Boron-dipyrromethene based multi-anionic sensor and a specific cationic sensor for Fe3+

Article abstract of DOI:10.1039/c4tc00317a

Symmetrical and unsymmetrical phenylhydrazone substituted boron-dipyrromethenes were synthesized by treating 3,5-diformyl boron-dipyrromethene with phenylhydrazine/2,4-dinitrophenylhydrazine in ethanol at reflux temperature. The X-ray structure of the unsymmetrical boron-dipyrromethene (BODIPY) was shown to have an almost extended planar orientation of BODIPY with 2,4-dinitrophenylhydrozone unit at the 3-position and ethyl acetal at the 5-position with small torsional angles. A strong absorption band at ～600 to 700 nm and a weak emission band at ～620 to 710 nm were observed for the phenylhydrazone substituted BODIPYs. The compounds showed interesting absorption properties in DMSO and DMF solvents compared to other solvents by shifting ～150 nm towards the red region. Anion binding studies indicated that the unsymmetrical phenylhydrazone substituted BODIPY could be applied as a sensor for F^-, CH₃COO^- and H ₂PO₄^- ions as confirmed by various spectroscopic studies. Furthermore, the unsymmetrical phenylhydrazone substituted BODIPY had the unique ability of acting as a two step fluorescence enhanced chemodosimetric sensor for Fe³⁺ ion. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00317a

Journal of
Materials Chemistry C
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Boron-dipyrromethene based multi-anionic sensor
and a specific cationic sensor for Fe3+†
Cite this: J. Mater. Chem. C, 2014, 2,
5576
*
Vellanki Lakshmi and Mangalampalli Ravikanth
Symmetrical and unsymmetrical phenylhydrazone substituted boron-dipyrromethenes were synthesized
by treating 3,5-diformyl boron-dipyrromethene with phenylhydrazine/2,4-dinitrophenylhydrazine in
ethanol at reflux temperature. The X-ray structure of the unsymmetrical boron-dipyrromethene
(BODIPY) was shown to have an almost extended planar orientation of BODIPY with
2,4-dinitrophenylhydrozone unit at the 3-position and ethyl acetal at the 5-position with small torsional
angles. A strong absorption band at ꢀ600 to 700 nm and a weak emission band at ꢀ620 to 710 nm
were observed for the phenylhydrazone substituted BODIPYs. The compounds showed interesting
absorption properties in DMSO and DMF solvents compared to other solvents by shifting ꢀ150 nm
towards the red region. Anion binding studies indicated that the unsymmetrical phenylhydrazone
Received 17th February 2014
Accepted 14th March 2014
substituted BODIPY could be applied as a sensor for Fꢁ, CH3COOꢁ and H2PO4 ions as confirmed by
ꢁ
DOI: 10.1039/c4tc00317a
various spectroscopic studies. Furthermore, the unsymmetrical phenylhydrazone substituted BODIPY
www.rsc.org/MaterialsC
had the unique ability of acting as a two step fluorescence enhanced chemodosimetric sensor for Fe3+ ion.
exclusive sensor for Fꢁ ion.7 However, the aldehyde functional
group on the BODIPY core enables the synthesis of novel
Introduction
substituted BODIPYs which will be useful for a range of appli-
cations.8 Here we report the synthesis of phenylhydrazone and
There are numerous applications of boron-dipyrromethene
(BODIPY) dyes in elds ranging from materials to medicine
because of their excellent thermal, chemical and photochemical
stability, high molar absorptivity, high uorescence quantum
yields and low sensitivity to both solvent polarity and pH.1 The
properties of BODIPY dyes can be ne tuned by introducing
appropriate substituents at the BODIPY core. Thus, BODIPY
dyes are amenable for functionalization, and the functionalized
BODIPYs have been used to synthesize a wide variety of
substituted BODIPYs for various applications.2 For example,
halogenated BODIPYs have been used as synthons for the
synthesis of a variety of polyarylated BODIPYs3a–c and cascade
type BODIPY arrays.3d,e Ziessel et al. developed a route to
introduce ethynyl functional groups in place of uorides in
BODIPY and used these systems to produce very interesting
light-harvesting systems.4 We recently reported the synthesis of
3,5-diformyl BODIPYs under simple reaction conditions and
used them to prepare a pH-based optical sensor5 as well as for
sensing CNꢁ ion by using various spectroscopic methods.6 We
also reported the synthesis of 3,5-bis(dipyrromethanyl) BODI-
PYs by treating 3,5-diformyl BODIPYs with excess pyrrole in the
presence of mild acid and demonstrated their use as an
2,4-dinitrophenylhydrazone substituted BODIPYs 1 and
2
respectively by treating 3,5-diformyl BODIPY 3 with appropriate
phenylhydrazines in alcohol at reuxing temperature. Our
studies indicated that compound 1 cannot sense anions
whereas compound 2 can bind to anions such as Fꢁ, CH3COOꢁ,
ꢁ
and H2PO4 as conrmed by optical and NMR techniques.
Furthermore, we also showed that BODIPY 2 acts as a two step
uorescence enhanced sensor which is unique and different
from other uorescence enhanced Fe3+ sensors reported in the
literature.9
Results and discussion
The phenylhydrazone substituted BODIPYs 1, 2a and 2b were
synthesized as outlined in Scheme 1. Compound 1 was
synthesized by treating 3,5-diformyl BODIPY5 3 with two
equivalents of phenylhydrazine in ethanol at reuxing temper-
ature for 5 h. TLC analysis of the reaction mixture indicated the
disappearance of spots corresponding to the precursors and the
appearance of one desired major violet spot along with two
unidentied minor spots. The crude compound was subjected
to alumina column chromatography and afforded the desired
compound 1 in 62% yield as a blue solid. Compound 2a was
prepared similarly by treating 3 with two equivalents of
2,4-dinitrophenylhydrazine in ethanol at reux for 5 h whereas
compound 2b was prepared by changing the solvent from
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400
076, India. E-mail: ravikanth@chem.iitb.ac.in
† Electronic supplementary information (ESI) available: Experimental methods,
spectral data, uorescence, absorption spectral traces. CCDC 945253. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4tc00317a
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intramolecular hydrogen bonding between NH and NO2 groups
1
in compound 2. The H NMR spectra of compounds 1 and 2a
recorded in DMSO-d6 had the same number of signals with
slight shis in their chemical shis except for the NH proton
which experienced a downeld shi compared to that recorded
in CDCl3. For example, the NH proton of compound 1 which
appeared at 8.49 ppm in CDCl3 experienced a 3 ppm downeld
shi and appeared at 11.5 ppm in DMSO-d6. Similarly, the NH
proton of compound 2a and 2b was also downeld shied by ꢀ1
ppm in DMSO-d6 compared to that recorded in CDCl3.
Furthermore, the magnitude of the downeld shi of the NH
proton of compound 1 was much larger compared to that of
compounds 2a and 2b when the solvent was changed from
CDCl3 to DMSO-d6 (Fig. 1a). This is attributed to the strong
intermolecular hydrogen bonding present between the hydra-
zone NH of compound 1 and the oxygen atom of the solvent.
However, in compounds 2a and 2b, since the hydrazone NH is
already engaged in intramolecular hydrogen bonding with the
NO2 group, an additional 1 ppm downeld shi was observed
Scheme 1 Synthesis of compounds 1, 2a and 2b.
ethanol to methanol. The crude compounds were subjected to due to intermolecular hydrogen bonding interaction with the
alumina column chromatography and afforded pure 2a and 2b solvent. Furthermore, compounds 1, 2a and 2b were also
in 17 and 26% yields respectively. Interestingly, when we used characterized by 19F and 11B NMR techniques. In the 19F NMR
2,4-dinitrophenylhydrazine, we obtained only monohydrazone spectra, a broad signal at ꢁ138 ppm was observed for
substituted BODIPY and the aldehyde at the 5-position was compound 1 but a typical quartet at ꢀ ꢁ138 ppm was observed
converted to acetal. We did not notice the formation of any for compounds 2a and 2b . In the 11B NMR spectra, a triplet at
dihydrazone substituted compound in this reaction even when ꢀ1.0 ppm was observed for compounds 1, 2a and 2b which was
we used excess equivalents of 2,4-dinitrophenylhydrazine. slightly downeld shied compared to that observed for other
Compounds 1, 2a and 2b were freely soluble in common organic BODIPYs such as meso-phenyl BODIPY10 (0.5 ppm).
solvents and the molecular structures were conrmed by
HR-MS, NMR and the crystal structure determined for terized by X-ray diffraction analysis. Single crystals of 2a suit-
compound 2a. able for X-ray analysis were grown by vapour diffusion of
Compounds 1, 2a and 2b were characterized in detail by 1H, petroleum ether into CHCl3 solution under atmospheric pres-
The structure of compound 2a was unambiguously charac-
1
1
19F, 11B and H–1H COSY NMR spectroscopy. The H NMR of sure over a period of 15 days. Compound 2a was crystallized†
compounds 1, 2a and 2b were recorded in a hydrogen bond with monoclinic symmetry and a C2/c space group. The crystal
donating solvent (HBD) such as CDCl3 and a hydrogen bond structure is depicted in Fig. 2. The structure is shown to have an
accepting solvent (HBA) such as DMSO-d6 and a comparison of almost extended planar orientation of BODIPY with a phenyl-
compounds 1 and 2a in two different solvents is presented in hydrazone unit on one side and ethyl acetal on the other side,
Fig. 1a. Furthermore, the 1H–1H COSY NMR spectrum of and small torsional angles of 8.1ꢂ (C15,C16,C17, N3) and 4.1ꢂ (N3,
compound 2a is presented in Fig. 1b. In the 1H NMR spectrum N4, C18, C19) respectively. The dipyrrin core of compound 2a
of compound 1 in CDCl3, four pyrrole signals due to the BODIPY exhibits a planar conformation having a central six-membered
core (f, g, h, i type) appeared as two sets of signals at 6.81 and ring fused with two adjacent ve-membered rings. The two
7.11 ppm because of symmetric substitution; the alkene CH uoride atoms present on the boron atom of the central six-
proton appeared as a singlet at 8.20 ppm and the NH proton membered ring as well as the alkoxy substituents of the acetal
1
appeared as a singlet at 8.49 ppm. In the H NMR spectrum of functional group are perpendicular to the dipyrrin core. The
compound 2a, the four pyrrole protons (f, g, h, i type) appeared dihedral angle between the meso-substituted phenyl group and
as four different sets due to unsymmetric substitution. These dipyrrin unit is 69ꢂ and the B–N bond distances are approxi-
˚
protons were identied with the help of cross peak correlations mately 1.54 A which indicates the usual delocalization of the
observed in the 1H–1H COSY NMR spectrum (Fig. 1b). The positive charge on both the nitrogen atoms as noted for other
alkene CH proton and acetal CH proton of compound 2a were BODIPYs.10 Furthermore, compound 2a displays some
˚
observed at 8.61 and 5.98 ppm respectively. All the aryl signals secondary intramolecular interactions such as C–H/F (2.65 A)
˚
were also identied based on cross-peak correlations observed between B–F and the H–C17 carbon, and N4–H/O3 (1.96 A)
in the 1H–1H COSY NMR spectrum. Similar NMR features to between N4–H and ortho-substituted NO2 group in dini-
those observed for compound 2a were observed for compound trophenylhydrazone unit which probably plays a crucial role in
2b. The most interesting feature in the spectrum of compound 2 the planarization of the molecule (Fig. 2). In addition, multiple
is the NH proton which appeared as a singlet at 11.6 ppm. The C–H–/F and N–H/F intermolecular interactions were also
NH proton in compound 2 was down eld shied by 3.2 ppm observed due to the strong electronegativity of F and N atoms
compared to that of compound 1 supporting the strong which led to a supramolecular head to tail assembly (Fig. 3). For
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Fig. 1 (a) Comparison of 1H NMR spectra of compounds 1 and 2a in CDCl3 and DMSO-d6. (b) 1H–1H correlation spectrum of compound 2a in
CDCl3.
instance, out of the two uorides which were arranged above at a higher wavelength due to the presence of two phenyl-
and the below the plane of BODIPY, one uoride was involved in hydrazone units leading to more p-delocalization compared to
intermolecular hydrogen bonding with the NH atom of one the one phenylhydrazone unit present in compound 2. The
adjacent molecule while the other uoride was involved in solvent studyshowed that compounds 1 and 2 exhibited a regular
intermolecular hydrogen bonding with the hydrogen atom of hypsochromic shi with an increase of the polarity of the solvent
the meso-tolyl CH3 group of another BODIPY molecule with as noted for BODIPY systems.10 However, we noted a large
˚
˚
bond distances in the range of 2.51 A to 2.64 A (Fig. 3).
bathochromic shi in the absorption band of compounds 1 and
The absorption properties of compounds 1 and 2a were 2 in hydrogen bonding accepting solvents such as DMSO and
studied in different solvents of varying polarity. A comparison of DMF. The absorption band at 682 nm in CHCl3 observed for
the absorption spectra of compounds 1 and 2a recorded in CHCl3 compound 1 was bathochromically shied by 50 nm in DMSO
and DMSO is presented in Fig. 4 and the relevant absorption data and appeared at 712 nm (Fig. 5a). Interestingly, the absorption
measured in different solvents are presented in Table 1. A typical band observed for compound 2 experienced a large red shi of
strong S0 / S1 transition in the 600–650 nm region with a 172 nm (Fig. 5b) when the solvent was changed from CHCl3 to
vibronic component at higher energy along with a broad S0 / S2 DMSO. The red shis observed for compounds 1 and 2 in DMSO/
transition at ꢀ400 nm were observed for compounds 1 and 2 as DMF were attributed tentatively to the stabilization of the energy
were observed for meso-phenyl BODIPY.10 Compound 1 absorbs states due to hydrogen bonding interaction between the NH of
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Fig. 4 Comparison of the absorption spectra of compound 1 in (a)
CHCl3 (dashed line) and (b) DMSO (small dashed line), and compound
2a in (c) CHCl3 (solid line) and (d) DMSO (dotted line).
Fig.
2 Crystal structure of compound 2a with intramolecular
hydrogen bonding interactions. The solvent molecules are omitted for
clarity.
Table 1 Photophysical data for compounds 1 and 2a
Compound 1 Compound 2a Compound 1 Compound 2a
Solventa
lmax (log 3max
)
FWHM
the compounds with the solvents resulting in the reduction of
the HOMO–LUMO gap. The large red shi observed for
compound 2 supports the stronger hydrogen bonding interac-
tion between the solvent and compound 2.
Toluene 691 (4.55)
CHCl3
MeOH
CH3CN 690 (4.57)
DMF
593 (6.06)
592 (6.06)
587 (6.04)
586 (6.00)
762 (6.05)
764 (6.05)
910
950
1033
971
1407
1506
2773
682 (4.56)
684 (4.54)
2774
3070
2884
3691
3676
703 (4.50)
712 (4.47)
Anion binding studies
DMSO
a
It has been shown in the literature that dinitrophenylhydrazone
substituted uorophores bind to anions.11 To test the ability of
compounds 1 and 2 to sense anions such as Fꢁ, Clꢁ, Brꢁ, Iꢁ,
N3ꢁ, NO3ꢁ, HSO4ꢁ, ClO4ꢁ, HPO42ꢁ, H2PO4ꢁ, HCO3ꢁ, CH3COOꢁ,
we rst qualitatively carried out studies by adding excess
equivalents of various anions to 1 and 2 in CH3CN and followed
the changes by absorptionꢁspectroscopy (Fig. 6). The addition of
Fꢁ, CH3COOꢁ and H2PO4 ions to compound 2a resulted in a
colour change from violet to light transparent green with a
signicant bathochromic shi (ꢀ160 nm) in the absorption
maxima whereas all the other anions listed above did not show
Dielectric constant: toluene
¼
2.4, chloroform (CHCl3)
¼
4.8,
methanol ¼ 32.7, acetonitrile (CH3CN) ¼ 37.5, dimethylformamide
(DMF) ¼ 36.7, dimethyl sulfoxide (DMSO) ¼ 46.7.
any change in the absorption peak maxima indicating that
compound 2a acts as an optical sensor for three anions, namely,
ꢁ
Fꢁ, CH3COOꢁ and H2PO4 ions (Fig. 6). The addition of these
three anions to compound 1 in CH3CN also changed the posi-
tion of the absorption band but the magnitude of the shi was
much less (50 nm) and the absorption band was very much
Fig. 3 Supramolecular assembly of compound 2a through intermolecular-hydrogen bonding network (between H10b/F2 and F1/H4).
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the p-electron delocalization extends all over the molecule
leading to a signicant bathochromic shi. A job plot analysis
of sensor 2a with Fꢁ interaction supports the 1 : 1 binding
stoichiometry (Fig. 7c). The binding constant evaluated from
the Benesi–Hildebrand equation was found to be 3 ꢃ 103, 2.6 ꢃ
103 and 9 ꢃ 103 for Fꢁ, H2PO4ꢁ and CH3COOꢁ ions respectively
on interaction with compound 2a. We also followed the inter-
action of Fꢁ ion with 2a by uorescence spectroscopy as shown
in Fig. 7b. It is clear from Fig. 7b that the intense uorescence
peak at 609 nm was completely quenched upon addition of 8
equiv. of Fꢁ to compound 2a. However, we noted the signicant
change in the uorescence spectrum of compound 2a upon
addition of 2 equiv. of Fꢁ ion. Furthermore, the addition of Fꢁ,
AcO and H2PO4ꢁ ions to compound 2a changes the colour of the
solution from violet to faded green, whereas no colour change
was noted in the presence of other anions (Fig. 8a).
Fig. 5 Absorption spectra of (a) compound 1 and (b) compound 2a in
various solvents.
To further elucidate the nature of the interaction between
1
the anions and compound 2a, we carried out a H NMR titra-
tion of compound 2a in CD3CN upon addition of TBAF ion,
and the 1H NMR spectral changes are displayed in Fig. 9. Upon
the gradual addition of Fꢁ ion, the NH signal at 11.5 ppm
disappeared completely supporting the deprotonation mecha-
nism. The removal of the proton from NH by the anion results
in an increase of the electron density on nitrogen which is then
propagated through delocalization into the 2,4-dinitrophenyl
motif resulting in an increase of the electron density in the
entire molecule. The increase in the electron density causes a
shielding effect leading to upeld shis of protons such as Ha–
Hf which are closer to the anion binding site. The other
protons of the BODIPY moiety which are far from the anion
Fig. 6 Absorption spectra of compound 2a (5 ꢃ 10ꢁ6 M) in the
presence of various anions (50 equivalents) in CH3CN.
1
binding site showed negligible shis. Thus, H NMR titration
broadened. Hence, systematic anion sensing experiments were
not carried out further with compound 1. Thus, we carried out
the absorption spectral titration of compound 2a with
increasing amounts of Fꢁ, CH3COOꢁ and H2PO4ꢁ ions and the
systematic changes in the absorption spectrum of compound 2a
on addition of increasing amounts of tetrabutylammonium
uoride (TBAF) in CH3CN are shown in Fig. 7a. The addition of
Fꢁ to a CH3CN solution of compound 2a resulted in a decrease
of the absorption band at 586 nm and the appearance of a new
band at 747 nm with a clear isosbestic point at 613 nm indi-
cating that compound 2a acts as a Fꢁ sensor in solution. This
was also clearly evident in the plot which shows the changes in
the absorption bands at 586 and 747 nm versus the amount of
Fꢁ ion (Fig. 7a inset). Similar absorption spectral changes with
clear isosbestic points were also noted forꢁthe systematic addi-
tion of CH3COOꢁ (Fig. S14†) and H2PO4 (Fig. S15†) ions to
compound 2a supporting the conclusion that compound 2a
also acts as a sensor for these two anions.
The large bathochromic shis of the absorption band
maxima of compound 2a upon binding with Fꢁ, CH3COOꢁ and
H2PO4ꢁ ions was tentatively attributed to the removal of the NH
proton of the dinitrophenylhydrozone group (Scheme S1†) by
the anion. This resulted in an increase of the electron density on
nitrogen and participates in delocalization with the dini-
trophenyl group leading to a more stable delocalized structure.
Thus, the electrons are rearranged in the whole molecule and
studies conrmed that NH was involved in binding with the Fꢁ
ion and that anion binding causes deprotonation which
increases the delocalization in the molecule and results in a
large red shi of the absorption band and upeld shis in the
1H NMR.
The binding of 2a with Fꢁ ion was also further probed by
following the changes in the reduction of compound 2a upon
addition of Fꢁ ion using square wave voltammetry. Fig. 10
shows the systematic changes in the reduction waves of
compound 2a upon the addition of increasing amounts of
uoride ion. As is clear from Fig. 10 the addition of
increasing amounts of Fꢁ ion to 2a resulted in a decrease of
the intensity of the reduction waves at ꢁ0.48 V and ꢁ0.82 V
with a gradual increase in the reduction wave appearing at
ꢁ0.66 V. The complete disappearance of the reduction wave at
ꢁ0.48 V with an increase in the intensity of the reduction
wave at ꢁ0.66 V indicates that the interaction of Fꢁ ion with
2a makes the boron-dipyrromethene unit of 2a relatively more
electron rich due to deprotonation of the hydrozone unit.
Similar observations were made for the anions AcOꢁ and
H2PO4ꢁ. However, no change in the reduction of 2a was
observed upon addition of any other anions. Thus, the elec-
trochemical study was in agreement with the absorption and
uorescence spectroscopic studies and supports the conclu-
sion that 2a can also ꢁbe used as an electrochemical sensor for
Fꢁ, AcOꢁ and H2PO4 ions.
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Fig. 7 (a) Absorption spectra of compound 2a (5 ꢃ 10ꢁ6 M), with different concentrations of Fꢁ (TBAF) solution (0–8 equiv.) in CH3CN. Inset is a
plot of A586/A747 vs. [Fꢁ]. (b) Emission spectral changes of BODIPY 2a (5 mM) upon addition of increasing equivalents of Fꢁ ion (lex ¼ 530 nm). (c)
Job plots of 2a (at 586 nm) with Fꢁ anion. The total concentration of the host and guest is 1.0 ꢃ 10ꢁ4 mol Lꢁ1 in CH3CN.
equivalents (100 equivalents) of Fe3+ ion whereas no consider-
Cation sensing studies
able changes in the position of the absorption band were
observed upon the addition of any other metal ions. Similarly,
To obtain an insight into the sensing properties of compound
2a towards various metal ions, we studied the effects of the
the changes in the uorescence spectra of compound 2a upon
addition of different metal ions such as Li+, Na+, K+, Ca2+, Mg2+,
the addition of different metal ions are shown in Fig. 11b.
Mn2+, Cr3+, Pb2+, Co3+, Pd2+, Cu2+, Hg2+, Cd2+, Ni2+, Zn2+, Fe2+,
Compound 2a exhibited negligible changes in the uorescence
Fe3+ to compound 2a in CH3CN solution and followed the
intensities upon the addition of metal ions such as Li+, Na+, K+,
changes by absorption and uorescence spectroscopy. As
shown in Fig. 11a, the absorption band of compound 2a was
hypsochromically shied by 62 nm upon the addition of excess
Ca2+, Mg2+, Mn2+, Cr3+, Pb2+, Co3+, Pd2+, Cu2+, Hg2+, Cd2+, Ni2+,
Zn2+ but the addition of Fe2+ resulted in a considerable intensity
Fig. 8 Color change upon addition of excess equivalents of (a) various anions (tetrabutylammonium salts) and (b) various cations to BODIPY 2a (1
ꢃ 10ꢁ5 M) in CH3CN solution in daylight.
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Fig. 9 Partial 1H NMR spectrum of compound 2a (2 ꢃ 10ꢁ2 M) in the presence of 1.5 equivalents of Fꢁ ion in CD3CN.
chemodosimetric sensor for Fe3+ which is different from the
other reported9 chemodosimetric sensors for metals ions. This
unique property of compound 2a is due to the presence of two
different substituents at the 3- and 5-positions, namely acetal
and dinitrophenylhydrozone groups. The two different substit-
uents on BODIPY 2a were cleaved in a step-wise manner in the
presence of Fe3+ and the changes that occurred due to the
cleavage of the substituents are clearly observed (Fig. 8b) from
the intensity enhancement of the uorescence spectra. Thus, we
demonstrated that the weakly uorescent compound 2a was
converted to the highly uorescent compound 3 via compound
4 in the presence of Fe3+ ion as described below.
The recognition of Fe3+ by compound 2a was rst scrutinized
by UV-vis spectroscopy in CH3CN. In the rst step, upon addi-
tion of 0 to 1 equivalent of Fe3+ to compound 2a, the absorption
band at 586 nm was shied bathochromically to 599 nm with a
color change from violet to dark blue. An isosbestic point at 595
nm (Fig. 12a) was clearly observed upon increasing the
concentration of Fe3+ up to 1 equivalent thus supporting the
presence of two species in equilibrium. We reasoned that the
two species which were in equilibrium were compound 2a and
its acetal hydrolyzed product, compound 4. In the presence of
the strong Lewis acidic Fe3+ (pKa ¼ 2.83),12 the hydrolysis of the
acetal group of compound 2a to aldehyde was facilitated by
Fig. 10 Square wave voltammograms recorded at a scan rate of 50
mV sꢁ1 of BODIPY 2a (2 mM) in the presence of increasing amounts of
Fꢁ (TBAF) ions (0–1.2 equiv.) in CH3CN containing 0.1 M TBAP as the
supporting electrolyte.
change in the uorescence spectra. However, a maximum
uorescence intensity enhancement with
a 60 nm hyp-
sochromic shi was observed for Fe3+ (Fig. 11b). Thus, the metal
sensing studies indicated that compound 2a was highly selec-
tive for Fe3+ which is attributed to the lower pKa of Fe3+
compared to other metal ions.11 Our studies also indicated that
compound 2a can be used as a two step uorescence enhanced
Fig. 11 (a) Absorption spectra and (b) emission spectra of compound 2a excited at 488 nm in the presence of excess equivalents of various metal
ions.
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Fig. 12 (a) Absorption and (b) emission spectral changes of BODIPY 2a (10 mM) upon titration with 0–1 equivalent of Fe3+ (lex ¼ 488 nm) in
CH3CN. (Inset: fluorescence colour change of 2a under a UV lamp (lex ¼ 365 nm) upon addition of 1 equivalent of Fe3+.)
1
coordinated acidic water molecules to form compound 4. We chromatography and its identity was conrmed by HRMS, H,
carried out the HR-MS analysis of compound 2a in the presence 19F and 11B NMR analysis. Under a UV lamp, the isolated
of one equivalent of Fe3+ and observed a molecular ion peak at compound 3 was highly green uorescent (Fig. 13b) as reported
541.1242 (M + Na)+ thus conrming the formation of compound earlier.5 The quantum yield (Ff) and singlet state lifetime (s) of
4. The sensitivity of compound 2a towards Fe3+ was further isolated compound 3 were in agreement with the reported data.5
investigated by uorescence studies in CH3CN. The uores- We then carried out systematic uorescence titration studies by
cence titration studies carried out by the addition of 0 to 1 adding 1 to 45 equivalents of Fe3+ to the in situ generated
equivalent of Fe3+ to compound 2a in CH3CN are shown in compound 4 in CH3CN with a time interval of 1 min for each
Fig. 12b. In the absence of Fe3+, compound 2a was very weakly addition. As shown in Fig. 13b, the uorescence band at 627 nm
uorescent (Ff ¼ 0.006) with a uorescence maxima at 609 nm. corresponding to compound 4 experienced a hypsochromic shi
Upon the addition of increasing amounts of Fe3+ to compound by 73 nm and appeared at 554 nm corresponding to compound 3
2a, the weak uorescence band of compound 2a at 609 nm was with a 1000-fold enhancement in the uorescence intensity, and
shied bathochromically to 627 with a 10-fold intensity the intensity enhancement was saturated by the addition of 45
enhancement (Ff ¼ 0.041) which indicates the hydrolysis of the equivalents of Fe3+. Although the systematic titration required
acetal group in compound 2a and the formation of compound about 1.5 h for the completion of the reaction, we noted that the
4. The weakly red uorescent compound 2a was changed to a conversion of 2a to 3 requires only 5 min upon one time addition
relatively more uorescent compound upon the addition of of 45 equivalents of Fe3+ ion. The sensitivity of Fe3+ towards
increasing amounts of Fe3+ ion up to 1 equivalent which was compound 2a was further examined by measuring the lowest
clearly noted by the naked eye as well as under a UV lamp concentration of the analyte using the linear dynamic response.
(Fig. 12b).
The limit of detection (LOD) was found to be 126 nM (Fig. S24†).
In step 2, we added 1 to 45 equivalents of Fe3+ to the in situ However, similar titration experiments in phosphate buffer
generated compound 4 in CH3CN and followed the changes by solution (PBS) failed indicating that compound 2a cannot be
absorption spectroscopy. Upon the addition of increasing used for sensing Fe3+ ion in water. All these observations suggest
amounts of Fe3+ from 1 equivalent to 45 equivalents, the that compound 2a exhibits two stages of uorescence
absorption band at 599 nm due to compound 4 was shied enhancement depending on the amount of Fe3+. We also carried
hypsochromically by 62 nm and appeared at 537 nm with a clear out uorescence titration studies with the addition of Fe2+
.
isosbestic point at 544 nm (Fig. 13a) supporting the existence of However, the addition of excess amounts of Fe2+ to compound
two species. The absorption spectral features of the species 2a resulted in the formation of only compound 4 but not
formed in the presence of 45 equivalents of Fe3+ matched with compound 3 indicating that compound 2a doesn't exhibit a two
the absorption features of compound 3 reported earlier.5 step uorescence enhanced chemodosimetric sensing behav-
Furthermore, compound
3
was isolated by column iour towards Fe2+. The hydrolysis of dinitrophenylhydrazone in
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Journal of Materials Chemistry C
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Fig. 13 (a) Absorption and (b) emission spectral changes of BODIPY 2a (10 mM) upon titration with 1–45 equivalents of Fe3+ (lex ¼ 488 nm) in
CH3CN. (Inset: fluorescence colour change of 2a UV lamp (lex ¼ 365 nm) upon addition of 45 equivalents of Fe3+.)
compound 2a was expected in the presence of Fe3+ since it is well extended conjugation with the BODIPY unit. Both symmetrical
established that Fe3+ can hydrolyze Schiff bases by metal and unsymmetrical BODIPYs absorb and emit at ꢀ600 nm with
promoted hydrolysis mechanism.13 Thus, we conclude that low quantum yields. The absorption properties of BODIPYs in
compound 2a can only be used as a two step uorescence DMSO and DMF solvents were distinctly different from those in
enhanced chemodosimetric sensor for Fe3+. To test the sensi- other solvents. The unsymmetrical BODIPY was shown to act as a
tivity of compound 2a, we also carried out systematic absorption sensor for Fꢁ, CH3COOꢁ and H2PO4ꢁ by means of a deprotona-
studies on compound 2a in an acetate buffer solution over a pH tion mechanism. Because of the two different substituents, the
range of 2.2–13 (Fig. S23†). The absorption spectra of compound unsymmetrical BODIPY dye was shown to have the unique ability
2a as a function of the pH in an acetate buffer solution exhibited of acting as a two step uorescence enhanced chemidosimetric
decrease in the intensity of the respective absorption bands with sensor for Fe3+ unlike other reported9 Fe3+ chemodosimetric
increasing pH without signicant changes in the peak maxima. sensors. This feature enables the unsymmetrical BODIPY dye 2a
Since the absorption band of compound 2a was already bath- to be used for the detection of different amounts of Fe3+ in
ochromically shied and appeared at 613 nm in a highly polar solution.
aqueous medium compared to other non-polar solvents like
toluene and chloroform, no signicant shis in the absorption
peak maxima were expected with a change in pH of the buffer
Experimental section
medium. As the pH increased, the changes in the intensity of the
General
absorption bands were minimal. Similarly, the emission studies
Chemicals. All general chemicals and solvents were procured
from S.D. Fine Chemicals, India. Column chromatography was
performed using silica gel obtained from Sisco Research
Laboratories, India. All the solvents used were of analytical
grade and were puried and dried by routine procedures
immediately before use.
of compound 2a also showed a signicant decrease in the
intensity of the uorescence band with negligible shis in the
peak maxima at different pH.
Conclusions
Instrumentation. All the NMR spectra (d values, ppm) were
In summary, we used 3,5-diformyl BODIPY to synthesize recorded with a 400 MHz spectrometer. Tetramethylsilane
symmetrical and unsymmetrical substituted BODIPYs with (TMS) was used as an external reference for recording 1H (of
phenylhydrazone/2,4-dinitrophenylhydrozone and acetalgroups residual proton; d ¼ 7.26 ppm) and 13C (d ¼ 77.2 ppm) spectra in
at the 3- and 5-positions under simple reaction conditions. The CDCl3. Chemical shi multiplicities are reported as s ¼ singlet,
crystal structure of the solved unsymmetrical BODIPY with acetal d ¼ doublet, t ¼ triplet, q ¼ quartet and m ¼ multiplet. 11B and
and 2,4-dinitrophenylhydrozone substituents at the 3- and 19F NMR spectra were recorded on Bruker spectrometer oper-
5-positions, respectively, revealed that the substituents are in ated at 128.3 and 376.4 MHz. All the NMR measurements were
5584 | J. Mater. Chem. C, 2014, 2, 5576–5586
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Journal of Materials Chemistry C
carried out at room temperature in deuterochloroform (CDCl3). by alumina column chromatography using petroleum ether–
The HRMS spectra were recorded with a Bruker maxis impact ethylacetate (94 : 6) to afford the pure compound as a blue solid.
282 001.00081 and Q-Tof micro mass spectrometer using the
Method 2. A solution of compound 3 (0.04 mmol) in ethanol
electron spray ionization method, TOF analyser. Absorption (10 ml) was added dropwise to a solution of 2,4-nitro-
and steady state uorescence spectra were recorded with Varian phenylhydrazine (0.08 mmol) in ethanol (20 ml) with stirring
Cary-Eclipse instruments. For UV-vis and uorescence titra- overnight at room temperature. The solvent was removed by
tions, a stock solution of BODIPY 2 (5 ꢃ 10ꢁ6 M, 1 ꢃ 10ꢁ5 M) evaporation and the resultant crude compound was puried by
was prepared by using HPLC grade acetonitrile (CH3CN). The alumina column chromatography using petroleum ether–eth-
quantum yields (Ff) were calculated using sulforhodamine ylacetate (94 : 6) to afford the pure compound as a blue solid.
reference (F ¼ 0.69 in ethanol, lexc ¼ 530 nm)14 for compound 1
Compound 1. Yield 62%. 1H NMR (400 MHz, CDCl3, d in ppm)
and 3 in CH3CN. All Ff are corrected for changes in refractive 2.47 (s, 3H, CH3), 6.81 (d, J ¼ 4.40 Hz, 2H, py), 6.92 (t, J ¼ 7.34
index. All the anions were used as tetrabutylammonium salts Hz, 2H, ph), 7.09–7.15 (m, 6H, ph + py), 7.24–7.33 (m, 6H, ph),
and cations were used as perchlorate salts. The tetrabuty- 7.41(d, J ¼ 8.07 Hz, 2H, Ar), 8.20 (s, 2H, aldehyde-CH), 8.49 (s,
lammonium uoride (TBAF), tetrabutylammonium acetate, 2H, NH); 13C NMR (100 MHz, CDCl3, d in ppm) 21.60, 113.56,
sodium dihydrogen phosphate (NaH2PO4) and iron perchlorate 117.24, 121.64, 129.21, 129.37, 129.50, 130.65, 131.69, 140.29,
(Fe(ClO4)3$xH2O) solutions were prepared (1 ꢃ 10ꢁ2 M) in 143.38, 151.62. 19F NMR (376.49 MHz, CDCl3, d in ppm)
CH3CN. The association constant of the anionic complex ꢁ138.03 (q, J(F, B) ¼ 64.0 Hz); 11B NMR (128.38 MHz, CDCl3, d in
formed in solution was estimated using the standard Benesi– ppm) 1.13 (t, J(B, F)
Hildebrand equation. The limit of detection (LOD) for the Fe3+ (C30H25BF2N6Na) 519.2280 (M + H)+, found 519.2290 (M + H)+.
ions was calculated to be three times the standard deviation for
Compound 2a. Yield: 17%. 1H NMR (400 MHz, CDCl3, d in
¼
30.8 Hz); HRMS calcd for
the average measurements of ten blank samples by the slope ppm) 1.26 (t, 6H, CH3), 2.49 (s, 3H, tol-CH3), 3.68–3.77 (m, 2H,
(LOD ¼ 3s/K).15 The solution containing BODIPY 1 (10 mM) was CH2), 3.78–3.87 (m, 2H, CH2), 5.99 (s, 1H, CH), 6.80 (d, J ¼ 4.40
placed in a quartz cell (1 cm width), and anion, Fe3+ solutions Hz, 1H), 6.93 (d, J ¼ 4.40 Hz, 1H, py), 6.97 (d, J ¼ 4.40 Hz, 1H,
were added in an incremental fashion. Their corresponding UV- py), 7.18 (d, J ¼ 4.03 Hz, 1H, py), 7.35 (d, J ¼ 8.07 Hz, 2H, Ar),
vis and uorescence spectra were recorded at 298 K. For 1H 7.45 (d, J ¼ 8.07 Hz, 2H, Ar), 8.09 (d, J ¼ 9.54 Hz, 1H, ph), 8.38
NMR titration, the spectra were measured using the 400 MHz (dd, J ¼ 9.54, 2.20 Hz, 1H, ph), 8.61 (s, 1H, aldehyde-CH), 9.17
NMR spectrometer. A solution of 2a in CD3CN was prepared (2 (d, J ¼ 2.57 Hz, 1H, ph), 11.55 (s, 1H, NH); 13C NMR (100 MHz,
ꢃ 10ꢁ2 M), and a 0.4 ml portion of this solution was transferred CDCl3, d in ppm) 15.38, 21.66, 63.36, 99.64, 117.20, 117.89,
into a 5 mm NMR tube. A small aliquot of TBAF in CD3CN was 118.63, 123.51, 129.46, 130.15, 130.44, 130.75, 130.94, 132.31,
added in an incremental fashion, and their corresponding 135.80, 136.78, 139.45, 141.63, 144.07, 146.03, 148.96, 159.35;
spectra were recorded.
19F NMR (376.49 MHz, CDCl3, d in ppm) ꢁ137.96 (q, J(F, B) ¼
64.0 Hz); 11B NMR (128.38 MHz, CDCl3, d in ppm) 0.93 (t, J(B, F)
¼ 30.8 Hz); HRMS calcd for (C28H27BF2N6O6Na): 615.1951 (M +
Na)+, found 615.1961 (M + Na)+.
X-ray diffraction studies
Compound 2b. Yield 26%. 1H NMR (400 MHz, CDCl3, d in
A single crystal X-ray structural study was performed using a ppm) 2.49 (s, 3H, CH3), 3.52 (s, 6H, CH3), 5.87 (s, 1H, CH), 6.76
CCD Oxford Diffraction XCALIBUR-S diffractometer equipped (d, J ¼ 4.27 Hz, 1H, py), 6.96 (t, J ¼ 4.58 Hz, 1H, py), 7.20 (d, J ¼
with an Oxford Instrument with a low-temperature attachment. 4.58 Hz, 1H, py), 7.36 (d, J ¼ 7.63 Hz, 1H, Ar), 7.46 (d, J ¼ 7.93 Hz,
Data were collected at 150(2)
K
using graphite-mono- 1H, Ar), 8.10 (d, J ¼ 9.77 Hz, 1H, ph), 8.39 (dd, J ¼ 9.46, 2.44 Hz,
˚
chromoated Mo-Ka radiation (la ¼ 0.71073 A). The strategy for 1H, ph), 8.63 (s, 1H, CH), 9.17 (d, J ¼ 2.44 Hz, 1H, ph), 11.57 (s,
the data collection was evaluated by using the CrysAlisPro CCD 1H, NH); 13C NMR (100 MHz, CDCl3, d in ppm) 21.68, 54.64,
soware. The data were collected by standard ‘phi-omega scan’ 98.49, 117.20, 1128.14, 118.55, 123.51, 129.48, 130.15, 130.44,
techniques and were scaled and reduced using CrysAlisPro RED 130.79, 130.91, 132.01, 135.87, 139.32, 141.73, 144.03, 146.21,
soware. Structure solutions for compound 1 was obtained 149.26, 157.72; 19F NMR (376.49 MHz, CDCl3, d in ppm) ꢁ138.44
using direct methods (SHELXS-97)16 and rened using full- (q, J(F, B) ¼ 64.0 Hz); 11B NMR (128.38 MHz, CDCl3, d in ppm)
matrix least-squares methods on F2 using SHELXL- 97.17 The 0.92 (t, J(B, F) ¼ 30.8 Hz); HRMS calcd for (C26H23BF2N6O6Na):
positions of all the atoms were obtained by direct methods. All 587.1638 (M + Na)+, found 587.1636 (M + Na)+.
non-hydrogen atoms were rened anisotropically. The
hydrogen atoms were placed in geometrically constrained
positions and rened with isotropic temperature factors,
Acknowledgements
generally 1.2 Ueq. of their parent atoms.†
MR and VL acknowledge the nancial support from the Council
of Scientic and Industrial Research, Govt. of India.
Synthesis of compounds
Method 1. A solution of compound 3 (0.04 mmol) in ethanol
(10 ml) was added dropwise to a solution of phenylhydrazine/
2,4-nitrophenylhydrazine (0.08 mmol) in ethanol (20 ml) with
stirring at reux. Aer stirring for 6 h, the solvent was removed
by evaporation and the resultant crude compound was puried
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