Synthesis, Structure, and Optical Studies of Donor-Acceptor-Type Near-Infrared (NIR) Aza-Boron-Dipyrromethene (BODIPY) Dyes

Article abstract of DOI:10.1002/asia.201600167

Six donor-acceptor-type near-infrared (NIR) aza-boron-dipyrromethene (BODIPY) dyes and their corresponding aza-dipyrrins were designed and synthesized. The donor moieties at the 1,7-positions of the aza-BODIPY core were varied from naphthyl to N-phenylcarbazole to N-butylcarbazole. The 3,5-positions were also substituted with phenyl or thienyl groups in the aza-BODIPYs. Photophysical, electrochemical, and computational studies were carried out. The absorption and emission spectra of aza-BODIPYs were significantly redshifted (≈100 nm) relative to the parent tetraphenylaza-BODIPY. Fluorescence studies suggested effective energy transfer (up to 93 %) from donor groups to the aza-BODIPY core in all of the compounds under study. Time-dependent (TD)-DFT studies indicated effective electronic interactions between energy donor groups and aza-dipyrrin unit in all the aza-BODIPYs studied. The HOMO-LUMO gap (ΔE) calculated from cyclic voltammetry data was found to be lower for six aza-BODIPYs relative to their corresponding aza-dipyrrins. Donations accepted: Donor-acceptor-type near-infrared aza-BODIPYs were designed and synthesized. The presence of donor groups at the 1,7-position of the aza-BODIPYs caused significant bathochromic shifts in their absorption and emission maxima. Efficient energy transfer from donor moieties to the aza-BODIPY core was observed in all compounds studied.

Full text of DOI:10.1002/asia.201600167

DOI: 10.1002/asia.201600167
Full Paper
Donor–Acceptor Systems
Synthesis, Structure, and Optical Studies of Donor–Acceptor-Type
Near-Infrared (NIR) Aza–Boron-Dipyrromethene (BODIPY) Dyes
[a]
[b]
[b]
[c]
[a]
Naresh Balsukuri, Mohsin Y. Lone, Prakash C. Jha, Shigeki Mori, and Iti Gupta*
Abstract: Six donor–acceptor-type near-infrared (NIR) aza–
boron-dipyrromethene (BODIPY) dyes and their correspond-
ing aza–dipyrrins were designed and synthesized. The donor
moieties at the 1,7-positions of the aza–BODIPY core were
varied from naphthyl to N-phenylcarbazole to N-butylcarba-
zole. The 3,5-positions were also substituted with phenyl or
thienyl groups in the aza–BODIPYs. Photophysical, electro-
chemical, and computational studies were carried out. The
absorption and emission spectra of aza–BODIPYs were sig-
nificantly redshifted (ꢀ100 nm) relative to the parent tetra-
phenylaza–BODIPY. Fluorescence studies suggested effective
energy transfer (up to 93%) from donor groups to the aza–
BODIPY core in all of the compounds under study. Time-de-
pendent (TD)-DFT studies indicated effective electronic inter-
actions between energy donor groups and aza–dipyrrin unit
in all the aza–BODIPYs studied. The HOMO–LUMO gap (DE)
calculated from cyclic voltammetry data was found to be
lower for six aza–BODIPYs relative to their corresponding
aza–dipyrrins.
Introduction
have small Stokes shifts (10–25 nm) and narrow emission in
[
6]
the 520–600 nm range. Synthetic modifications such as the ri-
[
7]
Sunlight is one of the best sources of alternative energy, and
the absorption of the wide-spectrum solar energy is one of the
gidification of difluoroboryl unit, extended aromatic conjuga-
[
8]
[9]
tion, and substitution of electron-rich aromatic groups in
BODIPYs have resulted in redshifted absorption and emission
of such molecules. When the meso-carbon is replaced by
[1]
challenges addressed by the scientists. Artificial light-harvest-
ing antenna systems have been a matter of interest for de-
cades and have inspired research to create better molecules
a meso-nitrogen atom in the dipyrromethene core, then such
[
2]
[5,10]
for this purpose. To mimic natural light-harvesting systems
like chlorophylls, many model compounds based on porphyrin
molecules are called aza–BODIPYs.
The chemistry of aza–
BODIPYs has seen significant growth in the last decade, and
several derivatives in which the thiophene rings have occupied
the 1,7- or 3,5-, or 2,6-positions of the aza–BODIPY core have
[
3]
or phthalocyanines have been synthesized and studied.
Among these model systems, donor–acceptor (D–A)-type por-
[
11]
phyrin–BODIPY (BF -chelated dipyrromethene) conjugates are
been reported. Thiophene substitution, particularly at the
3,5-position of aza–BODIPYs, causes significant redshifts in
their absorption and emission owing to increased conjugation
2
popular because selective excitation of the donor (BODIPY) is
[4]
possible in such systems. BODIPYs have good photostability,
a high molar absorption coefficient, and high quantum yields
in the molecule. Aza–BODIPYs have also been used as chemo-
[
5]
[12a,e]
in the visible region. However, meso-aryl-substituted BODIPYs
sensors;
1,7-bis(2-pyridyl)-3,5-diphenylaza–BODIPY showed
2
+
high selectivity for Hg sensing with redshifted absorption
[
12b]
[
a] N. Balsukuri, Dr. I. Gupta
Indian Institute of Technology Gandhinagar
Village Palaj, Simkheda, Gandhinagar
Gujarat-382355 (India)
and emission maxima.
The crown-ether-substituted aza–
BODIPYs have been used as visible sensors for the detection of
[
12c]
shellfish toxin.
Aza–BODIPYs with thiophene rings at the
E-mail: iti@iitgn.ac.in
1,7-positions and fluorene/N-methylcarbazole substituents at
[
b] M. Y. Lone, Dr. P. C. Jha
School of Chemical Sciences
Central University of Gujarat
Gandhinagar, Gujarat (India)
the 3,5-phenyl positions have been used as fluorescence sen-
2
+
[12d]
sors for Hg ions.
The supramolecular self-assemblies of
aza–BODIPY-fullerene and aza–BODIPY-zinc phthalocyanines
have been successfully synthesized, and electron-transfer stud-
[
c] Dr. S. Mori
Integrated Centre for Sciences
Ehime University
[
13]
ies were reported by D’Souza and co-workers. Burgess and
co-workers have reported fluorescein-attached aza–BODIPYs as
[
14]
Matsuyama 790-8577 (Japan)
energy-transfer cassettes and boron-chelated aza–BODIPYs.
Supporting information for this article can be found under http://
The chromophores with absorption and emission into the
near-infrared (NIR) region are very much in demand for cell
recognition and in vivo imaging, because deep penetration
dx.doi.org/10.1002/asia.201600167. It contains characterization data such
1
13
19 11
as HRMS as well as H, C, F, B NMR, and fluorescence spectra. The
energy and coordinates used for DFT calculations of compounds 19–24 are
also included.
[
15]
into biological tissues is possible by NIR fluorescence. Aza–
Chem. Asian J. 2016, 11, 1572 – 1587
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BODIPYs are very promising NIR chromophores that can be
[
16a–e]
used in materials and biology as photosensitizers.
Benzan-
[
17]
nulated aza–BODIPY
have been tested as photosensitizers
for applications in organic solar cells. Aza–BODIPYs display sig-
nificantly redshifted (ꢀ100 nm) absorption and emission rela-
tive to the parent BODIPYs.
Owing to their excellent photophysical properties, aza–BODI-
PYs have been used for applications in photovoltaics, optoelec-
[18]
tronics, bioimaging, and photodynamic therapy. The parent
,3,5,7-tetraphenylaza–BODIPY (TPAB) absorbs and emits
1
around 650 and 672 nm, respectively, and several approaches
have been employed to shift the emission further into the NIR
range with decent quantum yield efficiency. To make NIR-
active aza–BODIPYs, several synthetic modifications (Figure 1)
Figure 2. Donor—acceptor-type aza–BODIPYs 19–24 reported in this work
(data recorded in toluene).
thesis, characterization, X-ray structure, optical, and electro-
chemical studies of donor–acceptor-type aza–BODIPYs. All six
aza–BODIPYs exhibited redshifted emission maxima and effi-
cient energy transfer from donor units (N-phenylcarbazole/N-
butylcarbazole/naphthalene) to the acceptor unit (boron–aza–
dipyrrin core). Also, computational studies were carried out to
provide complementary spectroscopic insight into the six-aza–
BODIPY molecules to reach semiquantitative estimates of the
maximum absorption (lmax) of these challenging dyes.
Results and Discussion
Synthesis and Characterization
Figure 1. Selected aza–BODIPYs reported in the literature.
The synthetic procedures for chalcones 1–6 and compounds
7
–12 are depicted in Table 1. Chalcones 2–6 were prepared by
[
20]
the aldol condensation method. 2-Naphthaldehyde (1 equiv)
and 2-acetylthiophene (1 equiv) were heated under reflux con-
ditions in a mixture of methanol/water (1:1) in the presence of
NaOH for two hours. The desired chalcone 2 precipitated as
a yellow solid. It was purified by silica gel column chromatog-
raphy using a mixture of dichloromethane (CH Cl )/hexane.
[
19a]
have been reported: extension of the p conjugation,
chela-
[
19b]
tion of the boron atom by rigid aromatic groups,
attach-
[19c]
ment of donor groups onto phenyl rings,
the aza–BODIPY core by electron-withdrawing/releasing aro-
substitution at
[
19d]
matic rings,
and replacement of F atoms by alkynyl/aryl
2
2
[
19e]
groups.
In comparison to TPAB, the synthesis of such mole-
Chalcone 2 was obtained in 51% yield, and chalcones 3–6
were synthesized in a similar manner. Compounds 7–12 were
cules involves a multistep process with lower yields, thereby
limiting their applications. We were interested in creating a vari-
ety of NIR aza–BODIPYs by means of a simple straightforward
synthetic method (Figure 2). In this paper, we report the syn-
[
11]
prepared by means of a 1,4-Michael addition reaction. Chal-
cone 1 (1 equiv) and nitromethane (4 equiv) were heated to
reflux in a mixture of methanol/trimethylamine (3:1) for
Chem. Asian J. 2016, 11, 1572 – 1587
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Table 1. Synthesis of chalcones 1–6 and compounds 7–12.
Table 2. Synthesis of aza–dipyrrins 13–18 and aza–BODIPYs 19–24.
1
2
R
R
Chalcone
Compound
1
2
R
R
Aza–dipyrrin
Aza–BODIPY
1
2
(reported)
(51%)
7 (76%)
1
1
3 (29%)
4 (11%)
19 (20%)
8 (67%)
9 (63%)
20 (21%)
21 (15%)
3
4
(67%)
(72%)
1
1
5 (29%)
6 (18%)
10 (61%)
22 (14%)
5
6
(64%)
11 (53%)
12 (51%)
1
1
7 (46%)
8 (21%)
23 (23%)
24 (30%)
(reported)
2
4 hours. As the reaction progressed, the color of the reaction
mixture changed from yellow to black. The crude compound 7
was purified by silica gel column using a mixture of ethyl ace-
tate/hexane in 76% yield. By following the same procedure,
compounds 8–12 were synthesized in 51–67% yields.
fourteen aromatic protons of the N-butylcarbazole ring ap-
peared between d=8.83 to 7.02 ppm, and the aliphatic pro-
tons appeared as four sets of signals between d=4.20 to
19
The synthetic procedures for aza–dipyrrins and aza–BODIPYs
are shown in Table 2. Aza–dipyrrins 13–18 were prepared by
heating compounds 7–12 to reflux in ethanol with an excess
0.90 ppm. The F NMR spectra of aza–BODIPYs 19–24 were re-
corded in CDCl , and the fluorine resonance signals split into
3
quartets owing to the coupling with the adjacent boron atom
[14]
11
19
amount of ammonium acetate for 48 hours. The resulting
blue precipitate was then washed with ethanol to obtain pure
aza–dipyrrins 13–18 as blue solids in 11–46% yields. Aza–dipyr-
rin 13 (1 equiv) was treated with BF ·OEt (15 equiv) in a mix-
( B, I=3/2) and only one F signal was observed owing to the
[
9]
symmetrical nature of the molecules (Table 3). For com-
19
pounds 19–24 the F quartet appeared between d=À130.38
11
to À139.32 ppm (Table 3). The B signals of aza–BODIPYs 19–
24 appeared as triplets between d=1.04 to 1.47 ppm.
3
2
ture of dry CH Cl /DIEA (diisopropylethylamine; 1:1) for 24 h
2
2
[14]
19
under an inert atmosphere. The crude aza–BODIPY 19 was
purified by silica gel column chromatography and obtained as
a metallic green solid in 20% yield. Aza–BODIPYs 20–24 were
also prepared in 14–30% yields by following the same proce-
dure.
Interestingly, the F signals of 3,5-thienyl-substituted aza–
BODIPYs were slightly downfield-shifted relative to the 3,5-
phenyl-substituted compounds (Table 3). A similar trend was
1
Compounds 1–24 were characterized by HRMS as well as H
Table 3. Chemical-shift values of heteronuclear NMR spectroscopic data
13
recorded in CDCl3.
and C NMR spectroscopy; the data are shown in the Support-
ing Information. Aza–BODIPYs 19–24 were also characterized
19
11
Compound
F signal (d) [ppm]
B signal (d) [ppm]
19
11
by F and B NMR spectroscopy (see the Supporting Informa-
1
2
9
0
À131.59 (q, 2F)
À139.32 (q, 2F)
À131.18 (q, 2F)
À138.58 (q, 2F)
À130.38 (q, 2F)
À137.34 (q, 2F)
1.18 (t, 1B)
1.47 (t, 1B)
1.04 (t, 1B)
1.37 (t, 1B)
1.05 (t, 1B)
1.30 (t, 1B)
1
tion). A representative H NMR and HOMO-COSY spectrum of
aza–BODIPY 24 is shown in Figure 3. The characteristic b-pyr-
role protons and a-thiophene protons appeared as singlets at
d=7.16 and 8.39 ppm, respectively. The other b-thiophene
proton signals appeared between d=7.58 and 7.25 ppm. The
21
22
2
2
3
4
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Figure 4. ORTEP diagrams of a) aza–dipyrrin 13 and b) aza–dipyrrin 14. Ther-
mal ellipsoids are shown at the 50% probability level (hydrogen atoms are
omitted for clarity).
sion angles (N3-C8-C35-C40, N1-C1-C9-C10) between the aza–
dipyrrin core and two phenyl rings were 23.3(3) and 22.4(3)8,
respectively. These dihedral angles were higher than the re-
ported values of the 1,7-diphenyl-3,5-diferrocenyl-aza–dipyr-
[
21]
rin (19.81 and 21.258, respectively). Intramolecular hydrogen
bonding was observed in 13 between the N3-H-N1 (D-H-A)
with a bond angle of 119.318, and the hydrogen-bond length
was 2.16 Š. Single crystals of compound 14 were obtained by
slow evaporation of a chloroform solution over a period of one
week. Aza–dipyrrin 14 gave violet block crystals with a mono-
clinic crystal system in the C (no. 5) space group. In com-
2
pound 14 two naphthyl units were arranged in a perpendicular
manner, and the dihedral angles (C6-C7-C23-C24, C4-C3-C13-
C14) between the aza–dipyrrin core and naphthyl units were
1
1
1
3
Figure 3. H NMR and H– H COSY spectra of aza–BODIPY 24 in CDCl .
4
3.5(6) and 43.0(6)8, respectively. The torsion angles (N3-C8-
C33-C34, N1-C1-C9-C10) between the aza–dipyrrin core and
two thienyl groups were 4.8(10) and 8.0(8)8, respectively. Also,
the torsion angles (N3-C8-C33-S2, N1-C1-C9-S1) between the
aza–dipyrrin core and two thienyl groups that involved thio-
phene atoms were 15.7(8) and 4.2(7)8, respectively. The torsion
1
1
observed for B chemical shifts in aza–BODIPYs 19–24, which
suggests the increased delocalization of electron density in the
aza–dipyrrin core of 3,5-thienyl-substituted compounds, there-
1
9
by resulting in the slight downfield shifts of the
F
11
and B signals.
Table 4. Crystal structure data and refinement parameters of 13 and 14.
Single-Crystal X-ray Diffraction Studies
Compound 13
Compound 14
The single-crystal X-ray structures of aza–dipyrrins 13
CCDC 1016009) and 14 (CCDC 1425995) were
solved. ORTEP views of aza–dipyrrins 13 and 14 are
given in Figure 4; X-ray structural parameters and se-
lected bond lengths and bond angles are presented
in Tables 4 and 5, respectively. Single crystals of com-
Empirical formula
Formula weight
Crystal color, habit
Crystal dimensions [mm]
Crystal system
Lattice type
a [Š]
b [Š]
c [Š]
a [8]
b [8]
C
41
H
28Cl
3
N
3
36.5 3 2
C H23.5Cl1.5N S
(
669.05
621.41
violet, block
green, block
0.250”0.200”0.130
triclinic
0.200”0.030”0.030
monoclinic
C-centered
27.580(10)
7.565(3)
13.879(5)
–
92.131(6)
–
2893.8(18)
4
1.426
3.555
1.10
28459/8623
0.0466
primitive
9.523(4)
13.112(7)
13.426(7)
103.569(9)
99.317(7)
92.669(6)
1601.9(13)
2
1.387
3.219
1.09
31077/9544
0.0339
pound 13 were obtained by slow evaporation of the
chloroform/n-heptane solution over a period of one
week. Aza–dipyrrin 13 gave green block crystals with
a triclinic crystal system in the P 1¯ (no. 2) space
g [8]
3
group. The aza–dipyrrin core of compound 13 was
V [Š ]
Z
D
m [cm
essentially planar, which is similar to the reported fer-
À3
calcd [gcm
]
[21]
rocenyl-substituted aza–dipyrrin compound.
The
À1
]
2
two naphthyl units were found in syn conformation,
and the dihedral angles (C5-C6-C25-C34, C4-C3-C15-
C16) between the aza–dipyrrin core and naphthyl
units were 42.4(3) and 42.6(3)8, respectively. The tor-
Goodness-of-fit on F
Reflections collected/unique
R(int)
Space group
P 1¯ (no. 2)
2
C (no. 5)
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Table 5. Comparison of selected bond lengths [Š] and bond angles [8] of
3 and 14.
1
Compound 13
Compound 14
N1ÀC1
1.344(3)
1.395(3)
1.319(3)
1.338(3)
1.388(3)
1.346(3)
122.61(17)
105.77(17)
108.46(18)
108.31(17
109.60(18)
126.04(18)
124.93(18)
126.12(19)
105.64(18)
110.69(17
107.20(17)
1.321(5)
1.405(6)
1.307(5)
1.358(5)
1.369(5)
1.368(5)
122.3(4)
106.4(3)
108.0(3)
109.4(3)
107.7(4)
124.8(4)
124.0(4)
125.9(4)
105.2(4)
113.0(4)
105.1(3)
N1ÀC4
N2ÀC4
N2ÀC5
C5ÀN3
C8ÀN3
C5-N2-C4
C5-C6-C7
N3-C5-C6
C5-N3-C8
N3-C8-C7
N2-C5-N3
N2-C4-C3
N1-C4-N2
C2-C3-C4
N1-C1-C2
C1-N1-C4
Figure 6. Comparison of the absorption spectra of aza–BODIPYs 19–24 in
CH Cl .
2
2
dipyrrins 13–18 exhibited an intense absorption band in the
region of 590–662 nm in toluene, which corresponded to p–p*
transitions that originated from the dipyrrin ligand. Another
small absorption band appeared in the 290–345 nm range in
all the dipyrrins under study. The extinction coefficients of aza–
angles between the aza–dipyrrin core and phenyl rings (13)
were larger with respect to the aza–dipyrrin core and thio-
phene ring (14), thus indicating better electronic interactions
between smaller thiophene rings and the aza–dipyrrin core in
compound 14. Non-coplanar arrangements of naphthyl rings
with regard to the aza–dipyrrin core indicated less effective
electronic interactions between the two moieties in com-
pounds 13 and 14.
À1
À1
dipyrrins 13–18 were in the range of 27000 to 61000m cm ,
values that were much lower than the corresponding aza–
BODIPYs 19–24. The UV/Vis absorption maxima of aza–dipyr-
rins 13–18 were dependent on the nature of substituents at
[
11]
the 3,5- and 1,7-positions of the dipyrrin core. For example,
in compounds 13, 15, and 17, the 1,7-substitutents were
varied from naphthyl to N-phenylcarbazole to N-butylcarbazole
and 3,5-diphenyl substituents remained common to all of
them. It is clear from Table 6 that moving from naphthyl (13)
to N-phenylcarbazole (15) at the 1,7-positions resulted in
a 28 nm redshift in the absorption maxima of the major ab-
sorption band, whereas only a 3 nm redshift was observed in
the absorption maxima of the major absorption band upon
moving from N-phenylcarbazole (15) to N-butylcarbazole (17)
as 1,7-substitutents. Similarly, for compounds 14, 16, and 18
the 3,5-dithienyl substituents were kept common and varying
the 1,7-substitents from naphthyl (14) to N-butylcarbazole (18)
resulted in a 36 nm redshift in the absorption band. Clearly,
the N-butylcarbazole group introduced about 31–35 nm batho-
chromic shifts in the absorption maxima relative to naphthyl
groups in aza–dipyrrins. This could be attributed to the elec-
tron-donating nature of the bulky carbazole ring in these com-
Photophysical Studies
Absorption Studies
The absorption studies of aza–dipyrrins 13–18 and aza–BODI-
PYs 19–24 were carried out in four different solvents that
ranged from polar to nonpolar. A comparison of absorption
spectra of 13–18 in CH Cl is presented in Figure 5. Also, a com-
2
2
parison of absorption spectra of 19–24 is presented in
Figure 6; data are given in Table 6. The absorption spectra of
aza–dipyrrins 13–18 exhibited the p–p* transitions of dipyrrin
[
22]
ligand with two absorption bands in the visible region. Aza–
[
9]
pounds. The wavelengths of major absorption bands of 3,5-
dithienyl-substituted compounds 14, 16, and 18 were signifi-
cantly redshifted (41 to 46 nm) relative to their 3,5-diphenyl-
substituted counterparts 13, 15, and 17 respectively. The sig-
nificant bathochromic shifts were ascribed to the smaller tor-
sion angles and higher electron-donating ability of thienyl
[
12]
groups versus phenyl groups within the compounds. The
absorption bands of 13–18 were sharp with full width at half-
maxima (fwhmabs) varying from 82 to 116 nm in toluene
(
Table 6). Also, the absorption maxima for 13–18 were insensi-
tive to solvent polarity and varied slightly (2–8 nm) upon
changing the solvent from toluene to tetrahydrofuran. The ab-
sorption spectra of aza–BODIPYs 19–24 showed a similar pat-
tern to their corresponding aza–dipyrrins 13–18. The major ab-
Figure 5. Comparison of the absorption spectra of aza–dipyrrins 13–18 in
CH
2 2
Cl .
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(
19) to N-phenylcarbazole (21) to N-butylcarbazole (23), 21–
8 nm bathochromic shifts were observed in their absorption
Table 6. Absorption data of compounds 13–24 in various solvents.
3
Compound Solvent
l
abs [nm], e (logE)
fwhm
nm] [cm ]
maxima. Their absorptions were sharp with full-width at half-
maxima (fwhmabs) varying from 41 to 80 nm in CH Cl (Table 6).
À1
[
2
2
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
0
toluene
dichloromethane 586 (4.67)
chloroform
tetrahydrofuran
toluene
590 (4.72)
82
81
80
82
84
86
86
86
87
88
85
87
82
90
90
89
2458
The absorption maxima for 19–24 varied slightly (2–8 nm)
upon changing the solvent polarity from nonpolar to polar;
this trend was in line with the reported BODIPY chromo-
2449
2397
2492
2253
2315
2299
2330
2441
2446
2337
2401
1997
2222
2215
2200
3094
3125
3082
3569
2651
2855
2758
2805
1262
1352
1344
1326
748
589 (4.63)
585 (4.54)
626 (4.70)
[
5a,22]
phores.
The solvent polarity affected the broadening of
dichloromethane 623 (4.67)
the absorption band, and fwhmabs values were lower in nonpo-
chloroform
tetrahydrofuran
toluene
625 (4.63)
621 (4.59)
618 (4.42)
lar solvent and slightly higher in polar solvents for compounds
[5a,22]
19–24.
The extinction coefficients of 19–24 were in the
À1
À1
range of 63000–140000m cm , which were much larger
than the corresponding aza–dipyrrins, thus making these dyes
potentially useful for biological applications. Among the aza–
BODIPYs 19–24, a maximum redshifted absorption at 750 nm
was observed for 24 (Table 6). The absorptions of aza–BODIPYs
19–24 demonstrate that simple substitution by electron-rich
aromatic moieties at the 1,7-positions can lead to significant
bathochromic shifts rather than making conformationally re-
stricted aza–BODIPYs, which are difficult to synthesize.
dichloromethane 613 (4.35)
chloroform
tetrahydrofuran
toluene
615 (4.32)
616 (4.29)
655 (4.61)
dichloromethane 653 (4.57)
chloroform
tetrahydrofuran
toluene
654 (4.55)
652 (4.53)
sh 501(3.93), 621 (4.38)
116
dichloromethane sh 506 (3.84), 618 (4.30) 116
chloroform
tetrahydrofuran
toluene
sh 503 (3.77), 621 (4.25) 115
sh 502 (3.77), 616 (4.15) 130
sh 509 (4.25), 662 (4.78) 109
dichloromethane sh 510 (4.19), 658 (4.70) 116
chloroform
tetrahydrofuran
toluene
sh 514 (4.14), 661 (4.66) 113
sh 514 (4.02), 658 (4.53) 114
653 (4.96)
Fluorescence Studies
53
56
56
55
38
Fluorescence studies of all of the aza–BODIPYs (19–24) were
carried out in various solvents of different polarities and emis-
sion quantum yields were calculated; emission data are pre-
sented in Table 7, and emission spectra are given in Figure 7.
dichloromethane 648 (4.91)
chloroform
tetrahydrofuran
toluene
650 (4.85)
649 (4.80)
sh 654 (4.43), 716 (5.10)
dichloromethane sh 652 (4.41), 714 (5.06)
40
40
794
789
chloroform
sh 653 (4.36), 716 (5.03)
tetrahydrofuran
toluene
dichloromethane 668 (4.80)
chloroform
tetrahydrofuran
toluene
sh 652 (4.32), 712 (4.97)
674 (4.83)
38
71
80
76
74
41
41
40
39
–
–
–
–
54
68
60
67
756
1612
1871
1761
1709
766
771
751
735
–
2
2
2
2
1
2
3
4
670 (4.78)
670 (4.58)
sh 667 (4.47), 735 (5.14)
dichloromethane sh 668 (4.46), 733 (5.08)
chloroform
tetrahydrofuran
toluene
sh 667 (4.31), 733 (5.03)
sh 669 (4.30), 731 (4.97)
598 (4.44), 712 (4.64)
dichloromethane 602 (4.46), 712 (4.57)
–
–
–
chloroform
tetrahydrofuran
toluene
dichloromethane 613 (4.38), 752 (4.91)
chloroform
606 (4.43), 711 (4.51)
606 (4.25), 713 (4.35)
595 (4.38), 750 (4.94)
Figure 7. Comparison of the emission spectra of aza–BODIPYs 19–24 in tolu-
ene.
971
1224
1076
1214
601 (4.37), 750 (4.89)
606 (4.35), 748 (4.80)
tetrahydrofuran
The typical mirror-image relationship was observed for the
lowest-energy absorption band and the emission spectra of all
[
23]
these compounds (19–24). The emission maxima for com-
[
11]
sorption band designated as the S –S transition was observed
pounds 19–24 were redshifted relative to that of TPAB. The
emission maxima of aza–BODIPYs 19, 21, and 23 were com-
pared with the corresponding TPAB in chloroform; 19 showed
a small shift, whereas 21 and 23 exhibited redshifts of 49 and
102 nm, respectively (Table 7). The emission quantum yields of
21 and 23 were lower than TPAB (F =0.34, CHCl ), whereas it
0
1
[18b]
between 653–750 nm in aza–BODIPYs 19–24.
In addition to
this, a shoulder that corresponded to the 0–1 vibrational tran-
sition appeared in the 519 to 590 nm range for compounds 20
and 22–24. The absorption maxima of 3,5-dithienyl-substituted
compounds 20, 22, and 24 were significantly redshifted (38 to
f
3
6
3 nm in toluene) relative to their 3,5-diphenyl-substituted
was comparable with that of 19. Similarly, the emission
maxima of aza–BODIPYs 20, 22, and 24 were compared with
the corresponding 1,7-diphenyl-3,5-dithienylaza–BODIPY in
counterparts 19, 21, and 23, respectively. This trend was con-
sistent with the reported data of 3,5-dithienyl-substituted aza–
[
11]
[11]
BODIPYs. Also, by varying the 1,7-substituents from naphthyl
chloroform; compound 20 showed a 12 nm blueshift, where-
Chem. Asian J. 2016, 11, 1572 – 1587
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Table 7. Emission data of aza–BODIPYs 19–24 in different solvents.
[a]
Compound
Solvent
l
em [nm]
F
f
fwhm
Stokes shift
[nm]
À1
À1
[nm]
[cm
]
[cm
]
[
b]
1
2
2
2
2
2
9
0
1
2
3
4
toluene
dichloromethane
chloroform
tetrahydrofuran
toluene
dichloromethane
chloroform
tetrahydrofuran
toluene
dichloromethane
chloroform
tetrahydrofuran
toluene
dichloromethane
chloroform
tetrahydrofuran
toluene
dichloromethane
chloroform
tetrahydrofuran
toluene
dichloromethane
chloroform
680
682
681
677
726
728
729
723
711
743
721
710
744
745
744
743
756
797
774
784
771
795
779
785
0.47
45
49
46
46
35
35
36
35
61
84
94
56
37
39
37
36
64
77
79
81
53
74
68
73
967
1053
987
995
616
665
679
770
1197
1522
1761
1154
666
702
666
652
1112
1228
1306
1316
889
1179
1108
1179
27
34
31
28
10
14
13
11
37
75
51
40
9
12
11
12
44
85
63
71
21
45
29
37
608
0.25
0.36
0.25
0.14
0.13
0.13
0.11
0.31
0.01
0.14
0.01
0.09
0.02
0.09
0.03
0.04
0.01
0.02
0.01
0.04
0.01
0.02
0.01
769
700
637
192
269
249
213
772
1511
1055
840
164
219
201
220
817
1497
1144
1270
363
754
496
630
tetrahydrofuran
[
f f
a] Zinc–phthalocyanine (F =0.30, toluene) used as standard, lex =650 nm. [b] TPAB (F =0.34, chloroform) used as standard, lex =610 nm.
as 22 and 24 exhibited redshifts of 3 and 30 nm, respectively
Table 7). The emission quantum yields of 20, 22, and 24 were
550–700 nm, it can act as acceptor unit and three aromatic
moieties (naphthalene, N-phenylcarbazole, and N-butylcarba-
zole) can act as donor units. An energy transfer from donor
moieties to the aza–BODIPY core was expected. Indeed, all of
the compounds (19–24) exhibited the energy-transfer phe-
nomenon from donor units to the aza–BODIPY core (see the
Supporting Information). A comparison of emission spectra of
aza–BODIPY 24 and a 2:1 mixture of N-butylcarbazole and
TPAB is shown in Figure 8. In this system, the N-butylcarbazole
unit acts as an energy donor and the aza–BODIPY core acts as
an energy acceptor. Therefore, upon excitation of N-butylcar-
bazole unit at 320 nm in compound 24, the dominant emis-
sion was observed from the aza–BODIPY core in the range of
740–890 nm. The free N-butylcarbazole molecule emits in the
(
lower than the corresponding aza–BODIPY (F =0.44, toluene).
f
Among the three donor groups at the 1,7-positions of aza–
BODIPYs 19–24, the naphthyl rings clearly had no or little
effect on the emission maxima of 19 and 20. However, N-phe-
nylcarbazole and N-butylcarbazole groups influenced 38–
9
9 nm bathochromic shifts in the emission maxima of 21–24;
maximum redshifts occurred in compounds 23 and 24 with
,7-N-butylcarbazole groups. Solvatochromic studies revealed
1
that aza–BODIPYs 19, 20, and 22 showed small shifts (2–6 nm)
in emission maxima by changing the polarity of the solvents
(
Table 7), whereas changing the solvent polarity resulted in sig-
nificant shifts (16–30 nm) in the emission maxima for aza–BOD-
IPYs 21, 23, and 24. The observed Stokes shifts for compounds
19–24 were smaller (9 to 44 nm) in nonpolar solvents and
higher (34 to 85 nm) in polar solvents (Table 7). Except for
compounds 20 and 22, the rest of the aza–BODIPYs showed
much higher (34–45 nm) Stokes shifts than the parent TPAB
compound (22 nm in CH Cl ). In polar solvents, the redshifted
2
2
emission maxima reflect larger dipoles of the charge-transfer
CT) excited states, which was in agreement with the report by
(
[
22]
Boens et al. The quantum yields of aza–BODIPYs 19–24 were
higher in nonpolar solvents (i.e., toluene) and lower in polar
solvents (i.e., tetrahydrofuran); this is due to the large dipole
moment difference between the CT excited state and the
ground state, which in turn facilitate internal conversion in
[
22]
polar media.
Energy-transfer studies of aza–BODIPYs 19–24 were carried
out in toluene. Since the aza–BODIPY core absorbs around
Figure 8. Comparison of the normalized emission spectra of aza–BODIPY 24
in a 2:1 mixture of corresponding monomers in toluene (lex =320 nm).
Chem. Asian J. 2016, 11, 1572 – 1587
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[
26]
range of 340–450 nm. It is very clear from Figure 8 that upon
311G(2d,p) level of approximation. Vibrational spectra were
determined analytically at the same level of theory to make
sure that the given structure is at its true energy minima. Verti-
excitation of a 2:1 mixture of N-butylcarbazole and TPAB at
3
20 nm, the point at which N-butylcarbazole absorbs strongly,
[
27]
emission was observed exclusively from the N-butylcarbazole
molecule around 340–368 nm and no emission was observed
from TPAB. However, in the case of aza–BODIPY 24, the excita-
tion of the N-butylcarbazole unit at 320 nm resulted in the
major emission from the aza–BODIPY core around 771 nm and
the emission from N-butylcarbazole units was drastically
quenched (Figure 8). The energy-transfer efficiency (ETE [%]) is
a measure of total energy transfer from donor to acceptor irre-
spective of the mechanism and it can be calculated by the for-
cal linear-response TD-DFT approximation was employed to
calculate electronic transition energies at the TD-BMK/6-
[
28]
311+G(2d,p) level of theory. The effect of dichloromethane
was systematically modeled for all the steps by means of the
[
29]
conductor-like polarizable continuum model (C-PCM).
The
choice of functional and basis set was made from the TD-DFT
[
30]
benchmarking of aza–BODIPYs.
[24]
mula mentioned below.
The quantum yields of the aza–
Geometrical Parameters
BODIPY unit in all the compounds (19–24) were calculated at
two different wavelengths, that is, excited at the donor (where
the donor absorbs strongly) and excited at the acceptor (aza–
BODIPY core). The data are presented in Table 8. ETE gives an
idea of the extent of energy transfer including the loss in
energy by non-radiative decays during the energy-transfer pro-
Structural parameters obtained as a result of geometry optimi-
zation at the PBEPBE/6-311G(2d,p) level of approximation was
taken into consideration, and different conclusions may be
drawn by scrutinizing the data. The choice of functional and
[31]
basis set was made from the literature as it yields the small-
[
24]
cess. The calculated values of ETE were in the range of 70%
in 19) to 93% (in 22), thus indicating efficient energy transfer
from donor units to acceptor units in all the compounds
Table 8). X-ray crystal structures of the key precursors (13 and
4) of aza–BODIPYs (19 and 20, respectively) showed that
est mean absolute errors for bond lengths and valance angles.
Although the crystal data for the molecules of interest is not
yet available, calculated results for all the dye molecules in
terms of structural parameters are incorporated into Tables 9
and 10. Owing to solid-state effects, the experimental struc-
(
(
1
naphthyl moieties (donor) and the aza–dipyrrin core (acceptor)
were not coplanar, therefore the possibility of energy transfer
through other mechanisms is not very favorable. However,
more detailed photophysical and electrochemical studies will
be required to understand the mechanism of energy transfer
in these compounds.
tures cannot be visualized perfectly of C symmetry, therefore
2
the average measured structural parameters for the two halves
of the molecule are taken into account. The designation of
bond lengths and valance angles can be seen in Figure 9. It is
clear from the calculated results
that the effect of substitution on
the geometrical parameters of the
aza–BODIPY core is negligible. To
Computational Methodology
be more precise, the utmost devia-
Among the whole panel of synthesized molecules, theoretical
attention was paid to aza–BODIPY moieties because of their
absorbance in the near-infrared region. All the simulations of
aza–BODIPY derivatives (19–24) were accomplished with the
tion shown in terms of bond
lengths and valance angles is
Figure 9. Representation of
essential structural parameters
0.01 Š and 0.918, respectively.
These marginal deviations echo in terms of bond lengths and
[25]
valence angles of aza–BODIPY
molecules.
Gaussian 09 program package
Geometry optimization was
the strong influence of substitution
carried out without symmetry constraints at the PBEPBE/6-
on the absorption properties.
Table 8. Energy transfer data of aza–BODIPYs 19–24 in toluene.
[a]
Compound
Excitation (lex) [nm]
Emission (lem) [nm]
Donor [F
f
]
Acceptor [F
f
]
ETE [%]
[a]
Zn-phthalocyanine
TPAB
650
610
610
675, 741
673
680
679
726
727
711
388, 709
744
393, 746
756
–
–
–
0.30
0.34
0.47
0.33
0.14
0.13
0.31
0.25
0.09
0.09
0.04
0.03
0.04
0.03
–
–
70
[b]
1
2
2
2
2
2
9
0
1
2
3
4
2
90
650
90
650
20
650
20
650
20
650
20
–
–
–
–
–
88
81
93
84
92
2
3
3
3
400, 755
771
771
3
Ff of acceptor in the molecule excited at the donor
of acceptor in the molecule excited at the acceptor
[
a] ETE [%]=Ff
”100
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Table 9. Structural parameters computed at the PBEPBE/6-311G(2d,p) level of approximation.
[a]
Results
Aza–BODIPY
Bond lengths [Š]
a
b
c
d
e
f
g
h
Theoretical
19
1.40
1.40
1.40
1.40
1.40
1.40
1.57
1.56
1.57
1.57
1.56
1.56
1.37
1.37
1.37
1.37
1.37
1.37
1.42
1.42
1.42
1.42
1.41
1.42
1.39
1.39
1.39
1.39
1.40
1.39
1.44
1.44
1.44
1.44
1.45
1.44
1.32
1.33
1.32
1.33
1.33
1.33
1.40
1.39
1.40
1.39
1.40
1.40
2
2
2
2
2
0
1
2
3
4
[
a] See Figure 9 for the identification of the bond lengths.
Table 10. Structural parameters computed at the PBEPBE/6-311G(2d,p)
level of approximation.
Table 11. Comparison between theoretical and experimental lmax [nm].
PCM-TD-BMK/6-311+G(2d,p)//PCM-PBEPBE/6-311G(2d,p) level of approxi-
mation was employed to calculate the theoretical values.
Results
Aza–BODIPY
Valance angles [8]
aBNC
aFBF
aNCN
Aza–BODIPY
19
20
21
22
23
24
Theoretical
19
110.03
110.54
110.94
110.56
110.55
110.20
121.98
122.39
122.02
122.32
122.10
122.37
124.56
124.47
124.25
124.21
124.10
124.03
Theoretical lmax
Oscillatory strength (f)
Experimental lmax
614
0.95
648
672
0.94
714
647
0.84
668
692
0.88
733
676
0.96
712
705
1.10
750
2
2
2
2
2
0
1
2
3
4
Absorption Spectra Studies
Theoretical simulations are often viewed as an effi-
cient means to offer complementary spectroscopic
insight. For this purpose, singlet–singlet transitions of
aza–BODIPY dyes 19–24 were simulated at the PCM-
TD-BMK/6-311+G(2d,p)//PCM-PBEPBE/6-311G(2d,p)
level of approximation to achieve semi-quantitative
estimates of the lmax of these challenging dyes. Out
of the experimentally explored solvents, only CH Cl
2
2
was modeled for simulations, as solvent polarities did
not show any significant shift in the absorption
maxima. Before comparing the experimental and the-
oretical lowest-energy transitions (lmax) of the candi-
date dye molecules, it is better to analyze the nature
of these transitions.
This was done by determining the topologies of
the Kohn–Sham orbitals involved in the transitions.
The frontier molecular orbitals (FMOs) for the mole-
cules of interest (19–24) can be found in Figure 10
and the maximum wavelength absorption (lmax) in
Table 11. For molecules 19, 20, 22, 23, and 24 the
lmax is exclusively HOMO to LUMO electronic promo-
tion, and for 21 it is an amalgam of HOMO to LUMO
and HOMOÀ2 to LUMO with dominance of the
former. The significant electronic conjugation be-
tween the aza–BODIPY core and the arms is responsi-
ble for the delocalization of the FMOs. It is evident
from Figure 9 that the absorption of aza–BODIPY
(
19–24) corresponds to charge transfer from the
arms to the central core.
In addition, there is also a significant overlap ob-
Figure 10. Six molecular orbitals for aza–BODIPYs (19–24) obtained at the PCM-TD-BMK/
served between the occupied and virtual orbitals. It 6-311+G(2d,p) level of approximation.
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is apparent from the similar topologies of the LUMO of the
candidate dyes (19–24) that the aza–BODIPY core acts as
a strong accepting group, which suggests that the addition of
an additional electron-accepting moiety is not ideal for induc-
ing large charge transfer. Although a number of absorption
bands were observed in the dye molecules (19–24), the peaks
that corresponded to the lowest-energy transition (lmax) were
selected for comparison. The lmax values of aza–BODIPY mole-
cules 19–24 are summarized in Table 11. The experimentally
obtained lmax values of 20, 22, 23, and 24 are above 700 nm,
and for 19 and 21 they are at 648 and 668 nm, respectively.
These absorptions reflect the HOMO–LUMO gap, which is also
supported by DFT studies. Although a similar trend for band-
gap increments is followed, discrepancies are observed from
the experimental values. To be more precise, the maximum de-
viation obtained is 45 nm for 24, and the minimum deviation
obtained is 21 nm for 21. The consistent underestimation of
lmax for all the molecules is due to the inherent limitations of
vertical approximation.
waves were observed for compounds 19–24 in the CVs.
Among aza–dipyrrins, the naphthyl-substituted aza–dipyrrins
13 and 14 showed the lowest oxidation potentials (Table 12).
The reduction potentials of aza–dipyrrin 15–18 (with N-phenyl-
carbazole and N-butylcarbazole groups) were comparatively
higher than 13 and 14 (with naphthyl groups), thus making
them difficult to be reduced relative to the latter. Compounds
19–24 showed two reduction waves, the first around À0.36 to
À0.53 V and the second around À1.04 to À1.28 V. Compounds
19–24 showed two oxidation waves, the first around 0.68 to
1.09 V and the second around 1.32 to 1.36 V. The oxidation po-
tential of aza–BODIPYs followed a similar trend, which was ob-
served with aza–dipyrrins 13–14. The naphthyl-substituted
aza–BODIPYs 19 and 20 showed the lowest oxidation poten-
tials among the group. The oxidation potential values of aza–
BODIPYs 19–24 were higher than those of aza–dipyrrins 13–
18, thus indicating that aza–BODIPYs 19–24 were difficult to
oxidize. Owing to difluoroboron (BF ) complexation, aza–BODI-
2
PYs 19–24 became electron deficient, and the oxidation values
were shifted higher. This suggests that the reduction process
was more feasible for aza–BODIPYs 19–24 than aza–dipyrrins
Electrochemical Studies
[11]
13–18.
The HOMO–LUMO energies were determined by
The redox potentials of compounds 13–24 were recorded in
using the measured redox potentials with the potential of Fc/
À1
+
[32]
dry CH Cl at a scan rate of 50 mVs using 0.1m tetrabutylam-
Fc as reference energy; they are listed in Table 12. The ob-
tained frontier orbital energies for 13–18 and 19–24 are
shown in Figures 12 and 13, respectively. The energetic stabili-
zation for the LUMO was comparatively higher in aza–BODIPYs
19–24 than their corresponding aza–dipyrrins 13–18. The in-
creased stabilization of the LUMO was attributed to the com-
2
2
monium perchlorate (TBAP) as supporting electrolyte under N2
atmosphere. The redox potential data and comparison of re-
duction waves are shown in Table 12 and Figure 11, respective-
ly. Unlike ferrocene, which showed only one oxidation wave at
[
31]
0
.38 V, compounds 13–18 showed one reversible and one ir-
reversible oxidation wave in their cyclic voltammograms (CV).
Also, compounds 13–18 exhibited three reversible reduction
waves between À0.38 to À1.35 V in the CVs. Compounds 19–
plexation with the BF moiety in aza–BODIPYs 19–24. On com-
2
paring the effect of the substituent at the 1,7-position in com-
pounds 13–24, it was observed that the LUMO of compounds
with N-phenylcarbazole groups (15, 16, 21, and 22) were
slightly more stabilized than the others. The data given in
Table 12 clearly indicate that the energies of the HOMO were
slightly higher for aza–BODIPYs 19–24 than those of the corre-
sponding aza–dipyrrins 13–18. The HOMO–LUMO gaps (DE)
for aza–BODIPYs 19–24 were smaller than those of the corre-
sponding aza–dipyrrins 13–18, which is supported by the
2
4 exhibited two oxidation and two reduction waves in the
cyclic voltammograms. Compound 19 showed one reversible
and one irreversible oxidation potential in the CV; however,
two irreversible oxidation waves were observed for com-
pounds 20–22. One quasi-reversible oxidation and one irrever-
sible oxidation wave were observed for compounds 23 and
24. In the range of À0.36 to À1.28 V, two reversible reduction
Table 12. Electrochemical redox data (V versus SCE) of compounds 13–24 in CH
2
Cl
2
that contained 0.1m TBAP as supporting electrolyte recorded at
À1
5
0 mVs scan speed and energy levels calculated from optical and CV data.
Compound
E
1/2(ox) [V]
E
1/2(red) [V]
HOMO [eV]
LUMO [eV]
DE [eV] (from CV)
Optical DE [eV]
I
II
I
II
III
ferrocene
0.38
0.71
0.77
1.03
0.93
0.94
0.99
0.76
0.68
1.09
1.03
0.97
1.02
–
–
–
–
–
–
–
–
13
14
15
16
17
18
9
0
1
2
3
4
1.29
1.33
1.26
1.27
1.29
1.33
1.32
1.35
1.34
1.35
1.32
1.36
À0.55
À0.59
À0.38
À0.42
À0.42
À0.47
À0.50
À0.42
À0.37
À0.36
À0.53
À0.48
À0.85
À1.24
À1.29
À1.18
À1.20
À1.33
À1.35
À1.15
À1.17
À1.09
À1.04
À1.28
À1.25
À5.13
À5.19
À5.45
À5.35
À5.36
À5.41
À5.09
À5.11
À5.35
À5.30
À5.37
À5.44
À3.87
À3.83
À4.04
À4.00
À3.84
À3.74
À3.92
À4.00
À4.05
À4.06
À3.89
À3.94
1.26
1.36
1.41
1.35
1.52
1.67
1.17
1.11
1.30
1.24
1.48
1.50
1.94
1.83
1.85
1.76
1.76
1.71
1.74
1.66
1.73
1.62
1.56
1.54
À0.84
À0.77
À0.74
À0.87
À0.86
1
2
2
2
2
2
–
–
–
–
–
–
Chem. Asian J. 2016, 11, 1572 – 1587
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Full Paper
À1
Figure 11. Comparison of the reduction waves of 13–24 in CH
2
Cl
2
containing 0.1m TBAP as a supporting electrolyte. Recorded at 50 mVs scan speed (V
versus SCE).
trend observed in their optical properties. The absorption
maxima of aza–BODIPYs 19–24 were redshifted relative to
their corresponding aza–dipyrrins 13–18. The electrical gaps of
compounds 13–24, which were obtained from CV measure-
ments, do not correlate with the observed absorption proper-
ties. According to electrochemical data, compounds 14
(
among the aza–dipyrrins) and 20 (among the aza–BODIPYs)
should have the most redshifted absorption if one assumes
a pure HOMO–LUMO transition. However, this is not the case,
and these electrochemical studies confirm that no pure
HOMO–LUMO transitions are present in the absorption pro-
cess. The optical bandgap was also calculated for compounds
Figure 12. HOMO–LUMO energy levels of aza–dipyrrins 13–18 calculated
from CV data.
[
33]
13–24 from the absorption data (Table 12). Among the three
donor groups at the 1,7-positions of compounds 13–24, the
naphthyl-substituted compounds (13, 14, 19, and 20) exhibit-
ed higher optical bandgaps than the other compounds. The
lowest optical bandgap was found to be 1.54 eV for com-
pound 24, which showed the maximum redshifted absorption
among all the aza–BODIPYs, thus indicating that the N-butyl-
carbazole moiety has the maximum effect on the electronic
spectra of these compounds.
Conclusion
In summary, we have described the synthesis of NIR aza–BODI-
PYs and aza–dipyrrins that contain energy-donor groups (e.g.,
N-butylcarbazole, N-phenylcarbazole, napthyl) at their 1,7-posi-
tions. Also, the 3,5-positions of the aza–BODIPYs were substi-
Figure 13. HOMO–LUMO energy levels of aza–BODIPYs 19–24 calculated
from CV data.
Chem. Asian J. 2016, 11, 1572 – 1587
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Full Paper
tuted with phenyl or thienyl groups. Fluorescence studies indi-
cated efficient energy transfer from the donor moieties to the
aza–BODIPY core in all the compounds studied. Compared to
the parent TPAB, significant bathochromic shifts were observed
in the optical spectra of all compounds owing to the presence
of donor moieties. Furthermore, the presence of thienyl
groups resulted in better electronic interactions between the
aza–BODIPY core and thiophene rings, which in turn caused
redshifts in the electronic spectra of such compounds relative
to their 3,5-phenyl counterparts. DFT studies indicated effective
electronic interactions between the donor and acceptor moiet-
ies in all the compounds under study. Electrochemical studies
showed lower HOMO–LUMO gaps (DE) for aza–BODIPYs rela-
tive to the corresponding aza–dipyrrins owing to boron com-
plexation. With this work we demonstrated that simple substi-
tution with energy-donor groups on aza–BODIPYs can induce
large redshifts in their electronic spectra, and this could be an-
other strategy to create NIR dyes.
acted in a mixture of methanol (28.00 mL)/water (28.00 mL) per
the general procedure. The desired product 2 was purified by silica
gel column using 45% CH Cl /hexane as the solvent mixture.
2
2
Yellow solid, yield: 3.42 g (50%); R =0.23 (silica, CH Cl /hexane
f
2
2
1
1
:3); H NMR (CDCl , 500 MHz): d=8.6 (d, J=15.5 Hz, 1H), 8.23 (d,
3
J=8 Hz, 1H), 7.84–7.89 (m, 1H), 7.66 (d, J=5 Hz, 1H), 7.55 (m, 1H),
13
7
.46–7.52 (m, 3H), 7.15 ppm (m, 4H); C NMR (CDCl , 125 MHz):
3
d=181.93, 145.56, 140.98, 134.06, 133.78, 132.2, 132.02, 131.81,
130.92, 128.8, 128.36, 127.05, 126.37, 125.47, 125.15, 124.32,
+
123.54 ppm; HRMS (ESI-Q-TOF): m/z calcd for C17
H
13OS : 265.0687
+
[M+H] ; found: 265.0706.
Compound 3: N-Phenylcarbazole aldehyde (1.00 g, 3.68 mmol),
acetophenone (0.43 mL, 3.68 mmol), and NaOH (0.015 g) were re-
acted in a mixture of methanol (4.00 mL)/water (4.00 mL) by fol-
lowing the general procedure. The crude product 3 was purified
by silica gel column using 45% CH Cl /hexane as the solvent mix-
2
2
ture. Yellow solid, yield: 0.66 g (48%); R =0.21 (silica, CH Cl /
f
2
2
1
hexane 1:1.5); H NMR (CDCl , 500 MHz): d=8.15 (d, J=7.5 Hz, 2H),
3
8
.07 (d, J=7 Hz, 2H), 7.87–7.93 (m, 3H), 7.65 (m, 3H), 7.61 (m, 1H),
7.54 (m, 2H), 7.48 (d, J=8.5 Hz, 2H), 7.42 (m, 2H), 7.31 ppm (m,
1
3
2
1
H); C NMR (CDCl , 125 MHz): d=190.35, 143.64, 140.45, 139.71,
3
38.18, 133.76, 132.95, 129.93, 128.73, 128.56, 127.19, 126.14,
Experimental Section
123.71, 122.58, 120.44, 120.41, 109.8 ppm; HRMS (ESI-Q-TOF): m/z
+
+
calcd for C H NO : 374.1545 [M+H] ; found: 374.1538.
27
20
Instrumentation and Reagents
Compound 4: N-Phenylcarbazole aldehyde (1.00 g, 3.68 mmol), 2-
acetylthiophene (0.39 mL, 3.68 mmol), and NaOH (0.015 g) were re-
acted in a mixture of methanol (4.00 mL)/water (4.00 mL). The
crude product 4 was purified by column chromatography on silica
Unless otherwise mentioned, all the reagents and solvents were
purchased from Aldrich, Acros Organics, or Merck and used with-
out further purification. Solvents such as dichloromethane (CH Cl )
2
2
and diisopropylethylamine (DIEA) were dried as per the standard
procedure. Silica gel (60–120 mesh size) used for column chroma-
tography was procured from Merck. The NMR spectra of com-
pounds were recorded using a Bruker Avance III 500 MHz NMR
spectrometer at IIT Gandhinagar. The HRMS data for all the com-
pounds were recorded (in positive ion mode) using a Waters
Synapt-G2S ESI-Q-TOF Mass instrument at IIT Gandhinagar. Absorp-
tion spectra were recorded using a Shimadzu UV-1700. Cyclic vol-
tammetry (CV) studies were carried out with an electrochemical
system (CH1660E, CH instruments, USA) that utilized a three-elec-
trode configuration that consisted of a glassy carbon (working
electrode), platinum wire (auxiliary electrode), and saturated calo-
mel electrode (SCE; reference electrode). The experiments were
gel using 45% CH
yield: 0.76 g (54%); R
(CDCl , 500 MHz): d=8.14 (d, J=8 Hz, 2H), 7.90–7.95 (m, 2H), 7.86
2 2
Cl /hexane as the solvent mixture. Yellow solid,
1
=0.22 (silica, CH Cl /hexane 1:1); H NMR
2 2
f
3
(d, J=8.5 Hz, 2H), 7.69 (d, J=4.5 Hz, 1H), 7.64 (d, J=8.5 Hz, 2H),
7.46–7.50 (m, 3H), 7.41 (m, 2H), 7.30 (t, J=7.5 Hz, 2H), 7.19–
13
7.24 ppm (m, 1H); C NMR (CDCl , 125 MHz): d=181.85, 145.52,
3
142.90, 140.44, 139.76, 134.10, 133.56, 131.93, 129.98, 128.35,
127.16, 126.16, 123.72, 122.15, 120.46, 120.43, 109.82 ppm; HRMS
+
+
(ESI-Q-TOF): m/z calcd for C25H18NOS : 380.1109 [M+H] ; found:
380.1111.
Compound 5: N-Butylcarbazole aldehyde (2.00 g, 7.95 mmol), ace-
tophenone (0.93 mL, 7.95 mmol), and NaOH (0.03 g) were reacted
in a mixture of methanol (8.64 mL)/water (8.64 mL). The crude
product 5 was purified by column chromatography on silica gel
using a 40% CH Cl /hexane mixture. Yellow solid, yield: 1.79 g
carried out under nitrogen atmosphere in dry CH Cl using 0.1m
2
2
tetrabutylammonium perchlorate as supporting electrolyte at IIT
Gandhinagar. All potentials were calibrated versus SCE by the addi-
2
2
1
(
63%); Rf =0.24 (silica, CH Cl /hexane 1:2); H NMR (CDCl3,
2 2
+
tion of ferrocene as an internal standard with E1/2 (Fc/Fc )=0.38 V
500 MHz): d=8.35 (s, 1H), 8.12 (d, J=8 Hz, 2H), 8.03–8.07 (m, 3H),
versus SCE
7
2
.77 (d, J=8.5 Hz, 1H), 7.57 (m, 2H), 7.47–7.52 (m, 3H), 7.40 (m,
H), 7.27 (m, 1H), 4.29 (t, J=7.5, 2H), 1.82–1.88 (m, 2H), 1.35–1.43
13
(
m, 2H), 0.94 ppm (m, 3H); C NMR (CDCl , 125 MHz): d=190.72,
3
General Procedure for Chalcones 1–6
1
1
46.57, 142.03, 141.01, 138.87, 132.42, 128.57, 128.47, 126.33,
25.92, 123.41, 122.79, 121.60, 120.60, 119.71, 119.14, 109.18,
A mixture of aromatic aldehyde (1 equiv) and acetophenone
(
1 equiv) or 2-acetylthiophene (1 equiv) was dissolved in methanol/
109.17, 43.08, 31.12, 20.55, 13.87 ppm; HRMS (ESI-Q-TOF): m/z
calcd for C H NO : 354.1858 [M+H] ; found: 354.1835.
25 24
+
+
water (1:1). After five minutes, NaOH (0.1 equiv) was added, and
the reaction mixture was heated to reflux for 2 h. During the
course of the reaction, the product precipitated as a yellow solid.
Then the reaction mixture was extracted with CH Cl2 and dried
over anhydrous Na SO . The solvent was evaporated by using
a rotary evaporator under reduced pressure. The crude product
was subjected to silica gel column chromatography for purifica-
tion.
Compound 6: N-Butylcarbazole aldehyde (3.00 g, 11.93 mmol), 2-
acetylthiophene (1.28 mL, 11.93 mmol), and NaOH (0.05 g) were re-
acted in a mixture of methanol (13.00 mL)/water (13.00 mL). The
crude product 6 was purified by silica gel column using a 40%
CH Cl /hexane mixture. Yellow solid, yield: 2.52 g (58%); R =0.19
2
2
4
2
2
f
1
(
silica, CH Cl /hexane 1:3); H NMR (CDCl , 500 MHz): d=8.35 (s,
2 2 3
1H), 8.13 (d, J=7.5 Hz, 2H), 8.08 (d, J=15.5 Hz, 1H), 7.90 (d, J=
3.5 Hz, 1H), 7.76 (d, J=8.5 Hz, 1H), 7.65 (d, J=4.5 Hz, 1H), 7.45–
7.50 (m, 2H), 7.40 (m, 2H), 7.28 (d, J=7.5 Hz, 1H), 7.18 (m, 1H),
Compound 1: Compound 1 was prepared per the reported proce-
dure.
[20a]
Compound 2: 1-Naphthaldehyde (4.00 g, 3.47 mL, 25.61 mmol), 2-
acetylthiophene (2.76 mL, 25.61 mmol), and NaOH (0.10 g) were re-
4.29 (m, 2H), 1.85 (q, J=7.5 Hz, 2H), 1.36–1.43 (m, 2H), 0.94 ppm
(m, 3H); C NMR (CDCl , 125 MHz): d=182.16, 146.13, 145.72,
3
13
Chem. Asian J. 2016, 11, 1572 – 1587
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1
1
3
42.05, 141.00, 133.27, 131.33, 128.17, 126.37, 126.35, 125.72,
23.4, 122.78, 121.65, 120.61, 119.73, 118.53, 109.19, 109.17, 43.07,
1.12, 20.55, 13.88 ppm; HRMS (ESI-Q-TOF): m/z calcd for
4H), 7.26 (m, 2H), 7.12 (m, 1H), 4.88–4.92 (m, 1H), 4.75–4.80 (m,
13
1H), 4.29–4.35 (m, 1H), 3.38–3.49 ppm (m, 2H); C NMR (CDCl3,
125 MHz): d=189.7, 143.56, 140.73, 138.17, 138.15, 137.29, 134.60,
132.56, 129.12, 128.46, 127.52, 126.17, 123.49, 120.45, 120.27,
+
+
C H NOS : 360.1422 [M+H] ; found: 360.1420.
23
22
1
20.26, 109.92, 79.25, 42.13, 39.23 ppm; HRMS (ESI-Q-TOF): m/z
+
+
calcd for C H N O S : 441.1273 [M+H] ; found: 441.1283.
26
21
2
3
General Procedure for Compounds 7–12
Compound 11: Chalcone 5 (1.00 g, 2.82 mmol) and nitromethane
A chalcone (1 equiv) was dissolved in a mixture of methanol/trie-
(0.60 mL, 11.28 mmol) were reacted in a mixture of methanol
thylamine (Et N; 3:1). After 5 min, nitromethane (4 equiv) was
(3.40 mL)/Et
rified by silica gel column using a 6% ethyl acetate/hexane mix-
ture. Pale yellow oil, yield: 0.75 g (64%); R =0.23 (silica, CH Cl
, 500 MHz): d=8.05 (d, J=8 Hz, 1H),
7.97 (s, 1H), 7.91 (d, J=7 Hz, 2H), 7.53 (t, J=7.5 Hz, 1H), 7.40–7.46
(m, 3H), 7.31–7.37 (m, 3H), 7.19–7.23 (m, 1H), 4.89–4.92 (m, 1H),
4.74–4.88 (m, 1H), 4.40 (m, 1H), 4.23 (t, J=7.5 Hz, 2H), 3.47–3.60
(m, 2H), 1.80 (t, J=7.5 Hz, 2H), 1.32–1.38 (m, 2H), 0.92 ppm (t, J=
3
N (1.16 mL, 8.3 mmol). The desired product 11 was pu-
3
added into the reaction flask. Then the reaction mixture was
heated to reflux for 24 h. As the reaction progressed, the color
changed from light yellow to black. The reaction mixture was ex-
tracted with ethyl acetate and dried over anhydrous Na SO . The
/
2
f
2
1
hexane 1:1); H NMR (CDCl
3
2
4
solvent was evaporated by using a rotary evaporator under re-
duced pressure. The desired product was subjected to silica gel
column chromatography for purification.
1
3
7
1
1
2
4
.5 Hz, 3H); C NMR (CDCl , 125 MHz): d=197.25, 140.89, 139.97,
Compound 7: Chalcone 1 (2.00 g, 7.74 mmol) and nitromethane
3
36.62, 133.44, 128.71, 128.08, 125.97, 125.00, 123.24, 122.46,
20.44, 119.19, 118.96, 109.23, 108.87, 80.34, 42.92, 42.32, 31.15,
(
(
1.65 mL, 30.96 mmol) were reacted in a mixture of methanol
9.40 mL)/Et N (3.20 mL, 22.83 mmol). The desired product 7 was
3
+
0.56, 13.88 ppm; HRMS (ESI-Q-TOF): m/z calcd for C H N O
:
purified by silica gel column using a 6% ethyl acetate/hexane mix-
26 27
2
3
+
15.2022 [M] ; found: 415.2028.
ture. Brown oil, yield: 1.89 g (76%); R =0.22 (silica, CH Cl /hexane
f
2
2
1
1
:1); H NMR (CDCl , 500 MHz): d=8.21 (d, J=8 Hz, 1H), 7.91 (d,
Compound 12: Chalcone 6 (0.42 g, 1.17 mmol) and nitromethane
3
J=7.5 Hz, 2H), 7.87 (d, J=8 Hz, 1H), 7.77 (d, J=7 Hz, 1H), 7.50–
(0.25 mL, 4.68 mmol) were dissolved in a mixture of methanol
7
3
1
1
1
.60 (m, 3H), 7.38–7.44 (m, 4H), 5.16 (m, 1H), 4.85–4.93 (m, 2H),
(1.50 mL)/Et
purified by silica gel column using a 6% ethyl acetate/hexane mix-
ture. Pale yellow oil, yield: 0.25 g (51%); R =0.22 (silica, CH Cl
, 500 MHz): d=8.70 (d, J=8 Hz, 1H),
7.97 (s, 1H), 7.69 (d, J=3.5 Hz, 1H), 7.60 (d, J=4.5 Hz, 1H), 7.45 (t,
J=7.5 Hz, 1H), 7.37 (d, J=8.5 Hz, 1H), 7.34 (s, 2H), 7.21 (t, J=
3
N (0.50 mL, 3.45 mmol). The desired product 12 was
1
3
.60 ppm (m, 2H); C NMR (CDCl , 125 MHz): d=196.98, 136.39,
3
35.16, 134.24, 133.59, 133.09, 131.02, 129.28, 129.00, 128.75,
28.58, 128.48, 128.14, 128.07, 127.22, 126.97, 126.10, 125.62,
25.30, 123.53, 122.43, 78.86, 41.48, 33.65 ppm; HRMS (ESI-Q-TOF):
f
2
/
2
1
hexane 1:1); H NMR (CDCl
3
+
+
m/z calcd for C H NO : 320.1287 [M] ; found: 320.1292.
2
0
18
3
7
4
7
1
1
.5 Hz, 1H), 7.08 (m, 1H), 4.89–4.93 (m, 1H), 4.76–4.80 (m, 1H),
Compound 8: Chalcone 2 (2 g, 7.56 mmol) and nitromethane
.34–4.40 (m, 1H), 4.22-4.25 (m, 2H), 3.51 (m, 1H), 1.80 (q, J=
(
(
1.61 mL, 30.24 mmol) were reacted in a mixture of methanol
13
.5 Hz, 2H), 1.33–1.40 (m, 2H), 0.92 ppm (m, 3H); C NMR (CDCl3,
25 MHz): d=190.21, 143.70, 140.83, 139.96, 134.26, 132.32,
29.01, 128.25, 125.95, 124.92, 123.17, 122.36, 120.41, 119.11,
9.20 mL)/Et N (3.10 mL, 22.3 mmol). The desired product 8 was pu-
3
rified by column chromatography on silica gel using a 6% ethyl
acetate/hexane mixture. Yellow oil, yield: 1.34 g (54%); R =0.20
f
1
118.92, 109.21, 108.83, 80.15, 50.69, 42.92, 39.83, 31.12, 20.54,
(
silica, CH Cl /hexane 1:1); H NMR (CDCl , 500 MHz): d=8.20 (d,
2
2
3
+
1
3.85 ppm; HRMS (ESI-Q-TOF): m/z calcd for C H N O S :
24 25 2 3
J=8.5 Hz, 1H), 7.86 (d, J=8 Hz, 1H), 7.76 (m, 1H), 7.66 (d, J=
+
4
21.1586 [M+H] ; found: 421.1587.
3
7
3
1
1
.5 Hz, 1H), 7.57–7.62 (m, 2H), 7.51 (m, 1H), 7.38–7.42 (m, 2H),
.08 (d, J=4.5 Hz, 1H), 5.10–5.16 (m, 1H), 4.85–4.94 (m, 2H),
1
3
.52 ppm (m, 2H); C NMR (CDCl , 125 MHz): d=189.83, 135.55,
3
General Procedure for Aza–dipyrrins 13–18
34.86, 134.35, 134.23, 132.31, 130.99, 129.28, 128.54, 128.29,
27.00, 126.12, 125.31, 123.55, 122.40, 78.71, 42.02, 33.81 ppm;
The precursor compound (7–12; 1 equiv) was dissolved in ethanol
(15 mL), and NH OAc (35 equiv) was added to it. The reaction mix-
+
+
HRMS (ESI-Q-TOF): m/z calcd for C H NO S : 326.0851 [M] ;
1
8
16
3
4
found: 326.0831.
ture was allowed to stir for 48 h. Towards completion of the reac-
tion, the product precipitated as blue solid. The crude product was
filtered and washed with cold ethanol to obtained pure aza–dipyr-
rin.
Compound 9: Chalcone 3 (2.00 g, 5.35 mmol) and nitromethane
(
(
1.14 mL, 21.40 mmol) were reacted in a mixture of methanol
6.50 mL)/Et N (2.20 mL, 15.78 mmol). The desired product 9 was
3
purified by silica gel column chromatography using a 35% CH Cl /
Compound 13: Compound 7 (2.26 g, 8.78 mmol) was treated with
2
2
hexane mixture. Pale yellow oil, yield: 1.47 g (63%); R =0.19 (silica,
NH OAc (23.69 g, 307.43 mmol) in ethanol (20.00 mL) per the gen-
f
4
1
CH Cl /hexane 1.5:1); H NMR (CDCl , 500 MHz): d=8.12 (d, J=
eral procedure. Blue solid, yield: 1.42 g (29%); R =0.24 (silica,
2
2
3
f
1
7
6
4
.5 Hz, 2H), 7.96 (d, J=8 Hz, 2H), 7.58–7.61 (m, 1H), 7.47–7.54 (m,
CH Cl /hexane 1:1.5); H NMR (CDCl , 500 MHz): d=8.28 (d, J=
2
2
3
H), 7.35–7.42 (m, 4H), 7.26–7.29 (m, 2H), 4.90–4.94 (m, 1H), 4.77–
8 Hz, 2H), 8.01 (d, J=7.5 Hz, 4H), 7.81 (d, J=6.5 Hz, 2H), 7.71 (d,
J=8 Hz, 2H), 7.55-7.60 (m, 6H), 7.45-7.51 (m, 6H), 7.22 ppm (d, J=
13
.81 (m, 1H), 4.34–4.39 (m, 1H), 3.48–3.60 ppm (m, 2H); C NMR
13
(
CDCl , 125 MHz): d=196.68, 140.69, 138.30, 137.36, 136.34, 133.75,
6.5 Hz, 4H); C NMR (CDCl , 125 MHz): d=154.99, 150.43, 142.49,
3
3
1
1
29.05, 128.85, 128.10, 127.60, 125.98, 123.42, 120.33, 120.08,
133.8, 132.27, 131.9, 131.18, 130.15, 129.67, 129.24, 128.3, 128.11,
126.65, 126.26, 125.86, 125.43, 125.03, 118.87 ppm; HRMS (ESI-Q-
TOF): m/z calcd for C H N : 550.2283 [M+H] ; found: 550.2283.
40 28 3
09.75, 41.56, 39.01 ppm; HRMS (ESI-Q-TOF): m/z calcd for
+
+
+
+
C H N O : 435.1709 [M+H] ; found: 435.1690.
28
23
2
3
Compound 10: Chalcone 4 (3.00 g, 7.90 mmol) and nitromethane
Compound 14: Compound 8 (2.10 g, 1.69 mmol) and NH OAc
4
(
(
1.70 mL, 31.60 mmol) were reacted in a mixture of methanol
(17.50 g, 227.17 mmol) were reacted in ethanol (20.00 mL) per the
9.60 mL)/Et N (3.20 mL, 23.30 mmol). The desired product 10 was
general procedure. Blue solid, yield: 0.43 g (11 %); R =0.22 (silica,
3
f
1
purified by silica gel column using a 35% CH Cl /hexane (35%)
CH Cl /hexane 1:1.5); H NMR (CDCl , 500 MHz): d=8.24 (d, J=
2
2
2
2
3
mixture. Pale yellow oil, yield: 2.13 g (61%); R =0.18 (silica, CH Cl /
8.5 Hz, 2H), 7.80 (d, J=6.5 Hz, 2H), 7.69 (d, J=7 Hz, 2H), 7.60 (d,
J=2.5 Hz, 2H), 7.56 (d, J=7 Hz, 2H), 7.52 (d, J=4.5 Hz, 2H), 7.44
(m, 4H), 7.19–7.22 (m, 4H), 7.07 ppm (s, 2H); C NMR (CDCl3,
f
2
2
1
hexane 1.5:1); H NMR (CDCl , 500 MHz): d=8.12 (d, J=8 Hz, 2H),
3
13
7
.71 (d, J=3 Hz, 1H), 7.65 (m, 1H), 7.47–7.52 (m, 4H), 7.34–7.39 (m,
Chem. Asian J. 2016, 11, 1572 – 1587
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1
1
1
25 MHz): d=150.19, 149.02, 142.10, 137.19, 133.78, 131.86,
31.00, 129.62, 128.81, 128.78, 128.29, 128.14, 127.46, 126.23,
25.86, 25.43, 125.01, 118.72 ppm; HRMS (ESI-Q-TOF): m/z calcd for
a rotary evaporator before subjection to silica gel column chroma-
tography.
Compound 19: Aza–dipyrrin 13 (0.2 g, 0.36 mmol) and BF ·OEt
+
+
3
2
C H N S : 562.1412 [M+H] ; found: 562.1415.
36
24
3 2
(
(
0.71 mL, 5.61 mmol) were reacted in a mixture of dry CH Cl
0.25 mL)/dry DIEA (0.7 mL) per the general procedure. The desired
2
2
Compound 15: Compound 9 (2.00 g, 4.6 mmol) and NH OAc
4
(
12.40 g, 161 mmol) were reacted in ethanol (20.00 mL) per the
product was purified by column chromatography on silica gel and
eluted with 40% CH Cl /hexane. Evaporation of the solvent mix-
general procedure. Blue solid, yield: 1.04 g (29%); R =0.22 (silica,
CH Cl /hexane 1:1); H NMR (CDCl , 500 MHz): d=8.33 (d, J=
7
f
2
2
1
2
2
3
ture afforded 19 as a metallic green solid. Yield: 0.05 g (23%); R =
f
.5 Hz, 4H), 8.09 (d, J=7.5 Hz, 4H), 8.03 (d, J=7.5 Hz, 4H), 7.65 (d,
1
0
.22 (silica, CH Cl /hexane 1:1.5); H NMR (CDCl , 500 MHz): d=8.27
2
2
3
J=8 Hz, 4H), 7.59 (t, J=7.5 Hz, 4H), 7.53 (d, J=7 Hz, 2H), 7.45 (d,
(
d, J=9 Hz, 2H), 8.13 (d, J=4.5 Hz, 4H), 7.85 (m, 2H), 7.79 (d, J=
13
J=8 Hz, 4H), 7.32 (s, 2H), 7.19 (m, 4H), 7.14 ppm (m, 4H); C NMR
CDCl , 125 MHz): d=155.50, 141.85, 140.65, 137.62, 132.70, 132.09,
8
2
1
1
.5 Hz, 2H), 7.66 (d, J=6.5 Hz, 2H), 7.51 (m, 10H), 7.33 (t, J=8 Hz,
(
1
1
13
3
H), 7.12 ppm (s, 2H); C NMR (CDCl , 125 MHz): d=159.35,
3
30.54, 130.43, 130.37, 129.28, 126.69, 126.64, 123.53, 120.39,
20.24, 120.03, 115.51, 114.73, 109.96, 109.75 ppm; HRMS (ESI-Q-
46.71, 144.40, 133.86, 131.65, 131.57, 131.01, 130.32, 129.81,
29.77, 129.45, 128.71, 128.54, 126.48, 125.83, 125.08, 123.14 ppm;
+
+
TOF): m/z calcd for C H N : 780.3127 [M+H] ; found: 780.3129.
19
11
5
6
38
5
F NMR (470.4 MHz, CDCl ): d=À131.59 ppm (q, 2F); B NMR
3
Compound 16: Compound 10 (1.50 g, 3.28 mmol) and NH OAc
(160 MHz, CDCl ): d=1.18 ppm (t, 1B); HRMS (ESI-Q-TOF): m/z
3
calcd for C H BF N : 598.2266 [M+H] ; found: 598.2244.
40 27 2 3
4
+
+
(
8.86 g, 114.98 mmol) were reacted in ethanol (20.00 mL) per the
general procedure. Blue solid, yield: 0.47 g (18%); R =0.20 (silica,
f
1
Compound 20: Aza–dipyrrin 14 (0.20 g, 0.35 mmol) and BF
3
·OEt
2
CH Cl /hexane 1:1); H NMR (CDCl , 500 MHz): d=8.30 (d, J=8 Hz,
2
2
3
(
0.69 mL, 5.46 mmol) were reacted in a mixture of dry CH Cl
2
2
4
2
H), 8.08 (d, J=7.5 Hz, 4H), 7.62–7.76 (m, 7H), 7.57 (d, J=5 Hz,
H), 7.43 (d, J=8 Hz, 4H), 7.18 (m, 7H), 7.14 ppm (m, 4H); C NMR
13
(0.25 mL)/dry DIEA (0.70 mL) per the general procedure. The de-
sired product was purified by column chromatography on silica gel
and eluted with 40% CH Cl /hexane. Evaporation of the solvent
(
CDCl , 125 MHz): d=149.45, 141.45, 140.63, 137.62, 137.01, 132.53,
3
2
2
1
1
30.38, 129.10, 128.85, 127.62, 126.62, 125.99, 123.51, 120.23,
mixture afforded 20 as a metallic green solid. Yield: 0.06 g (28%);
20.01, 115.06, 109.73 ppm; HRMS (ESI-Q-TOF): m/z calcd for
1
+
+
R
8
2
2
4
1
1
f
=0.19 (silica, CH Cl /hexane 1:1.5); H NMR (CDCl , 500 MHz): d=
2
2
3
C H N S : 792.2256 [M+H] ; found: 792.2230.
52
34
5 2
.45 (d, J=4 Hz, 2H), 8.23 (d, J=7.5 Hz, 2H), 7.84 (d, J=8.5 Hz,
H), 7.77 (d, J=8.5 Hz, 2H), 7.67 (d, J=5 Hz, 2H), 7.62 (d, J=7 Hz,
H), 7.48–7.53 (m, 4H), 7.28–7.31 (m, 2H), 7.25 ppm (d, J=5 Hz,
H); C NMR (CDCl , 125 MHz): d=149.69, 146.71, 143.10, 134.16,
33.84, 133.33, 133.27, 133.21, 131.96, 131.65, 130.03, 129.90,
29.58, 129.35, 128.50, 126.44, 125.84, 125.79, 125.04, 122.63 ppm;
Compound 17: Compound 11 (0.70 g, 1.69 mmol) and NH OAc
4
(
4.55 g, 59.15 mmol) were reacted in ethanol (20.00 mL) per the
general procedure. Blue solid, yield: 0.58 g (46%); R =0.22 (silica,
13
f
1
3
CH Cl /hexane 1:2); H NMR (CDCl , 500 MHz): d=8.83 (s, 2H), 8.34
2
2
3
(
d, J=8.5 Hz, 2H), 8.02 (d, J=8 Hz, 4H), 7.88 (d, J=7.5 Hz, 2H),
7
8
4
.55 (t, J=7.5 Hz, 4H), 7.46 (m, 2H), 7.35-7.41 (m, 4H), 7.31 (d, J=
.5 Hz, 2H), 7.27 (s, 2H), 7.01 (t, J=7.5 Hz, 2H), 4.20 (t, J=7.5 Hz,
H), 1.76–1.82 (m, 4H), 1.33-1.39 (m, 4H), 0.91 ppm (t, J=7.5, 6H);
19
11
F NMR (470.4 MHz, CDCl ): d=À139.32 ppm (q, 2F); B NMR
3
(
160 MHz, CDCl ): d=1.47 ppm (t, 1B); HRMS (ESI-Q-TOF): m/z
3
+
+
calcd for C H BF N S : 610.1395 [M+H] ; found: 610.1362.
1
3
36 23
2
3 2
C NMR (CDCl , 125 MHz): d=154.83, 149.90, 143.89, 140.83,
3
1
1
2
7
40.41, 132.63, 129.72, 129.06, 127.17, 126.54, 125.49, 125.20,
Compound 21: Aza–dipyrrin 15 (0.50 g, 0.64 mmol) and BF
(1.26 mL, 9.98 mmol) were reacted in a mixture of dry CH
3
·OEt
Cl
2
23.30, 123.14, 120.72, 118.71, 113.41, 108.78, 108.63, 42.91, 31.13,
2
2
+
0.51, 13.84 ppm; HRMS (ESI-Q-TOF): m/z calcd for C H N
40.3753 [M+H] ; found: 740.3720.
:
(0.45 mL)/dry DIEA (1.30 mL) per the general procedure. The de-
sired product 21 was purified by silica gel column chromatography
using a 45% CH Cl /hexane mixture. Green solid, yield: 0.08 g
52
46
5
+
Compound 18: Compound 12 (1.00 g, 2.37 mmol) and NH OAc
2
2
4
1
(
15%); Rf =0.21 (silica, CH Cl /hexane 1:1); H NMR (CDCl3,
2 2
(
6.40 g, 82.95 mmol) were reacted in ethanol (20.00 mL) per the
5
00 MHz): d=8.91 (s, 2H), 8.35 (dd, J=8.5 Hz, J=1.5 Hz, 2H), 8.09
general procedure. Blue solid, yield: 0.37 g (21%); R =0.21 (silica,
CH Cl /hexane 1:2); H NMR (CDCl , 500 MHz): d=8.81 (s, 2H), 8.32
f
1
(m, 4H), 7.99 (d, J=8.5 Hz, 2H), 7.47–7.51 (m, 6H), 7.39–7.44 (m,
2
2
3
6
1
H), 7.07 (m, 4H), 4.26 (d, J=7.5 Hz, 4H), 1.80–1.86 (m, 4H), 1.34–
(
d, J=8.5 Hz, 2H), 7.89 (d, J=7.5 Hz, 2H), 7.65 (d, J=3.5 Hz, 2H),
13
.42 (m, 4H), 0.93 ppm (t, J=7.5, 6H); C NMR (CDCl , 125 MHz):
7
7
.51 (d, J=5 Hz, 2H), 7.35-7.41 (m, 4H), 7.31 (d, J=8.5 Hz, 2H),
.22 (t, J=4.5 Hz, 2H), 7.13 (s, 2H) 7.02 (t, J=7.5 Hz, 2H), 4.20 (t,
3
d=151.75, 140.60, 130.42, 129.29, 126.68, 126.63, 126.00, 120.25,
1
9
1
20.02, 115.20, 109.74, 52.34 ppm; F NMR (470.4 MHz, CDCl ): d=
3
J=7.5 Hz, 4H), 1.76–1.82 (m, 4H), 1.32–1.38 (m, 4H), 0.90 ppm (t,
J=7.5 Hz, 6H); C NMR (CDCl , 125 MHz): d=149.62, 148.89,
1
1
1
11
1
3
À131.18 ppm (q, 2F); B NMR (160 MHz, CDCl ): d=1.04 ppm (t,
3
3
+
1
B); HRMS (ESI-Q-TOF): m/z calcd for C H BF N : 828.3110
43.47, 140.81, 140.41, 137.60, 128.61, 128.22, 127.14, 126.90,
25.51, 125.02, 123.27, 123.11, 121.36, 120.77, 118.72, 113.32,
08.78, 108.66, 42.90, 31.14, 29.72, 20.51, 13.86 ppm; HRMS (ESI-Q-
56 37
2
5
+
[M+H] ; found: 828.3121.
Compound 22: Aza–dipyrrin 16 (0.32 g, 0.4 mmol) and BF ·OEt
(0.79 mL, 6.24 mmol) were reacted in a mixture of dry CH Cl
3
2
+
+
TOF): m/z calcd for C H N S : 752.2882 [M] ; found: 752.2860.
4
8
42
5
2
2
2
(
0.30 mL)/dry DIEA (0.80 mL) per the general procedure. The de-
sired product 22 was purified by silica gel column chromatography
General Procedure for Aza–BODIPYs 19–24
using a 45% CH Cl /hexane mixture. Green solid, yield: 0.05 g
2
2
1
In a clean, dry, two-necked, round-bottomed 100–250 mL flask,
aza–dipyrrin (19–24, 1 equiv) was taken under inert atmosphere.
Dry CH Cl (11 equiv) and dry diisopropylethylamine (DIEA;
(14%); Rf =0.19 (silica, CH Cl /hexane 1:1); H NMR (CDCl3,
2 2
500 MHz): d=8.44 (d, J=4 Hz, 2H), 8.31 (d, J=8 Hz, 4H), 8.09 (m,
4H), 7.68 (m, 6H), 7.47 (m, 4H), 7.31 (m, 2H), 7.28 (s, 2H), 7.23 ppm
2
2
13
11 equiv) were added into the flask. After 15 min, BF ·OEt2
(m, 8H); C NMR (CDCl , 125 MHz): d=150.06, 145.80, 141.81,
3
3
(
15.6 equiv) was added to the reaction mixture and allowed to stir
140.42, 138.92, 134.08, 133.42, 133.36, 132.08, 130.91, 130.69,
130.01, 126.70, 126.11, 123.69, 120.35, 120.28, 118.66, 109.75 ppm;
for 24 h at room temperature. The color of the reaction mixture
changed from blue to green when the product formed, then it was
quenched with water and extracted with dichloromethane. The
solvent was dried over anhydrous Na SO and evaporated using
19
11
F NMR (470.4 MHz, CDCl ): d=À138.58 ppm (q, 2F); B NMR
3
(160 MHz, CDCl ): d=1.37 ppm (t, 1B); HRMS (ESI-Q-TOF): m/z
3
+
+
calcd for C H BF N S : 840.2238 [M+H] ; found: 840.2207.
2
4
52 33
2
5 2
Chem. Asian J. 2016, 11, 1572 – 1587
www.chemasianj.org
1585
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Compound 23: Aza–dipyrrin 17 (0.50 g, 0.67 mmol) and BF ·OEt2
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6
1
768–6816; b) A. Satake, Y. Kobuke, Org. Biomol. Chem. 2007, 5, 1679–
691.
(
(
1.32 mL, 10.45 mmol) were reacted in a mixture of dry CH Cl2
0.50 mL)/dry DIEA (1.30 mL) per the general procedure. The de-
2
[
4] a) T. K. Khan, M. Brçring, S. Mathur, M. Ravikanth, Coord. Chem. Rev.
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sired product 23 was purified by silica gel column chromatography
using a 30% CH Cl /hexane mixture. Green solid, yield: 0.13 g
(
5
7
4
0
1
1
013, 257, 2348–2387; b) F. Li, S. I. Yang, Y. Ciringh, J. Seth, C. H. Martin,
2
2
D. L. Singh, D. Kim, R. R. Birge, D. F. Bocian, D. Holten, J. S. Lindsey, J.
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1
23%); Rf =0.19 (silica, CH Cl /hexane 1:2); H NMR (CDCl3,
2 2
00 MHz): d=8.91 (s, 2H), 8.34 (m, 2H), 8.09 (m, 4H), 7.99 (d, J=
.5 Hz, 2H), 7.47–7.51 (m, 6H), 7.39–7.44 (m, 6H), 7.07 (m, 4H),
.26 (t, J=7.5 Hz, 4H), 1.80–1.86 (m, 4H), 1.34–1.42 (m, 4H),
1
3
.93 ppm (t, J=7.5, 6H); C NMR (CDCl , 125 MHz): d=158.59,
3
[
51.75, 145.51, 145.07, 141.30, 140.95, 132.22, 130.42., 129.52,
28.49, 127.40, 126.03, 124.00, 123.55, 123.23, 122.16, 120.83,
1
19.26, 117.19, 109.21, 108.99, 43.04, 31.17, 20.53, 13.86 ppm;
1
9
11
[6] a) V. Lakshmi, M. Ravikanth, Dyes Pigm. 2013, 96, 665–671; b) T. K. Khan,
S. K. Jana, M. R. Rao, M. S. Shaikh, M. Ravikanth, Inorg. Chim. Acta 2012,
F NMR (470.4 MHz, CDCl ): d=À130.38 ppm (q, 2F); B NMR
3
(
160 MHz, CDCl ): d=1.05 ppm (t, 1B); HRMS (ESI-Q-TOF): m/z
3
3
83, 257–266.
+
+
calcd for C H BF N : 788.3736 [M+H] ; found: 788.3718.
5
2
45
2
5
[7] a) H. Kim, A. Burghart, M. B. Welch, J. Reibenspies, K. Burgess, Chem.
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N. Ono, H. Yamada, H. Uno, Tetrahedron 2011, 67, 3187–3193; c) C.
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Ikeda, T. Maruyama, T. Nabeshima, Tetrahedron Lett. 2009, 50, 3349–
Compound 24: Aza–dipyrrin 18 (0.20 g, 0.26 mmol) and BF ·OEt2
(
(
sired product 24 was purified by silica gel column chromatography
using a 30% CH Cl /hexane mixture. Green solid, yield: 0.06 g
(
5
8
(
1
3
0.51 mL, 4.05 mmol) were reacted in a mixture of dry CH Cl2
2
0.20 mL)/dry DIEA (0.50 mL) per the general procedure. The de-
3
351.
8] a) L. H. Thoresen, H. Kim, M. B. Welch, A. Burghart, K. Burgess, Synlett
998, 11, 1276—1278; b) A. Burghart, H. Kim, M. B. Welch, L. H. Thore-
2
2
[
1
28%); Rf =0.18 (silica, CH Cl /hexane 1:2); H NMR (CDCl3,
2 2
1
00 MHz): d=8.90 (s, 2H), 8.41 (d, J=3.5 Hz, 2H), 8.35 (m, 2H),
.00 (d, J=8 Hz, 2H), 7.61 (d, J=5 Hz, 2H), 7.39–7.44 (m, 6H), 7.28
sen, J. Reibenspies, K. Burgess, F. Bergstrçm, L. B. Š. Johansson, J. Org.
Chem. 1999, 64, 7813–7819; c) K. Rurack, M. Kollmannsberger, J. Daub,
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Hochstrasser, K. Burgess, Chem. Eur. J. 2003, 9, 4430–4441; f) O. Buyuk-
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m, 2H), 7.23 (s, 2H), 7.05 (m, 2H), 4.27 (m, 4H), 1.81–1.87 (m, 4H),
1
3
.34–1.42 (m, 4H), 0.92 ppm (m, 6H); C NMR (CDCl , 125 MHz):
3
d=149.05, 145.76, 143.64, 141.20, 140.91, 134.58, 132.53, 132.46,
1
1
1
32.40, 130.65, 129.61, 127.27, 125.94, 123.71, 123.45, 123.23,
21.94, 120.88, 119.18, 116.69, 109.09, 108.92, 42.97, 31.14, 20.50,
1
9
3.83 ppm; F NMR (470.4 MHz, CDCl ): d=À137.34 ppm (q, 2F);
3
[
9] a) P. E. Kesavan, I. Gupta, Dalton Trans. 2014, 43, 12405–12413; b) P. E.
Kesavan, S. Das, M. Y. Lone, P. C. Jha, S. Mori, I. Gupta, Dalton Trans.
2015, 44, 17209–17221.
1
1
B NMR (160 MHz, CDCl ): d=1.30 ppm (t, 1B); HRMS (ESI-Q-TOF):
3
+
+
m/z calcd for C H BF N S : 800.2864 [M] ; found: 800.2874.
4
8
41
2
5 2
CCDC-1016009 (13) and 1425995 (14) 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.
[10] a) M. Tasior, D. F. O’Shea, Bioconjugate Chem. 2010, 21, 1130–1133;
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2
6
Acknowledgements
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Liu, S. Bai, L. Lin, X. Feng, Org. Biomol. Chem. 2009, 7, 4120.
Financial support from the Science and Engineering Research
Board (SERB) (EMR/2015/000779) of the Government of India,
is greatly acknowledged. I.G. thanks Prof. H. Furuta of Kyushu
University, Japan for X-ray measurements. N.B. thanks IIT Gan-
dhinagar for a fellowship and infrastructural support. P. C.J. ac-
knowledges CUG for providing basic computational facilities
and UGC, New Delhi for startup grants. M.Y.L. thanks the UGC
for financial assistance.
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[
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Keywords: density functional calculations · donor–acceptor
systems · dyes/pigments · energy transfer · X-ray diffraction
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Manuscript received: February 10, 2016
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