Functionalized BODIPY with various sensory units-a versatile colorimetric and luminescent probe for pH and ions

Article abstract of DOI:10.1039/c2dt30446e

A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives containing ion- and pH-sensory units have been successfully designed and synthesized. One of the compounds was structurally characterized by X-ray crystallography. Owing to the presence of an ICT absorption band, one of the compounds was found to show pronounced solvatochromic behavior in different organic solvents. Their emission energies in various solvents show a linear dependence on the Lippert solvent parameter. The cation-binding properties of the complexes with different metal ions (alkali metal, alkaline earth metal and transition metal ions) have been studied using UV-vis and emission spectroscopies. A 1:1 complexation to metal ions (Li⁺, Na ⁺, Mg²⁺, Ba²⁺, Zn²⁺, Cd ²⁺) was found for the compound with one azacrown moiety in acetonitrile while another one with two azacrown moieties was shown to form 1:2 complexes with Zn²⁺ and Mg²⁺ cations. Their stability constants have been determined by both UV-vis and emission spectrophotometric methods. By introducing triarylborane moieties into the meso position and the 2-position of the BODIPY skeleton, different electronic absorption spectral changes together with an emission diminution were observed in response to fluoride ions. Ditopic binding study of 5, which was functionalized with both azacrown and triarylborane moieties, showed emission enhancement in the presence of Mg²⁺ and F^-. These findings suggest that these BODIPY derivatives are capable of serving as versatile colorimetric and luminescence probes for pH, cations and F^-.

Full text of DOI:10.1039/c2dt30446e

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PAPER
Functionalized BODIPY with various sensory units – a versatile colorimetric
and luminescent probe for pH and ions†
Zhendong Yin,a,b Anthony Yiu-Yan Tam,b Keith Man-Chung Wong,b Chi-Hang Tao,b Bao Li,a
Chun-Ting Poon,b Lixin Wua and Vivian Wing-Wah Yam*a,b
Received 24th February 2012, Accepted 13th July 2012
DOI: 10.1039/c2dt30446e
A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives containing ion- and
pH-sensory units have been successfully designed and synthesized. One of the compounds was
structurally characterized by X-ray crystallography. Owing to the presence of an ICT absorption band, one
of the compounds was found to show pronounced solvatochromic behavior in different organic solvents.
Their emission energies in various solvents show a linear dependence on the Lippert solvent parameter.
The cation-binding properties of the complexes with different metal ions (alkali metal, alkaline earth
metal and transition metal ions) have been studied using UV-vis and emission spectroscopies. A 1 : 1
complexation to metal ions (Li+, Na+, Mg2+, Ba2+, Zn2+, Cd2+) was found for the compound with one
azacrown moiety in acetonitrile while another one with two azacrown moieties was shown to form 1 : 2
complexes with Zn2+ and Mg2+ cations. Their stability constants have been determined by both UV-vis
and emission spectrophotometric methods. By introducing triarylborane moieties into the meso position
and the 2-position of the BODIPY skeleton, different electronic absorption spectral changes together with
an emission diminution were observed in response to fluoride ions. Ditopic binding study of 5, which was
functionalized with both azacrown and triarylborane moieties, showed emission enhancement in the
presence of Mg2+ and F−. These findings suggest that these BODIPY derivatives are capable of serving as
versatile colorimetric and luminescence probes for pH, cations and F−.
physiological regulation. Voltage-gated Na+ channels are respon-
Introduction
sible for the initiation and propagation of action potentials.6
Host–guest interactions have received much attention since the
turn of the last century.1 Most of the attention has been focused
on the design of suitable host molecules based on host–guest
interactions for highly selective molecular recognition.
Especially for ions and molecules, the recognition and sensing
process is of great importance in chemistry, biology, physiology,
and environmental science. In recent years there has been a
growing interest in the study of molecular probes for metal ions,
such as alkali metal ions2 and transition metal ions.3 For
example, sodium ion is considered as an essential element4 and
classified as a “dietary inorganic macro-mineral” for animals5
due to the involvement of signal transmission of nerve impulses.
Neuromodulation of electrical excitability is a fundamental
mechanism in many aspects of learning, memory, and
Fluorescent dyes have been widely studied in the past several
decades in many fields, such as in biological imaging,7 chemo-
sensing,8 and molecular switching.9 The coordination com-
pounds of boron(III) have attracted recent attention10,11 for the
design and synthesis of new fluorescent dyes. In particular,
BODIPY is one of the families of the widely used fluorescent
dyes.12,13 Because of their remarkable optical properties, includ-
ing high absorption coefficients, narrow emission bands, high
emission quantum yields and high photostability, they have been
frequently used in biological labeling and sensing. Herein, we
report the design, synthesis and spectroscopic studies of a series
of BODIPY derivatives with various sensory units. Drastic color
and spectral changes have been observed by the variation of
pH value as well as in the presence of various ions. Ditopic
binding properties of a BODIPY derivative with azacrown and
triarylborane moieties have also been studied.
aState Key Laboratory of Supramolecular Structure and Materials and
College of Chemistry, Jilin University, Changchun 130012, P.R. China
bInstitute of Molecular Functional Materials (Areas of Excellence
Scheme, University Grants Committee (Hong Kong)) and Department of
Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong.
E-mail: wwyam@hku.hk
Experimental section
Materials and reagents
†Electronic supplementary information (ESI) available. CCDC 867744
(1). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2dt30446e
Tetra-n-butylammonium perchlorate, lithium perchlorate, sodium
perchlorate, magnesium(II) perchlorate, barium(II) perchlorate,
11340 | Dalton Trans., 2012, 41, 11340–11350
This journal is © The Royal Society of Chemistry 2012

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zinc(II) perchlorate, and cadmium(II) perchlorate were obtained
from Aldrich Chemical Co. Acetonitrile was distilled over
calcium hydride before use. All other reagents and solvents were
of analytical grade and were used as received. 2-Methylpyrrole
was prepared according to reported procedures.14 4,4-Difluoro-8-
(4-hydroxyphenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene,
4,4-difluoro-8-(4-bromophenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-
s-indacene and 4,4-difluoro-8-phenyl-3,5-dimethyl-4-bora-3a,4a-
diaza-s-indacene were synthesized according to reported pro-
cedures.15 Safety Note. Caution! Metal perchlorate salts are
potentially explosive. Only small amounts of these materials
should be handled and with great caution.
MS (EI+): m/z: 617.2; Anal. Calcd (%) for C34H38·BF2N3O5: C,
66.13; H, 6.20; N, 6.81; found: C, 66.37; H, 6.34; N, 6.71.
2: 1H NMR (500 MHz, CDCl3, 298 K, relative to Me4Si)/ppm:
δ = 3.60–3.70 (m, 32H; –OCH2CH2N– and –OCH2CH2O–),
3.80 (t, 8H; –OCH2CH2N–), 6.70 (d, J = 8.9 Hz, 4H; 3′,5′-m-
Ar), 6.75 (d, J = 4.4 Hz, 2H; pyrrole protons at 2,6-position),
6.85 (d, J = 4.4 Hz, 2H; pyrrole protons at 1,7-position), 6.90 (d,
J = 8.6 Hz, 2H; 8-o-Ar), 7.20 (d, J = 16.0 Hz, 2H; vinyl protons
at 3′,5′-position), 7.35 (d, J = 8.6 Hz, 2H; 8-m-Ar), 7.50 (d, J =
8.9 Hz, 4H; 3′,5′-o-Ar), 7.60 (d, J = 16.0 Hz, 2H; vinyl protons
at 3,5-position). MS (EI+): m/z: 923.0; Anal. Calcd (%) for
C51H61·BF2N4O9·0.5(EtOH): C, 66.03; H, 6.82; N, 5.92; found:
C, 66.08; H, 6.69; N, 5.64.
4,4-Difluoro-8-(4-trimethylsilylethynylphenyl)-3,5-dimethyl-4-
bora-3a,4a-diaza-s-indacene. A mixture of 4,4-difluoro-8-(4-
bromophenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (3.2 g,
8.56 mmol), trimethylsilylacetylene (6 mL, 42.78 mmol), CuI
(81 mg, 0.43 mmol) and [Pd(PPh3)2Cl2] (300 mg, 0.43 mmol)
in triethylamine (30 mL) was stirred and heated to reflux under
nitrogen overnight. After removal of the solvents under reduced
pressure, the residue was purified via silica gel chromatography
(petroleum ether–dichloromethane 3 : 1 v/v) to give the desired
product as a red solid (2.25 g, 67%). 1H NMR (400 MHz,
CDCl3, 298 K, relative to Me4Si)/ppm: δ = 0.27 (s, 9H; –CH3),
2.65 (s, 6H; –CH3), 6.27 (d, J = 4.1 Hz, 2H; pyrrole protons at
2,6-position), 6.67 (d, J = 4.1 Hz, 2H; pyrrole protons at 1,7-
position), 7.43 (d, J = 8.4 Hz, 2H; 8-o-Ar), 7.56 (d, J = 8.4 Hz,
2H; 8-m-Ar).
Syntheses
The synthetic pathway of compounds 1–5 is shown in Scheme 1.
1 and 2 were synthesized according to a modification of a
literature procedure.16 4,4-Difluoro-8-(4-hydroxyphenyl)-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene (321 mg, 1 mmol) and
4-formyl-N-phenylaza-15-crown-5 (323 mg, 1 mmol) were dis-
solved in dry toluene (50 mL). Piperidine (0.5 mL) and acetic
acid (0.5 mL) were then added to the mixture. The reaction
mixture was heated to reflux in the presence of molecular sieves
for 24 h. After cooling to room temperature, the solvent was then
removed under reduced pressure to afford the crude product con-
taining 1 and 2, which were purified by column chromatography
on silica gel using ethyl acetate–petroleum ether (from 3 : 1 to
4 : 1 v/v) as an eluent to give compounds 1 (154 mg, 25% yield)
and 2 (166 mg, 18% yield).
4,4-Difluoro-8-(4-ethynylphenyl)-3,5-dimethyl-4-bora-3a,4a-
diaza-s-indacene. 4,4-Difluoro-8-(4-trimethylsilylethynylphenyl)-
3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (216 mg, 0.55 mmol)
was dissolved in degassed THF (30 mL). Tetra-n-butylammo-
nium fluoride (0.66 mL, 1 M in THF) was injected slowly at
−78 °C. The reaction mixture was stirred at −78 °C for 30 min.
After evaporation of the solvent, the residue was re-dissolved in
dichloromethane (30 mL) and washed with deionized water
twice (20 mL × 2). The organic layer was dried over anhydrous
sodium sulfate. Subsequent filtration and removal of the solvent
yielded the crude product, which was then purified by flash
1: 1H NMR (500 MHz, CDCl3, 298 K, relative to Me4Si)/ppm:
δ = 2.65 (s, 3H; –CH3), 3.60–3.70 (m, 16H; –OCH2CH2N– and
–OCH2CH2O–), 3.80 (t, 4H; –OCH2CH2N–), 5.60 (s, br, 1H;
–OH), 6.25 (d, J = 3.95 Hz, 1H; pyrrole proton at 1-position),
6.65 (t, 3H; pyrrole protons at 7-position and 5′-m-Ar), 6.80
(d, J = 4.5 Hz, 1H; pyrrole proton at 6-position), 6.85 (d, J =
4.5 Hz, 1H; pyrrole proton at 1-position), 6.90 (d, J = 8.31 Hz,
2H; 8-m-Ar), 7.25 (d, J = 16.1 Hz, 1H; vinyl proton at 5′-position),
7.35 (d, J = 8.24 Hz, 2H; 8-o-Ar), 7.45 (d, J = 8.59 Hz, 2H;
5′-o-Ar), 7.50 (d, J = 16.1 Hz, 1H; vinyl proton at 5-position).
Scheme 1 Synthetic route for the target molecules 1–5.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 11340–11350 | 11341

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column chromatography on silica gel using dichloromethane–
was dissolved in degassed THF (30 mL). Tetra-n-butylammo-
nium fluoride (1 mol dm−3 in THF, 0.9 mL) was injected slowly
at −78 °C. The reaction mixture was stirred at −78 °C for
30 min. After evaporation of the solvent, the residue was re-dis-
solved in dichloromethane (30 mL) and washed with water twice
(20 mL × 2). The organic layer was dried over anhydrous
sodium sulfate. Subsequent filtration and removal of the solvent
yielded the crude product, which was then purified by flash
column chromatography on silica gel using dichloromethane–
hexane (1 : 5 v/v) as an eluent to give the pure product (245 mg,
petroleum ether (5 : 1 v/v) as an eluent to give the pure
1
product (245 mg, 90%). H NMR (400 MHz, CDCl3, 298 K,
relative to Me4Si)/ppm: δ = 2.65 (s, 6H; –CH3), 3.21 (s, 1H;
–CuCH), 6.28 (d, J = 4.1 Hz, 2H; pyrrole protons at 2,6-
position), 6.68 (d, J = 4.1 Hz, 2H; pyrrole protons at 1,7-
position), 7.46 (d, J = 8.2 Hz, 2H; 8-o-Ar), 7.60 (d, J = 8.2 Hz,
2H; 8-m-Ar).
3: To a solution of 4,4-difluoro-8-(4-ethynylphenyl)-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene (200 mg, 0.62 mmol),
[Pd(PPh3)4] (46 mg, 0.4 mmol) and CuI (7.6 mg, 0.4 mmol) in
Et3N (40 mL) was added 4-iodophenyldimesitylborane (367 mg,
0.81 mmol). The reaction mixture was stirred under reflux for
16 h. After evaporation of the solvent under reduced pressure,
the residue was purified by flash column chromatography on
silica gel using dichloromethane–hexane (1 : 5 v/v) as an eluent
to give 3 (130 mg, 31%) as an orange solid. 1H NMR
(400 MHz, CDCl3, 298 K, relative to Me4Si)/ppm: δ = 2.00 (d,
12H; –CH3), 2.32 (s, 6H; –CH3), 2.66 (s, 6H; –CH3), 6.28 (d,
J = 4.2 Hz, 2H; pyrrole protons at 2,6-position), 6.72 (d, J =
4.1 Hz, 2H; pyrrole protons at 1,7-position), 6.84 (s, 4H; mesityl),
7.46–7.55 (m, 6H; –C6H4–), 7.64 (d, J = 8.2 Hz, 2H; –C6H4–).
MS (EI+): m/z: 644.65; Anal. Calcd (%) for C43H40B2F2N2: C,
80.14; H, 6.26; N, 4.35; found: C, 80.37; H, 6.11; N, 4.12.
1
86%). H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si)/
ppm: δ = 2.63–2.73 (d, 6H; –CH3), 3.18 (s, 1H; –CuCH), 6.33
(d, J = 3.8 Hz, 1H; pyrrole proton at 6-position), 6.80 (d, 2H;
pyrrole protons at 1,7-position), 7.45–7.54 (m, 5H; –C6H5).
4: To a solution of 4,4-difluoro-8-phenyl-2-ethynyl-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene (200 mg, 0.62 mmol),
[Pd(PPh3)4] (72 mg, 0.06 mmol) and CuI (12 mg, 0.063 mmol)
in Et3N (40 mL) was added p-iodophenyldimesitylborane
(564 mg, 1.25 mmol). The mixture was then heated under reflux
for 16 h. After evaporation of the solvent, the residue was
purified by column chromatography on silica gel using dichloro-
methane–hexane (1 : 5 v/v) as an eluent to give 4 as a red solid
1
(171 mg, 42%). H NMR (400 MHz, CDCl3, 298 K, relative to
Me4Si)/ppm: δ = 2.0 (s, 12H; –CH3), 2.30 (s, 6H; –CH3), 2.68
(s, 3H; –CH3), 2.76 (s, 3H; –CH3), 6.33 (d, J = 4.2 Hz, 1H;
6-H), 6.80 (d, J = 4.2 Hz, 1H; 7-H), 6.82 (s, 4H, mesityl),
6.84 (s, 1H; 1-H), 7.42 (d, J = 8.1 Hz, 2H; mesityl), 7.47 (d, J =
8.1 Hz, 2H; mesityl), 7.48–7.55 (m, 5H; –C6H5). MS (EI+): m/z:
644.77; Anal. Calcd (%) for C43H40B2F2N2: C, 80.14; H, 6.26;
N, 4.35; found: C, 79.72; H, 6.32; N, 4.30.
4,4-Difluoro-8-phenyl-2-iodo-3,5-dimethyl-4-bora-3a,4a-diaza-
s-indacene. 4,4-Difluoro-8-phenyl-3,5-dimethyl-4-bora-3a,4a-
diaza-s-indacene (500 mg, 1.7 mmol) and pyridinium iodochlor-
ide (491 mg, 2.0 mmol) were dissolved in degassed dichloro-
methane (200 mL). The reaction mixture was stirred under N2 in
the dark overnight. Upon quenching by addition of deionized
water, the organic layer was extracted with dichloromethane and
dried over anhydrous sodium sulfate. After subsequent filtration
and solvent evaporation, the residue was purified by flash
column chromatography on silica gel using dichloromethane–
hexane (1 : 5 v/v) as an eluent to give the product as a red solid
3-[2-(4-Aza-phenyl-15-crown-5)ethenyl]-4,4-difluoro-8-[4-bromo-
phenyl]-5-methyl-3a,4a-diaza-4-bora-s-indacene. The compound
was synthesized by a procedure similar to that used for 1 except
that 4,4-difluoro-8-(4-bromophenyl)-3,5-dimethyl-4-bora-3a,4a-
diaza-s-indacene (1.87 g, 50 mmol) was used instead of 4,4-
difluoro-8-(4-hydroxyphenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-
indacene. Yield: 1.32 g, 30%. 1H NMR (400 MHz, CDCl3,
298 K, relative to Me4Si)/ppm: δ = 2.66 (s, 3H; –CH3),
3.60–3.71 (m, 16H; –OCH2CH2N– and –OCH2CH2O–), 3.80 (t,
4H; –OCH2CH2N–), 6.25 (d, 1H, J = 3.9 Hz; pyrrole proton at
2-position), 6.58 (d, J = 3.9 Hz, 1H; pyrrole proton at 1-posi-
tion), 6.67 (d, J = 8.9 Hz, 2H; 5′-m-Ar), 6.75 (d, J = 4.6 Hz, 1H;
pyrrole proton at 6-position), 6.90 (d, J = 4.6 Hz, 1H; pyrrole
proton at 7-position), 7.30 (d, J = 16.1 Hz, 1H; vinyl proton at
5′-position), 7.38 (d, J = 8.3 Hz, 2H; 8-o-Ar), 7.47–7.56 (m, 3H;
vinyl proton at 5-position and 5′-o-Ar), 7.61 (d, J = 8.3 Hz, 2H;
8-m-Ar).
1
(358 mg, 50%). H NMR (400 MHz, CDCl3, 298 K, relative to
Me4Si)/ppm: δ = 2.65 (d, 6H; –CH3), 6.34 (d, J = 4.0 Hz, 1H;
pyrrole proton at 6-position), 6.79 (d, J = 4.0 Hz, 1H; pyrrole
proton at 7-position), 6.85 (s, 1H; pyrrole proton at 1-position),
7.44–7.54 (m, 5H; –C6H5).
4,4-Difluoro-8-phenyl-2-trimethylsilylethynyl-3,5-dimethyl-4-
bora-3a,4a-diaza-s-indacene. To a solution of 4,4-difluoro-8-
phenyl-2-iodo-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (437 mg,
1.04 mmol), Pd(PPh3)4 (60 mg, 0.05 mmol) and CuI (10 mg,
0.05 mmol) in triethylamine (50 mL) was added trimethylsilyla-
cetylene (1.5 mL, 10.4 mmol). The reaction mixture was stirred
at room temperature under N2 overnight. After evaporation of the
solvent, the residue was purified by column chromatography on
silica gel using dichloromethane–hexane (1 : 5 v/v) as an eluent
to give the product as a red solid (350 mg, 86%). 1H NMR
(400 MHz, CDCl3, 298 K, relative to Me4Si)/ppm: δ = 0.21
(d, 9H; –CH3), 2.67 (d, 6H; –CH3), 6.31 (d, J = 4.1 Hz, 1H;
pyrrole proton at 6-position), 6.77 (d, 2H; pyrrole protons at 1,7-
position), 7.46–7.55 (m, 5H; –C6H5).
3-[2-(4-Aza-phenyl-15-crown-5)ethenyl]-4,4-difluoro-8-[4-tri-
methylsilylethynylphenyl]-5-methyl-3a,4a-diaza-4-bora-s-indacene.
The compound was synthesized by a procedure similar to
that used for 4,4-difluoro-8-(4-trimethylsilylethynylphenyl)-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene except that 3-[2-(4-aza-
phenyl-15-crown-5)ethenyl]-4,4-difluoro-8-[4-bromophenyl]-5-
methyl-3a,4a-diaza-4-bora-s-indacene (400 mg, 0.6 mmol) was
1
4,4-Difluoro-8-phenyl-2-ethynyl-3,5-dimethyl-4-bora-3a,4a-diaza-
s-indacene. 4,4-Difluoro-8-phenyl-2-trimethylsilylethynyl-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene (350 mg, 0.89 mmol)
used. Yield: 360 mg, 86%. H NMR (400 MHz, CDCl3, 298 K,
relative to Me4Si)/ppm: δ = 0.28 (s, 9H; –CH3), 2.66 (s, 3H;
–CH3), 3.60–3.71 (m, 16H; –OCH2CH2N– and –OCH2CH2O–),
11342 | Dalton Trans., 2012, 41, 11340–11350
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3.78 (t, 4H; –OCH2CH2N–), 6.23 (d, J = 4.0 Hz, 1H; pyrrole
proton at 2-position), 6.58 (d, J = 4.0 Hz, 1H; pyrrole proton
at 1-position), 6.66 (d, J = 8.9 Hz, 2H; 5′-m-Ar), 6.74 (d, J =
4.6 Hz, 1H; pyrrole proton at 6-position), 6.89 (d, J = 4.6 Hz,
1H; pyrrole proton at 7-position), 7.29 (d, J = 16.1 Hz, 1H;
vinyl proton at 5′-position), 7.42–7.47 (d, 2H; 5′-o-Ar),
7.47–7.52 (m, 3H; vinyl proton at 5-position and 5′-o-Ar),
7.53–7.59 (d, J = 8.3 Hz, 2H; 8-m-Ar).
spectrophotometer, while the emission titration was performed
on a Horiba Jobin Yvon Fluorolog-3 Spectrofluorometer. For the
determination of the emission quantum yields Φ in CH3CN,
quinine sulphate in 1 N H2SO4 (Φ = 0.546) was used as the
reference standard.17
The concentrations of 1 and 2 employed in the UV-vis
and emission titration studies were typically fixed at 1.0 ×
10−5 mol dm−3. The electronic absorption spectral titration was
performed on a Varian Cary 50 UV-vis spectrophotometer at
room temperature, while the emission titration was performed on
a Horiba Jobin Yvon Fluorolog-3 Spectrofluorometer. Support-
ing electrolyte (0.1 M nBu4NClO4) was added to maintain a con-
stant ionic strength of the sample solution. The pKa values of the
amino group on the azacrown and the phenolic group are found
based on eqn (1):18
3-[2-(4-Aza-phenyl-15-crown-5)ethenyl]-4,4-difluoro-8-[4-ethynyl-
phenyl]-5-methyl-3a,4a-diaza-4-bora-s-indacene. The compound
was synthesized by a procedure similar to that used for 4,4-
difluoro-8-(4-ethynylphenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-
indacene except that 3-[2-(4-aza-phenyl-15-crown-5)ethenyl]-
4,4-difluoro-8-[4-trimethylsilylethynylphenyl]-5-methyl-3a,4a-
diaza-4-bora-s-indacene (287 mg, 0.41 mmol) was used. Yield:
1
214 mg, 83%. H NMR (400 MHz, CDCl3, 298 K, relative to
log½ðAmax ꢀ AÞ=ðA ꢀ AminÞꢁ ¼ pH ꢀ pKa
ð1Þ
Me4Si)/ppm: δ = 2.66 (s, 3H; –CH3), 3.20 (s, 1H; –CuCH),
3.62–3.70 (m, 16H; –OCH2CH2N– and –OCH2CH2O–), 3.78
(t, 4H; –OCH2CH2N–), 6.23 (d, J = 4.0 Hz, 1H; pyrrole proton
at 2-position), 6.59 (d, J = 4.0 Hz, 1H; pyrrole proton at 1-posi-
tion), 6.66 (d, J = 8.9 Hz, 2H; 5′-m-Ar), 6.76 (d, J = 4.6 Hz, 1H;
pyrrole proton at 6-position), 6.90 (d, J = 4.6 Hz, 1H; pyrrole
proton at 7-position), 7.29 (d, J = 16.0 Hz, 1H; vinyl proton at
5′-position), 7.45–7.56 (m, 5H; vinyl proton at 5-position and 5′-
o-Ar), 7.57–7.61 (d, J = 8.3 Hz, 2H; 8-m-Ar).
Based on eqn (1), a plot of log[(Amax − A)/(A − Amin)] against
pH gave a straight line with a y-intercept, which equals pKa.
Binding constants for 1 : 1 complexation were obtained by a
nonlinear least-squares fit19 of the absorbance (A) or emission
intensity (I) versus the concentration of the ions added (Cm)
according to the following equation:
Alim ꢀ Ao
A ¼ Ao þ
fCo þ Cm þ 1=Ks
5: The compound was synthesized by a procedure similar to
that used for 3 except that 3-[2-(4-aza-phenyl-15-crown-5)-
ethenyl]-4,4-difluoro-8-[4-ethynylphenyl]-5-methyl-3a,4a-diaza-
4-bora-s-indacene (130 mg, 0.21 mmol) was used. Yield:
2Co
ð2Þ
2
1=2
ꢀ ½ðCo þ Cm þ 1=KsÞ ꢀ 4CoCmꢁ
g
where Ao and A are the absorbance of the complex at a selected
wavelength in the absence and presence of the metal cations,
respectively, Co is the total concentration of the BODIPY-con-
taining azacrown, Cm is the concentration of the added ions, Alim
is the limiting value of absorbance at saturation level, and Ks is
the stability constant. For emission titration studies, eqn (2) can
be modified to give eqn (3), written as
1
111 mg, 56%. H NMR (400 MHz, CDCl3, 298 K, relative to
Me4Si)/ppm: δ = 2.02 (s, 12H; –CH3), 2.32 (s, 6H; –CH3), 2.67
(s, 3H; –CH3), 3.62–3.72 (m, 16H; –OCH2CH2N– and
–OCH2CH2O–), 3.79 (t, 4H; –OCH2CH2N–), 6.24 (d, J = 3.9 Hz,
2H; pyrrole proton at 2-position), 6.63 (d, J = 3.9 Hz, 1H;
pyrrole proton at 1-position), 6.67 (d, J = 8.9 Hz, 2H; 5′-m-Ar),
6.80 (d, J = 4.6 Hz, 2H; pyrrole proton at 6-position), 6.84 (s, 4H;
mesityl), 6.91 (d, J = 4.6 Hz, 1H; pyrrole proton at 7-position),
7.30 (d, J = 16.0 Hz, 1H; vinyl proton at 5′-position), 7.45–7.57
(m, 9H; vinyl proton at 5′-position and Ar), 7.64 (d, J = 8.2 Hz,
2H; Ar). MS (EI+): m/z: 949.79; Anal. Calcd (%) for
C60H63B2F2N3O4: C, 75.87; H, 6.69; N, 4.42; found: C, 75.87;
H, 6.68; N, 4.42.
Ilim ꢀ Io
I ¼Io þ
fCo þ Cm þ 1=Ks
2Co
ð3Þ
2
1=2
ꢀ ½ðCo þ Cm þ 1=KsÞ ꢀ 4CoCmꢁ
g
where Io and I are the emission intensity of the BODIPY com-
pounds at a selected wavelength in the absence and presence of
ions, respectively, and Ilim is the limiting value of emission inten-
sity at saturation level.
Physical measurements and instrumentation
Single crystals of 1 suitable for X-ray diffraction studies were
grown by the layering of n-hexane onto a dichloromethane solu-
tion of the complex. The single-crystal X-ray diffraction data for
compound 1 were collected on a Rigaku R-AXIS RAPID
imaging plate diffractometer with graphite-monochromated
Mo-Kα (λ = 0.71073 Å). The structure was solved by direct
methods employing the SHELXS-97 program.20 Full-matrix
least-squares refinement on F2 was used in the structure refine-
ment. The positions of H atoms were calculated based on riding
mode with thermal parameters equal to 1.2 times that of the
associated C atoms and participated in the calculation of final
R-indices. In the final stage of least-squares refinement, all
non-hydrogen atoms were refined anisotropically. Crystallo-
graphy and structural refinement data are given in Table S1.†
1H NMR spectra were recorded on a Bruker DPX-400
(400 MHz) or a Bruker DPX-500 (500 MHz) FT-NMR spectro-
meter in CDCl3 at 298 K and chemical shifts (δ, ppm) were
reported relative to tetramethylsilane (Me4Si). Elemental ana-
lyses of the compounds were performed on a Flash EA 1112
elemental analyzer at the Changchun Institute of Applied Chem-
istry, Chinese Academy of Sciences. EI mass spectra were
recorded on a Thermo Scientific ITQ 1100™ GC/MSn. Positive-
ion electrospray-ionization (ESI) mass spectra were obtained
on a Finnigan LCQ mass spectrometer. MALDI-TOF MS was
performed on an Autoflex speed TOF/TOF mass spectrometer
(Bruker Daltonics, Leipzig, Germany). Electronic absorption
spectra were obtained using
a Varian Cary 50 UV-vis
This journal is © The Royal Society of Chemistry 2012
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CCDC-867744 (1) contains the supplementary crystallographic
data for this paper.
contact distance of 1.97 Å between the hydroxy group and the
azacrown unit is observed.
Solvent-dependent UV-vis absorption and emission studies
Results and discussion
The electronic absorption spectra of 1 in various solvents at
298 K show intense absorption bands at 619–631 nm and mode-
rately intense bands at 350–500 nm. The electronic absorption
spectra of 1 in several solvents are shown in Fig. 1a, and the
photophysical data in different solvents are listed in Table 1.
With reference to previous work on p-aminostyryl-linked
BODIPY derivatives,16 the absorption bands at 380–450 nm with
molar extinction coefficients in the order of 104 dm3 mol−1 cm−1
in the high-energy absorption band are assigned to the S0 → S2
and S0 → S3 transitions. The intense low-energy absorption
bands with molar extinction coefficients in the order of
105 dm3 mol−1 cm−1 at about 619–631 nm are assigned to the
0 → 0 vibrational band of the strong S0 → S1 transition of 1 with
some charge-transfer character from the azacrown moiety to the
BODIPY moiety, as supported by the slight sensitivity towards
solvent polarity (Fig. 1a). A shoulder at approximately 585 nm
is assigned to the S0 → S1 vibrational band of the same
transition.
X-ray crystal structure
Fig. S1a† shows the perspective view of the molecular structure
of 1 showing a tetrahedral BF2N2 geometry. The selected bond
lengths and angles are listed in Table S2.† The bond lengths for
B–N and B–F and the N–B–N and F–B–F bond angles are in
good agreement with previously reported data.21 The shorter
bond length of 1.330(3) Å for C(17)–C(18) in the crystal struc-
ture indicates that C(17)–C(18) is a double bond in trans geome-
try formed by the condensation reaction between 4,4-difluoro-
3,5-dimethyl-8-(4-hydroxyphenyl)-4-bora-3a,4a-diaza-s-indacene
and 4-formyl-N-phenylaza-15-crown-5. Similar to other
BODIPY derivatives,21b,c the phenyl and indacene planes are
approximately perpendicular to each other with a dihedral angle
of 76.17°, resulting from the steric hindrance between the 1,7-
hydrogen atoms at the 1,7-positions on the indacene moiety and
the hydrogen atom at the 8-position of the meso-phenyl moiety.
The dihedral angle of 18.70° between indacene and the styryl
group suggests the less effective conjugation within the entire
chromophore. Although there are no π–π stacking interactions, a
dimeric arrangement is observed between two adjacent mole-
cules with intermolecular hydrogen bonds (Fig. S1b†). A short
In contrast, the emission energy of 1 is found to be sensitive
to the solvent polarity. This assignment with charge transfer
character is supported by its sensitivity towards solvent polarity.
Upon increasing the solvent polarity from chloroform to DMF,
Fig. 1 (a) UV-vis absorption spectra and (b) normalized emission spectra of 1 in selected solvents at 298 K.
Table 1 Summary for electronic absorption and emission energies for 1 in various solvents
Solvent
λabs/m
λem/nm
Δνˉ/cm−1
Refractive index (n)
Dielectric constant (ε)
Δf a
Ethyl acetate
Acetone
619
619
619
620
627
626
616
629
634
618
616
620
617
623
680
707
717
686
681
700
666
723
729
694
699
690
677
690
1449
2011
2234
1552
1265
1689
1219
2067
2055
1722
1928
1636
1436
1559
1.37
1.36
1.34
1.40
1.45
1.42
1.35
1.43
1.48
1.36
1.33
1.39
1.38
1.41
6.1
20.7
35.9
17.5
4.8
8.9
4.3
36.7
46.5
24.6
32.7
20.45
19.92
7.58
0.20
0.28
0.30
0.26
0.15
0.22
0.17
0.28
0.26
0.29
0.31
0.28
0.28
0.21
Acetonitrile
1-Butanol
Chloroform
Dichloromethane
Diethyl ether
DMF
DMSO
Ethanol
Methanol
1-Propanol
2-Propanol
THF
n2ꢀ1
a
εꢀ1
Δf ¼ f ðεÞ ꢀ f ðn2Þ ¼ 2εþ1 ꢀ 2n2þ1
.
11344 | Dalton Trans., 2012, 41, 11340–11350
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a significant red shift with ΔEem of ca. 1030 cm−1 is observed
(Fig. 1b). This indicates that this structureless emission band
originates from an intramolecular charge transfer (ICT) excited
state due to the presence of the electron-donating nitrogen atom
of the azacrown.16,22b The emission energies are found to shift to
the red with an increase in the dielectric constant of the solvent
from chloroform to DMF: CHCl3 (681 nm) > THF (691 nm) >
MeCN (719 nm) > DMF (722 nm). This may be attributed to the
significant increase in the dipole moment of the excited state,
which is stabilized in a polar solvent medium, relative to its
ground state. It is found that the Stokes shift is well correlated
with the Lippert solvent parameter (Fig. 2).23
been observed with increasing HClO4 concentration in aceto-
nitrile at room temperature. The observation of the weak
BODIPY emission (Φ = 0.018) in 1 before protonation is due to
a photoinduced electron transfer (PET) quenching pathway
from the amino group. The revival of the vibronic-structured
emission band together with significant emission enhancement
(Φ = 0.541 after protonation) is ascribed to the elimination
of the PET quenching pathway as a result of the decrease of
the electron-donating ability upon protonation of the amino
group of 1. The emission spectral changes of 1 are shown in
Fig. 3b.
n
Upon the addition of Bu4NOH, the lowest energy absorption
band at 619 nm is found to shift to 646 nm, while the absorption
band at 550 nm shows a drop in absorbance. Two well-defined
isosbestic points at 565 and 640 nm are observed (Fig. 4a). Such
a bathochromic shift is attributed to the better π-conjugation
between the phenolate and the BODIPY moieties upon deproto-
nation of the hydroxy group. Upon excitation at the isosbestic
wavelength at 477 nm, the emission intensity decreases (Φ =
0.007) upon addition of nBu4NOH due to enhanced PET
quenching exerted by the anionic phenolate moiety (Fig. 4b).
The pKa value has been determined by studying under different
pH conditions. The pKa values of the amino group on the
azacrown and the phenolic group are found to be 2.0 and
8.99, respectively, from the inflection point of the plot of
log[(Amax − A)/(A − Amin)] against pH (Fig. 5).
Acid–base titration studies
Compound 1 exhibits drastic color changes from blue to yellow
in acetonitrile upon addition of HClO4. Upon increasing the acid
concentration, the electronic absorption spectra of 1 show a drop
in the absorbance of the lowest energy absorption band at
619 nm with a growth of the higher energy absorption band at
561 nm. Two well-defined isosbestic points at 478 and 575 nm
are observed, indicating a clean conversion of 1 to its protonated
form.22,24 The UV-vis spectral traces are shown in Fig. 3a. The
drastic color changes are ascribed to the protonation of the
tertiary amino group in 1, in which the electron-donating ability
of the tertiary amino group is decreased, leading to a blue shift
in transition energy as a result of the decrease in the charge trans-
fer character.
Cation-binding properties
The corresponding emission spectroscopic study has also been
investigated. A dramatic emission enhancement at 576 nm has
The cation-binding properties of 1 with alkali metal ions (Li+
and Na+), alkaline earth metal ions (Mg2+ and Ba2+) and tran-
sition metal ions (Zn2+ and Cd2+) have been investigated by the
electronic absorption spectrophotometric method. Upon addition
of monovalent Li+ and Na+ to an acetonitrile solution of 1
n
(0.1 M Bu4NClO4), the S0 → S1 transition band shows a slight
blue shift in energy. In contrast, addition of divalent alkaline
earth metal ions (Mg2+ and Ba2+) and transition metal ions (Zn2+
and Cd2+) to 1 gives drastic color changes from purple to pink
together with dramatic UV-vis absorption spectral changes, in
which the S0 → S1 transition band shows a significant blue shift
in energy from ca. 619 to ca. 567 nm with well-defined isos-
bestic points at ca. 484 and 583 nm. Fig. 6a and 7a show the
UV-vis absorption spectral changes of 1 in acetonitrile solution
(0.1 M nBu4NClO4) upon addition of Mg2+ and Zn2+ ions,
Fig. 2 Plot of the Stokes shiftsΔνˉ(in cm−1) of 1 in various solvents
against the Lippert solvent parameter Δf (Δf = f(ε) − f(n2)).
Fig. 3 (a) UV-vis absorption spectral changes of 1 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of HClO4.. (b)
Emission spectral changes of 1 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of HClO4.
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Fig. 4 (a) UV-vis absorption spectral changes of 1 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of nBu4NOH.
(b) Emission spectral changes of 1 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of nBu4NOH.
Fig. 5 A plot of pH vs. log[(Amax − A)/(A − Amin)] monitored at λ = 619 nm (■) and its linear fit (–) for 1.
Fig. 6 (a) UV-vis absorption spectral changes of 1 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Mg(ClO4)2.
The insert shows a plot of absorbance vs. [Mg2+] monitored at λ = 619 nm and its theoretical fit (—). (b) Emission spectral changes of 1 (concentration
= 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Mg(ClO4)2.
Fig. 7 (a) UV-vis absorption spectral changes of 1 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Zn(ClO4)2.
The insert shows a plot of absorbance vs. [Zn2+] monitored at λ = 619 nm and its theoretical fit (—). (b) Emission spectral changes of 1 (concentration
= 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Zn(ClO4)2.
11346 | Dalton Trans., 2012, 41, 11340–11350
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respectively. The experimental data show a nice agreement with
the theoretical fits (inserts of Fig. 6a and 7a). The identity of the
1 : 1 Mg2+–1 adduct has further been supported by positive-ion
ESI-MS experiments (Fig. S2†). The binding constants (log Ks)
of 1 with various metal ions in acetonitrile have been determined
and summarized in Table 2. Similar to the addition of acid,
various metal ions encapsulated in the cavity of the azacrown
would lower the electron-donating ability of the amino group,
leading to an increase in the HOMO–LUMO gap. In general, 1
shows more drastic UV-vis absorption spectral changes towards
divalent metal ions. This can be attributed to the higher binding
affinity as well as the higher charge density of the divalent metal
ions which would render the amino group more electron-
deficient.
in energy (Fig. 8a and 9a). In contrast to 1, no well-defined iso-
sbestic points are observed in the UV-vis spectral profile. This is
possibly attributed to the occurrence of two binding processes
with stoichiometries of both 2 : 1 and 1 : 1. The 2 : 1 binding
mode has also been supported by the positive-ion ESI and MAL-
DI-TOF mass spectral studies of 2 upon addition of Mg2+, which
showed an ion cluster at m/z 484 corresponding to the [2·2Mg −
2H]+ adduct (Fig. S3†) and an ion cluster at m/z 1269 corres-
ponding to the [2·2Mg·3ClO4]+ adduct (Fig. S4†), respectively.
Also, the Hill plot suggests that the interaction of 2 with Mg2+ is
cooperative, as revealed by a Hill coefficient of 1.90 with an
overall binding constant (log K) of 3.55. On the other hand,
binding of Zn2+ gives a Hill coefficient of 1.05, suggesting that
the two Zn2+ binding processes are completely independent of
each other.
The emission studies upon addition of metal ions have also
been studied in acetonitrile solutions of 1. Upon the addition of
monovalent metal ions, such as Li+ and Na+, a slight increase in
emission intensity is observed due to the small binding affinity
of the azacrown for the monovalent metal ions. In contrast,
addition of divalent metal ions leads to significant emission
enhancement of a vibronic-structured band at ca. 580 nm upon
excitation at the isosbestic wavelength. Fig. 6b and 7b show the
emission spectral changes upon addition of Mg2+ and Zn2+.
Similarly, 2 shows dramatic emission enhancement upon
addition of Mg2+ (Fig. 8b) and Zn2+ (Fig. 9b). The photolumi-
nescence quantum yields of the complex have been determined
and summarized in Table 3. The observation of emission
enhancement in 1 and 2 upon addition of divalent metal ions can
Compound 2 with two azacrown moieties also exhibits inter-
esting cation-binding behaviors. Upon addition of Mg2+ and
Zn2+, the S0 → S1 transition band shows a significant blue shift
Table 2 Stability constants of 1 for various metal ions in acetonitrile
(0.1 M nBu4NClO4) at 298 K by the UV-visible spectrophotometric
method
Metal ions added
log Ks
LiClO4
NaClO4
Mg(ClO4)2
Ba(ClO4)2
Zn(ClO4)2
Cd(ClO4)2
2.44 0.03
2.02 0.02
2.32 0.02
3.10 0.01
2.39 0.01
2.82 0.01
Fig. 8 (a) UV-vis absorption spectral changes of 2 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Mg(ClO4)2.
(b) Emission spectral changes of 2 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Mg(ClO4)2.
Fig. 9 (a) UV-vis absorption spectral changes of 2 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Zn(ClO4)2.
(b) Emission spectral changes of 2 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of Zn(ClO4)2.
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be ascribed to the blocking of the intramolecular PET quenching
process induced by the encapsulation of the cation by the aza-
crown moiety, as a result of the decrease in the electron-donating
ability of the amino group (Fig. 10). Alternatively, coordination
of divalent metal ions would cause an increase in rigidity of the
molecule, resulting in less efficient relaxation of the excited state
to ground state via non-radiative processes. As a result, the
lowest energy emissive excited state is changed from the ICT to
the S0 → S1 excited state that emits at higher energy.
Fluoride-binding properties
It is known that triarylborane is a selective and sensitive moiety
for fluoride ion-binding by utilization of the empty pπ orbital on
the boron center to form four-coordinate triarylfluoroborate com-
pounds. BODIPY derivatives 3 and 4 with dimesitylboron moi-
eties were synthesized to study the fluoride ion-binding
properties. Interestingly, 3 and 4 show different electronic spec-
tral changes towards fluoride ions in acetonitrile solution (0.1 M
nBu4NClO4). Upon addition of fluoride ions to 3, the absorption
band at 335 nm diminishes in intensity with the growth of an
absorption band at 260 nm with small perturbation of the low-
energy BODIPY absorption, which is mainly located at 511 nm
(Fig. 11a). In contrast, 4 with a triarylborane moiety at the 2-pos-
ition of the BODIPY skeleton shows the growth of a new
absorption tail at 592 nm with a diminution of the absorption
band at 548 nm, which is assigned as the S0 → S1 transition
band of the BODIPY chromophore (Fig. 11b). A drastic color-
ation change is also observed from mauve to purple. The
Table 3 The emission quantum yields of compound 1–5 and their ion-
bound adduct
Complex
Φ
Complex
Φ
1
0.018
0.541
0.055
0.033
0.476
0.078
0.031
0.255
0.007
2
0.025
0.469
0.094
0.017
0.007
0.227
0.009
0.007
0.171
0.006
0.169
1–H+
2–Mg2+
2–Zn2+
3
1–Li+
1–Na+
1–Mg2+
1–Ba2+
1–Zn2+
1–Cd2+
1–OH−
3–F−
4
4–F−
5
5–Mg2+
5–F−
5–F−–Mg2+
Fig. 10 Molecular orbital energy diagrams which show the relative energetic dispositions of the frontier orbitals of the fluorophore and the receptor
in (left) the analyte-free situation and (right) the analyte-bound situation.
Fig. 11 UV-vis absorption spectral changes of (a) 3 and (b) 4 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of
nBu4NF. The inserts show the corresponding plot of absorbance against [F−] monitored at λ = (a) 333 nm and (b) 542 nm and their theoretical fits
(—). Emission spectral changes of (c) 3 and (d) 4 (concentration = 10−5 mol dm−3) in acetonitrile (0.1 M nBu4NClO4) upon addition of nBu4NF.
11348 | Dalton Trans., 2012, 41, 11340–11350
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difference in the spectral observation of 3 and 4 in response
to fluoride ions is attributed to the different degree of pertur-
bation on the BODIPY excited state. In compound 3, the intro-
duction of the triarylborane moiety at the meso position
has negligible effect on the BODIPY excited state due to the
large dihedral angle between the planes of the dimesitylboryl-
phenylethynylphenyl (Mes2BC6H4CuCC6H4–) moiety and the
indacene, as revealed by the X-ray crystal structure of 1. This
leads to less effective π-conjugation between the triarylborane
and the BODIPY moieties. In contrast, the triarylborane in 4
shows more effective π-conjugation with BODIPY since the
presence of the ethynyl spacer minimizes steric repulsion
between these two moieties. As a result, F− complexation
can influence the BODIPY excited state by enhancement of
the ICT character since the triarylfluoroborate is a better donor
than triarylborane. This leads to the appearance of a new low-
energy absorption tail. Upon excitation at the isosbestic wave-
lengths, both compounds show emission diminution (3: Φ =
0.017 → 0.007; 4: Φ = 0.227 → 0.009) due to the enhanced
photoinduced electron transfer upon fluoride ion complexation
(Fig. 11c and d).
Conclusion
A series of BODIPY derivatives containing ion- and pH-sensory
units have been successfully synthesized and characterized. One
of the compounds has been structurally characterized by X-ray
crystallography, in which the existence of intermolecular hydro-
gen bonding between the hydroxy group and the azacrown
moiety leads to a dimeric structure. Owing to the presence of
ICT absorption, 1 showed a structureless ICT emission band in
acetonitrile solutions that is found to be highly sensitive towards
solvent polarity. Due to the presence of the hydroxy group and
the azacrown moiety in compound 1, its electronic absorption
and emission properties are found to be sensitive towards acid,
base and divalent cations by perturbation of its ICT excited state.
Compound 2 with two azacrown moieties forms 1 : 2 adducts
with Mg2+ and Zn2+. The fluoride ion-binding properties of 3
and 4 were also studied. It was found that incorporation of the
triarylborane moiety would lead to different electronic absorp-
tion spectral changes. Ditopic binding study of 5, which was
functionalized with both azacrown and triarylborane moieties,
showed emission enhancement in the presence of Mg2+ and F−.
These findings suggest that these BODIPY derivatives are
capable of serving as versatile colorimetric and luminescence
probes for pH, cations and F−.
Ditopic binding properties
The ditopic binding properties of 5 with azacrown and triaryl-
borane moieties have been investigated. Similar to that of 1, 5
could form a 1 : 1 adduct in the presence of Mg2+. This has
been supported by the positive-ion ESI mass spectral studies of
5 upon addition of Mg2+, which showed an ion cluster at m/z
972.5 corresponding to the [5·Mg − H]+ adduct (Fig. S5†).
Addition of both Mg2+ and F− also gives significant emission
enhancement (Fig. 12). On the basis of the Mg2+ and F− binding
studies in 1 and 3 respectively, in which both binding
processes are known to have opposite effect on the emission
properties in that Mg2+ binding gives emission enhancement
while F− binding gives emission diminution, the net increase in
emission intensity upon ditopic binding suggests that photo-
induced electron transfer quenching arising from the addition
of F− cannot substantially suppress the emission enhancement
arising from the addition of Mg2+. This can be supported by the
observation that 5 with excess Mg2+ showed negligible emission
diminution (Φ = 0.171 → 0.169) even upon addition of F−
(Fig. 12).
Acknowledgements
V.W.-W.Y. acknowledges support from The University of
Hong Kong and Jilin University. We also acknowledge support
from the State Key Laboratory of Supramolecular Structure and
Materials in Jilin University and the University Grants Commit-
tee Areas of Excellence Scheme (AoE/P-03/08). A.Y.-Y.T.
acknowledges the receipt of a University Postdoctoral Fellow-
ship, and C.-T.P. the receipt of a Postgraduate Studentship,
both from The University of Hong Kong. Dr H.-L. Wong and
Mr F. K.-W. Hau are thanked for their help in ESI-MS measure-
ment. Ms L. Wang is thanked for her help in MALDI-TOF MS
measurement.
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11350 | Dalton Trans., 2012, 41, 11340–11350
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