Conformationally restricted aza-dipyrromethene boron difluorides (aza-BODIPYs) with high fluorescent quantum yields

Article abstract of DOI:10.1002/asia.201301362

A simple approach to the highly fluorescent near-infrared aza-BODIPY dyes with higher fluorescence quantum yields (up to 0.81 in toluene) in comparison with their known analogues is presented. Our approach is based on the restricted rotations of the 1,7-p

Full text of DOI:10.1002/asia.201301362

FULL PAPER
DOI: 10.1002/asia.201301362
Conformationally Restricted Aza-Dipyrromethene Boron Difluorides (Aza-
BODIPYs) with High Fluorescent Quantum Yields
Lijuan Jiao,* Yayang Wu, Yin Ding, Sufan Wang,* Ping Zhang, Changjiang Yu, Yun Wei,
Xiaolong Mu, and Erhong Hao*[a]
Abstract: A simple approach to the
highly fluorescent near-infrared aza-
BODIPY dyes with higher fluores-
cence quantum yields (up to 0.81 in tol-
uene) in comparison with their known
analogues is presented. Our approach
is based on the restricted rotations of
the 1,7-phenyl groups to the mean
plane of the aza-BODIPYs, which is
achieved through the installation of
bulky substituents on the 1,7-phenyl
groups of aza-BODIPYs and results in
a reduced nonradiative relaxation pro-
cess in solution. The large torsion
angles between the 1,7-phenyl groups
and the aza-BODIPY core (f1 and f2
in these novel conformationally re-
stricted aza-BODIPYs) were con-
firmed by X-ray diffraction studies.
Keywords: BODIPY · N ligands ·
dyes/pigments
heterocycles
·
fluorescence
·
Introduction
yield (0.36 in CHCl3) and is now widely used in bioprobes
for imaging and sensing.[12]
Highly fluorescent dyes emitting in the far-red or near-infra-
red (NIR) region are preferred for biological imaging be-
cause they offer maximum light penetration through skin
and tissue with minimum scattering[1] and have found impor-
tant applications in materials and medical sciences, for ex-
ample, as biological sensing and imaging agents. Aza-dipyr-
romethene boron difluoride (aza-BODIPY), a structural an-
alogue of BODIPY,[2–6] has recently gained increased re-
search attention due to its efficient synthesis and the fine
tuning of its photochemical properties through structural
modifications.[7–11] For example, aza-BODIPY A1 (Figure 1)
absorbs and emits at l=650 and 682 nm, respectively,
whereas the luminophore of aza-BODIPY with the longest
wavelength (emitting at l=850 nm) reported to date has
been achieved through pyrrolidyl substitution combined
with full chelation of the boron atom of aza-BODIPY.[11c] In
contrast, the simple installation of two methoxyl groups on
the para-position of the 3,5-phenyl groups (aza-BODIPY
A2, Figure 1) can also redshift the absorption and emission
to l=688 and 723 nm, respectively. Aza-BODIPY A2, de-
veloped by OꢀShea et al., has a good fluorescence quantum
[a] Prof. Dr. L. Jiao, Y. Wu, Y. Ding, Prof. Dr. S. Wang, P. Zhang, C. Yu,
Y. Wei, X. Mu, Prof. Dr. E. Hao
The Key Laboratory of Functional Molecular Solids
Ministry of Education
Anhui Key Laboratory of Molecule-Based Materials
School of Chemistry and Materials Science
Anhui Normal University
Wuhu, 241000 (China)
E-mail: jiao421@mail.ahnu.edu.cn
sfwang@mail.ahnu.edu.cn
haoehong@mail.ahnu.edu.cn
Figure 1. Chemical structure and photophysical properties of representa-
tive BODIPYs and aza-BODIPYs. f and f1–f4 are the torsion angles be-
tween the aromatic rings and the plane of the central chromophores.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301362.
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Lijuan Jiao, Sufan Wang, Erhong Hao et al.
In comparison with BODIPYs, which generally show high
fluorescent quantum yields, aza-BODIPYs generally give
a much lower fluorescent quantum yield (less than 0.5) in
common solvents, despite the approximately 90 nm redshifts
in their main absorption band with respect to the BODIPY
analogues. Lindsey et al. have reported that the relative ro-
tation of the meso-aryl group to the mean plane of the di-
pyrrin (defined as the dihedral angle f in meso-aryl BODI-
PYs) can greatly affect the excited-state dynamics of the
dyes (Figure 1).[13] For example, the fluorescence quantum
yield of BDP1 is much lower than that of its more substitut-
ed analogue BDP2.[14] These differences are attributed to
two methyl groups on the meso-phenyl that prevent free ro-
tation of the phenyl group and thus reduce the loss of
energy from excited states through nonradiative molecular
relaxation.
the tuning of the relative rotation of phenyl groups to the
BODIPY core.[18] Fluorescence changes in A1 were record-
ed in relation to the variation of the ratio of methanol to
glycerol. A fluorescence enhancement was observed as the
ratio of glycerol to methanol was increased (Figure 2). This
With this consideration in mind, we decided to study the
influence of the torsion angles (f1–f4, Figure 1) on the fluo-
rescent quantum yields of aza-BODIPYs. In contrast to
BODIPYs, there are four torsion angles to be considered in
aza-BODIPY. Of these four angles, f1 and f2 in the solid-
state conformation of the reported aza-BODIPYs are very
small (e.g., f1 and f2 of aza-BODIPY A2 are 14.4 and 7.38,
respectively; Table 1), whereas f3 and f4 are very large due
Figure 2. Fluorescence changes in the spectrum of aza-BODIPY A1 as
the ratio between methanol and glycerol was varied. Excitation at l=
610 nm.
indicates that the restricted conformation of aza-BODIPY
could indeed lead to fluorescence enhancement.
Encouraged by this positive result, we synthesized confor-
mationally restricted 1,7-phenyl aza-BODIPYs C1–C3,
which were achieved in three steps from an aldol condensa-
tion between aldehydes and ketones, a subsequent Michael
addition of nitromethane, and a final condensation with am-
monium acetate and BF3 complexation reactions
(Scheme 1). Most of these reactions gave high yields, except
for the final condensation of ammonium acetate, which only
gave yields of around 10% when performed in alcohol or in
Table 1. Torsion angles in reference compound A2[4a] and C1–C3 ob-
tained from x-ray crystallography.
f1 [8]
f2 [8]
f3 [8]
f4 [8]
A2
C1
C2
C3
14.4
62.9
71.0
66.0
7.3
38.8
45.5
19.5
35.5
33.9
34.8
19.5
30.9
86.1
71.0
67.0
[10b]
À
to the C H···F hydrogen bonding.
In addition, the copla-
narity of the 3,5-phenyl groups with the core of aza-
BODIPY is essential to retain the long wavelength absorp-
tion and emission properties of these dyes, as demonstrated
in aza-BODIPY B1 by Carreira and Zhao[15], in aza-
BODIPY B2 by Burgess and OꢀShea[16] and Kovtun
et al.[11c], and in 3,5-di-, 1,7-di-, and 1,3,5,7-tetrathiophene
aza-BODIPYs by Xiao et al.[17]. However, 3,5-phenyl-re-
stricted aza-BODIPYs B1 and B2 exhibit reduced fluores-
cent quantum yields in comparison with analogue A2.
Therefore, to achieve highly fluorescent NIR aza-BODIPY
dyes, we investigated aza-BODIPYs C1–C3, in which bulky
groups were only installed on the 1,7-phenyl groups with the
3,5-phenyl groups left unmodified; aza-BODIPYs A1 and
A2 were used as reference compounds.
Results and Discussion
Initially, we investigated the sensitivity of the fluorescence
of reference compound A1 to the solvent viscosity of the
system, given that BDP1 and its derivatives have been used
as excellent fluorescent probes for solvent viscosity through
Scheme 1. Syntheses of aza-BODIPYs C1–C3.
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solvent-free conditions. By changing the solvent to acetic
acid, the yields were improved to 20 to 30%. All new com-
pounds were characterized by using NMR spectroscopy and
HRMS.
Single crystals of aza-BODIPYs C1–C3 suitable for X-ray
analysis were obtained by slow evaporation of solutions of
these compounds in dichloromethane. As shown in Figure 3
Figure 4. Intermolecular packing of aza-BODIPY C2.
Table 2. Photophysical properties of aza-BODIPYs in different solvents.
[a]
Solvent
labs [nm]
(loge)
lem
Stokes shift
ff[a,b]
[nm]
[cmÀ1
]
C1 toluene
638 (4.86)
665
666
660
705
704
704
723
724
721
684
682
673
723
723
720
636
733
696
609
698
743
641
744
729
672
722
645
599
704
710
0.76Æ0.02
0.71Æ0.02
0.33Æ0.01
0.81Æ0.01
0.73Æ0.02
0.51Æ0.02
0.67Æ0.02
0.71Æ0.03
0.48Æ0.02
0.44Æ0.02
0.34Æ0.01
0.17Æ0.02
0.39Æ0.02
0.36Æ0.01
0.26Æ0.02
chloroform 635 (4.89)
methanol
C2 toluene
chloroform 671 (4.96)
methanol
C3 toluene
chloroform 687 (4.94)
methanol
A1 toluene
chloroform 650
methanol
A2 toluene
631 (4.88)
676 (4.99)
669 (4.97)
691 (4.98)
685 (4.95)
654
645
693
chloroform 688
methanol
685
[a] Aza-BODIPYs C1 and A1 were excited at l=610 nm, aza-BODIPYs
C2, C3, and A2 were excited at l=620 nm. [b] The fluorescence quantum
yields were calculated by using aza-BODIPY A1 in CHCl3 (f=0.34) as
the standard.
Figure 3. X-ray crystal structures of aza-BODIPYs C1–C3.
molecular hydrogen bonds were also observed, which facili-
tate the crystal packing in a head-to-tail fashion for aza-
BODIPY C2 as shown in Figure 4.
and Table 1, the aza-BODIPY cores in the three compounds
remain essentially unperturbed and planar. As expected,
target molecules C1–C3 have larger torsion angles in the
solid state (f1 =f2 =718 in 1,7-(2,4,6-trimethylphenyl)aza-
BODIPY C2; f1 =66 and f2 =678 in 1,7-(2,6-dichloropheny-
l)aza-BODIPY C3) than those of aza-BODIPY A2 (14.4
and 7.38 for f1 and f2, respectively). This indicates that the
installation of methyl or chloro groups on the 1,7-phenyl
groups can restrict the rotation of the 1,7-phenyl groups rel-
ative to the aza-BODIPY core.
Photophysical properties of aza-BODIPYs C1–C3 and
reference compounds A1 and A2 were measured in toluene,
chloroform, and methanol as listed in Table 2. The fluores-
cence quantum yields of conformationally restricted dyes
C1–C3 are nearly double those of aza-BODIPYs A1 and A2
in the three different solvents. For example, in toluene, the
fluorescence quantum yields of aza-BODIPYs C1–C3 are
0.76, 0.81, and 0.67, respectively, which are the highest re-
ported fluorescence quantum yields for aza-BODIPYs so
far.
In comparison with aza-BODIPY A1, aza-BODIPY C1
shows blueshifts of 14 to 16 nm and 13 to 9 nm in the ab-
sorption and emission, respectively. Similarly, blueshifts of
16 to 17 nm and 16 to 19 nm in the absorption and emission
were observed for aza-BODIPY C2 in comparison with aza-
A strong intramolecular interaction between the two fluo-
rine atoms of the BF2 moiety and the 2,6-protons of the 3,5-
phenyl groups were observed in aza-BODIPYs C1–C3, with
contacts ranging from 2.15 to 2.62 ꢂ. These hydrogen bonds
affect the f3 and f4 angles of aza-BODIPYs C1–C3 in the
solid state, which may restrict the free rotation of phenyl
groups at the 3- and 5-positions.[10b,19] Multiple C H···F inter-
À
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Lijuan Jiao, Sufan Wang, Erhong Hao et al.
BODIPY A2. These results, in agreement with the X-ray
analysis, indicate that the installation of 2,6-dimethyl sub-
stituents on the 1,7-phenyl groups indeed leads to an in-
crease in the dihedral angles f1 and f2 for aza-BODIPYs
C1 and C2 compared with aza-BODIPYs A1 and A2, which
results in a blueshift in the absorption and the emission
spectra in solution.
However, aza-BODIPY C3, which has electron-withdraw-
ing chloro substituents attached to the 2,6-positions of the
1,7-phenyl groups, shows redshifted spectra in comparison
with aza-BODIPY C2, which in turn gives a very similar ab-
sorption and emission spectra to those of widely used aza-
BODIPY A2, as shown in Figure 5. Therefore, aza-
BODIPY C3 with a higher fluorescence quantum yield
Figure 6. Ground state frontier orbitals for A1 and C1.
Ha and Hb on the 1,7-phenyl groups of A1 is 3.7 ꢂ, which
would have no effect on the free rotation of these 1,7-
phenyl groups. In contrast, the separation between protons
Ha and Hb on 1,7-phenyl groups on C1 is only 0.7 ꢂ, which
would greatly restrict the rotation of the 1,7-phenyl groups
(Figure S2 in the Supporting Information). There is an obvi-
ous electron delocalization from the aza-BODIPY core to
the 1,7-phenyl groups for A1, whereas only negligible distri-
bution of electron density from the aza-BODIPY core to
the 1,7-phenyl groups was observed for C1 in both the
ground and excited state (Figure 6 and Figure S1 in the Sup-
porting Information). This is in good agreement with the
blueshift observed in the spectra of C1 in solution.
Conclusion
Figure 5. Normalized absorption (a) and emission spectra (b) of aza-
BODIPYs A1 (black), A2 (blue), C1 (red), C2 (green), and C3 (magen-
ta) in toluene. Inset: Image of A1 and C1 in CHCl3 under daylight irradi-
ation conditions.
In summary, by introducing methyl and chloro substituents
on the 1,7-phenyl groups of aza-BODIPYs, three novel con-
formationally restricted aza-BODIPYs were synthesized and
characterized by using X-ray analysis and DFT calculations.
These conformationally restricted aza-BODIPYs all give
large torsion angles between the 1,7-phenyl groups and the
aza-BODIPY core, with the planarity of the core structure
remaining unaffected. These bulky groups prevent the free
rotation of the 1,7-phenyl groups of aza-BODIPYs, reduce
the nonradiative relaxation process, and result in a signifi-
cantly high fluorescence quantum yield (up to 0.81 in tolu-
ene) in three different solvents studied.
would be a good substituent for aza-BODIPY A2 in various
applications.
DFT calculations were performed for A1 and C1
(Figure 6; Figures S1–S3 and Table S1 in the Supporting In-
formation). A small out-of-plane distortion of the aza-dipyr-
rin frame (w within 6–108 for A1 and C1) was observed
(Figure S3 in the Supporting Information). In the optimized
ground-state conformation, the separation between protons
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Lijuan Jiao, Sufan Wang, Erhong Hao et al.
Compound 2c was synthesized from 1c (6.1 g, 20 mmol) and obtained as
a yellow oil (89% yield, 6.5 g). 1H NMR (300 MHz, CDCl3): d=7.94 (d,
J=8.5 Hz, 2H), 7.38–7.12 (m, 3H), 6.92 (d, J=8.5 Hz, 2H), 5.31–5.27 (m,
1H), 5.11–4.94 (m, 2H), 3.86 (s, 3H), 3.73–3.62 ppm (m, 2H); 13C NMR
(75 MHz, CDCl3): d=195.0, 163.9, 134.6, 130.4, 130.1, 129.4, 129.3, 129.2,
113.9, 76.3, 55.5, 38.5, 35.5 ppm; HRMS (APCI): m/z calcd for
C17H15Cl2NO4: 368.0451; found: 368.0450 [M+H]+.
Experimental Section
The NMR spectroscopic experiments were performed by using
a 300 MHz NMR spectrometer at RT. Chemical shifts (d) are given in
ppm relative to TMS. High-resolution mass spectra (HRMS) were ob-
tained by using an APCI-TOF spectrometer in positive mode. UV/Vis
absorption spectra and fluorescence emission spectra were recorded by
using commercial spectrophotometers (Shimadzu UV 2450 and Hita-
chi F4500, 190–900 nm scan range). The slit width was set at 2.5 nm for
excitation and 5.0 nm for emission. Relative fluorescence quantum yields
were calculated by using A1 in CHCl3 (f=0.34) as the standard. All f
values are corrected for changes in refractive index by using a previously
reported method.[20] Crystals of aza-BODIPYs C1–C3 suitable for X-ray
analysis were obtained by slow evaporation of their solutions in dichloro-
methane. X-ray intensity data measurements were carried out by using
a SMART APEX II CCD diffractometer with graphite-monochromated
radiation (MoKa =0.71073 ꢂ) at 297 K.[21, 22] CCDC 958519 (C1),
CCDC 958520 (C2), and CCDC 958521 (C3) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
General synthetic procedure for C1–C3
The synthesis of C1 is used as an example. Ammonium acetate (7.5 g,
100 mmol) was added to 2a (2.2 g, 6.6 mmol) in acetic acid (20 mL). The
resulting mixture was heated at reflux for 4 h, then the solvent was re-
moved under vacuum. The residue was purified by using column chroma-
tography on silica (eluent dichloromethane/hexane (1:1 v/v)) to give the
aza-dipyrromethene as a metallic blue-black powder (33% yield, 0.61 g)
that was directly used for the subsequent BF2 complexation reaction. The
aza-dipyrromethene (0.54 g, 1 mmol) in dichloromethane (100 mL) was
treated with triethylamine (2 mL) and borontrifluoride diethyletherate
(3 mL). The resulting mixture was stirred at RT for 6 h, then quenched
by addition of distilled water (100 mL). The organic layers were com-
bined, washed with water (2ꢃ100 mL), dried over anhydrous sodium sul-
fate, and evaporated to dryness under vacuum. Purification was per-
formed by using column chromatography on silica (eluent dichlorome-
thane/hexane (1:2 v/v)) to give C1 as a green solid (91% yield, 0.53 g).
1HNMR (300 MHz, CDCl3): d=8.07–8.04 (m, 4H), 7.51–7.48 (m, 6H),
6.87 (s, 4H), 6.67 (s, 2H), 2.28 (s, 6H), 2.14 ppm (s, 12H); 13C NMR
(75 MHz, CDCl3): d=159.6, 146.8, 146.0, 137.7, 136.8, 131.6, 129.7, 129.4,
128.7, 128.2, 123.9, 110.0, 21.1 ppm; HRMS (APCI): m/z calcd for
C38H34BF2N3: 581.2814; found: 581.2821 [M+H]+.
General synthetic procedure for 1a–c
The synthesis of 1a is given as an example. KOH (3 g) was added to
2,4,6-trimethylbenzaldehyde (10.0 mL, 0.067 mol) and acetophenone
(7.8 mL, 0.067 mol) in anhydrous methanol (50 mL). The mixture was
stirred at RT for 1 h. The precipitate was filtered, washed with methanol,
and dried under vacuum to give 1a as a light yellow solid (89% yield,
1
15.0 g). H NMR (300 MHz, CDCl3): d=8.02–7.97 (m, 3H), 7.60–7.51 (m,
3H), 7.16 (d, J=15.6 Hz, 1H), 6.94 (s, 2H), 2.41 (s, 6H), 2.32 ppm (s,
3H); 13C NMR (75 MHz, CDCl3): d=190.5, 143.3, 138.6, 138.2, 137.2,
132.8, 131.6, 129.4, 128.7, 128.6, 127.2, 21.3, 21.2 ppm; HRMS (APCI): m/
z calcd for C18H18O: 251.1430; found: 251.1428 [M+H]+.
Compound C2 was synthesized from 2b (0.8 g, 2.4 mmol) and ammonium
acetate (3.8 g, 100 mmol) and obtained as a greenish solid (22% overall
yield, 0.34 g). 1H NMR (300 MHz, CDCl3): d=8.08 (d, J=8.4 Hz, 4H),
7.01 (d, J=8.4 Hz, 4H), 6.85 (s, 4H), 6.65 (s, 2H), 3.89 (s, 6H), 2.27 (s,
6H), 2.13 ppm (s, 12H); 13C NMR (75 MHz, CDCl3): d=160.2, 153.1,
148.6, 143.1, 136.0, 135.5, 127.1, 126.8, 124.3, 117.3, 113.8, 54.6, 20.2,
20.1 ppm; HRMS (APCI): m/z calcd for C40H38BF2N3O2: 642.3098;
found: 642.3095 [M+H]+.
Compound 1b was obtained from 2,4,6-trimethylbenzaldehyde (10.0 mL,
0.067 mol) and 4-methoxyacetophenone (10.1 g, 0.067 mol) as a light
1
yellow solid (91% yield, 17.1 g). H NMR (300 MHz, CDCl3): d=8.01 (d,
J=8.4 Hz, 2H), 7.92 (s, 1H), 7.16 (d, J=15.9 Hz, 1H), 6.97 (d, J=8.3 Hz,
2H), 6.92 (s, 2H), 3.88 (s, 3H), 2.39 (s, 6H), 2.31 ppm (s, 3H); 13C NMR
(75 MHz, CDCl3): d=188.8, 163.4, 142.5, 138.4, 137.1, 131.8, 131.1, 130.9,
129.3, 127.2, 113.9, 55.5, 21.3, 21.1 ppm; HRMS (APCI): m/z calcd for
C19H20O2: 281.1536; found: 281.1536 [M+H]+.
Compound C3 was synthesized from 2c (0.9 g, 2.5 mmol) and ammonium
acetate (3.8 g, 100 mmol) and obtained as a greenish solid (16% over
two steps, 0.28 g). 1H NMR (300 MHz, CDCl3): d=8.12 (d, J=8.0 Hz,
4H), 7.31 (d, J=7.8 Hz, 4H), 7.17 (m, 2H), 7.03 (d, J=8.0 Hz, 4H), 6.87
(s, 2H), 3.90 ppm (s, 6H); 13C NMR was not available due to poor solu-
bility; HRMS (APCI): m/z calcd for C34H22BF2Cl4N3O2: 694.0600; found:
694.0602 [M+H]+.
Compound 1c was obtained from 2,6-dichlorobenzaldehyde (8.7 g,
0.05 mol) and 4-methoxyacetophenone (7.5 g, 0.05 mol) as a light yellow
solid (95% yield, 14.5 g). 1H NMR (300 MHz, CDCl3): d=8.05 (d, J=
9.0 Hz, 2H), 7.85 (d, J=18.1 Hz, 1H), 7.68 (d, J=15.2 Hz, 1H), 7.40 (d,
J=9.0 Hz, 2H), 7.24–7.18 (m, 1H), 6.99 (d, J=9.1 Hz, 2H), 3.90 ppm (s,
3H); 13C NMR (75 MHz, CDCl3): d=188.4, 163.7, 137.0, 135.2, 132.8,
131.1, 130.6, 130.4, 129.7, 128.8, 113.9, 55.5 ppm; HRMS (APCI): m/z
calcd for C16H12Cl2O2: 307.0287; found: 307.0285 [M+H]+.
Acknowledgements
This work is supported by the National Nature Science Foundation of
China (grant nos. 21072005, 21272007, and 21372011) and the National
Science Foundation of Anhui Province (1208085MB29).
General synthetic procedure for 2a–c
The synthesis of compound 2a is used as an example. Diethylamine
(15 mL) and nitromethane (15 mL) were added to 1a (5.0 g, 20 mmol) in
anhydrous methanol (50 mL). The resulting solution was heated at reflux
for 24 h, then concentrated under vacuum to afford 2a (93% yield,
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Kiyose, H. Kojima, T. Nagano, Chem. Asian J. 2008, 3, 506; c) L.
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42, 622; d) C. Yu, Y. Xu, L. Jiao, J. Zhou, Z. Wang, E. Hao, Chem.
Eur. J. 2012, 18, 6437; e) Z. Zhang, B. Xu, J. Su, L. Shen, Y. Xie, H.
Tian, Angew. Chem. 2011, 123, 11858; Angew. Chem. Int. Ed. 2011,
50, 11654.
1
5.8 g). H NMR (300 MHz, CDCl3): d=7.93 (d, J=7.6 Hz, 2H), 7.56–7.53
(m, 1H), 7.46–7.43 (m, 2H), 6.85 (d, J=9.0 Hz, 2H), 4.93–4.75 (m, 3H),
3.53 (d, J=1.9 Hz, 2H), 2.44 (s, 6H), 2.24 ppm (s, 3H); 13C NMR
(75 MHz, CDCl3): d=197.2, 137.6, 137.0, 136.2, 135.5, 133.5, 132.7, 131.3,
129.9, 128.8, 128.0, 78.2, 40.5, 33.9, 21.5, 21.2, 20.7 ppm; HRMS (APCI):
m/z calcd for C19H21NO3: 312.1594; found: 312.1593 [M+H]+.
Compound 2b was synthesized from 1b (5.6 g, 20 mmol) and obtained as
a yellow oil (84% yield, 5.7 g). 1H NMR (300 MHz, CDCl3): d=7.89 (d,
J=6.0 Hz, 2H), 6.92 (d, J=9.0 Hz, 2H), 6.83 (s, 2H), 4.89–4.75 (m, 3H),
3.85 (s, 3H), 3.44 (brs, 2H), 2.43 (s, 6H), 2.22 ppm (s, 3H); 13C NMR
(75 MHz, CDCl3): d=195.7, 163.8, 137.6, 136.9, 135.6, 132.9, 131.3, 130.4,
129.9, 129.4, 113.9, 78.3, 55.5, 40.1, 34.1, 21.5, 21.2, 20.7 ppm; HRMS
(APCI): m/z calcd for C20H23NO4: 342.1700; found: 342.1699 [M+H]+.
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Published online: && &&, 0000
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ÝÝ These are not the final page numbers!

FULL PAPER
To dye for: Novel conformationally
restricted aza-BODIPYs with bulky
groups at the 1,7-phenyl groups were
synthesized and characterized by X-ray
diffraction studies and DFT calcula-
tions. These dyes show significantly
higher fluorescence quantum yields
(up to 0.81 in toluene) with respect to
the known analogues.
Fluorescent Probes
Lijuan Jiao,* Yayang Wu, Yin Ding,
Sufan Wang,* Ping Zhang,
Changjiang Yu, Yun Wei,
Xiaolong Mu, Erhong Hao*
&&& — &&&
Conformationally Restricted Aza-
Dipyrromethene Boron Difluorides
(Aza-BODIPYs) with High Fluores-
cent Quantum Yields
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Chem. Asian J. 2014, 00, 0 – 0
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ