Turning on MLCT phosphorescence of iridium(III)-borane conjugates upon fluoride binding

Article abstract of DOI:10.1021/om2008937

The heteroleptic complexes (C∧N)₂Ir^III (C∧N: ppy, 2; btp, 3), bearing a 4-(dimesitylboryl)benzoate (Bbz) auxiliary ligand, and their fluoride adducts were prepared and characterized. While the PL spectra of 2 and 3 exhibit very weak emission in solution, the addition of fluoride turned on (C∧N)₂Ir-centered phosphorescence (λ _em 527 nm for 2 and 617 nm for 3). Experimental and DFT calculations suggest that the weak emission is ascribable to a photoinduced electron transfer from the MLCT state of the (C∧N)₂Ir moiety to the Bbz ligand and is switched on by fluoride binding to the boron center of the Bbz ligand.

Full text of DOI:10.1021/om2008937

Communication
pubs.acs.org/Organometallics
Turning On MLCT Phosphorescence of Iridium(III)−Borane
Conjugates upon Fluoride Binding
Ramesh S. Vadavi,† Hyungjun Kim,‡ Kang Mun Lee,‡ Taewon Kim,† Junseong Lee,§ Yoon Sup Lee,‡
and Min Hyung Lee*,†
†Department of Chemistry and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea
‡Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
§Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea
S
* Supporting Information
ABSTRACT: The heteroleptic complexes (C∧N)2IrIII (C∧N:
ppy, 2; btp, 3), bearing a 4-(dimesitylboryl)benzoate (Bbz)
auxiliary ligand, and their fluoride adducts were prepared and
characterized. While the PL spectra of 2 and 3 exhibit very
weak emission in solution, the addition of fluoride turned on
(C∧N)2Ir-centered phosphorescence (λem 527 nm for 2 and
617 nm for 3). Experimental and DFT calculations suggest
that the weak emission is ascribable to a photoinduced
electron transfer from the MLCT state of the (C∧N)2Ir moiety
to the Bbz ligand and is switched on by fluoride binding to the boron center of the Bbz ligand.
he detection of anions such as fluoride and cyanide has
recently attracted great interest due to their detrimental
Chart 1
T
effects on humans and the environment. Among various
chemosensors that have been developed, triarylborane
receptors which utilize a strong Lewis acid−base interaction
between the boron atom of triarylborane and anions have been
found to be very efficient for sensing small nucleophilic anions.1
While most of the design principles for the sensing of anions
using triarylboranes are based on absorption, color, and
fluorescence changes upon anion binding, phosphorescent
sensors based on heavy-metal complexes have also attracted
great attention due to their advantageous photophysical
properties, such as large Stokes shifts, long emission lifetimes,
and high quantum efficiency, thereby providing high signal-to-
noise ratios and easy separation of signal.2
sufficiently perceivable. In addition, fluoride binding in the
reported borane−Ir(III) conjugates was accompanied by a
phosphorescence decrease, i.e., a turn-off response toward
fluoride binding, although Huang and co-workers recently
demonstrated that a triplet−singlet switch in the Ir(III)
complex III bearing a borane−C∧N conjugated ligand could
provide an efficient ratiometric probe.8
Since turn-on responses would be beneficial from an
analytical point of view,11,12 it is highly desirable to design
and prepare “OFF−ON”-type phosphorescent sensors. To this
end, we turned our attention to the design of nonemissive
heteroleptic Ir(III) complexes. If the introduction of a
triarylborane moiety into the LX ligand of the heteroleptic
Ir(III) complex leads to a nonemissive lowest excited state,
anion binding may induce (C∧N)2Ir-centered phosphorescence.
To test this hypothesis, we prepared the heteroleptic
(C∧N)2IrIII complexes 2 and 3 (C∧N = 2-phenylpyridinato-
C2,N (ppy), 2-(2-benzothienyl)pyridinato-C2,N (btp)) bearing
a (4-dimesitylboryl)benzoate (Bbz) auxiliary ligand and
In this regard, triarylboranes containing transition-metal
moieties of Pt(II),3,4 Re(I),5 Ru(II),6 Cu(I),4 Hg(II),7 and
Ir(III)8,9 have been utilized successfully as phosphorescent
sensors for anions over the past few years. Among these
sensors, cyclometalated Ir(III) complexes are particularly
intriguing, due to their easily tunable phosphorescence color
depending on the type of cyclometalating ligand (C∧N), high
quantum efficiency, and high stability under ambient
conditions.10 In the reported examples to date, the borane
moiety is linked directly to a C∧N ligand to form a conjugate
receptor ligand (Chart 1). Changes in the electronic structure
of the C∧N ligand before and after anion binding induce
3
3
changes in the triplet (3MLCT, LC, LLCT) signal of the
complex, thereby realizing detection of the anion.
However, one drawback of such a sensing scheme may arise
from the difficulty in predicting whether changes in the
emission wavelength or intensity upon anion binding will be
Received: September 26, 2011
Published: December 14, 2011
© 2011 American Chemical Society
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Communication
investigated their photophysical response upon fluoride bind-
ing.
The dimesitylboryl (BMes2)-substituted benzoic acid BbzH
(1) was prepared in good yield (72%) by reaction between the
lithium salts derived from (4-BrPh)BMes2 and solid CO2 after
an acidic workup. Reaction of 1 with the dimeric iridium(III)
compound [(ppy)2Ir(μ-Cl)2Ir(ppy)2] (2a) afforded the hetero-
leptic (ppy)2Ir(Bbz) (2) in moderate yield (65%) (Scheme 1).
a
Scheme 1.
Figure 1. Crystal structure of [2F]− in [NEt4][2F] (molecule A only)
(40% thermal ellipsoids). The H atoms, solvent molecules, and
[NEt4]+ are omitted for clarity.
to the boron center of the Bbz moiety accompanied by
substantial pyramidalization (∑C−B−C = 338.6°). In contrast,
there is no noticeable change in the bonding geometry around
the Ir center in terms of bond lengths and angles after fluoride
binding.
To examine the fluoride ion binding properties, UV−vis
titrations were first carried out with 2 (Figure 2, left).
a
Legend: (i) for 2, Na2CO3, 2-ethoxyethanol, 120 °C, 65%, for 3,
NEt3, toluene, 110 °C, 75%; (ii) Et4NF, toluene, 25 °C, 95% ([2F]−),
93% ([3F]−).
While a similar reaction of 1 with the dimeric btp iridium(III)
starting compound resulted in very low yield, the use of the
monomeric btp starting compound 3a afforded the correspond-
ing (btp)2Ir(Bbz) complex 3 in good yield (75%). The
identities of 2 and 3 have been characterized by NMR
spectroscopy, elemental analysis, and X-ray diffraction.
1
While the H and 13C NMR spectra showed the expected
resonances corresponding to the (C∧N)2Ir and Bbz moieties,
the crystal structure of 2 confirmed the formation of a borane−
Ir(III) conjugate (Figure S1 in the Supporting Information).
The boron atom of the triarylborane adopts a trigonal-planar
geometry (∑C−B−C = 359.9°), and the benzoate is bound to the
Ir atom via bidentate O∧O chelation. The Ir-centered moiety
bears two ppy ligands having a trans disposition of the pyridine
rings, similar to the case for other (ppy)2Ir(LX) com-
plexes.13−15 Geometrical parameters such as bond lengths
and angles around the Ir atom are in a range similar to that
reported for (ppy)2Ir(O∧O) complexes containing three-atom
chelates.13
To gain insight into the fluoride binding, neutral compounds
2 and 3 were converted into their fluoride adducts (Scheme 1).
Treatment of 2 and 3 with Et4NF in toluene smoothly led to
tetraethylammonium salts of fluoroborate: [2F]− and [3F]−,
respectively. Along with the 11B NMR signal at δ +5 ppm, the
19F NMR signal at δ −177 ppm confirmed the formation of a
four-coordinate triarylfluoroborate. Moreover, the crystal
structure of [2F]− was determined by an X-ray diffraction
study (Figure 1). The asymmetric unit contains two
independent molecules, denoted as A and B. Interestingly,
despite very similar geometrical parameters, they exhibit Λ
(molecule A) and Δ (molecule B) configurations for the Ir
moiety (Figure S2 in the Supporting Information for molecule
B). The structure of molecule A shows a fluoride coordinated
Figure 2. (left) Spectral changes in the UV−vis absorption of 2 in
CH2Cl2 (2.80 × 10−5 M) upon addition of Bu4NF ((0−4.24) × 10−5
M). The inset shows the absorbance at 333 nm as a function of [F−].
The line corresponds to the binding isotherm calculated with K = 8.0
× 106 M−1. (right) PL spectra of 2 in aerated CH2Cl2 (2.87 × 10−5 M;
λex 420 nm) upon addition of Bu4NF ((0−4.30) × 10−5 M).
Compound 2 features a broad absorption band in the region
of 300−370 nm which was quenched upon addition of
incremental amounts of fluoride. Comparison with the
absorption spectra of 1 and [2F]− (Figures S3 and S4 in the
Supporting Information) indicates that the band mainly arises
from the dominant π(Mes)−pπ(B) transition16 in the Bbz
ligand and absorption quenching is associated with fluoride
binding to the boron center of the Bbz ligand. The high
fluoride binding constant (K) of 8.0 × 106 M−1 was estimated
from the 1:1 binding isotherm.
While the lower energy band at 404 nm results from spin-
allowed metal-to-ligand charge transfer (1MLCT), the band at
464 nm which tails to over 500 nm can be assigned to a mixture
3
of spin-forbidden MLCT (3MLCT) and π−π* transition of
the ppy ligand (3LC). Both MLCT bands fall in a region similar
to that reported for other (ppy)2Ir(L) complexes (L =
acetylacetonate,14,15 benzoate,13 bipyridine17). Interestingly,
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the MLCT bands remain almost unchanged after fluoride
complexation, indicating that fluoride binding at the Bbz
moiety has little effect on the electronic structure of the
(ppy)2Ir moiety. Next, changes in the PL spectrum of 2 upon
fluoride addition were investigated. Compound 2 exhibits a
very weak emission centered at 525 nm (λex 420 nm, τ = 1.44
μs) (Figure 2, right). Comparison with PL spectra of other
(ppy)2Ir(L) complexes such as (ppy)2Ir(benzoate)13 suggests
that the weak emission corresponds to (ppy)2Ir-centered
phosphorescence originating from mixed 3MLCT and 3LC
states. The structureless band shape also indicates the major
of the Bbz ligand is not likely to occur in 3 because of the lower
3MLCT energy and is also not responsible for the weak
phosphorescence of 2. Otherwise, all higher energy emission
bands in 2 should have been quenched except for the 3LX band,
as usually found for nonemissive (C∧N)2Ir(DBM) (C∧N =
ppy,14,19 dfppy;20 DBM = dibenzolylmethane) complexes.
Instead, it can be suggested that the weak emission is a
consequence of an intramolecular photoinduced electron
transfer (PET) process between the MLCT state as a donor
and the Bbz as an acceptor. A similar PET process from an
organic fluorophore to borane has been recently observed in a
turn-on fluorescence sensor.12
3
involvement of MLCT in the emission. Remarkably, upon
addition of fluoride, the intensity of the band gradually
increases while retaining essentially the same emission
wavelength (λem 527 nm, τ = 1.38 μs for [2F]−). This result
indicates that the formation of [2F]− activates efficient
phosphorescence.
DFT calculation results also support the involvement of the
PET process in the weak emission of 2. Orbital analyses on the
optimized ground-state structure of 2 show that while the
HOMO of 2 predominantly resides on the Ir atom and phenyl
ring of the ppy ligand, the LUMO is localized on the Bbz ligand
with a large contribution from the empty pπ(B) orbital and the
LUMO+1 bears a major contribution from the pyridyl ring of
the ppy ligand (Figure 4). These features indicate that the
Similar absorption and emission features are also observed
for 3 upon fluoride addition (Figure 3). While the absorption
Figure 4. HOMO, LUMO, and LUMO+1 of 2 (top) and [2F]−
(bottom) (isovalue = 0.04).
Figure 3. (left) Spectral changes in the UV−vis absorption of 3 in
CH2Cl2 (2.58 × 10−5 M) upon addition of Bu4NF ((0−4.67) × 10−5
M). The inset shows the absorbance at 334 nm as a function of [F−].
The line corresponds to the binding isotherm calculated with K = 9.0
× 106 M−1. (right) PL spectra of 3 in aerated CH2Cl2 (2.68 × 10−5 M;
λex 470 nm) upon addition of Bu4NF ((0−4.00) × 10−5 M).
MLCT excited state formed from the HOMO−LUMO+1
transition may be attenuated by electron transfer to the Bbz
acceptor, leading to quenching of the MLCT emission. In the
case of [2F]−, on the other hand, the LUMO and LUMO+1 are
mainly located on the pyridyl ring of the ppy ligand, as has
usually been observed in emissive (ppy)2Ir complexes.
band at 334 nm gradually decreases as a result of fluoride
binding to the boron center of the Bbz ligand (K = 9.0 × 106
M−1), the red phosphorescence of 3 (λem 615 nm, τ = 1.25 μs),
which is very weak in intensity, increases upon fluoride addition
(λem 617 nm, τ = 1.61 μs for [3F]−). Moreover, the
phosphorescence spectra clearly show mixed 3MLCT and
3LC (a shoulder) excited states in character, as typically
observed in the emissive (btp)2Ir complexes.14,18 The turn-on
response for 3 (10-fold) is shown to be more pronounced than
that for 2 (5.5-fold).
Finally, the weak phosphorescence band of 3 was not
significantly affected in the presence of small amounts (1 equiv)
of other anions such as Cl−, Br−, I−, OAc−, NO3 , and ClO4 ,
as usually observed in the triarylborane-based receptors, but the
addition of excess amounts of Cl− and Br− (>2 equiv) led to a
gradual increase in the emission intensity (Figure S10,
Supporting Information). Since the absorption band of the
Bbz moiety remained unaffected, this observation may be
related to the slow dissociation of Ir(O∧O) chelation due to the
high affinity of Ir(III) toward halides.
−
−
To investigate the origin of the weak emission of 2 and 3,
first the PL spectra with high-energy excitation were obtained.
Irradiation of 2 and 3 at 330 nm resulted in an intense Bbz-
centered fluorescence (λem 434 nm), indicating that the excited
states of the Bbz ligand are not strongly coupled with those of
the (C∧N)2Ir moiety (Figure S5, Supporting Information).
Next, the PL spectrum of 2 at 77 K showed an intense 3MLCT
band centered at 510 nm (Figure S6, Supporting Information).
We also prepared Gd(Bbz)3·2H2O and obtained its phosphor-
escence spectrum, which showed a phosphorescence band at
545 nm (Figure S8, Supporting Information). These findings
In conclusion, we have demonstrated that the addition of
fluoride to the weakly emissive heteroleptic (C∧N)2IrIII
complexes (2 and 3) bearing a 4-(dimesitylboryl)benzoate
auxiliary ligand (Bbz) turns on (C∧N)2Ir-centered phosphor-
escence. It was suggested that fluoride binding to the Bbz
ligand inhibits the PET process responsible for weak emission
3
of 2 and 3 while switching on (C∧N)2Ir-centered MLCT
phosphorescence. Although 2 and 3 didn't work in water, the
concept described in this report could be utilized in designing
highly sensitive turn-on phosphorescent sensors including
3
3
suggest that the energy transfer from the MLCT to LX state
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving experimental, UV−vis
and PL data, computational details, and crystallographic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: lmh74@ulsan.ac.kr.
■
Chem. Eur. J. 2006, 12, 2758−2771. Sole,
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