Effective tuning of HOMO and LUMO energy levels by p-n diblock and triblock oligomer approaches

Article abstract of DOI:10.1021/jo0521596

A novel series of oligomers consisting of thiophene as a p-type unit and oxadiazole as an n-type unit were separately synthesized. On the basis of the characterization of photophysical and electrochemical properties, the structure-property relationships of the oligomers were investigated. Cyclic voltammogram studies showed that changing the number of thiophene and oxadiazole units could effectively modulate the electronic properties of the p-n diblock and triblock oligomers. The effect of molecular regiochemistry on electronic properties is also investigated. The observed electronic properties were consistent with theoretical calculations. These systems serve as excellent examples, demonstrating the band gap control principle in the p-n heterostructure oligomers.

Full text of DOI:10.1021/jo0521596

Effective Tuning of HOMO and LUMO Energy Levels by p-n
Diblock and Triblock Oligomer Approaches
Jun-Hua Wan,† Jia-Chun Feng,† Gui-An Wen,† Wei Wei,† Qu-Li Fan,† Chuan-Ming Wang,†
Hong-Yu Wang,† Rui Zhu,† Xiang-Dong Yuan,† Chun-Hui Huang,† and Wei Huang*,‡,§
Institute of AdVanced Materials (IAM), Fudan UniVersity, Shanghai 200433, China, Institute of AdVanced
Materials (IAM), Nanjing UniVersity, Nanjing 210093, China, and Department of Chemical and
Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent,
Singapore 119260, Republic of Singapore
wei-huang@fudan.edu.cn; chehw@nus.edu.sg
ReceiVed October 17, 2005
This paper was withdrawn by the Editor on August 25, 2006 (J. Org. Chem. 2006, 71, 7124).
A novel series of oligomers consisting of thiophene as a p-type unit and oxadiazole as an n-type unit
were separately synthesized. On the basis of the characterization of photophysical and electrochemical
properties, the structure-property relationships of the oligomers were investigated. Cyclic voltammogram
studies showed that changing the number of thiophene and oxadiazole units could effectively modulate
the electronic properties of the p-n diblock and triblock oligomers. The effect of molecular regiochemistry
on electronic properties is also investigated. The observed electronic properties were consistent with
theoretical calculations. These systems serve as excellent examples, demonstrating the band gap control
principle in the p-n heterostructure oligomers.
Introduction
and obtained several series of p-n diblock conjugated poly-
mers.6 The variation of the length of the p and n segments in
the block polymer chain afforded the possibility of tuning the
HOMO and LUMO energy levels and emissive wavelength.
These polymers show improved photoluminescent and elec-
π-Conjugated thiophene-based polymers and oligomers have
been investigated extensively in the past decade because of their
potential applications in semiconductor devices such as thin film
organic field-effect transistors,1 organic light-emitting diodes,2
photovoltaic cells,3 and molecular rectifiers.4 Most of these
applications would benefit from a full understanding of charge-
transport properties, which depend on the morphology of thin
films and the electronic structures.5 Therefore, developing a
methodology to modulate the HOMO and LUMO energy levels
of the elemental conjugated units is indispensable. In our
previous work, we have developed the p-n diblock concept
(1) (a) Dimitrakopoulos, C. D.; Malenfant, R. L. AdV. Mater. 2002, 14,
99. (b) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.;
Casado, J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc. 2002, 124, 4184.
(c) Katz, H. E.; Bao, Z. N.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (d)
Heeney, M.; Bailay, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.;
Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078. (e) Horowitz,
G. AdV. Mater. 1998, 10, 3. (g) Garnier, F. Acc. Chem. Res. 1999, 32, 209.
(h) Rogers, J. A.; Bao, Z.; Dodabalopur, A.; Grone, B.; Raju, V. R.; Katz,
H. E.; Kuck, V.; Ammundson, k. J.; Drzaic, P. Proc. Natl. Acad. Sci. U.S.A.
2001, 98, 4817.
(2) (a) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471. (b)
Barbarella, G.; Melucci, M.; Sotgiu, G. AdV. Mater. 2005, 17, 1581 and
references therein. (c) Barbarella, G.; Favaretto, L.; Zambianchi, M.; Pudova,
O.; Arbizzani, C.; Bongini, A.; Mastragostino, M. AdV. Mater. 1998, 10,
551. (d) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714.
* To whom correspondence should be addressed. Tel: +86 21 5566 4188/
4198. Fax: +86 21 6565 5123/5566 4192.
† Fudan University.
‡ Nanjing University.
§ National University of Singapore.
10.1021/jo0521596 CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/08/2006
J. Org. Chem. 2006, 71, 2565 2565

Wan et al.
SCHEME 1. Structures of Diblock and Triblock Oligomers
troluminescent properties. However, the improvement is still
lower than the expected level. The poor improvement may be
attributed to the alternating distribution of the p segment and n
segment in the polymer chain. The electron deficient unit
inserted into the p-type polymer chain will partially act as the
hole-blocking unit as a result of its high electron deficiency.
Conversely, the hole-transporting unit will lower the electron
mobility.
Actually, the above-mentioned drawback can be resolved by
the diblock oligomer that has two separate blocks, which is
consistent with the p-type and n-type units, respectively. The
p-n diblock oligomer,7 which is an idea analogous to the
semiconductor p-n junction, combined easy charge injection
with current rectification properties. Additionally, in comparison
with the conjugated copolymers, the monodisperse conjugated
oligomers, possessing well-defined conjugation lengths and
structures, are characterized by structural uniformity, ease of
purification, and characterization.8 Furthermore, deep electron
traps possibly occur in polymeric systems as a result of chain
entanglements or structural defects.9 Recently, a thiophene-
thiazole diblock oligomer has been synthesized, and the
rectification properties have been investigated in a molecular
rectifier device.4 However, the report about the systematic study
for the electronic properties of the p-n diblock system is still
lacking. No general concept for the p-n diblock oligomer
system has been established as yet. In this article, a novel series
of diblock oligomers (T2O, T2O2, T4O2), consisting of an
electron-rich thiophene unit and an electron-deficient oxadiazole
unit with different unit lengths, are successfully synthesized.
Furthermore, the electronic properties of these oligomers are
investigated systematically. For the investigation of the effect
of molecular regiochemistry on the electrochemical and optical
properties, another series of n-p-n (OT2O) and p-n-p
(T2O2T2) triblock oligomers were also synthesized (Scheme 1).
Results and Discussion
Synthesis and Characterization. Generally, the oligomer
syntheses involve three procedures, that is, the synthesis of the
oxadiazole monomers, the synthesis of the thiophene monomers,
and the coupling reaction.
Schemes 2 and 3 show the synthetic routes of the oxadiazole
monomers. The common synthetic sequence of 4-bromo-2,5-
bis(octyloxy)benzoic acid methyl ester 2 involves three steps,10
which is too complicated (Scheme 2). We developed a one-pot
synthetic route for compound 2 from 1,4-dibromo-bis(octyloxy)-
benzene 1. Compound 2 with 60% yield is prepared by treating
1 with n-BuLi and treated further with liquid dimethyl carbonate,
which was used more conveniently than gaseous CO2. Moreover,
the monobromo-substituted oxadiazole dimer was synthesized
successfully (Scheme 3). To the best of our knowledge, no
monobromo-substituted oxadiazole dimer has been synthesized
as yet. To obtain compound 8, the mixture of compound 3 and
benzohydrazide in pyridine is added into the solution of
terephthaloyl dichloride in anhydrous THF slowly by syringe.
Compound 8 was obtained with 45% yield after the two
byproducts were removed by silica gel chromatography. After
cyclodehydration of compound 8, monobromo-substitued oxa-
diazole dimer 9 was obtained with 73% yield.
The synthesis of the thiophene monomers 13 and 14 are
depicted in Scheme 3. After crystallizing from ether at -78
°C, 4,4′-dioctyl-2,2′-bithiophene was obtained with 70% yield
from 3-octylthiophene by the oxidative coupling of the lithiated
derivative in the presence of copper chloride. The n-BuLi/Bu3-
SnCl sequence carried out on monobromo and dibromo deriva-
tives, obtained by the reaction of 1 and 2 equiv of NBS on
compound 10 in CHCl3/HOAC (1:1), lead to the stannanes 13
and 14. The stannanes 13 and 14 were used without further
purification as a result of their decomposition on silica gel.
Scheme 4 shows the synthetic route to the oligomers. The
oligomers are obtained with moderate yield through the coupling
(3) (a) Videlot, C.; Kassmi, A. Z.; Fichou, D. Sol. Energy Mat. Sol. Cells
2000, 63, 69. (b) Bettingnies, R. D.; Nicolas, Y.; Blanchard, P.; Levillain,
E.; Nunzi, J.-M.; Roncali, J. AdV. Mater. 2003, 15, 1939. (c) Liu, J.;
Kadnikova, E. N.; Liu, Y.; McGehee, M. D.; Frechet, J. M. J. J. Am. Chem.
Soc. 2004, 126, 9486. (d) Campos, L. M.; Tontcheva, A.; Gunes, S.; Sonmez,
G.; Neugebauer, H.; Sariciftci, N. S.; Wudl, F. Chem. Mater. 2005, 17,
4031.
(4) (a) Ng, M.-K.; Lee, D.-C.; Yu, L. P. J. Am. Chem. Soc. 2002, 124,
11862. (b) Jiang, P.; Morales, G. M.; You, W.; Yu, L. P. Angew. Chem.,
Int. Ed. 2004, 43, 4471.
(5) (a) Facchetti, A.; Yoon, M.-H.; Sten, C. L.; Katz, H. Z.; Mark, T. J.
Angew. Chem., Int. Ed. 2003, 42, 3900. (b) Turbiez, M.; Frere, P.; Allain,
M.; Videlot, C.; Ackermann, J.; Roncali, J. Chem.sEur. J. 2005, 11, 3472.
(c) Kiriy, N.; Bocharova, V.; Kiriy, A.; Stamm, M.; Krebs, F. C.; Adler,
H.-J. Chem. Mater. 2004, 16, 4765.
(6) Yu, W.-L.; Meng, H.; Pei, J.; Huang, W. J. Am. Chem. Soc. 1998,
120, 11808.
(7) In normal polymer concepts, the term “block polymer” means “the
polymer with several blocks that contain a large number of repeating units”.
The most common block polymers are the diblock and triblock polymers,
represented by AnBm and AnBmAn or AxByCz (A, B, and C represent different
repeating units), respectively. However, the polymer with the structure ...
ABABAB ... is named an alternating copolymer instead of a block polymer.
Also, the term “oligomer” means “the polymer with less repeating units
and a low molecular weight compared with that of a common polymer”. In
our manuscript, we introduced the terms “diblock oligomer” and “triblock
oligomer” because they possess the same structure as the diblock and triblock
polymers. The difference is the block length of the oligomer is much less
than that of a polymer. For example, the oligomers with the AAABBB and
AAABBBAAA structures can be named diblock and triblock oligomers,
respectively. In addition, the p-type unit and n-type unit refer to the electron-
rich segment and electron-deficient segment, respectively.
(8) (a) Klubek, K.; Vaeth, K. M.; Tang, C. W. Chem. Mater. 2003, 15,
4352. (b) Lee, S.-H.; Nakamara, T.; Tsutsui, T. Org. Lett. 2001, 3, 2005.
(c) Pei, J.; Wang, J.-L.; Cao, X.-Y.; Zhou, X.-H.; Zhang, W.-B. J. Am.
Chem. Soc. 2003, 125, 9944.
(9) Wu, C.-C.; Liu, T.-L.; Hung, W.-Y.; Lin, Y.-T.; Wong, K.-T.; Chen,
R.-T.; Chen, Y.-M.; Chien, Y.-Y. J. Am. Chem. Soc. 2003, 125, 3710.
(10) Ding, J.; Day, M.; Robertson, G.; Roovers, J. Macromolecules 2002,
35, 3474.
2566 J. Org. Chem., Vol. 71, No. 7, 2006

EffectiVe Tuning of HOMO and LUMO Energy LeVels
SCHEME 2a
SCHEME 4a
a Reaction conditions: (i) Pd(PPh3)4, 3%, 100 °C, toluene; (ii) NBS,
CHCl3/AcOH.
a Reaction conditions: (i) n-BuLi, -78 °C, Me2CO3, THF; (ii)
NH2NH2‚H2O, MeOH; (iii) benzoyl chloride, pyridine; (iv) POCl3, 80 °C;
(v) 3, pyridine; (vi) the solution of 3 and benzohydrazide in pyridine, THF.
FIGURE 1. 1H NMR spectra in the aromatic range for oligomers.
SCHEME 3a
The structure and purity of the oligomers were verified by
1H and 13C NMR, MALDI-TOF MS, and elemental analysis.
For all oligomers, the two triplet signals on the 1H NMR spectra
arising from between δ 4.00 and 4.11 ppm belong to the protons
of the first methylene at the 3-position of the thiophene rings.
The single signal arising from about δ 6.86 ppm originates from
the proton neighboring the thiophene ring of the 1,4-bis-
(octyloxy)benzene ring in the oligomers. Therefore, it is easy
to identify the oligomers through the relative integration of these
1
1
characteristic signals in the H NMR spectra. In the H NMR
spectra of Br-T2O2, the signal arising from δ 7.03 ppm
disappears, which was assigned to the R proton of the terminal
thiophene ring of T2O2 (Figure 1). It is convenient to monitor
the brominating process through the change of this characteristic
1
signal in the H NMR spectra.
Optical Properties. Spectroscopic properties of the oligomers
were investigated with UV-vis absorption and fluorescence
emission. Figure 2a shows the absorption spectra of the
oligomers in toluene solutions. The oligomers, T2O2, T4O2, and
T2O2T2, show two distinct absorption bands. The absorption
bands in the longer wavelength range (above 360 nm) come
from the thiophene units,11 and the remarkable enhancement of
the absorption intensity in T4O2 relative to T2O2 reflects the
a Reaction conditions: (i) n-BuLi/TMEDA, CuCl2; (ii) 2.0 equiv NBS,
CHCl3/HOAC; (iii) 1.0 equiv NBS, CHCl3/HOAC; (iv) n-BuLi, Bu3SnCl.
of the thiophene monomers and the corresponding oxadiazole
monomers by Stille reaction. As a result of the difficulty in
obtaining the pure monobromo-substituted tetrathiophene mono-
mer through the ordinary route, T4O2 was obtained through a
Stille coupling reaction of the stannane 14 with compound Br-
T2O2, which was afforded by the bromination of T2O2 with
NBS.
(11) Chen, S. Y.; Liu, Y. Q.; Qiu, W. F.; Sun, X. B.; Ma, Y. Q.; Zhu,
D. B. Chem. Mater. 2005, 17, 2208.
J. Org. Chem, Vol. 71, No. 7, 2006 2567

Wan et al.
TABLE 1. Optical Properties of the Oligomers
max, abs (nm)
λ
λ
max, em (nm)
a
a
oligomer
toluenea
CH2Cl2
filmb
372
319/405
319/394
397
toluenea
CH2Cl2
filmb
Egc (eV)
T2O
T2O2
T4O2
OT2O
T2O2T2
362
363
441
446
495
471
451
449
464
508
476
471
470
498
524
492
483
2.98
2.90
2.77
2.86
2.91
312/373
314/377
376
312/373
316/381
378
315/376
294/377
320/393
a Measured in solution (1 × 10-5 M). b Measured in solid thin film on quartz plates prepared by spin coating from solution. c Optical band gap derived
from the onset of UV-vis absorption spectra of oligomer solutions.
FIGURE 2. Absorption and fluorescence spectra for oligomers (a) in toluene solution and (b) in CH2Cl2 solution, respectively.
FIGURE 3. Fluorescence spectra for oligomers (a) in solid thin film and (b) for T2O2 in different environments, respectively.
corresponding increases in the number of thiophene units in
the oligomers. Other absorption bands of T2O2, T4O2, and
T2O2T2 (around 300 nm) come from the π-π* transition of
the oxadiazole groups.12 This implies that the electronic interac-
tions between the oxadiazole units and the thiophene units are
rather limited in their ground states.9 Nevertheless, both T2O
and OT2O display no remarkable absorption bands in the shorter
wavelength range.
The absorption spectra of the oligomers in the polar solvent
CH2Cl2 (Figure 2b) show no remarkable differences. No
noticeable solvatochromic shift is observed for any of the
oligomers. For all oligomers, it is worth noting that no
absorption bands are observed in the longer wavelength region
(400-600 nm), which would correspond to the charge transition
from the electron-rich thiophene unit to the electron-deficient
oxadiazole unit. It can be seen from Table 1 that the optical
band gap of T2O2 is smaller with respect to that of T2O,
indicating a greater delocalization of electronic charge in T2O2.
In the same way, the relatively smaller band gap of T4O2 with
respect to T2O2 ascribes to two more thiophene rings in T4O2
compared with T2O2. Interestingly, T2O2 has a different optical
band gap with its regioisomer OT2O. The data in Table 1 also
reveal that this interesting fact exists between T4O2 and its
regioisomer T2O2T2. These results confirm that changing the
length of the thiophene and oxadiazole units as well as altering
the molecular regiochemistry could effectively modulate the
band gaps of the oligomers.
(12) (a) Shu, C.-F.; Dodda, R.; Wu, F.-I.; Liu, M. S.; Jen, A. K.-Y.
Macromolecules 2003, 36, 6698. (b) Xu, B.; Pan, Y. C.; Zhang, J. H.; Peng,
Z. H. Synth. Met. 2000, 114, 337. (c) Peng, Z. H.; Zhang, J. H. J. Chem.
Mater. 1999, 11, 1138. (d) Lee, Y.-Z.; Chen, X.; Chen, S.-A.; Wei, P.-K.;
Fann, W.-S. J. Am. Chem. Soc. 2001, 123, 2296.
Figure 2a,b shows the normalized fluorescence emission
spectra of the oligomers in toluene and CH2Cl2. All of
2568 J. Org. Chem., Vol. 71, No. 7, 2006

EffectiVe Tuning of HOMO and LUMO Energy LeVels
1/2(R)
TABLE 2. Electrochemical Properties and Calculated Energy
Differences of the Oligomers
side (T2O f T2O2) on the reduction potential (ET O
)
2
2
) -2.39 V vs Ag/Ag+). This result
1/2(R)
-2.06 V and EOT O
2
reductiona
oxidationa
demonstrates that the molecular regiochemistry can also drasti-
cally modulate the reduction potential. This finding also
confirms the above hypothesis that strong electronic interaction
between two adjacent oxadiazole rings exists. Cyclic voltam-
metric reduction potential values can be used as a surrogate for
LUMO energy levels. The results suggest that the LUMO of
those materials can be effectively adjusted by changing the
oxadiazole ring number and the molecular regiochemistry.
b
b
Epa/Epc
Epc/Epa
LUMO/HOMOc band gapd
oligomer
(V)
(V)
(eV)
(eV)
T2O
T2O2
T4O2
OT2O
T2O2T2
-2.52/-2.36 0.83/0.52
-2.13/-1.99 0.78/0.52
-2.13/-2.03 0.61/0.44
-2.51/-2.28 0.76/0.64
-2.32/-2.16 0.83/0.72
-2.37/-5.37
-2.72/-5.39
-2.70/-5.15
-2.40/-5.30
-2.60/-5.38
3.00
2.67
2.45
2.90
2.78
a Determined by cyclic voltammetry in CH2Cl2 (for oxidation) and THF
(for reduction) with Ag/AgNO3 (0.1 M) as a reference electrode. Scan rate:
On sweeping anodically, all of these materials, except
T2O2T2, undergo a reversible multielectron (two or three)
oxidation originating from the thiophene units. Unlike their
reduction potentials, the oxidation potentials of diblock oligo-
mers are sensitive to the variation of the thiophene ring number
in the thiophene units. The oxidation potential of T4O2 is shifted
by 0.13 V toward less positive values with respect to that of
200 mVs-1 b Epa and Epc stand for anodic peak potential and cathodic peak
.
potential, respectively. c HOMO/LUMO ) Eonset + 4.7 eV. d Cyclic vol-
tammetric band gap derived from the difference between HOMO and LUMO
energy levels.
theoligomers exhibit a notable bathochromic shift (5-20 nm),
that is, 18 nm for T2O2 and 20 nm for T2O2T2. The solvato-
chromism may result from an intramolecular charge transfer
(ICT) in their excited states.13 Two adjacent oxadiazole units
have higher electron affinities than the single or isolated
oxadiazole unit. Consequently, the ICTs in T2O2 and T2O2T2
are more effective than those in T2O and OT2O. In the case of
diblock oligomers, with the increase in the number of thiophene
units in T4O2 relative to T2O2, the bathochromic shifts result
from the formation of a highly extended π-delocalized system.
Figure 3a shows the fluorescence emission spectra of solid
thin films on quartz plates prepared by spin coating from
solution. Compared to the spectra in CH2Cl2 solution, the
prominent red shifts (12-34 nm) of emission and (9-32 nm)
of absorption (Table 1) are observed for all oligomers. The
relatively large Stokes shift reveals that intermolecular associa-
tion and aggregation exists.
1/2(O)
1/2(O)
T2O2 (ET O
) 0.65 V and ET O
) 0.52 V vs Ag/
2
2
4 2
Ag+; Figure 4c), while the changing of the oxadiazole ring did
not change the oxidation potentials remarkably (ET O1/2(O) ) 0.66
2
V vs Ag/Ag+). It is demonstrated that the HOMO level of the
diblock oligomers could be effectively adjusted by changing
the thiophene number. In the same way, the oxidation potential
could be drastically modulated by changing the molecular
regiochemistry. Moving the oxadiazole unit of T4O2 into the
core of the thiophene unit from the terminal side drastically
1/2(O)
cut down the oxidation potential (ET O
) 0.52 V and
4
2
1/2(O)
ET O T
) 0.77 V vs Ag/Ag+; Figure 4d).
2
2 2
As shown in Table 3, the potential differences between the
first and the second oxidation, ∆Ep) Ep1 - Ep2, show remark-
able dependence on the molecular regiochemistry and length
of the p-type unit. OT2O had markedly higher ∆Ep values than
its regioisomer T2O2. Adding an electron-deficient oxadiazole
unit to T2O produces a marked increase of ∆Ep, that is, 30 mV
between T2O and T2O2 and 220 mV between T2O and OT2O.
The difference in the ∆Ep values reflects the Coulombic
repulsion (and, hence, the difficulty in stabilizing the dication)
between the positive charges in the dicationic state. Therefore,
adding an electron-deficient unit to the conjugated oligomer
could reduce the Coulombic repulsion effect. Compared with
the diblock oligomer T2O2, the triblock oligomer OT2O showed
better delocalization of the positive charges of the dication.
All of the oligomers exhibit only one maximum emission,
and this emission maximum was clearly independent of the
excitation wavelength. This indicates the existence of an efficient
energy transfer from the oxadiazole moiety to the thiophene
chromophore.9a
Electrochemical Properties. The redox properties of these
materials were determined by cyclic voltammetry in CH2Cl2
and THF solutions of 0.1 M tetrabutylammonium hexafluoro-
phosphate (TBAPF6) using a Pt wire as the counter electrode
and a Ag/AgNO3 (0.1 M) electrode as the reference electrode.
The detailed data are listed in Table 2, and the cyclic
voltammograms are given in Figure 4. When scanning cathodi-
cally, all the oligomers displayed reversible reduction processes.
In the case of the diblock oligomers, the increase in the length
of the oxadiazole unit changed the reduction potential of those
materials remarkably. The reduction potential of T2O2 is shifted
by 0.38 V toward less negative values with respect to that of
T2O (ET O 1/2(R) ) -2.06 V14 and ET O1/2(R) ) -2.44 V vs Ag/
The band gap of the oligomers derives from the difference
between the HOMO and the LUMO energy levels. As expected,
T2O has the biggest HOMO-LUMO gap of the series. The
relatively smaller HOMO-LUMO gap of T2O2 with respect
to that of T2O ascribes to the much lower LUMO energy level
of T2O2. T4O2 has the smallest HOMO-LUMO gap of the
series as a result of it possessing the highest HOMO energy
level. The HOMO-LUMO gap of the triblock oligomers (OT2O
and T2O2T2), as expected, lies between those of T2O and T4O2.
The reason T4O2 has a larger HOMO-LUMO gap than that of
the regioisomer T2O2T2 is due to the much lower HOMO level
of T4O2 with respect to that of T2O2T2. In the same way, the
higher LUMO level of T2O2 with respect to that of OT2O
contributes to the relatively larger HOMO-LUMO gap of T2O2.
However, we find that the band gap of the triblock T2O2T2 is
larger than that of the simple diblock T2O2. This point is
opposite to the known band gap control principle in the p-n
diblock oligomer system.
2
2
2
Ag+; Figure 4a). However, the increase in the thiophene ring
number of these materials did not change the reduction potential
remarkably. It can be seen from Figure 4b and Table 2 that the
addition of one oxadiazole ring from the terminal thiophene
side of T2O (T2O f OT2O) had a much smaller effect than
the addition of one oxadiazole ring from the terminal oxadiazole
(13) (a) Strehmel, R.; Sarker, A. M.; Malpert, T.-H.; Strehmel, V.; Seifert,
H.; Neckers, D. C. J. Am. Chem. Soc. 1999, 121, 1226. (b) Lu, H. F.; Chan,
H. S. O.; Ng, S.-C. Macromolecules 2003, 36, 1543. (c) Li, L.; Collard, D.
M. Macromolecules 2005, 38, 372.
All oligomers undergo both remarkably reversible oxidation
and remarkably reversible reduction processes except T2O2T2,
(14) E1/2(R), E1/2(O) ) (Epa + Epc)/2: half-wave reduction and oxidation
potentials.
J. Org. Chem, Vol. 71, No. 7, 2006 2569

Wan et al.
FIGURE 4. Cyclic voltammograms of the oligomers measured in dichloromethane (for oxidation) and THF (for reduction). Scan rate: 200 mVs-1
.
suggesting the potential for bipolar charge transport properties.15
However, most of the respective diblock copolymers show an
irreversible oxidation process.3
and T2O2T2) are delocalized to some degree on the whole
molecule (see Supporting Information). The trend of the
calculated HOMO and LUMO energy levels and the difference
(∆E) between them correlates well with that obtained from the
electrochemical measurement and the optical band gap (Table
2). This indicates that our calculation can be used to predict
the electronic structures of the system. The calculated HOMO
and LUMO levels were gathered in Table 4. Calculated data
for the diblock oligomers show that, as expected, the LUMO
level decreases with an increase in the length of the oxadiazole
unit, and the HOMO level increased with an increase in the
length of the thiophene unit.
Theoretical Calculations. The electronic structures of all
oligomers were investigated using the density functional
theoretical method B3LYP16 at the 6-31G* basis set calculations
of the Gaussian 03 program.17 The B3LYP functional is a
combination of Becke’s three-parameter hybrid exchange func-
tional and the Lee-Yang-Parr correlation functional. The alkyl
and alkoxy of the molecules were displaced by methyl and
methoxy as a result of their minor influence on the electronic
and optical properties of the conjugated molecules. The contours
of the HOMO and LUMO of the diblock oligomers are plotted
in Figure 5. We can see that the HOMO and LUMO of the
diblock T2O are delocalized among the whole molecule. With
the increasing number of thiophene or oxadiazole rings of the
T2O2 and T4O2 systems, the HOMO and LUMO tend to be
localized on the thiophene and oxadiazole moieties of the
systems, respectively. These results demonstrate that the HOMO
and LUMO energy levels of T2O2 and T4O2 can be modulated
independently by the thiophene and oxadiazole moieties.
However, the HOMO and LUMO of triblock oligomers (OT2O
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2004.
(15) Li, X.-.C.; Liu, Y.; Liu, M. S.; Jen, A. K.-Y. Chem. Mater. 1999,
11, 1568.
(16) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B: Solid State 1988, 37, 785.
2570 J. Org. Chem., Vol. 71, No. 7, 2006

EffectiVe Tuning of HOMO and LUMO Energy LeVels
FIGURE 5. Molecular orbital contours of the HOMO (A) and LUMO (B) for the T2O molecule, the HOMO (C) and LUMO (D) for the T2O2
molecule, and the HOMO (E) and LUMO (F) for the T4O2 molecule.
TABLE 3. Cyclic Voltammetric Data of Oligomers for Oxidation
Potential
triblock oligomers, the HOMO and LUMO energy levels of the
p-n diblock oligomers completely localized on the thiophene
and oxadiazole unit, respectively. Changing the thiophene or
oxadiazole ring number could independently tune the HOMO
or LUMO energy level in the p-n diblock oligomers. Thus,
the diblock oligomers were more effectively adjusted in regard
to their HOMO/LUMO energy levels than the triblock oligo-
mers. Additionally, the electronic properties could be drastically
modulated by molecular regiochemistry. The study, therefore,
provided fundamental insight into the improved design and
understanding of p-n heterostructure oligomer semiconductors.
oligomer
Ep1a (V)
Ep2a (V)
∆Ep (mV)
T2O
T2O2
OT2O
0.83
0.78
0.76
0.91
0.88
1.07
80
100
310
a For the conditions, see Table 2.
TABLE 4. HOMO/LUMO Energy Levels and Gapsa
HOMO
(eV)
LUMO
(eV)
∆E
(eV)
band gap
(eV)
Eg
(eV)
oligomer
T2O
T2O2
T4O2
-5.18
-5.23
-5.04
-1.69
-2.11
-2.12
3.49
3.12
2.92
3.00
2.67
2.45
2.98
2.90
2.77
Acknowledgment. This work was financially supported by
the National Natural Science Foundation of China under Grants
60325412 and 90406021, the Shanghai Commission of Science
and Technology under Grants 03DZ11016 and 04XD14002, and
the Shanghai Commission of Education under Grant 03SG03.
We would also like to thank Professor Z. M. Su and Dr. G. C.
Yang, Northeast Normal University, for their helpful discussion
about theoretical calculations.
a ∆E calculated by the DFT method (B3LYP/6-31G*).
Conclusions
A new series of p-n diblock and triblock oligomers consist-
ing of an electron-rich thiophene unit and an electron-deficient
oxadiazole unit were synthesized and characterized. The p-n
diblock oligomers exhibited excellent band-gap controlling
properties. Changing the number of the thiophene and oxadia-
zole rings could effectively modulate the electronic properties
of these oligomers. Density functional theory provided evidence
for the observed electronic properties. In comparison with
Supporting Information Available: Detailed synthetic proce-
dures, 1H and 13C NMR spectra for all oligomers, molecular orbital
contours of the HOMO and LUMO for OT2O and T2O2T2, and a
table of atom coordinates used in the calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO0521596
J. Org. Chem, Vol. 71, No. 7, 2006 2571