Organic fluorescent compounds that display efficient aggregation-induced emission enhancement and intramolecular charge transfer

Article abstract of DOI:10.3390/molecules23061446

A series of symmetric sulfone-linked organic fluorescent compounds (1a-c) was synthesized and characterized. V-shaped 1a-c were designed as aggregate of intramolecular charge transfer (ICT) and aggregation-induced emission enhancement (AIEE) processes. The 1a-c emitted intense blue violet lights in normal solvents. A large red shift of the emission wavelength and dramatic decrease of emission efficiency occurred with increasing solvent polarity. The 1a-c will function well as electron transport and blue light-emitting materials through theoretical calculations.

Full text of DOI:10.3390/molecules23061446

molecules
Article
Organic Fluorescent Compounds that Display
Efficient Aggregation-Induced Emission
Enhancement and Intramolecular Charge Transfer
Ruibin Hou 1,2 , Baohua Zhao , Yan Xia 1,2,* and Dongfeng Li 1,*
ID
1
1
School of Chemistry and Life Science, Changchun University of Technology, Changchun 130012, China;
hrb1018@163.com (R.H.); zbh1969@126.com (B.Z.)
Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, China
2
*
Correspondence: hmz20130521@163.com (Y.X.); lidongfeng@mail.ccut.edu.cn (D.L.);
Tel.: +86-431-8571-6671 (Y.X.); +86-431-8571-6671 (D.L.)
Academic Editor: Jwo-Huei Jou
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 29 May 2018; Accepted: 8 June 2018; Published: 14 June 2018
Abstract: A series of symmetric sulfone-linked organic fluorescent compounds (1a–c) was synthesized
and characterized. V-shaped 1a
and aggregation-induced emission enhancement (AIEE) processes. The 1a
violet lights in normal solvents. A large red shift of the emission wavelength and dramatic decrease of
emission efficiency occurred with increasing solvent polarity. The 1a will function well as electron
–
c
were designed as aggregate of intramolecular charge transfer (ICT)
–
c
emitted intense blue
–c
transport and blue light-emitting materials through theoretical calculations.
Keywords: 1,3,4-oxadiazole; aggregation-induced emission enhancement; blue light-emitting material
1
. Introduction
Fluorescent organic molecules are widely used as active elements in organic light-emitting diodes
(OLED), chemosensors, and bioprobes [1–5]. However, although most of these fluorescent organic
molecules possess planar conjugated structures and display reasonable emission in dilute solutions,
their emission is weak or nonexistent in the solid state because of aggregation-caused quenching.
Such quenching limits the applications of fluorescent organic molecules [
groups have independently reported an interesting phenomenon called aggregation-induced emission
AIE), wherein aggregated molecules emit more intense fluorescence than their dilute solutions [ 11].
6–8]. Recently, multiple
(
9–
Propeller-shaped molecules such as hexaphenylsilole, tetraphenylethene, and quinoline-malononitrile
and their derivatives are known for their unique ability to promote AIE [12].
The integration of AIE and intramolecular charge transfer (ICT) into one molecule has attracted much
attention. This type of luminophore can emit strong fluorescence not only in the aggregated form or
solid state but also in dilute solution [13–15]. For example, Tang and coworkers developed a new donor
(D)–acceptor (A) system that exhibits dual photoluminescence (PL) activity, including ICT and AIE [16].
However, the integration of AIE and ICT into one molecule is still rare and poorly understood, and thus
warrants further study to clarify the relationship between molecular structure and optical properties.
Herein, we report a series of new V-shaped D-
blue emission in both solution (nonpolar solvents) and the solid state. We integrated a sulfone into
because it has been demonstrated that sulfones possess a special V-shaped structure, which is
helpful to weaken stacking. A 1,3,4-oxadiazole was also included in 1a because it is an attractive
electron-accepting material that has received considerable interest for use in OLEDs.
π-A-π-D fluorophores (1a–c) that exhibit strong
1a–c
π–
π
–c
Molecules 2018, 23, 1446; doi:10.3390/molecules23061446
www.mdpi.com/journal/molecules

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2
. Results and Discussion
2
.1. Synthesis of Target Compounds 1a–c
The synthetic routes to 1a
diphenylsulfone ( ) was synthesized according to a previous report [13]. The target compounds
were obtained by condensation reactions between intermediate and 4-alkoxybenzoic acid in
phosphoryl chloride under reflux. Compounds 1a were all white crystals and their yields ranged
from 37% to 43%. The structures of 1a were characterized by H and C-NMR spectroscopy,
mass spectrometry, and elemental analysis.
–c are described in Scheme 1. The intermediate p,p’-dihydrazinocarbonyl
4
1
a
–
c
4
–
c
1
13
–
c
Scheme 1. Synthetic route to 1a–c.
2
.2. The Optical Properties of Compounds 1a–c
The optical properties of 1a in solution and the solid state were investigated by UV-vis
–
c
absorption and PL spectroscopies. The data revealed that the alkyl chain did not influence the
properties of these compounds. Figure S5 presents the UV-vis absorption spectra of the same
concentration of 1a
–
c
in different solvents. The absorption maxima (
λab) exhibit a slight red shift of a
few nanometers with increasing solvent polarity. In contrast, the PL maxima of 1a
–c
vary dramatically
with changes in solvent polarity, as shown in Figure 1 and Figure S6. The PL intensity gradually
decreased with increasing solvent polarity and fluorescent quantum yields (Qys) decreased from
0.42 to 0.05 when the solvent was changed from toluene to DMSO. Emission maxima (λem) red-shifted
from 387 nm in toluene to 468 nm in DMSO. The red shift and decreased QY can be ascribed to
stabilization of the excited state. This behavior originates from the strong ICT character from the
alkoxyl benzene to the sulfone. This result indicates that the ICT excited state has a larger dipole
moment than the ground state, which is caused by substantial charge redistribution. Because of the
dipole–dipole interaction of a polar solvent and excited state, the ICT state is more stable in a polar
solvent than in a nonpolar one.
To determine whether 1a
mixtures in THF/water and DMF/water mixtures with different water fractions (ƒw) were measured
Figure 2 and Figure S7). Figure 2 reveals that 1a in pure THF solution emitted intense indigo blue
–c are AIE-active, fluorescence emission spectra of their diluted
(
fluorescence at 427 nm, which was dramatically quenched when a small amount of water was added,
as evidenced by its weak emission in the THF/water mixtures with ƒw ≤ 40 vol%. Meanwhile,
λem red-shifted to about 449 nm upon addition of water, which was mainly attributed to the increase of
solvent polarity and enhancement of the ICT effect. If a hydrophobic environment was created inside
the aggregates, the ICT effect should be alleviated, and the fluorescence should recover because of the
AIE effect. The pure DMF solution of 1a displayed weak blue fluorescence at λem = 458 nm. When ƒw
of the DMF/water mixture was low, λem of 1a was dramatically quenched and slightly blue-shifted.
When ƒw of the DMF/water mixture was high, λem of 1a was red-shifted and the fluorescence intensity
did not recover. In this case, the solvent polarity influences the AIE effect. These data illustrate our

Molecules 2018, 23, 1446
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speculation that 1a is a luminogen with both ICT and AIE characteristics. Both 1b and 1c exhibited
similar AIE effects, as shown in Figure S7.
−
5
−1
Figure 1. PL emission spectra of 1a in different solutions at the same concentration (10 mol/L ,
excitation wavelength: 322 nm).
Figure 2. ) plot of
−1
peak intensity of 1a versus ƒw in the THF/water mixtures (10 mol L , excitation wavelength: 322 nm).
(a
) PL spectra of 1a in THF/water mixtures with different water fractions (ƒw); and (
b
−
5
·
Such AIE properties were also confirmed by the vivid image of the solid powders under 365 nm
UV illumination, as depicted in Figure 3. The solid powder and 1a dissolved in toluene emitted strong blue
fluorescence. Conversely, the emission spectra of the solution and film samples were different, considering
the broad emission peaks and red shift in Figure 3. Compound 1a exhibited dual-state emission behaviors
and that demonstrated its fluorescence was strong in both solution and the aggregated state.
Figure 3. PL spectra of 1a. Inset are photographs of ground powder of 1a and 1a in toluene under
3
65 nm illumination.

Molecules 2018, 23, 1446
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2
.3. The Electrochemical Properties of 1a–c
The electrochemical properties of 1a
–c in CHCl were investigated by CV. The electrochemical data
3
are summarized in Table 1. Figure 4 displays representative CV traces for the oxidation and reduction
of 1a, demonstrating that 1a undergoes a quasi-reversible reduction at the cathodic potential associated
with the reduction of the electron-deficient sulfone moiety to form an anion radical. Compound 1a also
exhibits an irreversible oxidation process corresponding to the removal of electrons from the terminal
alkoxy phenyl group to form a radical cation.
−
3
Figure 4. Cyclic voltammograms of 1a in CHCl (1 × 10 M).
3
Table 1. Photophysical, physical, and electrochemical data for 1a–c.
a
a
b
Ered/peak
(V)
ox/peak
dEgopt
(eV)
e
λ
λmax,em
(nm)
λ
E1
Tg/T
( C)
max,Abs
nm)
max,em
c
5d
Comp.
Φ
F
◦
(
(nm)
(V)
1
a
323
326
328
412
411
411
420
423
423
c
0.42
0.36
0.32
−0.73
−0.74
−0.76
0.62
0.75
0.78
4.18
4.21
4.26
119/396
156/383
145/386
1
b
1
c
a
. b Measured for film. Measured in CHCl
Measured in CHCl
3
3
with quinine sulfate as a standard (QY = 0.54) [17].
d
Calculated using Eg = 1240/λ onset. e Analyzed by DSC/TGA at a heating rate of 10 C·min under N2.
abs
◦
−1
2
.4. Theoretical Calculations
All calculations were carried out using the Gaussian 09 package [18]. Calculations of the
electronic ground state of compound 1a
with the B3LYP exchange-correlation functional [20
GaussView 5.0.8 was used to calculate the interface of the structures and manipulate orbital.
–
c
were carried out using density functional theory (DFT) [19
]
,
21] together with the 6–31 G basis set [22].
DFT was used to calculate molecular orbitals to understand the photophysical behavior of the
synthesized compounds. To estimate the charge-carrier injection barriers, we investigated the changes
that occurred upon frontier orbital substitution. Figure 5 shows the highest occupied molecular
orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and energy gap (E ) of 1a in toluene,
g
THF and DMSO. The electron density distributions of representative 1a change considerably between
the HOMO and LUMO. The electron density distributions of the HOMO are mostly located over the
g
alkoxy benzene groups, whereas the LUMOs are mainly concentrated on the sulfone core. E between
the HOMO and LUMO decreases from 4.03 to 3.97 eV from toluene to DMSO, which is consistent with
the slight red shift of 1 nm of the UV-vis absorption peak in polar solvents. Generally, this type of
dye molecule with such an electron distribution indicates ICT character, which is consistent with the
experimental results.

Molecules 2018, 23, 1446
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Figure 5. Electron density contours and orbital energies calculated for the HOMO and LUMO of 1a at
the Beck3LYP/DZP level in toluene, THF and DMSO.
The calculated vertical ionization potential IP (IP
v
), adiabatic IP (IP
a
), vertical electron affinity
(
v a
EA ), and adiabatic electron affinity (EA ) for 1a are listed in Table 2. The reorganization energy (λ)
can be used to estimate the charge transport rate and balance between holes and electrons; these results
are also provided in Table 2. The reorganization energy for electron transport can be expressed
in terms of the electron extraction potential (EEP): λe = EEP
−
v
EA . EEP represents the electron
extraction potential, which is the energy difference between M and MM- (anionic) when considering
M-. It was found that representative 1a had a low λe value of 0.28, which is similar to that of
tris(8-hydroxyquinolinato)aluminum (λe = 0.276) [23] and good electron transfer ability. These results
indicate that 1a may be a promising electron transport material.
Table 2. Ionization potentials, electron affinities, extraction potentials, internal reorganization energies,
and ∆ = |λ − λe| (in eV) for the studied complexes.
h
Comp.
IP (v)
IP (a)
HEP
EA (v)
EA (a) EEP
λ
λ
∆
hole
electron
1
a
6.21
6.18
6.17
6.07
6.07
6.07
5.90
5.90
5.90
2.35
2.29
2.29
2.49
2.49
2.49
2.63
2.63
2.63
0.31
0.28
0.27
0.28
0.34
0.34
0.03
0.06
0.07
0.034
1
b
1
c
Alq3
IP (5.8)
EA (3.0)
0.242
0.276
The thermal properties of 1a–c were evaluated by thermal gravimetric analysis (TGA)
and differential scanning calorimetry (DSC) under nitrogen atmosphere. (Table 1 and Figure S4).
The TGA revealed that the compounds possessed relatively high thermal stability with an initial
◦
weight loss (5%) temperature of 383–396 C. The glass transition temperatures of 1a
–
c
, as shown in
◦
◦
◦
Table 1, were 119 C, 156 C and 145 C, respectively. The excellent thermal stability of 1a–c
should aid
the formation of amorphous films.
3
. Experimental
General Information
Ultraviolet-visible (UV-vis) spectra were recorded on a Lambda 25 spectrophotometer.
Fluorescence spectra were obtained using a Shimadzu RF-5301PC fluorescence spectrophotometer.
Cyclic voltammetric studies were carried out using a CHI 852C instrument with CHCl as the solvent
3
−
10 M) and 0.1 M Bu NClO as the supporting electrolyte. Counter and working electrodes
4 4
3
(

Molecules 2018, 23, 1446
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consisted of a Pt wire and glass carbon, respectively, and the reference electrode was Ag/AgCl.
The thermal stability of the target compounds was characterized using a Shimadzu DTG-60H
thermogravimetric analyzer.
Starting compound
2
and p,p’-Dihydrazinocarbonyl diphenylsulfone (3) were synthesized
according to literature method [13].
Representative procedure for the synthesis of 1a
–
c
: A mixture of compound (2.2 mmol),
2
p,p’-Dihydrazinocarbonyl diphenylsulfone (1 mmol), and phosphorous oxychloride (8 mL) was heated
at reflux for 8 h, and then poured onto crushed ice after cooling to room temperature. The white solid
was obtained. The crude product was separated on a flash silica gel column (CHCl /Ethyl acetate)
2
to afford 1a–c.
p,p’-Bis[2-(4-Ethoxyphenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone (1a): White powder, yield: 40%.
◦
1
Mp 329 C (DSC). H-NMR (400 MHz, CDCl ):
7
δ
(ppm) 1.44 (t, J = 7.2 Hz, 6H), 4.11 (q, J =7.2 Hz, 4H),
.01 (d, J = 8.8 Hz, 4H), 8.04 (d, J = 9.0 Hz, 4H), 8.12 (d, J = 9.0 Hz, 4H), 8.27 (d, J = 8.8 Hz, 4H); 13C-NMR
100 MHz, CDCl3): (ppm) 14.65, 63.85, 115.12, 115.61, 127.67, 128.60, 128.79, 128.94, 143.37, 162.21,
3
(
δ
62.51, 165.45; FTIR (KBr, cm 1): 3079, 2920, 1615, 1489, 1066, 842; TOF-Ms (m/z) [M + H] = 596.1648;
−
+
1
Anal. Calcd for C H N O S: C, 64.64. H, 4.41. Found: C, 64.71. H, 4.48.
32
26
4
6
◦
p,p’-Bis[2-(4-Butoxyphenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone (1b): White powder, yield: 43%. Mp 325 C
1
(
DSC). H-NMR (400 MHz, CDCl ):
δ
(ppm) 0.98 (t, J = 7.2 Hz, 6H). 1.52 (m, 4H), 1.79 (m, 4H), 4.03
3
(
4
1
[
t, J = 6.4 Hz 4H), 7.01 (d, J = 8.8 Hz, 4H), 8.04 (d, J = 8.8 Hz, 4H), 8.13 (d, J = 8.8 Hz, 4H), 8.28 (d, J = 8.8 Hz,
13
H); C-NMR (100 MHz, CDCl ): δ (ppm) 13.79, 19.19, 31.14, 68.06, 115.14, 115.54, 127.67, 128.60, 128.80,
3
−1
28.92, 143.37, 162.43, 162.50, 165.47; FTIR (KBr, cm ): 3080, 2920, 1620, 1486, 1069, 840; TOF-Ms (m/z)
M + H] = 651.1227; Anal. Calcd for C H N O S: C, 66.45. H, 5.27. Found: C, 66.46. H, 5.34.
+
36 34 4 6
p,p’-Bis[2-(4-Hexoxyphenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone (1c): White powder, yield: 22%.
◦
1
Mp 282 C (DSC). H-NMR (400 MHz, CDCl ):
δ
(ppm) 0.91 (t, J = 7.2 Hz, 6H), 1.38 (m, 24H),
3
1
4
6
.83 (m, 4H), 4.05 (t, J = 6.4 Hz, 4H), 7.03 (d, J = 8.8 Hz, 4H), 8.06 (d, J = 8.8 Hz, 4H), 8.14 (d, J = 8.8 Hz,
H), 8.29 (d, J = 8.8 Hz, 4H); 13C-NMR (100 MHz, CDCl3):
δ (ppm) 14.04, 22.63, 25.72, 29.12, 31.59,
−
1
8.44, 115.20, 115.59, 127.73, 128.67, 128.87, 128.98, 143.42, 162.49, 165.54; FTIR (KBr, cm ): 3080, 2924,
+
1622, 1486, 1069, 844; TOF-Ms (m/z) [M + H] = 707.1256; Anal. Calcd for C H N O S: C, 67.97, H,
40
42
4
6
5
.99. Found: C, 67.99. H, 6.03.
4
. Conclusions
A series of D-π-A-π-D type sulfone derivatives with alkoxy D groups and sulfone A units was
produced. The experimental results demonstrate that these V-shaped compounds display the features
of both ICT and AIE. These blue emitters are solution processible and exhibit good film-forming ability
and high thermal stability, marking them promising as electron transport materials.
Supplementary Materials: Figures S1–S4: Characterization of compounds 1a
–
c
, Figure S5: Absorption spectra
of 1a , Figure S6: PL emission spectra of 1b, Figure S7: PL emission spectra of 1a,
–
c
1b in DMF/H O mixtures,
2
Figure S8: Cyclic voltammograms of 1a, Figure S9: Electron density contours and orbital energies calculated for
the HOMOs and LUMOs of 1b,1c.
Author Contributions: Study design, data analysis and writing manuscript, R.H.; Literature search, data analysis
and manuscipt modification, B.Z.; Data collection and data analysis, manuscript modification, Y.X.; Study design,
direct and writing, D.L.
Funding: This research was funded by [the National Science Foundation of China] grant number [21442004 and
2
01502008] and [the National Natural Science Foundation of Jilin Province] grant number [20101548].
Acknowledgments: This work was supported by the National Science Foundation of China (Nos. 21442004 and
01502008) and the National Natural Science Foundation of Jilin Province (grant No. 20101548).
2
Conflicts of Interest: The authors declare no conflict of interest.

Molecules 2018, 23, 1446
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Sample Availability: Samples of the compounds are available from the authors.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).