comparison between the
oxygen quenching of the caged system and the archetypical
Ir(ppy)3 (ppy = 2-phenylpyridine) shows a remarkable shielding
effect for luminescence quenching by oxygen for the caged
system.
Scheme 1 Synthetic pathway of the Ir(III)-cage complex 4. Reagents:
(i) 4-carboxyphenylboronic acid, Pd[PPh3]4, CsF; (ii) DCC, tris(2-
aminoethyl)amine, HOBt; (iii) (1) IrCl3, CF3CO2Ag, (2) LiOH; (iv)
LiOH.
The synthesis of the caged complex 4 was achieved by using
a tripodal approach (Scheme 1). The partly protected phenyl-
pyridine derivative 1 was chosen as the starting material in
order to assure an adequate chemoselectivity to avoid the
formation of structural isomers during the subsequent
DCC/HOBt coupling (see ESIw for full synthetic details and
explanation of the abbreviations). The building block 1 was
synthesized using a Suzuki coupling under mild conditions
(CsF in refluxing methanol) in order to avoid the methyl ester
hydrolysis which happens under basic aqueous conditions.17
The phenylpyridine derivative 1 was consecutively coupled
with tris(2-aminoethyl)amine (Tren) via DCC/HOBt coupling
giving the tripodal ligand 2, which was reacted with IrCl3 in
ethylene glycol at 180 1C. The crude product was found to be a
mixture of free ligand and hemicaged Ir(III) complexes with
different substituents on the hemicage ligand (i.e. methyl and
ethylene glycol esters), which are formed by transesterification
under the harsh reaction conditions.18 The methyl ester 3 was
hydrolysed and consecutively used for the synthesis of the
caged complex 4 via DCC coupling with Tren.
1
The H-NMR spectrum of hemicaged complex 3 (Fig. 1A)
shows four different signals for the two methylene units,
indicating the restrained flexibility in the ethylene unit of the
Tren, causing the geminal protons to be non-equivalent and
diastereotopic in the chiral environment of the Ir(ppy)3 unit.
The three ethylene units of the tripodal ligand are equivalent
as only four signals are observed in the corresponding region
of the spectrum. The same observation holds for the caged
complex 4 (Fig. 1B), i.e. the aliphatic protons give eight
different signals. Together with the observation of one single
set of phenylpyridine protons, this proves the structure of
4 to be highly symmetric, with the 3-fold symmetry of the
fac-Ir(ppy)3 unit being conserved.
By 2D-NMR experiments (COSY, ROESY, HMQC,
HMBC) all signals were assigned and the structure of the
caged complex was determined by characteristic NOE signals.
The through-space NOE couplings between both amide
protons with 3 or 4 methylene protons (Fig. 2B) clearly show
the tripodal capped structure of both Tren moieties. Interestingly,
Laboratory of Supramolecular Chemistry and Technology,
MESA+ Institute for Nanotechnology, University of Twente,
PO Box 217, 7500 AE, Enschede, The Netherlands.
E-mail: a.h.velders@utwente.nl; Fax: +31 53489-4645
w Electronic supplementary information (ESI) available: Experimental,
synthesis and characterization details, NMR and UV-Vis absorption/
fluorescence spectra of 3 and 4. See DOI: 10.1039/c0cc01178a
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Table 1 Photophysical properties of hemicaged complex 3, cage
complex 4 and Ir(ppy)3, measured in DMF at 25 1C
lema/nm
f
fair
t0a/ns
ta/ns
kq/Mꢂ1 sꢂ1
4
570
580
523
0.46
0.48
0.71
0.06
0.05
0.03
1270
1198
1873
168
120
82
5.2 ꢁ 109
1.4 ꢁ 1010
2.4 ꢁ 1010
3
Ir(ppy)3
a
lexc = 462 nm.
In deaerated solutions a lower quantum yield f0 is observed
both for 3 and 4, compared to Ir(ppy)3. However, in aerated
solutions, the luminescence quantum yield f of the caged
complex appears higher than that of 3 and twice as high of
that of the quantum yield of the Ir(ppy)3. A similar behaviour
is shown also by the excited state lifetimes in deaerated (t0)
and aerated (t) solutions.
In order to evaluate the shielding effect against oxygen
quenching, the fluorescence emission of samples containing
different concentrations of oxygen was registered and plotted
against the concentration of oxygen (Fig. 3) according to the
Stern–Volmer equation (1)
Fig. 1 1H-NMR (d7-DMF) of the Ir-hemicaged 3 (A) and Ir-caged 4
(B), with proton numbering scheme and peak assignment. *Residual
solvents.
the aromatic region of the ROESY spectrum of 4 (Fig. 2A)
indicates a different orientation of the two types of amide
groups. The amide protons situated on the phenyl rings (H1)
are oriented towards the inner part of the molecule, as proven
from the cross peak between this amide proton and the phenyl
singlet (H2); the amide protons on the pyridine rings (H8) are
oriented towards the outside, as proven from a cross peak with
the pyridine doublet (H6). In the pseudo-octahedrical
fac-Ir(ppy)3 complex, the Ir–C C bonds are significantly shorter
(2.0246 A) than the three Ir–N bonds (2.1325 A).19 Therefore,
the opposite orientations of the amide groups in 4 suggest that
a capped trigonal pyramid cage is formed,20 the top part being
smaller and having the bulky oxygens pointing outwards, and
a wider bottom with the amide oxygens oriented inwards.
The photophysical properties of 4 are reported in Table 1
together with those of the hemicaged 3 and the archetypical
Ir(ppy)3 for comparison purposes. The hemicaged and caged
compounds both show a red-shift of the absorption and
emission, with respect to Ir(ppy)3 (see ESIw), as expected
for phenylpyridines with electron withdrawing substituents.1
I0
¼ 1 þ kqt0½O2ꢀ
ð1Þ
I
where I0 and I are the intensities without and with the
quencher, respectively, kq is the quenching constant, t0 the
excited state lifetime in the absence of quencher, and [O2] the
molar concentration of oxygen, the quencher. From the
Stern–Volmer plots (see Fig. 3) the quenching constants kq
were determined to be 5.2 ꢁ 109 Mꢂ1 sꢂ1, 1.4 ꢁ 1010
M
ꢂ1 sꢂ1
and 2.4 ꢁ 1010
M
ꢂ1 sꢂ1 for 4, 3 and Ir(ppy)3, respectively. This
implies a dramatic decrease of the quenching constant for the
caged complex of almost 80% with respect to that of Ir(ppy)3,
while the hemicaged complex shows a 40% decrease of
quenching. In the pioneering work on caged ruthenium
complexes, a similar decrease has been found by Barigelletti
et al. for a Ru(II)-caged system with bipyridine ligands,16
attributing the observed decrease in oxygen quenching to an
effective shielding of orbitals of the caged complex. The here
presented results with the phenylpyridine ligand cage are even
Fig. 2 Selected areas of the ROESY spectrum of 4. Characteristic
through-space couplings of amide protons with the aromatic (A) and
aliphatic protons (B) are indicated by the red (phenylic amide) and
blue (pyridine amide) boxes. *d7-DMF.
Fig. 3 Stern–Volmer plot of oxygen quenching of Ir(ppy)3 (blue
rhombs), hemicaged complex
3
(yellow triangles) and caged
complex 4 (magenta squares) in DMF at 25 1C.
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more remarkable considering the fact that the Ru(II)-caged
complex is bulky and rigid with a tripodal aromatic capping
group, whilst the structure of complex 4 looks relatively
flexible with the tripodal tri(ethylene)amine capping and does
not seem to present high steric constraints. On the other hand,
the NMR data of 4 clearly show sharp sets of signals and no
indication of fluxional behaviour of the tripodal capping
structures, so the structure might actually be rather rigid,
corroborating the photophysical properties of the caged
iridium(III) structure.
discussions. The Strategic Research Orientation BioNano
from MESA+, and NanoNed, the Dutch Nanotechnology
program from the Ministry of Economic affairs, are acknowl-
edged for financial support.
Notes and references
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The reason for the reduced oxygen quenching shown by
caged metal complexes is not fully understood.16 It is known
from the literature that the HOMO of Ir(III) cyclometallated
complexes is mostly localized between the Ir and the phenyl
ring, whilst the LUMO mainly resides on the pyridine ring.1
The presence of electron withdrawing groups on the phenyl-
pyridine ligand induces a change in the localization of the
electron density in the excited state on different positions in the
pyridine ring, or even on its substituents, resulting in a higher
shielding effect towards quenching species, e.g. oxygen,
when the latter are further shielded to the environment by
substituents like the bulky tripodal Tren. The origin of the
shielding effect in iridium(III) complexes is currently further
investigated by systematically varying the substituents on the
phenylpyridine ligands in a series of hemicaged and caged
structures, combined with computational studies.
In conclusion, we have presented here the synthesis and
characterization of the first iridium(III) complex with a caged
ligand structure. The compound 4 shows a truncated trigonal
pyramid cage structure with 3-fold symmetry that has been
fully analyzed by solution-state NMR spectroscopy. The
iridium-caged structure 4 is less subject to luminescence
quenching by molecular oxygen compared to the archetypical
Ir(ppy)3 complex. Iridium(III) complexes are in general more
luminescent complexes, with higher quantum yields and longer
lifetimes, than the well-known ruthenium complexes because
of higher ligand splitting which pushes the metal centered
(MC) levels so high in energy that usually they are not
thermally accessible and therefore they do not affect the
emission properties.1 The possibility of shielding iridium
complexes from oxygen opens promising perspectives for the
synthesis of bright luminophores for applications in oxygen-
rich environments, e.g. biological systems,2,9 and current
investigations are focussed in this direction.
The authors acknowledge Prof. Luisa De Cola (Westfalische
¨
Wilhelms-Universitat, Munster, Germany) for valuable
20 J. C. Knight, S. Alvarez, A. J. Amoroso, P. G. Edwards and
N. Singh, Dalton Trans., 2010, 39, 3870–3883.
¨
¨
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6728 Chem. Commun., 2010, 46, 6726–6728
This journal is The Royal Society of Chemistry 2010