FRET-based probe for fluoride based on a phosphorescent iridium(iii) complex containing triarylboron groups

Article abstract of DOI:10.1039/c1jm00071c

An excellent F^- probe (complex 1) based on carbazole-fluorene- carbazole (CzFCz) as a fluorescent donor and a cationic Ir(iii) complex unit containing dimesitylboryl (Mes₂B) groups as a phosphorescent acceptor has been designed and synthesized. Several reference compounds, such as complex 2 which is similar to complex 1 but without Mes₂B groups, fluorescent donor CzFCz, and phosphorescent acceptors A1 and A2, were also synthesized in order to better understand the influence of Mes₂B groups on the excited state properties and fluorescence resonance energy transfer (FRET) in this system. The introduction of Mes₂B groups on the ligands of the Ir(iii) complex unit can lead to a red-shifted and more intense absorption, facilitating efficient FRET from the fluorescent donor to the phosphorescent acceptor. Complex 1 displayed highly efficient orange-red phosphorescent emission with an emission peak at 584 nm in CH₂Cl ₂ solution at room temperature. The emission wavelength of complex 1 in film is red-shifted to 600 nm with a shoulder at 650 nm, and its quantum efficiency in film was measured to be 0.15 under excitation at 450 nm. Utilizing the specific Lewis acid-base interactions between boron atom and F^-, the binding of F^- to complex 1 can change its excited state and suppress FRET, quenching the phosphorescent emission from the Ir(iii) complex and enhancing the fluorescent emission from CzFCz. Thus, a visual change in the emission color from orange-red to blue was observed. Optical responses of complex 1 to F^- revealed that it can be used as a highly selective, colorimetric and ratiometric optical probe for F^- utilizing the switchable phosphorescence and fluorescence.

Full text of DOI:10.1039/c1jm00071c

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Volume 21 | Number 21 | 7 June 2011 | Pages 7489–7828
ISSN 0959-9428
PAPER
Wei Huang et al.
FRET-based probe for fluoride based on a phosphorescent iridium(III) complex
containing triarylboron groups

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PAPER
FRET-based probe for fluoride based on a phosphorescent iridium(III)
complex containing triarylboron groups†
*
Wenjuan Xu, Shujuan Liu, Huibin Sun, Xinyan Zhao, Qiang Zhao, Shi Sun, Shan Cheng, Tingchun Ma,
*
Lixia Zhou and Wei Huang
Received 6th January 2011, Accepted 28th February 2011
DOI: 10.1039/c1jm00071c
An excellent Fꢀ probe (complex 1) based on carbazole-fluorene-carbazole (CzFCz) as a fluorescent
donor and a cationic Ir(III) complex unit containing dimesitylboryl (Mes2B) groups as a phosphorescent
acceptor has been designed and synthesized. Several reference compounds, such as complex 2 which is
similar to complex 1 but without Mes2B groups, fluorescent donor CzFCz, and phosphorescent
acceptors A1 and A2, were also synthesized in order to better understand the influence of Mes2B groups
on the excited state properties and fluorescence resonance energy transfer (FRET) in this system. The
introduction of Mes2B groups on the ligands of the Ir(III) complex unit can lead to a red-shifted and
more intense absorption, facilitating efficient FRET from the fluorescent donor to the phosphorescent
acceptor. Complex 1 displayed highly efficient orange-red phosphorescent emission with an emission
peak at 584 nm in CH2Cl2 solution at room temperature. The emission wavelength of complex 1 in film
is red-shifted to 600 nm with a shoulder at 650 nm, and its quantum efficiency in film was measured to
be 0.15 under excitation at 450 nm. Utilizing the specific Lewis acid–base interactions between boron
atom and Fꢀ, the binding of Fꢀ to complex 1 can change its excited state and suppress FRET,
quenching the phosphorescent emission from the Ir(III) complex and enhancing the fluorescent emission
from CzFCz. Thus, a visual change in the emission color from orange-red to blue was observed. Optical
responses of complex 1 to Fꢀ revealed that it can be used as a highly selective, colorimetric and
ratiometric optical probe for Fꢀ utilizing the switchable phosphorescence and fluorescence.
Fꢀ, have been adopted as an efficient approach for fluoride
Introduction
detection.4 Some three-coordinated boron compounds with
a donor–p conjugation–acceptor (D–p–A) chemical structure, in
which the dimesitylboryl (Mes2B) group was adopted as the
electron acceptor and receptor for Fꢀ, were reported to be highly
selective fluorescent probes for Fꢀ. In the presence of Fꢀ, the
strong B–Fꢀ interaction can interrupt the extended p conjuga-
tion of these organoboron derivatives, thereby causing
a dramatic change in the photophysical properties.
As we all know, fluoride (Fꢀ) is an important microelement in the
human body.1 Water fluoridation or addition of fluoride to
toothpaste has become a widespread practice because of the
beneficial effects of this anion in dental health. High doses of this
anion are, however, dangerous and can lead to dental or skeletal
fluorosis. Due to the possible toxicity of this anion, the design of
fluorescent Fꢀ probes is an area of active investigation, and
a great deal of effort has been devoted to the design of molecular
receptors containing binding sites to Fꢀ.2 Original efforts focused
on receptors that interact with Fꢀ via hydrogen bonds,3 but the
selectivity is often not good. Recently, specific Lewis acid–base
interactions, such as the strong affinity of a boron atom toward
Currently, organic luminophores are widely used as fluores-
cent probes. In addition to purely organic luminophores, most
recently, the use of phosphorescent heavy-metal complexes as
probes has also attracted considerable interest,5 as a result of
their advantageous photophysical properties, such as sensitivity
of emission properties to the changes of local environment,
evident Stokes shifts for easy separation of excitation and
emission, significant single-photon excitation in the visible range
and relatively long lifetimes (from the order of microseconds to
milliseconds) compared to those of purely organic luminophores.
In addition, compared with purely organic luminophores, the
excited-state properties of phosphorescent heavy-metal
complexes are more complicated and depend on the metal
centers, chemical structures and triplet state energy levels of the
Key Laboratory for Organic Electronics
& Information Displays
(KLOEID) and Institute of Advanced Materials (IAM), Nanjing
University of Posts and Telecommunications, Nanjing, 210046, P. R.
China. E-mail: wei-huang@njupt.edu.cn; iamqzhao@njupt.edu.cn; Fax:
+86 25 8586 6008; Tel: +86 25 8586 6008
† Electronic supplementary information (ESI) available: Graphs
showing response of UV-vis absorption spectra of 1 to various anions
(Fig. S1) and response of PL spectra of 1 to various anions (Fig. S2),
and the calculation of Fꢀ binding constant from the UV-vis titration
data (Fig. S3). See DOI: 10.1039/c1jm00071c
7572 | J. Mater. Chem., 2011, 21, 7572–7581
This journal is ª The Royal Society of Chemistry 2011

ligands. If a heavy-metal complex contains a specific binding site
in its ligand for an analyte, the presence of this analyte can lead
to dramatic variations in its photophysical and excited state
properties, realizing the detection. According to this principle, we
have successfully realized excellent phosphorescent probes for
Hg2+, anions and homocysteine based on Ir(III) complexes.6
Introducing Mes2B groups into the ligands of phosphorescent
heavy-metal complexes can realize phosphorescent Fꢀ detection
through the specific interaction of a boron atom with Fꢀ, which
can induce variations in the excited state properties of heavy-
metal complexes. Most recently, several phosphorescent probes
for Fꢀ based on heavy-metal complexes containing a Mes2B
group have been reported by us and other groups.6c,e,7 Consid-
ering their rich photophysical properties and excellent sensing
ability, it is necessary to further exploit this class of interesting
phosphorescent probes, especially ratiometric probes. The
ratiometric detection can increase the sensitivity by measuring
the ratio changes of the fluorescence intensities at two different
wavelengths, and minimize the external and environment influ-
ences.8 In order to realize the ratiometric Fꢀ probes based on
phosphorescent heavy-metal complexes, there are often two
approaches adopted. The first approach is to attach a Mes2B
group onto an appropriate position of a complex in which there
are dual signaling pathways, and the B–Fꢀ binding can change
the excited state properties of the complexes and induce the
interconversion between the two signaling pathways, realizing
the ratiometric detection of Fꢀ.7e Secondly, if there is no dual
signaling pathway for the heavy-metal complexes and the inter-
action of the boron atom with Fꢀ possibly quenches the phos-
phorescent emission, a donor–acceptor (D–A) structure is
usually adopted. Effective ratiometric detection can be realized
through the variation of fluorescent resonance energy transfer
(FRET) induced by Fꢀ which determines the emission intensity
of D and A. For this approach, the linking between D and A can
be conjugated (Fig. 1a) or nonconjugated (Fig. 1b). As for the
conjugated linking, the energy donor acts as the ligand of the
heavy-metal complex (A) and participates in the excited state of
the complex (see Fig. 1a). However, for the nonconjugated
linking, the heavy-metal complex is tethered to the energy donor
with an alkyl chain (Fig. 1b). Thus, the donor has no significant
influence on the excited state of the complex. For both kinds of
probes, in the absence of Fꢀ, the FRET is efficient and strong
phosphorescence from the complex is observed. Binding to Fꢀ
may change the excited state of the complex and suppress FRET,
quenching the phosphorescent emission from A and enhancing
the fluorescent emission from D. Thus, the ratiometric detection
Scheme 1 Chemical structures of compounds synthesized in this work.
of Fꢀ can be realized. Most recently, we have reported a ratio-
metric Fꢀ probe based on an Ir(III) complex with conjugated D–
A structure as shown in Fig. 1a.6e In this work, we paid attention
to exploitation of an excellent ratiometric Fꢀ probe based on an
Ir(III) complex with nonconjugated D–A structure as shown in
Fig. 1b, and further investigated the effect of B–Fꢀ binding on
the energy transfer from donor to acceptor.
Herein, wesynthesizedacomplex1basedoncarbazole-fluorene-
carbazole (CzFCz) as the fluorescent donor and a Mes2B group-
functionalized cationic Ir(III) complex as the phosphorescent
acceptor (see Scheme 1). In addition, several reference compounds
(shown in Scheme 1), such as complex 2 without Mes2B groups,
fluorescent donor CzFCz, and phosphorescent acceptors A1 and
A2, were also synthesized in order to better understand the influ-
ence of Mes2B groups on the excited state properties and FRET of
the complex. Finally, the binding of Fꢀ to complex 1 changes the
excited state properties of the phosphorescent acceptor, inducing
variation in FRET from donor to acceptor, and realizes an excel-
lent colorimetric and ratiometric probe for Fꢀ with switchable
phosphorescence and fluorescence signals.
Experimental section
Characterization
NMR spectra were recorded on a Bruker Ultra Shield Plus
400 MHz NMR instrument (1H: 400 MHz, 13C: 100 MHz).
Mass spectra were obtained on a Bruker autoflex MALDI-TOF/
TOF mass spectrometer and Thermo Scientific ESI-MS
spectrometer. The UV-visible absorption spectra were recorded
on a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer.
Photoluminescent spectra were measured using a RF-5301PC
spectrofluorophotometer. The quantum efficiency for 1 was
measured in degassed CH2Cl2 using fac-Ir(ppy)3 (fac-tris(2-
phenylpyridine)iridium) as the standard.
Computational details
The calculation was performed using the Gaussian 03 suite of
programs.9 The optimizations of complex structures were per-
formed using B3LYP density functional theory (DFT). The
LANL2DZ basis set was used to treat the iridium atom, whereas
Fig. 1 Phosphorescent Fꢀ probes based on heavy-metal complexes
containing a Mes2B group with conjugated (a) and nonconjugated (b)
donor–acceptor structures.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7572–7581 | 7573

the6-31G* basissetwasused totreatallotheratoms. Thecontours
of the highest occupied molecular orbitals and lowest unoccupied
molecular orbitals (HOMOs and LUMOs) were plotted.
were added to DMSO (10 mL). The solution was stirred at 60 ꢂC
for 5 h. After the reaction was complete, the mixture was poured
into H2O (40 mL), and then was extracted three times with ethyl
acetate. The combined organic layers were washed with brine
and dried over anhydrous MgSO4. The solvent was removed
under reduced pressure. The crude product was purified by
column chromatography on silica by using ethyl acetate/petro-
leum ether (1 : 3) as eluent to yield a white solid (74%). 1H NMR
(400 MHz, CDCl3, 298 K): d ¼ 8.61 (d, J ¼ 4.59 Hz, 1H), 8.36
(d, J ¼ 7.92 Hz, 1H), 7.85–7.80 (m, 2H), 7.52–7.29 (m, 10H), 4.73
(t, J ¼ 7.38 Hz, 2H), 1.90–1.84 (m, 4H), 1.74–1.67 (m, 2H),
1.25–1.04 (m, 14H), 0.83 (t, J ¼ 6.97 Hz, 3H), 0.6–0.49 (m, 4H);
13C NMR (100 MHz, CDCl3, 298 K): d ¼ 152.48, 150.81, 149.96,
148.70, 142.72, 139.16, 136.86, 136.67, 130.33, 126.20, 124.76,
123.77, 123.32, 122.58, 121.61, 121.31, 120.17, 110.30, 55.70,
45.34, 40.30, 40.17, 31.88, 30.03, 29.95, 29.27, 26.52, 23.69, 22.72,
14.21.
Materials
All reagents, unless otherwise specified, were obtained from
Sigma-Aldrich, Acros, and Alfa and used as received. All
solvents were purified before use. All reactions were performed
under nitrogen atmosphere.
Synthesis
9-Hexyl-carbazol-3-boronic acid, 2-(4-bromophenyl)quinoline
(Brpq), 2-(4-(dimesitylboryl)phenyl)quinoline (Bpq), and 2,7-
dibromo-9-octyl-9H-fluorene were prepared according to the
literature procedures.10–12
1-Hexyl-2-(pyridin-2-yl)-1H-benzo[d]imidazole
(Pbi-C6)13.
CzfpbiCz. To a mixture of BrfpbiBr (0.71 g, 1 mmol), 9-hexyl-
carbazol-3-boronic acid (0.62 g, 2.1 mmol), and Pd(PPh3)4 (0.006
g, 0.005 mmol), a degassed mixture of toluene, aqueous 2 M
potassium carbonate and ethanol (2 : 1 : 1 in volume) 3 mL was
added. The mixture was vigorously stirred at 70 ꢂC for 24 h.
After the mixture was cooled to room temperature, it was washed
with brine, extracted three times with CH2Cl2, dried over anhy-
drous Na2SO4. The solution was concentrated. The crude
product was purified by column chromatography on silica by
using ethyl acetate/petroleum ether (1 : 3) as eluent to yield
a white solid (0.63 g, 60%). 1H NMR (400 MHz, CDCl3, 298 K):
d ¼ 8.54 (d, J ¼ 4.37 Hz, 1H), 8.39 (d, J ¼ 1.26 Hz, 2H), 8.19 (d,
J ¼ 7.70 Hz, 2H), 7.83–7.78 (m, 5H), 7.73–7.67 (m, 5H),
7.55–7.39 (m, 7H), 7.74–7.17 (m, 6H), 4.67 (t, J ¼ 7.70 Hz, 2H),
4.35 (t, J ¼ 7.13 Hz, 4H), 1.95–1.88 (m, 4H), 1.75–1.66 (m, 2H),
1.53–1.44 (m, 2H), 1.52–1.35 (m, 2H), 1.46–1.28 (m, 16H),
1.21–1.11 (m, 14H), 0.88 (t, J ¼ 6.91 Hz, 9H); 13C NMR (100
MHz, CDCl3, 298 K): d ¼ 151.62, 148.75, 141.06, 141.00, 139.58,
136.84, 132.87, 128.65, 127.93, 126.95, 126.30, 126.33, 125.94,
125.39, 124.79, 123.84, 123.52, 123.40, 123.11, 121.70, 120.61,
120.04, 119.94, 119.01, 118.92, 110.38, 109.05, 108.99, 55.35,
45.46, 43.39, 40.77, 40.66, 32.07, 31.97, 31.76, 30.22, 30.07, 29.85,
29.81, 29.51, 29.42, 29.38, 29.15, 27.15, 26.63, 22.84, 22.75, 22.71,
14.28, 14.22, 14.19.
KOH (0.28 g, 5.1 mmol) and 2-(2-pyridyl)benzimidazole (0.50 g,
2.55 mmol) were added to ionic liquid (20 mL), and the mixture
was stirred magnetically for 5 min. Then, 1-bromohexane (0.7
mL, 5.1 mmol) was introduced in a single portion, the stirring
was continued for 5 h. After the reaction was complete, the
product was extracted with Et2O (3 ꢁ 5 mL). The combined
ethereal phases were evaporated under reduced pressure, and the
crude product was purified by column chromatography on silica
by using ethyl acetate/petroleum ether (1 : 3) as eluent to yield
colorless oil product (0.46 g, 65%). 1H NMR (400 MHz, CDCl3,
298 K): d ¼ 8.68 (d, J ¼ 4.0 Hz, 1H), 8.39 (d, J ¼ 8.0 Hz, 1H),
7.86–7.82 (m, 2H), 7.45 (dd, J ¼ 6.7, 2.0 Hz, 1H), 7.36–7.28
(m, 3H), 4.82 (t, J ¼ 8.0 Hz, 3H), 1.87 (dt, J ¼ 15.0 Hz, 7.6 Hz,
2H), 1.37–1.25 (m, 6H), 0.85 (t, J ¼ 7.1 Hz, 3H); 13C NMR
(100 MHz, CDCl3, 298 K): d ¼ 150.93, 150.08, 148.76, 142.80,
136.89, 136.78, 124.82, 123.80, 123.32, 122.60, 120.22, 110.37,
45.58, 31.46, 30.13, 26.62, 22.64, 14.12.
1-(6-Bromohexyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole (Br-
pbi)13. KOH (2.86 g, 51 mmol) and 2-(2-pyridyl)benzimidazole
(2 g, 10.2 mmol) were added to ionic liquid (20 mL), and the
mixture was stirred magnetically for 5 min. Then, 1,6-dibromo-
hexane (8 mL, 51 mmol) was introduced in a single portion, the
stirring was continued for 5 h. After the reaction was complete,
the product was extracted with Et2O (3 ꢁ 20 mL). The combined
ethereal phases were evaporated under reduced pressure, and the
crude product was purified by column chromatography on silica
by using ethyl acetate/petroleum ether (1 : 3) as eluent to yield
CzFCz. To a mixture of 2,7-dibromo-9,9-dioctyl-fluorene
(0.55 g, 1 mmol), 9-hexyl-carbazol-3-boronic acid (0.62 g, 2.1
mmol), and Pd(PPh3)4 (0.006 g, 0.005 mmol), a degassed mixture
of toluene, aqueous 2 M potassium carbonate and ethanol
(2 : 1 : 1 in volume) 3 mL was added. The mixture was vigorously
1
colorless oil product (2.2 g, 60%). H NMR (400 MHz, CDCl3,
ꢂ
298 K): d ¼ 8.70 (d, J ¼ 4.80 Hz, 1H), 8.41 (d, J ¼ 7.97 Hz, 1H),
7.88–7.33 (m, 2H), 7.46–7.44 (m, 1H), 7.37–7.30 (m, 3H), 4.84
(t, J ¼ 7.6 Hz, 2H), 3.38 (t, J ¼ 6.8 Hz, 2H), 1.95–1.88 (m, 2H),
1.86–1.79 (m, 2H), 1.53–1.44 (m, 2H), 1.42–1.35 (m, 2H); 13C
(100 MHz, CDCl3, 298 K): d ¼ 150.79, 149.94, 148.76, 142.71,
136.94, 136.69, 124.82, 123.88, 123.40, 122.68, 120.23, 110.28,
45.36, 33.88, 32.64, 29.94, 27.81, 26.07.
stirred at 70 C for 24 h. After the mixture was cooled to room
temperature, it was washed with brine, extracted three times with
CH2Cl2, dried over anhydrous Na2SO4. The solution was
concentrated. The crude product was purified by column chro-
matography on silica by using ethyl acetate/petroleum ether
(1 : 3) as eluent to yield a white solid (0.62 g, 70%).1H NMR (400
MHz, CDCl3, 298 K): d ¼ 8.40 (d, J ¼ 1.6 Hz, 1H), 8.21 (d,
J ¼ 7.7 Hz, 1H), 7.84–7.80 (m, 4H), 7.73–7.70 (m, 4H), 7.52–7.43
(m, 6H), 7.29–7.25 (m, 2H), 4.35 (t, J ¼ 7.2 Hz, 4H), 2.14–2.10
(m, 4H), 1.96–1.88 (m, 4H), 0.89 (t, J ¼ 7.1 Hz, 6H), 0.79 (t,
J ¼ 6.9 Hz, 6H); 13C NMR (100 MHz, CDCl3, 298 K):
1-(6-(2,7-Dibromo-9-octyl-fluoren-9-yl)hexyl)-2-(pyridin-2-yl)
benzoimidazole (BrfpbiBr). Br-pbi (1.4 g, 4.0 mmol), 2,7-
dibromo-9-octyl-9H-fluorene (4.0 mmol) and KOH (20 mmol)
7574 | J. Mater. Chem., 2011, 21, 7572–7581
This journal is ª The Royal Society of Chemistry 2011

d ¼ 151.83, 141.09, 141.01, 140.05, 139.62, 133.03, 126.26,
125.90, 125.46, 123.55, 123.19, 121.81, 120.64, 120.00, 119.00,
118.97, 109.01, 108.97, 55.44, 43.39, 40.73, 32.10, 31.99, 31.78,
29.87, 29.83, 29.54, 29.44, 29.40, 29.16, 27.17, 22.86, 14.28.
7.70 (m, 4H), 7.58 (d, J ¼ 8.1 Hz, 1H), 7.58 (d, J ¼ 8.1 Hz, 1H),
7.39–7.23 (m, 7H), 7.06–6.94 (m, 3H), 6.74 (t, J ¼ 7.4 Hz, 1H),
6.60 (d, J ¼ 8.3 Hz, 1H), 6.50–6.48 (m, J ¼ 8.0 Hz, 8H), 6.34 (s,
1H), 6.28 (s, 1H), 4.69–4.45 (m, 2H), 2.24 (d, J ¼ 13.6 Hz, 12H),
2.24 (s, 24H), 1.06–0.81 (m, 8H), 0.73 (t, J ¼ 6.9 Hz, 3H); 13C
NMR (100 MHz, CDCl3, 298 K): d ¼ 168.97, 168.39, 150.54,
150.05, 148.56, 147.96, 147.65, 146.91, 146.48, 146.22, 146.20,
146.13, 140.71, 140.53, 139.57, 139.29, 139.25, 138.44, 137.88,
137.53, 137.06, 137.01, 135.28, 129.79, 129.24, 128.89, 127.78,
127.73, 126.80, 126.73, 126.64, 125.73, 125.55, 125.10, 124.60,
124.52, 124.36, 124.11, 123.91, 123.41, 117.09, 116.30, 115.77,
110.89, 44.83, 30.24, 28.83, 24.79, 21.35, 20.43, 20.38, 13.25; MS
(ESI-MS) [m/z]: 1376.50 (A1 ꢀ PF6)+.
General synthesis of complex
A mixture of 2-ethoxyethanol and water (3 : 1, v/v) was added to
a flask containing IrCl3$3H2O (1 mmol) and the appropriate
C^N ligand (2.5 mmol). The mixture was refluxed for 24 h. After
cooling, the solid precipitate was filtered to give crude cyclo-
metalated iridium(III) chloro-bridged dimer. The solution of
cyclometalated iridium(III) chloro-bridged dimer (0.079 mmol)
and the appropriate N^N ligand (0.158 mmol) in CH2Cl2–
MeOH [30 mL, 2 : 1 (v/v)] was heated to reflux. After 4 h, the
solution was cooled to room temperature and then a 10-fold
excess of potassium hexafluorophosphate was added. The
suspension was stirred for 2 h and then was filtered to remove
insoluble inorganic salts. The solution was evaporated to dryness
under reduced pressure. It was chromatographed by using
CH2Cl2/acetone (50 : 1) to afford the solid in 70% yield.
1
Complex A2. H NMR (400 MHz, CDCl3, 298 K): d ¼ 8.22–
8.14 (m, 5H), 8.03 (d, J ¼ 7.2 Hz, 1H), 7.96 (dd, J ¼ 8.8 Hz, 3.3
Hz, 1H), 7.87 (dd, J ¼ 8.0 Hz, 4.2, 1H), 7.72 (dd, J ¼ 8.1 Hz, 1.1
Hz, 1H), 7.60 (d, J ¼ 7.0 Hz, 1H), 7.41–7.27(m, 5H), 7.16–7.06
(m, 4H), 6.95 (ddd, J ¼ 8.7 Hz, 6.9 Hz, 1.4 Hz, 1H), 6.87–6.80 (m,
3H), 6.60 (dd, J ¼ 7.1 Hz, 3.9 Hz, 2H), 6.49 (d, J ¼ 8.4 Hz, 1H),
4.58–4.30 (m, 2H), 1.38–0.91 (m, 8H), 0.76 (t, J ¼ 7.1 Hz, 3H);
13C NMR (100 MHz, CDCl3, 298 K): d ¼ 170.77, 169.97, 151.84,
151.37, 148.71, 147.79, 147.61, 147.23, 147.08, 147.02, 145.71,
140.51, 140.06, 139.32, 137.99, 136.10, 134.82, 134.56, 131.09,
130.91, 130.83, 129.01, 127.68, 127.61, 127.56, 127.07, 126.94,
126.68, 126.41, 126.16, 126.07, 125.20, 125.03, 124.68, 123.05,
122.96, 118.10, 117.55, 116.77, 111.78, 45.67, 31.26, 29.48, 25.76,
22.39, 14.25; MS (ESI-MS) [m/z]: 880.17 (A2 ꢀ PF6)+.
Complex 1. 1H NMR (400 MHz, CDCl3, 298 K): d ¼ 8.44 (d,
J ¼ 4.72 Hz, 2H), 8.22 (t, J ¼ 7.69 Hz, 3H), 8.12–8.08 (m, 2H),
7.90–7.64 (m, 14H), 7.55–7.35 (m, 12H), 7.30–7.26 (m, 3H),
7.21–7.10 (m, 6H), 6.70–6.82 (m, 2H), 6.73–6.63 (m, 2H), 6.48 (s,
4H), 6.36 (s, 4H), 4.36–4.28 (m, 6H), 2.24 (s, 6H), 2.13 (s, 6H),
1.97–1.86 (m, 8H), 1.60 (s, 12H), 1.57(s, 12H), 1.47–1.26 (m,
19H), 1.21–1.10 (m, 13H), 0.88 (t, J ¼ 6.91 Hz, 9H); 13C NMR
(100 MHz, CDCl3, 298 K): d ¼ 169.67, 169.27, 151.83, 151.74,
151.36, 151.03, 149.65, 148.72, 148.53, 147.55, 147.26, 147.05,
146.78, 141.12, 141.08, 140.88, 140.22, 140.15, 139.53, 138.81,
138.11, 137.99, 137.88, 136.30, 132.55, 132.44, 128.61, 127.79,
127.69, 127.42, 126.75, 125.94, 125.63, 125.35, 124.95, 124.64,
124.14, 123.62, 123.07, 121.70, 120.59, 120.01, 119.11, 118.79,
117.96, 117.18, 116.51, 111.87, 109.28, 109.10, 55.38, 45.50,
43.39, 40.86, 40.51, 32.06, 31.94, 31.75, 31.71, 30.14, 29.83, 29.79,
29.69, 29.49, 29.38, 29.15, 29.12, 27.14, 27.10, 25.53, 24.09, 23.93,
22.95, 22.73, 22.70, 22.66, 21.42, 21.32, 14.17, 14.14; MS
(MALDI-TOF) [m/z]: 1097.632 (1 ꢀ CzfpbiCz-PF6)+.
Results and discussion
Synthesis and characterization of complex 1 and reference
compounds
Scheme 2 shows the synthetic route to complex 1 and reference
compounds CzFCz, 2, A1 and A2. It includes four steps. The
first is the synthesis of cyclometalated C^N ligand (Rpq). Ligand
L2 and precursor 2-(4-bromophenyl)quinoline (Brpq) were
€
easily obtained in good yield by a Friedlander condensation
reaction.10 Then, ligand L1 (Bpq) was synthesized in about 60%
yield by lithiating Brpq, followed by the addition of dimesi-
Complex 2. 1H NMR (400 MHz, CDCl3, 298 K): d ¼ 8.43 (d,
J ¼ 9.8 Hz, 2H), 8.17–8.05 (m, 7H), 7.98 (d, J ¼ 7.8 Hz, 1H),
7.88–7.75 (m, 8H), 7.66–7.43 (m, 11H), 7.35–6.88 (m, 12H),
6.78–6.70 (m, 4H), 6.49–6.57 (m, 2H),6.30 (d, J ¼ 8.4 Hz, 1H),
4.18–4.61 (m, 6H), 2.13–1.86 (m, 9H), 1.43–1.12 (m, 32H), 0.87
(t, J ¼ 7.1 Hz, 6H), 0.79 (t, J ¼ 6.9 Hz, 3H); 13C NMR (100 MHz,
CDCl3, 298 K): d ¼ 170.38, 169.68, 154.44, 151.93, 151.81,
151.14, 147.62, 147.25, 147.00, 146.72, 145.61, 141.16, 141.12,
141.06, 141.01, 140.15, 140.12, 137.62, 136.17, 132.55, 132.50,
130.96, 130.73, 130.54, 127.55, 127.42, 127.33, 126.95, 126.08,
125.34, 125.32, 123.61, 123.55, 123.08, 123.02, 120.08, 119.15,
119.11, 118.84, 118.78, 117.38, 116.50, 109.28, 109.08, 43.41,
31.95, 31.74, 31.71, 30.15, 29.83, 29.39, 29.36, 29.16, 29.11, 27.14,
27.10, 22.74, 22.68, 22.66, 14.17, 14.15; MS (ESI-MS) [m/z]:
1655.00 (2 ꢀ PF6)+.
tylboron fluoride (Mes2BF) at ꢀ78 C.6c The second step is the
ꢂ
synthesis of N^N ligand CzfpbiCz and Pbi-C6. Pbi-C6 was
synthesized by N-alkylation reaction from 2-(pyridin-2-yl)benz-
imidazole. The oligomeric N^N ligand CzfpbiCz was obtained
by Suzuki coupling reaction from BrfpbiBr and 9-hex-
ylcarbazole-3-boronic acid.11,14 And the donor compound
CzFCz was also synthesized by a similar Suzuki coupling reac-
tion. In order to obtain the target complex 1 and other reference
complexes, the dinuclear cyclometalated Ir(III) chloro-bridged
precursor [Ir(Rpq)2Cl]2 with Rpq as cyclometalated ligand was
synthesized firstly using the same method as that reported by
Nonoyama.15 Finally the cationic Ir(III) complexes were
routinely synthesized with high yields of about 70% by reacting
cyclometalated Ir(III) chloro-bridged precursor with the appro-
priate N^N ligands. The structures of the target complex and
1
reference complexes were characterized by H NMR spectros-
copy, 13C NMR spectroscopy, MALDI-TOF-MS spectroscopy
1
Complex A1. H NMR (400 MHz, CDCl3, 298 K): d ¼ 8.33–
8.18 (m, 3H), 7.98–7.93 (m, 2H), 7.83 (d, J ¼ 7.9 Hz, 1H), 7.76–
This journal is ª The Royal Society of Chemistry 2011
and ESI-MS spectroscopy.
J. Mater. Chem., 2011, 21, 7572–7581 | 7575

Scheme 2 The synthetic route to the reference compounds and the target complex. Reagents and conditions: i) NaOH, EtOH, reflux, overnight; ii) n-
BuLi, THF, ꢀ78 ꢂC, 1 h; iii) Mes2BF, THF, ꢀ78 ꢂC 1h; iv) KOH, [Bmim]BF4, 20 ꢂC, 5–6 h; v) KOH, DMSO, 60 ꢂC, 6 h; vi) Pd(PPh3)4, 2 M K2CO3 (aq),
toluene, EtOH, 70 ꢂC, 24h; vii) IrCl3$H2O, 2-ethoxyethanol/H2O ¼ 3/1, 110 ꢂC, 24 h; viii) CH2Cl2–CH3OH ¼ 2 : 1, 40 ꢂC, 4 h; ix) KPF6, room
temperature, 1 h.
7576 | J. Mater. Chem., 2011, 21, 7572–7581
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Photophysical properties
solution with emission lifetime of 2.81 ms and quantum efficiency
of 1.1 relative to F [fac-Ir(ppy)3] ¼ 1. The emission band is
similar to that of A1, and so can be assigned to emission from the
phosphorescent acceptor in 1. The high quantum efficiency can
be assigned to the effects of the Mes2B unit and FRET. Besides,
very weak emission bands at around l ¼ 419 nm were also
observed, which were similar to those from fluorescent donor
CzFCz (see Fig. 3). These results indicate that efficient FRET
from the fluorescent donor to the phosphorescent acceptor takes
place. The reference complex 2 without Mes2B groups shows an
intense emission at 554 nm from the Ir(III) complex unit, and the
obvious blue emission from CzFCz was also observed, indicating
less efficient FRET from the donor to the acceptor compared to
complex 1. Hence, the introduction of Mes2B groups into the
C^N ligands results in the red-shift of the Ir(III) complex emission
and more efficient FRET from the fluorescent donor to the
phosphorescent acceptor.
The photophysical properties of complex 1 and reference
compounds were investigated through UV-vis absorption and
photoluminescence (PL) spectroscopy in order to clarify the
FRET from the fluorescent donor to the phosphorescent
acceptor and the influence of the Mes2B group on the photo-
physical properties of the complex.
1) Absorption spectra. The absorption spectra of the
complexes (1, 2, A1 and A2) and the normalized PL spectrum of
CzFCz are shown in Fig. 2, and the data are summarized in Table
1. From Fig. 2, we can see that the absorption spectrum of 1 can
be regarded as the spectral sum of CzFCz and A1, indicating
weak ground state interaction between them. 1 shows two intense
absorption bands centered at l ¼ 300 and 345 nm with molar
extinction coefficients (3) of ꢃ104 and weak absorption at
l ¼ 400–550 nm with 3 of ꢃ103 in CH2Cl2 solution. The intense
absorption bands below l ¼ 400 nm are related to the singlet
ligand-centered p–p* transitions from the cyclometalated C^N
ligand (Bpq) and N^N ligand of the Ir(III) complex and donor
CzFCz. The weak absorption at l ¼ 400–550 nm can be assigned
to a mixture of spin-allowed ligand-to-ligand charge transfer
(LLCT) transition, metal-to-ligand charge transfer (MLCT)
The photoluminescence spectra of 1 under different conditions
were also measured and are shown in Fig. 4. The emission of 1
has little dependence on the solvent polarity and temperature. At
77 K, almost no donor emission was observed, indicating more
efficient FRET at low temperature. According to previous
3
studies, the emission from the ligand-centered LC(p–p*) state
often displays vibronic progressions and little dependence on
solvent polarity and temperature, while those from CT states are
often broad, featureless, and sensitive to solvent polarity and
temperature.17 The emission peaks of 1 show little dependence on
solvent polarity and temperature (see Fig. 4), indicating that the
3
3
transition and spin-forbidden ligand-centered LC and MLCT
transitions.16 Comparing the spectra of compounds containing
Mes2B (1 and A1) to those without Mes2B (2 and A2), we can find
that the introduction of Mes2B groups results in significantly red-
shifted and more intense absorption. This may be assigned to the
enhanced pp–p* conjugation between the vacant p orbital of the
boron atom and the p conjugated skeleton.
2) Photoluminescence spectra. From Fig. 3, we can see that 1
shows an intense emission band at l ¼ 584 nm in CH2Cl2
Fig. 3 The normalized photoluminescence spectra of CzFCz, 1, 2, A1
and A2 measured in CH2Cl2 solution.
Fig. 2 The absorption spectra of 1, 2, A1, A2 and CzFCz and the PL
spectrum of CzFCz measured in CH2Cl2 solution.
Table 1 Photophysical data
labs/nm
(log3)
lem/nm
(CH2Cl2)
Compound
1
2
A1
A2
CzFCz
300 (4.98), 345 (5.10), 460 (3.75)
338 (4.99), 440 (3.76)
291 (4.71), 345 (4.72), 460 (3.70)
272 (4.64), 332 (4.55), 440 (3.70)
303 (4.36), 344 (4.58)
584
394, 415, 554
584
554
Fig. 4 The normalized photoluminescence spectra of probe 1 measured
394, 415
under different conditions.
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From Fig. 6, we can see that the absorbances at l ¼ 300 and
345 nm decreased gradually upon addition of Fꢀ to 1 in CH2Cl2
solution. In addition, the absorbances in the range of l ¼ 400–
550 nm increased slightly. An obvious isosbestic point at l ¼ 383
nm was observed. To quantify the Fꢀ concentration in solution,
the change in absorption spectra (see Fig. 6) is correlated to the
analyte concentration using f ¼ (A0 ꢀ A)/A0, where A and A0 are
the absorbances at l ¼ 345 nm in the presence and absence of Fꢀ,
respectively. The calibration factor (f) is defined to minimize the
influence of the absorption background in the absence of Fꢀ.
Fig. 7 shows f as a function of Fꢀ concentration in CH2Cl2
solution. From Fig. 7, we can see that 1 gives a linear response to
Fꢀ in the range 0–50 mM and the correlation coefficient is 0.99,
which is suitable for Fꢀ monitoring and quantification in the
range 0–50 mM. Furthermore, using the UV-vis titration data,
the binding constants K1 and K2 of probe 1 were determined to be
8.31 ꢁ 105 and 9.00 ꢁ 103 Mꢀ1, respectively (see Fig. S3†).
The response of probe 1 to Fꢀ was further investigated by PL
spectroscopy with l ¼ 383 nm (isobestic point) as the excitation
wavelength (shown in Fig. 8). Upon addition of Fꢀ to 1, the
emission intensity at l ¼ 584 nm assigned to the phosphorescent
acceptor decreased accompanied by a blue-shift to l ¼ 554 nm
gradually, whereas the emission intensity at l ¼ 392–419 nm
assigned to the fluorescence donor (CzFCz) increased
Fig. 5 Temperature dependence of PL spectra of 1 in neat films spin-
coated on quartz plates.
3LC state dominates the excited states. In addition, no vibronic
progressions were observed, indicating that the excited state of
the complex is complicated and has some CT character.
The emission properties of 1 in film was also investigated. The
emission band in film is red-shifted to 600 nm with a shoulder at
650 nm (see Fig. 5). No emission from the fluorescent donor was
observed, indicating more efficient FRET in film than in solu-
tion. And the quantum efficiency for 1 in film was measured to be
0.15 under excitation at l ¼ 450 nm. The dependence of solid-
film emission spectra for 1 on temperature was also studied (see
Fig. 5). With the decrease of temperature from 295 K to 12 K, no
evident shift of emission wavelength was observed. However, it is
noteworthy that the emission intensity was enhanced signifi-
cantly, indicating the reduced non-radiative channels for excited
states at low temperature.
3) FRET from fluorescent donor to phosphorescent acceptor.
From the results obtained above, we can see that the introduc-
tion of Mes2B groups into the complex can improve the efficiency
of FRET from the fluorescent donor to the phosphorescent
acceptor significantly. The efficiency of FRET depends on the
spectral overlap between the emission spectrum of the donor and
the absorption spectrum of the acceptor. In order to clarify the
different efficiencies of FRET for 1 and 2, the overlap between
the absorption spectra of phosphorescent guests A1 or A2 and
the PL spectrum of fluorescent donor CzFCz was compared.
From Fig. 2 we can see that the absorption spectrum of A1 has
better overlap with the emission spectrum of CzFCz than A2
does, which can explain the more efficient FRET observed for
complex 1. The better spectral overlap for A1 can be assigned to
the red-shifted and more intense absorption bands induced by
the introduction of Mes2B groups. Hence, we can see that the
introduction of Mes2B groups will be an effective way to design
excellent optoelectronic materials utilizing efficient intra-
molecular FRET.
Fig. 6 Change in the UV-vis absorption spectra of 1 (20 mM) in CH2Cl2
solution with various amounts of Fꢀ.
Optical responses of 1 to Fꢀ
It has been demonstrated that the vacant p orbital on the boron
atom in triarylborane derivative coordinates selectively with
fluoride anions, giving rise to changes in both absorption and
emission spectra.7 In the present study, the ability of probe 1 to
complex with Fꢀ was investigated using UV-vis absorption and
emission titration experiments.
Fig. 7 f as a function of Fꢀ concentration for 1 (20 mM) in CH2Cl2
solution at room temperature. (f ¼ (A0 ꢀ A)/A0, where A and A0 are the
absorbances at l ¼ 345 nm in the presence and absence of Fꢀ, respec-
tively, and the correlation coefficient is 0.99).
7578 | J. Mater. Chem., 2011, 21, 7572–7581
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Fig. 10 The normalized PL spectra of 1 with excess Fꢀ, CzFCz and 2.
Fig. 8 Change in the PL spectra of 1 (20 mM) in a CH2Cl2 solution with
various amounts of Fꢀ. lex ¼ 383 nm. Inset: emission color observed in
a CH2Cl2 solution of 1 (20 mM) in the absence (left) and presence (right)
of 10 equiv. Fꢀ.
Fig. 10 shows the normalized PL intensity of 1 with excess Fꢀ
along with CzFCz and 2. The complex 1-2Fꢀ shows a phospho-
rescent emission peak at 554 nm, which is similar to the phos-
phorescent emission of 2 and A2. This indicates that the
complexation of the Mes2B group with Fꢀ interrupts the conju-
gation of the p orbital of the boron atom with the p conjugated
skeleton. In addition, the blue emission at l ¼ 390–419 nm for
1-2Fꢀ is similar to that of donor CzFCz. Although the emission
bands of complex 1-2Fꢀ are similar to those of 2 without Mes2B
groups, the intensity ratio of blue fluorescence to orange phos-
phorescence is obviously increased compared with that of 2. This
is because the complexation of 1 with Fꢀ quenches the emission
from the Ir(III) complex unit partly and affects the FRET from
the donor to the acceptor.
simultaneously, realizing the ratiometric detection for Fꢀ. Such
ratiometric detection can be assigned to the changed FRET
efficiency from the donor to th eacceptor induced by the binding
of Fꢀ.7e The ratios of emission intensities at l ¼ 584 and 419 nm
(I584nm/I419nm) exhibit a dramatic change from 33.0 to 0.4. Such
a large change of emission intensity ratios at two wavelengths is
desirable for ratiometric fluorescent probes, as the sensitivity as
well as the dynamic range of ratiometric probes are controlled by
the emission ratio. Furthermore, the difference in two emission
wavelengths is very large (165 nm). This difference not only
contributes to the accurate measurement of two emission
intensities, but also results in a huge ratiometric value. More
importantly, the emission color of solution was changed from
orange-red to blue, realizing naked-eye detection (Fig. 8, inset).
Hence, excellent ratiometric and colorimetric Fꢀ detection was
realized.
High selectivity is necessary for an excellent probe. Herein, the
selective binding studies of probe 1 were then extended to other
anions in CH2Cl2 solution. As shown in Fig. 11, only the addi-
tion of Fꢀ results in a prominent change of I584nm/I419nm, whereas
ꢀ
the addition of other anions (Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, NO3ꢀ, H2PO4
,
CH3COOꢀ) causes little changes. Therefore, probe 1 displayed
a high selectivity in sensing Fꢀ. Similarly, achieving high selec-
tivity for the analyte of interest over a complex background of
potentially competing species is a challenge in probe develop-
ment. Thus, the competition experiment was also carried out by
adding Fꢀ to solutions of 1 in the presence of other anions. As
shown in Fig. 11, whether in the absence or presence of other
anions, obvious spectral changes were observed for 1 upon
addition of Fꢀ, indicating that the sensing of Fꢀ by 1 is hardly
affected by other anions.
As shown by the emission titration curves (Fig. 9), probe 1
needs approximately 2 equivalents of Fꢀ to reach saturation
point. Considering that there are two boron centers in one probe
molecule, it is possible for 1 to form a 1 : 2 complex with 2
equivalents of Fꢀ because the two Mes2B groups are electroni-
cally separated in the ground state and spatially distant from
each other.
Sensing mechanism of 1
In order to better understand the excited state properties and
sensing mechanism of probe 1, molecular orbital calculations for
a model complex (10) of 1 without donor CzFCz and the adduct
10-2Fꢀ were performed using DFT and time-dependent density
functional theory (TDDFT) calculations (see Fig. 12, Fig. 13 and
Table 2). For 10, the lowest triplet state (T1) originates from
HOMO / LUMO (43%) and HOMO / LUMO + 2 (45%).
The HOMO distribution primarily resides on the phenyl of the
cyclometalated ligands and the Ir(III) center. The LUMO distri-
bution is dominated by the 2-(pyridin-2-yl)benzimidazole (pbi)
fragment, and the LUMO + 2 distribution primarily resides on
the Bpq. Hence, the lowest triplet state apparently possesses
Fig. 9 Fluorescent titration curve of the ratio of emission intensity at l
¼ 584 nm and l ¼ 419 nm (I584nm/I419nm) versus the number of equiva-
lents of Fꢀ.
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J. Mater. Chem., 2011, 21, 7572–7581 | 7579

Table 2 Calculated lowest triplet states for 10 and 10-2Fꢀ
State
T1
Excitation
10
HOMO / LUMO (43%)
HOMO / LUMO + 2 (45%)
HOMO / LUMO (69%)
10-2Fꢀ
T1
Fig. 11 Emission response of probe 1 (20 mM) in the presence of various
anions (4 equiv) in CH2Cl2 solution. Bars represent the ratio of emission
intensity at l ¼ 584 nm and l ¼ 419 nm (I584nm/I419nm). Black and red
represent I584nm/I419nm before and after the addition of anions, respec-
tively. Blue bar represents I584nm/I419nm after addition of Fꢀ to the solu-
tion of 1 containing other anions. 1, Fꢀ; 2, Clꢀ; 3, Brꢀ; 4, Iꢀ; 5, NO3ꢀ; 6,
ꢀ
H2PO4ꢀ; 7, CH3COOꢀ; 8, ClO4
.
Fig. 14 The sensing mechanism of probe 1.
is efficient FRET from the donor to the acceptor, leading to the
intense phosphorescent emission from the Ir(III) complex unit.
However, the binding of Fꢀ can cause the significant variations in
photophysical properties of the Ir(III) complex unit and subse-
quent FRET efficiency between the donor and the acceptor,
leading to the restored blue fluorescence from the donor CzFCz
(see Fig. 14).
Fig. 12 Calculated orbital distributions of 10.
Conclusion
In summary, a FRET-based Fꢀ probe (1) based on carbazole-
fluorene-carbazole as the fluorescent donor and a Mes2B group-
functionalized cationic Ir(III) complex as the phosphorescent
acceptor has been designed and synthesized. Compared with the
reference compounds, the introduction of Mes2B groups on the
ligands of the Ir(III) complex unit can lead to red-shifted and
more intense absorption and phosphorescence emission. In
addition, the FRET efficiency from the fluorescent donor to the
phosphorescent acceptor can be enhanced significantly. The
binding of probe 1 with Fꢀ changes the excited-state properties of
the Ir(III) complex unit and suppresses the FRET from the donor
to the acceptor, enhancing the blue emission from the fluorescent
donor. So, this complex can be used as a highly selective,
colorimetric and ratiometric phosphorescent probe for Fꢀ
utilizing the switchable phosphorescence and fluorescence. This
work may be very useful for the further design of excellent
ratiometric and colorimetric probes utilizing switchable phos-
phorescent and fluorescent emissions.
Fig. 13 Calculated orbital distributions of 10-2Fꢀ.
ligand-centered (C^N ligand) character (3LC), which mixes with
[pphenyl / p*N^N] (3LLCT), and [dp(Ir center) / p*N^N
(3MLCT) transitions. This is in accordance with the emission
]
3
character, which apparently possesses a mixture of LC and CT
transitions. Such complicated excited states lead to the intense
phosphorescent emission at l ¼ 584 nm. After binding with Fꢀ,
the T1 of 10-Fꢀ mainly originates from the HOMO / LUMO
transition. The HOMO distributions primarily reside on the
Mes2B–Fꢀ fragment, and the LUMO distribution resides on the
pbi fragment. So, the T1 of 10-2Fꢀ can be assigned to the 3LLCT
transition from the Mes2B group to the N^N ligand. We tenta-
tively think that the formation of this transition is responsible for
the quenching of the original emission at 584 nm and the final
emission band at 554 nm of the adduct 1-2Fꢀ is assigned to the
emission from A2 without Mes2B groups.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (973 Program, 2009CB930601),
National Natural Science Foundation of China (Project No.
50803028 and 20804019), Natural Science Foundation of Jiangsu
Province of China (BK2009427), Natural science fund for
colleges and universities in Jiangsu Province (10KJB430010),
Hence, according to the spectral properties, emission titration
experiment and theoretical calculation results, we can summarize
the sensing mechanism as follows. Before binding with Fꢀ, there
7580 | J. Mater. Chem., 2011, 21, 7572–7581
This journal is ª The Royal Society of Chemistry 2011

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This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7572–7581 | 7581