Aza-BODIPY Derivatives Containing BF(CN) and B(CN)2 Moieties

Article abstract of DOI:10.1002/cplu.201600479

Two novel aza-BODIPY derivatives are synthesized, with the fluorine atoms in the BF₂ moiety replaced by cyano groups. The introduction of cyano groups changes the phenyl substituents on the 3,5 positions from parallel to antiparallel. The HOMO/

Full text of DOI:10.1002/cplu.201600479

Accepted Article
Title: Novel aza-BODIPY derivatives containing BF(CN) and B(CN)2
structures
Authors: Tian-yi Li, Zaifei Ma, Zhi Qiao, Christian Körner, Koen
Vandewal, Olaf Zeika, and Karl Leo
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Novel aza-BODIPY derivatives containing BF(CN) and B(CN)2
structures
Tian-yi Li*, Zaifei Ma, Zhi Qiao, Christian Körner, Koen Vandewal Olaf Zeika, and Karl Leo[a]
Abstract: Two novel aza-BODIPY derivatives with the fluorine
atoms in the BF2 moiety replaced by cyano groups are synthesized.
The introduction of cyano groups changes the phenyl substituents
on the 3, 5 positions from parallel to antiparallel. The HOMO/LUMO
energy levels are stabilized gradually when increasing the number of
cyano groups and the photophysical properties show corresponding
shifts. With high thermal stability, the derivatives can be purified by
sublimation and to prepare vacuum deposited thin films. Our
research extends the family of aza-BODIPY with cyano substituted
derivatives.
Figure 1. The core structure of BODIPY and aza-BODIPY.
the 2,6 positions with the aryl substituents on 3,5 positions.
The resulting rigid dyes presented high molar extinction
coefficients and even further red shifted absorptions.[10]
Recently, benzannulated aza-BODIPYs with fused rings on
1,2 positions were prepared and are promising as NIR
absorbing donor materials in organic photovoltaics.[11]
However, all these modifications of aza-BODIPY are
limited on the dipyrrolemethene section. In this contribution,
we therefore focus on the BF2 moiety and replaced the
fluorine atoms with cyano groups, aiming to develop a
versatile synthetic route for aza-BODIPY derivatives. As
shown in Scheme 1, the well-studied benzannulated aza-
The development of boron dipyrromethene (BODIPY) dyes
has been an attractive topic in numerous research fields,
such as organic photovoltaics,[1] thin film transistors,[2]
electroluminescence,[3] energy transfer cassettes,[4] and
chiroptics.[5] However, the absorption and emission peaks
of traditional BODIPY dyes are restricted to the range of
480‒540 nm.[6] In order to extend to the red or near infrared
(NIR), many efforts have been undertaken to modify the
BODIPY molecular structures. One of the most effective
methods is replacing the meso-carbon atom with a nitrogen
atom to form aza-BODIPY dyes (Figure. 1). Normally, aza-
BODIPY dyes present large bathochromic shifts compared
to their BODIPY analogues without damaging their
outstanding photophysical properties such as high molar
extinction coefficient, sharp Gaussian shaped absorption
bands and decent photostability.[7]
BODIPY
described method from the precursor pre-BDP [12]
exchange of the fluorine atom with a cyano group was
carried out by reacting and trimethylsilyl cyanide together
with a Lewis acid according to route (i) and (ii). Aza-
BODIPY and two equiv of Lewis acid were added in
1
was synthesized according to a previously
.
The
1
1
anhydrous DCM and stirred at reflux for 10 min. Then, ten
equiv of trimethylsilyl cyanide were added into the system
dropwise at room temperature. We found that, when
aluminium chloride was used as the Lewis acid, the final
product was
2 in spite of the excessive trimethylsilyl
cyanide and the reaction can be completed within one hour.
A prolongation of the reaction time to overnight did not
influence the final product. When the boron trifluoride
Several molecular modifications of aza-BODIPYs, with
the goal to realize certain properties and to accomplish
specific applications have been reported. Electron-donating
moieties were introduced in 1,3,7,5 positions to achieve
further bathochromic shifts in both absorption and
emission.[8] Also, heterocyclic rings were attached to these
positions to form chemical sensors and indicators.[9] In case
of substitution of the 2,6 positions by bromine atoms, the
production of singlet oxygen was increased, which was
favourable for photodynamic therapy.[8b] Aza-BODIPYs with
etherate was employed,
2
is observed at the beginning of
disappeared
started to accumulate. The complete
into was observed after 10 h of stirring.
the reaction. However, after several hours,
2
gradually and
conversion of
3
1
3
This observation reveals that the replacement of two
fluorine atoms with cyano groups can be considered as a
two-step process. The strong electron deficiency and
relatively small molecular volume of boron trifluoride
etherate play a significant role in this exchange reaction.
Moreover, we tried to synthesize
3 from pre-BDP directly
extended structures were synthesized by connecting
with five equiv of boron trifluoride etherate and ten equiv of
meso
1
7
N
N
B
N
HN
6
2
N
N
N
N
B
3
5
-
re
p
BDP
F
F
F
F
iii
)
(
-
z
a a
B DIPY
B DIPY
O
O
N
B
N
B
N
ii
( )
i
( )
[a]
Li, Dr. Ma, Qiao, Dr. Körner, Prof. Dr. Vandewal, Dr. Zeika, Prof. Dr.
Leo
N
N
N
N
N
N
B
F
NC CN
F
F
C
N
Dresden Integrated Center for Applied Physics and Photonic
Materials (IAPP)
Technische Universität Dresden
George-Bähr-Str.1, 01069 Dresden, Germany
E-mail: tianyi.li@iapp.de
3
i
( )
2
1
-
-
.
,
,
,
,
t
E
O
2
;
AlCl
TM
S C
N D M ii / iii BF
C
TM
N D M
S C C
( )
(
)
3
3
Scheme 1. The molecular structures and synthetic route of
aza-BODIPY derivatives with cyano groups.
Supporting information for this article is given via a link at the end of
the document.
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trimethylsilyl cyanide via route (iii). As a result, the yield of
route (iii) is around 56%, which is similar to that of route (i)
and (ii). Consequently, with a yield of around 60% from
pre-BDP to 1, the yield of 3 is doubled when using route
(iii) instead of route (i). The newly synthesized aza-
BODIPY derivatives are fully characterized by 1H NMR, 13C
NMR, 19F NMR and MS. Single crystal X-ray diffraction
measurements are carried out to determine the exact
molecular structures.
Figure 3. The ORTEP structures of
1, 2 and 3 with the
thermal ellipsoids set at 60% probability (up: top view; down:
side view; C: black, N: blue, F: green, B: dark yellow).
The thermal properties of
thermal gravimetric analysis (TG, Figure. S3). Both of the
derivatives display good thermal stability with
decomposition temperatures over 300 . Therefore, these
derivatives can be purified by vacuum gradient sublimation
at a temperature lower than 250 with reasonable yield
2 and 3 are investigated by
a
℃
℃
around 60%. The photostability of all the compounds is
studied in DCM solution under the radiation of 6000 W m-2
Figure 2. The 19F NMR spectra of the compounds.
for 12 h (Figure S5). Aza-BODIPY
stability and the extinction coefficient keeps as constant
during the radiation. The derivative , whose extinction
coefficient decreases around 16%, has better photo stability
than
1 shows the best photo
19F NMR spectra provide the most obvious evidence for
the demonstration of the new derivatives containing two,
3
one and zero fluorine atoms. As illustrated in Figure. 2,
1
presents a set of structured signals with four main peaks
and several splitting peaks in the range of ‒130.0 to ‒130.3
ppm. When one fluorine atom is replaced, the peaks in the
2
.
120000
80000
40000
0
0.6
0.4
0.2
0.0
spectrum of
out. Only four peaks with similar strength are observed in
the region from ‒130.1 to ‒130.6 ppm. For , there is no
2
broaden and the fine structures are smeared
1 in solution
2 in solution
3 in solution
1 in film
2 in film
3 in film
3
resonance signal, telling that both of the fluorine atoms are
replaced by cyano groups.
The molecular structures of
Figure. 3. The structure of
1
,
2
and
3 are illustrated in
1
has been reported by
Kobayashi and Lukyanets et.al.[13] Single crystal samples of
2
and 3 for X-ray diffraction measurements are obtained by
slow diffusion of iso-hexane into the DCM solutions. The
boron centres adopt a slightly disordered tetrahedral
300
400
500
600
700
800
900
1000
geometry. The B-F1 bond (1.38 Å) in
coordination sphere. For both and
2
is the shortest in the
Wavelength (nm)
2
3, B-N1 (1.55 Å) and
B-N2 (1.55 Å) have almost the same length, but slightly
shorter than those of , indicating that replacing the fluorine
1
Figure 4. The absorption spectra of
1, 2 and 3 in DCM
atoms with cyano groups strengthens the B-N bonds. The
bond lengths between boron and cyano groups (B-C1: 1.60,
solution and in solid state (50 nm thin film on glass).
1.59 Å for
2
and
3
; B-C2: 1.60 Å for
3
) are the longest
, it is
The absorption spectra of
diluted DCM solution and for vacuum deposited thin films
(Figure. 4, Table 1). In solution, both derivatives and
have similar absorption profiles as , consisting of an
1, 2 and 3 are recorded in both
(Table S2). When looking into the structure of
1
interesting to note that the two phenyl moieties are in the
parallel positions and the corresponding H atoms had
intermolecular interactions with both F atoms in the BF2
2
3
1
extremely intensive single absorption band in the long
wavelength region from 600 nm to 750 nm and a broad but
weak absorption tail extending from 500 nm to the UV
region. This observation demonstrates that the introduction
of cyano groups has only minor influence on the photo-
moiety. In contrast, for
2
and
3
, these two phenyl rings are
can have
antiparallel to each other and the only F atom in
2
intermolecular interaction with both of ortho-H atoms in the
phenyl rings.
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induced electronic transitions. However, it is worth to note
that the absorption peaks present interesting shifts with the
number of cyano groups. When introducing one cyano
Figure 5. The frontier molecular orbital positions of
from CV measurements (right) and DFT calculations (left)
together with the plots of HOMOs and LUMOs.
1, 2 and
3
group, the main absorption peak of
shifts by 11 nm from 711 nm to 722 nm, as compared to
Surprisingly, further introduction of the second cyano group
in brings a small hypsochromic shift of 5 nm and the
2
bathochromically
In order to obtain a deeper insight in the frontier
molecular orbitals, both cyclic voltammetry (CV)
measurements and density functional theory (DFT)
calculations are conducted (Figure 5, Figure S7 and S8).
1
.
3
absorption peak is found at 717 nm. This reveals that the
HOMO-LUMO energy gap decreases with the introduction
of one cyano group. However, this gap does not decrease
The aza-BODIPY
oxidation potential at 0.86 V. A similar reversible wave is
observed for with a positively shifted oxidation potential at
0.95 V. This oxidation becomes partial reversible for and
the oxidation peak potential is further shifted to 1.05 V. For
the redox potential in the negative region is reversible and
these waves become irreversible for and . The reduction
peaks are found at ‒0.78 V, ‒0.65 V and ‒0.59 V
respectively. The HOMO energy level of is measured to
1 has a reversible redox wave with an
2
further with the introduction of a second cyano group and
2
3
remains the smallest gap. The optical properties of all the
compounds in the solid state are investigated using vacuum
thermal deposited thin films with a thickness of 50 nm. The
obvious broadening and bathochromic shifts observed for
all the compounds indicate that these three dyes have J-
aggregation effects.[14] The absorption coefficients of these
three dyes are all higher than 1×105 cm-1 (Figure S4).
Furthermore, the molar extinction coefficients decreased
along with the increasing number of cyano groups in both
solution and film. The theoretical calculation results below
will give evidence for the decreasing absorbing abilities.
The emission spectra of all the compounds were also
recorded in DCM solutions (Figure S6) and present perfect
mirror-image relationship with the absorption spectra. The
emission peaks were in the NIR region from 745 to 752 nm
with rather small stokes shifts in the range of 32 to 35 nm.
1
2
3
1
be ‒5.22 eV. When one of the fluorine atom is replaced, the
HOMO decreases by 0.09 eV to ‒5.31 eV. Adding a second
cyano group, the HOMO is stabilized to ‒5.40 eV with
further decrease of 0.09 eV. The LUMO energy level
however shows a different trend. First it decreases by 0.15
eV from ‒3.65 eV for
to ‒3.85 eV for with only 0.05 eV. Consequently, the
smallest energy band gap was found for with one cyano
group. The energy gap of with two cyano groups is in the
middle. This result explains the observed hypsochromic
shift in the absorption spectra between compounds and
1 to ‒3.80 eV for 2, but then decrease
3
2
3
2
3.
Table 1 The spectroscopic data of
1
,
2
and
3
From the theoretical calculation results, both the HOMO
and LUMO are mainly located on the upper aza-diisoindole
moieties and the phenyl substituents. This is the reason for
the almost identical absorption profiles of these three
compounds. However, due to the subtle disturbance from
cyano groups, the electron density contribution to the
HOMOs from BF2, BF(CN) and B(CN)2 are 0.12%, 0.11%
and 0.18% respectively. These numbers become 1.36%,
Thin
film
Solution
Dye
λem
λabs/n ε/M-1cm-1 ΔEopt
λabs [O.D.]
/ nm [%][d]
ΔEopt/eV[
(△λStokes)/
nm[c]
m[a]
/eV[b]
[a]
b]
1
2
711
722
717
121 500
96 360
92 250
1.66
1.62
1.64
745 (34)
754 (32)
752 (35)
763 [0.50]
766 [0.50]
766 [0.42]
1.36
1.38
1.41
3.54% and 5.45%. This means aza-BODIPY
biggest overlap between HOMO and LUMO and the overlap
decreases gradually in and . Thus, the lower extinction
1 has the
3
[a]
The absorption spectra were measured in 1×10-5 mol L-1 DCM
2
3
solutions; [b] The band gaps were calculated with the onset wavelength of
coefficients for the derivatives are explained. Although the
exact trend in LUMO energy levels is too small to be
revealed by the CV measurements, the stepwise decrease
in HOMO energy levels by 0.1 eV is reproduced.
the absorption spectra λonset by ΔEopt=1240/λonset
were recorded in 5×10-5 mol L-1 DCM solutions;
;
[c]The emission spectra
[d]
The absorption was
measured according to the optical density (O.D.) and the transmission
rate can be determined as T=10(-O.D.)×100%.
In conclusion, we have successfully developed a novel
synthetic route for aza-BODIPY derivatives with BF(CN)
and B(CN)2 structures and have prepared two NIR dyes
2
and based on benzannulated aza-BODIPY. Due to the
3
lack of F‒H interactions, the phenyl substituents on 3,5
positions turned from parallel into antiparallel in both aza-
BODIPY derivatives. The absorption maxima of the
derivatives
and 6 nm respectively compared to the aza-BODIPY
increasing the number of cyano groups, the HOMO energy
level decreases gradually. However, the LUMO of is lower
than , making the energy gap of the derivative with one
cyano group the lowest. Similar as the former research in
2
and
3
display bathochromic shifts of 11 nm
1. By
2
3
1
2
3
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replacing fluorine by alkyne fragments in BODIPY,[15]
neither electron donating nor electron withdrawing moieties
linked to boron makes significant disturbing on the optical
and FMOs properties. Furthermore, both the new
derivatives present an outstanding thermal stability and can
be purified by vacuum sublimation. This results in thin film
of high purity and quality, indicating them suitable as
candidates for vacuum processed NIR organic electronic
devices. Considering the growing research interest of aza-
BODIPY, more functional aza-BODIPY derivatives with
BF(CN) and B(CN)2 structures can be developed and
present realistic applications in various fields of
optoelectronics and beyond.
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fellowship.
Keywords: aza-BODIPY • Cyano substituents • Near infrared •
Sublimation • Tuneable FMOs
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Tian-yi Li*, Zaifei Ma, Zhi Qiao, Christian
Körner, Koen Vandewal Olaf Zeika, and
Karl Leo
Page No. – Page No.
Novel aza-BODIPY derivatives
containing BF(CN) and B(CN)2
structures
Novel aza-BODIPY derivatives: The fluorine atoms in the BF2 moiety in aza-
BODIPY dyes is for the first time replaced by one or two cyano groups and two
corresponding derivatives are achieved with the presence of different Lewis acids.
The outstanding photophysical properties are kept in the derivatives. The frontier
molecular orbital energy levels can be tuned with different numbers of cyano
groups.
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