Design, synthesis, and photoelectric properties of v-shaped organic fluorescent compounds with a 1,3,4-oxadiazole moiety

Article abstract of DOI:10.1177/1747519819831881

A series of symmetric, silicon-linked organic fluorescent compounds with two electron-deficient 1,3,4-oxadiazole units were synthesized and characterized. The compounds possessed a V-shaped structure with a silicon atom, which weakened π–π stacking, promoting aggregation-induced emission. The compounds were fluorescent in both solution and solid-state thin films. The efficient fluorescent behavior of the materials was confirmed through optical and electrochemical measurements. The compounds displayed excellent thermal stability, with decomposition temperatures exceeding 400 °C. Amorphous films of the compounds possessed high morphological stability. These results indicate that the compounds may be promising emissive and electron-transporting materials.

Full text of DOI:10.1177/1747519819831881

831881CHL0010.1177/1747519819831881Journal of Chemical ResearchHou et al.
research-article2019
Research Paper
Journal of Chemical Research
2019, Vol. 43(1-2) 3–7
© The Author(s) 2019
Article reuse guidelines:
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Design, synthesis, and photoelectric properties
of V-shaped organic fluorescent compounds
with a 1,3,4-oxadiazole moiety
https://doi.org/10.1177/1747519819831881
DOI: 10.1177/1747519819831881
journals.sagepub.com/home/chl
Rui-Bin Hou1,2, Ji-Ying Su1,2, Ling-Ling Zhang1,
Dong-Feng Li1 and Yan Xia1,2
Abstract
A series of symmetric, silicon-linked organic fluorescent compounds with two electron-deficient 1,3,4-oxadiazole units
were synthesized and characterized. The compounds possessed a V-shaped structure with a silicon atom, which weakened
π–π stacking, promoting aggregation-induced emission. The compounds were fluorescent in both solution and solid-
state thin films. The efficient fluorescent behavior of the materials was confirmed through optical and electrochemical
measurements. The compounds displayed excellent thermal stability, with decomposition temperatures exceeding
400°C. Amorphous films of the compounds possessed high morphological stability. These results indicate that the
compounds may be promising emissive and electron-transporting materials.
Keywords
1,3,4-oxadizoles, electron-transporting materials, fluorescent materials, thermal stability, V-shaped structure
in optical applications because of their high brightness, ther-
Introduction
mal stability, and amorphous film-forming capability.22–24
Organic fluorescent materials have been the subject of great
Si-based tetrahedral organic molecules and polymers have
interest because of their potential applications in many
been intensively investigated as electroluminescent materi-
aspects of modern life, such as lighting, displays, bioprobes,
als. For example, Partee et al.25 and Zhen et al.26 recently
and chemosensors.1–5 Generally, the luminescence properties
reported a series of tetrahedral luminescent materials with
of organic molecules in the solid state are determined mainly
SiAr4 cores. Meanwhile, 1,3,4-oxadiazoles are classic het-
by their chemical structures. Most fluorescent organic mole-
erocyclic compounds that have also attracted substantial
cules with highly conjugated planar structures emit reasona-
interest for use in OLEDs because of their favorable elec-
bly intense fluorescence in dilute solutions but emit weak or
tronic transmission properties. Many kinds of oxadiazole-
nonexistent emission in the solid state because of aggrega-
based materials have been synthesized and used in different
tion-induced quenching.6–9 However, some groups have
applications, and some are considered to be potential blue
recently reported the unique phenomenon of aggregation-
emitters for OLEDs.27,28
induced emission (AIE).10,11 Propeller-shaped molecules
such as tetraphenylethene, hexaphenylsilole, and quinoline–
malononitrile, along with their derivatives, are known to dis-
play AIE.12–17 Molecular design optimization can be used to
increase molecular luminous efficiency and stability, leading
to organic light-emitting diodes (OLEDs) with improved
optoelectronic properties.18–21
1Key Laboratory School of Chemistry and Life Science, Changchun
University of Technology, Changchun, P.R. China
2Advanced Institute of Materials Science, Changchun University of
Technology, Changchun, P.R. China
Corresponding authors:
Dong-Feng Li andYan Xia, Key Laboratory School of Chemistry and Life
Science, Changchun University ofTechnology, Changchun, 130012, P.R. China.
Emails: lidongfeng@ccut.edu.cn; hmz20130521@163.com
Considering all organic luminescent materials, silicon
(Si)-based compounds have been reported to perform well

4
Journal of Chemical Research 43(1-2)
CH3
Si
CH3
CH3
Cl Si Cl
CH3
CH3
Si
CH3
KMnO4
CH3OH
Na,Toluene
reflux
+
Cl
H
H3C
CH3
3C
HO
H
CO2
2C
H O
H2SO4
Pyridine,
reflux
2
2
3
CO2H
CH3
Si
CH3
R
CH3
Si
CH3
NH2NH2
6
H2O
H
3CO2C
CO2CH3
H2NHNOC
CONHNH2
POCl3
4
5
CH3
Si
CH3
N
N
N
N
O
O
R
R
1a-d
:
=
1a
R
R
R
R
H
:
=
1b
H5
OC2
OC4
OC6
=
=
:
1c
H9
:
1d
H13
Scheme 1. Synthetic route of 1a–d.
The PL spectrum of compound 1a in CHCl3 contains two
emission peaks at around 340 and 400 nm (Figure 2(a)).
The emission spectra of compounds 1b–d in CHCl3 also
contain one similar broad peak. Interestingly, the emis-
sion maxima of compounds 1b–d are red-shifted to
382 nm compared with that of 1a because of the alkoxy
chain substituents. The fluorescence quantum yields
(QYs) of 1a–d measured at room temperature in dilute
CHCl3 were moderate with the values of 0.71, 0.72, 0.72,
and 0.72, respectively, compared with the PL intensity of
a standard solution of quinine sulfate (QY= 0.55). The
emission spectra of the films were quite different to
those for the solutions with regard to the two main emis-
sion peaks, as demonstrated in Figure 2(b). The emission
maxima of compounds 1b–d red-shifted with increasing
length of the alkyl chain. The intensity of one of the
emission peaks increased, while that of the other
decreased as the alkyl chain lengthened. This phenome-
non may originate from the dihedral angle of the mole-
cules increasing with the length of the alkoxy chain.
The electrochemical properties of 1a–d in dimethylfor-
mamide (DMF) were investigated by cyclic voltammetry
(CV) as summarized in Figure 3 and Table 1. Figure 3 dis-
plays representative CV traces for the oxidation and reduc-
tion of 1c. The CV curves reveal that 1c undergoes a
quasi-reversible reduction at the cathodic potential associ-
ated with the reduction of an electron-deficient oxadiazole
moiety to form an anion radical. Compound 1c also exhib-
its an irreversible oxidation process corresponding to the
removal of an electron from the peripheral phenyl group to
give a radical cation.
Results and discussion
Continuing our interest in V-shaped luminescent materi-
als,29,30 in this article, we report the development of a
series of V-shaped fluorescent molecules (1a–d) contain-
ing a naphthalene-linked 1,3,4-oxadiazole and an Si core
atom, which weakens π–π stacking interactions. The
length of the alkoxy chain attached to naphthalene was
changed to study the effect on the photochemical and
electrochemical properties of the materials in solution
and in the solid state.
Starting compounds 2–5 were synthesized according to
the reported methods.31,32 The synthetic procedure followed
to obtain target compound 1a–d is summarized in Scheme
1, and the details are given in the section “Experimental.”
An important intermediate (5) with a silylbenzohydrazide
moiety was prepared quantitatively by the reaction of 4
with excess hydrazine monohydrate under reflux. The reac-
tion of 5 and 2-naphthoic acid 6a or 6-alkoxy-2-naphthoic
acid 6b–d in phosphorus oxychloride under reflux gives
compound 1a–d as white crystals in yields of 32%–39%.
The structures of 1a–d were characterized by H and 13C
nuclear magnetic resonance (NMR) spectroscopy, mass
spectrometry, and elemental analysis.
1
The ultraviolet (UV)-Vis absorption properties of com-
pounds 1a–d in CHCl3 solution are presented in Figure 1. The
absorption spectra contain a wide peak in the region of 315–
330nm originating from the π–π* absorption of the conju-
gated naphthalene-phenyl 1,3,4-oxadizole moiety. The
absorption spectra of compounds 1b–d possessing alkoxy
groups displayed bathochromic shifts of 15nm compared with
that of compound 1a without a substituent. These results illus-
trate that the alkoxy chain can lower the energy of the π–π*
Calculations of 1a–d were carried out using the Gaussian
09 (G09) package.35 GaussView 5.0.8 was used for the visu-
alization of the structures and orbital manipulation. The cal-
culated highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (LUMO), and energy gap for
each of 1a–d are displayed in Figure 4. The calculated energy
gaps agreed well with the experimental photophysical data,
suggesting that density functional theory (DFT) calculations
(Egopt
transition of the molecules. The optical band gaps
of
)
the series were calculated from the relevant absorption onsets
and found to be 3.88, 3.76, 3.76, and 3.76eV, respectively.33
The photoluminescence (PL) spectra of compounds
1a–d in CHCl3 solution and in solid-state films obtained
by spin-coating a CHCl3 solution are shown in Figure 2.

Hou et al.
5
can be used to predict the photophysical behavior of these
complexes to some extent. The electron density of the
HOMO of 1a–d is predominantly localized on the naphtha-
lene donor moiety and diffusively distributed over the π-
conjugation onto the 1,3,4-oxadiazole ring.
Experimental
NMR spectra were recorded in CDCl3 with a Bruker
AV-400 spectrometer. Chemical shifts were referenced rel-
ative to tetramethylsilane (δH/δC =0). Infrared (IR) spectra
were recorded on a Perkin Elmer 2400 spectrophotometer
(using KBr pressed disks). Mass spectra were collected on
an Autoflex Speed™ matrix-assisted laser desorption/
ionization time-of-flight (MALDI-TOF) spectrometer.
Elemental analyses were performed on an Elementar Vario
EL CHN elemental analyzer. UV-Vis spectra were recorded
on a Lambda 25 spectrophotometer in CHCl3. Fluorescence
spectra were obtained from a Shimadzu RF-5301PC fluo-
rescence spectrophotometer in CHCl3. CV was conducted
using a CHI 852C instrument with 10−3 M solutions using
CH2Cl2 as the solvent (10−3) and containing 0.1M Bu4NPF6
as the supporting electrolyte. Counter and working elec-
trodes consisted of a Pt wire and a Pt disk, respectively, and
the reference electrode was Ag/AgCl. The thermal stability
of the target compounds was characterized using a Diamond
TG/DTA thermogravimetric analyzer. Starting compounds
2–5 were synthesized according to the methods described
in the literature.31,32 The commercially available 6-alkoxy-
2-naphthoic acids 6b–d were purchased from J&K
Chemical Company.
The thermal stability of a material is important for device
longevity. Figure 5 and Table 1 show the results of thermo-
gravimetric analysis (TGA) of compounds 1a–d. The TGA
analysis revealed that the compounds possessed relatively
high thermal stability with an initial weight loss (5%) tem-
perature of 400–434°C. The glass transition temperature (Tg)
values of 1a–d were in the range of 87–100°C. The thermal
stabilities of the compounds progressively decreased as the
substituent alkoxy chain lengthened. The high thermal stabil-
ity of 1a–d would benefit the formation of amorphous films.
In summary, we developed a facile and effective method
to synthesize V-shaped organic fluorescent compounds.
These synthesized compounds do not aggregate in the con-
densed state as shown by their similar properties either in
solution or in film states. These emitters are solution process-
able and exhibit good film-forming ability as well as high
thermal stability, making them attractive for use in OLEDs.
1.2
1a
1b
1c
1d
1.0
0.8
0.6
0.4
0.2
0.0
General procedure for the preparation of
compounds 1a–d
Amixture of the appropriate 2-naphthoic acid 6a–d (2mmol),
4,4′-(dimethylsilanediyl)di(benzohydrazide) (1mmol), and
phosphorus oxychloride (10mL) was heated under reflux for
16h under N2, then cooled to room temperature, and poured
onto crushed ice. The resulting dark green solid precipitate
was filtered and then washed thoroughly with distilled ice
water. The solid was recrystallized from DMF/H2O to obtain
the pure final compound.
Dimethylbis{4-[5-(naphthalen-2-yl)-1,3,4-oxadiazol-
2-yl]phenyl}silane (1a): White solid (Yield: 32%), m.p.
249.60°C (by differential scanning calorimetry (DSC)); 1H
NMR (400MHz, CDCl3): δ 0.69 (s, 6H), 7.55–7.64 (m,
4H), 7.74 (d, J=8.0Hz, 4H), 7.88–7.94 (m, 2H), 8.00 (d,
290
300
310
320
330
340
350
360
370
380
Wavelength(nm)
Figure 1. UV-Vis absorption spectra of compounds 1a–d in
CHCl3 solution.
UV: ultraviolet.
1000
䠄㼍䠅
(b)
500
1a
1b
1a
800
1b
1c
400
1c
1d
1d
600
400
200
0
300
200
100
0
300
400
500
600
700
800
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
Figure 2. Normalized PL spectra of compounds 1a–d in (a) CHCl3 and (b) as films.
PL: photoluminescence.

6
Journal of Chemical Research 43(1-2)
J=8.0Hz, 4H), 8.15–8.25 (m, 6H), 8.64 (s, 2H); 13C NMR 1079, 830, 779; MALDI-TOF MS m/z (%)=601.1372
(100MHz, CDCl3): δ −2.50, 121.38, 123.45, 124.92, (M++1, 100); anal. calcd for C38H28N4O2Si: C, 75.97; H,
126.39, 127.31, 127.56, 128.18, 129.03, 129.26, 133.09, 4.70; found: C, 75.62; H, 4.76%.
134.95, 134.98, 142.52, 164.85, 165.09; IR (KBr), νmax
/
Bis{4-[5-(6-ethoxynaphthalen-2-yl)-1,3,4-oxadiazol-
cm−1: 2968, 1626, 1497, 1408, 1346, 1292, 1250, 1118, 2-yl]phenyl}dimethylsilane (1b): White solid (Yield: 35%),
1
m.p. 227.21°C (by DSC); H NMR (400MHz, CDCl3):
δ 0.67 (s, 6H), 1.50 (t, J=6.8Hz, 6H), 4.18 (q, J=6.8Hz,
-5
6x10
4H), 7.16 (d, J=2.0Hz, 2H), 7.21 (dd, J=8.9, 2.3Hz, 2H),
-5
5x10
7.71 (d, J=8.0Hz, 4H), 7.85 (t, J=9.8Hz, 4H), 8.11–8.19
(m, 6H), 8.53 (s, 2H); 13C NMR (100MHz, CDCl3):
-5
4x10
δ −2.50, 14.89, 63.86, 106.90, 119.01, 120.49, 123.99,
-5
3x10
124.99, 126.31, 127.34, 127.88, 128.40, 130.52, 134.95,
-5
2x10
136.55, 142.37, 158.90, 164.57, 165.27; IR (KBr), νmax
/
-5
1x10
cm−1: 2981, 1629, 1555, 1543, 1504, 1399, 1259, 1209,
1115, 1045, 811, 779, 731; MALDI-TOF MS
m/z (%)=689.1630 (M++1, 100); anal. calcd for
C42H36N4O4Si: C, 73.23; H, 5.27; found: C, 73.19;
H, 5.32%.
0
-5
-1x10
-5
-2x10
Bis{4-[5-(6-butoxynaphthalen-2-yl)-1,3,4-oxadiazol-
2-yl]phenyl}dimethylsilane (1c): White solid (Yield: 39%),
m.p. 214.16°C (by DSC); H NMR (400MHz, CDCl3):
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E/v(vs.SCE)
1
δ 0.68 (t, 6H), 1.02 (t, J=7.2Hz, 6H), 1.49–1.62 (m, 4H),
1.81–1.92 (m, 4H), 4.12 (t, J=6.8Hz, 4H), 7.18
(d, J=1.6Hz, 2H), 7.23 (dd, J=8.8Hz, 2.4Hz, 2H), 7.72
(d, J=8.0Hz, 4H), 7.86 (t, J=9.8Hz, 4H), 8.13–8.21 (m,
6H), 8.55 (s, 2H); 13C NMR (100MHz, CDCl3): δ −2.49,
Figure 3. CV of 1c (1×10−3 M) in 0.1M tetrabutylammonium
hexafluorophosphate in CH2Cl2 at a scan rate of 0.1Vs−1
(inset: differential pulse voltammogram of 1c under the same
conditions).
CV: cyclic voltammetry.
Table 1. Photophysical, physical, and electrochemical data.
Compound
aAmax,Abs (nm)
aλmax,em (nm)
ΦF
Ered/peak (V)
Eox/peak (V)
Egopt
EgCal
Td
Tg
1
1a
1b
1c
1d
316
329
330
330
366
382
382
382
0.712
0.715
0.716
0.716
−0.65
−0.88
−0.89
−0.89
1.36
1.53
1.53
1.51
3.98
3.83
3.82
3.82
4.43
4.15
4.15
4.16
416
409
406
402
87.2
99.1
95.8
92.6
Egopt
ΦF is the fluorescence quantum yield, which was measured in CHCl3 using quinine sulfate as a standard (ΦF =0.55);34
is the optical band gap,
which was calculated using Eg =1240/λabsonset; EgCal is the calculated energy gap; Td represents the temperature at which 5% weight loss occurred; Tg
represents the glass transition temperature.
aMeasured in CHCl3.
Figure 4. Contour plots of the HOMO and LUMO for complexes 1a–d.
HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.

Hou et al.
7
(No. 21442004 and 21502008) and the Education Office of Jilin
province (No. 2016320).
120
110
100
90
Supplemental material
The supplemental material with 1H and 13C NMR spectra, TOF-MS
data, cyclic voltammograms, and TGA data for 1a–d is available
online.
80
70
60
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Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
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The author(s) disclosed receipt of the following financial support
for the research, authorship and/or publication of this article: This
work was supported by the National Science Foundation of China