Polycyclic imide derivatives: Synthesis and effective tuning of lowest unoccupied molecular orbital levels through molecular engineering

Article abstract of DOI:10.1021/ol1024103

A series of fluoranthene-fused imide derivatives were facilely developed through a Diels-Alder reaction followed by decarbonylation. The investigation of their photophysical and electrochemical properties demonstrated that their LUMO levels were effective

Full text of DOI:10.1021/ol1024103

ORGANIC
LETTERS
2010
Vol. 12, No. 23
5522-5525
Polycyclic Imide Derivatives: Synthesis
and Effective Tuning of Lowest
Unoccupied Molecular Orbital Levels
through Molecular Engineering
Lin Ding, Han-Ze Ying, Yan Zhou, Ting Lei, and Jian Pei*
Beijing National Laboratory for Molecular Sciences (BNLMS), the Key Laboratory of
Bioorganic Chemistry and Molecular Engineering, College of Chemistry and
Molecular Engineering, Peking UniVersity, Beijing 100871, China
jianpei@pku.edu.cn
Received October 6, 2010
ABSTRACT
A series of fluoranthene-fused imide derivatives were facilely developed through a Diels-Alder reaction followed by decarbonylation. The investigation
of their photophysical and electrochemical properties demonstrated that their LUMO levels were effectively tuned from -3.2 to -3.8 eV through the
introduction of a fused imide unit, which provides a platform to design new air-stable and solution-processable n-type materials.
For the past decades, intensive attention has been paid to
organic semiconducting materials due to their potential
applications in plastic displays, sensors, and RF-ID tags.1
Among these organic semiconducting materials, air-stable
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diodes, bipolar transistors, and complementary circuits,2
especially for application in complementary circuits due to
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noise margin.3 Recently, some n-type organic materials, such
as metallophthalocyanine and fullerene derivatives,4 naph-
thalene and perylene imides,5 and oligothiophene derivatives
with cyano or fluoro substituents,6 have been intensively
investigated and exhibit good n-type performance in opto-
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10.1021/ol1024103  2010 American Chemical Society
Published on Web 11/11/2010

to the LUMO-level tuning of the molecules.8 The LUMO
level can be further lowered by introducing ethoxycarbonyl
and cyano substituents. What is more, the LUMO level can
be drastically reduced after modifying the molecules from
two directions at the same time.
Scheme 1. Synthesis of Cyclopentadienone Derivatives
Here, we chose a Diels-Alder reaction followed by
decarbonylation as the crucial step for constructing our
polycyclic molecules because of its high efficiency and facile
operation process. Scheme 1 illustrates the synthetic ap-
proaches to four cyclopentadienone derivatives as the precur-
sors of the subsequent reaction. A cyaniding reaction between
compound 19 and cuprous cyanide afforded 1,2-dihy-
droacenaphthylene-5,6-dicarbonitrile 210 in 52% yield. Treat-
ment of compound 2 with chromium trioxide gave 3 in 72%
yield. A condensation between compounds 411 and 3
produced cyclopentadienone 5 in 61% yield. Following a
similar condensation reaction, we also obtained three other
cyclopentadienones 6, 7, and 8.
The synthesis of their corresponding imide compounds and
the structures of the eight imide derivatives are illustrated
in Scheme 2. Such molecular design gives the diversity of
our synthesis of our imides. For example, with the introduc-
tion of R1, R2, and R3, we realized the multifunctionalization
of the fluoranthene-fused imide detivatives. The Diels-Alder
reaction flollowed by decarbonylation of cyclopentadienones
with various pyrrole-2,5-dione derivatives at 180 °C afforded
imides 9-16 in 18-50% yields overall for two or three steps
(see Supporting Information).
electronic devices; however, the skeleton for the construction
of n-type materials is still limited. Therefore, it is still a
challenge to develop new air-stable and solution-processable
n-type organic siemiconducting materials.
In our previous contribution, we developed a series of
fluoranthene derivatives in organic field-effect transistors
(OFETs) with good performance.7 In fact, the LUMO level
of a compound is crucial for the electron-transport property
and the stability in air. In this contribution, we facilely
developed a new series of fluoranthene-fused imide deriva-
tives to tune their LUMO levels to make these compounds
show good stability in air. Such a fluoranthene-fused imide
molecular design allows probing the role of the imide unit
in tuning of the photophysical and electrochemical properties.
Our investigation indicates that the LUMO levels can be
finely tuned from two directions by introducing electron-
withdrawing groups. Although the pentafluorophenyl group
is located at the node of the conjugated system, it contributes
All new fluoranthene-fused imides show good solubility
in common organic solvents at room temperature, which is
above 10 mg/mL in CHCl3. Their structures and purity were
fully characterized and verified by 1H and 13C NMR,
elemental analysis, and MS (see Supporting Information).
The investigation of the thermal properties of all new imides
indicated that they exhibited good stability with decomposi-
tion temperature higher than 350 °C in nitrogen atmosphere.
The photophysical properties of these imides were inves-
tigated in dilute CH2Cl2 solutions and in thin films. Their
Scheme 2. Synthesis and Structures of the Corresponding Imides 9-16
Org. Lett., Vol. 12, No. 23, 2010
5523

Table 1. Photophysical and Electrochemical Properties of Compounds 9-16 in Solutions and in Thin Films
d
compd λonset, λmax abs.a [nm] (logε) λmax emis.b [nm] λmax abs.c [nm] λmax emis.c [nm] Ered [V] EHOMO [eV] ELUMO [eV] Eg(opt) [eV]
9
450, 403 (4.57)
460, 413 (4.62)
429, 393 (4.69)
466, 404 (4.53)
475, 408 (4.53)
433, 403 (4.53)
488, 413 (4.48)
500, 410 (4.52)
469
484
457
510
517
438
545
553
408
421
398
410
416
405
413
412
488
508
476
520
525
495
523
571
-1.27
-1.11
-1.05
-1.00
-0.88
-0.89
-0.73
-0.63
-6.00
-6.11
-6.38
-6.06
-6.22
-6.48
-6.29
-6.36
-3.16
-3.32
-3.38
-3.43
-3.55
-3.54
-3.70
-3.80
2.84
2.79
3.00
2.73
2.67
2.94
2.59
2.56
10
11
12
13
14
15
16
a λonset (left) and λmax (right) in dichloromethane solution (5 × 10-5 M). b In dichloromethane solution (5 × 10-5 M). c In thin films. d Reference electrode:
Ag/AgCl. Onsets in dichloromethane solution (1 × 10-3 M).
photophysical data in solutions and in thin films are sum-
marized in Table 1. Figure S1 (Supporting Information)
illustrates the absorption features of these eight imides in
CH2Cl2 solutions and in thin films. As shown in Figure S1,
For 9, 11, 12, and 15 with an octyl group in their structures,
relative to 9, the onset of the absorption for 11 blue-shifted
from 450 to 429 nm, and absorption maximum λmax also blue-
shifted almost 10 nm, which was due to the introduction of
the ethoxycarbonyl group; while for 12 and 15, their
absorption maximum λmax exhibited the successive red-shift,
which was in agreement with the increase of the electron-
withdrawing ability thus effecting increase of the conjugation
length. Compared to 9, the onset and absorption maximum
λmax of the pentafluorophenyl-substituted derivative 10
showed 10 nm red-shift because the pentafluorophenyl group
is located at the node of the molecule which has little
contribution to the photophysical properties of the com-
pounds. For the pentafluorophenyl-substituted derivatives,
they have the same wavelength-shift tendency with the octyl-
substituted derivatives as shown in Table 1.12 Compared to
the solution state, the absorption wavelengths of all the
compounds have a little red-shift in the thin film as shown
in Table 1 and Figure S1 (Supporting Information), because
of the aggregation of the compounds in the solid state.
The fluorescence spectra of these compounds were also
investigated as shown in Figure S2 (Supporting Information).
Compared to 9, the emission maximum λmax exhibited blue-
shift from 469 to 457 nm for 11; however for 12 and 15,
they exhibited the successive red-shift from 469 to 510 nm
and 545 nm; while for 10, it changed a little, the emission
maximum λmax red-shifted from 469 to 484 nm. The results
are consistent with the wavelength-shift tendency of their
absorption spectra and can be explained with the same
reasons mentioned above.
Figure 1. CV curves of the double-site modified fluoranthene
derivatives in CH2Cl2.
the absorption of 9 revealed a strong absorption band at 304
nm, a weaker one at 338 nm and a broad one at 403 nm.
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All these eight imides were investigated by cyclic
voltammetry in CH2Cl2. Two reduction bands were
observed for these compounds. The first reduction band
can be attributed to the monoanion and the second band
can be attributed to dianion formation of the conjugated
fluoranthene-fused imide chromophore.13 From the CV
curve, the reduction band shifted gradually by introducing
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Org. Lett., Vol. 12, No. 23, 2010

electron-withdrawing substituents (Figures 1 and S3,
Supporting Information). The onset of the first reduction
band is directly relevant to the LUMO level as shown in
Table 1. The CV investigation indicated that the LUMO
level is -3.16 eV for 9, and -3.32 eV for 10, which
indicated that the introduction of the pentafluorophenyl
group locating at the node of the molecule lowered the
LUMO level of the molecule.8 The introduction of the
electron-withdrawing group reduced the LUMO level of
11 to -3.38 eV. The LUMO levels were further lowered
to -3.43 eV for 12 and -3.70 eV for 15 due to the
introduction of the cyano substituents. Our investigation
demonstrated that the most efficient way to lower the LUMO
level of the compounds was the modification at the R3 position,
especially for introducing two cyano substituents. Such results
were due to two effects: one is the strong electron-withdrawing
ability of cyano groups and the other is the location of the R3
group in the para-position of the largest π-conjugated core of
the molecules, in contrast to the R2 group in the ortho-position
and the R1 group in the node position. The LUMO levels can
be continuously lowered by double-site modification. When the
ethoxycarbonyl group and perfluorophenyl group were em-
ployed together, the LUMO level could be -3.54 eV for 14.
While cyano and perfluorophenyl groups were combined, the
LUMO level could be -3.55 eV for 13 and -3.80 eV for 16.
The molecules with such electronic energy level are potential
good candidates for air-stable n-type semiconductors.
Figure 2. LUMO and HOMO distribution obtained by calculation
method (B3LYP/6-311+g(d,p)//B3LYP/6-31G(d)).
In conclusion, we have successfully synthesized a series
of polycyclic fluoranthene-fused imide derivatives through
the Diels-Alder reaction as the crucial step in the synthetic
strategy. Such molecular design gives a deversity approach
to develop new polycyclic imide derivatives for n-type
materials through two-directional functionalization. Our
investigation in detail indicates that the photophysical and
electrochemical properties of these eight imides are ef-
fectively tuned by introduction of some functional substit-
uents. For example, the LUMO energy levels of the
fluoranthene core are modulated through introducing electron-
withdrawing substituents. The LUMO levels are drastically
reduced from -3.2 to -3.8 eV after modifying the molecules
from three different positions using high electron-withdraw-
ing groups, so they are in agreement with the LUMO level
of the typical n-type materials.
Furthermore, the LUMO and HOMO distributions were
demonstrated by calculations applying the method of B3LYP/
6-311+g(d,p)//B3LYP/6-31G(d). The HOMO is located
mainly on the bis(methoxyphenyl) benzene-conjugated core
and a little on the fluoranthene imide conjugated core. For
the ethoxycarbonyl substituted derivative, HOMO is located
only on the fluoranthene imide π-system because of the absence
of the electron-donating methoxyphenyl group. For the cyano
substituted derivatives, HOMO is located only on the dimeth-
oxyphenyl benzene motif because of the strong electron-
withdrawing ability of cyano groups. The LUMO is mainly
located on the fluoranthene imide group for all the compounds
(Figure 2). The above calculation is consistent with the results
obtained by absorption and PL spectra.
Acknowledgment. This work was supported by the Major
StateBasicResearchDevelopmentProgram(No.2009CB623601)
from the Ministry of Science and Technology and National
Natural Science Foundation of China.
Supporting Information Available: Experimental pro-
cedures, 1H and 13C NMR, MS data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL1024103
Org. Lett., Vol. 12, No. 23, 2010
5525