Synthesis, spectral, electrochemical properties, and photovoltaic performance of structurally constrained BODIPY dyes with 4,4- dimethyltriphenylamine at the 2,6-positions

Article abstract of DOI:10.1016/j.tetlet.2013.08.088

Novel symmetrical D-A-D type of BODIPY derivatives (DTPA-BDP) were synthesized successfully. The introduction of chromophoric units makes the molecule absorption wavelengths spanning the ranges of 300-800 nm and shifts the emission to 793 nm in red and near-infrared region. Bulk heterojunction solar cells with DTPA-BDPs as electron donor and PC₆₁BM as electron acceptor exhibited photovoltaic properties with a power conversion efficiency of 0.77%, an open-circuit voltage of 0.70 V.

Full text of DOI:10.1016/j.tetlet.2013.08.088

Accepted Manuscript
Synthesis, Spectral, Electrochemical Properties and Photovoltaic Performance
of structurally constrained BODIPY dyes with 4,4-dimethyltriphenylamine at
the 2,6-positions
Hongbin Zhao, Bangying Wang, Junxu Liao, Hongke Wang, Guiping Tan
PII:
S0040-4039(13)01474-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.08.088
TETL 43451
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
31 January 2013
11 August 2013
20 August 2013
Please cite this article as: Zhao, H., Wang, B., Liao, J., Wang, H., Tan, G., Synthesis, Spectral, Electrochemical
Properties and Photovoltaic Performance of structurally constrained BODIPY dyes with 4,4-
dimethyltriphenylamine at the 2,6-positions, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.
2013.08.088
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1
Tetrahedron Letters
journal homepage: www.elsevier.com
Synthesis, Spectral, Electrochemical Properties and Photovoltaic Performance of
structurally constrained BODIPY dyes with 4,4-dimethyltriphenylamine at the 2,6-
positions
Hongbin Zhaoa, b, Bangying Wang a, b, Junxu Liaoa, Hongke Wang a, Guiping Tan a
a
College of Chemistry and Environmental Engineering, Dongguan University of Technology, Guangdong 523808, People’s Republic of China
College of Chemistry, Xiangtan University, Hunan 411105, People’s Republic of China
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Novel symmetrical D-A-D type of BODIPY derivatives (DTPA-BDP) were synthesized
successfully. The introduction of chromophoric units make the molecules absorption
wavelengths spanning the ranges of 300-800 nm and shift the emission to 793 nm in red and
near-infrared region. Bulk heterojunction solar cells with DTPA-BDPs as electron donor and
PC61BM as electron acceptor exhibited photovoltaic properties with a power conversion
efficiency of 0.77 %, an open-circuit voltage of 0.70 V.
Available online
Keywords:
BODIPY derivatives ;
Symmetrical type
Near-infrared
Photovoltaic properties
1
Recently, the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes
(BODIPYs) have received increasing attention due to their
outstanding optical properties such as high absorption
coefficients, relatively sharp emission bands, high
substituents and enhance the coplanarity;8 (3) introducing
electron-donating groups.9 Recently, there were some reports
on BODIPY derivatives be used in dye-sensitizd solar cells
2,10
(DSCs).
However, to the best of our knowledge, up to
fluorescence quantum yields, excellent chemical and
date, the reports on using BODIPY derivatives in such
molecular system for construction bulk heterojunction (BHJ)
are still rare. 11
1
photochemical stability, high solubility,
which make
BODIPY-based systems as the most promising candidate for
light-harvesting applications in solar cells,2 biological
labeling3 and cell imaging.4 Recently, they have been
introduced to render the dye sensitive to the environment.5
Additionally, straightforward chemical synthesis and
structural robustness have enabled finetuning of optical
properties of BODIPY dyes via systematic structural
variations. There have been a lot of researches focusing on the
development of new fluorophores with two photon absorption
cross sections as well as high fluorescence quantum yields.6
In order to modulate the optical properties of BODIPY dyes,
the following strategies are usually elaborated: (1) extension
of π-conjugation of the structure;7 (2) rigidifying the
molecular structure to suppress the free rotation of extended
In this paper, in order to extend the conjugation of the
BODIPY dyes, triphenylamine groups were introduced to
BODIPY by attaching to the pyrrole rings on the 2,6-axis to
form the novel symmetrical D-A-D type of DTPA-BDP. The
absorption spectral range of these novel DTPA-BDP were
shifted to infrared region. And BHJ solar cells were fabricated
with DTPA-BDP as donor materials and PC61BM as acceptor
materials. It should be noted that adequate solubilizing side
chains ensure DTPA-BDP good film ability when blended
with PC61BM.
DTPA-BDPs were synthesized according to the route
illustrated in Scheme 1. Firstly, Dipyrromethanes 1 were
prepared in a mild way which was reported by our group in
 Corresponding author. Tel.: (+86)-138-2722-3583; fax: (+86)-0769-2286-1233; e-mail: zhaohbhanlf@163.com.

2
Tetrahedron Letters
12
regions at 648nm, 653nm, 559nm, respectively, is attributed
to the intramolecular charge transfer (ICT) interaction
between the donor units and the acceptor units in the main
chain. Compared with the absorption spectrum in CH2Cl2
solution, the dyes in solid film displayed a red-shift
absorption peak at 313 nm and 666nm, 676nm, 573nm,
respectively (Figure 2, Table 1). We could see that DTPA-
BDP a, DTPA-BDP b and DTPA-BDP c display similar
absorption features.
2010. Subsequently, the dipyrromethanes were oxidated by
2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ) and then
treated with BF3OEt2 to form the important intermediates
BODIPY 2. Bromination of BODIPY 2 at 2,6-positions of
pyrrole rings affording the corresponding bromides 3. Finally,
the desired DTPA-BDPs were obtained by Suzuki coupling
reactions between bromides 3 and triphenylamine boronic
acid. Compared with the traditional synthetic method,13 this
strategy restrain the generation of accessory substances
effectively and promote the yields of target products. Using
this synthetic strategy, we successfully prepared three DTPA-
BDPs: DTPA-BDP a, DTPA-BDP b and DTPA-BDP c.
Figure 2. Normalized absorption spectra of DTPA-BDP a-c in
CH2Cl2 solution (left) and thin films on quartz surface (right).
Table1. Photophysical Properties of DTPA-BDP a-c in CH2Cl2 at
Room Temperature.
solutiona
filmb
Entry
λmax
(nm)
λem
(nm)
λmax
(nm)
λ onset
(nm)
lg ε
310,648 5.2 , 4.9
310,653 5.1, 4.7
310,559 5.1, 4.6
793
743
681
313,666
313,676
313,573
824
820
696
a
b
c
Scheme 1. Synthesis of DTPA-BDP a-c.
The color conversions from BODIPY 2 to DTPA-BDP dyes
under ambient light in the process of synthesizing DTPA-BDPs
were shown in Figure 1. It indicated that the absorption
wavelengths changed by introducing functional groups to
BODIPY. And we believe that such straightforward
functionalization and derivatization methodologies are likely to
enhance their impressive versatility. 14
a. In CH2Cl2, b. Deposited onto quartz substrate by spin-coating technique
from CH2Cl2 solution
The fluorescence properties of all BODIPY dyes were
studied in CH2Cl2 solution at room temperature.
A
comparison of the emission spectra of these dyes was shown
in Supporting Information and the normalized fluorescence
spectra of DTPA-BDP a-c were shown in Figure 3. BODIPY
dyes with 4,4-dimethyltriphenyl amine at the 2,6-positions
resulted in near-infrared and deep-red emissive BODIPY
copolymeric dyes with emission maxima of 793nm, 743nm
and 681nm, respectively. It can be explained as the basis of
photoinduced energy transfer from the BODIPY π→π*
excited state to the lower lying singlet excited state of the
triphenylamine and/or to the existence of new nonradiative
pathways form the BODIPY π→π* state to the ground state.
On the other hand , DTPA-BDP a-c gave low fluorescent
quantum yields. These could be attributed to the increased
internal conversion according to the energy gap law that
states the non-radiative deactivation probability of S0-S1
increases as the energy gap of S0→S1 decreases in a highly
extended conjugating system.
Figure 1. The color conversions of the DTPA-BDP dyes under
ambient light
With these novel DTPA-BDPs in hand, we subsequently
studied the optical properties of all three DTPA-BDPs and
found that these covalently assembled light harvesters showed
two major absorption bands which were derived from two
different chromophores.
In the D-π-D type (Donor-π bridge-Donor) dyes DTPA-
BDPs, a central BODIPY core with a considerable electron-
accepting nature was connected with two triphenylamino
groups. We expected that a strong red emission by direct two
photon excitation would be achieved by using the strong
fluorescent BODIPY dyes and high electron-donating groups.
An efficient π conjugated spacer is one of the most important
reasons for contributing to facilitate the electronic flow. With
this in mind, we tested their absorption spectra using a
ultraviolet-visible spectrometer. In CH2Cl2 solution, DTPA-
BDP a-c showed two obvious absorption peaks. The first one
at ~310 nm corresponds to the π→π* transition in short
wavelength regions. The second band of long wavelength
Consistent with the literature,15 the installation of new
substituents on the BODIPY core greatly affects the
spectroscopic charateristics of the chromophore. In
comparison to the starting BODIPY 2, the installation of
bromo-substituents onto the BODIPY core lead to an up to 41
nm red-shift of the absorption maximum and an up to 48 nm
red-shift of the emission maximun as shown in Supporting
Information. These may be due to the heavy atom effect,
which facilitates the S1→T1 intersystem crossing. Similar to it,
the installation of triphenylamino groups at 2,6 positions also

3
scan speed of 50 mV/s, Fc+/Fc refers to the ferricinium/ferrocene
couple used as internal reference
show large red-shift of the absorption and emission spectra.
Thus, the different arylated moieties at the meso position of
BODIPY 2 and BODIPY 3 do not significantly affect the
absorption and emission spectral maxima, only for DTPA-
BDPs, the electron-donating oxoalkyl groups make DTPA-
BDP a and b have longer maximum absorption and emission
wavelength. This difference may arise from the fact that the
arylated moiety is not coplanar with the BODIPY core due to
steric hindrance with the substitution position is structurally
unique.
By neglecting any entropy change during light absorption,
the reduction potential vs. NHE (E red), which corresponds to
the lowest unoccupied molecular orbital (LUMO vs. NHE),
can be obtained from Eoxonset and Eg .
Table 2. Solution Electrochemical Properties of DTPA-BDP a-c.
a
b
d
Eg
Eox
Eox c
(v)
1.12
1.08
1.09
Ered
HOMO/LUMOe
(eV)
-5.36/-3.44
-5.34/-3.44
-5.35/-3.16
onset
p
Entry
(eV)
1.92
1.90
2.19
(eV)
0.96
0.94
0.95
(v) vs.
-0.96
-0.96
-1.24
a
b
c
a. Energy band gap, determined from UV-vis absorption spectra. At
absorption maxima (Eg =1240/λmax)
b. Eoxonset, onset oxidation potential
c. Eoxp , oxidation peak potential
d. E red the reduction potential ,was calculated from Eoxonset –Eg.
e. HOMO = -(Eox
+ 4.4 eV); LUMO = HOMO + Eg eV.
onset
The above-discussed results indicated that the photophysical
and electrochemical properties of these dyes were excellent
and their performance as electron donor in BHJ solar cells
would be feasible. We expect that these dyes would become
valuable candidates to the applications of BHJ solar cells as a
series of new metal-free organic dyes.
BODIPY derivatives BHJ solar cells were fabricated using
DTPA-BDP a-c as the electron donor materials and fullerene
derivatives as the electron acceptor materials through the
conventional spin-coating process. The cell structure was
ITO/ PEDOT:PSS (30 nm)/donor:acceptor (80 nm)/LiF (0.7
nm)/Al(100 nm). The effective device area was 7×10-2 cm2, as
defined by a shadow mask. Figure 5 shows typical current
density-voltage (J-V) characteristics of two optimized devices
at a ratio of 1:3 between DTPA-BDPs and PC61BM under
1,000 W m–2 air mass 1.5 global (AM 1.5 G) illumination.
Device parameters such as short-circuit photocurrent densities
(JSC), open-circuit voltage (VOC), fill factor (FF) and solar
energy-to-electricity conversion yields (PCE) were deduced
from the J–V characteristics and summarized in Table 3. A
maximal PCE of 0.77 % with an VOC of 0.70 V, a JSC of 3.46
mA cm-2 and a FF of 0.32% was achieved in the DTPA-BDP
a:PC61BM-based device. All the PCE of BHJ solar cells were
obtained without the thermal annealing. The FF values of this
series of solar cells, from 0.32 to 0.23, which are relatively
lower than the devices employing the state-of-the-art dye
N719 (FF = 0.61),17 suggesting the charge transfer within the
cells was hindered once the charges in the active layer are
formed.18 Device performance was also very sensitive to
atmosphere (oxygen and moisture) exposure, which likeky
resulted in the presence of traps and worsening of fill factor.
Figure 3. Normalized fluorescence spectra of DTPA-BDP a (black,
λexc = 648 nm), DTPA-BDP b (red, λexc = 653 nm ), DTPA-BDP c
(blue, λexc = 559 nm ) recorded in CH2Cl2.
Furthermore, In order to investigate the electrochemical
behaviors of DTPA-BDP a-c, the electronic states of
compounds DTPA-BDP a-c were studied through cyclic
voltammetry in CH2Cl2 (Figure 4) and the results were
summarized in Table 2. The potentials were reported versus
ferrocene as internal standard and potentials were calibrated
by addition of 0.63 V to the potential (vs. SCE) vs. Fc/Fc+.16 It
can be seen that the Eoxonset and Eoxp of the three dyes were all
measured as around 0.95 V and 1.1 V (Table 2), indicating
that the different branched chains of electron donating groups
had no effect on the Eox. The corresponding HOMO (highest
occupied molecular orbital) energy level of DTPA-BDPs were
calculated (-5.36eV, -5.34 eV and -5.35eV, respectively) from
the equation EHUMO =Eoxonset+4.4 eV. Three dyes were actually
located at very low energy.The important factor for this was
that stronger electron donating group would result in more
intensive ICT process.
Figure 4. Cyclic voltammograms of DTPA-BDP a-c in CH2Cl2,
containing 0.1M nBu4NPF6 as the supporting electrolyte recorded at

4
Tetrahedron Letters
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Figure 5. Current density-voltage (J-V) characteristics of the devices
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Donor (DTPA- BDP a, DTPA- BDP b, DTPA- BDP c,)/C61−PCBM
BHJ Film.
Jsc (mA
cm-2)
3.46
Voc
(V)
0.70
0.40
0.54
FF
%
0.32
0.24
0.23
PCE
%
0.77
0.09
0.10
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a
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In Conclusion, three novel BODIPY dyes (DTPA-BDP a-c)
with 4,4-dimethyltriphenylamine at the 2,6-positions were
successfully synthesized, which displayed PCEs 0.77% with a
maximum Voc of 0.70V. Moreover, the interesting optical
and electrochemical properties of these BODIPY dyes were
also investigated. All dyes are emissive in near-infrared
region. We believed that BODIPY dyes with 4,4-
dimethyltriphenylamine at the 2,6-positions would improve
light-harvesting assemblies. The further molecular design of
molecular disks can enhance the PCE values of BHJ solar
cells by optimizing the organized structures of donors for
efficient charge mobility in the active layers.
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Acknowledgments
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The authors acknowledge the financial support from
Natural Science Foundation of China (No.21342003 and
No.51076032).
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Supplementary Material
Experimental procedures, structural proofs, and spectral
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Leave this area blank for abstract info.
Synthesis, Spectral, Electrochemical Properties
and Photovoltaic Performance of structurally
constrained BODIPY dyes with
4,4-dimethyltriphenylamine at the 2,6-positions
Hongbin Zhaoa, b, Bangying Wang a, b, Junxu Liaoa, Hongke Wang a, Guiping Tan a
a
College of Chemistry and Environmental Engineering, Dongguan University of Technology, Guangdong 523808, People’s Republic of China
College of Chemistry, Xiangtan University, Hunan 411105, People’s Republic of China
b