Synthesis, spectral, electrochemical, and anion binding properties of 3,5-bis(dipyrromethanyl) boron-dipyrromethenes

Article abstract of DOI:10.1021/ic202758r

Four new boron-dipyrromethenes (BODIPYs) containing dipyrromethanyl substituents at 3,5-positions, bis(3,5-dipyrromethanyl) BODIPYs 5-8, were synthesized by treating their corresponding 3,5-diformyl BODIPYs 1-4 with excess pyrrole under mild acid catalyze

Full text of DOI:10.1021/ic202758r

Article
pubs.acs.org/IC
Synthesis, Spectral, Electrochemical, and Anion Binding Properties of
3,5-Bis(Dipyrromethanyl) Boron-Dipyrromethenes
Sheri Madhu and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: Four new boron-dipyrromethenes (BODIPYs) containing dipyrromethanyl substituents at 3,5-positions, bis(3,5-
dipyrromethanyl) BODIPYs 5−8, were synthesized by treating their corresponding 3,5-diformyl BODIPYs 1−4 with excess
pyrrole under mild acid catalyzed reaction conditions. The compounds 5−8 are stable and freely soluble in common organic
solvents. One-dimensional, two-dimensional NMR, high resolution mass spectrometry (HRMS), absorption, fluorescence, and
electrochemical techniques were used to characterize the compounds. The spectral and electrochemical studies indicated that
dipyrromethanyl groups at 3,5-positions of BODIPY are less electron deficient compared to formyl groups at the same positions.
The anion binding studies indicated that bis(3,5-dipyrromethanyl) BODIPY compounds containing four pyrrole NH groups
showed preferential binding with F− ion over other anions, as confirmed by using NMR, fluorescence, and electrochemical
studies.
diformyl BODIPYs under simple reaction conditions and
demonstrated their use as pH-based optical sensors.8 However,
aldehyde functional groups on BODIPY core are very useful to
synthesize several new types of BODIPYs, and herein we report
the synthesis of four novel 3,5-bis(dipyrromethanyl) BODIPYs
5−8 by treating 3,5-diformyl BODIPYs 1−4 with excess
pyrrole in the presence of mild acid. The 3,5-bis-
(dipyrromethanyl) BODIPYs 5−8 containing four pyrrole
NH protons are expected to bind anions which can be sensed
by following changes in the spectral and electrochemical
properties of BODIPY unit. Our studies with various anions
indeed indicated that the dyes 5−8 can act as exclusive sensors
for F− ion.
INTRODUCTION
■
The synthetic chemistry of boron-dipyrromethenes (BODI-
PYs) has rapidly grown in recent years owing to their numerous
applications in biological labeling and cell imaging, and also as
artificial light harvesters, sensitizers for solar cells, fluorescent
sensors, molecular photonic wires, and solid state dye lasers.1
The remarkable properties of BODIPY fluorophores are their
pronounced stability, high absorption coefficients, narrow
emission profiles, and very good emission quantum yields
which reach unity in the best cases. The main reasons for the
rapid growth of synthetic chemistry of BODIPYs are their ease
of synthesis and potential for derivatization. Several new types
of BODIPY dyes have been synthesized recently by using
functionalized BODIPYs as these dyes are amenable for
functionalization at all positions.2 For example, BODIPYs are
subjected to halogenation3 at 3,5- and 1,7-positions and,
halogenated BODIPYs have been used as building blocks to
synthesize several new types of BODIPY derivatives4 including
cascade type BODIPY arrays.5 Similarly, the BODIPY systems
containing active methyl groups at 3,5-positions have been used
to synthesize π-extended conjugated BODIPY systems.6
Recently, Ziessel et al. developed a route to introduce ethynyl
functional groups in place of fluorides of BODIPY and used
these for the synthesis of very interesting light harvesting
systems.7 We recently reported the facile synthesis of 3,5-
RESULTS AND DISCUSSION
■
The required 3,5-diformyl BODIPYs 1−4 were prepared in a
two-step, one-pot reaction starting with corresponding meso-
aryl dipyrromethanes as reported earlier.8 The diformyl
compounds 1−4 were treated with 40 equiv of pyrrole in
dichloromethane in the presence of a catalytic amount of
BF3·OEt2 at room temperature for 15 min (Scheme 1). Thin
layer chromatography (TLC) analysis indicated the disappear-
Received: December 23, 2011
Published: March 20, 2012
© 2012 American Chemical Society
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1−4, the compounds 5−8 showed further downfield shifts
(Figure 1b). Furthermore, the compounds 5−8 showed one
quartet in 19F NMR indicating that fluorines are under a
chemically equivalent environment. However, in 19F NMR,
compounds 5−8 showed ∼10 ppm downfield shift compared
to 9 but 5 ppm upfield shift compared to diformyl BODIPYs
1−4 (Figure 1c). Thus, the NMR studies clearly indicated that
the presence of dipyrromethanyl groups at 3- and 5-positions
alters the π-delocalization of BODIPY core but less significant
compared to formyl groups at the same positions of the
BODIPY core which we attributed to the less electron
withdrawing nature of the dipyrromethanyl groups in
compounds 5−8 compared to formyl groups in compounds
1−4.
Scheme 1. Synthesis of 3,5-Bis(dipyrromethanyl)-BODIPYs
5−8
Spectral and Electrochemical Properties of Com-
pounds 5−8. The compounds 5−8 were characterized further
by absorption, fluorescence, and cyclic voltammetry techniques.
The absorption properties of compounds 5−8 were studied in
four different solvents of varying polarity, and the relevant data
of compounds 5−8 along with 3,5-diformyl BODIPY 1 and 3,5-
unsubstituted BODIPY 9 in CHCl3 are presented in Table 1.
The comparison of normalized absorption spectra of
compounds 1, 5, and 9 recorded in CHCl3 is presented in
Figure 3a. In general, the BODIPYs such as 9 exhibit one
strong S0 → S1 transition at ∼500 nm with one vibronic
component on the higher energy side and an ill defined band at
∼400 nm corresponding to a S0 → S2 transition.9 Compounds
5−8 showed similar absorption features like BODIPY 9.
However, the following differences and similarities were noted
in absorption spectra of compounds 5−8 when compared to 1
and 9; (1) S0 → S1 transition in compounds 5−8 experienced
bathochromic shift compared to 9 but showed a slight
hypsochromic shift compared to 1; (2) compounds 5−8
showed relatively broader absorption spectra with a bandwidth
∼900 cm−1 compared to 1 and 9; (3) the extinction coefficients
of compounds 5−8 are lower compared to 9 but higher as
compared to 1; (4) the absorption band in compounds 5−8
showed slight blue shift in polar solvents like CH3OH
compared to nonpolar solvents like toluene. This observation
is in line with the general behavior of other BODIPYs like 9
and diformyl BODIPYs 1−4.
The fluorescence properties of compounds 5−8 were studied
in four different solvents of varying polarity using both steady
state and time-resolved fluorescence techniques. The compar-
ison of steady state fluorescence spectra of compounds 5, 1,
and 9 is presented in Figure 3b and the relevant data are
included in Table 1. The following observations were noted by
close inspection of Figure 3b and data presented in Table 1: (1)
the compounds 5−8 showed relatively broad and strong
fluorescence band which is bathochromically shifted compared
to 9 and 1; (2) compounds 5−8 exhibited relatively larger
Stokes shifts compared to BODIPYs 9 and 1; (3) the quantum
yields (Φf) of compounds 5−8 are higher than unsubstituted
BODIPY 9 but significantly lower than their corresponding
diformyl BODIPYs 1−4. Furthermore, the quantum yields are
dependent on the type of meso-substituent, and it is very low
for BODIPYs containing an electron donating meso-aryl
substituent that may be involved in photoinduced electron
transfer with the BODIPY core; (4) the fluorescence band of
compounds 5−8 were hypsochromically shifted and quantum
yields were decreased with an increase in the polarity of the
solvent; (5) compounds 5−8 showed single exponential decay
(Supporting Information, Figure S21) and their singlet state
ance of spots corresponding to 3,5-diformyl compounds 1−4
and by the appearance of new less polar spots corresponding to
the required 3,5-bis(dipyrromethanyl) BODIPY 5−8. Column
chromatographic purification on silica afforded stable corre-
sponding 3,5-bis(dipyrromethanyl) BODIPY 5−8 as red solid
powder. The compounds 5−8 are soluble in all commonly used
solvents and were characterized by mass, NMR, UV−vis,
fluorescence, and electrochemical techniques. The composition
of the compounds 5−8 were confirmed by high resolution mass
spectrometry (HRMS) and elemental analysis. The compounds
5−8 were characterized in detail using one- and two-
dimensional (1D and 2D) NMR techniques, and the
comparison of 1H, 19F, and 11B NMR spectra of 5 with
corresponding diformyl BODIPY 1 and 3,5-unsubstituted meso-
aryl BODIPY 9 is shown in Figure 1. A representative spectrum
of 1H−1H COSY NMR for compound 5 is presented in Figure
2. We observed earlier8 that the presence of two electron
withdrawing formyl groups at the 3 and 5 positions of BODIPY
core in compounds 1−4 alters the π-delocalization significantly
which is reflected in the downfield shifts of type a and type b
pyrrole protons of BODIPY core compared to unsubstituted
BODIPY 9 (Figure 1a). In compounds 5−8, the type a and
type b pyrrole protons of BODIPY core appeared as doublets at
1
∼6.40 and ∼6.70 ppm respectively as confirmed by H−1H
COSY NMR (Figure 2) because of symmetric environment like
diformyl BODIPYs 1−4. However, these protons shifted
upfield compared to 9 as well as compared to their
corresponding 3,5-diformyl BODIPYs 1−4 supporting the
less electron-withdrawing nature of dipyrromethanyl groups in
5−8. The 12 pyrrole protons and two meso CH protons of
dipyrromethanyl units appeared as sets of two multiplets at
∼6.50 ppm and ∼6.80 ppm and four pyrrole NH protons
appeared as broad singlet at ∼8.47 ppm. In 11B NMR,
compounds 5−8 showed a typical triplet like diformyl
BODIPYs 1−4. However, compared to diformyl BODIPYs
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1
Figure 1. Comparison of (a) H, (b) 11B, and (c) 19F NMR spectra of compounds 9, 1, and 5 in the selected region recorded in CDCl3.
lifetimes (τf) were larger than 9 but lower than diformyl
BODIPYs 1−4 and (6) the radiative Kr and nonradiative rates
Knr were in agreement with their quantum yields. Thus,
absorption and fluorescence studies of 5−8 indicated that the
presence of dipyrromethanyl groups at 3,5-positions of
BODIPY alter the electronic properties compared to
unsubstituted BODIPY, and the magnitude is comparatively
less than their corresponding diformyl BODIPYs supporting
their less electron withdrawing nature compared to formyl
groups in BODIPY 1−4.
The electrochemical properties of BODIPYs 5−8 were
studied by cyclic voltammetry in CH2Cl2 using tetrabutylam-
monium perchlorate as the supporting electrolyte. A compar-
ison of first reduction waves of compounds 1, 5, and 9 is shown
in Figure 4, and the relevant data are included in the Table 1.
The 3,5-unsubstituted BODIPYs like BODIPY 9 show one
oxidation and two reductions.9 Our earlier study8 on 3,5-
diformyl BODIPYs such as BODIPY 1 indicated that these
compounds because of their electron deficient nature exhibit
only two reversible reductions which are at much lower
potentials compared to 9. Compounds 5−8 also showed one
reversible and one quasi-reversible reduction as the dipyrro-
methanyl groups are also electron deficient in nature but less
effective as compared to formyl groups. Thus, the reduction
potentials of compounds 5−8 shifted toward more negative
compared to diformyl BODIPYs 1−4 indicating that
compounds 5−8 were difficult to reduce compared to
compounds 1−4. However, the BODIPYs 5−8 are easier to
reduce by ∼200 mV compared to 9 supporting the electron
withdrawing nature of dipyrromethanyl groups present at the
3,5-positions of BODIPYs 5−8.
Anion Binding Studies. In recent years, anion sensing and
recognition has grown into an area of great interest in
supramolecular and biological chemistry.10 Most synthetic
anion chemosensors generally involve the covalent linking of
an optical-signaling chromophoric fragment to a neutral anion
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Figure 3. Comparison of normalized (a) absorption and (b) emission
spectra of compound 5 along with 1 and 9 recorded in chloroform
(excitation wavelength used was 488 nm).
selective fluoride sensor as confirmed by NMR, optical, and
electrochemical studies.
The fluoride anion binding to 5 was first followed by H
1
Figure 2. H−1H COSY spectrum of compound 5 in CDCl3.
1
NMR titration of 5 with increasing amounts of fluoride ion in
CDCl3. We noticed more numbers of signals and systematic
shifts for NH and other dipyrromethanyl protons of 5 by the
addition of increasing amounts of tetrabutylammonium fluoride
to compound 5. Hence we used 1H−1H COSY NMR to
identify all signals which appeared upon the addition of F− ion
to compound 5. The changes in the chemical shifts of NH
protons on addition of F− up to 2 equiv to compound 5 are
shown in Figure 5a; the comparison of changes in the chemical
shifts of dipyrromethanyl core protons, BODIPY core protons,
and meso-aryl protons before and after addition of F− to
receptor containing urea, thiourea, amide, phenol, or pyrrole
subunits, which can provide one or more hydrogen bond donor
sites for selective binding and sensing of some anions, especially
11
−
for F−, AcO−, and H2PO4 . Thus, the earlier studies showed
that pyrroles are excellent hydrogen bond donor groups in
synthetic anion receptor systems, and the pioneering work of
Sessler,12 Gale,13 and others14 established pyrrole as an effective
anion receptor group. Since BODIPY compounds 5−8 contain
four pyrrole groups and one fluorescent signaling unit, we
thought that these systems can be used as effective anion
sensors. We explored the anion binding properties of BODIPY
5 with various anions including F−, Cl−, Br−, I−, OH−, AcO−,
1
compound 5 is shown in Figure 5b and H−1H COSY NMR
spectrum of 5 + F− is shown in Figure 5c. As it is clear from
Figure 5a, upon increasing addition of F− ion to 5, the NH
−
−
ClO4 , H2PO4 , and our studies clearly showed that 5 is highly
Table 1. Photophysical and Electrochemical Data of Compounds 5−8, along with 1 and 9
a
a
compound
solvent
λabs (nm)
λem (nm)
Δνst (cm−1
)
ε0 (ε)
ϕ
τ (ns)
Kr (109 s−1
)
Knr (109 s−1
)
Ered(V)
I
Ered(V) II
9
1
2
3
4
5
6
7
8
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
501
546
550
544
548
537
543
536
541
517
556
560
554
570
562
568
563
560
618
329
325
332
704
828
811
895
627
4.74
4.09
3.77
4.25
4.06
416
0.03
0.31
0.30
0.09
0.51
5.9
0.059
0.053
0.055
0.064
1.9
−0.800
−0.132
−0.067
−0.148
−0.107
−0.659
−0.672
−0.623
−0.650
−1.820
−1.040
−0.979
−1.055
0.117
0.127
0.650
5.5
1.4
0.12
0.17
0.07
1.6
1.3
1.4
0.070
0.131
0.050
0.550
0.638
0.664
−1.661
−1.566
−1.665
−1.589
4.00
4.24
3.71
a
ε (extinction coefficient), λabs (absorption maxima), λem (emission maxima), Δν (Stokes shift), Φ (quantum yield), τ (lifetime), kr (radiative decay),
knr (nonradiative decay).
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plot of the fluorescence intensity at 565 nm versus the molar
equivalents of F− ion used in the titration which also indicates
that the fluorescence intensity reaches a plateau upon addition
of 30 equiv of F− ion. Furthermore, when excess amount of F−
ion was added, we did not observe any further changes in the
fluorescence intensity and compound 5 is stable. The
significant change in the fluorescence spectrum of compound
5 was noted upon addition of around 2 equiv of F− ion, and the
detection limit for F− ion is between 0.020 and 0.035 mM
under our experimental conditions. The Job plot analysis using
fluorescence spectroscopy also supported 1:2 stoichiometry for
5 and F− ions interaction (Supporting Information, Figure
S33). From the Benesi−Hildebrand equation, the binding
constant of 18000 M−1 was evaluated for receptor−anion
complexation. The formation of a hydrogen bond between
pyrrole NH and F− assists in the photoinduced electron
transfer (PET) resulting in quenching of the fluorescence in
compound 5. Furthermore, the addition of 30 equiv of F− ion
to compound 5 changes the color of the solution from pink to
faded pink, whereas in the presence of other anions such as Cl−,
Figure 4. Comparison of reduction waves of cyclic voltammograms of
compounds 1, 5, and 9 in dichloromethane containing 0.1 M TBAP as
supporting electrolyte recorded at 50 mV s−1 scan rate.
protons which appeared earlier as broad singlet at 8.47 ppm
experienced a downfield shift and appeared as a doublet at
∼12.35 ppm upon addition of 2 equiv of F−. The continuous
downfield shift of NH protons and the occurrence of a doublet
with increasing F− ion concentration indicated the strong
coupling between NH proton and F− because of NH···F−
hydrogen bond formation. Similarly, the 12 pyrrole protons of
dipyrromethanyl group (c, c′, d, d′, e, e′) also experienced shifts
Br−, I−, OH−, AcO−, ClO4 , H2PO4 , there is no change in the
pink color of compound 5 (Supporting Information, Figure
S35). Attempts have been made to extract the bound F− ion
from the receptor by washing with water. However, the
fluorescence signal of the original compound was not
recovered, indicating the F− ion is strongly bound to
compound 5.
−
−
1
on binding to F− ion (Figure 5b). In H NMR, as described
The specific binding of 5 with F− ion was further probed by
following changes in the first reduction of compound 5 upon
addition of F− ion using cyclic and square wave voltammetric
techniques. Figure 7 shows the systematic changes in the
reduction wave of compound 5 on increasing addition of
fluoride ion. As it is clear from Figure 7, the increasing addition
of F− ion to 5 resulted in a shift of potential toward less
negative by 150 mV indicating that F− ions binding to
dipyrromethanyl moieties of 5 make the boron-dipyrromethene
unit of 5 more electron deficient, and hence it undergoes
reduction at a lower potential. No change in the reduction of 5
was observed on addition of other anions. These results
indicate that 5 can also be used as specific electrochemical
sensor for fluoride ions.
above, the compound 5 exhibited two signals at 6.17 and 6.74
ppm corresponding to 10 and 4 protons of the dipyrromethanyl
group. However, on binding with F− ion, these protons
appeared as four sets of signals which appeared at slightly
1
higher field, and these signals were identified by using H−1H
COSY NMR studies (Figure 5c). In COSY of 5 + F−, the NH
protons at 11.7 ppm showed a cross peak with signals at 6.05
and 6.74 ppm, which in turn showed cross peaks with each
other and also with a signal at 6.13 ppm. Thus, we identified the
signals at 6.05, 6.13, and 6.74 ppm as (e, d′), (c, c′), and (d, e′)
types, respectively. The singlet at 6.22 ppm corresponding to
two protons was identified as meso-protons of dipyrromethanyl
group (f-type) which did not show any shift on addition of F−
ion. The two doublets that appeared at 6.45 ppm and 6.78 ppm
corresponding to type a and type b pyrrole protons of BODIPY
unit in compound 5 shifted by 0.3 ppm downfield and appeared
as doublets at 6.71 and 7.05 ppm respectively in 5 + F−. Thus,
NMR studies clearly indicated the binding of fluoride ion to the
pyrrole moieties of the dipyrromethanyl group of compound 5.
CONCLUSIONS
■
We used our previously synthesized 3,5-bisformyl BODIPYs as
key synthons to prepare novel 3,5-bis(dipyrromethanyl)
BODIPYs in one step under simple reaction conditions. The
compounds are stable and possess decent photophysical
properties. The spectral and electrochemical properties of
3,5-bis(dipyrromethanyl) BODIPYs indicated that dipyrrome-
thanyl groups are less electron deficient than formyl groups and
alter the properties of BODIPYs moderately. The presence of
four pyrrole NH groups however are very useful for binding of
anions, and our studies showed that these systems exhibit
specific selectivity for F− ion over other anions as verified by
NMR, fluorescence, and electrochemical properties. Further-
more, BODIPYs containing two dipyrromethanyl groups are
interesting synthons for the synthesis of several novel
compounds including macrocycles, and such synthetic efforts
are presently underway in our laboratory.
1
The Job’s plot analysis using H NMR confirmed the 1:2
stoichiometry for 5 and F− interaction (Supporting Informa-
tion, Figure S33).
The interaction of 5 with F− ion was investigated by
absorption spectroscopy. Upon addition of increasing amounts
of F− ion to compound 5, a slight increase in the intensity of
the absorption band with almost no shift in the peak maxima
was observed indicating that absorption spectroscopy is not a
very useful tool for studying 5 and F− interaction. However, we
observed significant changes in the fluorescence spectrum of
compound 5 on binding with F− ion. Figure 6 shows the
fluorescence spectral changes of 5 in CH2Cl2 upon addition of
increasing amounts of F− ion and the spectra were recorded
after 5 minutes. It is clear from Figure 6 that its intense
fluorescence peak at 565 nm is nearly quenched upon addition
of 30 equiv of F− to compound 5. Inset in Figure 6 shows the
EXPERIMENTAL SECTION
General Experimental. The chemicals such as BF3·Et2O, 2,3-
dichloro-5,6-dicyano-1,4- benzoquinone (DDQ) were used as
■
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Figure 5. 1H NMR spectra of 5 (1.3 × 10−2 M) in the presence of different concentrations of F− ions. Concentrations of F− ion was varied from 0 to
2.0 equiv. (a) The effect on NH protons; (b) the effect on pyrrole and aryl protons; and (c) 1H−1H COSY spectrum of compound 5 + F− in CDCl3.
obtained from Aldrich. All other chemicals used for the synthesis were
reagent grade unless otherwise specified. Column chromatography was
elemental analyses were performed on a ThermoQuest microanalysis
instrument, and the FTIR spectra were measured on Perkin−Elmer
spectrometer using KBr pellets. The fluorescence quantum yields (Φf)
were estimated from the emission and absorption spectra by a
comparative method at the excitation wavelength of 488 nm using
Rhodamine 6G (Φf = 0.88)15 as standard. The time-resolved
fluorescence decay measurements were carried out at a magic angle
using a picosecond diode laser based time correlated single photon
counting (TCSPC) fluorescence spectrometer from IBH, UK. All the
1
performed on silica (60−120 mesh). The H, 13C, 11B, and 19F NMR
spectra were recorded in CDCl3 on Bruker 400 MHz instrument using
tetramethylsilane (Si(CH3)4) as internal standard. 1H−1H COSY
experiments were performed on a Bruker 400 MHz instrument.
Absorption and steady state fluorescence spectra were obtained with
Perkin−Elmer Lambda-35 and PC1 photon counting spectrofluor-
ometer manufactured by ISS, USA instruments, respectively. The
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General Procedure for 3,5-Bis(dipyrromethane)-BODIPYs
(5−8). BF3·Et2O (0.175 mmol) was added to a solution of
corresponding BODIPYs 1−4 (1.75 mmol) and pyrrole (70 mmol)
in dichloromethane (100 mL) under N2 atmosphere. The reaction was
stirred at room temperature for 15 min. The reaction mixture was
washed with 0.1 M NaOH solution and water thoroughly. The
combined organic layers were dried over Na2SO4 and filtered. The
pyrrole was evaporated under a vacuum to give dark red oil. The crude
product was purified using silica gel column chromatography with
petroleum ether/ethylacetate (80:20) and afforded pure 3,5-bis-
(dipyrromethane)-BODIPYs 5−8 as a red solid.
3,5-Bis(dipyrromethanyl)-8-(Tolyl)- 4-bora-3a,4a-diaza-s-in-
1
dacene (5). Yield: 88 mg, 62%. H NMR (400 MHz, CDCl3, δ in
ppm): 2.44 (s, 3H; −CH3), 6.17−6.18 (m, 9H; py), 6.46 (d, 3J (H, H)
3
= 4.27 Hz, 2H; py), 6.74 (m, 5H; py), 6.79 (d, J (H, H) = 4.27 Hz,
Figure 6. Quenching of fluorescence intensity of compound 5 (5 μM)
during the titration with F− (TBAF). The molar equivalents of TBAF
used were 0, 1, 2, 4, 6, 8, 10, 12, 14, 16, 17, 18, 20, 22, 24, 26, and 28.
The inset shows a linear-correlation plot between the intensity at 565
nm versus [F−] (μM).
3
3
2H; py), 7.28 (d, J (H, H) = 7.94 Hz, 2H; Ar), 7.38 (d, J (H, H) =
8.25 Hz, 2H; Ar), 8.47 (br s, 4H; NH). 11B NMR (128.3 MHz, CDCl3,
δ in ppm): 1.41 (t, J (B−F) = 34 MHz, 1B). 19F NMR (282.2 MHz,
1
1
CDCl3, δ in ppm): −135.4 (q, J (F−B) = 35 MHz, 2F). 13C NMR
(100 MHz, CDCl3, δ in ppm): 21.6, 29.9, 37.8, 107.2, 108.8, 117.9,
118.7, 129.3, 129.6, 130.5, 131.2, 159.9. HRMS. Calcd for
C34H29BF2N6 [(M + 1)+]: m/z 571.2593. Found: m/z 571.2586. IR
(KBr, νmax): 532.1, 726.4, 746.1, 769.3, 887.3, 956.6, 1027.7, 1126.3,
1174.8, 1250.5, 1312.6, 1347.7, 1434.2, 1511.1, 1546.8, 1578.6, 1603.3,
1713.5, 3423.7. Anal. Calcd for C34H29BF2N6: C, 71.59; H, 5.12; N,
14.73. Found: C, 71.50; H, 5.34; N, 14.65.
3,5-Bis(dipyrromethanyl)-8-(4-bromophenyl)- 4-bora-3a,4a-
1
diaza-s-indacene (6). Yield: 150 mg, 53%. H NMR (400 MHz,
CDCl3, δ in ppm): 6.17−6.19 (m, 10H; py), 6.48 (d, 3J (H, H) = 4.58
Hz, 2H; py), 6.74 (m, 6H; py), 7.35 (d, 3J (H, H) = 8.55 Hz, 2H; Ar),
7.63 (d, 3J (H, H) = 8.55 Hz, 2H; Ar), 8.48 (br s, 4H; NH). 11B NMR
(128.3 MHz, CDCl3, δ in ppm): 1.36 (t, 1J (B−F) = 34 MHz, 1B). 19
F
1
Figure 7. (a) Cyclic voltammograms and (b) square wave voltammo-
grams of 5 (1.3 × 10−2 M) in the presence of different amounts of F−
ion. The F− concentration is (a) 0, (b) 0.5, (c) 1.5, (d) 2.0, (e) 2.5, and
(f) 3.0 mM, respectively.
NMR (282.2 MHz, CDCl3, δ in ppm): −135.5 (q, J (F−B) = 33.5
MHz, 2F). 13C NMR (100 MHz, CDCl3, δ in ppm): 37.9, 61.3, 63.8,
66.2, 107.3, 108.8, 117.9, 119.1, 125.1, 129.4, 130.8, 131.8, 132.7,
133.6, 143.1, 160.7. HRMS. Calcd for C33H26BrBF2N6 [(M + 1)+]: m/
z 635.1542. Found: m/z 635.1544. IR (KBr, νmax): 573.6, 718.7, 777.2,
884.6, 973.3, 1008.4, 1060.7, 1120.4, 1263.3, 1305.5, 1339.1, 1432.6,
1485.9, 1547.2, 1570.4, 1637.6, 1718.5, 3357.4. Anal. Calcd for
C33H26BrBF2N6: C, 62.39; H, 4.12; N, 13.23. Found: C, 62.12; H,
4.38; N, 13.12.
decays were fitted to single exponential. The good fit criteria were low
chi-square (1.0) and random distributions of residuals. Cyclic
voltammetric (CV) studies were carried out with BAS electrochemical
system utilizing the three electrode configuration consisting of a Glassy
carbon (working electrode), platinum wire (auxiliary electrode), and
saturated calomel (reference electrode) electrodes. The experiments
were done in dry dichloromethane using 0.1 M tetrabutylammonium
perchlorate (TBAP) as supporting electrolyte. Half wave potentials
were measured using differential pulse voltammetry (DPV) and also
calculated manually by taking the average of the cathodic and anodic
peak potentials. The HR mass spectra were recorded with a Q-Tof
micro mass spectrometer. For UV−vis and fluorescence titrations, the
stock solution of compound 5 (5 × 10−6 M) was prepared by using
spectroscopic grade CHCl3. The association constant of the anion
complex formed in solution has been estimated by using the standard
Benesi−Hildebrand equation, viz.,
3,5-Bis(dipyrromethanyl)-8-(4-methoxyphenyl)- 4-bora-
3a,4a-diaza-s-indacene (7). Yield: 88 mg, 79%. 1H NMR (400
MHz, CDCl3, δ in ppm): 3.90 (s, 3H; -OCH3), 6.18 (m, 9H; py), 6.48
(m, 2H; py), 6.75 (m, 5H; py), 6.82 (m, 2H; py), 7.01 (d, 3J (H, H) =
3
6.42 Hz, 2H; Ar), 7.46 (d, J (H, H) = 6.72 Hz, 2H; Ar), 8.47 (br s,
1
4H; NH). 11B NMR (128.3 MHz, CDCl3, δ in ppm): 1.39 (t, J (B−
F) = 33 MHz, 1B). 19F NMR (282.2 MHz, CDCl3, δ in ppm): −135.3
(q, 1J (F−B) = 33 MHz, 2F). 13C NMR (100 MHz, CDCl3, δ in ppm):
37.8, 55.6, 107.2, 108.8, 114.1, 117.8, 118.6, 126.4, 129.7, 131.1, 132.2,
133.8, 144.9, 159.6, 161.7. HRMS. Calcd for C34H29BF2N6O [(M +
1)+]: m/z 587.2542. Found: m/z 587.2554. IR (KBr, νmax): 536.0,
721.1, 740.5, 778.8, 886.4, 975.1, 1028.4, 1129.5, 1179.3, 1256.5,
1306.7, 1341.2, 1434.9, 1509.7, 1547.0, 1577.2, 1603.7, 1709.1, 3412.1.
Anal. Calcd for C34H29BF2N6O: C, 69.63; H, 4.98; N, 14.33. Found:
C, 69.72; H, 4.83; N, 14.15.
1
1
1
=
+
−
I − I
I − I
(I − I )K [A ]
1 0 a
0
1
0
3,5-Bis(dipyrromethanyl)-8-(3,4,5-trimethoxyphenyl)-4-
bora-3a,4a-diaza-s-indacene (8). Yield: 88 mg, 58%. 1H NMR
(400 MHz, CDCl3, δ in ppm): 3.85 (s, 6H; -OCH3), 3.93 (s, 3H;
where I0 is the intensity of the compound 5 before addition of anion,
where I is the intensity in the presence of A−, I1 is intensity upon
saturation with A−, and Ka is the association constant of the complex
formed. The Bu4NF solution was prepared (1 × 10−2 M) in CHCl3.
The solution containing compound 5 was placed in a quartz cell (1 cm
width), and Bu4NF solution was added in an incremental fashion.
Their corresponding UV−vis and fluorescence spectra were recorded
3
-OCH3), 6.17−6.19 (m, 9H; py), 6.49 (d, J (H, H) = 4.28 Hz, 2H;
py), 6.75 (m, 7H; py+Ar), 6.87 (d, 3J (H, H) = 4.27 Hz, 2H; py), 8.47
(br s, 4H; NH). 11B NMR (128.3 MHz, CDCl3, δ in ppm): 1.37 (t, 1J
(B−F) = 33 MHz, 1B). 19F NMR (282.2 MHz, CDCl3, δ in ppm):
−135.3 (q, 1J (F−B) = 33.5 MHz, 2F). 13C NMR (100 MHz, CDCl3,
δ in ppm): 22.9, 29.9, 37.9, 56.5, 61.2, 107.3, 108.0, 108.9, 117.9,
118.8, 129.3, 129.6, 131.1, 133.7, 153.2, 160.3. HRMS. Calcd for
C36H33BF2N6O3 [(M + 1)+]: m/z 647.2754. Found: m/z 647.2783. IR
(KBr, νmax): 534.1, 720.8, 740.1, 772.8, 879.4, 971.3, 1028.7, 1130.4,
1173.6, 1251.8, 1306.1, 1344.8, 1433.7, 1509.1, 1541.9, 1576.7, 1607.5,
1
at 298 K. In H NMR titration, the spectra were measured on 400
MHz NMR spectrometer. A solution of 5 in CDCl3 was prepared (5 ×
10−3 M), and a 0.4-mL portion of this solution was transferred to a 5-
mm NMR tube. A small aliquot of Bu4NF in CDCl3 was added in an
incremental fashion, and their corresponding spectra were recorded.
4291
dx.doi.org/10.1021/ic202758r | Inorg. Chem. 2012, 51, 4285−4292

Inorganic Chemistry
Article
1713.8, 3375.7. Anal. Calcd for C36H33BF2N6O3: C, 66.88; H, 5.14; N,
13.00. Found: C, 66.75; H, 5.32; N, 13.13.
13, 5656−5659. (f) Boens, N.; Leen, V.; Dehaen, W. Chem. Soc. Rev.
2012, 4111301172.
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5199−5201.
ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization data for all new compounds. Absorption,
fluorescence, Job’s plots. This material is available free of charge
via the Internet at http://pubs.acs.org.
(8) Madhu, S.; Rajeswararao, M.; Shaikh, M. S.; Ravikanth, M. Inorg.
Chem. 2011, 50, 4392−4400.
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W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W.
R.; Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2005, 109,
20433−20443.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ravikanth@chem.iitb.ac.in.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.R. thanks Board of Research in Nuclear Sciences (BRNS) for
financial support, and S.M. thanks IIT-Bombay for a fellowship.
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