Tetraphenylpyrazine-based AIEgens: Facile preparation and tunable light emission

Article abstract of DOI:10.1039/c4sc03365e

Research on aggregation-induced emission (AIE) has been a hot topic. Due to enthusiastic efforts by many researchers, hundreds of AIE luminogens (AIEgens) have been generated which were mainly based on archetypal silole, tetraphenylethene, distyrylanthrac

Full text of DOI:10.1039/c4sc03365e

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Tetraphenylpyrazine-based AIEgens: facile
preparation and tunable light emission†
Cite this: DOI: 10.1039/c4sc03365e
Ming Chen,a Lingzhi Li,a Han Nie,b Jiaqi Tong,a Lulin Yan,d Bin Xu,d Jing Zhi Sun,a
d
b
ab
abc
*
*
Wenjing Tian, Zujin Zhao, Anjun Qin
and Ben Zhong Tang
Research on aggregation-induced emission (AIE) has been a hot topic. Due to enthusiastic efforts by many
researchers, hundreds of AIE luminogens (AIEgens) have been generated which were mainly based on
archetypal silole, tetraphenylethene, distyrylanthracene, triphenylethene, and tetraphenyl-1,4-butadiene,
etc. To enlarge the family of AIEgens and to enrich their functions, new AIEgens are in high demand. In
this work, we report a new kind of AIEgen based on tetraphenylpyrazine (TPP), which could be readily
prepared under mild reaction conditions. Furthermore, we show that the TPP derivatives possess a good
thermal stability and their emission could be fine-tuned by varying the substituents on their phenyl rings.
It is anticipated that TPP derivatives could serve as a new type of widely utilized AIEgen, based on their
facile preparation, good thermo-, photo- and chemostabilities, and efficient emission.
Received 3rd November 2014
Accepted 11th December 2014
DOI: 10.1039/c4sc03365e
www.rsc.org/chemicalscience
luminescent in their dilute solutions, but emit efficiently when
aggregated or fabricated into solid lms.2 Due to their twisted
Introduction
conformation, the peripheral rotors, such as phenyl rings, of the
AIEgens rotate against the central stator, such as (hetero)
aromatic rings or vinyl groups, to annihilate the excitons in a
non-radiative fashion in solution, while in the aggregate state
this rotation is greatly restricted, which in turn opens up a
radiative channel and makes the molecules emissive.3 This
intriguing property enables AIEgens to be potentially applied in
highly efficient optoelectronic devices, uorescent sensors, cell
imaging, and so on.
Organic light-emitting materials are one of the key factors in the
development of efficient organic light-emitting diodes, organic
lasers, and uorescent sensors, etc. However, the thorny
problem encountered for the traditional organic light-emitting
molecules is the well-known concentration/aggregation-caused
quenching, which greatly limits the enhancement of device
performance. One of the methods to surmount this difficulty is
to let the aggregation play positive instead of negative roles in
enhancing the efficiency in molecule emission in the
condensed phases. Indeed, such a strategy has been success-
fully developed and is called aggregation-induced emission
(AIE).1
Thanks to enthusiastic efforts by many scientists, a lot of
AIEgens with versatile functionalities and diverse applications
Compared with traditional organic emitters, AIE luminogens
(AIEgens) are a burgeoning class of conjugated molecules with
propeller-shaped structures. They are weakly or non-
aMOE Key Laboratory of Macromolecular Synthesis and Functionalization,
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
310027, China. E-mail: qinaj@zju.edu.cn
bGuangdong Innovative Research Team, State Key Laboratory of Luminescent
Materials and Devices, South China University of Technology, Guangzhou 510640,
China
cDepartment of Chemistry, Institute for Advanced Study, Institute of Molecular
Functional Materials, and State Key Laboratory of Molecular Neuroscience, The
Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong
Kong, China. E-mail: tangbenz@ust.hk
dState Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, China
† Electronic
supplementary
information
(ESI)
available:
Synthesis,
Chart 1 Molecular structures of AIEgens of hexaphenylsilole (HPS),
tetraphenylethene (TPE), distyrylanthracene (DSA), triphenylethene,
tetraphenyl-1,4-butadiene (TPBD) and tetraphenylpyrazine (TPP).
characterization, TGA curves, PL spectra, and crystal structures of TPP and its
derivatives. CCDC 1031716–1031719. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4sc03365e
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have been developed (Chart 1).4,5 Currently, most AIEgens are
derivatives of silole,6 tetraphenylethene (TPE),7 distyrylanthracene
(DSA),8 triphenylethene,9 and tetraphenyl-1,4-butadiene (TPBD).10
Although these molecules could be regarded as the archetypal
AIEgens, their intrinsic disadvantages should not be ignored. For
example, silole and its derivatives are troublesome in preparation,
especially in their purication, and unstable under basic con-
ditions;10a TPE, DSA, triphenylethene, TPBD and their derivatives
contain double bonds, which may lead to potential photooxida-
tion and photobleaching. The involved E/Z isomerization of the
contained double bonds also complicates the mechanistic
understanding, though we and others have theoretically and
experimentally proven that the restriction of intramolecular
rotation (RIR) is the cause for the AIE effect.3 Thus, new AIE cores,
which are easy to synthesize and functionalize under mild reac-
tion conditions and stable upon exposure to light and heat and
under acidic and basic conditions, are highly desirable.11
In 2009, we developed a new AIEgen, 2,3-dicyano-5,6-diphenylpyr-
azine (DCDPP), which was synthesized under very mild reaction
conditions in high yield by simply heating a mixture of dione and
diamine in acetic acid.12 It is worth noting that there is no vinyl group
present in DCDPP, which makes it more stable and greatly simplies
mechanism elucidation. Furthermore, the pyrazine ring is an electron-
decient group and could enrich the properties of the resultant
AIEgens.13
Scheme
derivative.
1
Synthetic routes to tetraphenylpyrazine (TPP) and its
via three steps from commercially available bromo-substituted
aromatic boronic acid and phenylacetonitrile using three
successive catalytic systems of Ni/Zn, PhI(OH)OTs, and
AcONH4/AcOAc. Aerwards, the Suzuki coupling reaction
between the resultant intermediate and aromatic boronic acid
14 readily furnished the TPP derivatives with donor–p–acceptor
structures in satisfactory yields. Detailed synthetic procedures
are provided in the ESI.†
Inspired by the facile preparation and encouraged by the AIE
properties of DCDPP, in this work, we report a new type of
AIEgen, tetraphenylpyrazine (TPP), which could be synthesized
using a one-pot procedure under mild reaction conditions (its
structure is shown in Chart 1). It is worth noting that the TPP
core can be readily functionalized by varying the structure of the
starting molecules or using post-reactions of the bromo-
substituted TPP with aromatic boronic acid via Suzuki coupling,
which provides an ideal platform to ne-tune the light emission
of the resultant molecules from deep-blue to pure blue.
All TPP derivatives were characterized using 1H and 13C NMR
spectra (Fig. S1–S24, ESI†) and high resolution mass spectra
(Fig. S25–S30, ESI†). Satisfactory data corresponding to their
structures were obtained. These TPP derivatives structurally
exclude any unstable vinyl groups and are thermally stable.
Taking TPP as an example, the temperature for 5% loss of its
weight (Td) is 275 ꢀC, which is quite similar to that of HPS and
DSA, but is 62 ꢀC higher than that of TPE (Fig. S31, ESI†).
Interestingly, functionalization of TPP could further enhance Td
ꢀ
of the resultant products by 60 C (Fig. S32, ESI†), which will
increase the device stability if the compounds are applied in
Results and discussion
Synthesis of TPP and its derivatives
In general, there are two routes to prepare TPP and its deriva-
tives (Scheme 1). In route A, the commercially available starting
compound benzoin (1) or anisoin (2) could be readily converted
to TPP or its methoxy-substituted derivative, TPP-4M, in the
presence of acetic anhydride and ammonium acetate aer
reuxing in acetic acid for only 3.5 h,14 whereas route B is
similar to that in our previous report.12,15 Mixing benzil and 1,2-
diphenylethane 1,2-diamine in acetic acid and reuxing the
solution for 4 h produced TPP in moderate yield. More impor-
tantly, the produced TPP and its derivatives obtained via both
routes could be puried using recrystallization and no tedious
column chromatography was needed, making these routes the
simplest for the preparation of AIEgens.
To verify the universality of these routes, we applied them to
synthesize various TPP derivatives. In this paper, we will focus
on route A and demonstrate its advantages (Scheme 2).Route B
will be used in another work. The bromo-substituted interme-
diates 12 and 13, shown in Scheme 2, could be readily prepared Scheme 2 Synthetic routes to TPP-based AIEgens.
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this eld. It is worth noting that unlike HPS, TPP remains intact
under both acidic and basic conditions as suggested by their
remaining PL intensities in the aggregate states (Fig. S33, ESI†).
Moreover, unlike the photo-oxidizable DSA (Fig. S34, ESI†), no
new peak could be found in the 1H NMR spectrum of TPP aer it
was irradiated with a UV lamp at 365 nm with a power of 1.10
mW cmꢁ2 for 2 h (Fig. S35, ESI†), demonstrating that it
possesses good photostability.
Absorption of TPP-based AIEgens
Aer conrming the structures of TPP and its derivatives, we
investigated their photophysical properties. It is well-known
that pyrazine is an electron withdrawing group, thus, substitu-
tion of the phenyl rings of TPP with additional phenyl rings or
electron-donating methoxyl groups will extend the conjugation
and facilitate the charge transfer from the phenyl or methoxyl
groups to the central pyrazine ring, and in turn red-shi the
maximum absorption (lab) (Fig. 1 and Table S1, ESI†). Indeed,
from TPP to TPP-2P, TPP-2PM and TPP-2MPM, enlarging the
conjugated system and introducing D–A structures by incorpo-
rating phenyl and methoxyl groups have red-shied lab gradu-
ally from 338 to 362 nm. Meanwhile, when the methoxyphenyl
groups were directly attached to the pyrazine ring (TPP-4M, TPP-
2MP, and TPP-2MPM), longer lab values with very little differ-
ence (2–3 nm) were observed, suggesting that the charge
transfer plays a crucial role in controlling the absorption.
Aggregation-enhanced emission
By inspecting the structures of the resultant molecules, we can
nd that the phenyl rings are connected to the pyrazine core via
single bonds. According to the RIR mechanism of AIE, these
TPP derivatives could also feature the AIE effect. To conrm
this, we studied their photoluminescence (PL) behaviours
(Fig. 2 and S36–S40, ESI†). Fig. 2A shows the PL spectra of TPP
in THF–water mixtures with different water fractions (fw) as an
example. The emission of TPP is weak in THF solution and
increases slowly until fw reaches 60%. Aerwards, the emission
intensied swily. Other TPP derivatives behave similarly: they
Fig. 2 (A) PL spectra of TPP in THF–water mixtures with different
water fractions; lex ¼ 338 nm (10 mM). (B) Variation in the PL intensity of
TPP and its derivatives in THF–water mixtures with different water
fractions.
are weakly emissive in THF solution and in THF–water mixtures
with fw lower than 60%. When fw was beyond 60%, their emis-
sion enhanced remarkably. Since water is a poor solvent for TPP
and its derivatives, the addition of water will induce the
formation of nanoaggregates with sizes in the range of 31 to 287
nm (Table S2†). Thus, TPP and its derivatives feature the unique
aggregation-enhanced emission (AEE) characteristics.
Moreover, when carefully inspecting Fig. 2B, we could
observe that only the PL intensities of TPP and TPP-2P continue
to increase in the THF–water mixtures with a fw of 90%. The
decrease in PL intensity of the other TPP-based AIEgens is
related to the presence of methoxyl groups, which probably lead
to the formation of less emissive amorphous nanoaggregates
via random packing of the molecules in THF–water mixtures
with 90% water fraction.16
It is worth noting that the emission enhancement of TPP-2P,
TPP-2PM, TPP-2MPM and TPP-2MP is much higher than that of
TPP (Fig. 2B) due to the presence of additional phenyl rings,
Fig. 1 UV-vis absorption of TPP and its derivatives in THF.
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which will reinforce the RIR. The absolute quantum yield (FF)
measurements showed that these AIEgens possess high FF
values in the range of 8.3–30.7% and most of them are above
15% (Table S1, ESI†).
Interestingly, the highest emission enhancement was
recorded for TPP-4M in a THF/water mixture with a fw of 80%,
which is 49-fold higher than that in THF solution. Accordingly,
the highest absolute FF (30.7%) was also recorded for TPP-4M.
The reason might be related to the molecular packing as dis-
cussed below.
Single crystal packing
To understand the emission behaviours of these AIEgens, we
tried to grow their single crystals. Delightfully, single crystals of
TPP (CCDC 1031716), TPP-2P (CCDC 1031717), TPP-2PM (CCDC
1031718), and TPP-4M (CCDC 1031719) suitable for X-ray
diffraction analyses were obtained. The analysis showed that all
these TPP-based AIEgens adopt non-planar conformations with
twisted angles between the phenyl rotors and pyrazine stators in
the range of 33 to 66ꢀ (Fig. S41–S44, ESI†).
Fig. 4 PL spectra of the thin films of TPP and its derivatives. The
excitation wavelengths were selected based on the maximum
absorption in THF, as shown in Table S1.†
In a detailed study on the molecular packing in the single
crystals, we could nd that multiple intermolecular C–H/p
2MPM at ca. 460 nm, whereas, in the thin-solid lm, the
emission peaks of these AIEgens remain almost unchanged due
to the propeller-shaped molecular structures. Interestingly,
from solution to lm, the maximum emission peaks of TPP-2P
and TPP-2PM red-shied from 423 and 438 nm to 442 and 453
nm, whereas TPP-2MP and TPP-4M blue-shied from 455 and
433 nm to 440 and 428 nm, respectively. The red-shi of
emission peaks of TPP-2P and TPP-2PM implies that the
number of additional phenyl rings connected to the TPP core
plays a crucial role, which will probably cause additional
intermolecular interactions going from solution to lm. The
blue-shi of the emissions of TPP-2MP and TPP-4M indicates
that the charge-transfer from the methoxyl groups to the pyr-
azine ring was alleviated. Thus, these results suggest that the
emission of TPP-based AIEgens could be ne-tuned by varying
the substituents on TPP.
˚
interactions with distances in the range of 2.82–3.18 A existed
(Fig. 3). Such interactions could assist in locking the molecular
motion in the crystal lattice and reducing the non-radiative
deactivation of excitons.12 Moreover, the multiple C–H/p
interactions in TPP-4M are much stronger than those in TPP
because the interaction distances are much shorter in the
former, which helped to enhance the molecular rigidity, limit
the conformational freedom much more effectively and turn on
its emission to a greater magnitude.
Tunable blue emission
Thanks to the facile functionalization, different TPP-based
AIEgens emitting light from deep blue to pure blue could be
obtained. As shown in Fig. 4 and S45 as well as Table S1 (ESI†),
TPP in THF emits with a maximum peak at ca. 390 nm and TPP-
Conclusions
In conclusion, a new type of AIEgen, TPP and its derivatives, was
rationally designed according to the mechanism of RIR and the
compounds were readily synthesized under mild reaction
conditions. The TPP-based AIEgens simplify the mechanistic
understanding because they do not contain any controversial
vinyl groups. Furthermore, these AIEgens possess high thermo-,
photo- and chemostabilities because they contain no active
double bonds, as compared to TPE and DSA, and no instable Si–C
bonds as compared to HPS under basic conditions. The emission
of these AIEgens could be ne-tuned from deep-blue to pure blue
by varying the substituents on the pyrazine rings. It is worth
pointing out that the AEE-active TPP-based derivatives with
electron accepting substituents, such as cyano-groups could also
be readily prepared in high yield via method A (Scheme S1,
Fig. S46 and S47), further manifesting the universality of this
synthetic method. Moreover, thanks to the electron withdrawing
Fig. 3 Single crystal packing diagrams and intermolecular C–H/p
interactions of (A) TPP and (B) TPP-4M.
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ability of the pyrazine moiety, green or red emission is antici-
pated to be realized by attaching stronger electron donating
groups onto the periphery of TPP. Thus, we believe that these
TPP-based AIEgens will not only enrich the family of AIEgens and
nd wide application in light-emitting diodes, chemosensors and
bioprobes etc., but also attract considerable interest from scien-
tists in the areas of fundamental photophysical research and
materials science.
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This work was partially supported by the key project of the
Ministry of Science and Technology of China (2013CB834702),
the National Science Foundation of China (21490571, 21222402
and 21174120), and the Research Grants Council of Hong Kong
(16301614, N_HKUST604/14 and N_HKUST620/11). A.J.Q. and
B.Z.T. thank for the support from the Guangdong Innovative
Research Team Program (201101C0105067115).
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