Remarkable Mg2+-selective emission of an azacrown receptor based on Ir(iii) complex

Article abstract of DOI:10.1039/c002401e

A new aza-15-crown-5 ether-appended iridium complex was synthesized and showed promising on-off selective emission-triggering by inhibition of photoinduced electron transfer (PET) upon binding of Mg²⁺.

Full text of DOI:10.1039/c002401e

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Remarkable Mg2+-selective emission of an azacrown receptor based
on Ir(III) complexw
Jeremy Brandel, Masaki Sairenji, Kyoko Ichikawa and Tatsuya Nabeshima*
Received 3rd February 2010, Accepted 9th April 2010
First published as an Advance Article on the web 5th May 2010
DOI: 10.1039/c002401e
A new aza-15-crown-5 ether-appended iridium complex was
synthesized and showed promising on-off selective emission-
triggering by inhibition of photoinduced electron transfer
The absorption spectrum of 3 in CH3CN at 298 K shows a
classical intense high energy absorption band between 220–320 nm
assigned to intraligand (IL) (p–p* bpy and az-ppy) transitions,
and weaker absorption shoulders above 320 nm assigned to
metal-to-ligand charge-transfer (MLCT) (dp(Ir)–p*(bpy and
az-ppy)) transitions. The absorption spectra of 1 and 2 are
composed of similar IL and MLCT bands plus additional
bands centered at 480 nm for 2 (Fig. 2) and from 320–450 nm
for 1. These bands are assigned to an overlap of the MLCT
bands and intraligand charge transfer (ILCT) transitions
originating from the azacrown nitrogen atoms as suggested
by their disappearance upon protonation. These assignments
are consistent with literature data2,9–11 and in accordance with
our TD-DFT calculations (Fig. 3).
(PET) upon binding of Mg2+
.
Selective sensing of metal ions plays a crucial role in fields as
diverse as chemistry, biology, environmental and material
sciences. The ionophore–luminophore approach has become
of particular interest to design chemosensors as a convenient
and high-sensitivity strategy. Transition metal complexes such
as platinum(II), ruthenium(II) or rhenium(I) proved to have
many advantages as luminophores (large Stokes shift, long
excited-state lifetime) compared to organic emitters. Recently,
iridium complexes emerged as especially suitable candidates in
the design of new chemosensors, luminescent biological
labels, OLEDs, or molecular switches due to their exceptional
luminescent properties and the possibility to fine-tune these
properties by ligand substitution.1,2 The aza-crown ether motif
is of particular interest as an ionophore due to its on-off
luminescence switching upon cation binding through inhibition
of the PET process3 and ability to bind several biologically
relevant cations. Yet, very few studies have investigated the
potential of crown ether appended iridium complexes.4,5 We
report herein the synthesis of two novel heteroleptic iridium
complexes (1, 2) bearing two aza-15-crown-5 ethers. Complex
2 showed outstanding on-off luminescence triggered by the
specific binding of Mg2+, which would represent an interesting
chemosensor as magnesium is one of the most important
biologically relevant metals. In addition, complex 2 has
great potential as a molecular switch due to its very clear on
and off states.
The stability constants of 1 and 2 with a series of alkali and
alkaline-earth metal ions (M = Na+, K+, Mg2+, Ca2+
)
were determined by UV-visible titrations in acetonitrile with
increasing amounts of metal ions. The titrations of 1 by the
metal ions showed very small spectral variations, preventing
the determination of the stability constants and suggesting low
affinity. On the contrary, 2 showed a blue shift of the spectra
and the disappearance of the ILCT band upon coordination of
metal ions (Fig. 1), in agreement with their binding to the
aza-crown ether moiety. These results underline the crucial
impact of the substitution position of the binding group on its
coordination properties.
TD-DFT calculations were performed on model complexes,
in which the crown ether moiety was replaced by a dimethyl-
amino group to simulate the free complexes (M1, M2, see
ESIw) and by their protonated counterparts to simulate the
metallated complexes (M1-H, M2-H, see ESIw) to simplify
the calculations without altering the chromophoric part of
the receptor. This approach has been adopted successfully in
The aza-15-crown-5 ether phenylpyridines (az-ppy) used as
cyclometalating ligands were synthesized with yields of
40–42% by Pd-catalyzed cross-coupling reactions of the corres-
ponding bromophenylpyridine with aza-15-crown-5 ether
(Scheme 1).6 The cationic heteroleptic iridium(III) complexes
(1, 2) were obtained by a two-step procedure7 involving
the synthesis of a cyclometalated dichloro-bridged diiridium
dimer and its subsequent cleavage with a neutral 5,50-di-
methyl-2,20-bipyridine. The previously reported complex 38
was used as a reference to discuss the effects of substitution.
Graduate School of Pure and Applied Sciences, University of
Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: nabesima@chem.tsukuba.ac.jp; Fax: +81-29-853-4507;
Tel: +81-29-853-4507
w Electronic supplementary information (ESI) available: Experimental,
synthesis, spectrophotometric and DFT results for complexes 1 and 2
and their metal complexes. See DOI: 10.1039/c002401e
Scheme 1 Synthesis of complex 2. (a) Aza-15-crown-5 ether, Pd(OAc)2,
P(t-Bu)3, t-BuONa, toluene. (b) IrCl3ꢀ3H2O, 2-ethoxyethanol/H2O (3:1).
(c) 5,50-Dimethyl-2,20-bipyridine, AgClO4, 1,2-dichloroethane/CHCl3
(2 : 1).
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and log K2 = 2.8(1) for Mg2+, log K1 = 6.1(1) and log K2 =
3.2(2) for Ca2+. It can be noted that the spectral variations
upon coordination of K+ (r = 1.38 A) were negligible,
suggesting very low binding affinity and reflecting the mismatch
of size with the aza-15-crown-5 cavity (r = 0.85–1.1 A).14 The
variation of the binding constants with the nature of the cation
is in agreement with that reported for other systems bearing
aza-15-crown-5 ether ionophores.15 Considering that the two
free crown-ether binding sites are equivalent and independent
Fig. 1 Structure of the complexes.
1
(as suggested by H NMR) we can deduce a strong coordina-
tion of the first metal ion to 2 with negative cooperativity
towards the second binding (4K2/K1 o 1).16
The emission spectra in degassed acetonitrile showed a
complete extinction of luminescence of 1 and 2 (F = 0.00,
lex = 380 nm, Fig. 4) compared to the model complex 3
(F = 0.20, lem = 558 nm, lex = 400 nm)8 with no crown ether
moiety. This quenching effect is due to classic PET from the
nitrogen atom of the aza-crown ether to the iridium(III)
center.3 Complex 1 showed no significant increase of lumines-
cence upon binding of cations as a result of its low binding
ability. Cation binding by 2, bearing the aza-15-crown-5 ether
groups in the para-position (relative to the iridium atom) of
the az-ppy phenyl group, exhibited an outstanding on-switching
of the luminescence in the blue-green region upon specific
coordination of Mg2+ (F = 0.15, lem = 475 and 507 nm,
lex = 380 nm), whereas its Na+, K+ and Ca2+ complexes
remained non emissive (Fig. 4). The emission of cationic
iridium(III) complexes containing diimine ligands commonly
Fig. 2 UV-visible titration of 2 with magnesium perchlorate. [2] =
1.0 ꢂ 10ꢃ5 M, CH3CN. Inset shows the fit (line) of the experimental
data (dots) at 258 nm.
3
3
arises from a mixed excited-state containing IL and MLCT
transitions, that is responsible for the multiple emission
maxima (or shoulders) observed. Our results corroborated
recent studies on iridium complexes bearing benzylsulfonyl-17
and perfluorophenyl-18 phenylpyridine, which showed that the
position of the substitution plays a decisive role in the emission
wavelength tuning. In particular, the metallated aza-crown
ethers act like withdrawing substituents (as opposed to their
donating effect when they are metal-free), inducing a stabilisa-
tion of the HOMO of 2-Mg2+ and a subsequent blue shift of
51 nm compared to 3 (lem = 558 nm).
Fig. 3 Experimental absorption spectra of 1 ([1] = 1.00 ꢂ 10ꢃ5 M,
CH3CN) (line) and TD-DFT-simulated absorption spectra of its
model M1 (sticks). Isodensity plots of the 5 transitions with the biggest
oscillator strength (isodensity value 0.03).
This cation-triggered luminescence can be rationalised by
analogy with the kinetic models proposed by Schanze and
MacQueen for the interpretation of cation coordination on the
reported semi-empirical and TD-DFT calculations.12,13 The
calculated absorption transitions showed a good fit with the
experimental spectra for both the free (1 given as an example,
Fig. 3) and metallated complexes, suggesting the pertinence of
our model and the use of TD-DFT calculations to predict
absorption properties of such complexes. Moreover the plot of
the molecular orbitals corresponding to the five transitions
with highest oscillator strength of 1 confirmed the attribu-
tion of the absorption bands above 320 nm to a mixing
of ILCT (s8, s11, s12, s19) and MLCT (s13) transitions
(Fig. 3 and ESIw).
Statistical processing of the spectrophotometric data for 2
supported the formation of 1 : 1 and 2 : 1 M : L species corres-
ponding to the successive ligation of two metal ions to the
two crown ether moieties of the az-ppy with values of log
K1 = 3.93(4) and log K2 = 2.8(1) for Na+, log K1 = 4.58(4)
Fig. 4 Emission spectra of 2 and 2-Mn+. [2] = 1.0 ꢂ 10ꢃ5 M,
[Mn+]/[2] = 2000. lex = 380 nm. Solvent: degassed CH3CN. Inset:
Photograph of samples under ambient light (top) and UV irradiation
(bottom, lex = 365 nm).
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binding as a new powerful and convenient color and intensity
tuning strategy for iridium complexes. In order to draw a more
complete picture of the potential of these molecules a systematic
study of an extended series of complexes is under way.
The authors would like to acknowledge Prof. T. Arai and
Prof. Y. Nishimura from Graduate School of Pure and Applied
Sciences, University of Tsukuba for the measurement of the
luminescence lifetime of 2-Mg2+. This work was supported by
Grants-in-Aid for Scientific Research from the Minister of
Education, Culture, Sports and Technology, Japan.
Notes and references
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Scheme 2 Kinetic model of possible decay pathways of 2.
photophysics of a crown-ether-appended Re(I) complex19 and
by Tung and coworkers on a Pt(II)-crown ether complex.15 The
similar mechanism suggested for 2-Mn+ is described by
Scheme 2 and suggests that, in addition to the normal radia-
3
tive and nonradiative pathways (kr, knr), the MLCT excited
state can also decay via a nonradiative path involving the
dissociation of the crowned cation (koff) followed by the rapid
3
internal conversion to a non-emitting low-lying LLCT state
(kic).20 We suggest this dissociation pathway to be responsible
for the non-emissive nature of the metal complexes of 1 and
2 with the exception of 2-Mg2+. The specific on-triggering
of 2 by Mg2+ is due to its much higher charge density
(Na+: 0.05, K+: 0.10, Ca2+: 0.24, Mg2+: 0.75),21 inducing a
stronger interaction between Mg2+ and the aza-crown nitrogen
leading to normal radiative and nonradiative decay of the
3MLCT state (kr, knr) rather than to cation dissociation
(kon, koff). This strong binding of Mg2+ by the aza-crown
ether nitrogen is supported by the fact that the absorption
spectrum of 2-Mg2+ is almost identical to the protonated 2.
It can be noted that the long emission lifetime of 2-Mg2+
(t = 2.45 ms in acetonitrile) together with its high sensitivity
towards O2 quenching is in accordance with an emission
originating from phosphorescence resulting from the enhance-
ment of the spin–orbit coupling.4
In conclusion we report a new phosphorescent aza-15-
crown-5-ether appended heteroleptic iridium complex (2) showing
outstanding Mg2+ specificity and unambiguous on (2-Mg2+
)
and off (free 2) states (Fig. 3, Inset) with a good quantum yield,
suggesting promising future development in the fields of
chemosensors or molecular switches. Moreover, to the best
of our knowledge, this study is the first example of substitution
effect on chelation-induced luminescence properties in iridium
complexes. The drastic structure/properties differences between
the two complexes considered in this work suggested metal
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This journal is The Royal Society of Chemistry 2010
3960 | Chem. Commun., 2010, 46, 3958–3960