8-hydroxyquinoline-substituted boron-dipyrromethene compounds: Synthesis, structure, and OFF-ON-OFF type of pH-sensing properties

Article abstract of DOI:10.1021/jo200050a

A series of four novel 8-hydroxyquinoline-substituted boron-dipyrromethene derivatives, namely 4,4-difluoro-8-(5-(8-hydroxyquinoline))-3,5-dimethyl-4-bora- 3a,4a-diaza-s-indacene (1), 4,4-difluoro-8-(5-(8-hydroxyquinoline))-1,3,5,7- tetramethyl-4-bora-3a,4a-diaza-s-indacene (2), 4,4-difluoro-8-(5-azastyryl-(8- hydroxyquinoline))-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (3), and 4,4-difluoro-8-(5-azastyryl-(8-hydroxyquinoline))-1,3,5,7-tetramethyl-4-bora-3a, 4a-diaza-s-indacene (4), have been synthesized and characterized by a series of spectroscopic methods. The molecular structures of 1 and 2 have been determined by single-crystal X-ray diffraction analyses. The two methyl substituents attached at C-1 and C-7 positions of boron-dipyrromethene (Bodipy) in compound 2 was revealed to prevent the free rotation of the 8-hydroxyquinoline (8-HQ) moiety, resulting in an almost vertical 8-HQ-Bodipy configuration of this compound. This is obviously different from those for 1 with the dihedral angle between 8-hydroxyquinoline and Bodipy moieties of 65.44 and 66.79° due to the lack of methyl substituents in the latter compound. The intense fluorescence from the Bodipy subunit of these compounds was revealed to gradually get diminished along with either decreasing or increasing the pH value under acidic and basic conditions, respectively, in particular for 1, 2, and 4 because of the photoinduced intramolecular electron transfer from excited Bodipy moiety to 8-HQ unit and just an opposite process. This renders these compounds the first OFF-ON-OFF type of pH-dependent fluorescent sensors. Nevertheless, both the intrinsic fluorescence of these compounds and their fluorescent quenching properties along with the change in the pH value have been found to depend on the steric configuration as well as the linking group between 8-hydroxyquinoline and Bodipy moieties, revealing the effect of molecular structure on their fluorescence properties.

Full text of DOI:10.1021/jo200050a

ARTICLE
pubs.acs.org/joc
8-Hydroxyquinoline-Substituted BoronꢀDipyrromethene
Compounds: Synthesis, Structure, and OFFꢀONꢀOFF Type of
pH-Sensing Properties
Yuting Chen,†,‡,§ Hailong Wang,‡ Liang Wan,‡ Yongzhong Bian,‡ and Jianzhuang Jiang*,†,‡
†Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China
‡Department of Chemistry, University of Shandong, Jinan 250100, China
§Department of Chemistry, Dezhou University, Dezhou 253023, China
S Supporting Information
b
ABSTRACT: A series of four novel 8-hydroxyquinoline-substi-
tuted boronꢀdipyrromethene derivatives, namely 4,4-difluoro-
8-(5-(8-hydroxyquinoline))-3,5-dimethyl-4-bora-3a,4a-diaza-s-
indacene (1), 4,4-difluoro-8-(5-(8-hydroxyquinoline))-1,3,5,
7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (2), 4,4-difluoro-
8-(5-azastyryl-(8-hydroxyquinoline))-3,5-dimethyl-4-bora-3a,
4a-diaza-s-indacene (3), and 4,4-difluoro-8-(5-azastyryl-(8-
hydroxyquinoline))-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-
indacene (4), have been synthesized and characterized by a
series of spectroscopic methods. The molecular structures of 1
and 2 have been determined by single-crystal X-ray diffraction
analyses. The two methyl substituents attached at C-1 and C-7
positions of boronꢀdipyrromethene (Bodipy) in compound 2 was revealed to prevent the free rotation of the 8-hydroxyquinoline
(8-HQ) moiety, resulting in an almost vertical 8-HQ-Bodipy configuration of this compound. This is obviously different from those
for 1 with the dihedral angle between 8-hydroxyquinoline and Bodipy moieties of 65.44 and 66.79° due to the lack of methyl
substituents in the latter compound. The intense fluorescence from the Bodipy subunit of these compounds was revealed to
gradually get diminished along with either decreasing or increasing the pH value under acidic and basic conditions, respectively, in
particular for 1, 2, and 4 because of the photoinduced intramolecular electron transfer from excited Bodipy moiety to 8-HQ unit and
just an opposite process. This renders these compounds the first OFFꢀONꢀOFF type of pH-dependent fluorescent sensors.
Nevertheless, both the intrinsic fluorescence of these compounds and their fluorescent quenching properties along with the change
in the pH value have been found to depend on the steric configuration as well as the linking group between 8-hydroxyquinoline and
Bodipy moieties, revealing the effect of molecular structure on their fluorescence properties.
’ INTRODUCTION
its unique pyridyl N and ꢀOH structural characteristics.8,9
Nevertheless, since the end groups of pyridyl N and ꢀOH in
the 8-hydroxyquinoline moiety are able to act as a weak acid and a
weak base under acidic and basic conditions, respectively,
associated with the equilibrium between the protonated quino-
linium NHþ∼ꢀOH form and deprotonated quinolinate
N∼ꢀOꢀ form,10 this moiety appears to be able to act as the
pH indicator for both acidic and basic systems. However, despite
the large number of investigations on the fluorescent sensors
with the 8-HQ moiety as receptor for metal ions,11 there still
exists no exploration on the fluorescent sensors with 8-HQ
moiety as pH indicator to the best of our knowledge.
There has been a growing interest in the development of versatile
fluorescent chemosensors in recent years because of their potential
applications in chemical and biological target systems1 associated
with their easy signal transduction nature.2 The reasonable con-
struction for a highly efficient fluorescent sensor is to conjugate a
suitable fluorescent signaling unit with an efficient receptor for
targets to be detected. Among numerous fluorophores, 4,4-difluoro-
4-bora-3a,4a-diaza-s-indacene (Bodipy) has been revealed to be the
most attractive as a fluorescent signaling unit in constructing
fluorescent sensors toward heavy-metal ions,3 pH,4 solvent
polarity,5 and small molecules6 because of its favorable characteristics
such as pronounced photostability, high extinction coefficient, narrow
emission band, high fluorescence quantum yield, and the extra feature
of excitation/emission wavelengths in the visible region.7
For the purpose of preparing versatile fluorescent sensors for
the acidic and/or basic systems, novel fluorescent compounds
On the other hand, the 8-hydroxyquinoline moiety has been
usually employed as a functional receptor for metal ions owing to
Received: January 12, 2011
Published: April 08, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of 8-Hydroxyquinoline-Bodipy-Conjugated Derivatives 1ꢀ4a
a Key: (I) pyrrole, TFA, CH2Cl2, rt; (II) DDQ, CH2Cl2, rt; (III) DIEA, BF3 OEt2, rt; (IV) NH2NH2ꢀH2O, 10% PdꢀC, MeOH, CH2Cl2, reflux;
3
(V) acetic acid, MeOH, reflux.
simultaneously possessing the excellent Bodipy signaling unit
and the efficient 8-HQ receptor should be developed. However,
introduction of a Bodipy signaling unit at the C-2 position of
8-hydroxyquinoline, adjacent to the binding site of the pyridyl
N atom, was revealed to be still selective only to metal ions such
fluorescent pH sensors with potential applications in chemical and
biological fields.
’ RESULT AND DISCUSSION
as Hg2þ or Cu2þ 12
. Nevertheless, further investigation revealed
Design and Synthesis. Considering the potential effect of the
Bodipy unit attached at the C-5 position of 8-HQ moiety on the
ꢀOH group, two different Bodipy signaling units are directly
connected onto the C-5 position of 8-HQ moiety in compounds
1 and 2, respectively. With the aim of investigating the effect of
the linking group between the fluorophor and receptor on the
sensitive capacity of the target compounds, an aza-styryl group
with a CdN moiety was introduced between an 8-HQ receptor
and the Bodipy signaling unit in compounds 3 and 4, Scheme 1.
These four 8-hydroxyquinoline-substituted boronꢀdipyrromethene
compounds were synthesized according to the literature procedure.7
Satisfactory elemental analysis results have been obtained for the
whole series of four newly prepared compounds 1ꢀ4 after repeated
column chromatography and/or recrystallization. These four com-
pounds have also been characterized by MALDI-TOF mass and 1H
as well as 13C NMR spectroscopy (Figures S1ꢀS4, Supporting
Information). The molecular structures of 1 and 2 were clearly
revealed by single-crystal X-ray analyses.
Crystal Structures of 1 and 2. Single crystals of compounds 1
and 2 suitable for X-ray diffraction analysis were obtained by slow
diffusion of hexane into a CHCl3 solution of these compounds,
and the basic features are listed in Table S1 (Supporting In-
formation). Compound 1 crystallizes in the orthorhombic system
containing two molecules with slightly different configurations per
unit cell, Figure 1A,B. In contrast, compound 2 crystallizes in the
triclinic system with only one molecule per unit cell, Figure 1C.
In both compounds, the C9BN2 (Bodipy) framework consisting
of one central six-membered and two adjacent five-membered
rings is essentially flat, with the maximum deviation from the
least-squares mean plane being 0.023 and 0.035 Å in 1 and 0.099 Å
that substitution at the C-2 and/or C-4 positions in the 8-HQ
moiety mainly tunes the coordinating property of the pyridyl
N group, while substitution at the C-5 and/or C-7 positions
induces change in the binding ability of the ꢀOH group.13 As a
consequence, designing and preparing novel fluorescent sensors
in which the Bodipy unit is attached at the C-5 position are of
interest. In the present paper, four novel 8-hydroxyquinoline-
substituted boronꢀdipyrromethene derivatives, namely 4,4-
difluoro-8-(5-(8-hydroxyquinoline))-3,5-dimethyl-4-bora-3a,
4a-diaza-s-indacene (1), 4,4-difluoro-8-(5-(8-hydroxyquinoline))-
1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (2), 4,4-difluoro-
8-(5-azastyryl(8-hydroxyquinoline))-3,5-dimethyl-4-bora-3a,
4a-diaza-s-indacene (3), and 4,4-difluoro-8-(5-azastyryl-(8-
hydroxyquinoline))-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-
s-indacene (4), Scheme 1, were designed and synthesized. As
expected, the intense fluorescence of compounds 1, 2, and 4 has
been revealed to change depending on the pH value; the fluores-
cence gradually diminished with either decreasing or increasing pH
value of their solution in a mixed solvent of H2O and acetonitrile
(1:9) because of the protonated quinolinium NHþ∼ꢀOH form
and deprotonated quinolinate N∼ꢀOꢀ form of the 8-HQ moiety
under acidic and basic conditions, respectively. This in turn leads
to a photoinduced electron transfer from the excited Bodipy
moiety to the 8-HQ unit under acidic conditions and just a
reverse-electron transfer process under basic conditions respon-
sible for the gradual fluorescent quenching in the acidic and basic
systems. As a consequence, these compounds seem to represent
the first example of a novel OFFꢀONꢀOFF type of fluorescent
pH sensors, which will be helpful for developing novel versatile
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Table 1. Spectral Parameters of the 8-Hydroxyquinoline-
Bodipy-Conjugated Derivativees 1ꢀ4 in Different Solvents
λAbs
λem
Δνmax
BDP solvent (max/nm) log(εmax
)
(max/nm) (nm)
Φf
1
2
3
4
MeOH
CH3CN
THF
511
511
514
515
516
502
501
504
505
507
509
510
511
512
512
497
498
501
502
503
3.99
4.02
4.03
4.02
4.04
4.35
4.25
4.36
4.37
4.39
3.96
3.97
4.05
4.03
4.09
4.22
4.20
4.23
4.23
4.26
523
523
527
527
528
515
514
518
518
521
522
524
526
527
528
511
512
515
515
516
12
12
12
13
12
13
13
15
13
14
13
14
15
15
16
12
14
14
14
13
0.0028
0.1138
0.1733
0.2869
0.2506
0.0147
0.2090
0.3226
0.3809
0.3090
0.0004
0.0054
0.0128
0.0212
0.0866
0.0017
0.0870
0.0892
0.1306
0.1964
CH2Cl2
toluene
MeOH
CH3CN
THF
CH2Cl2
toluene
MeOH
CH3CN
THF
Figure 1. Molecular structures of 1 in top-view (A and B) and side-view
(D and E), and molecular structures of 2 in top-view (C) and in side-
view (F), with all hydrogen atoms omitted for clarity.
CH2Cl2
toluene
MeOH
CH3CN
THF
in 2, respectively. More interestingly, all of the CꢀC and CꢀN
bond lengths within the C9BN2 backbone locate in the range of
1.363ꢀ1.426 and 1.336ꢀ1.401 Å, respectively, without any clear
distinction between single and double bonds, indicating the
strongly delocalized π-system nature of the C9BN2 (Bodipy)
framework in both 1 and 2. However, this π-electron delocaliza-
tionisinterrupted between thetwoBꢀN bonds(1.535ꢀ1.555 Å),
which is in agreement with the results reported for other
Bodipys.14 As shown in Figure 1D,E, the dihedral angles between
the mean plane of 8-HQ unit and the C9BN2 moiety of the two
8-HQ-bodipy molecules with slightly different configurations in
the same unit cell of compound 1 are 66.79 and 65.44°, respec-
tively, suggestingthe inefficient electronic coupling between 8-HQ
and C9BN2 (Bodipy) units in this compound. However, owing to
the introductionof twomethyl groups onto theC-1andC-7 atoms
in the Bodipy moiety, this dihedral angle increases to 85.58° in
compound 2, Figure 1F, indicating the almost nonelectronic
coupling nature between the 8-HQ moiety and C9BN2 unit in
2.15 This difference in molecular configuration because of different
steric arrangements between 1 and 2 leads to different electron
transfer efficiencies between the 8-HQ unit and C9BN2 (Bodipy)
moieties in these two compounds, which in turn are responsible
for their different optical properties as detailed below.
Spectroscopic Properties of 1ꢀ4. The electronic absorption
and steady-state fluorescence spectra of compounds 1ꢀ4 were
measured in a variety of solvents with different polarities includ-
ing toluene, CH2Cl2, THF, CH3CN, and MeOH, and the data
are summarized in Table 1. The electronic absorption maxima of
these compounds are centered at 501ꢀ517 nm, ascribed to the
S0ꢀS1 transition of Bodipy moieties, along with high molar
absorption coefficients. The fluorescence emission spectra dis-
play a slightly Stokes-shifted (ca. 13 nm), mirror-symmetrical
band relative to the absorption of Bodipy unit, Figure S5
(Supporting Information), with the maximum emission wave-
lengths in the range of 510ꢀ528 nm. Moreover, along with the
decrease in the solvent polarity from MeOH, CH3CN, THF, and
CH2Cl2 to toluene, the maxima of both electronic absorption
and fluorescence emission bands for compounds 1ꢀ4 take a
slight shift to the lower energy direction, ca. 5 nm, Figure S6
(Supporting Information), indicating the very small change in
the dipole moment between the ground and excited state of
Bodipy moiety in these four compounds.16
CH2Cl2
toluene
However, in comparison with 1, the maxima of both absorp-
tion and emission bands of compound 2 in the series of solvents
including acetonitrile take an obvious blue-shift, ca. 10 nm,
Figure 2 and Table 1. Nevertheless, the fluorescence quantum
yield of compound 1 is also significantly lower than that for 2,
probably because of the nearly orthogonal configuration of the
latter compound which limits the effective interaction between
Bodipy and 8-HQ units and results in less excited energy loss
from the excited Bodipy moiety via nonirradiative decay. This is
also true for compound 3 relative to 4. These results clearly reveal
the effect of molecular configuration on the electronic absorption
and emission spectroscopic properties of 8-HQ-Bodipy-conju-
gated derivatives. It is worth noting that introduction of an aza-
styryl group with a CdN moiety between the 8-HQ unit and
Bodipy moiety in compound 4 seems to induce little shift, ca.
3 nm, in both the absorption and emission bands relative to those
of 2, Figure 2, suggesting the lower influence of the aza-styryl
group between the 8-HQ unit and Bodipy moiety on the maxima
of absorption and emission bands of this compound. However,
the fluorescence quantum yield of 4 is a bit lower than that for 2
because of the introduction of an aza-styryl group with the
electron-donating ꢀCdNꢀ bond between the 8-HQ and
Bodipy moieties in 4, which results in additional partial fluor-
escent quenching of Bodipy chromophore. This is also true for 3
and 1. Moreover, the effects of the molecular configuration and
the linking group between the 8-HQ moiety and Bodipy
chromophore on the spectroscopic properties are further clearly
validated by the obviously small fluorescence quantum yield and
more red-shifted in both absorption and emission bands for
compound 3 in comparison with those for 2, Figure 2 and
Table 1.
pH-Dependent Fluorescence Spectra of Compounds 1ꢀ4.
Before the pH-dependent photophysical properties of com-
pounds 1ꢀ4 can be expounded, it is necessary to give a very
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Figure 2. Electronic absorption (A) and fluorescence spectra (B) of compounds 1ꢀ4 at a concentration of 5 ꢁ 10ꢀ5 M in CH3CN with excitation
wavelength of 450 nm.
Figure 3. Dependence of the fluorescence emission spectra of compound 2 under acidic (A) and basic conditions (B), respectively, on the change of the
pH value at the concentration of 1.25 ꢁ 10ꢀ5 M in a mixed solvent of H2O and CH3CN (1:9) with an excitation wavelength of 450 nm.
brief description of the fluorescence properties of pH-dependent
fluorescent sensors in which Bodipy moiety is utilized as
fluorescent signaling unit. The fluorescent intensity of re-
ported pH sensors was found to change depending only on
the change of pH value in a monotonous way, which gets
increased or decreased along with the change (either increase
or decrease) in the pH value in a monotonous way due to the
only one photoinduced electron transfer mechanism between
receptor and Bodipy signaling unit, resulting in the OFFꢀON
or ONꢀOFF type of fluorescent pH sensors.17 There is
still no report on the pH sensor whose fluorescent intensity
changes in acidic and basic systems along with decreases and
increases in the pH value, respectively, based on the opposite
photoinduced electron transfer processes between a receptor
and Bodipy signaling unit.
To study the pH-dependent optical properties of compounds
1ꢀ4 with unique pyridyl N and ꢀOH structural characteristics,
the pH titration experiments were carried out in a mixed solvent
of H2O and CH3CN (v/v = 1:9) with a very small amount of
TFA for protonation of pyridyl N group and NaOH (0.1 M) used
for deprotonation of the ꢀOH group, respectively. As shown in
Figure S7 (Supporting Information), the electronic absorption
spectrum of compound 2 remains almost unchanged along with
changing (either decreasing or increasing) the pH value of the
system. In good contrast, the intense fluorescence from the
Bodipy subunit of 2 under acidic conditions gradually gets
diminished (despite the maximum wavelength of this emission
remaining almost unchanged) along with a decrease in the pH
value from 6.8 to 0.49, Figure 3A. Most interestingly, along
with an increase in the pH value from 6.8 to 13.1, the intense
fluorescence of the same compound under basic conditions
also gradually gets diminished in a similar manner as observed
under acidic conditions, Figure 3B. These results reveal the
typical characteristics of a pH-fluorescent probe through a
photoinduced electron transfer (PET) quenching process
between receptor and fluorescent signaling unit for this
compound.18 In other words, a photoinduced electron trans-
fer quenching process between the 8-HQ receptor and Bodipy
fluorescent signaling unit is responsible for the particular
fluorescent properties in compound 2 along with either decreasing
the pH from 6.8 to 0.49 or increasing the pH from 6.8 to 13.1. The
unique pH-dependent fluorescent property of 2 indicates the
potential application of this compound as a pH indicator under
both acidic and basic conditions. This, to the best of our knowl-
edge, representsthefirst exampleofa novelOFFꢀONꢀOFF type
of fluorescent pH sensor.
Similar to 2, compounds 1 and 4 also display the similar
fluorescence characteristics in slightly changed ranges of pH
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Figure 4. Proposed photoinduced electron transfer mechanism between Bodipy moiety and 8-HQ unit of compound 2 in protonated quinolinium,
neutral, and deprotonated quinolinate states, respectively.
under both acidic and basic conditions, Figures S8 and S9
(Supporting Information) and, therefore, can also act as an
OFFꢀONꢀOFF type of pH fluorescent sensors. However, their
sensitive properties toward the change of pH values appear to be
not as good as that for analogue 2 (Figure 3) because of the
relatively lower intrinsic fluorescent emission intensities for 1
and 4 in comparison with that for 2 as mentioned above due to
the difference in molecular structure of these compounds. In
particular, the intrinsic fluorescence of the compound 3 is too
weak to be suitable for pH sensors. These results clearly reveal
the effects of the molecular configuration and the linking group
between 8-HQ moiety and Bodipy chromophore on the pH-
dependent fluorescent properties of these compounds, which
will be helpful for the design and synthesis of novel efficient
fluorescent probes.
pH-Dependent PET Mechanism. For the purpose of under-
standing the interesting pH-dependent fluorescence character-
istics of these compounds, the following PET quenching
Figure 5. Energy levels and surfaces of frontier MOs of compound 2 in
protonated, neutral, and deprotonated states, respectively.
mechanism determined by the orbital energy levels of the excited
states of Bodipy moiety and 8-HQ unit is proposed with
as the receptor in 2, its HOMO is located on the phenoxide side
and LUMO on the pyridyl side.13 When the N atom is proto-
nated in the form of NHþ, the LUMO energy of electron-
deficient 8-HQ unit becomes lower than that of the excited
Bodipy moiety, Figure 4, leading to a d-PET quenching process.
In contrast, when the OH group of 8-HQ is deprotonated into
Oꢀ, the HOMO energy of 8-HQ moiety gets increased and
becomeshigher thanthatofexcited Bodipyfluorophore, Figure 4,
compound 2 as an example, Figure 4. In principle, Bodipy can
serve as both electron donor and acceptor upon photoexcitation.
In other words, the photoinduced electron can transfer from the
excited Bodipy moiety to the lowest unoccupied molecular
orbital (LUMO) of the 8-HQ (donor-excited PET; d-PET) or
just reversely from the highest occupied molecular orbital
(HOMO) of 8-HQ to the excited Bodipy moiety (acceptor-
excited PET; a-PET).19 As far as the 8-HQ moiety is concerned
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which induces the electron transfer from 8-HQ moiety to Bodipy
unit, giving rise to an reductive a-PET. These two opposite
photoinduced electron transfer processes are in turn responsible
for the special fluorescence characteristics of compound 2 along
with the change in the pH value under acidic and basic condi-
tions, resulting in a novel OFFꢀONꢀOFF type of pH fluor-
escent switch to acidic and basic conditions, respectively.
and the just opposite process responsible for the gradual
fluorescent quenching in the acidic and basic systems. This
renders these compounds the special OFFꢀONꢀOFF type of
pH-dependent fluorescent sensors. Nevertheless, single-crystal
X-ray diffraction analysis together with the DFT theoretical
calculations reveals the effect of molecular structure, actually
the dihedral as well as the linking group between the 8-HQ
receptor unit and Bodipy fluorescent signaling moiety, on the
intrinsic fluorescent property and in turn the pH-sensing prop-
erty of these compounds. The present result appears to represent
the first example of pH fluorescent sensor, with 8-HQ as receptor
and Bodipy as signaling unit, in particular with the OFFꢀONꢀ
OFF type nature, which should be helpful for designing and
preparing novel versatile fluorescent sensors with potential
applications in chemical and biological fields.
The above-mentioned two opposite PET mechanisms are
clearly validated by density functional theory (DFT) calculation
with the B3LYP/6-31G(d) method (see the Supporting Infor-
mation for computational details and the coordinates for calcu-
lated structures, Tables S2ꢀ4) based on the optimized molecular
structure of compound 2. The calculated frontier molecular
orbital (MO) energies and surfaces are shown in Figure 5. As
can be found, both the HOMO and LUMO of 2 in the neutral
state are located almost completely on the Bodipy moiety, while
the HOMO-1 and LUMOþ1 are located mainly on the 8-HQ
unit. As a result, the electronic transition between HOMO and
LUMO for 2 is limited only on the Bodipy moiety, leading to an
intensive intrinsic fluorescence from the bodipy moiety of this
compound. When the N atom of 8-HQ in compound 2 is
protonated in the form of NHþ, the LUMO energy of 8-HQ
moiety, ꢀ6.74 eV, becomes lower than that for the Bodipy unit in
the molecule of compound 2, ꢀ5.31 eV, which in turn results in
the electronic transfer from Bodipy moiety to 8-HQ moiety for
compound 2 under acidic conditions, Figure 5. When the OH
group of the 8-HQ moiety of 2 is deprotonated into Oꢀ in the
basic system, change occurs in the molecular structure of this
compound as exemplified by the dihedral angle between Bodipy
unit and 8-HQ moiety from ca. 90° in neutral state to ca. 60° in
negative state. Along with this structural change, the HOMO
energy of the 8-HQ moiety in the molecule of 2 gets increased to
be close to those of the HOMO and LUMO of Bodipy moiety. As
a consequence, the linear combination of these three MOs with
similar energy levels generates three new frontier MOs for this
compound in the negative state, namely LUMO (0.19 eV),
HOMO (ꢀ1.95 eV), and HOMO-1 (ꢀ2.96 eV), all of which
obviously have electronic distribution over both Bodipy and
8-HQ moieties, Figure 5. However, it must be pointed out that
the HOMO-1 and LUMO for compound 2 in the negative state
are mainly contributed from the Bodipy moiety, while the
HOMO mainly from the 8-HQ moiety on the basis of the
theoretical calculation result. As a result, the electron should
transfer from the 8-HQ unit to the Bodipy moiety in 2 in the
negative state, just in the opposite direction to that for the same
compound in the positive state. These results rationalize the
photoinduced electron transfer (PET) quenching mechanism
proposed for compound 2 as detailed in last section.
’ EXPERIMENTAL SECTION
Chemicals. Column chromatography was carried out on silica gel
(Merck, Kieselgel 60, 70ꢀ230 mesh) with the indicated eluents. All
other reagents and solvents were used as received. Dichloromethane was
freshly distilled from CaH2 under nitrogen. The compound of dipyrro-
methane was prepared according to the published procedures. The
compound of 4,4-difluoro-8-(4-aminophenyl)-1,3,5,7-tetramethyl-4-
bora-3a,4a- diaza-s-indacene (7) was prepared according to the literature
procedure.20
General Instruments. 1H and 13C NMR spectra (1H, 400 MHz,
and 13C, 100 MHz) were recorded on a Bruker DPX 400 MHz
spectrometer in CDCl3 with shifts referenced to SiMe4 (0.00 ppm).
MALDI-TOF mass spectra were taken on a Bruker BIFLEX III ultra-
highresolution Fourier transform ion cyclotron resonance (FT-ICR)
mass spectrometer with R-cyano-4-hydroxycinnamic acid as matrix.
Elemental analyses were performed on an Elementar Vavio El III.
Electronic absorption spectra were recorded on a U-4100 spectro-
photometer. Steady-state fluorescence spectroscopic studies were per-
formed on an F 4500 (Hitachi). The slit width was 2.5 nm for emission.
The photon multiplier voltage was 700 V. The relative fluorescence
quantum yields of Bodipy derivatives were obtained by comparing the
area under the corrected emission spectrum of the test sample with that
of a solution of 4,4-difluoro-8-(4-methylphenyl)-1,3,5,7-tetramethyl-4-
bora-3a,4a-diaza-s-indacene in CH2Cl2 with excitation wavelength of
450 nm, which has a quantum efficiency of 0.60 according to the
literature.20 The pH measurements were carried out with a PHS-25 pH
analyzer. Crystal data for 1 and 2 were collected on a Bruker SMART
CCD diffractometer with a Mo KR sealed tube (λ = 0.71073 Å) at 293 K,
and details of the structure refinement are given in Table S1 (Supporting
Information). CCDC-793813 and -793814 containing the supplemen-
tary crystallographic data for this paper can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
pH-Dependent Fluorescence Experiments. To determine
pH-dependent fluorescence properties of these compounds, the solu-
tions of 1ꢀ4 in a mixed solvent of H2O/CH3CN (1:9) (1.25 ꢁ 10ꢀ5 M)
were prepared. TFA and 0.1 M NaOH in a mixed solvent of H2O/
CH3CN (1:9), respectively, were employed to tune the pH values for the
purpose of investigating their pH-dependent fluorescence properties. All
of the measurements were conducted at 20 °C.
Synthesis of 8-Hydroxyquinoline-5-carboxaldehyde (5).
This known compound was synthesized by modifying the reported
procedure with improved reaction yield.21 Sodium hydroxide (10 g) in
water (8 mL) was added into the solution of 8-hydroxyquinoline (5 g)
in ethanol (20 mL) at 40 °C. After the mixture was heated to 65 °C,
cetyltrimethylammonium bromide (0.013 g) was added to this reaction
’ CONCLUSION
In summary, a series of four novel 8-hydroxyquinoline-sub-
stituted boron-dipyrromethene derivatives have been designed
and synthesized. Comparative studies reveal their special pH-
dependent fluorescent properties under acidic and basic condi-
tions. The intense intrinsic fluorescence of these compounds, in
particular compounds 1, 2, and 4, gradually gets diminished
along with decreasing and increasing pH values, respectively,
because of the protonated quinolinium NHþ∼ꢀOH form and
deprotonated quinolinate N∼ꢀOꢀ form of the 8-HQ moiety
under acidic and basic conditions, which leads to a photoinduced
electron transfer from the excited Bodipy moiety to 8-HQ unit
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The Journal of Organic Chemistry
ARTICLE
mixture, and chloroform (5 mL) was subsequently dropped within
20 min. The resulting black mixture was refluxed for another 8 h,
and then the volatiles were evaporated under reduced pressure.
The semisolid residue was poured into cold water (150 mL) and
the pH was adjusted to ca. 5 with acetic acid. The brown precipitate
obtained was filtered off and dried in air, which was further dissolved in
a relatively large amount of chloroform and then filtered again. The
filtrate was evaporated under reduced pressure, and the residue was
applied on a silica gel chromatography with CH3Cl/MeOH (10:1)
as eluent. The target compound was obtained as pale-yellow powder,
867 mg (19.3%).
Synthesis of 4,4-Difluoro-8-(5-(8-hydroxyquinoline)-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene (1). To the mixture
of 8-hydroxyquinoline-5-carboxaldehyde compound 5 (174 mg, 1
mmol) and 2-methylpyrrole (160 mg, 2.00 mmol) dissolved in CH2Cl2
(120 mL) was added one drop of TFA. The resulting mixture was then
stirred at room temperature under N2 atmosphere. When thin-layer
chromatography (TLC) monitoring (silica, CH2Cl2) indicated the
complete consumption of the aldehyde, a solution of DDQ (227 mg,
1 mmol) in CH2Cl2 (20 mL) was added, and the reaction mixture was
further stirred for another 30 min. After the addition of N,N-diisopro-
pylethylamine (DIEA) (2 mL) into the mixture, the resulting black-red
then chromatographed on a silica gel column with CH2Cl2 as eluent,
affording compound 6 as a red powder 121 mg (78%): 1H NMR
(CDCl3, 400 MHz) δ 7.34 (d, J = 8.2 Hz, 2H), 6.80 (d, J = 2.4 Hz, 2H),
6.74 (d, J = 8.2 Hz, 2H), 6.26 (d, J = 4 Hz, 2H), 3.98 (s, 2H), 2.67 (s, 6H);
13C NMR (100 MHz, CDCl3) δ 156.3, 148.7, 143.4, 134.4, 132.3, 130.1,
124.1, 118.9, 114.2, 14.8; MS (MALDI-TOF) an isotopic cluster peaking at
m/z310.94 [calcd for Mþ 311.14]. Anal. Calcd for C17H16BF2N3: C, 65.62;
H, 5.18; N, 13.51. Found: C, 65.53; H, 5.24; N, 13.48.
Synthesis of 4,4-difluoro-8-(5-azastyryl(8-hydroxyquino-
line))-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (3). To a
solution containing compound 5 (17 mg, 0.1 mmol) and 6 (32 mg,
0.1 mmol) in EtOH (5 mL) and CH2Cl2 (1 mL) was added one drop of
glacial acetic acid. The resulting reaction mixture was refluxed for 5 h
under N2 and then cooled to room temperature. The red precipitate was
filtrated and washed by EtOH. Recrystallization from CHCl3 and MeOH
yielded a pure sample of 3 as a black-red powder: 37 mg, 80% as orange-
red powder; 1H NMR (CDCl3, 400 MHz) δ 9.96 (d, J = 8.8 Hz, 1H), 8.86
(d, J = 28 Hz, 2H), 7.91 (d, J = 7.6 Hz, 1H), 7.64 (t, J = 8 Hz, 1H), 7.56 (d,
J =8 Hz, 2H), 7.33(d, J= 8 Hz, 2H), 7.24(overlappedwith strong residual
CHCl3 signal at δ 7.24, 1H), 6.78 (s, 2H), 6.28 (s, 2H), 2.65 (s, 6H); 13
C
NMR (100 MHz, CDCl3) δ 161.5, 157.4, 155.6, 154.2, 148.3, 142.3,
138.4, 135.8, 135.2, 134.5, 131.6, 131.4, 130.3, 127.0, 123.6, 122.7, 120.7,
119.3, 114.3, 109.3, 14.9; MS (MALDI-TOF) an isotopic cluster peaking
at m/z 466.16 [calcd for Mþ 466.18]. Anal. Calcd for C27H21BF2N4O: C,
69.55; H, 4.54; N, 12.02. Found: C, 69.36; H, 4.60; N, 11.89.
mixture was stirred for 5 min. Then BF3 OEt2 (2.0 mL) was added into
3
the reaction mixture, and stirring was continued for another 30 min.
After the reaction mixture was washed with water, the organic phase was
dried over anhydrous MgSO4, filtered, and evaporated. The residue was
chromatographed on a silica gel column using CHCl3/CH3OH (50:1)
as eluent. Repeated chromatography followed by recrystallization from
ethyl acetate and hexane gave the target compound 1 as black-green
crystals: 88 mg (24%); 1H NMR (CDCl3, 400 MHz): δ 8.81 (t, J = 4 Hz,
1H), 8.23 (m, J = 10 Hz, 1H), 7.54 (d, J = 7.6 Hz, 1H), 7.41 (m, J = 12.8
Hz, 1H), 7.23 (overlapped with strong residual CHCl3 signal at δ 7.24,
1H), 6.46 (d, J = 4 Hz, 2H), 6.20 (d, J = 4.4 Hz, 2H), 2.67 (s, 6H); 13C
NMR (100 MHz, CDCl3) δ 158.1, 153.6, 148.4, 139.3, 137.7, 135.7,
134.7, 133.8, 130.2, 130.1, 129.7, 128.2, 122.3, 121.7, 119.6, 109.1, 14.9;
MS (MALDI-TOF) an isotopic cluster peaking at m/z 363.2 [calcd for
Mþ 363.14]. Anal. Calcd for C20H16BF2N3O: C, 66.14; H, 4.44; N,
11.57. Found: C, 66.29; H, 4.31; N, 11.29.
Synthesis of 4,4-Difluoro-8-(5-azastyryl(8-hydroxyquino-
line))-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (4).
By using the above-described procedure with 7 instead of 6 as starting
material, compound4 was obtainedintheyield of41 mg(84%): 1H NMR
(CDCl3, 400 MHz) δ 9.98 (d, J = 8.8 Hz, 1H), 8.86 (d, J = 8.8 Hz, 2H),
7.93 (d, J = 8 Hz, 1H), 7.64 (t, J = 8.8 Hz, 1H), 7.39 (m, J = 30.4 Hz, 4H),
7.24 (overlapped with residual CHCl3 signal at δ 7.24, 1H), 5.99 (s, 2H),
2.56 (s, 6H), 1.49 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 161.2, 155.5,
153.2, 148.2, 143.1, 141.6, 138.3, 135.6, 135.2, 132.3, 131.6, 129.0, 126.9,
123.5, 122.7, 121.6, 121.2, 109.3, 14.7, 14.6; MS (MALDI-TOF) an
isotopic cluster peaking at m/z 494.28 [calcd for Mþ 494.21]. Anal. Calcd
for C29H25BF2N4O: C, 70.46; H, 5.10; N, 11.33. Found: C, 70.14; H,
5.33; N, 11.08.
Synthesis of 4,4-Difluoro-8-(5-(8-hydroxyquinoline)-
1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (2). By
employing the above-described procedure with 2,4-dimethylpyrrole
(190 mg, 2.00 mmol) instead of 2-methylpyrrole as starting material,
4,4-difluoro-8-(5-(8-hydroxyquinoline)-1,3,5,7-tetramethyl-4-bora-
3a,4a-diaza-s-indacene 2 was obtained as purple-red powder: 82 mg
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR and 13C NMR spec-
S
b
tra of compounds 1ꢀ4 in CDCl3; the electronic absorption and
fluorescent spectra and the spectral parameters of compounds
1ꢀ4 in different solvents; pH-dependent optical properties of
compounds 1, 2, and 4; the crystal data and structure refinements
of compounds 1 and 2; computational details for DFT calcula-
tion of compound 2 and tables of coordinates for calculated
structures. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
(21%); H NMR (CDCl3, 400 MHz) δ 8.82 (t, J = 4 Hz, 1H), 8.10
(m, J = 9.6 Hz, 1H), 7.42 (m, J = 26.4 Hz, 2H), 7.28 (d, J = 8 Hz, 1H),
5.93 (s, 2H), 2.56 (s, 6H), 1.10 (s, 6H); 13C NMR (100 MHz, CDCl3)
δ 155.9, 153.2, 148.6, 142.9, 138.3, 138.2, 133.7, 132.4, 127.9, 127.1,
122.8, 122.6, 121.4, 110.1, 14.6, 14.1; MS (MALDI-TOF) an isotopic
cluster peaking at m/z 391.10 [calcd for Mþ 391.17]. Anal. Calcd for
C22H20BF2N3O: C, 67.54; H, 5.15; N, 10.74. Found: C, 67.27; H, 5.46;
N, 10.56.
’ AUTHOR INFORMATION
Synthesis of 4,4-Difluoro-8-(4-aminophenyl)-3,5-dimeth-
yl-4-bora-3a,4a-diaza-s-indacene (6). By utilizing the above-
described procedure used to prepare compound 1 with 4-nitrobenzal-
dehyde (604 mg, 4.00 mmol) instead of 8-hydroxyquinoline-5-carbox-
aldehyde as starting material, 4,4-difluoro-8- (4-nitrophenyl)-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene was prepared. After the reaction
of 4,4-difluoro-8-(4-nitrophenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-in-
dacene (172 mg, 0.5 mmol) with NH2NH2ꢀH2O (0.7 mL, 80%) in
refluxing ethanol (30 mL) with Pd/C (10 mg) as catalyst under N2
atmosphere for 6 h, the resulting mixture was cooled to room tempera-
ture, and the catalyst and the solvent were removed by filtration and
evaporation under reduced pressure, respectively. The residue was
Corresponding Author
*E-mail: jianzhuang@ustb.edu.cn.
’ ACKNOWLEDGMENT
Financial support from the Natural Science Foundation of
China, Ministry of Education of China, and Fundamental
Research Funds for the Central universities, Beijing Municipal
Commission of Education, and University of Science and
Technology Beijing is gratefully acknowledged.
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The Journal of Organic Chemistry
ARTICLE
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