Design and synthesis of extended quinoxaline derivatives and their charge transport properties

Article abstract of DOI:10.1039/c6ra22677a

A scalable and convenient strategy is described to synthesize extended conjugation quinoxaline derivatives from phenylene-ethynylene arrays. By tuning the solvent, the compounds display bright emission solvatochromism. Furthermore, the fabricated FET devices possess good performance characteristics, with mobilities of 0.47 cm² V^-1 s^-1 and 0.99 cm² V^-1 s^-1. The mobilities are higher than those of corresponding to the alkyl substituted compounds, which can be proven by grazing incidence X-ray diffraction (GIXRD) measurement.

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DOI: 10.1039/C6RA22677A
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Design and Synthesis of Extended Quinoxaline Derivativesis and
Their Charge Transport Properties
Received 00th January 20xx,
Accepted 00th January 20xx
Junwei Yang, Haoyun Zhu, Yuli Huang, Wei Huang and Weizhi Wang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A scalable and convenient strategy is described to synthesize Due to this consideration, series of conjugation extended
conjugation extended quinoxaline derivatives from phenylene- quinoxalines are prepared, with the simple catalyst and facility
ethynylene arrays. By tuning the solvent, the compounds are isolation. Dicarbonyl derivatives, as intermediate products,
brightly emissive with an emission solvatochromism. have been obtained without much effort, which are useful
Furthermore, the fabricated FET devices possess nice building blocks capable of undergoing various chemical
performance, whose mobilities are 0.47 cm2 V-1 s-1 and 0.99 cm2 V- transformations.16
Different
π-conjugated
phenylene-
1 s-1, respectively. The mobilities are higher than those of the alkyl ethynylene arrays, as starting compounds, made a decision for
substituted corresponding compounds, which can be proved by the sequence of the conjugation chain of final products. The
the grazing incidence X-ray diffraction (GIXRD) measurement.
insertion of the electron-withdrawing quinoxalines on the
phenylene-ethynelene backbone influences the electronic
structures and various properties of the compounds. Alkyl
chains (n-butyl) are introduced to the compounds to improve
solubility.
Heterocyclic polycyclic aromatic hydrocarbons (hetero-PAHs)
have attracted increasing attention in their design, synthesis as
well as applications because of some fascinating features, such
as high stability, low energy absorption, and high charge
carrier mobility.1 Among enormous N-substituted heterocycles
that are pervasive in natural products, biologically active
agents2 and part structures in functional π-systems,3
quinoxalines are extraordinarily attractive.4 Many derivatives
have been proven to possess in vitro antiparasitic activities,5
anti-tumor activities,6 and used as 5-HT3 receptors,7 kinase
inhibitors.8 In addition, quinoxalines have also been applied as
building blocks for the development of semiconducting
materials,9 fluorescent probes,10 sensors,11 anion receptors12
and cavitands.13 In general, the reaction features the
condensation of 1,2-diamines with diketones to afford
quinoxaline derivatives.14 Recently, one-pot strategy has been
proposed to construct quinoxaline oligomers using expensive
metal catalysts, which possesses tedious isolation
procedures.15
Scheme 1. Synthetic Procedures of Compounds 5-12.
The oxidation of alkynes was catalyzed efficiently by PdCl2 to
afford intermediate products 5-8, in which DMSO acted as a
powerful oxidant (Scheme 1). The condensation reaction of
compounds 5-8 and 1,2-phenylenediamine successfully
afforded quinoxaline derivatives 9-12 in good yields, after plain
filtration of 5-8. Brunet products were avoided and white
solids finally precipitated after these two separate steps. In our
experimentation, the stepwise process was the key to obtain
pure products in high yields, which was verified by 1HNMR and
Address here. The State Key Laboratory of Molecular Engineering of Polymers,
Collaborative Innovation Center of Polymers and Polymer Composite Materials,
Department of Macromolecular Science, Fudan University, Shanghai 200433,
China.
E-mail: weizhiwang@fudan.edu.cn; Tel: +86021-65643836
†Electronic Supplementary Information (ESI) available: experimental section
including materials, measurements and characterizations, devices fabrication and
characterization; mass spectra, NMR data for new compounds, UV, PL and cyclic
voltammetry spectra of compounds and CCDC reference number of 10b: 1499076.
For ESI or other electronic format see DOI: 10.1039/x0xx00000x
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13CNMR (Fig. S1-S40). With the increase of the π-conjugated
skeleton, 11-12 are poorly soluble in acetic acid, leading to
their nearly complete precipitation from the reaction mixture.
The needle-like single crystal of 10b was obtained by
recrystallization from acetic acid. The X-ray analysis was
12b
396
418
62.33
a
b
Absorption maximum in dilute CHCl3 solutions. Quantum
yield estimated with quinine sulfate (ΦF = 54% in 0.1 M H2SO4)
conducted to establish the structure in Fig. 1 and Table S1
,
which led to elucidation of its solid state structure.
Fig. 1 The ORTEP drawing of 10b
The ultraviolet/visible
.
(UV/vis)
absorption
and
photoluminescence (PL) spectra of 9-12 in chloroform are
shown in Fig. S41, S42, and the data is compared in Table 1
.
The extended structural unit affects the position of the
absorption and emission band from 9 to 12, although the red-
shift is not obvious. However, the electron-donating effect of
n-butyl substituent results in
a redshift of the longer
wavelength absorption band to ca. 390 nm, which changes the
electron cloud density of the adjacent benzene ring. In
Fig. 2 Solvent-dependent emission spectra of (a) 12b and (b)
12a Inset: the optical photograph of corresponding
.
chloroform,
compound
12b
emits
deep
blue
compounds in different solutions.
photoluminescence with an emission maximum at 418 nm.
However, the emission changes to bright green in acetone
solutions. Depending on the variation of solvent, the result of
the solvent polarity on the emission is quite positive and
significant, ranging from 432 to 518 nm (Fig. 2a). The
pronounced positive emission solvatochromism is additionally
observable upon naked eyes, where the color changes from
The introduction of an electron-donating or electron-
accepting group directly affects the HOMO and LUMO levels of
the compound. The cyclic voltammetry curves of quinoxaline
derivatives 9-12 show no reversible peaks arising from the
oxidation potential, which was ascribed to the electron-
deficient pyrazine ring containing two sp2-type centers (Fig.
S43).18 In the anodic scan, the onset of oxidation for 9a-12a
occurred at 1.06, 0.88, 0.75 and 0.70 V, which corresponded to
HOMO values of -5.81,-5.63, -5.50 and -5.45 eV, respectively
blue (toluene) to yellow-green (DMSO)
(Fig. 2a). More
polarizable solvent served to stabilize the excited state more
than the ground state, which lessens the energy gap.17 We also
determined the photophysical of 12a in same solvents and
observed a similar bathochromic trend, exhibiting the PL
emission maxima in the range from 480 to 525 nm (Fig. 2b).
Table 1. Photophysical Properties of compounds 9a-12a and
9b-12b.
(
Table 2). The oxidation potential values of alkylated oligomers
follow the order 9b 10b 11b 12b, which presents the
same trend with the values of 9a-12a The extended
>
>
>
.
conjugation length slightly raises the HOMO level and
consequently reduces the band gap of the oligomers. The
energy band gaps of 9a-12a were calculated to be 3.28, 3.18,
3.15 and 3.12 eV, which were determined from the onset
wavelength of their UV absorptions. The HOMO-LUMO energy
gaps showed good correlations with their UV data and the
molecular extended conjugation chain.
a ,b
Sample
9a
UVa
345
358
359
359
390
394
395
PLa
ΦF
(%)
413
416
421
425
416
417
417
5.23
10a
11a
12a
9b
10.35
31.32
54.37
5.32
10b
11b
11.24
35.11
2 | J. Name., 2012, 00, 1-3
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the out-of-plane spacing of 11a increased to 5.66 Å, whose
π−π stacking distance rose to 3.50 Å as well (Fig. S44).
Table 2. Electrochemical Properties of compounds 9a-12a and
9b-12b.
ox
Sample Eonset
HOMO
-5.81
-5.63
-5.50
-5.45
-5.71
-5.61
-5.48
-5.44
λonset
378
389
394
397
390
395
396
398
ΔEg (eV) LUMO
9a
1.06
0.88
0.75
0.70
0.96
0.86
0.73
0.69
3.28
3.18
3.15
3.12
3.18
3.14
3.13
3.12
-2.53
-2.45
-2.35
-2.33
-2.53
-2.47
-2.35
-2.32
10a
11a
12a
9b
10b
11b
12b
ox
aAbbreviations: Eonset is the onset potential for oxidation.
ox
HOMO is calculated by the equation: HOMO = − e(Eonset
–
0.0468 V) − 4.8 eV. λonset is the onset wavelength of UV
absorptions. LUMO = ΔEg + HOMO.
The transfer integral and reorganization energy, which are
believed to be important for the mobility of semiconductors,
are both extremely based on the arrangement of the organic
molecules.19 To further inspect and compare the molecular
packing characteristics of quinoxaline derivatives with the
extended skeleton, two-dimensional grazing incidence X-ray
diffraction (2D-GIXRD) measurements were performed. As for
oligomer 12a, the clear (100) reflection arch appears along the
Fig. 4 (a) Transfer and (b) output characteristics of the 11a
devices (W = L = 1000 μm) at a drain source voltage (VDS) = 2.5
V. (c) Transfer and (d) output characteristics of the 12a based
FETs.
qz direction, with a qz value of 11.2 nm-1
(Fig. 3a). Furthermore,
a clear arch shape of the (010) diffraction peak in the in-plane
direction appears at qxy= 18.01 nm-1, which corresponds to the
π−π stacking distance of 3.49 Å. Obviously, 12a tends to pack
more orderly and tightly than 12b in neat films, which may be
due to the steric hindrance caused by alkyl chains. The obvious
(010) arch of compound 12b appears at qxy= 17.50 nm-1, whose
π−π distance is 3.59 Å (Fig. 3b). Compared with the variation in
π−π
The higher HOMO energy level suggests that 12 might have
great potential for use in field-effect transistors (FETs) as p-
type materials.21 Furthermore, N-Heteroatoms provide a new
method of tuning the intrinsic molecular electronic properties
and improving stability, which have been extensively studied in
organic thin film transistors.22 Quinoxaline derivativesis 11a-
12a and 11b-12b with high thermal stabilities are beneficial for
the processing of the electrical devices
(Fig. S45). We
evaluated the charge transport properties of the oligomers
11a-12a and 11b-12b by fabricating field-effect transistors in
the “top-contact top-gate” geometry. To this end, FETs using
the quinoxaline derivatives were fabricated using ion gels23 to
efficiently diminish the heat generated at work and to endow
low voltage operation.24 Besides, the top-gated quinoxaline
derivatives transistors prepared using ion gel gate dielectrics
has been rarely reported. The representative transfer curve
(ID-VG) of the FET with 11a is displayed in Fig. 4a and that of
the 12a is shown in Fig. 4c. The best results of 11a and 11b
devices exhibit mobilities up to 0.47 cm2 V−1 s−1 and 0.44 cm2
Fig. 3 The 2D-GIXRD pattern of (a) 12a and (b) 12b
.
V−1 s−1, respectively, with on/off ratios up to 103
While the significant increases of the mobilities were observed
on the 12a
12b based devices, 0.99 cm2 V−1 s−1 and 0.88 cm2
V−1 s−1 were respectively evaluated from the saturation regime
Fig. S47). In addition, the output characteristics (ID-VD) of the
aluminum-gated quinoxaline derivatives based FETs at five
different gate voltages (VG), as shown in Fig. 4b and Fig. 4d
(Fig. S46).
stacking, the alkyl substitute is of larger influence for the
spacing in out-of-plane direction. The intense reflections of the
(100) plane along the qz (out-of-plane) and a relative weak
(010) plane along qxy (in-plane) axes of 12b films can be
observed from Fig. 3b, which implies that 12b molecules
would prefer to have the edge-on structure.20 The ordered
edge-on structure of organic molecules prefers to be formed in
high performances FETs, which allows the molecules to
arrange along the direction in the conducting channel and then
get an efficient charge transport.19 In addition, the decreased
,
(
,
affirming the clear p-channel characteristics. The
structure−property relaꢀonship has been verified by a range of
analyses, especially using 2D-GIXRD. The substantial structural
conjugated chain made some differences. Compared to 12a
,
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In summary, series of conjugation extended quinoxaline
oligomers 9-12 have been successfully synthesized by a
convenient and efficient route with the simple procedure of
isolation. The compounds 12a and 12b display positive
emission solvatochromism, as revealed by detailed optical
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This work was financially supported by the National Natural Science
Foundation of China (21274027 and 20974022) and the Innovation
Program of Shanghai Municipal Education Commission (15ZZ002).
The FETs were fabricated and characterized in Fudan
Nanofabrication Laboratory. The synchrotron-based 2D-GIXRD
measurement was supported by Shanghai Synchrotron Radiation
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A scalable and convenient strategy is described to synthesize conjugation extended quinoxaline derivatives from phenylene-
ethynylene arrays. The fabricated FET devices possess nice performance, whose mobilities are 0.47 cm2 V-1 s-1 and 0.99 cm2 V-1 s-1,
respectively.
This journal is © The Royal Society of Chemistry 20xx
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