Coplanarity in the backbones of ladder-type oligo(p-phenylene) homologues and derivatives

Article abstract of DOI:10.1021/ol061762n

(Chemical Equation Presented) p-Tolyl-substituted ladder-type oligo(p-phenylene)s containing three, four, and five phenylene rings were readily synthesized. The uniform aryl substitution of these systems allowed us to determine the coplanarity of the π-co

Full text of DOI:10.1021/ol061762n

ORGANIC
LETTERS
2006
Vol. 8, No. 22
5029-5032
Coplanarity in the Backbones of
Ladder-type Oligo(p-phenylene)
Homologues and Derivatives
Ken-Tsung Wong,* Liang-Chen Chi, Shih-Chiang Huang, Yuan-Li Liao,
Yi-Hung Liu, and Yu Wang
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ntu.edu.tw
Received July 18, 2006
ABSTRACT
p-Tolyl-substituted ladder-type oligo(p-phenylene)s containing three, four, and five phenylene rings were readily synthesized. The uniform aryl
substitution of these systems allowed us to determine the coplanarity of the -conjugated backbones crystallographically. The intramolecular
annulations eliminate almost all of the conformational disorder and enhance the degree of -conjugation of the backbones, resulting in
significant red shifts in the absorption and emission maxima and lower oxidation potentials in the higher homologues.
π
π
In recent years, advances in organic optoelectronicssfor
example, organic light-emitting diodes (OLEDs), organic
solar cells, and organic thin-film transistors (OTFTs)shave
imposed substantial demands on the required electroactive
oligomeric and polymeric materials. For example, ladder-
type poly(p-phenylene)s (LPPPs)1 that possess coplanar
π-conjugated backbones, obtained through intramolecular
annulation, have received a considerable amount of research
interest; they have been exploited successfully as active
materials in electronic devices because of their favorable
properties, such as enhanced π-conjugation,2 high photo-
luminescent efficiency, and remarkable hole mobility.3 Un-
fortunately, the intrinsic polydispersity of polymers normally
impedes their detailed structural characterization and further
understanding of their structure-property relationships.
Monodisperse oligomers, with their well-defined molecular
sizes and structures, often can serve as model systems for
their polymeric counterparts. This approach has provided a
substantial amount of information relating to the electrical
and optical phenomena displayed by polymers.4 Although,
ladder-type p-phenylene oligomers [e.g., terphenylene (in-
denofluorene),5 pentaphenylene,6 and undecaphenylene7]
have been synthesized and characterized as discrete chro-
mophoric systems with which to probe the structure-
property relationships of LPPP. However, the coplanarity of
ladder-type oligo(p-phenylene) backbones usually attached
with different pendent alkyl and/or aryl groups to improve
their solubility remains speculative at higher numbers of
repeating units because of the lack of direct evidence for
(4) Mullen, K.; Wegner, G. Electronic Materials: the Oligomer Ap-
proach; Wiley-VCH: Weinheim, Germany, 1998.
(5) (a) Merlet, S.; Birau, M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157. (b)
Hadizad, T.; Zhang, J.; Wang, Z. Y.; Gorjanc, T. C.; Py, C. Org. Lett.
2005, 7, 795.
(1) (a) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (b) Grimsdale, A. C.;
Mu¨llen, K. Angew. Chem., Int. Ed. 2005, 44, 5592.
(2) (a) Setayesh, S.; Marsitzky, D.; Mu¨llen, K. Macromolecules 2000,
33, 2016. (b) Jacob, J.; Zhang, J.; Grimsdale, A. C.; Mu¨llen, K.; Gaal, M.;
List, E. J. W. Macromolecules 2003, 36, 8240.
(6) (a) Jacob, J.; Sax, S.; Piok, T.; List, E. J. W.; Grimsdale, A. C.;
Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 6987. (b) Jacob, J.; Sax, S.; Gaal,
M.; List, E. J. W.; Grimsdale, A. C.; Mullen, K. Macromolecules 2005,
38, 9933.
(7) Schindler, F.; Jacob, J.; Grimsdale, A. C.; Scherf, U.; Mu¨llen, K.;
Lupton, J. M.; Feldmann, J. Angew. Chem., Int. Ed. 2005, 44, 1520.
(3) Hertel, D.; Scherf, U.; Ba¨sseler, H. AdV. Mater. 1998, 10, 1119.
10.1021/ol061762n CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/28/2006

the molecular configuration; this information is accessible
only through computer simulation.8 In this Letter, we report
the synthesis of a homologous series of p-tolyl-substituted
ladder-type oligo(p-phenylene)s containing three, four, or five
phenylene units and exhibiting extensive π-conjugation. The
introduction of the p-tolyl groups as peripheral substituents
was beneficial not only to provide high thermal and
morphological stability but also to allow us to obtain solid-
state structures, making detailed analyses of the backbone
structures feasible. We have found that the photophysical
and electrochemical properties, as well as the thermal
stabilities, correlated well with the degree of π-conjugation.
boronic ester (3)9 gave the diester 4 in 92% yield. Subsequent
double addition of 4-methylphenyllithium onto the ester
groups of 4, followed by acid-catalyzed intramolecular
cyclization, gave the desired product 5Ph in 90% yield.
9,9-Ditolylfluorene-2,7-diboronic ester (5)10 was reacted
with ethyl 2-iodobenzoate under Suzuki coupling conditions
to give the diester 6 in 90% yield (Scheme 2). Following
Scheme 2. Synthesis of the Coplanar Tetra-p-phenylene 4Ph
Scheme 1 depicts the synthesis of the p-tolyl-substituted
coplanar ter-p-phenylene 3Ph.
Scheme 1. Syntheses of the Coplanar Ter-p-phenylene 3Ph
and Penta-p-phenylene 5Ph
the usual reaction sequencesdouble alkylation of the ester
groups followed by acid-catalyzed intramolecular cyclizations
we obtained the target product, the coplanar tetra-p-phenylene
4Ph in an isolated yield of 95%.
We characterized the morphological and thermal properties
of the homologues 3Ph-5Ph using differential scanning
calorimetry (DSC) and thermogravimetric analyses (TGA),
respectively; Table 1 summarizes the data. These homologues
Table 1. Properties of the p-Tolyl-Substituted Ladder-type
Oligo(p-phenylene)s 3Ph-5Ph
absa
λmax
PLb
λmax
(nm)
c
c
d
e
Tg
(°C)
Tc, Tm
(°C)
Td
(°C)
E1/2oxd
(V)
compound (nm)
3Ph
4Ph
5Ph
347 353, 369 159 234, 353 353 1.44
375 382, 403 225 301, 404 438 1.27
397 403, 427 293 n.d., 470 506 1.16, 1.59
Taking advantage of commonly used synthetic strategy
for the syntheses of ladder-type oligomers and polymers,5-8
Pd-catalyzed Suzuki coupling of diethyl 2,5-dibromotere-
phthalate (1) and phenylboronic acid afforded the diester 2
in an isolated yield of 98%. The reaction of the diester 2
with an excess amount of 4-methylphenyllithium, generated
in situ by reacting 4-bromotoluene with n-BuLi at -78 °C,
afforded a diol, which subsequently underwent ring closure
through Friedel-Crafts alkylation under acidic conditions
to generate the coplanar ter-p-phenylene 3Ph in 87% yield.
Its longer homologue, the coplanar penta-p-phenylene 5Ph,
was synthesized through a similar synthetic pathway (Scheme
1). The Suzuki coupling of 1 with 9,9-ditolylfluorene-2-
a In CHCl3. b In CHCl3, upon excitation at the absorption maximum.
c Analyzed using DCS. d Analyzed using TGA (5% weight loss). e In CH2Cl2
containing 0.1 M nBu4NPF6 as a supporting electrolyte; potentials were
recorded vs Ag/AgCl.
exhibit distinct glass transition temperatures (Tg) that are
higher than 159 °C. As a consequence, these novel homo-
logues can form homogeneous and stable amorphous films,
an essential property for any potential device application,
such as organic light-emitting diodes (OLEDs), upon thermal
(9) Chao, T.-C.; Lin, Y.-T.; Yang, C.-Y.; Hung, T. S.; Chou, H.-C.; Wu,
C.-C.; Wong, K.-T. AdV. Mater. 2005, 17, 992.
(10) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-F.; Lin, Y.-T.;
Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su, Y. O.; Lee, G.-
H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576.
(8) Xia, C.; Advincula, R. C. Macromolecules 2001, 34, 6922.
5030
Org. Lett., Vol. 8, No. 22, 2006

evaporation. We ascribe the amorphous behavior and high
values of Tg to the rigidity of the conjugated backbones and
the presence of the p-tolyl substituents, which effectively
suppress intermolecular interactions. The aryl substituents
impart the oligomers with a high tolerance to heat, as
indicated by their high decomposition temperatures. DSC
analyses also revealed that these homologues exhibit crystal-
lization temperatures (Tc) and melting points (Tm), which
suggested the feasibility of obtaining single crystals for X-ray
crystallographic analysis.
Gratifyingly, we obtained single crystals of each of 3Ph,
4Ph, and 5Ph that were suitable for X-ray diffraction
analyses.11 As an example, in Figure 1, we present the
Figure 2. Crystal packing of 5Ph in its unit cell (hydrogen atoms
and solvent molecules have been omitted for clarity).
tion in solution feasible. Figure 3 illustrates the electronic
absorption and photoluminescence (PL) spectra of the
coplanar oligo(p-phenylene)s 3Ph-5Ph in CHCl3; Table 1
summarizes the data. Both the absorption and emission
maxima were red-shifted significantly upon increasing the
number of repeating phenylene units, indicating that the
degree of π-conjugation was extended effectively in longer
homologues. In addition, the small Stokes shifts and the
mirror images of the absorption and emission spectra are
consistent with the molecular coplanarity and rigidity
observed in the solid-state structures. The absorption maxi-
mum of 4Ph (centered at 375 nm) was red-shifted by 43
nm relative to that of 9,9-di(p-tolyl)bifluorene (T2)9 (Figure
S-5, Supporting Information), indicating the effectiveness of
using this coplanar molecular configuration to enhance the
degree of π-conjugation of the backbone.
Figure 1. (a) Solid-state structure of 5Ph. (b) Top and (c) side
views of the structure with the p-tolyl substituents omitted for
clarity.
We used cyclic voltammetry (CV) to probe the electro-
chemical behavior of the homologues 3Ph-5Ph (Figure S-6,
Supporting Information); Table 1 summarizes the data. One
reversible oxidation potential was detected for both 3Ph and
4Ph, at 1.44 and 1.27 V (vs Ag/AgCl), respectively. The
homologue 5Ph exhibited two reversible oxidation potentials,
at 1.16 and 1.59 V, respectively. The large potential
difference (0.43 V) between these signals is indicative of
efficient radical cation delocalization within the conjugated
structure of the highest homologue, 5Ph, which we crystal-
lized from a tetrachloroethane solution.
To the best of our knowledge, 5Ph is the longest ladder-
type p-phenylene oligomer that has been characterized using
X-ray crystallography. The conjugated backbone of 5Ph
adopts a nearly coplanar configuration with plane-to-plane
twisting between phenylene rings of -2.3 (rings 1 and 2)
and 2.5° (rings 2 and 3). All of the phenylene rings are
slightly distorted from hexagonal because of the presence
of the annulated rigid and strained five-membered rings. The
molecular length of 5Ph was ca. 2.1 nm. As indicated in
Figure 2, the p-tolyl substituents occupied the top and bottom
faces of the conjugated molecular plane to effectively
suppress any interchromophore interactions. Although we
observe no evidence for effective chromophore aggregation,
interdigitation of the peripheral p-tolyl groups of neighboring
molecules appears to play an important role in crystal
packing.12
These homologues are readily soluble in chlorinated
solvents, rendering the study of structure-property correla-
(11) Table S-1 in the Supporting Information provides a summary of
the crystal data. Figure S-1 in the Supporting Information presents the crystal
structures of 3Ph and 4Ph.
(12) Crystal packing diagrams and the analyses of possible intermolecular
interactions of 3Ph, 4Ph, and dNpPh are provided in the Supporting
Information.
Figure 3. Normalized UV-vis absorption (dashed lines) and
photoluminescence (solid lines) spectra of the p-tolyl-substituted
ladder-type oligo(p-phenylene)s 3Ph-5Ph in CHCl3 at 25 °C.
Org. Lett., Vol. 8, No. 22, 2006
5031

backbone. The lower oxidation potentials observed upon
increasing the molecular length is consistent with the
effective extension of the degree of π-conjugation within
the backbones of these coplanar oligo(p-phenylene)s; such
higher HOMO energy levels are crucial for reducing the
energy barrier for hole injection from the anode electrodes
in active organic electronic devices. The lower oxidation
potential (1.27 V) of 4Ph relative to that (1.42 V) of 9,9-
di(p-tolyl)bifluorene (T2)9 is consistent with the coplanar
structure having an greater degree of π-conjugation; this
annulation approach appears to be a useful alternative method
for modulating the energy levels in such systems.
Figure 4. (a) Solid-state structure of dNpPh. (b) Top and (c) side
views of the structure with the p-tolyl substituents omitted for
clarity.
The highly efficient and versatile synthetic strategy can
be applied to the synthesis of other novel coplanar chro-
mophores, for example, for introducing new arenes, such as
naphthalene, as constituent units of the conjugated backbone
(Scheme 3). Although, in principle, the ring closure of the
cal and thermal stability while preserving the photophysical
properties of the indenofluorene unit.13
In summary, we have readily obtained p-tolyl-substituted
ladder-type oligo(p-phenylene)s incorporating three, four, and
five phenylene rings in high yields. The annulation of these
oligomers eliminates their conformational disorder and
enhances the degree of π-conjugation of their backbones;
we confirmed these features through the observation of
significant red shifts in the absorption and emission maxima,
as well as lower oxidation potentials, in the longer homo-
logues. We verified the limited twisting and resultant high
coplanarity of the conjugated backbone of the longest
homologue 5Ph through the use of X-ray crystallographic
diffraction analysis. In addition, we used similar strategies
to synthesize the novel coplanar chromophores dNpPh and
sInF to demonstrate the versatility of this synthetic method.
We are currently investigating the potential applications of
3Ph-5Ph and sInF as large band gap hosts and dNpPh as
a blue emitter in OLEDs; the results will be reported in due
course.
Scheme 3. Syntheses of the Coplanar Chromophore dNpPh
and the Spiro-Bridged Indenofluorene sInF
Acknowledgment. This study was supported financially
by the National Science Council and the Ministry of
Economic Affairs, Taiwan.
diol intermediate 7 could occur at either the C2 or C8
positions of the naphthalene unit to generate five- or six-
membered rings, respectively. The product obtained (dNpPh)
underwent cyclization only at the C2 position, as confirmed
through X-ray crystallography (Figure 4). The S-shaped
molecular backbone of dNpPh is relatively coplanar, with
the terminal naphthalene rings being twisted slightly (by ca.
6.8°, plane-to-plane) with respect to the central phenylene
ring. In addition, we also synthesized the spiro-bridged
coplanar ter-p-phenylene (indenofluorene) derivative (sInF).
The orthogonal configuration of the two coplanar ter-p-
phenylene branches in sInF further enhances its morphologi-
Supporting Information Available: Detailed experi-
mental procedures; spectroscopic characterization and NMR
spectra of new compounds; comparison of the absorption
and emission spectra of 4Ph and T2; cyclic voltammograms;
crystal data; crystal packing; crystal CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org
OL061762N
(13) Wong, K.-T.; Liao, Y.-L.; Lin, Y.-T.; Su, H.-C.; Wu, C.-C. Org.
Lett. 2005, 7, 5131.
5032
Org. Lett., Vol. 8, No. 22, 2006