Tailoring the photophysical and photovoltaic properties of boron-difluorodipyrromethene dimers

Article abstract of DOI:10.1016/j.cclet.2017.01.013

Five boron-difluorodipyrromethene (BODIPY) dimers have been designed and synthesized successfully via acid-catalysed condensation and Pd-catalysed cross-coupling reactions. The structural modification, including verifying the structures of the π-bridges,

Full text of DOI:10.1016/j.cclet.2017.01.013

Accepted Manuscript
Title: Tailoring the photophysical and photovoltaic properties
of boron-difluorodipyrromethene dimers
Authors: Wen-Xu Liu, Jian-Nian Yao, Chuan-Lang Zhan
PII:
S1001-8417(17)30035-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2017.01.013
CCLET 3962
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Chinese Chemical Letters
Received date:
Accepted date:
11-1-2017
11-1-2017
Please cite this article as: Wen-Xu Liu, Jian-Nian Yao, Chuan-Lang Zhan, Tailoring the
photophysical and photovoltaic properties of boron-difluorodipyrromethene dimers,
Chinese Chemical Letters http://dx.doi.org/10.1016/j.cclet.2017.01.013
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Original article
Tailoring the photophysical and photovoltaic properties of boron-
difluorodipyrromethene dimers
Wen-Xu Liu, Jian-Nian Yao, Chuan-Lang Zhan
Beijing National Laboratory of Molecular Science, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190 China
 Corresponding author.
E-mail address: clzhan@iccas.ac.cn
Graphical abstract
The photophysical and photovoltaic properties of BODIPY based dimers can be modulated effectively by facile structural
modifications.
ABSTRACT
Five boron-difluorodipyrromethene (BODIPY) dimers have been designed and synthesized successfully via acid-catalysed condensation and Pd-
catalysed cross-coupling reactions. The structural modification, including verifying the structures of the π-bridges, altering the positions the bridges
link (meso- or - positions), and regulating the molecular planarity, can modulate the photophysical properties and the aggregation behaviors of the
five dimers efficiently. Solution-processed organic solar cells were fabricated to evaluate the photovoltaic properties of these molecules further either
as acceptors or donors. When using as nonfullerene acceptor and blended with the polymer donor of PTB7, an open-circuit voltaic (Voc) of 1.12 and
1.08 V was achieved from the thiophene and benzodithiophene bridged BODIPY dimers, respectively. This Voc is among the top values achieved from
the non-fullerene organic solar cells so far.
Keywords:
BODIPY
Nonfullerene acceptor
Small molecule donor
Photovoltaic cell
Solution-processed
———

ARTICLE INFO
Received 24 November 2016
Received in revised form 12 December 2016
Accepted 26 December 2016
Available online
1. Introduction
The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPY) dyes, known as the “little sister” of porphyrins because of the similar
dipyrromethene backbone they possess [1, 2], have gained much attention in the field of labeling, liquid-crystalline materials, chemical
sensors, electroluminescence, energy transfer cassettes and dye-sensitized solar cells recently [3-12]. The wide range of the
applications of these dyes have a very close relationship with their excellent properties, such as facile functionalization of their
backbones, outstanding spectroscopic properties with maximum molar extinction coefficient (max) larger than 80103 L mol-1 cm-1 and
quantum yield (F) larger than 0.5, insensitive to the solvent polarity, together with excellent thermal and chemical stability [13-16].
Due to the advantages aforementioned as well as the suitable frontier energy levels of the BODIPY core, it has been regarded as
promising candidate as electron-deficient group in the regime of organic photovoltaic in the past decade. By rationally tailoring the
structures, such as extending the -system by attaching or fusing electron-donating aromatic building blocks at the  and  positions,
reinforcing intermolecular interaction by modifying the fragments at the meso position, power conversion efficiency (PCE) of near 5%
have been realized [17-27]. Very recently, the BODIPYs have been used as the building blocks to construct non-fullerene acceptors
and a moderate PCE of 1.5% has been achieved when blend with P3HT [28].
In this work, we concentrate on the structural modification of the BODIPY-based dimers with the goal of effectively modulating the
energy levels, absorption as well as the optoelectronic properties in the solid. To realize this, we synthesized a series of novel BODIPY
based dyes (Ph(m-BODIPY)2, T(m-BODIPY)2, BDT(T-m-BODIPY)2, BDT(-metBODIPY)2 and BDT(-metBODIPY)2 (Fig. 1)),
differing in the structure of the π-bridges and the positions they link to the BODIPY core as well as the substitutions on the BODIPY
core. Our results imply that the π-bridges at the meso position have just slight effects on the photophysical properties of the molecules
regardless of their electron-donor abilities and sizes. However, they do have significant effects on the aggregation behaviors in solid
and optoelectronic properties in bulk heterojunction solar cells. Besides, variation of the positions bridges linked to the terminal
BODIPY motifs and the dihedral angle between them have dramatically influences not only on the photophysical and electrochemical
properties, but on the packing behaviors in the solid state, thus the optoelectronic properties of the molecules.
2. Results and discussion
Fig. 2 shows the optimal conformations of the five molecules at the ground state simulated by Guassian 09 program at the B3LYP/6-
31G level. In the cases of Ph(m-BODIPY)2, T(m-BODIPY)2, and BDT(T-m-BODIPY)2, the dihedral angles between the bridge units
and the BODIPY planes are larger than 70. The twisted arrangement between the bridge units and the terminal BODIPY motifs is
independence of the structures of the bridges. Compared with BDT(T-m-BODIPY)2, the dihedral angle between the BODIPY and 4,8-
bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b’]dizhiophene (BDT) plane reduce to 50 in the case of BDT(-metBODIPY)2,
implying better planarity can be achieved when the BDT bridge is selected to link at BODIPY’s  position. Moreover, the comparison
between BDT(-metBODIPY)2 and BDT(-BODIPY)2 reveals that the severely steric congestion can be released further when the four
methyl groups on the BODIPY plane are replaced by H atoms. Therefore, the geometries of the targeted molecules can be tuned by a
series of structural evolution, thus modulating the photo-physical and optoelectronic properties consequently.
Fig. 3 depicts the absorption spectra of the five molecules both in dilute CH2Cl2 solution and in the solid state. The relevant data are
summarized in Table 1. Obviously, the absorption spectra of the five molecules in the solution cover a wide range of the visual light
region from 400 nm to 700 nm in solution. Specifically, for Ph(m-BODIPY)2, T(m-BODIPY)2, and BDT(T-m-BODIPY)2, they all
exhibit similar narrow Gaussian-shaped S0S1 absorption bands with the max at about 500 nm and max larger than 100103 L mol-1
cm-1, besides, a pronounced shoulder peak on the high-energy side of the main peak can also be observed, which is attributed to the
vibrational transition [14, 29]. Their optical phenomena are similar to those of BODIPY monomer, which implies the poor electronic
communication between the π-bridges and the BODIPY planes due to their highly twisted conformations. However, in contrast, the
absorption spectrum of BDT(-metBODIPY)2 is broader and the bandedge red-shifts significantly compared with the meso linked
counterpart (BDT(T-m-BODIPY)2) although its max red-shifts only a little (520 nm vs. 517 nm), this result indicates that when BDT
group is selected to link the BODIPY motifs at their  positions (2, 6-positions) directly, it can conjugate with the BODIPY planes to
some degree, despite that the existing of the methyl groups on the pyrrole rings deteriorate the electronic interaction between the bridge
and the BODIPY cores, which is consistent with the theoretical results. Furthermore, as the steric hindrance is removed effectively
when the methyl groups are removed, a broad intramolecular charge-transfer (ICT) peak which results from the strong donor-acceptor

(D-A) interaction between the BDT bridge and the BODIPY moieties, with a max of 50103 L mol-1 cm-1 appears at about 617 nm in
BDT(-BODIPY)2. When spun-cast into thin films, their absorption spectra all red-shift and broaden compared with the solution
counterparts due to the molecular aggregation and interaction in the solid [30, 31]. Moreover, for BDT(-BODIPY)2, a shoulder peak
could clearly be seen at 700 nm, which is related to better π–stacking in the solid due to its more planar conformation than BDT(-
metBODIPY)2. The optical band gaps are calculated to be 2.09, 2.11, 2.19, 1.99 and 1.54 eV for Ph(m-BODIPY)2, T(m-BODIPY)2,
opt
BDT(T-m-BODIPY)2, BDT(-metBODIPY)2 and BDT(-metBODIPY)2 respectively, according the formula: Eg = 1240/edge [32].
Here, edge refers to the onset values of the absorption spectra of the molecules in the thin films.
Cyclic voltammetry (CV) was applied to investigate the electrochemical properties of the five small molecules. The corresponding
CV traces are plotted in Fig. 4A. According to the onset of the oxidation and reduction potentials as well as the empirical equations:
EHOMO = e(4.4 + Eox); ELUMO = e(4.4 + Ered), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) energy levels are estimated to be 5.50 and 3.59 eV, 5.53 and 3.49 eV, 5.54 and 3.49 eV, 5.10 and3.32 eV,
and 5.10 and 3.62 eV for Ph(m-BODIPY)2, T(m-BODIPY)2, BDT(T-m-BODIPY)2, BDT(-metBODIPY)2 and BDT(-BODIPY)2,
respectively. The electrochemical band gaps of Ph(m-BODIPY)2, T(m-BODIPY)2, BDT(T-m-BODIPY)2, BDT(-metBODIPY)2 and
cv
BDT(-BODIPY)2, are calculated to be 1.91, 2.04, 2.05, 1.78 and 1.48 eV, respectively, according to the equation: Eg = ELUMO
–
opt
EHOMO, these values are well agree with the corresponding Eg values mentioned above. Judging from the electrochemical data, one
can find that the substitutions at the meso position has very slight impact on the frontier orbital energies, thus leading to comparable
bandgaps for Ph(m-BODIPY)2, T(m-BODIPY)2 and BDT(T-m-BODIPY)2. When the two BODIPY monomers are selected to link at
their 2,6-positions via the same π-bridge, the HOMO energies rise up conspicuously (about 0.4 eV) in the case of BDT(-
metBODIPY)2 and BDT(-BODIPY)2. Interestingly, the planar BDT(-metBODIPY)2 possesses much lower LUMO energy (3.62
eV) than the twisted BDT(-metBODIPY)2 (3.32 eV). All the modulations of the frontier energies resulting from the structural
modification leads to the decrease of the bandgap value from BDT(T-m-BODIPY)2 (2.05 eV) to BDT(-BODIPY)2 (1.48 eV).
Given to the appropriate energy levels of the five small molecules (Fig. 4B), solar cells with the structure of ITO/ poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) /PTB7:Ph(m-BODIPY)2 (or T(m-BODIPY)2 and BDT(T-m-
BODIPY)2)/Ca (20 nm)/Al (80 nm) and ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/PC71BM:
BDT(-metBODIPY)2 (or BDT(-metBODIPY)2)/Ca (20 nm)/Al (80 nm) were constructed to evaluate Ph(m-BODIPY)2, T(m-
BODIPY)2 and BDT(T-m-BODIPY)2 as the acceptor materials and BDT(-metBODIPY)2 and BDT(-BODIPY)2 functioning as the
donor
materials.
Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl]]3-fluoro-2-[(2-ehtylhexyl)carbonyl]-
thieno[3,4-b]thiophenediyl]] (PTB7) was employed as the donor here not only because of its appropriate HOMO and LUMO energy
but its excellent light capturing ability from 550 nm to 700 nm [33]. Unfortunately, Ph(m-BODIPY)2 exhibits poor solubility even in
dilute CHCl3, consequently, no perfect films can be obtained from the solution mixture of PTB7: Ph(m-BODIPY)2. Different
conditions, including processing solution, donor: acceptor weight ratio, concentration, the amount of additive and thermal annealing
were optimized to achieve the best performances for the others. During these procedures, we found that all the systems exhibited best
performances when using o-DCB as the processing solvent with a D:A weight ratio of 1:1. Commonly used solvent additive such as
DIO and 1-CN were used to optimize the performances of the devices further, however, no positive effect was observed, unfortunately.
The best photovoltaic parameters for each molecule are given in Table 2. The corresponding J-V curves are displayed in Figure 5 as
well. Compared with the negligible PCE of PTB7: BDT(T-m-BODIPY)2 based system (<0.04%), PTB7: T(m-BODIPY)2 based one
exhibits higher Jsc (0.10 vs. 0.72 mA cm-2) and an extremely high Voc of 1.12 V is also achieved, to the best of our knowledge, this Voc
is the highest value reported from PTB7 based solar cells and finally, a PCE of 0.24% is reached. The similar Voc values of BDT(-
metBODIPY)2: PC71BM and BDT(-BODIPY)2: PC71BM based devices are in accordance with their close HOMO energy. However,
BDT(-metBODIPY)2 based devices exhibit lower Jsc and FF than BDT(-BODIPY)2 based one. On the one hand, the absorption band
of BDT(-metBODIPY)2 covers only a narrow region from 450 nm to 600 nm while that of BDT(-metBODIPY)2 extends even to 800
nm, which contributes to the full use of the sunlight, since about 50% of the solar photons have energies corresponding to a wavelength
of 600-1000 nm, on the other, the presence of the tilt angles between the BODIPY planes and BDT plane hinders the molecules from
packing orderly in the solid state. As it is known that the packing ordering of the molecules in the solid has a close correlation with the
transportation of the charge carriers, thus the Jsc and the PCE of the solar cells [34, 35].
3. Conclusion
A series of BODIPY-based dimers have been synthesized and characterized. Our results clearly demonstrate that the modification at
the meso position has only slight effect on the absorption and electrochemical properties of the dimers, their aggregation behaviors and
optoelectronic behaviors can be modulated effectively, however. The thiophene π-bridge one gives superior performances to other
analogues when blended with PTB7 in organic solar cells. The Voc of 1.12 V is among the highest Voc values in single-junction
solution-processed organic solar cells. The modulation of the photophysical behaviors has also been realized by changing the positions
the π-bridges link to the BODIPY cores and the dihedral angle between them. The reduction of the dihedral angle from BDT(-
metBODIPY)2 to BDT(-BODIPY)2 not only improves the conjugation, brings down the LUMO energy and red-shifts the absorption
spectrum, but facilitates the ordered stacking of BDT(-BODIPY)2 in the solid, thus leading to an improvement of light-harvesting
ability and charge carriers transformation behaviours, which are corresponding to the enhancement of the performance of the solar cells
from 0.4% to about 1%. All the results reveal that the modulation of the photophysical properties of BODIPY dyes is achievable by

judicious structural modification, and BODIPY dyes are promising candidates for application in organic solar cells, functioning as both
the donor and the acceptor materials.
4. Experimental
4.1. Materials
All the materials used were received from commercial sources (Acros, Sigma, TCI or Stream) and were used without further
purification. Toluene was distilled from benzophenone ketyl under the protection of nitrogen prior to use in order to get rid of the trace
amount of water. Compounds 1, 2 and 6 were synthesized according to the reported literatures [21, 36, 37]. 2,6-Bis(trimethyltin)-4,8-
bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b’]dizhiophene (BDT-2Sn, compound 3) were purchased from Suna Tech Inc and
used as received.
4.2. General characterization methods
NMR spectra were measured on a Brucker AVANCE 300 MHz or 400 MHz, the samples were dissolved in chloroform-d or
dichloromethane-d2  = 0 ppm
Mass spectra (MALDI-TOF-MS) were conducted on a Bruker BIFLEX III mass spectrometer (Matrix: CCA). Elemental analysis
was conducted on a flash EA1112 analyser to analyse the content of C, H, and N, respectively.
UV-vis absorption spectra were recorded on Schimadzu UV 2600 absorption spectrometer. The concentration of the solutions used
here was 10-6 mol L-1, and the solid samples were prepared by spin-casting the solution on the quartzes. Cycle voltammetry (CV) was
measured on a computer-controlled Zennium electrochemical workstation at a scan rate of 100 mV s-1. A glassy carbon electrode, a Pt
wire and a Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively. The molecules were dissolved
in degassed anhydrous CH2Cl2 to achieve a concentration of 10-4 mol L-1 with 0.1 mol L-1 tetra-n-butylammonium hexafluorophosphate
(Bu4NPF6) as the supporting electrolyte.
4.3. Synthetic procedures (The routines are depicted in Fig. 5)
Synthesis of Ph(m-BODIPY)2: To a well-dried two neck round bottle with 100 mL CH2Cl2, 2,4-dimethylpyrrole (1.6 mL, 15.5 mmol)
and terephthalaldehyde (519.9 mg, 3.88 mmol) were charged. After N2 was purged for 1 hour, a small amount of trifluoroacetic acid
(TFA) was added, and the solution turned purple quickly. After one hour, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1.76 g, 7.76
mmol) was added at once. After 2 h, (i-Pr)2EtN (15 mL) was added to the reaction mixture followed by BF3-Et2O (15 mL) immediately.
The reaction was quenched by adding 100 mL water 1 h later. The mixture was extracted with CH2Cl2 and the organic phase was
collected and evaporated under vacuum. Finally, the crude product was purified by column chromatography (eluent: CH2Cl2/petroleum
= 3/2) to give a red solid as desired product (818.6 mg, yield: 37%). 1H NMR (300MHz, CDCl3): δ 7.52 (s, 4H), 6.01 (s, 4H), 2.58 (s,
12H), 1.55 (s, 12H). 13C NMR (75 MHz, CDCl3): δ 155.4, 143.0, 142.1, 135.1, 131.5, 128.0, 121.1, 14.6, 14.3. MS (MALDI-TOF):
calculated for 570.27, found m/z 570.2 (M+). Elemental anal. calcd. for C32H32B2F4N4 (%): C, 67.40; H, 5.66; N, 9.83. Found (%) C,
67.58; H, 5.46; N, 9.85.
Synthesis of T(m-BODIPY)2: The procedure was similar to that of Ph(m-BODIPY)2, except that thiophene-2,5-dicarbaldehyde
(543.8 mg, 3.88 mmol) was used instead of terephthalaldehyde. A reddish solid (670.7 mg, yield: 30%) was gotten after purification.
1H NMR (300 MHz, CDCl3): δ 7.16 (s, 2H), 6.05 (s, 4H), 2.56 (s, 12H), 1.88 (s, 12H). 13C NMR (75 MHz, CDCl3): δ 156.1, 143.4,
134.9, 133.9, 132.3, 127.4.121.6, 14.6, 13.5. MS (MALDI-TOF): calcd. for 576.23, found m/z 576.1 (M+). Elemental anal. calcd. for
C30H30B2F4N4S (%): C, 62.53; H, 5.25; N, 9.72. Found (%) C, 62.58; H, 5.16; N, 9.75.
Synthesis of compound 3: N-Iodo-succinimid (NIS) (300 mg, 1.33 mmol) was dissolved in 30 mL anhydrous CH2Cl2, then the
solution was added dropwise into the solution of compound 1 (429.0 mg, 1.33 mmol) in CH2Cl2. After the reaction was completed,
water (50 mL) was added, then the organic layer was extracted by CH2Cl2, the organic phase was evaporated to dry under vacuum. The
crude material was purified by silica gel column chromatography (eluent: CH2Cl2/petroleum = 1/2). A red solid was collected after
1
purification. (419.0 mg, yield: 70%). H NMR (300 MHz, CDCl3): δ 7.497.51 (m, 3H), 7.257.27 (m, 2H, overlapped with CDCl3),
6.04 (s, 1H), 2.64 (s, 3H), 2.57 (s, 3H), 1.38 (s, 6H).
Synthesis of compound 4: Liquid bromine (1 mL, excess) was dissolved in 50 mL anhydrous CH2Cl2, then the solution was added
dropwise slowly into the solution of compound 2 (804.2 mg, 3.00 mmol) in CH2Cl2 (100 mL). The reaction was monitored by TLC
every 30 min, after all the compound 2 was consumed, the organic layer was washed with aqueous solution of sodium thiosulfate, then
the organic layer was extracted with CH2Cl2, the organic phase was collected and evaporated to dry by a rotator evaporation equipment.
Purification was conducted by silica gel column chromatography (eluent: CH2Cl2/petroleum = 2/5) to afford a red solid (780.7 mg,
yield: 75%) as the product.1H NMR (400 MHz, CDCl3): δ 7.94 (s, 1H), 7.73 (s, 1H), 7.517.57 (m, 1H), 7.457.49 (m, 4H), 6.936.94
(d, J = 4 Hz, 1H), 6.82 (s, 1H), 6.536.54 (d, 1H, J = 4 Hz).
Synthesis of BDT(T-m-BODIPY)2: Compound 5 (180.9 mg, 0.20 mmol) and compound 6 (204.5 mg, 0.50 mmol), and 10mL
anhydrous toluene was added into a dry Schlenk tube. After purged with argon for 30 min, Pd(PPh3)4 (20 mg, 0.017 mmol) was
charged. Then the mixture was heated at 110 °C for 12 h. After cooling to room temperature, the mixture was poured into water and

extracted with CH2Cl2 three times. The organic phase was collected and washed with brine and water and then dried over Na2SO4. The
residue was then loaded on silica gel column chromatography (eluent: CH2Cl2/petroleum = 1/1) and recrystallized from EtOH/CHCl2
1
(1/10, v/v) to produce BDT(T-m-BODIPY)2 as a reddish powder (185.2 mg, 75%). H NMR (400 MHz, CD2Cl2): δ 7.63 (s, 2H),
7.257.27 (m, 4H), 6.876.90 (m, 4H), 5.98 (s, 4H), 2.792.82 (m, 4H), 2.43 (s, 12H), 1.67 (s, 12H), 1.601.64 (m, 2H), 1.301.40 (m,
16H), 0.770.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 155.9, 145.6, 143.6, 143.3, 142.7, 138.0, 137.3, 134.4, 134.0, 132.6, 131.4,
127.5, 127.2, 125.6, 122.3, 121.1, 118.6, 116.4, 41.7, 34.4, 32.6, 29.1, 26.0, 23.2, 14.6, 14.1, 13.6, 10.9. MS (MALDI-TOF): Calcd. for
1234.42, found m/z 1234.0 (M+). Elemental anal. calcd. for C68H72B2F4N4S6 (%): C, 66.11; H, 5.87; N, 1.75. Found (%) C, 66.02; H,
6.00; N, 1.81.
Synthesis of BDT(-metBODIPY)2: The synthetic procedure was similar to that of BDT(T-m-BODIPY)2, except that compound 3
(225.0 mg, 0.50 mmol) was used instead of compound 6. Purification on silica gel column chromatography (eluent: CH2Cl2/petroleum
= 2/3) following by recrystallization from a mixture of EtOH/CH2Cl2 (10/1, v/v) yielded BDT(-metBODIPY)2 as a brown-reddish
powder (203.1 mg, 83%). 1H NMR (300 MHz, CDCl3): δ 7.46-7.48 (m, 6H), 7.35 (s, 2H), 7.28-7.31 (m, 4H, overlapped with CDCl3),
7.247.28 (m, 2H, overlapped with CDCl3), 6.836.84 (d, 2H, J = 3.3 Hz), 6.03 (s, 2H), 2.812.83 (d, 4H), 2.63 (s, 6H), 2.59 (s, 6H),
1.631.65 (m, 2H), 1.41 (s, 6H), 1.39 (s, 6H), 1.261.32 (m, 16H), 0.880.94 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 156.2, 154.1,
145.2, 143.9, 143.4, 142.9, 142.1, 140.3. 137.9, 137.4, 135.4, 132.2, 130.8, 129.4, 129.1, 128.1, 127.9, 126.6, 125.5, 122.8, 121.7, 41.7,
34.2, 32.6, 29.0, 25.8, 23.2, 14.9, 14.6, 14.0, 13.5, 12.6, 10.9. MS (MALDI-TOF): calculated for 1222.51, found m/z 1222.0 (M+).
Elemental anal. calcd. for C72H76B2F4N4S6 (%): C, 70.69; H, 6.26; N, 4.58. Found (%): C, 70.62; H, 6.30; N, 4.35.
Synthesis of BDT(-BODIPY)2: The synthetic procedure was similar to that of BDT(T-m-BODIPY)2, except that compound 4
(182.5 mg, 0.50 mmol) was used instead of compound 6. Purification on silica gel column chromatography following by
1
recrystallization from a mixture of EtOH/CH2Cl2 (8/1, v/v) yielded BDT(-BODIPY)2 as a blue-black powder (171.1 mg, 77%). H
NMR (300 MHz, CDCl3): δ 8.28 (s, 2H), 7.98 (s, 2H), 7.54-7.64 (m, 12H), 7.27 (s, 2H, overlapped with CDCl3), 6.916.98 (m, 6H),
6.576.58 (d, 2H, J = 2.7 Hz), 2.872.89 (d, 4H), 1.691.73 (m, 2H), 1.271.47 (m, 16H), 0.810.99 (m, 12H). 13C NMR (100 MHz,
CDCl3): δ 147.3, 146.4, 145.2, 144.3, 143.1, 142.9, 142.1, 138.1, 137.9, 135.6, 134.5, 133.8, 132.0, 131.0, 130.9, 130.6, 128.2, 127.4,
125.9, 125.3, 122.9, 120.2, 41.8, 34.2, 32.6, 29.1, 25.9, 23.3, 14.0, 11.0. MS (MALDI-TOF): Calcd. for 1111.06, found m/z 1110.9 (M+).
Elemental anal. calcd. for C64H60B2F4N4S4 (%): C, 69.18; H, 5.44; N, 5.04. Found (%): C, 69.22; H, 5.36; N, 5.15.
4.4. Fabrication and characterization of the organic solar cells
Conventional organic solar cells with an architecture of: ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/active layer/Ca (20 nm)/Al (80 nm) were fabricated. The indium tin oxide (ITO) glasses were cleaned with detergent,
deionized water, acetone and isopropanol and then treated in a Novascan PSD-ultraviolet-ozone chamber for 1 hour and a layer of 30
nm PEDOT: PSS (Baytron P VP AI 4083, Germany) was spin-coated subsequently. After baking at 150 °C for 15 min in the air, the
glasses were transferred into a glove box. Then a blend solution (1,2-dichlorobenzene (o-DCB), chlorobenzene (CB) or chloroform
(CF)) of PTB7:Ph(m-BODIPY)2 (or T(m-BODIPY)2 and BDT(T-m-BODIPY)2) or BDT(-metBODIPY)2 (or BDT(-
metBODIPY)2):PC71BM with various concentrations was spun-cast to form the photosensitive layer, donor/acceptor weight ratio,
contents of 1,8-diiodooctance (DIO) (or 1-chloronaphthalene (1-CN)), thermal annealing temperatures were optimized to achieve the
best photovoltaic responses. The solutions were stirred at 60 C overnight prior to use. The Ca/Al cathode was deposited by vacuum
evaporation onto the photosensitive layer. The effective area was measured to be 6 mm2. The currentvoltage (JV) measurement of
the devices was measured using a Keithley 2400 Source Measure Unit in the glove box under white light illumination of simulated AM
1.5 G, 100 mW cm-2 using a xenon-lamp-based solar simulator (AAA grade, XES-70S1).
Acknowledgments
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NSFC, Nos.
91433202, 91227112 and 21221002) and Chinese Academy of Sciences (CAS, No. XDB12010200).
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Fig. 1. Chemical structure of five BODIPY based dyes.

Fig. 2. The optimal geometries of the five BODIPY derivatives of Ph(m-BODIPY)2 (A), T(m-BODIPY)2 (B), BDT(T-m-BODIPY)2 (C), BDT(-metBODIPY)2
(D) and BDT(-BODIPY)2 (E) using Gaussian 09 program.
Ph(m-BODIPY)
2
200
T(m-BODIPY)
2
150
100
50
BDT(T-m-BODIPY)
2
BDT(-metBODIPY)2
BDT(-BODIPY)2
0
0.6
0.4
0.2
0.0
400
500
600
Wavelength / nm
Fig. 3. Absorption spectra of the five dimers with concentration of 10-6 mol L-1 in CH2Cl2 solution (upper) and in the thin films (lower).
700
800
900

A
B
-3
-4
-5
-3.31
-5.15
1
4
2
5
3
-3.49 -2.9
-3.49
-5.53
-3.59
-5.50
-4.3
-4.7
-5.0
-5.54
-6
-2
-3
-4
-5
-6
-3.32
-3.62
-2.9
-3.91
-5.87
-4.3
-4.7
-5.0
-5.10
-5.10
-2
-1
0
1
2
Potential / V
Fig. 4. (A) CV curves of the five molecules in dilute CH2Cl2 solutions; (B) The energy diagram of the five molecules, PTB7, PC71BM as well as other
components used in the normal solar cells for comparisons. For clarity, from 1 to 5 represent Ph(m-BODIPY)2, T(m-BODIPY)2, BDT(T-m-BODIPY)2, BDT(-
metBODIPY)2 and BDT(-BODIPY)2 here, respectively.
Fig. 5. Synthesis routines of the dimers. Reaction conditions: (i) TFA, DDQ, (i-Pr)2EtN, BF3-Et2O, N2, CH2Cl2, yield: 37% and 30% for Ph(m-BODIPY)2 and
T(m-BODIPY)2, respectively; (ii) NIS, CH2Cl2, yield: 70%; (iii) Br2, CH2Cl2, yield: 75%; (iv) Pd(PPh3)4, N2, 120 C, toluene, 12 h, yield: 75%, 83% and 77%
for BDT(T-m-BODIPY)2, BDT(-metBODIPY)2 and BDT(-BODIPY)2, respectively.

Table 1 A summary of the optical and electrochemical properties of the five molecules.
edgec (nm);
Compd.
HOMO (eV)
LUMO (eV)
EgCV,e (eV)
absa (nm); log
absb (nm)
Egopt,d (eV)
590; 2.09
587; 2.11
566; 2.19
623; 1.99
805; 1.54
Ph(m-BODIPY)2
T(m-BODIPY)2
498; 5.11
519; 5.06
517; 5.13
520; 5.06
617; 4.68
545.5
544
534
532
630
5.50
5.53
5.54
5.10
5.10
3.59
3.49
3.49
3.32
3.62
1.91
2.04
2.05
1.78
1.48
BDT(T-m-BODIPY)2
BDT(-metBODIPY)2
BDT(-BODIPY)2
a Maximum absorption wavelength measured in dilute CH2Cl2. b Maximum absorption wavelength measured in solid. c Offset of the absorption spectra in solid. d
Optical band gap, Egopt = 1240/edge e Electrochemical band gap, Egcv = ELUMO – EHOMO
.
.
Table 2 Performances of the five molecules based solar cells under optimal conditions.
D:Aa
D:A (w:w)
1:1
Jsc (mA cm-2)
0.72
Voc (V)
1.12
1.08
0.48
0.53
FF (%)
29.9
34.7
24.0
29.1
PCEmax (%); PCEaved (%)
0.24; 0.20
b
PTB7:T(m-BODIPY)2
c
PTB7: BDT(T-m-BODIPY)2
1:1
1:1
1:1
0.10
3.72
6.17
0.04; 0.03
0.43; 0.40
0.95; 0.91
BDT(-metBODIPY)2: PC71BMc
BDT(-BODIPY)2: PC71BMc
a The donor materials and acceptor materials in the active layers; b Processing solvent: 1,2-dichlorobenzene (o-DCB) with a total concentration of 30 mg mL-1; c
Processing solvent: o-DCB with a total concentration of 40 mg mL-1; d Average values from 12 devices.