Synthesis and properties of 1-(4-aminophenyl)-2,4-dicyano-3-diethylamino-9, 9-diethylfluorenes: Potential fluorescent material

Article abstract of DOI:10.1246/cl.2008.570

Four new fluorenes bearing two electron donors and two electron acceptors were synthesized and found to emit blue fluorescence in both solution and solid states. Copyright

Full text of DOI:10.1246/cl.2008.570

570
Chemistry Letters Vol.37, No.6 (2008)
Synthesis and Properties of 1-(4-Aminophenyl)-2,4-dicyano-3-diethylamino-9,9-diethylfluorenes:
Potential Fluorescent Material
Xiao-Hang Chen, Zujin Zhao, Yang Liu, Ping Lu, and Yan-Guang Wangꢀ
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
(Received January 31, 2008; CL-080116; E-mail: orgwyg@zju.edu.cn)
Four new fluorenes bearing two electron donors and two
electron acceptors were synthesized and found to emit blue
fluorescence in both solution and solid states.
Table 1. Physical property data of compounds 1–4
Compd.
Tg/Tm/Tda/ꢁC
1
2
3
4
70/220/
275
82/179/
323
110/239/ 107/261/
324 355
ꢀabs./nm
Hexane/Film
266, 308, 307, 317, 265, 293, 293, 322,
320, 387/ 385/309, 320, 385/ 392/295,
264, 309, 320, 389 265, 296, 324, 387
Donor–acceptor (D–A) molecules have gained much
attention due to their potential application in electronic devices
such as light-emitting diodes1 and field-effect transistors.2 2,6-
Dicyanoanilines are typical A–D–A systems which have been
found to exhibit strong fluorescence in our previous studies.3
There are several publications involving the synthesis of this
class of compounds, but most of them did not reported their
optical properties.4 We herein report the synthesis and properties
of a new class of 2,6-dicyanoaniline derivatives with a fluorene
moiety: 1-(4-aminophenyl)-2,4-dicyano-3-diethylamino-9,9-di-
ethylfluorenes 1–4 (Scheme 1). These multifunctional molecules
contain two amino groups as the electron donors (D) and two
cyano groups as the electron acceptors (A).
Synthesis of compounds 1–4 is outlined in Scheme 1. Aryl
aldehydes 5 reacted with malononitrile by the Knoevenagel con-
densation in the presence of piperidine to afford 2-arylidenema-
lononitriles 6 in excellent yields. 2-(Indan-1-ylidene)malononi-
trile (8) was prepared by the condensation of indan-1-one (7)
and malononitrile according to the published procedure.5 6 react-
ed with 8 in the presence of piperidine (1.3 equiv) to afford 9
in moderate yields (41–62%). Ethylation of 9 with ethyl bromide
in the presence of sodium hydroxide gave 1–4 in good yields
(82–85%).6
321,
323,395
9859
(387 nm) (385 nm) (385 nm)
b
" /dm3 cmꢂ1 molꢂ1 16520
14631
9643
(392 nm)
ꢀemc/nm
Hexane/DCM
425/514/ 427/517/ 423/447/ 427/450/
568/467/ 561/458/ 457/446/ 461/451/
/MeOH/
Powder/Film
500
469
449, 464
450, 462
d
ꢁf
0.18/0.11 0.24/0.16 0.11/0.08 0.16/0.14
DCM/MeOH
Eonsetoxe/V
HOMO/
LUMO/
Egf/eV
0.89
5.29/2.41 5.28/2.40 5.34/2.46/ 5.35/2.47/
/2.88 /2.88 2.88 2.88
(5.35/1.94 (5.12/1.93
/3.41)g 3.19)g
0.88
0.94
0.95
aObtained from DSC and TGA measurements under N2. bMeasured in
hexane. cExcited at 313 nm. dUsing quinine sulfate monohydrate
(ꢁ313 ¼ 0:48) as a standard. eTwo platinum electrodes used as the
counter and working electrodes, respectively, and one Ag/AgCl elec-
f
trode used as the reference electrode. HOMO level values computed
ox
from the Eonset plus 4.4 eV. Eg values are calculated from the lowest
energy absorption edge of UV–vis absorption spectra. gValues are
computed by Gaussian 98.
110 ꢁC. Compounds 3 and 4 presented higher Tg than that of 1
and 2. Tgs of compounds 3 and 4 were rather higher than the
commonly used hole-transport materials, such as 4,40-bis[N-
(1-naphthyl)-N-phenylamino]biphenyl (NPB, Tg ¼ 95 ꢁC)
and N,N0-diphenyl-N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-
diamine (TPD, Tg ¼ 65 ꢁC).7 Compounds 2 and 3 have the
decomposed temperature (Td) higher than 300 ꢁC while that of
compound 1 is too low.
As next step, we investigated the physical properties of
compounds 1–4 and results were summarized in Table 1. The
glass-transition temperatures (Tg) of 1–4 ranged from 70 ꢁC to
CN
CN
CN
CN
CN
Piperidine
EtOH, rt
>95%
Ar
ArCHO +
NH2
CN
5a–5d
Piperidine
6a–6d
NC
Cyclic voltammograms (CVs) of the four compounds in
dichloromethane (DCM) are shown in Figure 1. One reversible
oxidation wave was observed for 1–4. The onset potentials
DMF
O
CN
105 °C
CN
9a–9d
Ar
NaOAC
EtOH, rt
84%
+
a: Ar = 4-dimethylaminophenyl
b: Ar = 4-diphenylaminophenyl
c: Ar = 9-ethyl-9H-carbazol-3-yl
d: Ar = 4-carbazol-9-ylphenyl
CN
onset
(Eox
vs. Ag/AgCl, Table 1) of 1–4 are 0.89, 0.88, 0.94,
7
8
100
50
0
-50
CN Et
N
CN Et
N
CN Et
N
1
3
2
4
Et
Et
Et
-100
-150
-200
-250
-300
EtBr
CN
CN
CN
Et
Et
Et
NaOH
Et
Et
Et
DMF, °C
N
N
N
R
R
Et
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1. R = Me
2. R = Ph
3
4
Potential / V
Scheme 1. Synthesis of compounds 1–4.
Figure 1. Cyclic voltammograms of compounds 1–4.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.6 (2008)
571
Hexane
Film
Powder
DCM
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
3
1
Methanol
250 300 350 400 450 500 550 600 650 700
250 300 350 400 450 500 550 600
1.0
1.0
4
0.8
2
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650 700250 300 350 400 450 500 550 600
Wavelength/nm
Figure 3. Computed HOMO and LUMO orbital surfaces of 1
and 2.
Figure 2. Normalized UV–vis absorption and PL emission of
1–4.
Table 2. The computed reorganization energies of some com-
pounds
and 0.95 V, respectively. Then, the estimated HOMO levels
onset
(HOMO = Eox
þ 4:4 eV) for 1–4 are 5.29, 5.28, 5.34, and
Molecule
ꢀꢂ
ꢀþ
5.35 eV, respectively. Eg and LUMO values are listed in
Table 1. All compounds have same Eg of 2.88 eV, which is
suitable to serve as blue light emitting material. No obvious
change was found when compound 4 was scanned for three
cycles, so this compound is electrochemically stable.
Benzene
Anthracene
NPB
0.41
0.20
0.19
0.29
0.30
0.14
0.29
0.12
2
investigated their photophysical, electrochemical, and thermo
properties. These molecules exhibited medium fluorescent quan-
tum yields with blue-light emission and high thermostability.
The preliminary calculation suggested that the new molecules
have good hole mobility. Further solid-state studies and single
layer device test are underway.
UV–vis absorption spectra of 1–4 were measured in dilute
hexane and film (Figure 2 and Table 1). There are three absorp-
tion bands in all of the four compounds. A same lowest weak en-
ergy absorption was found ranging from 360–420 nm, which can
be assigned to the intermolecular charge-transfer characteristic
from aminophenyl moiety to fluorene skeleton containing with
2,4-dicyano groups. The second absorption at 310–330 nm can
be attributed to the delocalized ꢂ–ꢂꢀ transition. The highest en-
ergy absorption band at 255–275 nm for 1 and 3 and 275–310 nm
for 2 and 4 is associated with the absorption of fluorene moiety.
The UV–vis absorption spectra in solid film are similar to that in
dilute hexane.
PL emission spectra were recorded in hexane, DCM, and
methanol solution as well as powder and film state at excitation
wavelength of 313 nm (Table 1 and Figure 2). All compounds
emitted blue light with the maximum emission wavelengths
ranged from 423–427 nm in hexane. Much larger positive solva-
tochromism was observed in the PL emission spectra of 1 and 2
than 3 and 4, this can be explained by the much stronger electron
donating ability of aminophenyl than the carbazolyl substituent.
Solid-state PL emission spectra of these compounds are red-
shifted comparing to that in hexane solution. The quantum yields
of 1–4 ranged from 0.11 to 0.24 measured in DCM (Table 1).
Molecular orbital calculations for 1 and 2 at the B3LYP/6-
31Gꢀ level showed that the HOMO and LUMO orbital are locat-
ed at different part of the molecule (Figure 3), this is coincidence
to the solvatochromism observed in PL emission. HOMO and
LUMO levels calculated by Gassian 988 are very close to that
of experiment (Table 1). The computed reorganization energies
We thank the Natural Science Foundation of Zhejiang
Province (no. R404109), and the Specialized Research Fund
for Doctoral Program of Higher Education (no. 20050335101).
References and Notes
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ꢀ
(anionic) and ꢀ (cationic) of 2 and several compounds are
þ
ꢂ
listed on Table 2. The lower ꢀ proved 2 is suitable to be used as
þ
hole-transport material by the theoretical calculation level. But
the ꢀ is not low enough for electron transport although there
ꢂ
are two cyano groups in the molecule.
In conclusion, we synthesized four new 1-(4-aminophenyl)-
2,4-dicyano-3-diethylamino-9,9-diethylfluorene derivatives and
Published on the web (Advance View) April 26, 2008; doi:10.1246/cl.2008.570