Synthesis and aggregation-induced emissions of thienyl substituted cyclobutene derivatives

Article abstract of DOI:10.1039/c4tc00221k

Thienyl substituted cyclobutenes with aggregation induced emission behaviour have been synthesized for the first time in good selectivity, opening the possibility of cyclobutenes for application in light emitting diodes.

Full text of DOI:10.1039/c4tc00221k

Volume 2 Number 26 14 July 2014 Pages 5067–5236
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
ISSN 2050-7526
COMMUNICATION
Yonggang Zhen, Wenping Hu et al.
Synthesis and aggregation-induced emissions of thienyl substituted
cyclobutene derivatives

Journal of
Materials Chemistry C
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Synthesis and aggregation-induced emissions of
thienyl substituted cyclobutene derivatives†
Cite this: J. Mater. Chem. C, 2014, 2,
5083
*
Xiaotao Zhang, Xiuqiang Lu, Yonggang Zhen, Jie Liu, Huanli Dong, Guangyao Zhao,
*
Ping He, Zongrui Wang, Lang Jiang and Wenping Hu
Received 3rd February 2014
Accepted 8th March 2014
DOI: 10.1039/c4tc00221k
www.rsc.org/MaterialsC
Thienyl substituted cyclobutenes with aggregation induced emission from rubrenes by traditional methods and suffer low selectivity
behaviour have been synthesized for the first time in good selectivity, and poor yields for cyclobutenes relative to rubrenes.18–20
opening the possibility of cyclobutenes for application in light emitting
In this work, we rstly established a modied synthetic
method to achieve cyclobutenes in a much higher yield and
selectivity compared with rubrenes. By changing the haloge-
nating agent to acid anhydride and using N,N-diisopropyle-
thylamine as the base, we obtained two new derivatives of
dimethylene-cyclobutene with excellent AIE properties, both of
which emit yellow-green light. Moreover, from the single crystal
structure, the aryl groups of cyclobutene show a classical rigid
structure with the intramolecular rotation being impeded,
which meet the requirements of AIE properties.
1,2-Diaryl-3,4-bis(diarylmethylene)-1-cyclobutene (4a and
4b) was synthesized in two steps as shown in Scheme 1. Firstly,
arylacetylene (1) reacted with diaryl ketone 2a and 2b to give the
intermediate product 3a and 3b in yields of 75% and 70%
respectively. Secondly, the dimerization of the propargyl alco-
hols afforded the target compound 4a and 4b in yields of 30%
and 31% respectively.
diodes.
Aggregation-caused quenching (ACQ), occurring when organic
materials are highly uorescent in their dilute solutions but
become weakly uorescent with the increase of solution
concentration or aggregation in the solid state, is a notorious
phenomenon in organic luminescent materials.1,2 Unfortu-
nately, for practical applications, such as organic light emitting
diodes (OLEDs), the solid state (powders, lms or crystals)
provides the best candidates for functional layers in electronic
devices.3–5 So luminescent materials with aggregation-induced
emission (AIE) properties have attracted much interest since the
debut of the AIE concept in 2001.6–13
The AIE phenomenon is caused by a restriction of vibrational
and rotational motion in aggregation states, keeping a distorted
conformation for a single molecule and conning the rotation
in the solid state by dense packing.14–16 1,2-Diphenyl-3,4-bis-
(diphenylmethylene)-1-cyclobutene (HPDMCb), which was iso-
lated from a commercial product, rubrene, was found to exhibit
AIE properties by Tang's group.17 Regrettably, HPDMCb is the
only cyclobutene compound that exhibits AIE properties and
has found application in OLEDs. It is interesting and important
to investigate substituent dependant AIE properties of cyclo-
butene derivatives. Meanwhile, there are few studies on the
synthesis of cyclobutene derivatives, which are oen separated
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail:
zhenyg@iccas.ac.cn; huwp@iccas.ac.cn
† Electronic supplementary information (ESI) available: Synthetic details of
compounds 4a and 4b, TGA of compounds 4a and 4b, oxidation cyclic
voltammogram of 4a and 4b, lifetime of compounds 4a. CCDC 973433. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4tc00221k
Scheme 1 Synthetic route of compounds 4a and 4b.
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Halogenating agents, such as SOCl2 and MesCl, were used in
the dimerization of propargyl alcohols in previous research of
rubrene.21–24 We swapped halogenating agents for acetic anhy-
dride, which is a good leaving group, then obtained compounds
4a and 4b in a 2–3 fold higher yield than previously reported.
The relative yield ratio of cyclobutenes 4a and 4b to the rubrene
analogue is 3 : 1 and 5 : 1 respectively. Generally, the electron-
donating aryl group can lead to cyclobutenes in high selectivity.
Surprisingly we demonstrated that electron-accepting aryl
substituents achieved cyclobutenes in much higher yield than
rubrenes.25
Usually, organic compounds with four-membered rings are
thought to be unstable because the tension of the ring is larger
than that of ve or six-membered rings, so rst we check the
stability of compounds 4a and 4b under ambient conditions. No
change was detected aer it was put on a shelf under normal
laboratory lighting for 18 months or exposed to the irradiation
of a UV light of 365 nm for 24 h. Their thermal stability was
measured under nitrogen atmosphere at a heating rate of 10 ꢀC
minÀ1. The decomposition temperature of 4a was 298 ꢀC while
that of compound 4b was 276 ꢀC, so both of them are thermally
stable (see Fig. S1 in ESI†).
Fig. 2 (A) PL image of solutions of 4a with different water fractions
under UV (365 nm) light. (B) PL spectra of 4a in different water–THF
(v/v) mixtures. (C) The dependence of the PL intensity on the
composition of the solution (inset: photo of the luminescent solid
powder under 365 nm). The concentration was kept at 30 mM,
excitation wavelength: 332 nm.
Fig. 1 shows the absorption spectra of compounds 4a and 4b
in CH2Cl2. The compounds exhibit similar absorption charac-
teristics. According to the maximum absorption edges, their
energy gaps are calculated to be 2.76 eV. According to their
cyclic voltammogram curves (see Fig. S2 in ESI†), their HOMO
levels are calculated to be À5.31 eV for compound 4a and À5.67
eV for compound 4b respectively. The LUMO values for 4a and
4b are calculated to be À2.55 eV and À2.91 eV, respectively.
Therefore, the introduction of peruorophenyl lowered the
HOMO and LUMO levels by 0.36 eV. The absorption peaks at
around 330 nm (4a) and 325 nm (4b) are the initial absorption
peaks, which could be used as the excitation wavelength in
order to observe their uorescence properties.
increased at the same dye concentration under identical
conditions. With the increasing water content, the emission
maximum wavelength of compound 4a exhibits a slight red
shi from about 510 nm to 540 nm, while that of compound 4b
became signicantly red shied by 145 nm from 405 nm to 550
nm, which is probably due to the increased solvent effects by the
introduction of uorinated phenyl groups and the electronic
interactions between the phenyl and uorinated phenyl rings.
Their PL peak intensity was up to 530-fold for 4a and 39-fold for
4b, which was measured at the same dye concentration under
identical measurement conditions. Compared to HPDMCb,
both exhibit a red shi of more than 40 nm, which corresponds
well with the fact that HPDMCb emits green light while
To conrm the AIE properties of these compounds we
measured their photoluminescence (PL) spectra. The dilute
solution in THF gave an almost at line parallel to the abscissa
(Fig. 2 and 3). In order to induce aggregation of the dye, a
nonsolvent of the dye is necessary. Water was chosen to induce
the aggregation of these compounds. When a large proportion
of water is added into the solution, the intensity of the PL peaks
Fig. 3 (A) PL image of solutions of 4b solutions with different water
fractions under UV (365 nm) light. (B) PL spectra of 4b in different
water–THF (v/v) mixtures. (C) The dependence of the PL intensity on
the composition of the solution (inset: photo of the luminescent solid
powder under 365 nm). The concentration was kept at 30 mM, exci-
tation wavelength: 340 nm.
Fig. 1 UV-vis spectra of compounds 4a and 4b.
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compounds 4a and 4b emit yellow-green light. Moreover, in the
solvent mixture with 90% water content, the uorescent inten-
sity of both compound 4a and 4b increased signicantly
whereas that of HPDMCb reduced sharply, indicating that
thienyl substituted cyclobutenes have excellent AIE properties.
To acquire more convincing results, the absolute uorescence
quantum yield was tested in a calibrated integrating sphere. The
uorescence quantum yield of compounds 4a and 4b in solution
was below the detection limit (<0.5%), while the solid state
showed a signicantly enhanced uorescence efficiency of 29%
for compound 4a and 11% for compound 4b respectively under
identical conditions. The introduction of peruorophenyl groups
can enhance the polarity of compound 4b, which possibly leads
to emission quenching in the solid state.
The decay proles of the uorescence lifetime26,27 of
compound 4a in solution, powder and crystal forms were recor-
ded individually (as shown in Fig. S3, ESI†) The lifetime in
solution is 0.63 Æ 0.01 ns, while lifetimes in powder and crystal
forms are 1.82 Æ 0.06 ns and 1.83 Æ 0.11 ns. The three fold
increase of lifetime also corresponds well with the AIE properties.
Single crystals of compounds 4a were obtained from petro-
leum through evaporation slowly at room temperature (CCDC
973433, Table S2 in ESI†). As shown in Fig. 4, it is obvious that
the intramolecular rotation process of the phenyl and thienyl
groups in compound 4a is impeded. The restriction of the
intramolecular rotation in the aggregates is a classic feature of
AIE materials, so compound 4a is rationalized to be a light
emitting material that can be used in photoluminescence and
electroluminescence devices. Since the molecular structure of
compound 4b is similar to compound 4a, and it exhibits good
AIE properties with yellow-green light emission.
Fig. 5 (A) Changes in current density and luminance with applied
biases. (B) Current efficiency vs. current density in a multilayer EL
device with the configuration ITO/NPB/HPDMCb/Alq3/LiF/Al.
In summary, we developed an efficient and selective
synthetic method towards thienyl substituted cyclobutenes 4a
and 4b, which exhibit red-shied AIE spectra in the region of
450–550 nm and better electroluminescence performance with
luminance up to ꢁ21 000 cd mÀ2 relative to phenyl substituted
cyclobutene, HPDMCb. Our present work provides a useful
guideline for the design of cyclobutene derivatives with AIE
properties. Further studies on the cyclobutenes for applications
in photoluminescence and electroluminescence devices is
under way in our lab.
The authors thank for Prof. Hongbing Fu, Dr Yishi Wu and
Ms Huiying Liu for discussions and measurements of uores-
cence lifetime and quantum yields for the cyclobutene deriva-
tives. The authors also acknowledge the nancial support from
the National Natural Science Foundation of China (51303185,
21021091, 51033006, 51222306, 51003107, 61201105, 91027043,
91222203, 91233205), the China-Denmark Co-project, TRR61
(NSFC-DFG Transregio Project), the Ministry of Science and
Technology of China (2011CB808400, 2011CB932300,
2013CB933403, 2013CB933500, 2014CB643600) and the
Chinese Academy of Sciences.
A multilayer OLED with a device conguration of ITO
(Indium Tin Oxides)/NPB (N,N'-Bis-(1-naphthalenyl)-N,N'-bis-
phenyl-(1,1'-biphenyl)-4,4'-diamine) (50 nm)/compound 4a
(40 nm)/Alq3 (8-Hydroxyquinoline aluminum salt) (20 nm)/LiF
(1 nm)/Al (120 nm) was fabricated using a vapor deposition
processes, in which NPB and Alq3 served as hole- and electron-
transport layers, respectively. The device is turned on at ꢁ14 V
and emits a green-yellow light at 540 nm (Fig. 5). Its luminance
reached ꢁ21 000 cd mÀ2 at ꢁ22 V, which is much higher than
that of HPDMCb (13 000 cd mÀ2). The maximum current effi-
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