Coreextended terrylene diimide on the bay region: Synthesis and optical and electrochemical properties

Article abstract of DOI:10.1021/ol202775b

Two novel coreextended terrylene diimides on the bay region (CETDIs) were synthesized via annulation of the four additional ethylene units or benzene units on the bay region of the terrylene diimide core. The optical and electrochemical properties of the

Full text of DOI:10.1021/ol202775b

ORGANIC
LETTERS
2011
Vol. 13, No. 24
6484–6487
Core-Extended Terrylene Diimide on the
Bay Region: Synthesis and Optical and
Electrochemical Properties
Qianqian Bai,† Baoxiang Gao,*,†,‡ Qi Ai,† Yonggang Wu,†,‡ and Xinwu Ba†,‡
College of Chemistry and Environmental Science, Hebei University, Baoding, 071002,
P. R. China, and Key Laboratory of Medicinal Chemistry and Molecular Diagnosis,
Ministry of Education, Hebei University, Baoding, 071002, P. R. China
bxgao@hbu.edu.cn
Received October 14, 2011
ABSTRACT
Two novel core-extended terrylene diimides on the bay region (CETDIs) were synthesized via annulation of the four additional ethylene units or
benzene units on the bay region of the terrylene diimide core. The optical and electrochemical properties of the two compounds were investigated.
These CETDIs exhibited broad absorption spectra with high extinction coefficients, which span a wide range in the ultraviolet and visible spectrum
from 300 to 700 nm. Furthermore, the redox process of the CETDIs increased from two waves to four waves, and the lowest unoccupied molecular
orbital (LUMO) levels were enhanced from À4.00 to À3.59 eV.
Perylene diimides (PDIs) have received considerable
attention in dye chemistry,1 supramolecular assemblies,2
and optoelectronic applications3 because of their excep-
tional properties, such as photochemical stabilities, high
quantum yields,4 and ability to self-assemble into ordered
supramolecular structures.5 Modifications of PDIs have
been concentrated on the bay regions, at which chemical
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r
10.1021/ol202775b
Published on Web 11/21/2011
2011 American Chemical Society

substitution can dramatically alter the intermolecular in-
teractions and the optical electronic properties.6,7 Further-
more, the core-extended of PDIs attract huge interest and
offer access to various biological8 as well as organic elec-
tronic applications.9,10 Core bay-extended PDIs include
annulated hydrocarbon aromatic rings11 and diverse hete-
rocycles such as sulfur-,12 pyridyl-, and N-heterocyclic13
substituted PDIs.
The higher PDI homologues, terrylene diimides (TDIs),
exhibit the same attractive properties, such as brilliant
color; high extinction coefficients, a high fluorescence
quantum yield of 90%, and high thermal, chemical, and
photochemical stabilities, which are generally required for
practical use.7a,14,15 However, core-extended terrylene dii-
mides on the bay region remain unexploited. Our current
interests are development of the core-enlarged terrylene
diimide on the bay region for use as stable fluorophores.16
The synthetic approach for CETDIs is outlined in
Scheme 1. Tetrabromoterrylenedicarboximide 4 was
prepared according to literature procedures.18 Under
Sonogashira conditions, the reaction of compound 4 with
1-dodecyne yielded the intermediate compound 5, which
was not isolated but was converted to the CETDI 2
through the addition of DBU to the reaction mixture.
Some cyclization of intermediate compound 5 to the final
product was observed prior to the addition of DBU.6c
Suzuki coupling of compound 4 with 3-isopropyl-
phenylboronic acid produced the terrylene diimides 6 with
a 63% yield. Intramolecular oxidative cyclodehydrogena-
tion reactions were then performed on compound 6 using
FeCl3 as an oxidant. With 40 equiv of FeCl3 (3.3 equiv per
hydrogen to be removed), the completely closed CETDI 3
was obtained within 30 min with a 70% yield.19,20 These
CETDI compounds show good solubility in common
organic solvents such as cyclohexane, dichloromethane,
chloroform, tetrahydrofuran, and toluene. The CETDIs
have been fully characterized by H NMR spectroscopy, C
NMR spectroscopy, and matrix-assisted laser desorption/
ionization mass spectrometry (MALDI- TOF-MS).
€
While this work was in progress, Klaus Mullen and co-
workers reported the synthesis and chiroptical properties
of a partially core-extended terrylene diimide.17 In this
paper, the synthesis of the fully core-extended terrylene
diimides on the bay region (CETDIs) was present, and
their optical and electrochemical properties were further
investigated.
The 1H NMR spectrum of CETDI 2 in CDCl3 (Figure
S1A in Supporting Information) shows the resolved reso-
nances of the fourteen aromatic protons with four signals.
However, eight protons H1 (ArÀCH2) of CETDI 2 show
two distinct signals at 4.37À4.32 and 4.19À4.14 ppm,
Scheme 1. Synthesis of Core-Extended Terrylene Diimidesa
13
which were assigned by the aid of 1HÀ C COSY NMR
data. It suggests that protons ArÀCH2 are not equivalent
€
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80 °C, 24h;(c) Pd2(dba)3, DPEPhos, K2CO3, H2O, toluene, ethanol, 95 °C,
36 h; (d) FeCl3, nitromethane/CH2Cl2, 25 °C for 0.5 h.
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three times, suggesting that the cyclodehydrogenation reaction is a
stepwise mechanism. However, this cyclodehydrogenation condition
using 20 equiv of FeCl3 (1.7 equiv per hydrogen to be removed) did
not produce the completely closed CETDI 3, even after the reaction time
was extended to 48 h. The formation of partially closed compound 6 is
due to the hindrance induced by the twisting of the terrylene diimides 6.
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Org. Lett., Vol. 13, No. 24, 2011
6485

due to conformational restrictions, with a possible contri-
bution from the anisotropic magnetic current. Further-
more, four C1 of CETDI 2 have the same chemical shift at
37 ppm (Figure S1C in Supporting Information).
has a significantly blue-shifted absorbance maximum at
419 nm with four low intensity bands at 592, 548, 479, and
453 nm. It is already known that an extension of the core of
PDIsinsomecases inducesashift toshorterwavelengthsin
their absorption spectra, which is in contrast to the exten-
sion along their long molecular axis.7,11a Furthermore, the
extinction coefficient of CETDI 2 is lower than that of
TDI 1 but still remains high (ε = 115 000 MÀ1 cmÀ1).
The absorption band of CETDIs is red-shifted after
the annulation of four additional benzene units in the
bay region of the terrylene diimide core. At the shorter
wavelength region, CETDI 3 shows a slightly red-shifted
absorption band with a comparable extinction coefficient
(Table 1). However, the longest absorbance maximum of
CETDI 3 is significantly red-shifted by 84 nm (from 592 to
674 nm) withhighly improved extinction coefficients (from
15000 to 91000 MÀ1 cmÀ1). It is noteworthy that the
CETDI 3 has seven absorption bands with high extinction
coefficients, which span a wide range of the UVÀvis
spectra from 300 to 700 nm. The broad spectrum and high
extinction coefficient of CETDI 3 make it a potential
compound for organic solar cells.12b,22
In contrast to CETDI 2, the chemical shifts of aromatic
protons H4 of CETDI 3 show two sets of signals, a doublet
signal at low field of 7.87 ppm with a large coupling
constant of 9.0 Hz and another doublet signal at high field
of 7.47 ppm with a small coupling constant of 7.2 Hz
(Figure S1B in Supporting Information). It is very surpris-
ing that the chemical shifts of the aliphatic CH and (CH3)2
groups of CETDI 3 respectively show two sets of signals
with a 0.4 to 1.8 ppm interval. The protons H3 of 2,6-
diisopropylphenyl on the imide show two sets of signals
with a 0.4 ppm interval, a doublet signal at low field of
2.24 ppm and another doublet signal at high field of
1.84 ppm. The protons H1 and H2 of isopropyl on the aro-
matic core have four sets of signals with 1.8 and 1.5 ppm
intervals, respectively. The protons H1 and H2 of the sub-
stituted group near the large aromatic core have a long
ppm interval. However, the protons H3 and H4 of the
substituted group far from the large aromatic core have a
short ppm interval. The ring-current effects of the large
aromatic π-systems, which result in the anisotropic mag-
netic current, are possibly responsible for the two sets of
signals of the same substituted group.21
Table 1. Optical and Electrochemical Properties of CETDIs
λ
max/nm
λem/nm EHOMO ELUMO Eg
(Φ)
Compd
(log ε)
eV a
eV a
eV b
1
652 (5.50), 600 (5.20),
556 (4.67)
667,
730
À5.58 À4.00 1.82
À5.72 À3.79 2.08
À5.29 À3.59 1.77
(0.90)
597,
2
3
592 (4.16), 548 (3.85),
479 (4.61) 453 (4.67),
4.19 (5.07), 404 (4.91)
674 (4.95), 614 (4.55),
5.73 (4.69), 532 (4.03),
429 (5.00), 406 (4.66)
650
(0.10)
689,
756
(0.55)
a Energy level of HOMO and LUMO according onset of first
oxidation potential and reduction potential: 1.18 V and À0.40 V for 1;
1.32 V and À0.61 V for 2; 0.89 V and À0.81 V for 3 . b Optical band gap
according UV/vis absorption.
The emission spectra of all CETDIs are recorded, and
the wavelength of the emission maxima is given in Table 1.
TDI 1 in chloroform exhibits a deep red fluorescence witha
quantum yield of90%.14 Annulation of the fouradditional
ethylene units on the core bay led tothe hypsochromic shift
of the emission maxima by 70 nm (from 667 to 597 nm),
leading to orange red fluorescence with a low quantum
yield of 10%. Further fusion of the four benzene units on
the core bay resulted in the red shift of emission maxima of
CETDI 3 by 92 nm (from 597 to 689 nm). The fluorescence
quantum yield of CETDI 3 increased by 55% but is still
lower than that of TDI 1. Furthermore, the two CETDI
compounds display small Stokes shifts of approximately
5 nm to 15 nm. In the case of the terrylene diimides
core twist, small Stokes shifts are possibly responsible for
the decrease in the fluorescence quantum yield. The small
Figure 1. UVÀvis spectra (top) and fluorescence emission spec-
tra (bottom) of CETDIs in chloroform.
The UVÀvis absorption spectra and fluorescence spec-
tra of the CETDIs in chloroform are shown in Figure 1,
and the photophysical dataaresummarizedinTable 1. The
UVÀvis spectra of the CETDIs show a multipeak band
with defined structures. Compared with TDI 1, CETDI 2
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Stokes shift results in a strong reabsorption effect which
lowers the fluorescence quantum yield.
three compounds were calculated as À5.29 to À5.58 eV and
À3.59 to À4.00 eV, respectively. The continuously enhanced
LUMO levels are ascribed to the terrylene diimides core
twist, which weaken the electronic coupling between two
of the electron-deficient imides.24 Compared with TDI 1,
CETDI 2 has lower HOMO levels. However, CETDI 3
shows enhanced HOMO levels, suggesting that the elec-
tron-donating property of CETDI 3 becomes stronger.
In conclusion, we reported the synthesis of the core bay-
extended terrylene diimides by the palladium-catalyzed
coupling reaction and ring annulation reaction. The op-
tical and electrochemical properties of the two compounds
were investigated. Fusedwithfour ethylene units in the bay
region of the terrylene diimides, CETDI 2 has a signifi-
cantly blue-shifted absorbance and emission maximum
with low extinction coefficients and fluorescence quantum
yields. CETDI 3, after annulation of an additional four
benzene units on the bay region of the terrylene diimides,
shows red-shifted absorbance and emission maxima with a
high extinction coefficient and moderate fluorescence
quantum yield. CETDI 3 has seven absorption bands with
high extinction coefficients, which span a wide range of the
UV and visible spectrum from 300 to 700 nm. Further-
more, the redox process of the CETDIs increased from two
to four waves, and the LUMO levels were enhanced from
À4.00 to À3.59 eV.
Figure 2. Cyclic voltammograms of compound CETDIs in
dichloromethane at a scan rate of 0.1 V/s.
The electrochemical properties of the CETDIs in di-
chloromethane were studied by cyclic voltammetry (CV).
Figure 2 shows the CV curves of the CETDIs. TDI 1 shows
one reversible oxidation potential at 1.35 V vs Ag/AgCl
and one reversible reduction potential at À0.56 V vs Ag/
AgCl. CETDI 2 exhibits one irreversible reduction poten-
tial at À0.89 V, whereas CETDI 3 exhibits two reversible
reduction potentials at À1.0 and À1.30 V, respectively.
These CETDIs show a continuous decrease of the first
reduction potential, indicating that the electron-accepting
power becomes weaker. CETDI 3 exhibits two reversible
oxidation potentials at 1.01 and 1.26 V respectively. The
lowest unoccupied molecular orbital (LUMO) and the high-
est occupied molecular orbital (HOMO) were estimated
from the onset of the first reduction potentials and first
oxidation potentials.23 The HOMO and LUMO levels of the
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (Nos.
20804012 and 20904008), Hebei Uinversity Natural Sci-
ence Funds for Distinguished Young Scholar, the Natural
Science Foundation of Hebei Province (No. B2009-
000169), and the Key Project of Chinese Ministry of
Education (No. 20100015).
Supporting Information Available. Detailed synthetic
procedures and characterization, general procedures, and
supporting spectroscopic and microscopic images. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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