Charge transfer in cross conjugated 4,8-dithienylbenzo[1,2-b:4,5-b′] dithiophene based organic sensitizers

Article abstract of DOI:10.1039/c3cc00159h

Two novel cross-conjugated isomers based on 4,8-dithienylbenzo[1,2-b:4,5- b′]dithiophene have been designed and successfully synthesized. It was found that the charge transfer interaction was much stronger in the benzodithiophene direction as compared wit

Full text of DOI:10.1039/c3cc00159h

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Charge transfer in cross conjugated
4,8-dithienylbenzo[1,2-b:4,5-b0]dithiophene
based organic sensitizers†
Cite this: Chem. Commun., 2013,
49, 3899
Received 8th January 2013,
Accepted 22nd March 2013
Shenghui Jiang, Xuefeng Lu, Gang Zhou* and Zhong-Sheng Wang*
DOI: 10.1039/c3cc00159h
www.rsc.org/chemcomm
Two novel cross-conjugated isomers based on 4,8-dithienylbenzo- established, the effect of additional conjugated pathways on
[1,2-b:4,5-b0]dithiophene have been designed and successfully charge delocalization and transfer in cross-conjugated systems
synthesized. It was found that the charge transfer interaction is still a challenge for chemists and physicists.
was much stronger in the benzodithiophene direction as compared
with the other perpendicular direction.
4,8-Dithienylbenzo[1,2-b:4,5-b0]dithiophene (DTBDT) consists
of four thiophene rings with a benzene core. Thus, it can be
functionalized within two perpendicular directions to build
Charge transfer in conjugated organic systems has been widely cross-conjugated organic semiconductors and endow them with
investigated to construct organic semiconductors for optoelectronic unique properties, such as broader absorption band, lower
devices, such as light-emitting diodes (LEDs),1 photovoltaic HOMO level and higher charge mobility. Therefore, DTBDT
devices,2 field-effect transistors (FETs),3 nonlinear optics,4 and has been studied as light-harvesting materials.13 For example,
fluorescent chemosensors.5 Among the various applications, it has been recently incorporated into copolymers to construct
dye-sensitized solar cells (DSSCs) have attracted extensive attention donor–acceptor polymers, and a power conversion efficiency (Z)
in the last two decades since the breakthrough by O’Regan and of 7.59% has been achieved for the corresponding polymer solar
6
Gratzel. The key component in a DSSC device is the sensitizer cells.13a However, the charge transfer in the two different con-
¨
which consists of an electron donor and acceptor connected by a jugation directions has not been studied so far. Therefore,
p-conjugated bridge. Such a push–pull configuration lowers the herein, we designed and synthesized two cross-conjugated
band gap of the semiconductor and greatly benefits the light- push–pull isomers (I1 and I2) and systematically compared the
harvesting capability.7 Most importantly, a facile charge transfer in charge transfer interactions via two perpendicular directions.
the chromophore can make the electrons transport from the donor
moiety to the acceptor moiety upon photoexcitation and then inject both isomers I1 and I2 (Fig. 1) start from 4,8-dehydrobenzo-
into the semiconductor electrode.
[l,2-b:4,5-b0]dithiophene-4,8-dione which contains four reactive
The cross-conjugated8 molecule is constructed from two sites at the a-position on the four thiophene rings. The
‘‘perpendicular’’ p-conjugated arms connected through a-position on the two thienyl groups is much more active than
The synthetic approaches (Schemes S1 and S2, ESI†) to
a
central aromatic core. Such a two-dimensional conjugation is the other two fused on the benzene core. Therefore, for isomer
currently receiving intense attention due to its unique opto- I1, after stirring with 4-hexylthienyl-2-lithium at 0 1C followed
electronic properties as compared with one-dimensional linear by reacting with SnCl2 in 10% hydrochloric acid, the key
conjugated systems. Cross-conjugated materials, such as spiro intermediate core 2 was produced in 65% yield. Two hexyl
compounds,9 tetrasubstituted benzenes,10 paracyclophanes,11 groups were incorporated in order to ensure the solubility and
and bisoxazole derived cruciforms,12 have been recently minimize the intermolecular interactions. Then the corres-
reported. However, much less attention has been paid to the ponding monoaldehyde-substituted derivative 3 was synthe-
charge transfer system in cross-conjugated organic semi- sized by refluxing with a Vilsmeier reagent. However, for
conductors. Although the basic concept for charge transfer isomer I2, to functionalize the a-position on the other two
in one-dimensional systems has been well developed and thiophene rings with less reactivity, two alkyl chains have been
incorporated firstly to block the more active sites of the
Department of Chemistry & Laboratory of Advanced Materials, Fudan University,
a-position on the thienyl rings linked to the benzene core with
a single bond. Lithiation of 2-hexylthiophene with n-butyl-
lithium at 0 1C followed by addition of 4,8-dehydrobenzo-
[l,2-b:4,5-b0]dithiophene-4,8-dione and subsequent stirring
2205 Songhu Road, Shanghai 200438, P. R. China.
E-mail: zhougang@fudan.edu.cn, zs.wang@fudan.edu.cn; Fax: +86-21-5163-0345;
Tel: +86-21-5163-0345
† Electronic supplementary information (ESI) available: Synthesis and character-
ization details for isomers I1 and I2. See DOI: 10.1039/c3cc00159h
with SnCl2 in hydrochloric acid produced the key core 5.
c
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Fig. 2 Absorption and PL spectra of isomers I1 and I2 in toluene solutions.
Fig. 1 Chemical structures of isomers I1 and I2.
benzo[1,2-b:4,5-b0]dithiophene is coplanar. Thus, the conjuga-
tion is more delocalized in the benzodithiophene direction in
However, our further introduction of an aldehyde group by a comparison to the dithienylbenzene direction, which results in
Vilsmeier reagent is not applicable due to the low reactivity. An stronger charge transfer interactions in I2 and bathochromi-
alternative procedure to obtain compound 6 was carried out upon cally shifted the absorption band. Moreover, when the isomeric
addition of a stoichiometric amount of n-butyllithium to the dyes are attached on nanocrystalline TiO2 films, isomers I1 and
solution of compound 5 in tetrahydrofuran (THF) at ꢀ78 1C. I2 display the hypsochromically shifted maximum absorption
Then quenching with N,N-dimethylformamide and subsequent band at 419 and 450 nm, respectively (Fig. S2, ESI†), due to the
acidic hydrolysis produced the corresponding monoaldehyde- deprotonation of the carboxylic acid.
substituted derivative 6. Subsequently, for both isomers I1 and
The photoluminescence (PL) spectra of the two cross-
I2, the electron donor, N,N-bis(4-hexyloxyphenyl)phenylamine, conjugated isomers in toluene solutions were also measured.
was attached via C–H bond activation,14 which provided the The maximum emission band is located at 564 nm for isomers
precursors. It should be noted that a recently developed C–H both I1 and I2. However, as illustrated in Fig. S3 (ESI†), both
bond activation14 was utilized to synthesize the precursors in high isomers display solvent-dependent PL spectra, and bathochromic
yield, which shortened the synthetic procedure by two steps. In shifts of 20 and 31 nm are observed in the PL spectra of isomers
the last step, the obtained precursors were converted to the I1 and I2, respectively, when the solvent polarity was increased
corresponding isomers I1 and I2 by Knoevenagel condensation from toluene to chloroform. Such a bathochromic shift is
in the presence of cyanoacetic acid and piperidine. All the target characteristic for an efficient charge transfer from the electron
compounds were characterized using 1H NMR, 13C NMR spectro- donor to the acceptor through the cross-conjugated bridge.15
scopy, and mass-spectrometry, and were found to be consistent The larger solvatochromic effect in I2 as compared with I1
with the proposed structures.
suggests stronger charge transfer interactions in I2.
Compounds I1 and I2 can easily dissolve in common organic
To further investigate the electronic properties of the two
solvents, such as dichloromethane, toluene, THF, and chloro- cross-conjugated compounds, the electrochemical behavior
form, and produce orange and deep red solutions, respectively. was studied using cyclic voltammetry (CV). As shown in
Fig. 2 shows the UV-vis absorption spectra of the resulting Fig. 3, both cross-conjugates exhibit one reversible oxidative
isomers in toluene solutions (ca. 10ꢀ5 M). It can be found wave at lower potential attributed to the oxidation of the
that both compounds exhibit two distinct absorption bands. triphenylamine moiety and one quasi-reversible oxidative wave
The absorption band in the high-energy region (o400 nm) at higher potential assigned to the oxidation of the DTBDT
corresponds to the p–p* electronic transition of the conjugated core. The first half-wave potential (E1/2) for I1 and I2, which is
backbone, and the other one in the low-energy region taken as the HOMO levels of the isomers, was determined to be
(>400 nm) can be assigned to an intramolecular charge transfer 0.77 and 0.81 V (vs. NHE, the same below), respectively.
from the electron donating unit to the electron-withdrawing Correspondingly, calculated from HOMO levels and the optical
group through the cross-conjugated bridge. As shown in Fig. 2, band gap,16 the LUMO energy level of I1 and I2 is ꢀ1.54 and
I1 displays the maximum absorption wavelength at 441 nm. ꢀ1.38 V, respectively. Moreover, the second E1/2 for I1 and I2 is
Upon changing the electron donor and acceptor to the other determined to be 1.27 and 1.33 V, respectively. It can be clearly
perpendicular direction, I2 demonstrates the maximum observed that both the first and the second oxidative potential
absorption wavelength at 476 nm with a bathochromic shift values of I2 are higher than those for I1. This is due to the more
of 35 nm. Such a significant bathochromic shift can be delocalized conjugated system in I2, which lowers the electron
explained by the molecular planarization. As shown in Fig. S1 density and results in a higher oxidative potential.17 The more
(ESI†), the dihedral angle between the 4,8-substituted delocalized conjugation in I2 also suggests stronger charge
thienyl ring and the benzene core is around 341, while the transfer interactions.
c
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17%, which is probably attributed to the spectral mismatch
between the simulated and standard AM1.5G solar light.
Since Voc is related to the conduction band position of TiO2
and the charge recombination rate in DSSCs, to explain the
difference of another important performance parameter (Voc),
the electron lifetime against charge density for the DSSCs
(Fig. S4, ESI†) was investigated since the relative conduction
band positions are generally identical for the DSSCs based on
sensitizers with similar chemical structures. At the same charge
density, the electron lifetime of I2 based DSSC is longer than
that of I1. Consequently, the reduced charge recombination
rate constant will reduce electron loss at an open circuit. When
more charge is accumulated in TiO2, the Fermi level moves
upward and Voc becomes larger.
In summary, two cross-conjugated isomers based on the
DTBDT unit were designed and synthesized. It was found that
the charge transfer interaction was much stronger in the
benzodithiophene direction as compared with the other dithienyl-
benzene direction. As a result, the DSSC based on isomer I2
exhibited a power conversion efficiency of 7.0% which was
much higher than that for the DSSC based on isomer I1. Our
findings provide insight into the push–pull effects in the cross-
conjugated system and will guide the design of the organic
semiconductors based on cross-conjugated materials.
Fig. 3 Cyclic voltammograms of isomers I1 and I2.
This work was financially supported by the National Basic
Research Program (2011CB933302) of China, the National
Natural Science Foundation of China (90922004 and
51273045), Shanghai Pujiang Project (11PJ1401700), STCSM
(12JC1401500), Shanghai Leading Academic Discipline Project
(B108), and Jiangsu Major Program (BY2010147).
Notes and references
Fig. 4 J–V curves for the DSSCs based I1 and I2. IPCE spectra are shown in the
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Chem. Commun., 2013, 49, 3899--3901 3901