Modulation of the spectroscopic property of Bodipy derivates through tuning the molecular configuration

Article abstract of DOI:10.1039/c1pp00001b

A series of six Bodipy derivatives, namely 4,4-difluoro-8-(4-amidophenyl)- 1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (1), 4,4-difluoro-8-(4- methylphenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (2), 4,4-difluoro-8-(4-nitrylphenyl)-1,3,5

Full text of DOI:10.1039/c1pp00001b

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PAPER
Modulation of the spectroscopic property of Bodipy derivates through tuning
the molecular configuration†
Yuting Chen,a,b,c Liang Wan,b Daopeng Zhang,b Yongzhong Bianb and Jianzhuang Jiang*a,b
Received 1st January 2011, Accepted 16th February 2011
DOI: 10.1039/c1pp00001b
A series of six Bodipy derivatives, namely
4,4-difluoro-8-(4-amidophenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (1),
4,4-difluoro-8-(4-methylphenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (2),
4,4-difluoro-8-(4-nitrylphenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (3),
4,4-difluoro-8-(4-amidophenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (4),
4,4-difluoro-8-(4-methylphenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (5), and
4,4-difluoro-8-(4-nitrylphenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (6) were structurally
characterized by single crystal X-ray diffraction analysis. Two methyl substituents attached at C-1 and
C-7 positions of boron-dipyrromethene (Bodipy) moiety in compounds 1–3 were revealed to prevent
the free rotation of the benzene moiety, resulting in a molecular configuration with an almost
orthogonal dihedral angle between the Bodipy and benzene moieties with the dihedral angle in the
range of 81.14–88.56◦. This is obviously different from that for 4–6 with a free-rotating benzene moiety
relative to the Bodipy core due to the lack of two methyl substituents in the latter series of compounds,
leading to an enhanced interaction between the Bodipy and benzene moieties for 4–6 in comparison
with 1–3. The resulting larger HOMO–LUMO gap for 1–3 than 4–6 results in a blue-shifted absorption
band for 1–3 relative to that for 4–6. Comparative studies over their fluorescence properties also disclose
the blue-shifted fluorescence emission band and corresponding higher fluorescence quantum yield for
1–3 relative to those of 4–6, revealing the effect of molecular configuration on the spectroscopic
properties of Bodipy derivatives. Comparison of the redox behaviors of these two series of Bodipy
compounds provides additional support for this point. In addition, the electron-donating/withdrawing
property of the para substituent of the benzene moiety was shown to exhibit a slight influence on the
electronic absorption and fluorescence emission properties of the Bodipy compounds.
tation/emission wavelengths in the visible region.6 In particular,
their fluorescent properties can be easily tuned by incorporating
Introduction
Since their first discovery by Treibs and Kreuzer in 1968,1 4,4-
different functional groups onto different peripheral positions of
difluoro-4-bora-3a,4a-diaza-s-indacenes (Bodipys) have been an
the Bodipy core, which meanwhile renders it possible to introduce
important class of fluorophores with extensive applications in
more recognition sites in Bodipy derivatives for a variety of
biochemical labeling,2 fluorescent sensors,3 sensitizers for solar
cells,4 and laser dyes,5 due to their advantageous photo-spectral
properties including high photostability, sharp absorption and
emission bands, relatively high absorption coefficient, high flu-
orescence quantum yields, and the extraordinary feature of exci-
analytes.7
Thus far, a wide variety of Bodipy derivatives with different
substituents at peripheral positions of the Bodipy core have
been synthesized and have found utilization for different analysis
purposes. Akkaya et al. revealed that when the methyl group(s) at
the C-3 and/or C-5 position(s) of Bodipy core is/are conjugated
with electron-rich aromatic aldehyde(s), the extension in the
p-electron conjugated system of the Bodipy derivatives results
in a pronounced bathochromic shift from UV-visible region to
near-infrared (NIR) region in both electronic absorption and
fluorescence spectra, leading to the expansion in the photo-
optical range of Bodipy derivatives.8 In a similar manner, fusion
of additional aromatic moieties onto the pyrrole rings can also
lead to the extension of the p-electron conjugated system for
aDepartment of Chemistry, Shandong University, Jinan, 250100, China.
E-mail: jianzhuang@ustb.edu.cn
bDepartment of Chemistry, University of Science and Technology Beijing,
Beijing, 100083, China
cDepartment of Chemistry, Dezhou University, Dezhou, 253023, China
† Electronic supplementary information (ESI) available: Molecule orbital
energy levels for 1–6; electronic absorption and fluorescent spectra
properties of compounds 1–6 in different solvents. CCDC reference
numbers 798782–798787. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1pp00001b
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the Bodipy compounds and therefore induces a spectroscopic
bathochromic shift.9 Nagano and co-workers found that the
substituents introduced onto the C-2 and/or C-6 position(s) of
the Bodipy core via electrophilic substitution reaction not only
modulate the spectroscopic properties of the Bodipy chromophore
but also provide water-soluble Bodipy compounds.10 The same
authors also reported that the substituents incorporated at the
C-1 and C-7 positions of Bodipy core increase the molecular
rigidity and minimize the decrease of fluorescence intensity,
resulting in a high fluorescence quantum yield.11 Sunahara et al.
disclosed that introduction of different aryl-substituents at the C-
8 position of the Bodipy core leads to very slight changes in the
electronic absorption and fluorescence emission bands of Bodipy
derivatives, with an almost orthogonal configuration between
the Bodipy moiety and aryl-group.12 According to Ulrich and
Ziessel, replacement of fluorine atoms with aryl,13 ethynylaryl,14
and ethynyl subunits15 in the Bodiopy compounds results in a
dramatic increase in the versatility of Bodipy dyes and opens
the way to the preparation of new diad and cascade-type Bodipy
dyes. However, systematic study towards revealing the effect of the
molecular configuration between the Bodipy and meso-aromatic
moieties on the optical properties of Bodipy derivatives through
single crystal X-ray diffraction analysis still remains rare despite
numerous reports on the effect of C-1/7-substituents preventing
free rotation of the meso-aromatic group from the Bodipy moiety.7a
In the present work, a series of six Bodipy derivatives,
and benzene moieties due to the presence of two methyl groups at
the C-1 and C-7 positions of Bodipy core, while the counterparts
4–6 without two methyl groups at the Bodipy core take the
configuration with a relatively small dihedral angle between
the Bodipy and benzene moiety in the single crystal state. This
leads to a larger HOMO–LUMO gap for 1–3 than that for 4–6,
resulting in a blue-shifted absorption band for the former series of
three compounds than the latter series, which is also responsible
for the blue-shifted fluorescence emission band and increased
fluorescence quantum yield for 1–3 relative to those of 4–6. These
results clearly reveal the effect of the molecular configuration of
Bodipy compounds on their spectroscopic properties.
Result and discussion
Intention and design
Bodipy derivates have attracted increasing attention due to their
excellent spectroscopic properties together with the ease of fluo-
rescent modulation by incorporating different functional groups
onto different peripheral positions of Bodipy core. Substituents at
the C-8 position of Bodipy derivates with an almost orthogonal
molecular configuration has been revealed to have little influence
on their absorption and fluorescence emission bands. However,
there still seems to be no systematic investigation towards un-
derstanding the effect of molecular configuration on the electron
transfer-originated spectroscopic properties of Bodipy derivates
based on the X-ray diffraction analysis. As a consequence, in the
present paper two series of Bodipy compounds with different
molecular configuration between the Bodipy fluorophore and
benzene moiety attached at the C-8 position have been prepared
for the purpose of investigating the configurational effect on the
electron absorption and fluorescence spectroscopic properties,
Scheme 1. In addition, amine, methyl, and nitro groups were
introduced onto the para-position of the C-8-attached benzene
ring, which renders it possible to further investigate the effect of the
electron-donating/withdrawing property of the benzene moiety at
namely
4,4-difluoro-8-(4-amidophenyl)-1,3,5,7-tetramethyl-4-
bora-3a,4a-diaza-s-indacene (1), 4,4-difluoro-8-(4-methylphenyl)-
1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (2), 4,4-dif-
luoro-8-(4-nitrylphenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-
s-indacene (3), 4,4-difluoro-8-(4-amidophenyl)-3,5-dimethyl-4-
bora-3a,4a-diaza-s-indacene (4), 4,4-difluoro-8-(4-methylphenyl)-
3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (5), and 4,4-difluoro-
8-(4-nitrylphenyl)-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene
(6), Scheme 1, were structurally characterized by single crystal
X-ray diffraction analysis. The compounds 1–3 were revealed to
employ an almost orthogonal configuration between the Bodipy
Scheme 1 Synthesis of the Bodipy compounds 1–6.
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Table 1 Crystal data and structure refinements of 1–6
Compound
1
2
3
4a
5
6
Formula
F.W.
Crystal system
C19H20BF2N3
339.19
Monoclinic
C20H21BF2N2
338.20
Monoclinic
P21
11.882(3)
8.409(2)
44.412(10)
90.00
91.766(4)
90.00
4435.4(18)
10
1.266
0.088
1780
0.0754
0.1882
1.001
C20H20BCl2F2N3O2
454.10
C17H16BF2N3
311.14
Monoclinic
C18H17BF2N2
310.15
Monoclinic
C2/c
6.554(3)
26.198(11)
9.732(4)
90.00
105.458(8)
90.00
1610.4(12)
4
1.279
0.091
648
0.0445
0.1031
1.006
C17H14BF2N3O2
341.12
Triclinic
Triclinic
¯
¯
Space group
P21/n
P1
P21/n
P1
˚
a/A
10.8454(10)
11.8897(11)
13.9990(13)
90.00
8.347(4)
11.611(6)
12.092(6)
74.438(9)
83.016(8)
68.946(8)
1053.1(9)
2
9.935(5)
23.900(5)
13.513(5)
90.000(5)
103.184(5)
90.000(5)
3124(2)
8
6.4804(7)
8.3217(9)
14.6304
80.583(2)
84.666(2)
85.576(2)
773.44(15)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
105.262(10)
90.00
1741.5(3)
4
1.294
0.092
712
0.0434
0.0870
0.993
3
˚
V/A
Z
D
cald/g cm-3
1.432
0.348
468
0.0628
0.1492
0.994
1.323
0.096
1296
0.0391
0.0944
1.008
1.465
0.113
352
0.0376
0.1034
0.993
m/mm-1
F(000)
R1 I > 2q
Rw2 I > 2q
S
a The data given here for 4 are in line with those reported previously.17
the C-8 position of Bodipy frame on the electron transfer nature
of Bodipy fluorophore.
two B–N single bonds, in coincidence with the result reported for
other Bodipys.18
It is worth noting that the presence and absence of the two
methyl groups at C-1 and C-7 positions of Bodipy core lead to
remarkably different molecular configurations in single crystal
state, principally in the dihedral angle between the benzene moiety
and Bodipy frame, between 1–3 and 4–6. As shown in Fig. 1, the
dihedral angle between the benzene and Bodipy moiety is 88.56
in 1, 81.14◦ in 2 (the average of 80.52, 80.62, 80.74, 82.13 and
82.20◦ in five molecules), and 82.93◦ in 3, indicating an almost
perpendicular configuration between the benzene and Bodipy
moiety. However, the corresponding dihedral angles decrease to
46.99◦ in 4 (the average of 48.25 and 45.73◦ in two molecules),
64.69◦ in 5, and 58.63◦ in 6, which are significantly smaller than
those in 1–3 since the two methyl substituents at C-1 and C-7
positions of Bodiy core in 1–3 prevent the benzene group from free
rotation around the Bodipy moiety. This rotation of the benzene
group with respect to the Bodipy moiety has an obvious influence
on the electron transfer nature within the Bodipy compounds,
which in turn is responsible for their different optical properties
as detailed below.
X-Ray single crystal structures
Single crystals of 1–3 suitable for X-ray diffraction analysis were
obtained by slow diffusion of hexane into the CHCl2 solution of
these compounds, while those of 4–6 obtained from natural solvent
evaporation of their CHCl2 solution. Compounds 1, 2, 5, and 6
are first structurally characterized by X-ray diffraction analysis.
However, X-ray diffraction analysis reveals the triclinic crystal
¯
system and P1space group for single crystals of 3 obtained in the
present case, which is different from the monoclinic system and
C2/c space group reported previously for the same compound,
suggesting a crystal structure of 3 revealed this time isomeric with
that reported previously.16 Both the space group and molecular
structure of compound 4 were revealed to be just the same as
those determined previously.17 There are five and two molecules
with slightly different configurations per unit cell for 2 and 4,
respectively. In contrast, only one molecule exists per unit cell for
the remaining four compounds 1, 3, 5, and 6. The crystal data of
the whole series of compounds 1–6 are listed in Table 1.
In these compounds, the central six-membered ring is essentially
coplanar with two fused pyrrole rings, Fig. 1, with the very small
maximum deviation from the least-squares mean plane of the
DFT calculations
Density functional theory (DFT) calculation was carried out using
PBE1PBE/6-31+G(d,p) based on the crystal structures of two
series of Bodipy derivatives. Since PBE1PBE displays excellent
performance in energy calculation of various molecules,19 the
calculation result obtained at this level in the present case is precise
enough for the discussion. All calculations were carried out using
the Gaussian 03 program20 on an IBM P690 system housed at
Shandong Province High Performance Computing Center.
The calculated energies of the highest occupied molecular
orbitals (HOMOs) and the lowest occupied molecular orbitals
(LUMOs) of these compounds are listed in Table 2. As can be seen,
after introducing two methyl substituents onto the C-1 and C-7
positions, both the HOMO and LUMO energies of 1–3 become
˚
˚
C9BN2 frame being 0.052 A for 1, 0.0404 A for 2 (actually the
˚
average of 0.033, 0.036, 0.042, 0.043, and 0.048 A in five molecules
˚
˚
per unit cell), 0.077 A for 3, 0.036 A for 4 (the average of 0.032
˚
˚
˚
and 0.04 A in two molecules), 0.022 A for 5, and 0.022 A for
6, respectively, Fig. 1, indicating the slightly less planar degree
of C9BN2 frame for 1–3 with two methyl substituents at the C-1
and C-7 positions in comparison with that for 4–6 without the
two methyl substituents on the Bodipy core. The bond lengths
within the C9BN2 backbone, without any clear distinction between
single and double bonds for 1–6, reveals the strongly delocalized
p-system nature of the C9BN2 frame in these six compounds.
However, this p-electron delocalization is interrupted between the
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Table 2 Calculated energy data, the recorded spectroscopic and electrochemical data of 1–6 in CH2Cl2
Compounds
HOMO/eV
LUMO/eV
LUMO–HOMO/eV
l
Ab(max)/nm
l
em(max)/nm
Uf
Eox/V
Ered/V
1
2
3
4
5
6
-2.33
-2.34
-2.93
-2.57
-2.64
-3.22
-5.69
-5.71
-6.15
-5.85
-5.93
-6.32
3.36
3.37
3.22
3.28
3.29
3.10
500
500
506
507
510
518
513
513
527
523
526
555
0.491
0.600
0.009
0.198
0.292
0.002
0.934
0.993
1.061
1.080
1.090
1.132
-1.64
-1.531
-1.394
-1.540
-1.490
-1.344
Fig. 1 Molecular structures of 1–6 in top-view (top) and side-view (bottom), respectively, with the atoms except for the ones of Bodipy and benzene
moieties omitted for clarity in side-view (bottom).
higher in comparison with those of corresponding counterpart in
4–6, namely 1 > 4, 2 > 5, and 3 > 6. However, the LUMO level for
1–3 increases to a larger extent than the HOMO level, resulting in
an enlarged HOMO–LUMO gap of 1–3 in comparison with 4–6
(Fig. S1, ESI†). On the basis of a previous study, the molecular
structure for the six Bodipy compounds 1–6 could be divided
into two parts, namely the Bodipy fluorophore and the benzene
moiety connected through C–C single bond in a twisted form.21
The HOMO for all these compounds is localized in the Bodipy
moiety regardless of the substituent at the C-8 position. However,
the LUMO of 1–6 exhibits significantdependence on the molecular
configuration of the Bodipy compounds. When the benzene and
Bodipy moieties are perpendicular to each other, the LUMO is
localized on the Bodipy frame. In contrast, when the benzene
moiety is coplanar with the Bodipy moiety in the compounds, the
LUMO becomes extensively delocalized into the benzene moiety.22
As a consequence, the change in the molecular configuration, i.e.
the dihedral angle between benzene and Bodipy moieties, leads
to different LUMO energy level and corresponding HOMO–
LUMO energy gap between 1–3 and 4–6, Fig. 2. This in turn
is responsible for the different electron absorption spectroscopic
features between these two series of Bodipy compounds as detailed
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Fig. 2 Orbital maps of LUMOs and HOMOs of 1–6.
below due to the nearly pure HOMO–LUMO electron transition
nature of S0–S1 transition in these Bodipy compounds.23
Tables 2 and S1 (ESI†). As shown in Fig. 3, these six compounds
show typical electronic absorption features of Bodipy derivates,
namely narrow spectral bands with two absorption maxima. The
electronic absorption maxima at ca. 500 nm can be attributed to a
strong S0–S1 transition, while the second maximum or shoulder at
the high energy side to the 0–1 vibrational transition. Along with
the decrease in the solvent polarity from MeOH to toluene, their
absorption maxima take slight shift (ca. 6 nm) to the lower energy
Electronic absorption properties
The electronic absorption spectra of 1–6 were recorded in a variety
of solvents with different polarity including toluene, CH2Cl2,
THF, CH3CN, DMF, and MeOH, and the data are compiled in
Fig. 3 The electronic absorption (A and B) and fluorescence spectra (C and D) of Bodipy compounds 1–6 at the concentration of 1 ¥ 10-5 M in CH2Cl2
and CH3CN, respectively, with the excitation wavelength of 450 nm.
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direction (Fig. S2, ESI†), indicating the relatively small change in
the dipole moment between the ground and excited states of these
compounds. The result is in line with the behavior of other Bodipy
derivates reported previously.24
to the larger transfiguration between the excited state and ground
state for the Bodipy compounds 4–6.25 This is clearly evidenced
by the smaller degree of red-shift in the fluorescence emission
band along with the change of the para-substituent at the benzene
moiety from NH2, CH3, to NO2 group for 1, 2, and 3, ca. 12 nm,
in comparison with that for 4, 5, and 6 along with the same order,
ca. 33 nm. Moreover, the fluorescence quantum yields of 1–3 are
significantly higher than those of the corresponding counterparts
4–6, further confirming the enhanced interaction between the
benzene and Bodipy moieties in the latter series of compounds
4–6 due to the free rotation of benzene moiety, which in turn leads
to a larger extent of excited energy loss from the excited Bodipy
moiety via non-irradiative decay for 4–6 than 1–3.26
It is worth noting that the main absorption band for 1–3 is
obviously blue-shifted in comparison with that for 4–6 due to
the bigger HOMO–LUMO gap of the former series of three
compounds, Table 2. Nevertheless, along with the change of the
para-substituent at the C-8 attached benzene moiety from NH2,
CH3, to NO2 group, the main absorption band in their electronic
absorption spectra takes a slight red-shift, ca. 5 nm, from NH2-
substituted 1 to NO2-substituted 3. However, along with the
same order, the main absorption band takes a more significant
degree of red-shift, ca. 11 nm, from NH2-substituted 4 to NO2-
substituted 6. As mentioned in the DFT calculation section, the
LUMO energy level is primarily located on the Bodipy core in
1–3 with almost perpendicular configurations between the Bodipy
core and the benzene moiety, indicating the little effect of benzene
moiety on the electronic absorption band of 1–3 involving the
S0–S1 transition. In contrast, the LUMO energy level in 4–6 with
smaller dihedral angles between the Bodipy core and the benzene
moiety is partly delocalized into benzene moiety, resulting in
the dependence of electronic absorption band involving the S0–
S1 transition on the benzene moiety in 4–6. As a consequence,
the change in the electron-donating/withdrawing ability of para-
substituent of benzene moiety from NH2 to NO2 via CH3 induces
a much more significant effect on the main electronic absorption
band for the series of 4–6 in comparison with 1–3, Fig. 3, revealing
the enhanced interaction between the Bodipy core and the benzene
moiety for 4–6.
It is also noteworthy that the electron-donating/withdrawing
property of the para-substituent of the benzene moiety has an
obvious effect on the fluorescence properties of 1–6. As can be
seen in Fig. 3 and Tables 2 and S1 (ESI†), the shift between the
two corresponding counterparts with the same para-substituent
of benzene moiety but different molecular conformation ranges
from ca. 10 nm for NH2-substituted 1 vs. 4, ca. 12 nm for CH3-
substituted 2 vs. 5, to ca. 28 nm for NO2-substituted 3 vs. 6. More
remarkably, the fluorescence quantum yield of the Bodipy com-
pounds is closely related to the electron-donating/withdrawing
properties of the benzene moiety in 1–6 due to their modulation
on the MO energy level as mentioned above, which in turn exhibits
influence on the electron transfer between the benzene and Bodipy
moieties. As listed in Tables 2 and S1,† along with the change in
the para-substituent of benzene moiety from the electron-donating
group of NH2 and CH3 for 1 and 2 to electron-withdrawing group
of NO2 for 3, the fluorescence quantum yield largely decreases
from 1 and 2 to 3 in apolar solvent such as toluene and CH2Cl2.
However, in polar solvents like methanol, DMF, acetonitrile, and
THF, the fluorescence quantum yield of 1 was markedly decreased
ca. 30-fold. In contrast, the corresponding fluorescence quantum
yield for 2 or 3 displayed a slight decrease comparable to that in
apolar solvent. This is also true for the series of compounds 4–6.
According to our calculation results, when the strong electron-
withdrawing NO2 group is introduced onto the para-position of
the benzene moiety in 3 and 6, the LUMO energy of the electron-
deficient benzene moiety gets decreased, being close to the LUMO
of Bodipy fluorophore. This induces an electron transfer from
the excited Bodipy moiety to the benzene moiety in these two
compounds, resulting in the fluorescence quenching of the Bodipy
fluorophore.27 In contrast, when the electron-donating CH3 group
is introduced onto the para-position of the benzene moiety in 2
and 5, the LUMO energy level of the benzene moiety is increased,
becoming further higher than that of Bodipy fluorophore, which in
turn inhibits the electron transfer from the LUMO of the excited
Bodipy fluorophore to the LUMO of the benzene moiety and
therefore switches on the fluorescence of Bodipy moiety for 2 and
5 in these six solvents, Tables 2 and S1 (ESI†).
Fluorescence properties
To systematically investigate the effect of the molecular configu-
ration and the electron-donating/withdrawing ability of benzene
moiety on the electron transfer-originated fluorescence properties
of Bodipy derivatives, the steady-state fluorescence emission
spectra of 1–6 were measured in the same six kinds of solvents
as done for their electronic absorption spectra and the data are
summarized in Tables 2 and S1 (ESI†). The fluorescence emission
spectra of 1–6 display a clearly Stokes-shifted band relative to
their main electronic absorption band with the Stokes-shift of ca.
13 nm for NH2- or CH3-substituted 1 and 2, ca. 20 nm for NO2-
substituted 3, ca. 17 nm for NH2- or CH3-substituted 4 and 5, and
37 nm for NO2-substituted 6, Fig. 3. Obviously, the fluorescence
emission band for 1–3 takes a relatively less extent of Stokes-
shift in comparison with corresponding counterpart in 4–6. This
is attributed to the nearly orthogonal configuration between the
Bodipy core and the benzene moiety associated with two methyl
substituents at the C-1 and C-7 positions of the Bodipy core in the
former series of three compounds, which prevents the free rotation
of a benzene moiety along the single C–C bond connecting the
benzene and Bodipy moieties. In contrast, in the latter series of
three compounds 4–6, the free rotation of the benzene ring relative
to the Bodipy moiety due to the absence of two corresponding
methyl groups renders it possible to induce an enhanced interac-
tion between the benzene and Bodipy moieties. As a consequence,
a relatively larger Stokes-shift in the fluorescence emission band
was observed for 4–6 than corresponding counterparts 1–3 due
This should also be true for the Bodipy compounds 1 and 4 with
a much stronger electron-donating group of NH2 incorporated
onto the para-position of the benzene moiety. However, as
mentioned above, the fluorescence properties of 1 and 4 are
revealed to be intensively dependent on the polarity of the solvents.
When 1 and 4 were dissolved into the apolar solvents like toluene
and CH2Cl2, intensive fluorescence was observed for these two
compounds, similar to 2 and 5. In contrast, when 1 and 4 were
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dissolved in polar solvents, including THF, CH3CN, DMF, and
MeOH, the intermolecular interaction between the NH2 group
and solvent molecules, in particular hydrogen bond interactions,
induced the photo-induced electron transfer between the excited
Bodipy moiety and the benzene moiety, as shown in 3 and 6,
resulting in fluorescence quenching from the Bodipy moiety for 1
and 4.16
between Bodipy moiety and benzene moiety, indicating the effect
of the molecular configuration on the redox properties of Bodipy
compounds. Nevertheless, as can be expected, along with the
change of the para-substituent from the electron-donating NH2
and CH3 group to the electron-withdrawing NO2 group, the quasi-
reversible reduction potential increases in the order of 1 < 2 < 3,
which is also true for the latter series of 4 < 5 < 6, Table 2,
indicating that the electron-withdrawing NO2 group increases the
redox potential, which is beneficial to the photo-induced electron
transfer between the excited Bodipy and nitrophenyl moieties.
Electrochemistry properties
The redox behaviors of 1–6 were studied by cyclic voltammetry
in CH2Cl2 and the electrochemical data are listed in Table 2, Fig.
4 and Fig. S4.† Within the electrochemical window of CH2Cl2,
all these compounds exhibited one irreversible one-electron ox-
idation and one quasi-reversible one-electron reduction, which
are attributed to removal from or addition of one electron to
the frontier molecular orbitals of 1–6. Comparison of the redox
behavior between the two counterparts with the same para-
substituent of benzene moiety but different dihedral angle between
Bodipy moiety and benzene moiety, namely 1 vs. 4 (-1.640 V vs.
-1.540 V), 2 vs. 5 (-1.531 V vs. -1.490 V) and 3 vs. 6 (-1.394 V
vs. -1.344 V) reveals that compounds 1–3 with almost orthogonal
configuration are harder to get one electron to be reduced than
the corresponding counterparts 4–6 with lower dihedral angles
Conclusion
Briefly summarizing above, a series of six Bodipy derivatives have
been structurally characterized by single crystal X-ray diffraction
analysis. The compounds 1–3 with two methyl groups at the C-
1 and C-7 positions of Bodipy core were revealed to employ the
relatively rigid molecular configuration with an almost orthogonal
dihedral angle between the Bodipy and benzene moieties. In
contrast, compounds 4–6 without two methyl substituents take
the configuration with a free-rotating benzene moiety relative to
the Bodipy core, leading to an enhanced interaction between these
two moieties in 4–6 in comparison with in 1–3. The resulting larger
HOMO–LUMO gap for 1–3 than for 4–6 leads to blue-shifted
Fig. 4 Cyclic voltammetry of NH2-substituted 1 and 4 as well as NO2-substituted 3 and 6 in CH2Cl2 containing 0.1 M [NBu4][ClO4] at the scan rate of
20 mV s-1.
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absorption and fluorescence emission bands, and in particular
higher fluorescence quantum yield for the former series of three
compounds, revealing the effect of molecular configuration on
the spectroscopic properties of Bodipy derivatives. This effect gets
additional support from the redox behavior of these two series
of Bodipy compounds. The present result appears to represent
the first systematic study towards understanding the effect of the
molecular configuration on the optical properties, which should
be helpful for designing and preparing novel Bodipy derivatives
with potential applications in chemical and biological fields.
CCD diffractometer with a Mo-Ka sealed tube (l = 0.71073
◦
˚
A) at 293 K, using a w scan mode with an increment of 0.3 .
Final unit cell parameters were obtained by global refinements
of reflections obtained from integration of all the frame data.
The collected frames were integrated using the preliminary cell-
orientation matrix. The SMART software was used for collecting
frames of data, indexing reflections, and determination of lattice
constants; SAINT-PLUS for integration of intensity of reflections
and scaling; SADABS for absorption correction; and SHELXL for
space group and structure determination, refinements, graphics,
and structure reporting. CCDC reference numbers 798782–798787
for six Bodipy compounds 1–6. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1pp00001b
Experimental
General chemicals
Synthesis 4,4-difluoro-8-(4-methylphenyl)-3,5-dimethyl-4-bora-
3a,4a-diaza-s- indacene (5)
Column chromatography was carried out on silica gel (Merck,
Kieselgel 60, 70–230 mesh) with the indicated eluents. All other
reagents and solvents were used as received. Dichloromethane was
freshly distilled from CaH2 under nitrogen. The compounds 1, 2, 3,
4, and 6 were prepared according to the published procedures.16–17
To the mixture of 4-methylbenzaldehyde (120 mg, 1 mmol) and 2-
methylpyrrole (160 mg, 2.00 mmol) dissolved in CH2Cl2 (100 mL),
one drop of TFA was added. 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 (40 mL) was added and the reaction
mixture was further stirred for another 15 min. After the addition
of N,N-diisopropylethylamine (DIEA) (2 mL) into the mixture for
5 min, the BF3-OEt2 (2.0 mL) was added into the reaction mixture
and stirring was continued for another 50 min. The resulting
mixture was evaporated, and the residue was chromatographed on
a silica gel column using CH2Cl2/hexane (1 : 2) as eluent. Repeated
chromatography followed by recrystallization from CH2Cl2 and
MeOH gave the target compound 5 as black-green crystals, 84 mg
(27%). 1H NMR (CDCl3, 400 MHz): d 7.375 (d, 2H, J = 7.2 Hz),
7.267 (d, 2H, J = 14.6 Hz), 6.716 (d, 2H, J = 2.4 Hz), 6.245 (d, 2H,
J = 3.2 Hz), 2.637 (s, 6H), 2.429 (s, 3H); MS (MALDI-TOF): an
isotopic cluster peaking at m/z 310.15, [Calcd. For M+ 310.14];
Anal. calcd for C18H17BF2N3: C, 69.71; H, 5.52; N, 9.03%. Found:
C, 69.05; H, 4.82; N, 9.74.
General instruments
1H NMR spectra were recorded on a Bruker DPX 400 spectrom-
eter (400 MHz) in CDCl3 using the residual solvent resonance
of CDCl3 at 7.26 ppm relative to SiMe4 (0.00 ppm). MALDI-
TOF mass spectra were taken on a Bruker BIFLEX III ultra-high
resolution Fourier transform ion cyclotron resonance (FT-ICR)
mass spectrometer with alpha-cyano-4-hydroxycinnamic acid as
matrix. Elemental analyses were performed on an Elementar
Vavio El III. Electronic absorption spectra were recorded on
a Hitachi U-4100 spectrophotometer. Steady-state fluorescence
spectroscopic studies were performed on an F 4500 (Hitachi). The
slit width was 2.5 nm for emission. The photon multiplier voltage
was 700 V. The relative quantum efficiencies of fluorescence 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.16
Acknowledgements
Electrochemical measurements were carried out with a BAS
CV-50W voltammetric analyzer. The cell comprised inlets for a
glassy carbon disk working electrode of 2.0 mm in diameter and a
silverwire counter electrode. The reference electrode was Ag/Ag+
{a solution of 0.01 M AgNO3 and 0.1 M [NBu4][ClO4] in acetoni-
trile}, which was connected to the solution by a Luggin capillary
whose tip was placed close to the working electrode. Results were
corrected for junction potentials by being referenced internally to
the ferrocenium/ferrocene (Fe+/Fe) couple [E1/2(Fe+/Fe) = +0.50
V vs. SCE]. Potentials in this paper are referenced to the SCE.
Typically, a 0.1 M solution of [NBu4][ClO4] in CH2Cl2 containing
0.5 M of sample was purged with nitrogen for 10 min, and then
the voltammograms were recorded at ambient temperature. The
scan rate was 20 mV s-1.
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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