Meso -(2-Benzimidazolyl)-substituted BODIPYs: Synthesis, structures and spectroscopic properties

Article abstract of DOI:10.1142/S1088424619501104

A series of meso-(2-benzimidazolyl)-substituted boron dipyrromethene (BODIPY) derivatives 3a-3c and 4 have been synthesized and characterized. The absorption and fluorescence bands of 3a are bathochromically shifted by 36 nm and 61 nm, respectively, compared with those of the meso-phenyl BODIPY in toluene. More importantly, the fluorescence quantum yields of these meso-(2-benzimidazolyl)-substituted BODIPYs (up to 0.45 in toluene) are much higher than those of the previously reported meso-heterocyclic BODIPYs. X-ray crystallographic analysis of the single crystal structure of 3a revealed that the dihedral angle of meso-benzimidazolyl ring and indacene plane (40.47) is smaller compared with that of the meso-tolyl substituted BODIPY (61.4). Replacement of the six-membered ring with a five-membered ring, as well as the absence of hydrogen at the imino-nitrogen, generated the reduced repulsion and the hydrogen bonding interaction. The increased planarity not only provided the substantial delocalization of π electrons and red shifted the absorption and emission bands but also enhanced the fluorescence quantum yield by reducing free rotation induced nonradiative deactivation pathway. Furthermore, 3,5-distyryl coupled BODIPY 4 exhibits a NIR fluorescence band at 712 nm with moderate quantum yield (φF > 0.3) in nonpolar and polar solvents, which indicate that meso-(2-benzimidazolyl) BODIPY acts as a good candidate for post modification toward NIR dyes for biological applications.

Full text of DOI:10.1142/S1088424619501104

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2019; 23: 1–8
DOI: 10.1142/S1088424619501104
Published at http://www.worldscinet.com/jpp/
meso-(2-Benzimidazolyl)-substituted BODIPYs: Synthesis,
structures and spectroscopic properties
Hu Gao, Chenhong Li and Zhen Shen*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210046, China
Dedicated to Professor Atsuhiro Osuka on the occasion of his 65th birthday.
Received 25 May 2019
Accepted 2 August 2019
ABSTRACT: A series of meso-(2-benzimidazolyl)-substituted boron dipyrromethene (BODIPY)
derivatives 3a–3c and 4 have been synthesized and characterized. The absorption and fluorescence
bands of 3a are bathochromically shifted by 36 nm and 61 nm, respectively, compared with those of the
meso-phenyl BODIPY in toluene. More importantly, the fluorescence quantum yields of these meso-
(2-benzimidazolyl)-substituted BODIPYs (up to 0.45 in toluene) are much higher than those of the
previously reported meso-heterocyclic BODIPYs. X-ray crystallographic analysis of the single crystal
structure of 3a revealed that the dihedral angle of meso-benzimidazolyl ring and indacene plane (40.47°)
is smaller compared with that of the meso-tolyl substituted BODIPY (61.4°). Replacement of the six-
membered ring with a five-membered ring, as well as the absence of hydrogen at the imino-nitrogen,
generated the reduced repulsion and the hydrogen bonding interaction. The increased planarity not
only provided the substantial delocalization of π electrons and red shifted the absorption and emission
bands but also enhanced the fluorescence quantum yield by reducing free rotation induced nonradiative
deactivation pathway. Furthermore, 3,5-distyryl coupled BODIPY 4 exhibits a NIR fluorescence band
at 712 nm with moderate quantum yield (FF > 0.3) in nonpolar and polar solvents, which indicate that
meso-(2-benzimidazolyl) BODIPY acts as a good candidate for post modification toward NIR dyes for
biological applications.
KEYWORDS: BODIPY, meso-(2-benzimidazolyl), near IR, intramolecular hydrogen bonds.
quantum yields, moderate redox potentials, negligible
INTRODUCTION
sensitivity to solvent polarity, excellent photostability,
The development of low-cost NIR region excitation
and high solubility in commonly used organic solvents
sources and detectors have inspired the design of new
with different polarities [3–7]. The conventional meso-
fluorophores or fluorescent materials with high molar
aryl substituted BODIPYs absorbed and emitted in the
absorption coefficients and fluorescence quantum yields
470–530 nm region [8]. Three main approaches have
in the 650–1000 nm region. Boron-dipyrromethene
been identified to shift the main absorption and emission
(BODIPY) dyes have received intensive studies over the
bands to long wavelengths [5, 8]: (i) introduction of aryl,
past two decades owing to their wide range of potential
vinyl, styryl and arylethynyl substituents at peripheral
applications in numerous research fields [1, 2]. This
positions, (ii) aromatic ring fusion of the pyrrole moiety,
has been related to their facile synthesis and structural
and (iii) substitution of a carbon atom with a nitrogen
versatility, excellent spectroscopic properties such
atom at the meso-position. Recently, it has been reported
as narrow Gaussian-shaped absorption and emission
that introduction of five membered heterocycles such
bands, high molar extinction coefficients and fluorescent
as pyrrolyl, furyl, thienyl and thiazolyl [9–13] rings
instead of six membered aryl groups at meso-position
of a BODIPY can also affect the optical properties to a
*Correspondence to: Zhen Shen, email: zshen@nju.edu.cn.
Copyright © 2019 World Scientific Publishing Company

2
H. GAO ET AL.
great extent. This is mainly because the meso-phenyl ring
situated almost orthogonally with the indacene plane and
made minor contributions to the conjugation of the whole
molecule. Replacement of the six-membered ring with
a five-membered ring as well as the absence of methyl
substituents at the 1,7-positions of a BODIPY can reduce
the dihedral angle of the meso-substituent and indacene
plane due to the reduced repulsion, thus participating in
the conjugation [10]. Although these meso-heterocyclic
BODIPYs exhibited red-shifted absorption and emission
bands, they were accompanied by a sharply decreased
fluorescence quantum yield (FF < 0.1 in toluene) on
account of the electron-donation effect and the twisting
motion of the meso-heterocyclic rings. This shortcoming
restricts their application for post modification to NIR
fluorophores. It is well known that imidazole has wide
applications in optical materials and biological fields
because of its π-conjugated cyclic system and bioactivity
[14]. Herein we describe the synthesis, structures and
spectroscopic properties of meso-(2-benzimidazolyl)
BODIPY derivatives 3a–3c. Meanwhile, we also
investigated its distyryl coupled dye 4 which emits
strongly in the NIR region with moderate fluorescence
quantum yield.
literature [15]. Recently, Werz and co-workers reported
a method for synthesizing this kind of dipyrromethanes
and their BF2-complexes with the participation of
benzimidazole auxochrome [16]. Compounds 1a–1c
were converted to their respective formaldehydes using
2-iodobenzoic acid (IBX) followed by in situ conden-
sation with 2-methylpyrrole. A mixed medium consis-
ting of DMSO and N-methyl-2-pyrrolidone (NMP) as
well as excess amounts of TFA was used to dissolve the
carbaldehyde intermediates and neutralize the evolving
basicity of products. Compounds 2a–2c were obtained
in 35–50% yields. Finally, the target BODIPYs 3a–3c
were obtained by adding only one-third amount of
.
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and BF3 OEt2
according to the literature in 50–58% yields. The
Knoevenagel condensation reaction of BODIPY 3a and
4-tert-butylbenzaldehyde affords the distyryl expanded
dye 4 in 75% yield [17]. The structures of 3a–3c and 4
1
have been characterized by H, 13C NMR and HR-MS
spectroscopy.
Crystal structure
The structure of 3a was confirmed by X-ray crystallo-
graphic analysis. An orange-colored single crystal
for 3a was obtained by slow diffusion of n-hexane
into a dichloromethane solution at room temperature.
Compound 3a was crystallized in the asymmetric
monoclinic unit cell with P21/c space group and the
crystal structure is shown in Fig. 1. The highly planar
BODIPY framework is composed of two pyrrole rings at
the periphery and a central six-membered ring containing
a coordinated boron atom. The dihedral angle between the
central indacene core and the meso-benzimidazolyl ring is
RESULTS AND DISCUSSION
Synthesis and characterization
The synthetic procedures for BODIPYs 3a–3c, 4
and the dipyrromethane precursors 2a–2c are described
in Scheme 1. The starting materials, 2-hydroxymethyl-
benzimidazole 1a–1c, were prepared according to the
R
R
1) IBX, DMSO,
rt, overnight
1) DDQ, DCM,
-20°C , 15min
R
N
N
NH
N
NH
H
2) 2-methylpyrrole,
NMP, TFA,
rt, 6-8h
N
H
OH
2) DBU, BF3·OEt2,
-78°C to rt, 1h
N
N
NH HN
B
1a R=H
1b R=F
1c R=CN
F
3a-c
F
2a-c
N
NH
PTSA, toluene, piperidine
Reflux, 30 min
N
F
N
F
B
3a
4
tBu
tBu
Scheme 1. Synthetic procedures and structures of BODIPYs 3a–3c and 4
Copyright © 2019 World Scientific Publishing Company
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meso-(2-BENZIMIDAZOLYL)-SUBSTITUTED BODIPYS
3
Fig. 1. X-Ray crystal structure of 3a with the thermal ellipsoids set at 50% probability level. (a) Top view; (b) Side view; (c) dimeric
structure formed by intermolecular hydrogen bonding. Solvent molecules are omitted for clarity
Fig. 2. The UV-vis absorption (a) and normalized emission (b) spectra of dyes 3a (black), 3b (green), 3c (blue) and 4 (red) in toluene
(c = 1 × 10-5 M) (lex = 515 nm for 3a–3c and lex = 645 nm for 4)
40.47°, which is smaller than that of meso-tolyl BODIPY
(61.4°), but larger than that of meso-pyrrolyl BODIPY
(33.94°) [10]. There are two intramolecular hydrogen
bonding interactions between the nitrogen atoms from
the benzimidazolyl ring and the proximal proton atoms
from the indacene core (N1–H10 = 2.589 Å, N2–H15 =
2.685 Å), which imposes restrictions on the twisting
motion of the benzimidazolyl ring and facilitates the
molecule to adopt a stable, partly coplanar conformation.
The enhanced planarity of the whole π-conjugated
structure would promote greater π-electron density
delocalization, and thus enhanced optical characteristics
[12]. In the solid state, the intermolecular hydrogen
Spectroscopic properties in solution
The photophysical properties of compounds 3a–3c and
4 were measured in five solvents of different polarity and
the data were summarized in Fig. S17 and Table S1. The
absorption and emission spectra of all the dyes in toluene
at the same concentration is shown in Fig. 2 and Table 1.
Compounds 3a–3c display absorption maxima at 533, 534
and 537 nm, respectively, with similar intensity, which
can be assigned to a typical S0–S1 transition of BODIPY
[18]. Other higher-energy absorption bands around
300–450 nm are shown broadly, which can be attributed to
S0–S2 or S0–S3 transition. Compound 4 exhibits a narrow
and strong absorption band at 676 nm with an obvious
shoulder at 621 nm, which is red shifted by 143 nm
relative to that of 3a. Meanwhile, the absorbance of 4 is
significantly increased as the molar absorption coefficient
…
bonding interactions N2-H N1 (1.963 Å) (N2-H of a
meso-benzimidazolyl moiety and N1 of another meso-
benzimidazolyl moiety) between the neighboring
BODIPY molecules forms a dimeric structure.
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H. GAO ET AL.
Table 1. Spectroscopic and photophysical properties of dyes 3a–3c and 4 in toluene at 298k
-1
em [nm] emax × 104 [M-1 cm ]
Dn [cm-1]
FF
tf [ns]
.
Solvents
l
abs [nm]
l
3a
Toluene
Toluene
Toluene
Toluene
533
534
537
676
578
582
588
712
5.65
5.7
1461
1544
1615
748
0.45
0.41
0.36
0.31
4.83
4.47
4.05
3.72
3b
3c
4
5.4
10.3
Fig. 3. Energy level diagrams and Kohn–Sham orbital representation of LUMOs (top) and HOMOs (bottom) of 3a–3c and 4 using
the CAM-B3LYP functional and 6-31G (d, p) basis set
-1
at 676 nm (e = 1.03 × 105 M-1 cm ) is 1.82-fold compared
aromatic heterocycle at the meso-position significantly
alters the optical characteristics of BODIPYs. In addition,
the FF values of compounds 3a–3c are much higher
than those of the previously reported meso-heterocyclic
BODIPYs. It is likely that the intramolecular hydrogen
bonds limit the free rotation of the benzimidazolyl ring,
thereby decreasing the incidence of nonradiative decay.
The distyryl coupled compound 4 displays a maximum
fluorescence peak at 712 nm in the NIR region and
maintains FF value over 0.3 in all investigated solvents,
which suggests that meso-(2-benzimidazolyl) BODIPY
can be post-modified to novel NIR BODIPY dyes.
Furthermore, the fluorescence decay profiles of 3a–3c
and 4 can be described by mono-exponential fits with
lifetimes. The radiative (kr) and nonradiative (knr) rate
constants are calculated using FF and fluorescence
lifetime values.
.
to that of 3a at 578 nm (e = 5.65 × 104 M-1 cm ). This
-1
.
is due to the increased π conjugation by coupling two
styryl-groups at 3,5-positions. In addition, the absorption
maximum of 3a–3c and 4 is slightly dependent on
the polarity of the solvent, with slight blue shifts with
the increase of solvent polarity, which is in line with the
properties of BODIPY systems.
The emission spectra were obtained by excitation at
optimal wavelength, which exhibit approximate mirror
symmetry with the main absorption bands. In toluene,
compounds 3a–3c show a maximum fluorescence
peak at 578, 582 and 588 nm, respectively, with decent
quantum yields (FF) of 0.45, 0.41 and 0.36, respectively.
The introduction of electron withdrawing groups (-F and
-CN) on the benzimidazolyl moiety results in a further
red shift in absorption and emission spectra, with a larger
Stokes shift and reduced FF value. Compared with the
meso-phenyl BODIPY, the emission band of 3a is red
shifted by 61 nm and the Stokes shift is increased by
683 cm-1 [19], which fully illustrates that the replacement
of the six-membered aryl group with the five-membered
It is well known that benzimidazole is sensitive to pH
changes and can be protonated in acidic environments
[20]. The pH-dependent absorption and emission spectra
were studied by titration of trifluoroacetic acid (TFA)
into the CH2Cl2 solution of 3a (Fig. S18). Upon addition
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meso-(2-BENZIMIDAZOLYL)-SUBSTITUTED BODIPYS
5
Table 2. Calculated electronic excitation energies, oscillator strengths, and eigenvectors
for the TD-DFT spectra of 3a–3c and 4 carried out using the CAM-B3LYP functional and
6-31G (d, p) basis set
Statea
Eb (eV)
l (nm)
f c
Orbitals(coefficient)d
S1
S2
S1
S2
S1
S3
S1
S2
2.96
3.69
2.89
3.62
2.86
3.86
2.24
3.26
420
336
429
343
434
322
554
381
0.457
0.354
0.444
0.413
0.441
0.357
0.721
1.217
H→L (97%), H-3→L (2%)
H-1→L (89%), H-3→L (6%)
H→L (97%)
3a
3b
3c
4
H-1→L (93%)
H→L (97%)
H-1→L (55%), H-3→L (40%)
H→L (97%)
H-1→L (90%), H-2→L (4%)
a Excited states. b Energy of excited states. c Oscillator strength (f < 0.02 are not included). d MOs
involved in the transitions with H and L denoting the HOMO and LUMO, respectively.
of TFA, the absorption intensity at 533 nm decreased
minor effect on the energy of the HOMO. However, the
meso-substituent has a significant impact on the energy
of the LUMO, since there is a large MO coefficient at
this position. The calculations for compounds 3a–3c
display that the delocalized HOMO distributes only over
the indacene core, while delocalized LUMO distributes
over the indacene core and partial meso-benzimidazolyl
moiety. As the electron withdrawing ability of the
substituent on the benzimidazolyl ring increases, both the
HOMO and LUMO energy levels are further stabilized
with a slightly decreased HOMO–LUMO gap. This
is consistent with the slight red shift of the absorption
and emission spectra of 3b and 3c. Compared with 3a,
the remarkable narrowing of the HOMO–LUMO gap of
compound 4 is mainly owing to the destabilization of the
HOMO via coupling with distylyl substituents, leading
to an apparent red shift of the absorption and emission
bands.
with a new band at 548 nm appearing concomitantly. The
fluorescence quantum yield of 3a decreased almost 4-fold
(22%) in the presence of 0.2 equiv. of TFA, and dropped
to 6% upon addition of 1 equiv. of TFA. This indicates
that the protonated benzimidazole moiety quenches
the fluorescence of the BODIPY significantly. This
phenomenon can be explained by theoretic calculations
(Fig. S19). In the neutral form, the HOMO energy level
of the benzimidazole subunit (-5.89 eV) is slightly lower
than that of the indacene moiety (-5.53 eV), while its
LUMO energy level (-0.22 eV) is much higher than that
of the indacene moiety (-2.51 eV). Therefore, no photo-
induced electron transfer (PET) will take place between
the benzimidazole and indacene core. However, after
protonation, the LUMO energy level (-5.42 eV) of the
protonated benzimidazole becomes much lower than that
of the indacene moiety. Thus, upon photoexcitation of
the indacene core, the intramolecular electron transfer
from the LUMO of the indacene moiety to the LUMO
of the protonated benzimidazole becomes energetically
favorable. The fluorescence of 3a in the protonated form
is quenched through an oxidative PET process.
CONCLUSION
In summary, we have synthesized and characterized a
series of meso-(2-benzimidazolyl) BODIPY derivatives
3a–3c and 4. The absorption and fluorescence bands
of 3a are centered at 533 nm and 578 nm, which
are bathochromically shifted by 36 nm and 61 nm,
respectively, compared with the meso-phenyl BODIPY
in toluene. Surprisingly, its fluorescence quantum yield
is up to 0.45 in toluene, which is much higher than those
of previously reported meso-heterocyclic BODIPYs. It is
likely that the intramolecular hydrogen bonds limit the
rotation of the benzimidazolyl ring, thereby decreasing
the incidence of nonradiative decay. In addition, the
distyryl-coupled BODIPY 4 absorbs at 676 nm and shows
a fluorescence band at 712 nm in toluene along with
FF value over 0.3 in both nonpolar and polar solvents,
DFT calculations
To derive an enhanced understanding of the electronic
and spectroscopic properties of 3a–3c and 4, density
functional theory (DFT) calculations at the CAM-
B3LYP/6-31G (d, p) level were carried out. The plots
and data for the HOMOs and LUMOs are shown in
Fig. 3 and Table 2. The general trends of the calculated
HOMO/LUMO levels and energy gaps are in line with
the experimentally observed trends in solution. In the
HOMO depictions, there is a nodal plane at the meso-
carbon in all meso-(2-benzimidazolyl) BODIPYs,
indicate that the meso-benzimidazole ring has only a
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6
H. GAO ET AL.
which suggests that meso-(2-benzimidazolyl) BODIPY
can be used as a good platform for post modification to
achieve NIR dyes for various applications in material and
biological sciences.
Preparation of BODIPYs (3a–3c). Dipyrromethane
2a (1.00 mmol, 1.0 equiv.) was dissolved in dry
dichloromethane (60.0 mL) under a N2 atmosphere. The
reaction mixture was cooled to -20°C. A suspension
of DDQ (1.1 mmol, 1.1 equiv.) in a mixed solution of
2.00 mL toluene and 2.00 mL dichloromethane was added
to the previous mixture. Stirring continued for 15 min at
-20°C. After cooling to -78°C, 1, 8-diazabicyclo[5.4.0]
undec-7-ene (DBU) (10.0 mmol, 10 equiv.) was added
dropwise under vigorous stirring. After stirring for 5 min,
EXPERIMENTAL
Instrumentation and materials
All chemical reagents were obtained from commercial
suppliers and used directly, unless otherwise mentioned.
The NMR spectra of all compounds were measured
on a Bruker DRX 500 or 400 spectrometer at 298k
and referenced to the residual proton signals of the
CD3Cl or DMSO-d6. The HR-MS data were recorded
(in negative ion mode) on an Agilent 6540 Q-TOF LC/
MS spectrometer. All the solvents employed for the
spectroscopic measurements were of UV spectroscopic
grade (Aldrich).
.
BF3 OEt2 (15.0 mmol, 15 equiv.) was added dropwise
and stirring continued for 5 min. Finally, the reaction
mixture was allowed to warm to room temperature
and stirred for a further 1 h. The reaction mixture was
quenched by distilled water. The resulting solution was
extracted with dichloromethane. The organic layer was
dried over Na2SO4 and evaporated to dryness under
vacuum. The crude residue was separated by column
chromatography (silica gel, petroleum ether/25% ethyl
acetate), eluting 3a as a strongly fluorescent fraction
and recrystallized from dichloromethane/n-hexane to
give 3a as an orange solid. Yield 168 mg (50%). 3a:
1H NMR (500 MHz, DMSO-d6) d: 7.78–7.69 (m, 2H),
7.36 (dd, J = 5.7, 3.6 Hz, 4H), 6.56 (d, J = 4.2 Hz, 2H),
2.59 (s, 6H). 13C NMR (126 MHz, DMSO-d6) d: 158.35,
144.94, 133.03, 131.81, 128.80, 123.58, 120.59, 14.74.
HR-MS (ESI-Q-TOF): m/z C18H14BF2N4-[M–H]- calcd.
335.1285, found 335.1288.
Synthesis
Preparation of dipyrromethanes (2b–2c). 2-Hydroxy-
methylbenzimidazole 1b (2.50 mmol, 1.0 equiv.) was
added to a solution of 2-iodoxybenzoic acid (IBX)
(2.75 mmol, 1.1 equiv.) in 5 mL dimethyl sulfoxide
(DMSO). The mixture was stirred at room temperature
under N2 atmosphere overnight. Next the reaction mixture
was diluted with 10.0 mL N-methyl-2-pyrrolidone
(NMP). Trifluoroacetic acid (TFA) (1.50 mmol, 0.6
equiv.) was added and subsequently the 2-methylpyrrole
(7.50 mmol, 3.0 equiv.) as liquid diluted with little NMP.
Stirring continued for 8 h at room temperature under a N2
atmosphere.Afterwards, the reaction mixture was washed
with saturated NaHCO3 solution first, then with saturated
NaCl solution to remove the remainders of DMSO and
NMP with high boiling points and extracted with ethyl
acetate. The organic layer was dried over Na2SO4 and
evaporated to dryness under vacuum. Finally, the crude
product was purified by column chromatography (silica
gel, dichloromethane/0.5% methanol) and recrystallized
from dichloromethane/n-hexane to give 2b as pink
Compounds 3b and 3c were obtained by following a
procedure similar to that of 3a in 53% and 58% yield,
1
respectively. 3b: H NMR (400 MHz, DMSO) d: 13.38
(b, 1H), 7.91–7.37 (m, 2H), 7.33 (d, J = 4.0 Hz, 2H),
7.30–7.13 (m, 1H), 6.55 (d, J = 4.1 Hz, 2H), 2.59
(s, 6H). 13C NMR (126 MHz, DMSO-d6) d: 160.23,
158.48, 158.34, 146.27, 132.98, 131.76, 128.50, 120.63,
112.18, 111.98, 99.53, 14.74. HR-MS (ESI-Q-TOF): m/z
C18H13BF3N4- [M-H]- calcd. 353.1185, found 353.1191.
3c: 1H NMR (500 MHz, DMSO-d6) d: 8.32 (s, 1H), 7.87
(d, J = 8.4 Hz, 1H), 7.73 (dd, J = 8.4, 1.5 Hz, 1H), 7.32 (d,
J = 4.2 Hz, 2H), 6.57 (d, J = 4.3 Hz, 2H), 2.60 (s, 6H). 13C
NMR (126 MHz, DMSO-d6) d: 159.07, 148.06, 133.09,
131.76, 127.85, 126.58, 120.88, 119.68, 105.20, 14.79.
HR-MS (ESI-Q-TOF): m/z C19H13BF2N5- [M–H]- calcd.
360.1238, found 360.1239.
1
powders. Yield 380 mg (50%). H NMR (400 MHz,
CDCl3) d: 8.74 (s, 2H), 7.39 (dd, J = 8.8, 4.7 Hz, 1H),
7.15 (dd, J = 8.9, 2.5 Hz, 1H), 6.96 (td, J = 9.2, 2.5 Hz,
1H), 5.94 (t, J = 3.0 Hz, 2H), 5.82–5.81 (m, 2H), 5.62 (s,
1H), 2.21 (s, 6H). HR-MS (ESI-Q-TOF): m/z C18H16FN4-
[M–H]- calcd. 307.1364, found 307.1366.
Preparation of BODIPY (4). A solution of meso-(2-
benzimidazolyl) BODIPY 3a (0.15 mmol, 1.0 equiv.)
and 4-tert-butylbenzaldehyde (0.375 mmol, 2.5 equiv.)
in a mixture of 30 mL toluene, p-toluenesulfonic acid
(PTSA) (3 mg) and pyridine (0.2 mL) were placed in a
round-bottom flask equipped with a Dean–Stark trap, and
the mixture was heated under reflux until it evaporated
to dryness. After cooling for 15 min, the resulting
combined solid was dissolved in dichloromethane and
washed with water. The organic layer was dried over
Na2SO4 and evaporated to dryness under vacuum, and the
crude residue was separated by column chromatography
(silica gel, petroleum ether/10% ethyl acetate) and
Compound 2c could not be isolated in pure form,
apparently, because of its high sensitivity to oxidation.
The synthesis follows the general reaction procedure
similar to that of 2b and the reaction time was 6 h.
The crude product, obtained in ca. 35% yield, was
submitted to the next BODIPY synthesis. HR-MS
-
(ESI-Q-TOF): m/z C19H16N5 [M–H]- calcd. 314.1411,
found 314.1402.
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meso-(2-BENZIMIDAZOLYL)-SUBSTITUTED BODIPYS
7
recrystallized from tetrahydrofuran/n-hexane to give 4
as purple powders. Yield 70 mg (75%). H NMR (500
approach was used to calculate the absorption properties
based on the time dependent (TD-DFT) method.
1
MHz, DMSO-d6) d: 7.81–7.72 (m, 4H), 7.65–7.58 (m,
6H), 7.53 (d, J = 8.5 Hz, 4H), 7.48 (d, J = 4.5 Hz, 2H),
7.39 (d, J = 4.5 Hz, 2H), 7.36 (dt, J = 7.2, 3.6 Hz, 2H),
1.32 (s, 18H). 13C NMR (126 MHz, DMSO-d6) d: 154.84,
152.73, 145.45, 138.23, 134.94, 133.43, 130.89, 127.28,
126.07, 125.19, 123.48, 117.89, 117.65, 34.68, 30.96.
HR-MS (ESI-Q-TOF): m/z C40H38BF2N4- [M–H]- calcd.
623.3163, found 623.3165.
Acknowledgments
We gratefully acknowledge the financial support
provided by the National Natural Science Foundation
of China (No. 21771102). The theoretical calculations
were carried out at the Center for High-Performance
Computing in Nanjing University.
Supporting information
X-ray structure determination
A full list of characterization data including HRMS,
1H, 13C NMR spectra, absorption, emission spectra, and
X-ray structure details of reported compounds (Figs
S1–S19 and Table S1) are given in the supplementary
material. This material is available free of charge via the
Internet at http://www.worldscinet.com/jpp/jpp.shtml.
Crystallographicdataforcompound3ahasbeendeposited
at the Cambridge Crystallographic Data Centre (CCDC)
under number CCDC-1909769. Copies can be obtained
on request, free of charge, via www.ccdc.cam.ac.uk/
data_request/cif or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44 1223-336-033 or email: deposit@ccdc.cam.
ac.uk).
The X-ray diffraction data of 3a were collected
on a Bruker Smart 85 Apex-II CCD diffractometer
with graphite monochromated Mo-Ka radiation (l =
0.71073 Å) using the w-2q scan mode. The structure was
solved by direct methods and refined on F2 with the full-
matrix least-squares method using SHELX programs.
All calculations and molecular graphics were carried out
using the SHELX-97 program package and the Mercury
program.
3a. C18H15BF2N4, CH2Cl2: an orange block-like crystal
of the approximate dimensions 0.22 × 0.19 × 0.17 mm3
was measured. Monoclinic, space group P2(1)/c, a =
11.069 (2) Å, b = 17.359 (3) Å, c = 10.1105 (16) Å, a =
90, b = 99.650 (5), g = 90, V = 1915.2 (6) Å3, Z = 4,
-3
.
F(000) = 864.0, r = 1.460 g cm , R1 = 0.0430, wR2 =
0.1223, GOF = 1.013, residual electron density between
0.254 and -0.379 eÅ-3.
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