Pyrrole-cored push-pull single chromophore

Article abstract of DOI:10.1016/j.tetlet.2010.02.029

We report a straightforward synthesis toward pyrrole-cored push-pull single chromophore in which the electron-donating (D) units are pyrrole groups and electron-withdrawing (A) moieties are diketone groups. Optical and electrochemical properties of the no

Full text of DOI:10.1016/j.tetlet.2010.02.029

Tetrahedron Letters 51 (2010) 2007–2009
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Pyrrole-cored push–pull single chromophore
*
Changduk Yang
School of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a straightforward synthesis toward pyrrole-cored push-pull single chromophore in which the
electron-donating (D) units are pyrrole groups and electron-withdrawing (A) moieties are diketone
groups. Optical and electrochemical properties of the novel pyrrole-based building block embedding car-
bonyl functionalities were studied in detail. On the redox behaviors, this resulting material not only pos-
sesses low-lying LUMO but also displays high-lying HOMO with fully reversible p-and n-doping, which
can establish it as a good candidate for use as electron/hole-transporting material in optical and elec-
tro-optical applications.
Received 18 January 2010
Revised 2 February 2010
Accepted 5 February 2010
Available online 10 February 2010
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
compoundis outlinedin Scheme1. 2,20-(1,3-Butadiyne-1,4-diyl)bis-
benzoic acid dimethyl ester (3) was easily prepared in three steps
One of the most important developments in the synthesis of
-conjugated materials has been the utilization of donor–acceptor
(isolated overall yield = 72%).17 The cycloaddition reaction of 3 with
p-octylphenylamine gave 2,20-[1-(4-octylphenyl)1-H-pyrrole-2,5-
diyl]bisbenzoic acid dimethyl ester (4) using copper(I) chloride.
Double intramolecular Friedel–Crafts acylation was performed by
the treatment of the diester 4 with concentrated sulfuric acid
165 °C,18,19 no product was isolated. This failure is presumably due
to the instability of the pyrrole moieties under the acid and high-
temperature condition. An alternative ring closure of the diester 4
to prepare the target molecule was therefore sought, probably it
can be achieved by means of a combination of saponification and Le-
wis acid-promoted intramolecular cyclization. Thus, 4 was hydro-
lyzed by treatment with KOH as strong base to obtain the
corresponding benzoic acid 5 which is a key intermediate material
of the target. Compound 5 was treated with oxalyl chloride, and then
intramolecular Friedel–Crafts acylation using aluminum chloride
gave the desired pyrrole-based building with diketone 6 (11%). It
has been reported20 that the method for the synthesis of the starting
five-membered heterocycle consisting of intramolecular cyclization
of 2-hetarylbenzoic acids and their derivatives under various condi-
tions suffer from a number of drawbacks such as low accessibility of
these benzoic acids, sometimes unpredictable reaction route,21 and
relatively low yields, most likely resulting in the observed low yield
for the last step of the synthesis of 6.
p
architectures within the backbone since the built-in intermolecu-
lar charge transfer can facilitate ready manipulation of the elec-
tronic structure (HOMO/LUMO levels), leading to small band gap
semiconducting materials1–6 with relatively high charge carrier
mobilities.7–14 Furthermore, such compounds are potentially; (i)
intrinsically good electrical conductors or semiconductors without
the need of any chemical or electrochemical doping reaction, (ii)
transparent in either the neutral or doped state and (iii) of interest
as new polymeric materials for nonlinear optics.15
Among the various
p-conjugated materials, the thiophene, oli-
gothiophenes, and derivatives have most widely been the elec-
tron-donating moieties used in constructing donor–acceptor
conjugated materials.1–3,5 Although pyrrole-based materials have
also attracted strong attention due to their high electrical conduc-
tivity, thermal, and environmental stabilities,16 to the best of
knowledge, there exist no examples of the pyrrole materials with
push-pull architecture within a single chromophoric unit.
Herein, we present the synthesis and characterization of a
push–pull material based on pyrrole via intramolecular Friedel–
Crafts acylation, which is the first example of pyrrole-cored
push–pull-type single chromophore system.
The absorption behavior of 6 was measured in solvents of vary-
ing polarity in order to investigate the solvatochromism of these
donor(D)–acceptor(A) molecules. Two prominent absorption bands
along with a shoulder are observed in the spectra of 6 in all
solvents: a broad lower intensity band at 500–520 nm range, a
higher intensity band at 408–422 nm range, and a shoulder at
2. Synthesis and characterization
The facile synthesisof the target molecule toward thedesired bis-
carbonyl-bridged
1-(4-octyl-phenyl)-2,5-diphenyl-1H-pyrrole
429–445 nm range. The high-energy absorption band can be asso-
*
ciated with p–p transitions of the D–A molecules. As depicted in
* Tel.: +82 52 217 2920; fax: +82 52 217 2909.
E-mail address: yang@unist.ac.kr
Figure 1a, the absorption maxima of 6 recorded in various solvents
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.029

2008
C. Yang / Tetrahedron Letters 51 (2010) 2007–2009
O
O
O
O
O
O
O
O
CuI/ PdCl2(pph3)2
Et3N
CuCl/TMEDA
K2CO3
Br
Si
O2
86%
MeOH
88%
Si
O
95%
2
3
1
O
O
O
O
O
O
O
O
O
OH
HO
KOH
CuCl
1. ClCOCOCl
2. AlCl3
11%
N
N
N
THF/MeOH/H2O
85%
NH2
C8H17
C8H17
C8H17
41%
C8H17
4
5
6
Scheme 1. Synthesis of pyrrole-based material with diketone.
Figure 1. UV–vis absorption spectra of 6 in solvents of varying polarity (a) and cyclic voltammogram of 6 in acetonitrile solution (b).
reveal moderate bathochromic shift upon increasing the polarity of
the solvent from 408 nm in hexane to 422 nm in acetonitrile. This
indicates that this molecule possesses unambiguously intramolec-
ular charge transfer (ICT) from the push (pyrrole) to the pull (ke-
tone) unit.22–26 However, the color change of 6 is not observable
by the human eye as a function of solvent polarity. It is known that
larger solvatochromic shifts are observed in the PL emission spec-
tra compared to those in the absorption spectra because of an ex-
cited state with much stronger ICT character and larger dipole
moments relative to the ground state.27,28 However, the PL proper-
ties of 6 cannot be analyzed in various solvents since the observed
PL intensities are too low due to internal quenching units (two
ketone).
effect of the nitrogen atom in the chromophore. This corresponds
to an electrochemical bandgap of 2.33 eV, which is slightly larger
than its optical bandgap determined from the onset of maximum
absorption wavelength (ꢁ2.15 eV). It has been demonstrated that
the electrochemical bandgaps in general are higher than the
optical bandgaps.29–31 The relatively high EA is lower than that
of 2-(4-biphenylyl)-5-(4-tert-butylpehnyl)-1,3,4-oxadiazole (PBD)
(ca. ꢀ2.4 eV), one of the most widely used electron injecting/trans-
port materials,32 and comparable with that of CN-PPV (3.10 eV)33
which shows good electron-transporting ability in polymeric LEDs.
Therefore, 6 which shows reversible reduction is a promising elec-
tron-transporting material.34,35 Moreover, the HOMO energy levels
of 6 matches well with one of the most widely used hole-trans-
porting materials ((4,40-bis(4-(N-(1-naphthyl)-N-phenylamino)-
phenyl)biphenyl (5.46 eV)).36 Thus, 6 also holds potential as a hole
injecting and hole transport material in OLEDs.
3. Photophysical properties
The redox behavior of 6 was investigated by cyclic voltammetry
(CV) against Ag/Ag+. As shown in Figure 1b, 6 exhibits clear revers-
ibility in both the p-doping and n-doping processes, with two
reduction peaks ꢀ1.25 and ꢀ1.61 V, which can presumably be as-
signed to the reduction of the diketone to the quinoidal dianion,
and an oxidation peak at 1.29 V. The reduction and oxidation on-
sets occurred at ꢀ1.15 and 1.18 V, from which the LUMO and
HOMO energy levels were estimated to be 3.25 and 5.58 eV,
respectively. The LUMO levels of 6 are somewhat raised in compar-
ison with LUMO levels of phenylene-based materials with diketone
reported previously.18,19 A likely rationale for this is the inductive
In summary, the pyrrole-based building block with diketone
bridge 6 as push-pull-type material was successfully prepared
via intramolecular Friedel–Crafts acylation with 2,20-[1-(4-octyl-
phenyl)1-H-pyrrole-2,5-diyl]bisbenzoic acid (5). The compound 6
shows good processability from solution. On the redox behaviors,
6 not only shows low-lying LUMO but also displays high-lying
HOMO with fully reversible p- and n-doping. Such attractive prop-
erties of 6 can establish it as a promising candidate for use in opti-
cal and electro-optical applications. A detailed investigation of
various devices fabricated from 6 is in progress and will be re-
ported in a future Letter.

C. Yang / Tetrahedron Letters 51 (2010) 2007–2009
2009
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This work was supported by the National Research Foundation
of Korea Grant funded by the Korean Government (MEST) (NRF-
2009-C1AAA001-2009-0093020) and the 2009 Research Fund of
the UNIST (Ulsan National Institute of Science and Technology).
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Supplementary data
Supplementary data (full experimental details for the synthesis
of all new materials are available. Supplementary data associated
with this article can be found, in online version) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.02.029.
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