Design and synthesis of iridium(iii) azacrown complex: Application as a highly sensitive metal cation phosphorescence sensor

Article abstract of DOI:10.1039/b511943j

A new metal cation probe 1 bearing a central Ir(iii) element and 1-aza-15-crown-5-ether substituted pyridyl pyrazolate as the chelate was synthesized. The octahedral molecular structure of 1 was confirmed using single crystal X-ray diffraction analyses. S

Full text of DOI:10.1039/b511943j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Design and synthesis of iridium(III) azacrown complex: application as a
highly sensitive metal cation phosphorescence sensor
Mei-Lin Ho,a Fu-Ming Hwang,b Pei-Nung Chen,b Ya-Hui Hu,a Yi-Ming Cheng,a Kung-Shih Chen,a
Gene-Hsiang Lee,a Yun Chi*b and Pi-Tai Chou*a
Received 22nd August 2005, Accepted 27th October 2005
First published as an Advance Article on the web 18th November 2005
DOI: 10.1039/b511943j
A new metal cation probe 1 bearing a central Ir(III) element and 1-aza-15-crown-5-ether substituted
pyridyl pyrazolate as the chelate was synthesized. The octahedral molecular structure of 1 was
confirmed using single crystal X-ray diffraction analyses. Subsequent photophysical study showed
yellow-green emission at ∼560 nm in both fluid solution and solid state at room temperature.
Remarkable differentiation in spectral properties upon metal cation (e.g. Ca2+) complexation makes
complex 1 a highly sensitive phosphorescence probe.
centered pp* excited states,7 enable both the HOMO and LUMO
Introduction
to reside predominantly on the azacrown substituted pyridyl
Detection of alkali or alkaline earth ions has great potential
pyrazolate segment. This architecture enhances the effectiveness
for practical applications in areas such as analytical chemistry,
of this design over others having more delocalized electronic
environmental chemistry and the biological sciences.1 One method
configurations; the latter should be less capable of recognizing the
is to use the so-called chemosensor that shows the basic molecular
cation due to the spreading of their electronic perturbation over
configurations such as chromophore–receptor or chromophore–
the whole complex. Thirdly, the azacrown fragment is attached
spacer–receptor. As the receptor can selectively bind with the
to an anionic pyrazolate chelate ligand, forming a neutral Ir(III)
guest cations, measurable and reversible changes in color and/or
metal complex. Such a charge-neutral characteristic is similar to
luminescence can be detected at the signalling unit (i.e. chro-
that of the Re(I) and Pt(II) based sensor complexes, but is in sharp
contrast to most of the Ru(II) based polypyridyl sensors,8 for which
the net cationic charge on the overall metal complex is expected to
reside, in part, at the azacrown ether site, giving a much reduced
sensitivity in recognizing metal cations.9
mophore) linking to the receptor, which allows the recognition and
quantification of the metal cation by conventional spectroscopic
methods.2 A common design contains a crown ether to serve as
the receptor, together with an organic dye or other metal based
fluorophores.3 Recently, extension was made to systems with third-
row transition-metal complexes.4 Work by a number of groups,
mainly pioneered by Schanze and co-workers,5 has established
the importance of utilizing long-lived phosphorescence. Typical
signalling involves the switching between non-emissive intra-
ligand (3IL) pp* and metal–to–ligand charge transfer (3MLCT)
states,6 so that a positive luminescent response vs. concentration
of cation can be obtained. Moreover, it may allow the design
of “light-controlled ion switches”, for which the cation ejection
from the crown ether can be triggered by the effective reduction
of electron donation at the receptor on excitation of the nearby
chromophore.
In this article, we present a novel system in which an azacrown
receptor is attached to the pyridyl pyrazolate chelate of a heterolep-
tic Ir(III) complex. This design provides three inherent advantages.
First, the Ir(III) metal atom forms a highly stable, octahedral coor-
dinated structure and induces strong phosphorescent emission due
to the heavy atom effect. Moreover, the ancillary cyclometalated
phenyl pyrazole ligands, for which the pp* energy levels are far
higher than those of the respective MLCT and other ligand-
Experimental
General information and materials
Elemental analyses and mass spectra (operating in FAB mode)
were carried out at the NSC Regional Instrument Centre at
1
the National Chiao Tung University, Taiwan. H and 13C NMR
spectra were recorded on a Varian Mercury 400 or an Inova-
500 MHz instrument; chemical shifts are quoted with respect to
an internal standard, Me4Si. All synthetic manipulations were
performed under a N2 atmosphere, while solvents were used as re-
ceived. Synthesis of 1-[4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-
13-yl)-phenyl] ethanone follows the procedures reported by Oka-
hara and co-workers,10 using the starting materials 4-N,N-bis(2-
hydroxyethyl)aminoacetophenone and triethylene glycol di(p-
toluenesulfonate). Triethylene glycol di(p-toluenesulfonate) was
prepared using the literature method,11 while 1-phenyl-3,5-
dimethyl pyrazole (pdpz)H was prepared from the condensation
of phenyl hydrazine hydrochloride with acetylacetone according
to literature procedures.12 Treatment of (pdpz)H with IrCl3·nH2O
in refluxing methoxyethanol afforded the chloride bridged dimer
[(pdpz)2IrCl]2 in 75% yield;13 it was then used for subsequent
reactions without further purification.
aDepartment of Chemistry, National Taiwan University, Taipei, 106, Taiwan.
E-mail: chop@ntu.edu.tw
bDepartment of Chemistry, National Tsing Hua University, Hsinchu, 300,
Taiwan. E-mail: ychi@mx.nthu.edu.tw
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Synthesis of [3-(4-aza-15-crown-5-phenyl)pyridyl(1,3-dione)]
3H), 1.61 (s, 3H), 1.57 (s, 3H). Anal. calcd. for C46H51IrN8O4: C,
56.83; H, 5.29; N, 11.53. Found: C, 57.01; H, 5.13; N, 11.73.
Single crystal X-ray diffraction data were acquired on a Bruker
Smart CCD diffractometer using k(Mo Ka) radiation (k =
To a stirred suspension of NaH (0.75 g, 31.3 mmol) in THF
(40 mL), was slowly added 15 mL of THF solution of 1-[4-
(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-phenyl]-ethanone
(2.6 g, 7.8 mmol) at room temperature. The solution was heated to
reflux, and ethyl picolinate (2.0 mL, 15 mmol) was added over the
course of 1 h. After the addition was completed, the temperature
was gradually lowered to room temperature. The solution was
continuously stirred for another 3 h and then quenched with
dilute HCl solution. After the removal of the solvent, the residue
was extracted with CH2Cl2 (3 × 60 mL). The extracts were
combined, washed twice with water, and dried over anhydrous
MgSO4 to afford a yellow solid (2.4 g, 5.4 mmol, 70%).
˚
0.71073 A). Data collection was executed using the SMART
program. Cell refinement and data reduction were made with
the SAINT program. The structure was determined using the
SHELXTL/PC program and refined using full-matrix least
squares. All non-hydrogen atoms were refined anisotropically,
whereas hydrogen atoms were placed at the calculated positions
andincluded inthe finalstage ofrefinements withfixedparameters.
Selected crystal data of 1: C58H77IrN8O8, M = 1206.48, triclinic,
space group P-1, a = 10.3194(7), b = 16.8549(12), c = 18.3003(13)
◦
˚
˚
A, a = 63.527(2), b = 89.840(2), c = 76.006(2) , V = 2744.4(3)
A , Z = 2, qcalcd = 1.460 gcm−1, F(000) = 1244, crystal size =
3
Selected spectral data. 1H NMR (500 MHz, CDCl3, 294 K):
d 16.87 (s, 1H), 8.69 (d, JHH = 7.5 Hz, 1H), 8.13 (d, JHH = 7.5 Hz,
1H), 7.98 (d, JHH = 8.7 Hz, 2H), 7.87 (dd, JHH = 7.5, 4.5 Hz,
˚
0.35 × 0.17 × 0.08 mm, k (Mo Ka) = 0.7107 A, T = 150(1) K, l =
2.495 mm−1, 12566 reflections collected (Rint = 0.0645), final R1[I
> 2((I)] = 0.0558 and wR2(all data) = 0.1406.
1H), 7.49 (s, 1H), 7.42 (dd, JHH = 7.5, 4.5 Hz, 1H), 6.68 (d, JHH
=
8.7 Hz, 2H), 3.77 (t, JHH = 6.0 Hz, 4H), 3.67 ∼ 3.63 (m, 12H), 3.61
Preparation of [(pdpz)2Ir(dappz)] (2)
(s, 4H).
A mixture of [(pdpz)2IrCl]2 (100 mg, 0.088 mmol), NMe2 substi-
tuted pyridyl pyrazole (dappzH, 51 mg, 0.19 mmol) and Na2CO3
(93 mg, 0.88 mmol) in 2-methoxyethanol (25 mL) was heated to
reflux for 4 h. Excess of water was added after cooling the solution
to room temperature. The precipitate was collected by filtration
and subjected to silica gel column chromatography using ethyl
acetate and methanol (5 : 1) as the eluent. Yellow-green powders
of [(pdpz)2Ir(dappz)] (2) were collected after washing with acetone;
yield: 50 mg, 0.063 mmol, 36%.
Synthesis of 3-(4-aza-15-crown-5-phenyl) pyridyl pyrazole
A solution of [3-(4-aza-15-crown-5-phenyl) pyridyl(1,3-dione)]
(1.7 g, 3.8 mmol) and 98% of hydrazine monohydrate (1.94 mL)
in 45 mL of anhydrous ethanol was refluxed for 12 h. Next, the
solvent was removed under vacuum, and the residue was dissolved
in CH2Cl2, washed twice with water, dried over anhydrous MgSO4,
and the solution was concentrated to dryness. The crude product
was purified by silica gel column chromatography (ethyl acetate
and methanol = 5 : 1, v/v), giving a light yellow powdery material
(azppz)H (1.2 g, 2.74 mmol, 71%).
Spectral data of 2. MS (FAB, 193Ir): actual m/z (calculated)
1
[assignment]: 799 (798.3) [M + 1]. H NMR (500 MHz, CD2Cl2,
Selected spectral data. 1H NMR (500 MHz, CDCl3, 294 K):
294 K): d 7.68 ∼ 7.61 (m, 3H), 7.52 (d, JHH = 8.5 Hz, 2H), 7.41
(dd, JHH = 9.0, 8.5 Hz, 2H), 6.96 ∼ 6.92 (m, 2H), 6.87 (s, 1H), 6.79
(t, JHH = 6.5 Hz, 1H), 6.77 ∼ 6.71 (m, 2H), 6.66 (d, JHH = 8.5 Hz,
2H), 6.42 (d, JHH = 7.5 Hz, 1H), 6.31 (d, JHH = 7.5 Hz, 1H), 5.98
(s, 1H), 5.94 (s, 1H), 2.89 (s, 6H), 2.79 (s, 3H), 2.74 (s, 3H), 1.62 (s,
3H), 1.57 (s, 3H). Anal. calcd. for C38H37IrN8: C, 57.20; H, 4.67;
N, 14.04. Found: C, 57.52; H, 4.43; N, 14.33.
d 8.6 (d, JHH = 4.5 Hz, 1H), 7.79 ∼ 7.72 (m, 2H), 7.61 (d, JHH
9.0 Hz, 2H), 7.21 (t, JHH = 4.5 Hz, 1H), 6.96 (s, 1H), 6.79 (d, JHH
9.0 Hz, 2H), 3.76 (t, JHH = 6.2 Hz, 4H), 3.66 ∼ 3.60 (m, 16H).
=
=
Preparation of [(pdpz)2Ir(azppz)] (1)
A mixture of [(pdpz)2IrCl]2 (100 mg, 0.088 mmol), azacrown
substituted pyridyl pyrazole (azppzH, 85 mg, 0.19 mmol) and
Na2CO3 (93 mg, 0.88 mmol) in 2-methoxyethanol (25 mL) was
heated to reflux for 4 h. Excess of water was added after cooling
the solution to room temperature. The precipitate was collected
by filtration and subjected to silica gel column chromatography
using ethyl acetate and methanol (5 : 1) as eluent. Yellow-green
crystals of [(pdpz)2Ir(azppz)] (1) were obtained from repeated
recrystallization using a mixture of THF and pentane at room
temperature (70 mg, 0.07 mmol, 41%).
Preparation of [Ph2B(azppz)] (3)
In a 50 mL reaction flask, a mixture of azacrown substituted
pyridyl pyrazole (azppzH, 219 mg, 0.5 mmol), 4.4 mL of 2.5 M
BPh3, and 20 mL of anhydrous THF solvent was heated to
reflux for 24 h. The solution was then concentrated to dryness
and an orange-red crystalline solid of 3 was obtained from
recrystallization using a mixture of CH2Cl2 and methanol (100 mg,
0.166 mmol, 33%).
Spectral data of 1. MS (FAB, 193Ir) actual m/z (calculated)
[assignment]: 973 (972.4) [M + 1]. H NMR (500 MHz, CD2Cl2,
Spectral data of 3. MS (FAB, 11B), actual m/z (calculated)
[assignment]: 602 (602.3) [M+]. 1H NMR (500 MHz, CD2Cl2, 294
K): d 8.47 (d, J = 7.5 Hz, 1H), 8.08 (m, 1H), 7.84 (d, J = 7.5 Hz,
1H), 7.32 (d, J = 8.5 Hz, 2H), 7.40 ∼ 7.37 (m, 1H), 7.30 ∼ 7.28
(m, 4H), 7.24 ∼ 7.18 (m, 6H), 6.95 (s, 1H), 6.68 (d, J = 8.5 Hz,
2H), 3.72 (t, J = 6.2 Hz, 4H), 3.61 ∼ 3.60 (m, 8H), 3.58 ∼ 3.55
(m, 8H). Anal. calcd. for C36H39BN4O4: C, 71.76; H, 6.52; N, 9.30.
Found: C, 71.40; H, 6.61; N, 9.27.
1
294 K): d 7.68 ∼ 7.61 (m, 3H), 7.52 (d, JHH = 8.5 Hz, 2H), 7.41
(dd, JHH = 9.0, 8.5 Hz, 2H), 6.97 ∼ 6.92 (m, 2H), 6.86 (s, 1H), 6.79
(t, JHH = 6.3 Hz, 1H), 6.78 ∼ 6.71 (m, 2H), 6.58 (d, JHH = 8.5 Hz,
2H), 6.42 (d, JHH = 7.5 Hz, 1H), 6.31 (d, JHH = 7.5 Hz, 1H), 5.98
(s, 1H), 5.94 (s, 1H), 3.68 (t, JHH = 6.0 Hz, 4H), 3.60 ∼ 3.57 (m,
8H), 3.56 (s, 4H), 3.52 (t, JHH = 6.0 Hz, 4H), 2.79 (s, 3H), 2.74 (s,
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Measurements
Steady-state absorption and emission spectra were recorded by a
Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920)
fluorimeter, respectively. Emission quantum yields were measured
at excitation wavelength kexc = 380 nm in CH2Cl2 at room
temperature. In this approach, coumarin 480 (Exciton, Ur = 0.93
in EtOH) was used as the reference. The association constant
Ka of 1 : 1 1/guest complex formation calculated by the UV-Vis
absorption method is obtained by the following equation.14
ꢀ
ꢁ ꢂ
ꢃ
A0
A0 − A
eM
1
=
+ 1
(1)
eM − ep
KaCg
where Cg is the added guest (e.g. Ca2+) concentration. A0 (eM)
and A (eP) denote the absorbance (molar extinction coefficient)
of free 1, and in solution after adding e.g. Ca2+, respectively, at
a selective wavelength. Eqn. (1) can be further extended to the
emission titration experiment expressed as14
ꢀ
ꢁ
F0
F0 − F
UMeM
1
=
+ 1
(2)
(UMeM − Upep) KaCg
where F0 (UM) and F (UP) denote the photoluminescence (quan-
tum yield) of free 1, and in solution after adding Ca2+, respectively,
at a selective wavelength.
Scheme 1 Synthetic scheme of 1 and molecular structures of 2 and 3.
with hydrazine monohydrate in ethanol solution (Scheme 1).
The subsequent reaction of (azppz)H with the chloride bridged
dimer complex [(pdpz)2IrCl]2 and slight excess of Na2CO3 in
refluxing methoxyethanol solution afforded the required iridium
chelate complex [(pdpz)2Ir(azppz)] (1), where (pdpz)H = 1-phenyl-
3,5-dimethyl pyrazole. Moreover, the analogous dimethylamino
substituted iridium derivative (2) and a BPh2 substituted complex
(3) (see Scheme 1) bearing identical azppz pyrazolate ligand were
synthesized from reaction with the boron reagent BPh3, and these
then served as the standards for photophysical measurements.
As indicated in Fig. 1, single crystal X-ray structural analysis
of 1 shows the expected octahedral geometry around the irid-
ium metal center, along with two N-phenyl pyrazole fragments
and one azacrown substituted pyridyl pyrazolate ligand. The
cyclometalated N-phenyl pyrazoles adopt an eclipse orientation,
while the azacrown pyrazolate chelate is located opposite to the
cyclometalated carbon atoms, with the spatial arrangement being
akin to those observed for other heteroleptic pyridyl pyrazolate
For the phosphorescence lifetime measurements in the microsec-
ond region, a third harmonic of an Nd:YAG laser 355 nm (∼8 ns)
was used as an excitation source. Emission decay was detected
by a photomultiplier tube and averaged over 500 shots using
an oscilloscope. Laser energy was reduced to <1 mJ pulse−1 to
prevent possible photochemical decomposition. For the lifetime
measurements of <10 ns, the fundamental train of pulses from
a Ti-sapphire oscillator (82 MHz, Spectra Physics) was used
to produce second harmonics (375 ∼ 425 nm, ∼120 fs) as
an excitation light source. The signal was detected by a time-
correlated single photon counting system (Edinburgh OB 900-L).
The system response time was determined to be ∼30 ps.
Computational methodology
All calculations were performed with the Gaussian03 package.15
Geometrical optimization on the electronic ground state was
carried out using the hybrid Hartree–Fock/Density functional
theory (HF/DFT) method, B3LYP.16 “Double-f” quality ba-
sis set consisting of Hay and Wadt’s effective core potentials
(LANL2DZ)17 was employed for iridium atom and 6-31G* basis18
for H, C, and N atoms. A relativistic effective core potential (ECP)
replaced the inner core electrons of Ir(III), leaving the outer core
(5s25p6) electrons and the 5d6 valence electrons. Time-dependent
DFT (TDDFT) calculations were then performed with the same
functional and basis set at the optimized geometry to obtain
electronic transition energies. Oscillator strengths were deduced
from the dipole transition matrix elements (for singlet states only).
Results and discussion
A multi-step synthetic pathway leading to the desired iridium
metal complexes is depicted in Scheme 1. First of all, an
azacrown substituted pyridyl pyrazole ligand, (azppz)H, was
obtained from the condensation reaction of an azacrown substi-
tuted acetophenone and ethyl picolinate, followed by treatment
Fig. 1 The X-ray crystal structure of 1; selected distances: Ir–C(25) =
1.984(6), Ir–C(36) = 2.020(6), Ir–N(1) = 2.155(5), Ir–N(2) = 2.112(5),
˚
Ir–N(5) = 2.023(5) and 2.021(5) A.
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complexes.19 Moreover, the azacrown fragment shows unusually
large deformation toward the iridium metal fragment, for which
the calculated dihedral angle between the adjacent p-phenyl group
and the O4N plane of azacrown is 117.5◦.
Table 1 Photophysical data of 1 and the respective cationic adducts
PL kmax
Q.Y.a
lifetime (s)a
Ka
1
560 nm
520 nm
518 nm
521 nm
520 nm
0.001 (0.22)
0.012 (0.20)
0.013 (0.23)
0.010 (0.20)
0.012 (0.18)
38 ns (8.2 ls)
42 ns (0.6 ls)
48 ns (0.8 ls)
41 ns (0.6 ls)
40 ns (0.5 ls)
1/Ca2+
1/Mg2+
1/Ba2+
1/Na+
4.2 × 104
1.6 × 105
5.3 × 103
4.7 × 103
As shown in Fig. 2, ion-free 1 in CH3CN exhibits a 318 nm
absorption band, accompanied by a shoulder at ∼375 nm. In
comparison, the boron complex 3 bearing the same pyridyl
pyrazolate ligand reveals two low-lying absorption bands with
peak wavelengths at 315 and 375 nm, the spectral features of which
are similar to that of complex 1. Due to lack of both MLCT and the
transition associated with cyclometalated phenyl pyrazole ligand
in boron complex 3, it is reasonable to assign both 315 and 375 nm
bands as the 1pp* transitions incorporating pyrazolates → pyridyl
types of charge transfer. Transitions associated with MLCT in
a data in parentheses are measured in the degassed solution.
Likewise, the corresponding observed lifetime decreased from 8 ls
(degassed) to 38 ns (aerated, see Table 1). The ∼1/9 diffusion
controlled rate of O2 quenching, in combination with a long
radiative decay rate of 3.5 × 104 s−1, leads to an unambiguous
conclusion that 1 exhibits predominantly the phosphorescence
resulting from the enhancement of Ir(III) spin–orbit coupling.
Upon addition of Ca2+, the absorption and emission titration
spectra of 1 (2.2 × 10−5 M) in CH3CN are shown in Figs. 2 and 3,
respectively. Increasing [Ca2+] leads to a hypsochromic shift of the
absorption profile, in which the appearance of an isosbestic point
at ∼295 nm verifies a two-species equilibrium. The 1 : 1 1/Ca2+
complexation was supported by a straight-line plot for the ratio
of absorbance, A0/(A0 − A), versus 1/[Ca2+] (see the experimental
section) throughout the titration, and an association constant Ka
of 4.0 × 104 M−1 was thus deduced in CH3CN. Likewise, drastic
changes on the Ca2+ phosphorescence titration spectra were also
observed. Upon excitation at the isosbestic point of 295 nm,
the 560 nm phosphorescence was gradually blue shifted toward
520 nm, accompanied by the increase of the emission intensity.
Taking the emission peak intensity of Ca2+-free and Ca2+-added
complex 1 to be F0 and F, respectively, a straight line plot of
F0/(F − F0) versus 1/[Ca2+] (see the experimental section) at e.g.
520 nm is depicted in the insert of Fig. 3. The deduced Ka value
of 4.2 × 104 M−1, within experimental error, is in agreement with
that extracted from the absorption titration.
1
1 are probably too weak and thus are hidden inside the pp*
bands. Further firm support is given in the theoretical approaches
(vide infra). Likewise, absorption features associated with the 3pp*
3
intra-ligand bands and MLCT could not be resolved although
an effective enhancement of the spin–orbit coupling from Ir is
expected.
Fig. 2 Absorption spectrum changes of 1 (2.2 × 10−5 M) upon addition
of various concentrations of anhydrous Ca(ClO4)2 in aerated CH3CN
solution (a) 0, (b) 1.45 × 10−5, (c) 1.83 × 10−5, (d) 1.84 × 10−5, (e) 2.13 ×
10−5, (f) 2.71 × 10−5, (g) 4.39 × 10−5, (h) 8.70 × 10−5, (i) 1.45 × 10−4
,
(j) 2.03 × 10−4 M. (---) absorption spectrum of 3 in CH3CN. Insert: the
plot of A0/A0 − A against 1/[Ca2+] at 320 nm.
It should be noted that both complexes 2 and 3 showed
negligible changes in their absorption and emission spectra upon
addition of Ca2+, manifesting the importance of the coexistence
of Ir and 1-aza-15-crown-5-ether in complex 1 toward the Ca2+
recognition. The results can be rationalized by weakening the
electron donating ability of the aza-nitrogen upon Ca2+/azacrown
complex formation and consequently greatly alters the photo-
physical properties of 1. This viewpoint can be firmly supported
by theoretical modelling. Presently, ab initio calculation on 1 is
formidable due to its structural complexity. Alternatively, the
dimethyl amino analogue 2 was selected, of which the major ligand
chromophores remain intact with respect to 1. Furthermore,
the protonated form of 2, 2H+, serves as a model for the Ca2+
bonded complex 1. We then applied density functional theory
incorporating B3LYP method with 6-31G* basis for non-metal
atoms and a relativistic effective core potential for the inner core
electrons of Ir(III) metal atom. The resulting frontier orbitals for
the low-lying transitions revealed drastic differences between 2
and 2H+. As shown in Fig. 4, the lowest triplet manifold for 2
exhibits predominantly HOMO → LUMO intraligand charge
transfer (ILCT) transition, in which HOMO and LUMO are
mainly located at the phenyl pyrazolate and pyridyl chromophores,
respectively. In sharp contrast, upon protonation (2H+) the
The emission spectrum of 1 in CH3CN is depicted in Fig. 3
and the corresponding relaxation dynamics are listed in Table 1.
The emission band with a peak wavelength at 560 nm revealed a
drastic oxygen quenching effect, the intensity of which decreased
from 0.22 in degassed CH3CN, to ∼1.0 × 10−3 upon aeration.
Fig. 3 The emission spectrum changes of 1 (2.2 × 10−5 M) upon addition
of various concentrations of anhydrous Ca(ClO4)2 in aerated CH3CN
solution (a) 0, (b) 3.15 × 10−6, (c) 3.25 × 10−6, (d) 5.40 × 10−6, (e) 5.57 ×
10−6, (f) 7.89 × 10−6, (g) 1.16 × 10−5, (h) 2.09 × 10−5, (i) 3.48 × 10−5
,
(j) 5.8 × 10−5, (k) 1.85 × 10−4 M. kex: 300 nm. Insert: the plot of F0/F −
F0 against 1/Cg at 520 nm.
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a salient change of emission from yellow-orange for the Ca2+-free,
complex 1 coated TLC plate to a bright green emission upon
complexation with the Ca2+ ion.
Conclusions
In conclusion, we demonstrate a novel metal ion sensor, complex 1,
based on the rare metal-ion sensitive phosphorescence. In this case,
the chromophores designed act as both recognition and signal-
transducing units, while the center heavy metal, i.e. Ir(III), serves
as a perturbing base to enhance the phosphorescence. Thus, the
current system is versatile in that functional derivatization can
be achieved with methods similar to those strategically designed
for the singlet pp* (i.e. fluorescence) ligand chromophores. In
view of drastic oxygen quenching, in a steady state approach,
one can then saturate the solution with N2 so that the enhanced
phosphorescence can also serve as an additional signalling to
distinguish it from the fluorescence interference. Alternatively, in a
time-resolved manner, due to its much longer lifetime (even in the
aerated solution) than that of typical fluorescence, the associated
phosphorescence can be obtained free from fluorescence interfer-
ences in the solution simply by acquiring the phosphorescence
only after a certain time delay of the excitation pulse. The success
in the recognition of Ca2+(aq) in the TLC plate demonstrates its
suitability for the future development of a practical device, such
as a metal ion sensor anchored on cellular membranes. We thus
believe that results presented in this study may spark a broad
range of interest in both fundamental approach and applications
relevant to the third-row transition metal complexes.
Fig. 4 Frontier orbitals of complexes 2 and 2H+, see text for detail
description.
transition switches greatly to a ligand–ligand charge transfer
(LLCT) incorporating cyclometalated phenyl pyrazole ligand
(HOMO) → p-dialkylamino-phenyl pyrazolate (LUMO). Assum-
ing that the differences in photophysical behavior between 2 and
2H+ can be likewise applied to 1 and 1/Ca2+ complex, changes
of absorption and emission spectra during Ca2+ titration are
thus be rationalized by the swap of lowest transition from ILCT
(p-dialkylaminophenyl pyrazolate → pyridine) in 1 to LLCT
(cyclometalated phenyl pyrazolate → p-dialkylaminophenyl pyra-
zolate) in 1/Ca2+ complex.
Similar titration experiments have been performed for the hard,
bivalent metal cation such as Mg2+ and Ba2+ and Ka values of
1.6 × 105 M−1 and 5.3 × 103 M−1, respectively, were obtained. In
another approach, negligible changes of absorption spectra were
observed for soft divalent ions like Hg2+. Titration experiments
were also performed for Na+ and the results indicated a Ka value
of 4.7 × 103 M−1 for the 1/Na+ complex formation. The resulting
metal ion dependent association strengths can be qualitatively
rationalized by the amount of charge density, q, specified as q =
g/(4/3pr3) where g and r are the corresponding formal charge (+1
Acknowledgements
This work was funded by the National Science Council of
Taiwan, R.O.C. under grants: NSC 93-2113-M-007-012 and NSC
93-2752-M-002-002-PAE. We are also grateful to the National
Center for High-Performance Computing for generous amounts
of computing time.
References
or +2) and radius, respectively. Evidently, the value of q(Mg2+
∼
1 (a) H.-G. Lo¨hr and F. Vo¨gtle, Acc. Chem. Res., 1985, 18, 65; (b) W.-S.
Xia, R. H. Schmehl and C.-J. Li, J. Am. Chem. Soc., 1999, 121, 5599;
(c) B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3; (d) A. P. de
Silva, D. B. Fox, A. J. M. Huxley and T. S. Moody, Coord. Chem. Rev.,
2000, 205, 41; (e) A. P. de Silva, D. B. Fox, T. S. Moody and S. M. Weir,
Pure Appl. Chem., 2001, 73, 503.
2 (a) K. S. Schanze, D. B. MacQueen, T. A. Perkins and L. A. Cabana,
Coord. Chem. Rev., 1993, 122, 63; (b) G. W. Gokel, W. M. Leevy and
M. E. Weber, Chem. Rev., 2004, 104, 2723; (c) S.-W. Lai and C.-M. Che,
Top. Curr. Chem., 2004, 241, 27.
3 (a) A. P. de Silva, H. Q. Nimal Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev.,
1997, 97, 1515; (b) P. D. Beer and E. J. Hayes, Coord. Chem. Rev., 2003,
240, 167.
4 (a) R. Slone, D. I. Yoon, R. M. Calhoun and J. T. Hupp, J. Am. Chem.
Soc., 1995, 117, 11813; (b) Y. Shen and B. P. Sullivan, Inorg. Chem., 1995,
34, 6235; (c) V. W.-W. Yam, C.-L. Chan, C.-K. Li and K. M.-C. Wong,
Coord. Chem. Rev., 2001, 216–217, 173; (d) V. W.-W. Yam, R. P.-L.
Tang, K. M.-C. Wong, X.-X. Lu, K.-K. Cheung and N. Zhu, Chem.
Eur. J., 2002, 8, 4066; (e) L. J. Charbonniere, R. F. Ziessel, C. A. Sams
and A. Harriman, Inorg. Chem., 2003, 42, 3466; (f) P. K. M. Siu, S.-W.
Lai, W. Lu, N. Zhu and C.-M. Che, Eur. J. Inorg. Chem., 2003, 2749;
(g) Q.-Z. Yang, L.-Z. Wu, H. Zhang, B. Chen, Z.-X. Wu, L.-P. Zhang
and C.-H. Tung, Inorg. Chem., 2004, 43, 5195.
0.75, Ca2+ ∼ 0.24, Na+ ∼ 0.10 and Ba2+ ∼ 0.13)20 correlates well
with the trend of Ka values deduced experimentally (see Table 1).
Furthermore, negligible spectral changes were observed for K+
throughout [K+] of 10−2 M. This result can simply be rationalized
by the mismatched sizes between K+ and the 1-aza-15-crown-5
ether.
In solid form 1 exhibits a 565 nm phosphorescent emission with
a quantum yield as high as 0.25 (sp ∼ 12 ls). Thus, from the
viewpoint of application, we have also examined whether a similar
recognition capability can be applied in the heterogeneous solid
film in aqueous solution. Note that 1 is insoluble in water. To
simplify the process, a silica-based TLC plate was used as a solid
support to soak 1 in CH3CN so that a complex 1 coated TLC plate
was prepared with an optical density of ∼1.0 at 350 nm. This plate
was then dipped into an aqueous solution (pH ∼ 7.0) containing
∼10−3 M CaClO4 for ∼30 s, and then vacuum-dried to remove
water. With the use of a commercially available UV-lamp (366
nm) as an excitation source, a photograph in TOC demonstrates
1 0 2 | Org. Biomol. Chem., 2006, 4, 98–103
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View Article Online
5 (a) D. B. MacQueen and K. S. Schanze, J. Am. Chem. Soc., 1991,
113, 6108; (b) C. E. Whittle, J. A. Weinstein, M. W. George and K. S.
Schanze, Inorg. Chem., 2001, 40, 4053.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X.
Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,
GAUSSIAN 03 (Revision C.02), Gaussian, Inc., Wallingford, CT,
2004.
6 (a) J. F. Michalec, S. A. Bejune, D. G. Cuttell, G. C. Summerton, J. A.
Gertenbach, J. S. Field, R. J. Haines and D. R. McMillin, Inorg. Chem.,
2001, 40, 2193; (b) J. D. Lewis, L. Bussotti, P. Foggi, R. N. Perutz and
J. N. Moore, J. Phys. Chem. A, 2002, 106, 12202; (c) J. D. Lewis and J. N.
Moore, Dalton Trans., 2004, 1376; (d) W.-S. Tang, X.-X. Lu, K. M.-C.
Wong and V. W.-W. Yam, J. Mater. Chem., 2005, 15, 2714.
7 A. B. Tamayo, B. D. Alleyne, P. I. Djurovich, S. Lamansky, I. Tsyba,
N. N. Ho, R. Bau and M. E. Thompson, J. Am. Chem. Soc., 2003, 125,
7377.
8 (a) Y. Shen and B. P. Sullivan, J. Chem. Educ., 1997, 74, 685; (b) P. D.
Beer, S. W. Dent, N. C. Fletcher and T. J. Wear, Polyhedron, 1996,
15, 2983; (c) M. E. Padilla-Tosta, J. M. Lloris, R. Martinez-Manez,
M. D. Marcos, M. A. Miranda, T. Pardo, F. Sancenon and J. Soto,
Eur. J. Inorg. Chem., 2001, 1475.
9 T. Lazarides, T. A. Miller, J. C. Jeffery, T. K. Ronson, H. Adams and
M. D. Ward, Dalton Trans., 2005, 528.
10 H. Maeda, S. Furuyoshi, Y. Nakatsuji and M. Okahara, Bull. Chem.
Soc. Jpn., 1983, 56, 212.
11 Y. Chen and G. L. Baker, J. Org. Chem., 1999, 64, 6870.
12 K. Y. Lee, J. M. Kim and J. N. Kim, Tetrahedron Lett., 2003, 44, 6737.
13 (a) M. Nonoyama, J. Organomet. Chem., 1975, 86, 263; (b) C.-H. Yang,
S.-W. Li, Y. Chi, Y.-M. Cheng, Y.-S. Yeh, P.-T. Chou, G.-H. Lee, C.-H.
Wang and C.-F. Shu, Inorg. Chem., 2005, 44, 7770.
14 (a) P.-T. Chou, G.-R. Wu, C.-Y. Wei, C.-C. Cheng, C.-P. Chang and F.-T.
Hung, J. Phys. Chem. B, 1999, 103, 10042; (b) P.-T. Chou, G.-R. Wu,
C.-Y. Wei, C.-C. Cheng, C.-P. Chang and F.-T. Hung, J. Phys. Chem.
B, 2000, 104, 7818.
16 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) C. Lee, W. Yang
and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
17 (a) T. H. Dunning and P. J. Hay, In Modern Theoretical Chemistry,
ed. H. F. Schaefer, III, Plenum Press, New York, 1976, vol. 3,
pp. 11; (b) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82,
270; (c) W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985, 82,
284; (d) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82,
299.
18 P. C. Hariharan and J. A. Pople, Mol. Phys., 1974, 27, 209.
19 (a) Y.-H. Song, S.-J. Yeh, C.-T. Chen, Y. Chi, C.-S. Liu, J.-K. Yu, Y.-H.
Hu, P.-T. Chou, S.-M. Peng and G.-H. Lee, Adv. Funct. Mater., 2004,
14, 1221; (b) F.-M. Hwang, H.-Y. Chen, P.-S. Chen, C.-S. Liu, Y. Chi,
C.-F. Shu, F.-I. Wu, P.-T. Chou, S.-M. Peng and G.-H. Lee, Inorg.
Chem., 2005, 44, 1344.
15 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.
20 Q.-Z. Yang, Q.-X. Tong, L.-Z. Wu, Z. X. Wu, L. P. Zhang and C. H.
Tung, Eur. J. Inorg. Chem., 2004, 1948.
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The Royal Society of Chemistry 2006
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