Solid-state emissive triarylborane-based BODIPY dyes: Photophysical properties and fluorescent sensing for fluoride and cyanide ions

Article abstract of DOI:10.1039/c1ob05959a

We disclose two novel BODIPY dyes, which contain the bulky substituent, [(4-dimesitylboryl)phenyl]ethynyl at 2- and 2,6-positions. The steric bulkiness of the boryl group is effective to suppress the intermolecular interaction in the solid state and thus

Full text of DOI:10.1039/c1ob05959a

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Solid-state emissive triarylborane-based BODIPY dyes: Photophysical
properties and fluorescent sensing for fluoride and cyanide ions†
Guang-Liang Fu, Hong Pan, Yi-Hong Zhao and Cui-Hua Zhao*
Received 14th June 2011, Accepted 12th September 2011
DOI: 10.1039/c1ob05959a
We disclose two novel BODIPY dyes, which contain the bulky substituent,
[(4-dimesitylboryl)phenyl]ethynyl at 2- and 2,6-positions. The steric bulkiness of the boryl group is
effective to suppress the intermolecular interaction in the solid state and thus these two compounds
display intense fluorescence not only in solution but also in the solid state. In addition, the BODIPY
dyes display sensitive fluorescence responses to fluoride and cyanide anions through the complexation
with the boron center of the boryl group and the subsequent decomposition of the BODIPY core,
illustrating their potential uses for the fluorescence sensing of fluoride and cyanide ions.
boron center with the p* orbital of the p-conjugated framework.11
Introduction
In addition, triarylboranes have shown promising applications as
The pirromethene–BF2 complex (BODIPY) is known as an
fluoride and cyanide sensors with high selectivity through the
complexation with these Lewis basic anions.10a,12–21 It is more
noteworthy that we and others have demonstrated that the steric
bulkiness of boryl groups arising from the aryl substituents, such
as Mes (2,4,6-trimethylphenyl) and Tip (2,4,6-triisopropylphenyl)
on the boron atom to get enough kinetic stability can effectively
prevent an intermolecular interaction.10 Although recent research
on tri-coordinate organoboron compounds has described a vast
of variety of molecular structures, little has been reported on the
boryl functionalized BODIPY dyes.19 We envisioned that the func-
tionalization of BODIPY core with the bulky dimesitylboryl group
might provide a new molecular design method to obtain solid-
state emissive BODIPY dyes. With these considerations, we herein
designed triarylborane-based BODIPY dyes 1 and 2 (Fig. 1),
which contain one and two [(4-dimesitylboryl)phenyl]ethynyls at
the 2- and 2,6 positions of the BODIPY core, respectively. We
comprehensively studied their photophysical properties, including
the solution and solid state photophysical properties. It was
found that the introduction of the bulky boryl group is very
extraordinary class of fluorescent dyes, which has attracted
remarkable research interest because of its unusual and excellent
properties, such as high fluorescence quantum yields, large molar
extinction coefficient, sharp emission spectra, and outstanding
chemical, thermal, and photochemical stabilities.1 To tune its
optical properties, a variety of strategies has been achieved by
functionalizing the BODIPY core at the meso- and/or 3,5-
positions, fusing some aromatic rings to the BODIPY core and
replacing the 8-carbon atom with a nitrogen atom to form aza-
BODIPY dyes.2–4 However, it may be noted that only very recently
the functionalization of the BODIPY core at the 2,6-positions
has gained attention and thus has not been fully investigated and
utilized.5–7 In addition, in spite of their high fluorescence quantum
yields in dilute solutions, most BODIPY dyes hardly fluoresce in
the solid state due to the severe fluorescence quenching arising
from their small Stokes shift (5–20 nm, in most cases) and high
planarity.8 As a result, the increased efforts have been devoted to
achieve BODIPY dyes that exhibit intense fluorescence even in
the solid state. To improve the solid-state fluorescence intensity
of BODIPY dyes, one effective strategy is to introduce sterically
bulky substituents to inhibit molecular aggregation.9
We and others have recently focused our attention on the
creation of solid-state fluorescent dyes utilizing boron elements.10
The incorporation of boron element into p-conjugated framework
can lead to the intriguing electronic and photophysical properties
owing to the pp–p* conjugation between the vacant p orbital on the
School of Chemistry and Chemical Engineering, Shandong University,
Shanda Nanlu 27, Jinan, 250100, People’s Republic of China. E-mail:
chzhao@sdu.edu.cn; Fax: 86 531 88564464; Tel: 86 531 88363756
† Electronic supplementary information (ESI) available: NMR spectra for
all new compounds, the titration of 1 and 2 with fluoride and cyanide ions.
See DOI: 10.1039/c1ob05959a
Fig. 1 The structures of compounds 1 and 2.
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Table 1 Photophysical properties of 1,2 and 6 in THF
abs /nma (log e)
em/nm
effective to prevent intermolecular interactions in the solid-state.
These two compounds exhibit intense fluorescence not only in
solution but also in the solid state. In addition, to demonstrate the
utility of these two highly emissive fluorescent BODIPY dyes, we
also investigated their application as the fluorescent chemosensor
for fluoride and cyanide ions. Both 1 and 2 display sensitive
fluorescence responses to fluoride and cyanide anions.
b
l
l
UF
1
2
6
535 (4.96)
576 (5.23)
573 (4.86c)
570
611
608
0.78
0.54
0.60c
a Only the longest maxima are shown. b Calculated using Rhodamine B as
a standard. c Cited from ref. 21
Results and discussion
Synthesis of triarylborane-based BODIPY dyes
The synthetic route to the BODIPY derivatives 1 and 2 is shown
in Scheme 1. First the iodination of the precursor BODIPY
3 with different equivalents of N-iodosuccinimide (NIS) could
afford 2-iodo-BODIPY 4 and 2,6-diiodo-BODIPY 5 selectively.
The monoiodide 4 was obtained as the major product in 76%
yield for the iodination with only 0.5 equivalent of NIS while
the iodination with excess NIS provided diiodide 5 with a high
yield (67%). With the monoiodide and diiodide in hand, the
following Pd(0)-catalyzed Sonogashira coupling reaction with [(p-
dimesitylboryl)phenyl]acetylene afforded the target compounds
in moderate yields, confirming that Pd(0)-catalyzed Sonogashira
coupling reactions were very efficient and versatile methods to
develop novel BODIPY dyes with longer conjugation length.22
The structures of these two BODIPY dyes 1 and 2 were fully
characterized by 1H NMR, 13C NMR and HRMS. And they are
stable in air and water and can be purified by silica-gel column
chromatagrophy.
Fig. 2 (a) Absorption and (b) emission spectra of 1,2 and 6 in THF.
(UF = 0.54), entering the red region. It is out of our expectation
that the absorption and emission spectra of 2 are almost same to
those of 6, in contrast to the significant bathochromism of the
general tri-coordinate organoboron compounds compared with
the parent p-conjugated framework.11 This presumably suggests
trivial contribution of the boryl groups to the frontier orbitals for
these borane-based BODIPY dyes.
Theoretical calculations
Scheme 1 Synthesis of triarylborane-containing BODIPY dyes 1 and 2.
To further elucidate the effect of the boryl groups on the electronic
structures and thus the photophysical properties of the borane-
based BODIPY dyes, we conducted theoretical calculations of
the compounds 2 and 6. The optimizations of the molecular
geometry and total energy calculation were carried out using
density functional theory (DFT) calculations at the B3LYP/6-
31G(d) level of theory. We also performed time-dependent density-
functional theory (TD-DFT) calculations of these two compounds
at the B3LYP/6-31G(d) theory. The pictorial drawing of their
molecular orbitals are shown in Fig. 3, and the calculated data are
summarized in Table 2.
The 2,6-bis{[(4-dimesitylboryl)phenyl]ethynyl} substituted
BODIPY 2 has a HOMO delocalized over the central BODIPY
core and the entire phenylethynyl arms, whereas its LUMO is
mostly localized on the central BODIPY moiety. The HOMO and
LUMO energy levels are -5.52 eV and -3.12 eV, respectively. The
Photophysical properties in solution
The UV/Vis absorption and emission spectra of compounds 1–2
were first characterized in THF solution, which are shown in Fig. 2
and the related data are summarized in Table 1. To elucidate the
effect of the dimesitylboryl group, the photophysical property data
of the reference compound 6 were also included for comparison.23
In THF solution, the BODIPY dye 1 exhibits an intense
absorption at 535 nm (log e = 4.96). In the fluorescence, it displays
a strong orangish yellow emission at 570 nm with a high quantum
yield (UF = 0.78). A large red shift ca. 40 nm was observed in
the absorption and emission from 2-monosubstituted BODIPY
1 to 2,6-disubstituted BODIPY 2, suggesting the more extended
conjugation in 2. Thus 2 shows intense fluorescence at 611 nm
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Table 2 Calculated Kohn–Sham molecular orbital energy levels and the
first excited-state energies for the BODIPY dyes 2 and 6
HOMO LUMO HOMO–LUMO Transition energya
[eV]
[eV]
gap [eV]
[eV] (l [nm])
f b
2
6
-5.52
-5.44
-3.12
-3.02
2.40
2.42
2.12 (586)
2.16 (575)
1.003
0.616
a The first excited-state transition energy. b Oscillator strength.
Fig. 3 Pictorial drawings of the HOMO and LUMO for (a) 2 and (b) 6,
calculated at the B3LYP/6-31G (d) level of theory.
calculated first excited state, mainly consisting of a HOMO →
LUMO transition, has an excitation energy of 2.12 eV (586 nm)
with an oscillator strength of 1.003. It is noteworthy that in sharp
contrast to the remarkable contribution of the boryl group to the
LUMO for the general tri-coordinate organoboron compound,11
the boryl groups in compound 2 have almost no contribution
to both the HOMO and the LUMO. As a result, the frontier
energy levels, the energy gap and the transition energy of 2 are
almost same as those of compound 6, which contains no boryl
groups. All the calculated results are in good agreement with the
experimental results, confirming the negligible effect of the boryl
groups on the electronic structures when they are introduced at
2,6-positions of the BODIPY core.
Fig. 4 UV/Vis absorption and emission spectra of (a) 1, (b) 2, (c) 6 in
THF and the solid state and (d) 2 and 6 in the spin-coated films. The insets
show the photographs of THF solution and powders of the compounds
under UV irradiation at 360 nm.
Photophysical properties in the solid state
It is intriguing to note that compounds 1 and 2 still retain intense
fluorescence even in the powder form while the powder of 6
hardly fluoresces. This observation prompted us to investigate
their photophysical properties in the solid state in details. Thin
films of compounds 1, 2 and 6 were prepared by spin-coating from
their dichloromethane solutions with ca. 3 mg mL-1 concentration
on the quartz plates, and their absorption and fluorescence spectra
were directly measured. The corresponding spectra are shown in
Fig. 4.
film state, suggesting the introduction of the bulky dimesitylboryl
group is very effective to enhance the solid state emission efficiency
of BODIPY dyes. In addition, it is interesting to note that the
emission band of 2 (lem = 627 nm) in the spin-coated film belongs
to the red light region. Our current molecular design provides
an efficient strategy to achieve solid-state red emissive materials,
which is of great interest for realizing full color display of organic
light-emitting devices (OLEDs).
When going from THF solution to the spin-coated film, the
triarylborane-containing BODIPY dyes 1 and 2 only exhibit trivial
red shifts (7 nm for 1, 8 nm for 2) in the absorption spectra.
Again no significant red shifts were observed for 1 and 2 in the
fluorescence spectra from THF solution to the spin-coated film
(22 nm for 1, 16 nm for 2), demonstrating the formation of neither
the aggregate in the ground state nor the excimer in the excited
state. In contrast, the absorption and emission spectra of 6 are
significantly red shifted by 48 nm and 44 nm, indicative of the
presence of the intense intermolecular interactions in the solid
state. It is most noteworthy that the fluorescence intensity of
2 is about 10 times stronger than that of 6 in the spin-coated
Fluorescent sensing of fluoride and cyanide ions
The selective detection of the fluoride ions has attracted great
attention because it is highly relevant to human health and
environment issues.24 Fluorescence sensing is one of the most
powerful methods due to its high sensitivity. The organoboron
compounds generally possess high selectivity for fluoride binding
over other anions, such as chloride, bromide, and iodide, owing
to the steric hindrance of bulky substituents on the boron center,
which not only ensures stability to water, including atmospheric
moisture, but also prevents the complexation of the boron center
with other large Lewis bases. Hereby, tri-coordinate conjugated
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organoboron compounds have attracted increased attention as
efficient fluoride ion sensors. Considering the intense fluorescence
of compounds 1 and 2, we next investigated their fluorescence
sensing abilities for fluoride anions.
At the same time, the absorption is remarkably blue-shifted to
448 nm. Similar results were also observed for the fluorescence
and absorption spectra of 2 upon addition of fluoride anions (Fig.
6, see ESI†). The changes in the UV-vis and fluorescence spectra
in the first step may be ascribed to the complexation of F- to the
boron center of the mesityboryl groups to form electron-donating
anionic four-coordinate boranate species groups.25 BODIPY dyes
containing electron-donating (4-subsitutedphenyl)ethynyl groups
at 2,6-positions generally display very weak emissions, presum-
ably due to the intramolecular charge transfer (ICT) from the
(p-boranate-substituted)phenylethynyl residues to the BODIPY
core.6 The further changes in the UV-vis and fluorescence spectra
in the second stage may result from the decomposition of BODIPY
moiety in the presence of fluoride anions, as evidenced by the
titration experimental results of compound 6 with fluoride anions
(Fig. 7). The addition of a TBAF solution to THF solution of
6 causes significant blue shift in the absorption and fluorescence
spectra. Moreover, the UV-vis and emission spectra of 6 in the
presence of excess fluoride anions are almost same to those
of 2 in the second state of the titration, in terms of not only
the absorption and emission maxima but also in the spectra
shape (Fig. 8). The decomposition of the BODIPY cores in the
presence of fluoride anions probably arise from a nucleophilic
displacement that breaks a B–N bond resulting in a B–F bond.5b,26
The assignment for the changes in the UV-vis and fluorescence
spectra of these BODIPY–borane dyes was further confirmed by
the titration experiments with 11B spectroscopy (see ESI†). Both
1 and 2 still exhibit a triplet signal in the 11B NMR spectra in the
first step of the titration, suggesting the binding of fluoride to the
borane moiety. In contrast, the triplet 11B NMR signal changed
into a quartet signal in the second step of the titration, indicative
of the decomposition of the BODIPY core through the fluoride
attack on the B–N bond to form a B–F bond.
The titration experiments of 1 and 2 were carried out in THF
solutions by using n-Bu4NF (TBAF) as the fluoride source. The
fluorescence spectra changes of 1 (4.28 mM) and 2 (2.61 mM)
are shown in Fig. 5 and Fig. 6, respectively. A two-step stepwise
response to the fluoride ions was observed for compound 1. Thus
upon addition of a TBAF solution, the emission band of 1 at
570 nm decreased gradually. This spectra change became saturated
when the concentration of TBAF amounted to 16.5 mM. This is
accompanied by a slight red shift (3 nm) in the absorption (see
ESI†), confirming the negligible effect of the boryl groups on the
electronic structures when they are introduced at 2,6-positions of
the BODIPY core. It is noteworthy that the further addition of
a large excess of TBAF caused the appearance of a significantly
blue-shifted emission band with maxima at 507 nm and 535 nm.
Fig. 5 Fluorescence spectra change of (a) 1 (4.28 mM in THF, lex
=
370 nm) upon addition of TBAF with the range of (a) 16.5 mM and (b)
216.0 mM.
Fig. 7 Fluorescence spectra change of 6 (3.66 mM in THF, lex = 370 nm)
upon addition of TBAF.
Fig. 8 UV/Vis absorption and fluorescence spectra of 2 (2.61 mM in
THF) and 6 (3.66 mM in THF) after addition of excess TBAF.
While the selectivity of dimesitylboranes for fluoride ions over
chloride, bromide, iodide and others is well documented, these
compounds are also well-known to be responsive to cyanide ions
Fig. 6 Fluorescence spectra change of 2 (2.61 mM in THF, lex = 370 nm)
upon addition of TBAF with the range of (a) 16.7 mM and (b) 120.0 mM.
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owing to their small size.20,21 Compounds 1 and 2 were also
evaluated for potential responses to cyanide anions. The addition
of tetrabutylammonium cyanide (TBACN) to a solution of 1 in
THF also first induced the gradual decrease in fluorescence (see
ESI†). The fact that the absorption spectrum is very similar to
that of the titration with TBAF when this fluorescence spectra
change became saturated, irrespective of the absorption maxima
and the absorption efficiency, suggests cyanide is first bound
to the dimesityboryl group to form a four-coordinate complex.
However, a new blue-shifted band did not appear with the further
addition of TBACN. Notably, the absorption became very weak,
suggesting the BODIPY moiety is presumably decomposed by the
cyanide anions in a different way compared to fluoride anions.5b
Similar results were obtained for the titration of 2 with TBACN.
It has been documented that the addition of TBAF resulted in the
decomposition of the BODIPY moiety directly and the addition
of TBACN gave rise to the formation of four-coordination borate
while the BODIPY core remained intact for a BODIPY–borane
dyad,19 For our BODIPY–borane dyes 1 and 2, both fluoride and
cyanide anions preferably complex with the boron center of the
boryl groups and then decompose the BODIPY moiety. Although
the reaction mechanism of fluoride and cyanide to decompose
BODIPY core might need further investigations, our current
results provide new insights into the stabilities of the borane-
functionalized BODIPY dye.
Synthesis
4,4-difluoro-2-iodo-1,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4a-
diaza-s-indacene (4). To a solution of 4,4-difluoro-1,3,5,7-
tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene 3 (243 mg,
0.75 mmol) in anhydrous CH2Cl2 (30 mL) was added NIS (112 mg,
0.5 mmol). The reaction mixture was stirred at room temperature
for 30 min. After addition of a saturated solution of NaCl,
the aqueous layer was extracted with CH2Cl2 (3 times). The
combined organic layer was dried over anhydrous Na2SO4, filtered,
and concentrated under reduced pressure. The resulting crude
product was subjected to a silica gel column chromatography (2/1
petroleum ether/CH2Cl2, Rf 0.41) to afford 170 mg (0.38 mmol)
of 4 in 76% yield as red solids: mp 158–160 ◦C; 1H NMR (CDCl3,
300 MHz): d (ppm) 1.38 (s, 6H), 2.57 (s, 3H), 2.63 (s, 3H), 6.04
(s, 1H), 7.25–7.28 (m, 2H), 7.49–7.51 (m, 3H); 13C NMR (CDCl3,
100 MHz): d (ppm) 14.5, 14.8, 15.8, 16.6, 84.3, 122.3, 127.9, 129.2,
129.3, 131.0, 131.9, 134.9, 141.6, 143.3, 145.1, 154.6, 157.8.
4,4-difluoro-2-{[(4-dimesitylboryl)phenyl]ethynyl}-1,3,5,7-tetra-
methyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene (1). To a mixture
of 4-difluoro-2-iodo-1,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4a-
diaza-s-indacene 4 (170 mg, 0.38 mmol), [(4-dimesitylboryl)-
phenyl]acetylene (160 mg, 0.46 mmol), Pd(PPh3)4 (22 mg, 0.019
mmol) and CuI (7.2 mg, 0.038 mmol) was added a degassed mixed
solvent of THF/NEt3 (3 : 1) (20 mL) under a stream of nitrogen.
The reaction mixture was stirred at room temperature overnight
and then quenched with saturated solution of NH4Cl. The aqueous
layer was extracted with CH2Cl2 (3 times). The combined organic
layer was dried over anhydrous Na2SO4, filtered, and concentrated
under reduced pressure. The resulting mixture was purified by a
silica gel column chromatography (2/1 petroleum ether/CH2Cl2,
Rf 0.35) to afford 98 mg (0.15 mmol) of 1 in 38% yield as orange
solids: mp > 300 ◦C; 1H NMR (CDCl3, 300 MHz): d (ppm) 1.40
(s, 3H), 1.50 (s, 3H), 1.99 (s, 12H), 2.30 (s, 6H), 2.58 (s, 3H), 2.70
(s, 3H), 6.04 (s, 1H), 6.81 (s, 4H), 7.26–7.3 (m, 2H), 7.39(d, J = 8.1
Hz, 2H), 7.45 (d, J = 8.1 Hz, 2H), 7.50–7.52 (m, 3H); 13C NMR
(CDCl3, 100 MHz): d (ppm) 13.2, 13.6, 14.5, 14.8, 21.2, 23.4,
84.4, 96.4, 115.9, 122.2, 127.0, 127.9, 128.2, 129., 129.3, 130.3,
130.7, 132.6, 134.7, 136.2, 138.8, 140.9, 141.6, 142.2, 142.8, 144.9,
156.6, 157.9; HRMS (MALDI/DHB): 670.3749 (M+); Calcd for
C45H44B2N2F2: 670.3726.
Conclusions
Two novel BODIPY dyes were obtained by the introduction of the
bulky substitutent, [(4-dimesitylboryl)phenyl]ethyl at the 2, and
2,6-positions. The steric bulkiness of the boryl group is effective
to suppress the intermolecular interaction in the solid state and
thus makes these two compounds keep intense fluorescence even
in the solid state. In addition, the BODIPY–borane dyes exhibit
sensitive responses to fluoride and cyanide anions, through the
complexation with the boron center of the boryl group and
subsequent decomposition of BODIPY core. Our preliminary
results will provide a basis for the further design of new BODIPY-
borane dyes, which is under way in our group.
Experimental
4,4-difluoro-2,6-bis{[(4-dimesitylboryl)phenyl]ethynyl}-1,3,5,7-
General
tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene
(2). This
compound was prepared essentially in the same manner as
described for 1 using 4-difluoro-2,6-diiodo-1,3,5,7-tetramethyl-
8-phenyl-4-bora-3a,4a-diaza-s-indacene 4 (140 mg, 0.24 mmol),
[(4-dimesitylboryl)phenyl]acetylene (252 mg, 0.72 mmol),
Pd(PPh3)4 (14 mg, 0.012 mmol) and CuI (0.024 mmol, 4.6 mg) in
20 mL THF/NEt3 (3 : 1) at room temperature. The purification by
a silica gel column chromatography (3/1 petroleum ether/CH2Cl2,
Rf 0.48) afforded 90 mg (0.09 mmol) of 2 in 40% yield as dark
1H and 13C NMR spectra were recorded with a Bruker 300
or 400 spectrometer in CDCl3 with TMS as internal reference.
UV-vis absorption spectra and fluorescence spectra measurement
were performed with a Hitachi UV-4100 spectrophotometer and
a Perkin Elmer LS-55 Luminescence spectrometer, respectively.
All reactions were carried out under nitrogen atmosphere. Com-
pounds 3,27 523 and [(p-dimesitylboryl)phenyl]acetylene28 were
prepared according to the literature.
◦
red solids: mp > 300 C;1H NMR (CDCl3, 300 MHz): d (ppm)
1.53 (s, 6H), 1.99 (s, 24H), 2.3 (s, 12H), 2.72 (s, 6H), 6.81 (s,
8H), 7.28–7.31 (m, 2H), 7.40 (d, J = 8.4 Hz, 4H), 7.46 (d, J =
8.4 Hz, 4H), 7.52–7.54 (m, 3H); 13C NMR (CDCl3, 100 MHz): d
(ppm) 13.4, 13.7, 21.2, 23.4, 83.9, 97.1, 126.7, 127.9, 128.2, 129.3,
129.5, 130.7, 131.4, 134.4, 136.2, 138.9, 140.9, 141.6, 142.7, 144.3,
Computation methods
All calculations were conducted by using the Gaussian 09
program.29
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145.8, 158.7; HRMS (MALDI/DHB): 1017.5811 (M+); Calcd for
C71H69B3N2F2: 1017.5811.
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Acknowledgements
The authors gratefully acknowledge the financial support from
the National Nature Science Foundation of China (Grant Nos.
21072117, 20802041), Nature Science Foundation of Shandong
Province (No Q2008B02), Science Foundation of Ministry of
Education of China (No 200804221009), and 973 Program
(2010CB933504).
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