Benzothiadiazole-based D-π-A-π-D organic dyes with tunable band gap: Synthesis and photophysical properties

Article abstract of DOI:10.1021/ol101754q

A series of symmetrical D-π-A-π-D molecules based on benzothiadiazole and oligo(thienylenevinylene)s were facilely developed. The investigation of their photophysical and electrochemical properties demonstrated that these compounds exhibited broad absorpt

Full text of DOI:10.1021/ol101754q

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4164-4167
Benzothiadiazole-Based D-π-A-π-D
Organic Dyes with Tunable Band Gap:
Synthesis and Photophysical Properties
Jin-Liang Wang, Qi Xiao, and Jian Pei*
Beijing National Laboratory for Molecular Sciences (BNLMS), the Key Laboratory of
Bioorganic Chemistry and Molecular Engineering, College of Chemistry and
Molecular Engineering, Peking UniVersity, Beijing 100871, China
jianpei@pku.edu.cn
Received July 28, 2010
ABSTRACT
A series of symmetrical D-π-A-π-D molecules based on benzothiadiazole and oligo(thienylenevinylene)s were facilely developed. The
investigation of their photophysical and electrochemical properties demonstrated that these compounds exhibited broad absorption covering
the whole visible range with appropriate energy levels for light-harvesting donors.
π-Conjugated organic molecules as alternatives to conjugated
polymers have attracted considerable attention in solution-
processable solar cells due to their monodisperse, reproduc-
ible preparation, and easy fuctionalization.1,2 The benzothi-
adiazole skeleton as a strong acceptor unit is usually
empolyed to modulate the HOMO or LOMO energy levels
for low band gap features in optoelectronic devices.3,4
In this contribution, we present a series of symmetrical
linear donor(D)-π-acceptor(A)-π-donor(D) oligomers with
benzothiadiazole as acceptor core and oligo(thienylenevi-
nylene)s (OTVs)5 with different size as donor segments. Such
D-π-A-π-D molecular design makes it possible to probe
the relationship between the extension of donor chro-
mophores and their corresponding photophysical and elec-
trochemical properties. It is desirable to modulate their
absorption bands to cover the whole visible region, which
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10.1021/ol101754q  2010 American Chemical Society
Published on Web 08/19/2010

might bring insight into the synthesis of good donor
candidates for solution-processable solar cells. Furthermore,
long aliphatic chain at both ends of the D-π-A-π-D rodlike
structures may help induce liquid crystal properties and better
molecular packing.
DMF facilely gave the monoaldehyde 4 in 86% yield.
BT2TVPh with all trans double bonds was obtained
Wittig-Horner reaction of diphosphonate 57 and monoal-
dehyde 4 at room temperature in 63% yield.
Scheme 2. Synthesis of BT3TVPh and BT2TV
Scheme 1. Synthesis of BT2TVPh
Scheme 1 illustrates the synthetic approaches to donor-
acceptor-donor chromophores BT2TVPh. A Wittig-Horner
reaction between 3,4,5-tris(hexyloxy)benzaldehyde 16 and
diethyl 2-thienylmethylphosphonate 25a afford 3 in 92%
yield. Treatment of 3 with n-BuLi followed by anhydrous
As shown in Scheme 2, to improve the solubility and
processability of BT3TVPh, multiple alkyl groups were
introduced at the acceptor core and the donor terminal alkoxy
groups were elongated.8 Reduction of dialdehyde 69 with
NaBH4 gave a diol, and then the crude diol was treated
triethylphosphate, iodide, and DBU to afford the diphos-
phonate 7 in overall 43% yield. Phosphonate 9 was prepared
from treating 3,4,5-tridodecyloxybenzyl bromide 810 with
triethylphosphate in 83% yield.11 11 with extending thienyl
vinylene segment was obtained from 9 and monoaldehyde
1012 through a Wittig-Horner reaction, which is converted
to extended monoaldehyde 12 using n-BuLi and anhydrous
DMF in 53% yield with two steps. Finally, a two-fold
Wittig-Horner reaction of 12 and 7 generated BT3TVPh
in 56% yield. To better understand the photophysical
properties of these D-A-D molecules, BT2TV as model
compound was afforded from treating 5 with 5-hexylth-
iophene-2-carbaldehyde13 in 73% yield.
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Org. Lett., Vol. 12, No. 18, 2010
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Table 1. Photophysical and Electrochemical Properties of these D-π-A-π-D Molecules in Solutions and in Thin Films
λmax
λmax
λmax
λmax
EHOMO
eV
,
ELUMO,
eV
compd
absa, nm (log ε) ema, nm absb, nm emb, nm ΦPL, % Eox(onset)d, V Ered(onset)d, V
Eg(cv), eV
Eg(opt), eV
387 (4.84)
539 (4.78)
419 (5.87)
559 (5.04)
460 (5.19)
580 (5.17)
623
656
678
415
552
430
573
462
595
692
21c
1e
0.65
0.48
0.44
-1.60
-1.30
-1.15
-5.36 -3.11
-5.19 -3.41
-5.15 -3.56
2.25
2.01
BT2TV
1.78
1.59
1.87
1.62
BT2TVPh
0.1
BT3TVPh
a In cyclohexane solution (5 × 10-6 M). b In thin films. c In cyclohexane solution and rhodamine B (ΦPL) 0.65 in ethanol) as the standard. d Reference
electrode: Ag/AgNO3. e In cyclohexane solution and BT2TV as standard.
other two compounds in common organic solvents. Their
structures and purity were fully characterized and verified
by 1H, COSY, and 13C NMR, elemental analysis, and
MALDI-TOF MS (see Supporting Information). The E-
configured double bonds of these new compounds were
confirmed from the values of the coupling constant (J)
between vinyl protons (ca. 16 Hz). Meanwhile, the protons
assigned to the benzothiadiazole core shifted upfield with
increasing concentration (see Figure S11 in Supporting
Information), which was caused by a shielding effect from
π-π stacking.
The investigation of the thermal properties of these new
compounds indicated that they exhibited good stability with
decomposition temperature higher than 350 °C in nitrogen
atmosphere. Figure S1 in Supporting Information showed
crystalline to liquid crystalline phase transition at about 98
°C for BT2TV and 55 °C for BT3TVPh and liquid
crystalline to isotropic liquid phase transition at 188 °C for
BT2TV and 125 °C for BT3TVPh. BT2TVPh exhibited
only single melting point transition at about 160 °C.
The photophysical properties of these D-π-A-π-D linear
molecules were investigated in dilute solutions and in thin
films. Their photophysical data in solutions and in thin films
are summarized in Table 1. As shown in Figure 1, all
and 580 nm for BT3TVPh. The peaks at 387-460 nm could
be assigned to the π-π* transition, and those at longer
wavelengths absorption peaks should result from an intramo-
lecular charge transfer (ICT) between the peripheral donor
part and the acceptor core group. The absorption bands
exhibited the successive red-shift in agreement with the
increase of the effective conjugation length and formation
of donor-acceptor type structures upon appending electron-
rich alkoxyl phenyl group to the electron-deficient cores.14
Meanwhile, the molar extinction coefficient also gradually
increased from BT2TV (0.60 × 105 M-1 cm-1 at 539 nm)
to BT3TVPh (1.48 × 105 M-1 cm-1 at 580 nm). Such high
molar extinction coefficients are favorable for absorbing the
solar radiation.
Figure 1 also shows the photoluminescent (PL) spectra of
these D-π-A-π-D molecules in cyclohexane solutions. A
emission maximum; λmax peaked at 623 nm for BT2TV, 656
nm for BT2TVPh, and 678 nm for BT3TVPh. The red-
shift trend of emission spectra confirmed that BT3TVPh had
the longest effective conjugation length. Moreover, the
fluorescence quantum yields (ΦPL) of these molecules in
cyclohexane solution successively decreased from 21% for
BT2TV to 0.1% for BT3TVPh due to extending thienylvi-
nylene segment. Interesting, the emission band of BT2TV
and BT2TVPh both came exclusively from the charge
transfer band when excited at short wavelength, which
indicated that a highly efficient intramolecular energy transfer
processed from the π-π* transition band to charge transfer
band. For BT3TVPh, residual fluorescence in the short
wavelength region was clearly observed due to its relative
lowest quantum efficiency of BT3TVPh.
The absorption spectra of our D-π-A-π-D molecules
were nearly independent of solvent polarity. However, the
emission spectra of these D-π-A-π-D molecules were
solvatochromic, which further confirmed the strong intramo-
lecular CT in these compounds (see Supporting Information).
Emission maximum λmax of BT2TVPh red-shifted from 654
nm in cyclohexane solution to 714 nm in DMF solution along
with successively decreased emission intensity because the
dipole moment is larger in the excited state than in the ground
Figure 1. Absorption and PL spectra of three molecules in
cyclohexane solution (5 × 10-6 M). PL spectra were recorded at
different excitation wavelengths shown in every parentheses.
Emission spectrum of BT3TVPh scaled up (×5).
molecules showed two distinct absorption bands in the range
of 300-500 and 500-700 nm in cyclohexane dilute solu-
tions. The absorption maximum λmax peaked at 387 and 539
nm for BT2TV, 419 and 559 nm for BT2TVPh, and 460
(14) Silvestri, F.; Marrocchi, A.; Seri, M.; Kim, C.; Marks, T. J.;
Facchetti, A.; Taticchi, A. J. Am. Chem. Soc. 2010, 132, 6108–6123.
4166
Org. Lett., Vol. 12, No. 18, 2010

state.15 Meanwhile, the emission of BT3TVPh was absent
in THF, indicating the stronger charge-transfer excited state
in relatively polar THF.
Figure 3. Cyclic voltammagram of D-A-D conjugated oligomers
films on Pt electrode in 0.1 M Bu4NPF6 in CH3CN solution with a
scan rate of 100 mV/s.
these onset of oxidative or reductive potentials, estimated
the HOMO and LUMO level were -5.36/-3.11 eV for
BT2TV, -5.19/-3.41 eV for BT2TVPh, -5.15/-3.56 eV
for BT3TVPh. Consequently, the band-gaps and LUMO
level were obviously reduced with the increase of the
effective conjugation length in our molecules and were in
agreement with these obtained from their absorption spectra
in thin films. These results indicated that our compounds
had promising electrochemical properties as donor materials
in organic solar cells.
Figure 2. Absorption and PL spectra of these D-A-D molecules
in thin films. PL spectra recorded under excitation at the absorption
λmax wavelength at room temperature.
Figure 2 shows the absorption and PL spectra of three
D-π-A-π-D molecules in the solid state. The thin films
used for absorption and emission measurements were
obtained by spin coating THF solutions (ca. 10 mg/mL) of
our molecules onto quartz plates at 1000 rpm. All compounds
exhibited good film forming properties. The absorption
spectra covered almost UV-vis region and showed almost
same features as those in dilution solution although their
absorption λmax obviously red-shifted, especially in longer
wavelength range due to the formation of J-aggregates.16
The results suggested our molecules are suitable for future
applications in organic solar cells. Table 1 illustrates the
energy band gap of these molecules, which were estimated
from the onset of the absorption spectra in thin films. As
shown in Figure 2, for BT2TV, weak emission with the
maximum λmax at 692 nm was observed; however, the
emission of BT2TVPh and BT3TVPh in thin film was
totally quenched due to the lower PL quantum yield of those
compounds in solution and strong intermolecular interactions
in thin films.17
In conclusion, we have facilely developed a new family
of linear benzothiadiazole-based D-π-A-π-D organic mol-
ecules in good yields. Their absorption and emission features
are significantly modulated by changing the length of OTV
segments in our molecules. BT2TVPh and BT3TVPh in
thin films show a broader and stronger absorption band in
the range of 300 to 800 nm covering the whole visible region.
Such D-π-A-π-D synthetic strategy with simple molecular
structures and extended absorption bands obviates the
difficulty in the preparation of longer OTVs. These photo-
physical and eletrochemical properties showed that our
molecules are good candidates for high-performance solution-
processable organic solar cells and organic field-effect
transistors. Further experiments to explore their liquid crystal
properties and organic solar cells are underway in our
laboratory.
As shown in Figure 3 and Table 1, the HOMO and LUMO
energy levels of these D-π-A-π-D molecules in thin films
were determined by cyclic voltammetry on a Pt electrode.
One single-electron quasi-reversible reduction were observed
at -1.66/-1.50 V for BT2TV, -1.52/-1.33 V for
BT2TVPh, and -1.40/-1.35 V for BT3TVPh. These
compounds exhibited irreversible oxidation process. From
Acknowledgment. This work was supported by the Major
State Basic Research Development Program (Nos. 2006CB921602
and 2009CB623601) from the Ministry of Science and
Technology and National Natural Science Foundation of
China.
Supporting Information Available: Experimental pro-
cedures, 1H and 13C NMR, and MS data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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T. J. Chem. Mater. 2009, 21, 2592–2594.
(17) Jenekhe, S. A.; Lu, L.; Alam, M. M. Macromolecules 2001, 34,
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