3,5-diformylboron dipyrromethenes as fluorescent pH sensors

Article abstract of DOI:10.1021/ic102499h

A series of boron dipyrromethene (BODIPY) dyes containing two aldehyde functional groups at the 3 and 5 positions have been synthesized in low-to-decent yields in two steps. In the first step, the meso-aryl dipyrromethanes were treated with POCl₃ in N,N-dimethylformamide to afford 1,9-diformylated dipyrromethanes. In the second step, the diformylated dipyrromethanes were first in situ oxidized with 2,3-dichloro-5,6-dicyano-1,4- benzoquinone and then reacted with BF₃·OEt₂ to afford 3,5-diformylboron dipyrromethenes. The X-ray structural analysis indicated that the aldehyde groups are involved in intramolecular hydrogen bonding with fluoride atoms, which may be responsible for the stability of the diformylated BODIPY compounds. The presence of two formyl groups significantly alters the electronic properties, which is clearly evident in downfield shifts in the ¹H and ¹⁹F NMR spectra, bathochromic shifts in the absorption and fluorescence spectra, better quantum yields, and increased lifetimes compared to 3,5-unsubstituted BODIPYs. Furthermore, 3,5-diformylboron dipyrromethenes are highly electron-deficient and undergo facile reductions compared to unsubstituted BODIPYs. These compounds exhibit pH-dependent on/off fluorescence and thus act as fluorescent pH sensors.

Full text of DOI:10.1021/ic102499h

ARTICLE
pubs.acs.org/IC
3,5-Diformylboron Dipyrromethenes as Fluorescent pH Sensors
Sheri Madhu,† Malakalapalli Rajeswara Rao,† Mushtaque S. Shaikh,‡ and Mangalampalli Ravikanth*,†
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
‡Department of Pharmaceutical Chemistry, Bombay College of Pharmacy, Santacruz (E), Mumbai 400 098, India
S Supporting Information
b
ABSTRACT: A series of boron dipyrromethene (BODIPY)
dyes containing two aldehyde functional groups at the 3 and 5
positions have been synthesized in low-to-decent yields in two
steps. In the first step, the meso-aryl dipyrromethanes were
treated with POCl3 in N,N-dimethylformamide to afford 1,9-
diformylated dipyrromethanes. In the second step, the difor-
mylated dipyrromethanes were first in situ oxidized with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone and then reacted with
BF3 OEt2 to afford 3,5-diformylboron dipyrromethenes. The X-ray structural analysis indicated that the aldehyde groups are
3
involved in intramolecular hydrogen bonding with fluoride atoms, which may be responsible for the stability of the diformylated
BODIPY compounds. The presence of two formyl groups significantly alters the electronic properties, which is clearly evident in
downfield shifts in the 1H and 19F NMR spectra, bathochromic shifts in the absorption and fluorescence spectra, better quantum
yields, and increased lifetimes compared to 3,5-unsubstituted BODIPYs. Furthermore, 3,5-diformylboron dipyrromethenes are
highly electron-deficient and undergo facile reductions compared to unsubstituted BODIPYs. These compounds exhibit pH-
dependent on/off fluorescence and thus act as fluorescent pH sensors.
’ INTRODUCTION
group can be used as a fluoresecent pH sensor. In this paper, we
report a simple route for the synthesis of 3,5-diformylboron
dipyrromethene dyes and demonstrate their use as fluorescent
pH sensors. Because the electron-withdrawing aldehyde groups
are placed directly in conjugation with electron-rich BODIPY,
these compounds are sensitive toward the pH and act as
fluorescent pH sensors. While we designed this project, Sathya-
moorthi et al.9a and recently Ziessel et al.9b reported the synthesis
of BODIPYs having a formyl group at the 3 position by 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation of
corresponding 3-methylboron dipyrromethenes (Scheme 1a).
Furthermore, while our work was in progress, Jiao et al.10
reported the synthesis of BODIPYs containing a formyl group
at the 2 position by subjecting the appropriate BODIPYs to the
modified VilsmeierꢀHaack conditions (Scheme 1b).
Fluorescent chemosensors that are responsive to changes in
the pH are widely used in analytical chemistry, physiology, and
biosciences.1 The fluorescence method offers several advantages
over other methods for physiological pH measurements because
of its high sensitivity. There are a wide range of fluorescent
indicator dyes that can sense pH changes.1 In recent years, a lot
of work has been focused on the synthesis of 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene-based2 (boron dipyrromethene or
BODIPY3) fluorescent probes and their application4 as selective
and efficient sensors of ionic species. BODIPY dyes possess
excellent qualities such as high photostability, high fluorescence
quantum yields, narrow emission band widths, relatively high
absorption coefficients, and excitation/emission wavelengths
above 500 nm.5 It is now well established that, by simple
modifications on the BODIPY dye, it is possible to tune the
absorption/emission peak maxima, which range from 500 to over
700 nm.5 BODIPY-based fluorophores have been applied as pH
sensors in organic solvents or aqueous/organic mixed media.6ꢀ8
BODIPY dyes bearing phenolic,6 (dialkylamino)phenyl,7 and
calix[4]arene8 subunits at meso positions have been used as pH
sensors, and these compounds showed deprotonation/proton-
ation-dependent fluorescence off/on switching. A perusal of the
literature reveals that aldehyde groups are generally not used for
fluorescent pH sensors. This is because the aldehyde group
present in the fluorophore is less sensitive toward pH variation,
unlike hydroxy/amine/carboxylate groups. However, if an alde-
hyde group is placed appropriately in conjugation with the
fluorophore, the fluorophore containing an aldehyde functional
However, their efforts to synthesize the diformylated BODI-
PYs under various reaction conditions remained unsuccessful,
which they attributed to the instability of the diformylated
BODIPYs.10 Here we showed a different approach to synthesiz-
ing the first examples of 3,5-diformylboron dipyrromethenes,
which are stable and isolated in 7ꢀ26% yield. Furthermore, we
also showed the use of 3,5-diformylboron dipyrromethenes as
fluorescent pH sensors, which exhibit fluorescence on and off
states under acidic and basic pH regions, respectively.
Received: December 14, 2010
Published: April 21, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
’ RESULTS AND DISCUSSION
triethylamine and BF3 OEt2 at room temperature for 10 min.
3
Thin-layer chromatographic analysis showed a fluorescent yel-
low spot of the desired compound. The crude compounds were
subjected to flash silica gel column chromatographic purification
and afforded pure 3,5-diformylboron dipyrromethenes 9ꢀ12 as
The synthesis of 3,5-diformylboron dipyrromethenes 9ꢀ12,
starting from their corresponding unsubstituted dipyrro-
methanes (DPMs) 1ꢀ4, is shown in Scheme 2. The required
dipyrromethanes 1ꢀ4 were prepared according to Lindsey’s
procedure11 by condensing the appropriate aldehyde with excess
pyrrole in the presence of a catalytic amount of trifluoroacetic
acid at room temperature. The DPMs were purified by flash silica
gel column chromatography and afforded pure compounds
1ꢀ4 as white solids in 56ꢀ76% yield. The 1,9-diformyl
dipyrromethanes12 [DPM(CHO)2] 5ꢀ8 were prepared in the
next step by treating DPMs 1ꢀ4 with the Vilsmeier reagent in
1,2-dichloroethane at reflux temperature, followed by column
chromatographic purification on silica, and afforded pure 1,9-
diformyl dipyrromethanes 5ꢀ7 as white solids and 1,9-diformyl
dipyrromethane 8 as a light-brown solid in 67ꢀ82% yield.
Compounds 5ꢀ8 were characterized by various spectroscopic
techniques, and the data for known compounds 5ꢀ7 are in
agreement with those in the literature.12 A signal at ∼10.7 ppm in
1H NMR and a molecular-ion peak in mass spectra confirmed the
identity of 1,9-diformyl dipyrromethanes 5ꢀ8. The target 3,5-
diformylboron dipyrromethenes 9ꢀ12 were prepared in two
steps in a one-pot reaction. In the first step, the appropriate
diformyl dipyrromethanes 5ꢀ8 were oxidized in dichloro-
methane with DDQ for 30 min, followed by treatment with
green solids in 7ꢀ26% yield. It is observed that BF3 OEt2 should
3
be added immediately after the addition of triethylamine, and any
time delay between the addition of these two reagents to the
reaction mixture resulted in decomposition of the compound.
The molecular-ion peaks in high-resolution mass spectra con-
firmed the identity of compounds 9ꢀ12. 1H, 13C, 19F, and 11
B
NMR spectroscopies have been used to characterize compounds
1
9ꢀ12 in detail. In H NMR, the absence of a signal at ∼7.94
ppm corresponding to 3,5 protons of BODIPY and the presence
of a typical singlet at ∼10.5 ppm corresponding to an aldehyde
proton support that formylation occurred at the 3 and 5
positions. The comparison of the 1H, 19F, and 11B NMR spectra
of 3,5-diformylboron dipyrromethene 9 with 3,5-unsubstituted
BODIPY13 13 is presented in Figure 1, and relevant NMR data of
compounds 9ꢀ12 along with compound 13 are presented in
Table 1. As is clear from Figure 1 and the data in Table 1, the
presence of two electron-withdrawing formyl groups at the 3 and
5 positions alters the π delocalization, which is reflected in the
1
1
downfield shifts in H, 19F, and 11B NMR. In H NMR, the
β-pyrrole protons Ha and Hb, which appear as two sets of signals at
6.55 and 6.90 ppm, respectively, in compound 13,13 experienced
downfield shifts and appeared at 7.11 and 7.19 ppm, respectively,
in compound 9. Compounds 10ꢀ12 also exhibited similar
downfield shifts of Ha and Hb, supporting alteration of the π
delocalization upon the introduction of two formyl groups at the
3 and 5 positions. In 11B NMR, compounds 9ꢀ12 showed a
typical triplet like compound 13. However, compounds 9ꢀ12
exhibited a 0.75 ppm downfield shift of the triplet and appeared
at 1.26 ppm, unlike compound 13, in which the triplet was
observed at ∼0.5 ppm in 11B NMR (Table 1). These kinds of
significant downfield shifts observed in 11B NMR were not very
Scheme 1. Synthesis of the Reported BODIPYs
1
common for BODIPYs.14 The downfield shifts observed in H
and 11B NMR spectra for compound 9 compared to compound
13 indicate that the strong electron-withdrawing formyl groups
make the BODIPY unit electron-deficient. The density func-
tional theory (DFT) studies discussed later also indicated that
the electron density is more toward formyl groups and the
Scheme 2. Synthesis of 3,5-Diformylboron Dipyrromethenes 9ꢀ12
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Figure 1. Comparison of (a) 1H, (b) 11B, and (c) 19F NMR spectra of compounds 9 and 13 in selected regions recorded in CDCl3.
1
Table 1. H, 19F, and 11B NMR Data of Compounds 9ꢀ12
which the presence of internal hydrogen bonding between fluo-
ride ions and amido protons resulted in a ꢀ16 ppm downfield
shift of the signal in 19F NMR. Furthermore, the 1H NMR spectra
recorded for compound 9 in five deuterated solvents of varying
polarity such as benzene, chloroform, acetone, acetonitrile, and
methanol clearly indicated that the aldehyde proton (Hc) is
shifted upfield by 0.6 ppm in methanol compared to benzene
(Figure S26 in the Supporting Information). Similar observa-
tions were made in 19F NMR spectra recorded for 9 in deuterated
chloroform and methanol (Figure S27 in the Supporting In-
formation). In chloroform, the quartet signal in the 19F NMR
spectrum was observed downfield (ꢀ130 ppm) because of the
presence of hydrogen bonding, while in methanol, the hydrogen
bonding is absent, which is reflected in an upfield shift of the
signal (ꢀ140 ppm). This supports the fact that the polar solvent
breaks hydrogen bonding, whereas hydrogen bonding is stabi-
lized in a nonpolar solvent. The 3,5-disubstituted BODIPYs16,17
are the first class of compounds where intramolecular hydrogen
bonding was established crystallographically. Because we in-
ferred hydrogen bonding in compounds 9ꢀ12 from the 19F
along with 13 Recorded in CDCl3
1H NMR
compound
Ha
Hb
Hc
19F NMR
11B NMR
13
9
6.55
7.11
7.07
7.14
6.84
6.90
7.19
7.20
7.21
7.18
ꢀ145.4
ꢀ130.8
ꢀ130.8
ꢀ130.7
ꢀ130.5
0.50
1.26
1.24
1.28
1.24
10.478
10.476
10.474
10.480
10
11
12
BODIPY ring remains electron-deficient in compound 9. The
most interesting observations were made in the 19F NMR spectra
of compounds 9ꢀ12. In all reported BODIPYs such as 13, the
19F NMR spectrum corresponds to a single quartet because of
coupling to 11B (I = /2 and J = 32 Hz) and resonances at
3
approximately ꢀ147 ppm.13
However, in some recently reported BODIPY compounds,15
the fluorines were found to be inequvalent and showed two sets
of multiplets at ∼ꢀ140 ppm. In complexes 9ꢀ12, we observed
only one quartet in 19F NMR, indicating that the fluorines are
under a chemically equivalent environment. However, com-
pounds 9ꢀ12 exhibited a 19F NMR signal at ∼ꢀ131 ppm,
a ꢀ15 ppm downfield shift compared to that of the other
reported BODIPYs.13 This unusual downfield shift was attrib-
uted to the presence of hydrogen bonding between fluoride ions
and an aldehyde group. This observation is in line with the
recently reported16 3,5-diamidoboron dipyrromethene dyes, in
1
and H NMR study, we attempted to grow single crystals of
compounds 9ꢀ12. We successfully obtained single crystals for
compound 11 by the slow evoporation of n-hexane/CHCl3 (1:1)
over a period of 1 week, which crystallized in a triclinic P1 unit
cell. The crystal structure of compound 11 is shown in Figure 2.
Compound 11 has a planar BODIPY framework, which is com-
prised of two pyrrole rings and a central six-membered boron
ring like that reported in meso-(phenyl)boron dipyrromethene
compound 14 (Table S1 in the Supporting Information). The
dihedral angle between the meso-aryl ring and the BODIPY core
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ARTICLE
Figure 3. Comparison of the reduction waves of the cyclic voltammo-
grams of compounds 9 and 13 in dichloromethane containing 0.1 M
tetrabutylammonium perchlorate as the supporting electrolyte recorded
at a 50 mV sꢀ1 scan rate.
Figure 2. ORTEP diagram (50% probability) of compound 11.
is 48°, which is closer to that of 14 (∼60°),13 indicating the
presence of free rotation of the meso-aryl group in compound 11.
The striking feature of compound 11 is the presence of intra-
molecular hydrogen bonding between the boron-bound fluoride
groups and the aldehyde groups present at the 3 and 5 positions
(Table S1 in the Supporting Information). Each fluoride in
compound 11 is involved in hydrogen bonding with one of
In addition, an ill-defined band at ∼400 nm corresponding to a
S0 f S2 transition is also present. The absorption bands possess
narrow spectral bandwidths in compounds 9ꢀ12. These absorp-
tion features are in agreement with those of other BODIPY dyes
such as 13. Similarly, compounds 9ꢀ12 showed a single fluor-
escence band with a bandwidth comparable to that of unsub-
stituted BODIPY 13. However, the data presented in Table 2
reveal the following differences and similarities between 3,5-
diformylboron dipyrromethenes 9ꢀ12 and 3,5-unsubstituted
BODIPY 13: (1) Compounds 9ꢀ12 exhibited a 40ꢀ50 nm
red shift in the S0 f S1 absorption band (Figure 4a), and the
molar absorption coeffiecients are 2ꢀ3 times lower compared to
that of 13. This is due to the presence of electron-withdrawing
formyl groups in compounds 9ꢀ12. (2) The absorption study of
compounds 9ꢀ12 in different solvents (Table S3 in the Support-
ing Information) indicated that the absorption properties are
greatly affected by the solvent polarity. The absorption band was
blue-shifted and its intensity was reduced with an increase in the
solvent polarity. Thus, compounds 9ꢀ12 shows strong absorp-
tion in less polar solvents like chloroform, which is shifted to blue
with a very weak and broad absorption in polar solvents like
methanol. This is due to the prevention of conjugation between
formyl groups and the BODIPY core in polar solvents like
methanol, but it is operational in nonpolar solvents like chloro-
form. Although BODIPY compounds such as 13, which does not
have formyl groups at the 3 and 5 positions, also exhibit slight
hypsochromic shifts with an increase in the solvent polarity, the
effects observed for compounds 9ꢀ12 were quite significant. (3)
Compounds 9ꢀ12 showed typical emission features of BODIPY
dyes, that is, narrow, slightly Stokes-shifted bands of mirror
image shape. The smaller Stokes shift suggests that the difference
between the ground and excited states is much less and these
BODIPYs undergo less structural reorganization in the excited
state, like any other reported BODIPY.18 (4) Compounds 9ꢀ12
showed bathochromically shifted emission bands compared to
that of 3,5-unsubstituted BODIPY 13 (Figure 4b). (5) Com-
pounds 9 and 10 showed decent quantum yields compared to
that of 13; compound 11 showed a lower quantum yield, and
compound 12 was completely nonfluorescent. The poor fluor-
escence behavior of compounds 11 and 12 is due to electron-rich
meso-aryl groups that are involved in photoinduced electron
transfer (PET) with the BODIPY core. (6) The fluorescence
the aldehyde protons with CꢀH F distances of 2.50 Å
3 3 3
(CꢀH1 F1) and 2.91 Å (CꢀH11 F2). The presence of
3 3 3
3 3 3
two formyl groups at the 3 and 5 positions also alters the bond
lengths and angles slightly compared to those of 14 (Table S1 in
the Supporting Information), which is attributed to the electron-
withdrawing nature of the formyl groups.
Compounds 9ꢀ12 were further characterized by cyclic vol-
tammetry, steady-state absorption and fluorescence, and time-
resolved fluorescence techniques. The electrochemical proper-
ties of compounds 9ꢀ12 along with 13 were followed by cyclic
voltammetry in CH2Cl2 using tetrabutylammonium perchlorate
as the supporting electrolyte. A comparison of the reduction
waves of compound 9 with those of compound 13 is shown in
Figure 3. Compounds 9ꢀ11 showed no oxidation waves but two
reversible reductions occurring at the boron dipyrromethene
unit (Table S2 in the Supporting Information). The absence of
oxidation in compounds 9ꢀ11 indicates the electron-deficient
nature of the boron dipyrromethene unit. Compound 12 showed
two oxidations at 0.81 and 1.32 V, which are due to the presence
of an electron-rich 3,4,5-trimethoxyphenyl group at the meso
position. The electron-deficient nature of the boron dipyrro-
methene unit in compounds 9ꢀ12 is clearly evident in the first
and second reduction potentials, which were shifted by ∼650 mV
toward less negative compared to that of 13, indicating that the
3,5-diformylboron dipyrromethenes 9ꢀ12 are very easy to
reduce. This significant shift in the reduction potentials of
compounds 9ꢀ12 compared to that of 13 supports the strong
electron-withdrawing effect of the formyl groups present at the 3
and 5 positions in 9ꢀ12 (Table S2 in the Supporting In-
formation). The absorption and fluorescence properties of
compounds 9ꢀ12 were studied in different solvents of varying
polarity (Table S3 in the Supporting Information), and the
relevant data of compounds 9ꢀ12 along with compound 13 in
CHCl3 are presented in Table 2. The absorption spectra of
compounds 9ꢀ12, in general, showed a characteristic strong
band corresponding to a S0 f S1 transition in the 540ꢀ550 nm
region with one vibronic component on the higher energy side.
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a
Table 2. Photophysical Data of Compounds 9ꢀ13 Recorded in CHCl3
compound
λabs (nm)
λem (nm)
Δνst (cmꢀ1
)
log ε
Φ
τ (ns)
kr (ꢁ109 sꢀ1
)
knr (ꢁ109 sꢀ1
)
13
9
501
546
550
544
548
517
556
560
554
570
618
329
325
332
704
4.74
4.09
3.77
4.25
4.06
0.03
0.31
0.30
0.09
0.51
5.9
0.059
0.053
0.055
0.064
1.90
0.117
0.127
0.650
10
11
12
5.5
1.4
a log(ε/molꢀ1 dm3 cmꢀ1) ꢀ molar extinction coefficient, λabs (absorption maxima), λem (emission maxima), Δν (Stokes shift), Φ (quantum yield), τ
(lifetime), kr (radiative decay), and knr (nonradiative decay).
Figure 4. Comparison of normalized (a) absorption and (b) emission spectra of compounds 9 (—) and 13 (---) recorded in chloroform.
band of compounds 9ꢀ11 was hypsochromically shifted and
quantum yields were decreased with an increase in the polarity of
the solvent (Table S3 in the Supporting Information). These
observations were in agreement with the general behavior of
other BODIPYs. Thus, the steady-state absorption and fluores-
cence study revealed that compounds 9ꢀ12 exhibit altered
properties compared to 13, which is due to the presence of
electron-withdrawing formyl groups at the 3 and 5 positions. The
time-resolved fluorescence studies were carried out in order to
understand their fluorescence properties in detail (Figure S28 in
the Supporting Information). Compounds 9ꢀ11 showed single-
exponential decay, and their singlet state lifetimes were larger
than that of compound 13. The fluorescence decay of com-
pounds 9ꢀ11 studied in different solvents also showed generally
Figure 5. Energies and electron distribution patterns in the HOMO
and LUMO states of molecules 13 (left) and 9 (right).
single-exponential decay, and the lifetimes were parallel with
their observed fluorescence quantum yields (Table S3 in the
Supporting Information).
To understand the basis of having distinct properties of 3,5-
diformylboron dipyrromethene at molecular and electronic levels,
ab initio calculations were carried out on compounds 9 and 13
using the Gaussian program19a with DFT,19b the B3LYP19c
method, and 6-31þG(d,p)19d as the basis set. The contours of
the electronic distribution in highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
statesonthesemoleculessuggestedthattherewere veryminute but
essential differences between compounds 9 and 13 (Figure 5).
Precisely, the HOMO states of the two molecules seem to be
superimposable in the BODIPY nucleus, while close examination
of the LUMO states of compounds 9 and 13 revealed that
electrons were more delocalized toward the 3,5-diformyl groups
in molecule 9, making the BODIPY nucleus slightly electron-
deficient. It was interesting to note that, although the 3,5-diformyl
groups stabilize both states of BODIPY, the energy gap is just
slightly less than that of molecule 13 (a difference of 0.013 eV was
observed). Such an effect could have arisen through extension
of π conjugation introduced in the system and the electron-
withdrawing ability of the 3,5-diformyl groups. Although the
HOMOꢀLUMO energies give a clear idea that 3,5-diformylboron
dipyrromethenes are easily reducible, which is in agreement
with our experimental results, the better indicators for the redox
properties are electron affinity (EA) and ionization potential (IP)
data. The EA is a fundamental property of atoms and molecules
and is defined as the energy difference between an uncharged
species and its negative ion.20 For both molecules 9 and 13, the
negative ions were generated by defining the value of charge
as ꢀ1 and multiplicity as 2. Similarly, the IP is defined as the
amount of energy required for removing one electron from a
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Table 3. Data Generated from the Computational Study for
Compounds 13 and 9
HOMO energy
(eV)
LUMO energy
(eV)
molecule
EA (eV)
IP (eV)
13
9
ꢀ6.1756
ꢀ6.2733
ꢀ3.1123
ꢀ3.2225
1.846
2.881
7.4465
8.6381
molecule and is generally computed as the energy difference
between its cationic and neutral states.21 The cations were
generated by defining the value of charge as 1 and multiplicity
as 2 for calculations. These ions were optimized with the above-
mentioned protocol. The heats of formation were corrected for
zero point for calculation of the EA and IP data. The computa-
tional data for the two molecules are given in Table 3. It is clear
from the data that the EA for molecule 9 is higher than that for
molecule 13; i.e., molecule 9 is more reducible compared to 13.
The IPs of the two molecules indicate that molecule 9 is difficult
to oxidize. Thus, the quantum-mechanics-based studies of these
molecules were found to be in accordance with the experi-
mental data.
It is well established in the literature that BODIPYs bearing
hydroxyaryl6/diaminoaryl7/calix[4]arene8 groups at the meso
position can be used as deprotonation/protonation-dependent
fluorescence off/on pH sensors. The phenolic, N,N-dialkylani-
line, and calix[4]arene derivatives sense the alkaline, acidic, and
near-neutral pH range, respectively. The low emission intensity
of the phenolate or uncharged dialkylaniline forms has been
attributed to charge transfer from phenolate or uncharged N,
N-dialkylaniline donors to the excited-state BODIPY acceptor
moiety. Interestingly, the fluorophore-bearing aldehyde group is
generally not used as the fluorescent pH sensor. This is because
of the electron-deficient nature of aldehyde, which prevents its
involvement in charge transfer with the fluorophore. However, in
BODIPY compounds 9ꢀ12, the aldehyde groups at the 3 and 5
positions are relatively more electron-rich because these groups
are directly involved in π conjugation of the BODIPY unit. Hence,
we assumed that the aldehyde groups in BODIPY compounds
9ꢀ12 are as senstive as the phenolic/(dialkylamino)phenyl group
toward the pH and can be used as fluorescent pH sensors. We
carried out systematic pH-dependent absorption studies on com-
pounds 9ꢀ12 and fluorescence studies on compounds 9ꢀ11 in an
acetate buffer solution over a pH range of 4.0ꢀ9.0 (Table S4 in the
Supporting Information), and the effect of the pH on the absorp-
tion and fluorescence of compound 9 is shown in Figure 6. The
absorption spectra of compounds 9ꢀ12 as a function of the pH in
an acetate buffer solution exhibited an increase of the respective
absorption bands with increasing Hþ concentration without
changes in the peak maxima, as shown in Figure 6a for compound
9. Because the absorption studies are performed in a polar medium,
the absorption bands of BODIPY are already hypsochromically
shifted compared to a nonpolar medium like benzene; hence, no
shifts in the absorption peak maxima are expected with changes of
the pH in the buffer medium. The emission peak maxima of
compounds 9ꢀ11 also remain unaltered in different pH solutions,
but their intensities were significantly altered. For example, at pH 8,
compound 9 was very weakly fluorescent. Furthermore, the
fluorescence switching process was also found to be reversible. It
was observed that compound 9 showed some emission at pH 7, but
the emission intensity decreased as the pH of the alkaline medium
was increased and compound 9 became completely nonfluorescent
Figure 6. (a) Absorption spectra of compound 9 (5 μM) in an aqueous
acetate buffer solution (0.1 M) as a function of the pH. (b) Fluorescence
spectra of compound 9 (5 μM) in an acetate buffer solution (0.1 M) as a
function of the pH. The excitation wavelength used was 488 nm. The
inset shows the plot of I/Imax vs pH.
at pH 9. However, the decrease in the emission intensity is not very
substantial as we move toward alkaline pH from pH 7, indicating
that compound 9 is less fluorescent even at neutral pH. Upon a
decrease from pH 7 to acidic pH (pH 4), the emission intensity was
enhanced by 17 times and a maximum changewas foundwithin the
pH range of 6.5ꢀ4.0 (pKa = 6.39 at pH 4).22 This is also clearly
reflected in the sigmoidal response observed in the plot of I/Imax vs
pH, where Imax is the maximum fluorescence intensity of com-
pound 9 and I is the fluorescence intensity at a particular pH.
Similar observations were made for compounds 10 and 11 (Table
S4 in the Supporting Information). However, compound 12 was
too weakly fluorescent to perform pH titration studies and
compound 13, which does not have formyl groups at the 3 and 5
positions, did not exhibit any changes in the fluorescence intensity
upon variation of the pH. These significant changes in acidic and
alkaline media represent two different “states”, where the fluores-
cence is “switched off” (state I) in an alkaline solution and
“switched on” (state II) in an acidic solution, as shown in Scheme 3.
In an alkaline solution, the aldehyde group is electron-rich and
engaged in PET quenching of the BODIPY excited state, and in an
acidic medium, the PET quenching is prevented because of
protonation of the aldehyde group. To confirm that 3,5-diformyl-
boron dipyrromethenes exist in two different states, we recorded
1H NMR spectra of compound 9 at basic and acidic media (Figure
S29 in the Supporting Information). NMR spectra of compound 9
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ARTICLE
Scheme 3. Schematic Representation of the Fluorescence
On/Off pH Sensor
steady-state fluorescence spectra were obtained with Perkin-Elmer
Lambda-35 and PC1 photon-counting spectrofluorometers manufac-
tured by ISS and USA Instruments, respectively. The fluorescence
quantum yields (Φf) were estimated from the emission and absorption
spectra by comparative method at the excitation wavelength of 488 nm
using Rhodamine 6G (Φf = 0.76)17a as standard. The time-resolved
fluorescence decay measurements were carried out at magic angle using
a picosecond diode-laser-based time-correlated single-photon-counting
fluorescence spectrometer from IBH, U.K. All of the decays were fitted
to a single exponential. The good fit criteria were low χ2 (1.0) and
random distributions of residuals. Cyclic voltammetric studies were
carried out with a BAS electrochemical system utilizing a three-electrode
configuration consisting of glassy carbon (working), platinum wire
(auxiliary), and saturated calomel (reference) electrodes. The experi-
ments were done in dry dichloromethane using 0.1 M tetrabutylammo-
nium perchlorate as the supporting electrolyte. Half-wave potentials
were measured using differential-pulse voltammetry and also calculated
manually by taking the average of the cathodic and anodic peak
potentials. The HRMS spectra were recorded with a Q-Tof micro mass
spectrometer. A single crystal of compound 11 (CCDC 768735) was
obtained from the slow evaporation of a n-hexane/CHCl3 solution. The
intensity data collection for compound 11 has been carried out on a
Nonius MACH3 four-circle diffractometer at 293 K. The structure
solution for compound 11 was obtained using direct methods (SHELXS-
97)23 and refined using full-matrix least-squares methods on F2 using
SHELXL- 97.24 For pH studies, the absorption and fluorescence
titrations were performed in an acetate buffer solution (0.1 M) contain-
ing a minimum amount of methanol [1:99 (v/v) CH3OH/buffer] at a
probe concentration of 5 μM. A few microliters of 0.1 and 0.01 M
solutions of HCl and/or NaOH were used to adjust the pH of the
solutions.
in basic and acidic pH media are quite different from each other. In
a basic medium, the aldehyde proton Hc appeared at 9.9 ppm,
which experienced a substantial upfield shift in an acidic medium
and appeared at 5.8 ppm, supporting the existence of compound 9
in two different states (Scheme 3). These changes are also clearly
evident in the color of the solutions at different pH under a UV
lamp. At acidic pH, the solution of 9 is bright fluorescent green,
which faded at basic pH (Figure S47b in the Supporting In-
formation). Thus, the pH-controlled fluorescence titration studies
indicate that 3,5-diformylboron dipyrromethenes can be used as
pH fluorescent sensors.
Computational Details. The computational studies were per-
formed with Gaussian 0319a installed on a Windows operating system.
The structures were energy-optimized using quantum mechanics with
DFT19b and B3LYP19c gradient-corrected correlation functional meth-
ods in conjugation with a standard 6-31G (p,d) basis set19d and
parameters. For the optimized structures, population analysis studies
were done. The redox properties of compounds 9 and 13 and EA and IP
data were also calculated.20,21 Simulations of the addition and removal of
electrons from BODIPY systems (compounds 9 and 13) were affected
by simply setting the charges and multiplicities as (ꢀ1, 2) and (1, 2).
With these charges and multiplicities and the 3D coordinates of neutrally
optimized BODIPY systems, geometry optimizations were carried out
for the respective anions and cations. From the heats of formation
obtained, the total energies (TEs) were calculated by zero-point and
basis-set corrections. The EA and IP data were calculated using the
following formulas:
’ CONCLUSIONS
We synthesized stable 3,5-diformylboron dipyrromethenes in
two steps starting from meso-aryl dipyrromethanes under simple
reaction conditions. The X-ray structural analysis indicated that
the two aldehyde groups are involved in intramolecular hydrogen
bonding with the fluoride atoms and stabilization of the com-
pounds. The formyl groups on the BODIPY framework alter the
electronic properties of the BODIPY unit siginificantly, which is
reflected in various spectroscopic and electrochemical properties.
The electrochemical studies as well as quantum-mechanical
studies at the DFT level indicated that the BODIPY unit in
diformylboron dipyrromethenes is highly electron-deficient and
readily undergoes facile reduction. The synthesized 3,5-difor-
mylboron dipyrromethenes were used as fluorescent pH sensors,
which is uncommon for aldehyde-containing fluorophores. Pre-
sently, we are using these 3,5-diformylboron dipyrromethenes
to synthesize further eloborated structures such as 3,5-bis-
(dipyrromethane)boron dipyrromethenes (Scheme S1 in the
Supporting Information) for various studies.
EA ¼ TEðneutralÞ ꢀ TEðanionÞ
IP ¼ TEðcationÞ ꢀ TEðneutralÞ
1,9-Diformyl-meso-(3,4,5-trimethoxyphenyl) Dipyrro-
methane. Dimethylformamide (5.1 mmol) was added to a 100 mL
three-neck, round-bottomed flask, cooled to 5ꢀ10 °C, and flushed with
nitrogen for 5 min. POCl3 (4.23 mmol) was added dropwise over 15 min
with stirring. Dry dichloroethane (10 mL) was then added, and the
mixture was stirred at room temperature for 15 min. The mixture was
cooled to 0 °C, meso-(3,4,5-trimethoxyphenyl)dipyrromethane (4; 4.23
mmol) in dry dichloroethane (20 mL) was added dropwise over 1 h, and
the solution turned to deep-purple. The solution was warmed to 50 °C
for 15 min and then allowed to cool to room temperature. A saturated
sodium carbonate solution (50 mL) was added, and the reaction was stir-
red vigorously at room temperature for 2 h. The mixture was extracted
’ EXPERIMENTAL SECTION
General Experimental Details. The 1,9-diformyl dipyrro-
methanes12 5ꢀ7 and BODIPY13 13 were synthesized by following the
literature procedures. BF3 Et2O and 2,3-dichloro-5,6-dicyano-1,4-ben-
zoquinone (DDQ) were used as obtained. All other chemicals used for
the synthesis were reagent-grade unless otherwise specified. Column
chromatography was performed on silica (60ꢀ120 mesh). The 1H, 13C,
11B, and 19F NMR spectra were recorded in CDCl3 using a Varian
VXR 400 spectrometer operating at the appropriate frequencies using
tetramethylsilnae [Si(CH3)4] as the internal reference. Absorption and
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ARTICLE
with ethyl acetate (3 ꢁ 100 mL), and the combined organic layers were
dried over Na2SO4, filtered, and evaporated. The crude compound was
subjected to silica gel column chromatography and eluted with petro-
leum ether/ethyl acetate (70:30), which afforded pure 1,9-diformyl-
meso-(3,4,5-trimethoxyphenyl) dipyrromethane (8) as a light-brown
151.1, 153.1, 153.6, 184.3. HRMS. Calcd for C20H17BF2N2O5 [(M ꢀ
F)þ]: m/z 395.1215. Found: m/z 395.1197. IR (KBr, νmax): 633.1,
765.0, 1074.2, 1175.1, 1232.2, 1269.7, 1335.8, 1379.5, 1464.9, 1539.2,
1570.3, 1675.4, 2922.7.
1
solid in 67% yield (0.8 g). Mp: 168ꢀ169 °C. H NMR (400 MHz,
’ ASSOCIATED CONTENT
CDCl3, δ in ppm): 3.76 (s, 9H; ꢀOCH3), 5.45 (s, 1H; ꢀCH),
6.07ꢀ6.09 (m, 2H; py), 6.63 (s, 2H; Ar), 6.85ꢀ6.87 (m, 2H; py),
9.12 (s, 2H; ꢀNH), 10.86 (br s, 2H; ꢀCHO). HRMS. Calcd for
C20H20N2O5 [(M þ 1)þ]: m/z 369.1450. Found: m/z 369.1436.
General Procedure for 3,5-Diformylboron Dipyrro-
methenes (9ꢀ12). 1,9-Diformyl dipyrromethanes 5ꢀ8 (1.7 mmol)
were dissolved in dichloromethane (200 mL) and oxidized with DDQ
(2.04 mmol) at room temperature. The reaction mixture was allowed to
stir at room temperature for 30 min. Triethylamine (68 mmol), followed
by BF3.Et2O (85 mmol), was added to the reaction mixture successively
without any time delay, and stirring was continued at room temperature
for an additional 30 min. The reaction mixture was evaporated, and the
crude product was purified using silica gel column chromatography with
petroleum ether/ethyl acetate (75:25), which afforded pure 3,5-difor-
mylboron dipyrromethenes 9ꢀ12 as blue-greenish solids.
S
Supporting Information. Spectral data of all compounds,
b
fluorescence data, tables of crystal and absorption data, and X-ray
crystallographic data for compound 11 inCIFformat. Thismaterial
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ravikanth@chem.iitb.ac.in.
’ ACKNOWLEDGMENT
M.R. acknowledges financial support by the DST and BRNS.
M.R.R. thanks the CSIR for their research fellowship. We thank
the DST-funded National Single Crystal X-ray Diffraction Facil-
ity for diffraction data. We thank the Department of Chemistry
and Sophisticated Analytical Instrument Facility (SAIF), Indian
Institute of Technology Bombay, for instrumentation. We also
thank Dr. Evans Coutinho, Bombay College of Pharmacy for
providing computational facility and valuable inputs.
3,5-Diformyl-8-tolyl-4-bora-3a,4a-diaza-s-indacene (9).
1
Yield: 80 mg, 14%. Mp: 242ꢀ243 °C (dec). H NMR (400 MHz,
CDCl3, δ in ppm): 2.52 (s, 3H; ꢀCH3), 7.12 (d, 3J(H,H) = 4.27 Hz, 2H;
py), 7.20 (d, 3J(H,H) = 4.27 Hz, 2H; py), 7.43 (d, 3J(H,H) = 7.94 Hz,
2H; Ar), 7.53 (d, 3J(H,H) = 7.94 Hz, 2H; Ar), 10.48 (br s, 2H; ꢀCHO).
1
11B NMR (100 MHz, CDCl3, δ in ppm): 1.26 (t, J(BꢀF), 1B). 19F
1
NMR (300 MHz, CDCl3, δ in ppm): ꢀ130.8 (q, J(FꢀB), 2F). 13C
NMR (100 MHz, CDCl3, δ in ppm): 21.8, 120.2, 129.9, 130.6, 131.4,
132.9, 137.9, 143.8, 151.0, 153.8, 184.4. HRMS. Calcd for C18H13-
BF2N2O2 [(M ꢀ F)þ]: m/z 319.1054. Found: m/z 319.1042. IR (KBr,
νmax): 760.0, 997.2, 1135.1, 1186.8, 1232.5, 1268.7, 1342.4, 1387.6,
1477.7, 1542.4, 1567.7, 1672.9, 2924.6.
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1
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diaza-s-indacene (12). Yield: 40 mg, 7%. Mp: 250ꢀ251 °C (dec).
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3H, p-OCH3), 6.84 (s, 2H; Ar), 7.18 (d, 3J(H,H) = 4.58, 2H; py), 7.21
(d, 3J(H,H) = 4.28, 2H; py), 10.48 (br s, 2H; ꢀCHO). 11B NMR (100
MHz, CDCl3, δ in ppm): 1.24 (t, 1J(BꢀF), 1B). 19F NMR (300 MHz,
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