Structural effects on the hole mobilities of indenothiophene-embedded homologs

Article abstract of DOI:10.1016/j.tetlet.2009.02.148

A systematic study of the relationship between the structures and the non-dispersive hole mobilities (up to 10^-3 cm² V^-1 s^-1) of a homologous series of amorphous indenothiophene-containing materials is described

Full text of DOI:10.1016/j.tetlet.2009.02.148

Tetrahedron Letters 50 (2009) 3422–3424
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Structural effects on the hole mobilities of indenothiophene-embedded
homologs
b,
b
b
Teng-Chih Chao a, Ken-Tsung Wong a, , Wen-Yi Hung , Tei-Hung Hou , Wei-Jiun Chen
*
*
a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
b Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A systematic study of the relationship between the structures and the non-dispersive hole mobilities (up
to 10ꢀ3 cm2 Vꢀ1 sꢀ1) of a homologous series of amorphous indenothiophene-containing materials is
Received 7 January 2009
Revised 17 February 2009
Accepted 19 February 2009
Available online 25 February 2009
described. The hole mobilities were dependent mainly on the length and rigidity of the
backbone and the peripheral substituents.
p-conjugated
Ó 2009 Elsevier Ltd. All rights reserved.
Recent developments in organic electronic and optoelectronic
devices, such as organic light-emitting diodes (OLEDs), solar cells,
electrochromic cells, and organic field effect transistors (OTFTs),
have imposed substantial demands on organic semiconducting
materials. Although such devices perform a diverse range of func-
tions, charge carrier transport is a common key issue for each sys-
tem, and its optimization remains a challenge for the development
of practical or commercial devices. The charge transport behavior
of amorphous organic materials can be described using Marcus
theory1:
a systematic study of the chemical structures and hole mobilities
of a homologous series of amorphous indenothiophene-based
conjugated materials (Scheme 1). Our results indicate that the hole
mobilities are dependent mainly on the length and rigidity of the
p-
p-
conjugated backbone, with the peripheral substituents playing a
subtle role in governing the hole-transporting behavior. We ob-
served hole mobilities of up to 10ꢀ3 cm2 Vꢀ1 sꢀ1 for a rigid, copla-
nar, amorphous indenothiophene-based derivative bearing small,
uniform peripheral aryl substituents.
2
2
h
p
H2da
ðD
G0 þ kÞ
4kkT
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ket
¼
exp ꢀ
4
pkkT
S
S
in which the rate of intermolecular electron hopping is governed by
the reorganization energy (k) and the charge transfer integral (Hda),
with the latter especially being the bottle-neck parameter.2 There-
Ar
Ar
Ar
Ar
Ar
Ar
fore, the molecular design of organic
p-conjugated charge carrier-
transporting materials that can reduce the reorganization energy
and enhance the electronic coupling factor during electron hopping
processes has emerged as an effective approach toward achieving
novel materials exhibiting high mobility. In this regard, molecules
that feature rigid, coplanar units are excellent candidates;3 for
example, polyacenes, especially pentacene, exhibit remarkable car-
rier mobility.4 Unfortunately, non-homogeneous crystalline sys-
tems that feature grain boundaries of various sizes often lead to
the generation of carrier traps, which may cause problems in device
applications. Chemical modification of the coplanar core structure
with sterically hindered peripheral substituents is an effective
means of suppressing crystal formation, leading to amorphous
materials that can form homogeneous thin films, while retaining
sufficient charge transporting character.5 In this Letter, we report
1TT Ar = 4-CH3Ph
S
S
S
Ar1
Ar1
Ar2 Ar2
Ar
Ar
Ar
Ar
2TT Ar1 = 4-CH3Ph, Ar2 =4-CH3Ph
2PT Ar1 = Ph, Ar2 = 4-CH3Ph
3PP Ar = Ph
3TT Ar = 4-CH3Ph
S
Ar
Ar1
1Ar2 Ar2
4PP Ar1 = Ph, Ar2 = Ph
4PT Ar1 = Ph, Ar2 = 4-CH3Ph
4TT Ar1 = 4-CH3Ph, Ar2 = 4-CH3Ph
* Corresponding authors. Tel.: +886 2 33661665; fax: +886 2 33661667 (K.-T.W).
E-mail addresses: kenwong@ntu.edu.tw (K.-T. Wong), wenhung@mail.ntou.
edu.tw (W.-Y. Hung).
Scheme 1. Structures of indenothiophene-derived homologs.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.148

T.-C. Chao et al. / Tetrahedron Letters 50 (2009) 3422–3424
3423
Oligothiophenes and their related derivatives are promising
materials for use in carrier-transporting applications.6 However,
increases the conformational flexibility, resulting in larger reorga-
nization energy and, weaker intermolecular interactions, which
could render carrier transport more difficult. Although the reduced
molecular lengths of 2TT and 2TP could diminish the complexity of
their molecular configurations, we detected no significant
enhancements in their hole mobilities. The homo-coupled
bis(indenothiophene)s, 3PP and 3TT, exhibit hole mobilities that
are comparable to those of their counterparts 2PT and 2TT. These
results indicate that hybridization of the characteristics of the
coplanar fluorene and indenothiophene moieties did not provide
any evident benefit to hole mobility. Figure 1 indicates that the ri-
gid and coplanar homologs 4PP, 4PT, and 4TT exhibit remarkably
improved hole mobilities (e.g., up to 10ꢀ3 cm2 Vꢀ1 sꢀ1) at high elec-
tric field, making them competitive with typical arylamine-based
hole transporting materials.10 This finding confirms the advantages
of rigidity and coplanarity of the molecular backbone that contrib-
ute to the carrier transportation properties. Importantly, the struc-
tures of the aryl substituents exert a significant effect on the hole
mobilities. Figure 2 depicts the hole mobilities plotted as functions
of the different types of substituents attached to common indeno-
thiophene-embedded backbones. The hole mobilities follow the or-
ders 4PP > 4PT > 4TT, 2PT > 2TT, and 3PP > 3TT.
conformational disorder of the
p-conjugated backbone of thio-
phene-containing materials has the potential to reduce the inter-
molecular electronic coupling efficiency, which in turn limits the
carrier mobility. We envisioned that the conformational flexibility
of the molecular backbone would be reduced significantly through
flattening of the thiophene moiety and adjacent arene ring(s) into a
coplanar configuration. In our approach, one (or more) sp3-hybrid-
ized carbon atom bearing two aryl substituents would serve as a
bridging unit flattening the molecular planes of the thiophene
and phenylene moieties into coplanarity to form an indenothioph-
ene skeleton, Scheme 1 illustrates the molecular structures of a
series of indenothiophene-derived homologs used for this current
work. For systematic investigation of the structure and mobility
correlation, indenothiophene derivatives 2TT, 2PT, 3PP, and 3TT
were synthesized (Scheme S1, supplementary data) in addition to
previously reported 1TT, 4PP, 4PT, and 4TT.7 The C2 position of
indenothiophene can be readily functionalized and further trans-
formed into a homologous series with structural features including
(1) hybridization with a coplanar fluorene ring gives 1TT, 2TT, and
2PT; (2) homo-couplings of indenothiophenes afford 3PP and 3TT;
and (3) sequential annulation with adjacent phenylene rings gives
the rigid and coplanar 4PP, 4PT, and 4TT. The introduction of aryl
groups as peripheral substituents provides high tolerance toward
thermal decomposition (analyzed by TGA) and high morphological
stability with distinct glass transition temperatures (analyzed by
DSC) (Table S1, supplementary data). Thus, homogeneous thin
films can be obtained through thermal evaporation under high
vacuum.
Molecular calculation of 4TT reveals an optimized ground state
geometry that is consistent with the experimental one determined
through the X-ray crystallographic analysis;7 the frontier molecu-
lar orbitals (HOMO and LUMO; Fig. 3) of 4TT are localized mainly
on the rigid and coplanar indenothiophene backbone without a
significant contribution from the peripheral tolyl substituents.
The result of MO calculation is consistent with the nearly identical
absorption and emission characteristics of molecules having the
same indenothiophene backbone but bearing different substitu-
ents (Fig. S2, supplementary data). In addition, the reorganization
energies of 4PP, 4PT, and 4TT upon hole-transportation were cal-
We conducted charge carrier mobility measurements for the
indenothiophene derivatives at ambient temperature using a
time-of-flight (TOF) technique. Clear constant-current plateaus in
typical photocurrent transients of the hole mobility measurements
(Fig. S1, supplementary data) were observed, which are the signa-
ture of non-dispersive carrier transport behavior. The calculated
field-dependent hole mobilities of the indenothiophene derivatives
are plotted as a function of T1/2 in Figure 1. We observed that the
*
culated at B3LYP/631G level with Jaguar 7.0 quantum chemistry
software to be 270 meV, 274 meV, and 270 meV, respectively.
These results indicate that the aryl substituents on the top and bot-
tom faces of the molecular plane behave only as effective spatial
hindrance groups to suppress crystallization and improve morpho-
logical stability. Thus, the smaller substituents (i.e., phenyl groups)
reduce the carrier hopping distance between the molecules,
leading to improved mobility.11 For the same substituents (e.g.,
two ditoyl units), we found that the hole mobility of 4TT was over
three times higher than those of 2TT and 3TT. The results again
suggest that longer backbones lead to more disordered molecular
conformations and, thus, lower hole mobilities.
universal Poole–Frenkel relationship [
l
/ exp(bT1/2), where b is
the Poole–Frenkel factor] is followed.8 The hole mobilities exist
in the range from 4 ꢁ 10ꢀ5 to 10ꢀ3 cm2 Vꢀ1 sꢀ1 for fields varying
from 105 to 6.7 ꢁ 105 V cmꢀ1. In contrast, the TOF transient for
electrons exhibited strong dispersive photocurrents.
The observed hole mobility of 1TT is comparable to, but slightly
lower than that of its pure hydrocarbon counterpart, terfluorene
T3,9 which we ascribe mainly to the relatively non-linear molecu-
lar backbone of 1TT resulting from the intrinsic molecular geome-
try of the thiophene ring. This deviation from linearity for 1TT
In order to probe the effect of structural features that govern the
hole mobilities, we also accomplished temperature and field-
1E-3
1E-3
4TT
4PT
4PP
3TT
4PP
4PT
4TT
3PP
3PP
2TT
2PT
1E-4
1E-4
3TT
2PT
2TT
1TT
TT
PP
PT
Type of substitutions
400
600
800
E
1/2 (V/cm)1/2
Figure 2. Hole mobilities (at E = 2.5 ꢁ 105 V cmꢀ1) plotted with respect to the
different types of substituents (TT: two ditoyl units; PT: one diphenyl and one
ditoyl unit; PP: two diphenyl units) attached to common indenothiophene-
embedded backbones.
Figure 1. Hole mobilities plotted against E1/2 for the indenothiophene-embedded
homologs.

3424
T.-C. Chao et al. / Tetrahedron Letters 50 (2009) 3422–3424
ene-based
sults indicate that their hole mobilities are influenced strongly by
the length and rigidity of their -conjugated backbones and by
p-conjugated materials are reported first time, the re-
p
the structures of the peripheral substituents. Hole mobilities of
up to 10ꢀ3 cm2 Vꢀ1 sꢀ1 were observed for a rigid, coplanar indeno-
thiophene-based derivative (4PP) bearing small, uniform periphe-
ral aryl substituents.
Acknowledgments
Figure 3. Frontier molecular orbitals HOMO (left) and LUMO (right) of 4TT
computed using the DFT (B3LYP/631G) method.
We thank Mr. Wei-Guang Liu for the aid in molecular calcula-
tion on the reorganization energy. The financial support of this re-
search was from the National Science Council of Taiwan.
Table 1
GDM carrier transport parameters for 4PP, 4PT, and 4TT
Supplementary data
Hole
l0 (ꢁ10ꢀ3 cm2/V s)
r
(ꢁ10ꢀ2 eV)
R
C (ꢁ10ꢀ4
)
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.02.148.
4PP
4PT
4TT
15
10
3
8.00
7.80
7.75
1.65
1.63
1.58
4
3.7
4.5
References and notes
1. (a) Marcus, R. A. Rev. Mod. Phys. 1993, 65, 599; (b) Cornil, J.; Beljonne, D.;
Calbert, J.-P.; Brédas, J.-L. Adv. Mater. 2001, 13, 1053.
2. (a) Brédas, J. L.; Calbert, J. P.; da Silvo Filho, D. A.; Cornil, J. Proc. Natl. Acad. Sci.
U.S.A. 2002, 99, 5804; (b) Lin, B. C.; Cheng, C. P.; You, Z.-Q.; Hsu, C.-P. J. Am.
Chem. Soc. 2005, 127, 66.
3. (a) Yamaguchi, S.; Xu, C.; Okamoto, T. Pure Appl. Chem. 2006, 78, 721; (b) Hertel,
D.; Scherf, U.; Bässeler, H. Adv. Mater. 1998, 10, 1119; (c) Walkim, S.; Bouchard,
J.; Simard, M.; Drolet, N.; Tao, Y.; Leclerc, M. Chem. Mater. 2004, 16, 4386; (d) Li,
Y.; Wu, Y.; Gardner, S.; Ong, B. S. Adv. Mater. 2005, 17, 849.
4. (a) Nelson, S. F.; Lin, Y.-Y.; Gundlach, D. J.; Jackson, T. N. Appl. Phys. Lett. 1998,
72, 1854; (b) Lee, J. Y.; Roth, S.; Park, Y. W. Appl. Phys. Lett. 2006, 88, 252106.
5. (a) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001,
123, 9482; (b) Jiang, J.; Kaafarani, B. R.; Neckers, D. C. J. Org. Chem. 2006, 71,
2155.
6. (a) Yoon, M.-H.; Facchetti, A.; Stern, C. E.; Marks, T. J. J. Am. Chem. Soc. 2006, 128,
5792; (b) Murphy, A. R.; Chang, P. C.; VanDyke, P.; Liu, J.; Fréchet, J. M.;
Subramanian, V.; DeLongchamp, D. M.; Sambasivan, S.; Fischer, D. A.; Lin, E. K.
Chem. Mater. 2005, 17, 6033; (c) Meng, H.; Bao, Z.; Lovinger, A. J.; Wang, B.-C.;
Mujsce, A. M. J. Am. Chem. Soc. 2001, 123, 9214.
7. (a) Wong, K.-T.; Hwu, T.-Y.; Balaiah, A.; Chao, T.-C.; Fang, F.-C.; Lee, C.-T.; Peng,
Y.-C. Org. Lett. 2006, 8, 1415; (b) Wong, K.-T.; Chao, T.-C.; Chi, L.-C.; Chu, Y.-Y.;
Balaiah, A.; Chiu, S.-F.; Liu, Y.-H.; Wang, Y. Org. Lett. 2006, 8, 5033.
8. (a) Bässler, H. Philos. Mag. B 1992, 65, 795; (b) Bässler, H. Phys. Status Solidi B
1993, 175, 15; (c) Borsenberger, P. M.; Pautmeier, L.; Bässler, H. J. Chem. Phys.
1991, 94, 5447.
dependent hole mobility measurements of 4PP, 4PT, and 4TT
(Fig. S3, supplementary data). The obtained carrier mobilities agree
with the prediction of Gaussian disorder model (GDM)8,12, which
assumes that charge transport in amorphous organic solids occurs
by means of hopping among uncorrelated localized sites with
Gaussian-distributed energetic disorder and intermolecular posi-
tional disorder. Table 1 summarizes the extracted data: disorder-
free mobility (
parameter of positional disorder (
dependent hole mobilities using Bässler formalism described as:
l0), degree of energetic disorder (
r), and the
R) from temperature- and field-
0 2
3
1
!
!
2
ꢀ
ꢁ
2
pffiffiffi
2
3kBT
r
r
kBT
@ 4
5
A
l
ðE; TÞ ¼
l
0 exp
ꢀ
exp
C
ꢀ
R2
E
where E is electric field, T is temperature, kB is the Boltzmann con-
stant, C is an empirical constant.
The obtained data indicate that the nature of peripheral substi-
tutions exhibits negligible influence on the energetic disorder, as
well as on the positional disorder. More significantly, the results
of temperature-dependent mobility measurement suggest that
the relative magnitudes of carrier mobility of a common indenothi-
ophene backbone with different substitutions are mainly governed
by l0 (4PP > 4PT > 4TT) (i.e., intrinsic intermolecular charge-trans-
fer characteristics), which is consistent with those observed for
mobilities.
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. (a) Strohriegl, P. Adv. Mater. 2002, 14, 1439; (b) Shirota, Y. J. Mater. Chem. 2000,
10, 1.
11. (a) Wu, C.-C.; Liu, W.-G.; Hung, W.-Y.; Liu, T.-L.; Lin, Y.-T.; Lin, H.-W.; Wong, K.-
T.; Chien, Y.-Y.; Chen, R.-T.; Hung, T.-H.; Chao, T.-C.; Chen, Y.-M. Appl. Phys. Lett.
2005, 87, 052103; (b) Fong, H. H.; So, S. K.; Sham, W. Y.; Lo, C. F.; Wu, Y. S.;
Chen, C. H. Chem. Phys. 2004, 298, 119.
12. (a) Poplavskyy, D.; Nelson, J. J. Appl. Phys. 2003, 93, 341; (b) Laquai, F.; Wegner,
G.; Bässler, H.; Heun, S. J. Appl. Phys. 2006, 99, 203712.
In summary, TOF measurements of the non-dispersive hole
mobilities of a homologous series of amorphous indenothioph-