Synthesis of thiophene-substituted aza-BODIPYs and their optical and electrochemical properties

Article abstract of DOI:10.1016/j.tet.2011.06.100

A series of novel thiophene-substituted aza-BODIPY dyes were synthesized by means of a standard procedure and complemented by a Stille-coupling of a brominated species with 2-tributylstannylthiophene. The optical as well as the electrochemical properties of the compounds were investigated and compared to result of density functional theory (DFT) calculations. The influence of the thiophene substituents is discussed in dependence of the position at the aza-BODIPY core regarding the HOMO and LUMO frontier orbitals. The different distributions of the HOMO and LUMO coefficients over the BODIPY core lead to a variable influence of the thiophene substituents on the HOMO and LUMO energies, being the origin of the tunable optical and electrochemical properties.

Full text of DOI:10.1016/j.tet.2011.06.100

Tetrahedron 67 (2011) 7148e7155
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of thiophene-substituted aza-BODIPYs and their optical
and electrochemical properties
*
Roland Gresser, Horst Hartmann , Marion Wrackmeyer, Karl Leo, Moritz Riede
€
€
€
Institut fur Angewandte Photophysik, George-Bahr-Str. 1, Technische Universitat Dresden, 01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel thiophene-substituted aza-BODIPY dyes were synthesized by means of a standard pro-
cedure and complemented by a Stille-coupling of a brominated species with 2-tributylstannylthiophene.
The optical as well as the electrochemical properties of the compounds were investigated and compared to
result of density functional theory (DFT) calculations. The influence of the thiophene substituents is dis-
cussed in dependence of the position at the aza-BODIPY core regarding the HOMO and LUMO frontier
orbitals. The different distributions of the HOMO and LUMO coefficients over the BODIPY core lead to
a variable influence of the thiophene substituents on the HOMO and LUMO energies, being the origin of the
tunable optical and electrochemical properties.
Received 23 March 2011
Received in revised form 20 June 2011
Accepted 27 June 2011
Available online 2 July 2011
Keywords:
Aza-BODIPY
Stille-coupling
NIR absorption
Cyclic voltammetry
DFT calculations
Ó 2011 Elsevier Ltd. All rights reserved.
R4
B
R3
R3
R1
R3
R3
R1
1. Introduction
N
B
R2
R2
R2
R2
Boron difluoride dipyrromethene derivatives (BODIPYs) have
attracted much attention due to their intense absorption and strong
fluorescence combined with an excellent photostability.1 Therefore,
a large range of divers substituted BODIPYs have been prepared and
extensively studied for various applications, such as fluorescence
probes, e.g., for biological imaging,2 as emitter material in organic
light emitting diodes,3 or as absorber material for organic solar
cells.4
N
N
N
N
R1
R1
F
F
F
F
BODIPY
AZABODIPY
Scheme 1. Representation of the BODIPY (difluoroboradiaza-indacene) and the aza-
BODIPY (difluoroboratriaza-indacene) structures.
derivatives are unknown at yet, on the other hand, we have syn-
thesized few examples 1aed of this series and studied their optical
and electrochemical properties more in detail. In addition to the
experimental studies, quantum chemical calculations by means of
the density functional theory (DFT) were applied to obtain deeper
insights into their absorption characteristics and energy levels,
which are essential for understanding the optoelectronic properties
of these compounds.
In contrast, corresponding aza-BODIPYs as aza-analogous
BODIPYs (Scheme 1) are described to a lower extent in the litera-
ture hitherto, obviously due to their less effective synthesis starting
from 1,3-bis(het)aryl-4-nitrobutanones5 or from 1-(het)aryl-5-
nitroso-pyrroles.6 Nevertheless, few aza-BODIPYs have been stud-
ied for some applications, such as sensitizers for singlet oxygen
generation,7 as near-infrared absorbers,8 as NIR-emitting chemo-
sensors for heavy-metal detection,9 or as fluorescent sensors for
polymer characterization.10
2. Results and discussion
2.1. Synthesis
Because thienyl-substituents at the BODIPY core give rise to
compounds with some interesting properties, e.g., to compounds
with remarkable long-wavelength absorptions11 and emissions,12
to redox-active polymers,13 to heavy-metal sensors,14 or to light-
collectors,15 on the one hand, and thienyl-substituted aza-BODIPY
The synthesis of the thiophene-substituted aza-BODIPYs 1aed
(see Scheme 2) follows a route developed by Rogers5 in 1943 and
later on applied for other aza-BODIPYs as well.5,6,7a,8c,9
In the crucial step of this method (outlined in Scheme 3),
4-nitro-1,3-di(het)arylbutanones 6 are reacted with ammonium
* Corrresponding author. E-mail address: hartmann@iapp.de (H. Hartmann).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.06.100

R. Gresser et al. / Tetrahedron 67 (2011) 7148e7155
7149
S
S
S
S
S
S
N
B
N
B
N
B
N
B
N
B
N
N
N
N
N
N
N
N
N
N
S
S
F
F
F
F
F
F
F
F
F
F
S
S
1c
1b
1d
1a
2
Scheme 2. Thiophene-substituted aza-BODIPYs 1aed investigated in comparison to the tetraphenyl-substituted parent compound 2.
acetate to the appropriate pyrroles and nitroso-pyrroles, as dem-
onstrated by Hall et al.,6 followed by a subsequent condensation of
both intermediates.
spectra are depicted in Fig. 1. The 1,3,5,7-tetraphenyl aza-BODIPY 2,
which was already described elsewhere,14,17 is shown here for
reasons of comparison.
O
Table 1
O
(a)
Spectroscopic characteristics of the dipyrromethines 7aed and aza-BODIPY com-
pounds 1aed, measured in dichloromethane
R2
+
R1
CH3
O
R1
R2
4
3
5
l
uva [nm] Extinction
[L molꢀ1 cmꢀ1
35,500
lmaxb [nm] Extinction
[L molꢀ1 cmꢀ1
43,000
lflu
Fd
]
]
[nm]c
(b)
R2
R2
7a 328
7b 308
7c 335
7d 297
1a 344
1b 320
1c 354
1d 270
633
625
663
590
718
683
742
650
d
d
d
d
d
d
d
d
NO2
R2
N
(c)
8-12%
36,000
37,500
42,500
35,000
29,000
40,000
18,500
37,500
45,000
40,000
120,500
78,000
110,000
40,000
O
NH
N
R1
R1
R1
7
740 (22) 0.44
725 (42) 0.10
764 (22) 0.11
6
78-85%
R2
(d)
R2
R1
d
d
R2
2-Thienyl Phenyl
a
b
c
d
a
N
Extinction coefficients are given for the most intense absorption in the UV re-
Phenyl
2-Thienyl
2-Thienyl 2-Thienyl
gion of the spectra.
Phenyl
Phenyl
N
N
b
The absorption maximum in the visible regime.
Stokes-shifts are specified in brackets.
Fluorescence quantum yield measured relative to the one of Rhodamine 101.
B
F
1a-c
c
R1
R1
F
d
Ph
Ph
Ph
The absorption spectra of all dipyrroazamethenes 7aed exhibit
two intense peaks, one in the UV region (which is the maximum in
case of 7d) and one in the visible region (usually the maximum).
The absorption maxima of the thiophene substituted dipyrroa-
zamethenes 7aec are found in the visible region at 633 nm,
625 nm, and 663 nm, respectively. These maxima are significantly
shifted into the red region compared to the 1,3,5,7-tetraphenyl
substituted dipyrroazamethene 7d, which absorbs at 589 nm.14,17
The extinctions of both absorption peaks in the UV and the visi-
ble are similar for all dipyrroazamethenes 7aed and found to be
between 35,000 and 37,000 L molꢀ1 cmꢀ1 for the first absorption
bands and between 40,000 and 45,000 L molꢀ1 cmꢀ1 for the second
bands, as can be seen in Table 1.
The complex formation of the dipyrroazamethenes 7 with boron
trifluoride has a tremendous influence on the absorption charac-
teristics of the resulting thiophene-substituted aza-BODIPYs 1aed.
It gives rise to a strong bathochromic shift, especially for the
compounds 1aec bearing the thiophene substituents in the 1,7- or/
and 3,5-positions (see Table 1). Such a bathochromic shift is known
in the BODIPY series to be caused by the complexation with boron
trifluoride17,18 but it is unusual strong for 1a and 1c with 85 nm and
79 nm, since for the 1,3,5,7-tetraphenyl aza-BODIPY 2 it amounts
only to 61 nm compared to the corresponding borondi fluoride-free
1,3,5,7-tetraphenyl dipyrroazamethene.17
As a result, by replacing the phenyl moieties in 1,3,5,7-positions
at the aza-BODIPY core by thiophene moieties (compare 1c and 2),
the absorption maximum can be shifted bathochromically by
92 nm. Surprisingly, compound 1d with the thiophene substituent
in the 2,6-positions at the aza-BODIPY core shows the most hyp-
sochromic and most broadened absorbance in this series with an
absorption maximum at 650 nm. This value is the same as for
compound 217 and indicates that the thiophene moieties at the 2,6-
positions do not have any influence on the visible spectral part.
However, in the UV part of the spectrum of 1d, the absorption band
N
(f)
(e)
(d)
7d
Br
1d
Br
N
N
B
Ph
F
F
8
Scheme 3. Synthetic routes to the thienyl-substituted aza-BODIPYs 1aed. (a) KOH in
EtOH, rt, (b) CH3NO2 in EtOH, K2CO3, reflux, (c) BuOH, NH4HOAc, reflux, (d) BF3$Et2O,
dichloroethane, DIPEA, reflux, (e) Br2, benzene, rt, (f) 2-tributylstannyl-thiophene,
Pd(PPh3)4, toluene, reflux.
The necessary 4-nitro-1,3-di(het)arylbutanones 6 are readily
available by a base-mediated Michael-addition of nitro methane to
a thiophene substituted chalcone 5. After aqueous work-up, the
reaction gave nearly quantitative yields of 4-nitro-1,3-di(het)aryl-
butanones 6, which were directly used without further purification.
The required substituted chalcones 5 were synthesized in the first
step via an aldol condensation of (het)arylketones 3 with (het)
arylaldehydes 4 in excellent yields.16
By using this procedure, the dipyrroazamethenes 7aec were
obtained in yields of 6e12%. Finally, the desired thiophene-
substituted aza-BODIPYs 1aec were prepared by reaction of the
azamethenes 7aec with boron trifluoride etherate under standard
conditions in good yields ranging between 71% and 82%.
For the synthesis of the star-shaped 2,5-dithienyl substituted
aza-BODIPY 1d an extended route using the dibromo aza-BODIPY
compound 8 as key intermediate has been used. This dibromo
compound 8 was prepared according to a route described by O’Shea
et al.7,17 and transformed in good yields into the target compound
1d by means of a Stille-coupling using 2-tributylstannyl-thiophene
as co-reactant.
2.2. Spectroscopic characteristics
The spectroscopic data of all aza-BODIPYs 1aed and their boron
free precursors 7aed are summarized in Table 1. The corresponding

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R. Gresser et al. / Tetrahedron 67 (2011) 7148e7155
with 78,000 L molꢀ1 cmꢀ1. The 2,6-dithienyl substitution in the
aza-BODIPY 1d exhibited with 40,000 L molꢀ1 cmꢀ1 the smallest
extinction coefficient, about one third that of 1a.
Besides of the star-shaped aza-BODIPY 1d, all other thienyl-
substituted derivatives 1aec emit in the red spectral region. The
corresponding fluorescence spectra were measured quantitatively
and are shown in Fig. 1c. As can be seen, the emission maxima vary
between 725 nm for 1b and 764 nm for 1c. The Stokes shift is in the
usual range but it is with 42 nm significantly larger for the com-
pound 1b than for the other compounds 1a and 1c with a Stokes
shift of 22 nm only. This data indicate, that the excited state ge-
ometries of the compounds 1a and 1c have, in contrast to the ge-
ometry of compound 1b, minor differences compared to the
corresponding ground state geometries.19
2.3. Electrochemical characterization
Both compound series 7aed and 1aed as well as compound 8
were analyzed with cyclic voltammetry (CV) to obtain the corre-
sponding oxidation and reduction potentials. The measurements
were performed in dichloromethane, using an Ag/AgCl reference
electrode with the ferrocene/ferrocinium couple (Fc/Fcþ) as in-
ternal redox standard.20
All examined compounds exhibit, as shown for the aza-BODIPYs
1aed in Fig. 2, reversible one-electron reduction and irreversible
one-electron oxidation waves. This fact points to the formation of
stable radical anions under the applied conditions. The reason for
the irreversible oxidation waves might be the unsubstituted
5-position at the thiophene ring, which is known to be reactive and
can undergo electrochemical polymerization via the corresponding
radical cations.21
Fig. 1. Spectroscopic characteristics for 7aed and 1aed; (a) absorption spectra of the
boron free compounds 7aed; (b) absorption spectra of the boron difluoride complexes
1aed; (c) fluorescence spectra of 1aec. Compound 1d did not show any measurable
fluorescence. For details on the wavelength and the extinction coefficients see Table 1.
is diminished, what is in contrast to the other thiophene-
substituted aza-BODIPYs 1aec.
The extinctions of the thiophene-substituted aza-BODIPYs 1aed
differ significantly in dependence of the substitution pattern. The
strongest extinction with 120,000 L molꢀ1 cmꢀ1 is found for the 3,5-
dithienyl-substituted aza-BODIPY compound 1a, followed by the
1,3,5,7-tetrathienyl derivative 1c with 110,000 L molꢀ1 cmꢀ1. The
1,7-dithienyl-substituted species 1b shows a weaker absorbance
Fig. 2. Cyclic voltammograms of the aza-BODIPYs 1aed measured in dichloromethane
against Ag/Agþ
.

R. Gresser et al. / Tetrahedron 67 (2011) 7148e7155
7151
From the redox potentials, listed in Table 2, it can be seen that
the boron difluoride-free dipyrroazamethenes 7aed show the
lowest oxidation and reduction potentials.
3,5-position for 1a result in an increase of 0.2 eV (in comparison to
2), whereas at the 1,7-position for 1b it amounts to 0.16 eV. In 1d the
influence of the thiophene substituents in the 2,6-position on the
HOMO energy is negligible, as no differences in the redox potentials
can be seen for 1d compared to 2.23 The largest effect was found for
the 1,3,5,7-tetrathienyl-substituted aza-BODIPY 1c with an in-
creased HOMO energy of 0.43 eV compared to 2. In contrast to the
HOMO energies, the LUMO energies of 1aed stay rather constant
with minor changes between 0 and 0.09 eV. This allows a system-
atically HOMO energy level tuning and, consequently, a control of
the band gap by the choice of appropriate substituents in this aza-
BODIPY system.
Table 2
Electrochemical data of the dipyrroazamethines 7aed, the aza-BODIPYs 1aed, and 8
a
a
EOX [V]
ERED [V]
HOMOb [eV]
LUMOb [eV]
D
Ec [eV]
7a
7b
7c
7d
8
1a
1b
1c
1d
0.41
0.44
0.35
0.65
1.07
0.68
0.72
0.47
0.90
ꢀ1.19
ꢀ1.18
ꢀ1.16
ꢀ1.16
ꢀ0.72
ꢀ0.76
ꢀ0.86
ꢀ0.84
ꢀ0.77
ꢀ5.19
ꢀ5.22
ꢀ5.13
ꢀ5.43
ꢀ5.85
ꢀ5.46
ꢀ5.50
ꢀ5.25
ꢀ5.68
ꢀ3.59
ꢀ3.60
ꢀ3.62
ꢀ3.62
ꢀ4.06
ꢀ4.02
ꢀ3.92
ꢀ3.94
ꢀ4.01
1.60
1.62
1.51
1.81
1.79
1.44
1.58
1.31
1.67
2.4. Quantum chemical calculations
a
Redox-potentials versus Fc/Fcþ of the first reduction (ERED) and oxidation (EOX);
measured in CH2Cl2/Bu4PF6(0.1 M) versus Ag/AgCl, scan rate 100 mVsꢀ1, with Fc/Fcþ
as internal standard.
DFT calculations for the aza-BODIPYs 1aed were performed (at
the b3lyp/6-31þg** level of theory)24 in order to gain a deeper in-
sight into the experimental optical and electrochemical observed
structureeproperty relationship.
b
Determined HOMO and LUMO energy values.
c
HOMOeLUMO gap from the obtained redox potentials.22
The electronic structures of the aza-BODIPYs 1aed provide
some general characteristics for understanding of substituent ef-
fects on the electrochemical and optical properties. To discuss this
correlation, the HOMO and LUMO orbitals are displayed, exem-
plarily for 1a in Fig. 4 (for compound 1bed see Supplementary
data). It can be seen that both the HOMO and LUMO are exclu-
The redox potentials of the aza-BODIPY compounds 1aed are
shifted to higher potentials compared to 7aed. This effect is caused
by the electron withdrawing character of the BF2 moiety in the aza-
BODIPYs 1aed and leads to a facilitated reduction of these com-
pounds whereas the oxidation becomes less favored. This trend is
intensified for the dibromo substituted aza-BODIPY compound 8,
which is easily reducible due to the electron negativity of the
bromine substituent.
sively characterized as
p orbitals, as expected. However, the dis-
tributions of the MO coefficients for both frontier orbitals show
essential differences.
By using the measured oxidation and reduction potentials, the
HOMO and LUMO energies of 7aed and 1aed were determined
with the potential of Fc/Fcþ as reference energy and are listed in
Table 2. The obtained frontier orbital energies for 1aed are dis-
played in Fig. 3. The energetic stabilization caused by the com-
plexation with BF3 is slightly more pronounced for the LUMO
energies than for the HOMO energies, as can be seen by calculating
the corresponding 1(HOMO)e7(HOMO) and 1(LUMO)e7(LUMO)
differences for 1aed and 7aed, respectively. Consequently, the
HOMOeLUMO gap DE listed in Table 2 for the aza-BODIPYs 1aed is
smaller than for the dipyrroazamethenes 7aed. This characteristic
is in accordance with the trend of the optical absorption mea-
surements (see above).
Fig. 4. Froniter orbitals of aza-BODIPY 1a.
In the LUMO the largest coefficients are localized at the aza-
BODIPY core with a strong contribution on the nitrogen atoms
and, particularly, at the azamethene bridge (in position 8, shown in
Fig. 4b). Corresponding to the larger LUMO coefficient at the ni-
trogen atoms, the complexation with boron trifluoride has
a stronger influence on the LUMO energy. Accordingly, the stabili-
zation of the energy levels due to the electron withdrawing char-
acter of the boron difluoride moiety is more pronounced for the
LUMO in 1a. As a consequence, a reduced energetic gap for 1a
compared to 7a can be recorded by cyclic voltammetry (see above).
In the HOMO orbital (shown in Fig. 4a), the coefficients are small
at the pyrrole nitrogen atoms, especially at the azamethene bridge
nitrogen atom due to a nodal plane. In contrast, the coefficients at
the residual aza-BODIPY core and on the (het)aryl-rings at the 3,5-
position are relatively large.
These observations demonstrate that substituents in the 3,5-
position at the aza-BODIPY core influence predominantly the
HOMO energies in these compounds. Accordingly, the HOMO en-
ergies can be increased more effectively than the LUMO energies by
attaching electron donating substituents at the aza-BODIPY core.25
Moreover, this effect is larger at the 3,5-position than at the 1,7-
position. Therefore, compound 1a shows a higher HOMO energy
than compound 1b. In compound 1c, the overall electron donating
Fig. 3. HOMO and LUMO energy values for the aza-BODIPYs 1aed, derived from the
CV measurements.
Comparing the HOMO energies of the parent aza-BODIPY 2 to
the thiophene substituted derivatives 1aed reveals that the 2,6-
dithienyl-substituted compound 1d has the same HOMO and
LUMO energies as 2, whereas the HOMO energies of the compounds
1aec are significantly increased. The thiophene substituents at the

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R. Gresser et al. / Tetrahedron 67 (2011) 7148e7155
character is enhanced due to the 1,3,5,7-tetrathienyl substitution
and the HOMO energy is further increased compared to 1a and 1b.
Furthermore, the influence of substituents on the frontier or-
bital energies depends on the dihedral angle between the aza-
BODIPY core and the substituents, as the conjugation decreases
with an increasing angle. This was demonstrated by Carreira et al.
who synthesized conformational restricted aza-BODIPYs and found
a strong bathochromic shift of the absorption maximum compared
to the unrestricted compounds.26 In compound 1d, the thienyl
substituents do not show any influence on the energy levels com-
pared to the 1,3,5,7-tetraphenyl aza-BODIPY 2. In the case of 1d
a steric hindrance of the thiophenes can be assumed, due to the
presence of adjacent phenyl rings at the aza-BODIPY core.
shifts in the series 1aec are induced by the reduced HOMOeLUMO
gap, as it was already suggested by the CV results.
The star-shaped aza-BODIPY compound 1d offers a discrepancy
compared to 1aec. Here, two transitions with a HOMO-2/LUMO
and HOMO/LUMO component, differing about 0.25 eV in energy,
are responsible for the absorption maximum (see Table 3).
Accordingly, the absorption band of 1d is caused by a superpo-
sition of two excitations giving rise to a broadening of the
absorption band for the 2,6-dithienyl-substituted aza-BODIPY
derivative 1d.
3. Conclusion
DFT geometry optimizations24,27 illustrate
a propeller-like
To summarize, a series of novel thiophene-substituted aza-
BODIPY dyes has been synthesized via a synthetic standard route
extended by a Stille-coupling of a 2,6-dibromo dipyrroazamethene
derivative with a stannylated thiophene. The new compounds were
systematically characterized in terms of the substitution pattern
with both experimental and theoretical methods.
It was found that the absorption maxima of the aza-BODIPY
compounds prepared can be shifted into the red spectral region
by replacing phenyl with thiophene moieties. This effect can be
attributed to an increase of the HOMO energies, while the LUMO
energies remain nearly constant, resulting in an overall reduced
gap. DFT calculations confirmed the experimental results and dis-
played predominantly a HOMOeLUMO transition for the absorp-
tion, except for the 2,6-thiophene derivative. For this compound,
the absorption originates from two overlapping transitions leading
to a broad and weak absorbance.
Analysis of the frontier orbitals revealed significant differences
in the distribution of the frontier orbital coefficients. According to
the distribution of the HOMO coefficients, the influence of the
thiophene substituents on the HOMO energies is stronger than on
the LUMO energies and varies with the substitution pattern. The
largest effect was found for the thiophene substitution at the
1,3,5,7- and 1,5-positions for the bis-thiophene-substituted aza-
BODIPY compounds. The thiophene substitution in the 2,6-
position has no influence on the optical and electrochemical
properties, which originated from weak conjugation caused by
steric hindrance.
Particularly with regard to applications, the thiophene-
substituted aza-BODIPYs indicate to be suitable for organic solar
cells applications because of their intense absorption and tunable
energy levels as well as for their reversible reduction. Due to the
versatility of the brominated aza-BODIPY compounds in Stille-
coupling reaction, as exemplified for compound 8, numerous
other substituted aza-BODIPY derivatives should be easily acces-
sible. New synthetic targets following that direction, together with
the testing of the optoelectronic properties, are currently under
investigation.
structure of the peripheral (het)aryl-rings in 1d with increased
dihedral angles between 56ꢁ and 60ꢁ. In contrast, the thiophene
moieties in the compounds 1aec exhibit smaller dihedral angles
between 14ꢁ and 19ꢁ and the phenyl rings in 1a and 1b cause angles
between 28ꢁ and 36ꢁ, respectively. Hence, this result confirms the
diminished influence of the thiophene substitution in 1d due to
steric hindrance.27
2.5. Calculated frontier orbital energies
The calculated HOMO energies agree well with the energies
derived from CV measurements with a maximum difference of only
0.02 eVe0.15 eV, with the largest deviation for 1c (see Table 2 and
Table 3). The differences in the LUMO energies with respect to the
experiments are higher, in the order of 0.39e0.54 eV. This origi-
nates to some extent from the idealized calculations in gas phase
and might be improved slightly by applying an appropriate solva-
tion model.28The energies of the absorption maxima of the aza-
BODIPYs 1aed were calculated with the time dependent DFT ap-
proach29 (at the b3lyp/6-31þg** level of theory)24 and were in good
agreement with the experimental value with a tolerance in the
range from 0.19 eV to 0.29 eV. These deviations can be attributed to
overestimated LUMO energies. However, the predicted oscillator
strengths for the absorption maxima reflect the measured trend of
the extinction correctly.
The DFT calculations revealed that the UV absorptions of the
aza-BODIPYs 1aec can be ascribed to transitions from the HOMO
and some of the lower lying occupied levels (HOMO-1, HOMO-3,
HOMO-4) to the LUMO and several higher lying unoccupied
levels (LUMOþ1, LUMOþ2). In accordance with the experiment, no
noteworthy oscillator strength in this spectral region was calcu-
lated for 1d. The absorption maxima in the visible range of the
compounds 1aec are predominantly HOMOeLUMO transitions,
although other excitations are involved to a minor extent, in case
a thiophene unit is present in the 1,7-position as for 1b and 1c (see
Table 3). Therefore, the experimentally observed bathochromic
Table 3
Results of the DFT calculations for the aza-BODIPYs 1aed
Energya [eV] (nm)
Oscillator
strengtha
Principle orbital
contributiona
Energya [eV] (nm)
2.01 (615.11)
Oscillator
strengtha
Principle orbital
contributiona
HOMO [eV]
LUMO [eV]
DE [eV]
1a
3.73 (331.84)
0.70
HOMO-3/LUMO
HOMO/LUMOþ1
HOMO/LUMOþ2
HOMO/LUMOþ1
HOMOeLUMOþ2
HOMO-4/LUMO
HOMO/LUMOþ1
HOMO/LUMOþ2
HOMO-2/LUMO (52%)
HOMO/LUMO (22%)
0.73
HOMO/LUMO (91%)
ꢀ5.44
ꢀ3.48
1.96
1b
1c
3.85 (317.26)
3.59 (344.55)
0.28
0.63
2.01 (614.60)
1.90 (651.57)
0.60
0.68
HOMO/LUMO (64%)
HOMO-1/LUMO (4%)
HOMO/LUMO (63%)
HOMO-1/LUMO (2%)
ꢀ5.58
ꢀ5.39
ꢀ3.53
ꢀ3.53
2.05
1.86
1d
2.30 (537.83)
0.49
2.05 (604.28)
0.32
HOMO-2/LUMO (36%)
HOMO/LUMO (48%)
ꢀ5.71
ꢀ3.47
2.24
a
The most intense transitions contributing to the absorption spectrum.

R. Gresser et al. / Tetrahedron 67 (2011) 7148e7155
7153
4. Experimental and computational details
4.1. General
potassium hydroxide (0.2 g, 3.56 mmol), dissolved in 5 mL
water, was added at rt. The mixture was stirred for 24 h and
the products precipitated were filtered of, washed with aque-
ous ethanol and dried. Recrystallization from ethanol gave 5 as
white solids in satisfactory yields. The spectroscopic data for all
compounds 5aed were in accordance to those reported in the
literature.34
All necessary starting materials were obtained from com-
mercial sources and used without further purification. Toluene
was dried over sodium and distilled right before use or stored
ꢀ
over molecular sieve 4 A. The solvents dichloroethane, dichloro-
1
ꢀ
methane, and n-butanol were dried over molecular sieves 4 A. H
and 13C NMR spectra were recorded in CDCl3 with a Bruker DRX
500 P instrument for 500.13 and 125.76 MHz, respectively. The
assignment of quaternary, secondary, and primary C atoms was
completed using DEPT spectra. Elemental analyses were esti-
mated with a Eurovektor Hekatech EA-3000 elemental analyzer
4.3. Preparation of the dipyrroazamethenes 7aec (general
procedure)
A
mixture of a chalcone 5 (100 mmol), nitro methane
(500 mmol), and K2CO3 (0.2 mmol), dissolved in ethanol
(100 mL), was heated to reflux for 6e12 h.35 After cooling at rt,
the solvent was removed in vacuum and the oily residue
obtained was dissolved in ethyl acetate and washed with water
(3ꢂ50 mL). The combined organic layers were washed with brine,
dried over sodium sulfate, and concentrated to give the target
compounds as a yellowish oily residue in nearly quantitative
yields, which were used in the next step without further
purification.
and the UVevis spectra with a Perkin Elmer
l 25 spectrometer.
The emission spectra were recorded on an Edingburgh spec-
trometer and the mass spectra with a Bruker Esquire-LC 00084
instrument. Melting points were determined with a Netzsch STA
449C instrument. Cyclic voltammograms were recorded with
a Metrohm
m-Autolab instrument in a single-component cell
under a nitrogen atmosphere. A typical three electrode configu-
ration with an inlaid platinum disk as working electrode, a plati-
A mixture of a 1,3-(bis-(het)aryl)-4-nitrobutanone 6 (1 mmol)
and ammonium acetate (20 mmol) was refluxed in n-butanol
(100 mL) for 24 h. After cooling to rt, the reaction mixture was
diluted with water and extracted three times with dichloro-
methane. The combined organic layers were washed with water
and brine, dried with sodium sulfate, and concentrated to give
the crude product as a dark blue-black solid. Purification by
column chromatography yields the desired products as coppery
shiny crystals.
num wire as counter electrode, and
a silver rod coated
electrochemically with AgCl was used. The potentials were
measured versus Ag/AgCl and referenced to ferrocene as an in-
ternal standard (E0(Fc/Fcþ)¼ꢀ4.78 eV to vacuum). The measure-
ments were performed with
a
scan rate of 100 mVsꢀ1 in
degassed dichloromethane (HPLC quality) containing tetra-n-
butylammonium hexafluorophosphate (TBAPF, 0.1 mol Lꢀ1) as
electrolyte.
The fluorescence quantum yields
F
were estimated by use the
By this procedure, the following compounds were synthesized:
comparative method described by Williams et al.30 Freshly pre-
pared solutions of Rhodamine 101 as standard in ethanol with five
different concentrations and compound 1a, 1b, and 1c in
dichloromethane are used for recording the absorption as well as
fluorescence spectra using a slit of 5 nm for each sample. The
4.3.1. 3-Phenyl-5-(thienyl-1H-pyrrol-2-yl)-(3-phenyl-5-thienyl-pyr-
rol-2-ylidene)-amine (7a). In a yield of 8%; mp: 262e263 ꢁC; 1H
NMR: ppm¼8.03 (d, J¼7.2 Hz, 4H), 7.60 (d, J¼3.7 Hz, 2H), 7.50 (d,
J¼4.9 Hz, 2H), 7.41 (t, J¼7.2 Hz, 4H), 7.34 (t, J¼7.3 Hz, 2H), 7.19 (t,
J¼3.5 Hz, 2H), 7.06 (s, 2H). 13C NMR: ppm¼149.3, 149.0, 142.2, 137.0,
133.5, 129.0, 128.8, 128.7, 128.2, 128.0, 127.3, 114.7. ESI-MS m/z (%):
[MþH] 462.1. Elemental analysis: calcd for C28H19N3S2: C 72.86, H
4.15, N 9.10, S 13.89; found: C 72.37, H 4.04, N 8.27, S 13.92.
fluorescence quantum yield of Rhodamine 101 is defined to be
F
¼1
according to reference.31,32 The excitation wavelengths for Rhoda-
mine 101 and for the compounds 1a, 1b, and 1c are selected as
550 nm, 655 nm, 630 nm, and 700 nm, respectively. The intensities
measured were corrected in each case by the lamp spectra.
With these preconditions the fluorescence quantum yields
F
4.3.2. 5-Phenyl-3-(thienyl-1H-pyrrol-2-yl)-(5-phenyl-3-thienyl-pyr-
rol-2-ylidene)-amine (7b). In a yield of 6%; mp: 257e260 ꢁC; 1H
NMR: ppm¼7.91 (d, J¼7.2 Hz, 4H), 7.86 (d, J¼2.4 Hz, 2H), 7.52 (t,
J¼7.1 Hz, 4H), 7.46 (d, J¼7.2 Hz, 3H), 7.42 (d, J¼4.1 Hz, 2H), 7.15
(t, J¼4.3 Hz, 2H), 7.09 (s, 2H). 13C NMR: ppm¼155.3, 149.1, 136.9,
135.9, 131.9, 130.1, 129.1, 127.9, 127.7, 127.2, 126.5. ESI-MS m/z
(%): [MþH] 462.1. Elemental analysis: calcd for C28H19N3S2: C
72.86, H 4.15, N 9.10, S 13.89; found: C 72.68, H 3.69, N 9.29, S
13.35.
were calculated by using the following equation in which Grad is
defined as the gradient from the plot of the integrated fluorescence
intensities over the optical densities at the excitation wavelength
and
h
the reactive index of the solvents:33
!
ꢀ
ꢁ
hX2
hR2
GradX
GradR
FX
¼
FR
The subscripts X and R refer to the probe and the standard
reference solution, respectively.
For all quantum chemical calculations the Gaussian03 package24
was used and the ab initio calculations of the orbital energies were
performed with density functional theory using the b3lyp func-
tional with the basis set 6-31þg(d,p). No symmetry constrains were
used for the geometry optimization. All optimized structures were
confirmed with subsequent frequency calculations to make sure
that a true minimum was reached. The absorption energies and the
principal orbital contributions were calculated with time de-
pendent DFT as implemented in the Gaussian package.
4.3.3. 3,5-(Bithienyl-1H-pyrrol-2-yl)-(3,5-bithienyl-pyrrol-2-
ylidene)-amine (7c). In a yield of 12%; mp: 312e315 ꢁC; 1H NMR:
ppm¼7.84 (d, J¼3.6 Hz, 2H), 7.58 (d, J¼3.6 Hz, 2H), 7.50 (d,
J¼4.9 Hz, 2H), 7.41 (d, J¼5.0 Hz, 2H), 7.19 (d, J¼4.9 Hz, 2H), 7.14
(d, J¼5.0 Hz, 2H), 6.93 (s, 2H). 13C NMR: ppm¼149.2, 148.9,
136.9, 136.6, 135.7, 128.8, 128.7, 127.9, 127.7, 127.4, 127.3, 113.0.
ESI-MS m/z (%): [MþH] 474.0. Elemental analysis: calcd for
C24H15N3S4: C 60.86, H 3.19, N 8.87, S 27.08; found: C 61.23, H
2.77, N 8.21, S 27.10.
4.2. Preparation of the thienyl-substituted chalcones 5
(general procedure)
4.4. Preparation of the aza-BODIPYs 1aec (general procedure)
To a solution of a corresponding azamethene derivative 7
(12.6 mmol) in dichloroethane (150 mL) diisopropylamine
(10.5 mL, 60.8 mmol) was added and the mixture stirred for
To a mixture of a (het)arylketone 3 (50 mmol) and a (het)
arylaldehyde
4 (50 mmol), dissolved in 25 mL ethanol,

7154
R. Gresser et al. / Tetrahedron 67 (2011) 7148e7155
1 h. Then BF3$OEt2 (7.5 mL, 60.5 mmol) was added at rt and
the resulting mixture refluxed until the starting material was
completely converted (ca. 2 h) to the corresponding product
(checked with TLC). The cold reaction mixture was diluted
with water and extracted twice with dichloromethane
(100 mL). The combined organic layers were dried with so-
dium sulfate and concentrated in vacuum. The crude products
were purified by flash chromatography using dichloromethane/
hexane as eluent to obtain the products as coppery shiny
crystals.
References and notes
1. (a) Ziessel, R.; Ullrich, G.; Harriman, A. New J. Chem. 2007, 31, 496e501; (b)
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T.; Watanabe, T.; Hasegawa, A.; Choyke, P. L.; Kobayashi, H. Nat. Med. 2009, 15,
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3. Hepp, A.; Ulrich, G.; Schmechel, R.; von Seggern, H.; Ziessel, R. Synth. Met. 2004,
146, 11e15.
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Commun. 2009, 1673e1675; (b) Rousseau, T.; Cravino, A.; Bura, T.; Ullrich, G.;
Ziessel, R.; Roncali, J. J. Mater. Chem. 2009, 19, 2298e2300.
By this procedure, the following compounds were prepared.
5. (a) Rogers, M. A. T. J. Chem. Soc. 1943, 590e596; (b) Davies, W. H.; Rogers,
M. A. T. J. Chem. Soc. 1944, 126e131; (c) Knott, E. B. J. Chem. Soc. 1947,
1196e1201.
6. Hall, M. J.; McDonnell, S. O.; Killoran, J.; O’Shea, D. F. J. Org. Chem. 2005, 70,
5571e5578.
7. (a) Killoran, J.; Allen, L.; Gallagher, J.; Gallagher, W. M.; O’Shea, D. F. Chem.
Commun. 2002, 1862e1863; (b) McDonnell, S. O.; Hall, M. J.; Allen, L. T.;
Byrne, A.; Gallaher, W. M.; O’Shea, D. F. J. Am. Chem. Soc. 2005, 127,
16360e16361; (c) Quartarolo, A. D.; Russo, N.; Sicilia, E. Chem.dEur. J. 2006,
12, 6797e6803; (d) Adarsh, N.; Avirah, R. R.; Ramaiah, D. Org. Lett. 2010, 12,
5720e5723.
4.4.1. Aza-BODIPY 1a. In a yield of 78%; mp: 297e298 ꢁC; 1H NMR:
ppm¼8.37 (d, J¼3.8 Hz, 2H), 8.05 (d, J¼8.3 Hz, 4H), 7.63 (d, J¼4.3 Hz,
2H), 7.42 (m, 3H), 7.27 (t, J¼4.0 Hz, 2H), 7.17 (s, 2H). 13C NMR:
ppm¼149.7, 145.6, 142.8, 134.0, 133.0, 132.1, 131.7, 129.8, 129.3,
129.2, 128.5, 118.6. ESI-MS m/z (%): [MþH] 474.0. Elemental analy-
sis: calcd for C28H18BF2N3S2: C 66.02, H 3.56, N 8.25, S 12.59; found:
C 66.22, H 3.25, N 7.74, S 11.61.
8. (a) Zhao, W.; Carriera, E. M. Angew. Chem., Int. Ed. 2005, 44, 1677e1678; (b)
Loudet, A.; Bandichhor, R.; Wu, L.; Burgess, K. Tetrahedron 2008, 64,
3642e3654; (c) Loudet, A.; Bandichhor, R.; Burgess, K.; Palma, A.; McDonnell,
S. O.; Hall, M. J.; O’Shea, D. F. Org. Lett. 2008, 10, 4771e4774; (d) Li, Y.; Dolphin,
D.; Patrick, B. O. Tetrahedron Lett. 2010, 51, 811e814.
4.4.2. Aza-BODIPY 1b. In a yield of 68%; mp: 272e274 ꢁC; 1H NMR:
ppm¼7.94 (m, 4H), 7.86 (dd, J¼3.6, 0.9 Hz, 2H), 7.51 (dd, J¼5.0,
0.9 Hz, 4H), 7.40 (t, J¼3.3 Hz, 6H), 7.14 (dd, J¼5.0, 1.2 Hz, 2H), 6.84 (s,
2H). 13C NMR: ppm¼159.5, 144.9, 138.3, 134.7, 131.5, 130.8, 130.3,
129.7, 129.4, 128.5, 128.3, ESI-MS m/z (%): [MþH] 510.1. Elemental
analysis: calcd for C28H18BF2N3S2: C 66.02, H 3.56, N 8.25, S 12.59;
found: C 66.12, H 3.67, N 7.51, S 12.25.
9. Coskun, A.; Yilmaz, D.; Akkaya, E. U. Org. Lett. 2007, 9, 607e609.
ꢁ
10. (a) Palma, A.; Tasor, M.; Frimannsson, D. O.; Vu, T. T.; Meallet-Renault, R.;
O’Shea, D. F. Org. Lett. 2009, 11, 3638e3641; (b) Yoshii, R.; Nagai, A.; Chujo, Y. J.
Polym. Sci., Part A: Polym. Chem. 2010, 48, 5348e5356.
11. (a) Schmidt, E. Y.; Trovimov, B. A.; Mikhaleva, A. I.; Zorina, N. V.; Protzuk, N. I.;
ꢁ
Petrushenko, K. B.; Ushakov, I. A.; Dvorko, M. Y.; Meallet-Renault, R.; Clavier, G.;
Vu, T. T.; Tran, H. T. T.; Pans, R. B. Chem.dEur. J. 2009, 15, 5823e5830; (b) Zrig, S.;
4.4.3. Aza-BODIPY 1c. In a yield of 82%; mp: 318e320 ꢁC; 1H NMR:
ppm¼7.06 (s, 2H), 7.19 (t, J¼5.0 Hz, 2H), 7.25 (t, 2H), 7.55 (d,
J¼5.0 Hz, 2H), 7.62 (d, J¼5.0 Hz, 2H), 7.92 (d, J¼3.7 Hz, 2H), 8.34 (d,
J¼3.8 Hz, 2H). 13C NMR: ppm¼151.8, 149.7, 148.2, 134.4, 133.9, 133.0,
131.5, 129.9, 129.7, 129.4, 129.1, 116.2. ESI-MS m/z (%): [MþH] 522.4.
Elemental analysis: calcd for C24H14BF2N3S4: C 55.28, H 2.71, N 8.06,
S 24.60; found: C 54.86, H 2.70, N 7.81, S 25.23.
ꢁ
Remy, P.; Andrioletti, B.; Rose, E.; Asselberghs, I.; Clays, K. J. Org. Chem. 2008, 73,
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Hartmann, H.; Leo, K.; Riede, M. Chem.dEur. J. 2011, 17, 2939e2947.
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1517e1520; (b) Benniston, A. C.; Copley, G.; Harriman, A.; Rewinska, D. B.;
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4.4.4. Synthesis of the aza-BODIPY 1d. The dibromo compound 8
(0.2 g, 0.3 mmol), prepared according to Ref. 1, and 2-(tributyl-
stannyl)thiophene (0.24 g, 0.61 g) were dissolved in dry toluene
under nitrogen. Tetrakis(triphenylphosphine)palladium(0) (0.14 g,
0.1 mmol) was added and the mixture refluxed over night, until no
starting material could be detected by TLC. The reaction mixture
was cooled to rt and evaporated to dryness. Purification by column
chromatography with chloroform yielded the product 1d in a yield
of 85% (169 mg); mp: >320 ꢁC; 1H NMR: ppm¼6.62 (dd, J¼3.6,
1.1 Hz, 2H), 6.85 (dd, J¼5.1, 3.6 Hz, 2H), 7.20 (dd, J¼5.1, 1.1 Hz, 2H),
7.31 (m, 8H), 7.34 (t, J¼7.4 Hz, 4H), 7.53 (d, J¼7.2 Hz, 4H), 7.54 (d,
J¼6.8 Hz, 4H). 13C NMR: ppm¼159.6, 145.1, 142.0, 133.5, 131.2, 130.9,
130.1, 130.0, 128.8, 128.7, 127.8, 127.7, 127.0, 126.8, 126.4, 125.8. ESI-
MS m/z (%): [MþH] 662.3. Elemental analysis: calcd for
C40H26BF2N3S2: C 72.62, H 3.96, N 6.35, S 9.69; found: C 72.86, H
3.70, N 6.13, S 9.23.
14. (a) Choi, S. H.; Pang, K.; Kim, K.; Churchill, D. G. Inorg. Chem. 2007, 46,
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Acknowledgements
The authors gratefully acknowledge the financial support by the
German Bundesministerium fur Bildung und Forschung (BMBF)
within the Innoprofile program (03IP602) and the computational
resources provided by the Center for Information Services and High
Performance Computing (ZIH), TU Dresden.
€
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2011.06.100.
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