Suppression of aggregation-induced fluorescence quenching in pyrene derivatives: Photophysical properties and crystal structures

Article abstract of DOI:10.1016/j.tetlet.2011.01.060

Two new pyrene-based fluorophores, namely 1-[4-(2,2-diphenylvinyl)phenyl] pyrene (PVPP) and 1,3,6,8-tetrakis[4-(2,2-diphenylvinyl)phenyl]pyrene (TPVPP), were synthesized through Suzuki coupling reaction and well characterized. PVPP successfully suppresses the fluorescence quenching of pyrene units in the solid state, displaying aggregation-induced enhanced emission. Despite the same substituent, TPVPP shows a different fluorescent behavior. On the basis of the crystal structures, the distinct optical behavior is discussed and clarified. The intermolecular C-H?π interaction has a dramatic effect on their photophysical properties in the solid state.

Full text of DOI:10.1016/j.tetlet.2011.01.060

Tetrahedron Letters 52 (2011) 1329–1333
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Suppression of aggregation-induced fluorescence quenching in pyrene
derivatives: photophysical properties and crystal structures
Zuo-Qin Liang a, Ye-Xin Li a, Jia-Xiang Yang b, Yan Ren a, Xu-Tang Tao a,
⇑
a State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
b Department of Chemistry, Anhui University, Hefei 230039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new pyrene-based fluorophores, namely 1-[4-(2,2-diphenylvinyl)phenyl]pyrene (PVPP) and 1,3,6,
8-tetrakis[4-(2,2-diphenylvinyl)phenyl]pyrene (TPVPP), were synthesized through Suzuki coupling reac-
tion and well characterized. PVPP successfully suppresses the fluorescence quenching of pyrene units in
the solid state, displaying aggregation-induced enhanced emission. Despite the same substituent, TPVPP
shows a different fluorescent behavior. On the basis of the crystal structures, the distinct optical behavior
Received 2 December 2010
Revised 30 December 2010
Accepted 14 January 2011
Available online 21 January 2011
is discussed and clarified. The intermolecular C–Hꢀ ꢀ ꢀ
p
interaction has a dramatic effect on their photo-
Keywords:
Pyrene
physical properties in the solid state.
Ó 2011 Elsevier Ltd. All rights reserved.
Fluorescence quenching
Aggregation-induced enhanced emission
Crystal structure
1. Introduction
is responsible for the AIE phenomenon. In this work, we designed
and synthesized two novel light emitting materials, namely
1-[4-(2,2-diphenylvinyl)phenyl]pyrene (PVPP) and 1,3,6,8-tetra-
kis[4-(2,2-diphenylvinyl)phenyl]pyrene (TPVPP), by introducing
1,1-diphenylvinyl onto the pyrene ring. Exhilaratingly, PVPP suc-
cessfully suppresses the fluorescence quenching of pyrene units
in the solid state, displaying aggregation-induced enhanced emis-
sion (AIEE). Despite the same substituent, TPVPP shows a distinct
fluorescent behavior. The different optical behaviors are thought
The design and synthesis of efficient, stable organic fluorescent
materials have been an active topic in the area of organic light-
emitting diodes (OLEDs).1 Among various materials, pyrene has
attracted significant attention as a candidate in OLEDs because of
its excellent fluorescent properties,2 such as long fluorescence life-
time and pure blue fluorescence. However, it shows a great pro-
pensity to form
p
aggregates/excimers in concentrated solution
aggregates/excimers
and the solid state,3 and the formation of
p
to be related to the intermolecular C–Hꢀ ꢀ ꢀ
p interactions, which is
causes the quenching of fluorescence, leading to low photolumi-
nescence (PL) quantum yields.4 As a result, many efforts have been
made to improve the photophysical properties of pyrene.5 So far,
the pyrene derivatives that have been reported still present some
degree of fluorescence quenching in the solid state.6 Therefore,
the preparation of compounds based on pyrene that emits effi-
ciently in the solid state is a continuing objective. In 2001, an
intriguing phenomenon of aggregation-induced emission (AIE) in
siloles was observed.7 Later, several other compounds bearing this
character have been reported.8 AIE-active materials are faintly
emissive in solutions, but their aggregates or solid states are
strongly luminescent. This interesting discovery opens a new path
to developing highly efficient solid emitter based on pyrene.9
Very recently, 4,40-bis(2,2-diphenylvinyl)-1,10-biphenyl (DPVBi)
and its analogs have been found to be AIE-active.10 It is believed
that 1,1-diphenylvinyl with unique three-dimensional structure
supported by an X-ray structural analysis.
2. Results and discussion
The synthetic routes of PVPP and TPVPP are shown in Scheme 1.
Compound 1 and 2 were prepared according to the method
reported in the literature.11 The boronic acid moiety of compound
3 was introduced by addition of n-butyllithium and trimethylb-
orate to the THF solution of compound 2. PVPP and TPVPP were
prepared by
a palladium-catalyzed Suzuki coupling reaction
between pyrene bromide and compound 3. The yields of PVPP
and TPVPP were up to 73% and 93%, respectively. Moreover, both
final products exhibit excellent thermal stabilities. Their decompo-
sition temperatures are found to be 405.7 °C for PVPP and 524.8 °C
for TPVPP.
To determine whether PVPP and TPVPP were AIEE-active or not,
spectrometric verification tests were performed in various frac-
tions of THF/water mixtures. Water was used as a nonsolvent for
PVPP and TPVPP in THF. Concentration of the solution was kept
⇑
Corresponding author. Tel.: +86 531 88364963; fax: +86 531 88364518.
E-mail address: txt@sdu.edu.cn (X.-T. Tao).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.060

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Z.-Q. Liang et al. / Tetrahedron Letters 52 (2011) 1329–1333
Scheme 1. Synthetic routes to PVPP and TPVPP.
at 5
l
M. The absorption spectra of PVPP in water/THF mixtures are
drastic decrease in aggregates. In the case of 90% volume fractions
of water addition, the fluorescence intensity of TPVPP suspension
particles is almost 10 times lower than that of TPVPP solution in
THF. As a result, TPVPP is AIEE-inactive.
shown in Figure 1a. Compound PVPP exhibits a prominent absorp-
tion band at about 354 nm in pure THF. This band is associated
with the intramolecular p–
p⁄ transitions. When the water content
was increased to 74%, the absorption spectra were slightly red-
shifted and exhibited Mie light scattering. At the same time, the
absorption intensity started to become weaker. It demonstrated
that molecules began to aggregate in the solvent with this compo-
sition. The absorption intensity continued to reduce with increas-
ing water content. This is because that the formation of solid
particles decreases the concentration of the solution and the
absorption intensity depends on the concentration of the com-
pound dissolved in the solvent.
When compound PVPP was forced to aggregate in the mixture
solvents of THF and water, a dramatic enhancement of fluores-
cence intensity was observed. The fluorescent behavior caused by
aggregate is illustrated in Figure 1b. The fluorescence intensity
increased abruptly, accompanied by a slight red-shift of the emis-
sion peak, when the content of the mixture was increased from
74% to 95%. The relationship between the PL peak intensity of PVPP
and the solvent composition of THF/water mixtures is depicted in
the inset of Figure 1b. The fluorescence of PVPP in aggregates is
apparently stronger than that of PVPP solution in THF. So PVPP is
AIEE-active.
A totally different fluorescence behavior between PVPP and
TPVPP (AIEE versus quenching) is obtained. To understand the rela-
tionship between optical properties and crystal structures, the
crystal structures of PVPP and TPVPP were determined by single
crystal X-ray diffraction analysis. The important crystallographic
data are listed in Table S1.
Colorless crystals of PVPP suitable for X-ray diffraction were
obtained by very slow evaporation of a dichloromethane/ethanol
solution. The molecular structure of PVPP and its packing arrange-
ment in single crystal are given in Figure 3. A noteworthy feature of
this molecule in crystal is its torsion conformation as shown in Fig-
ure 3a. The twist angle between the pyrene ring and the phenyl
ring (C17–C22) is 51.4°. The two terminal phenyl rings are almost
perpendicular to each other with a twist angle of 89.3°. The bigger
twist angle may be caused by the intramolecular C–Hꢀ ꢀ ꢀ
p interac-
tion. The C–Hꢀ ꢀ ꢀ interaction distance and the angle between
p
C(19)–H(19) and the terminal phenyl ring (C25–C30) are 2.88 Å
and 147°, respectively. In addition, the vinyl double bond and the
middle phenyl ring don’t lie in the same plane with a twist angle
of 11.3°. These twist angles can bring the steric hindrance effect
to hinder the tight intermolecular packing. In the packing struc-
ture, there are two types of stacking modes for pyrene moieties,
as shown in Figure 3b. One is that two neighboring pyrene moieties
are stacked in an off-set fashion, with a perpendicular distance of
3.53 Å and a sliding angle of 23.7°. The other is that two adjacent
pyrene moieties are completely separated by two neighboring
4-(2,2-diphenylvinyl)phenyl substituents. For the first stacking
mode, eight of the edge pyrenyl carbons in each PVPP are involved
The absorption and PL spectra of compound TPVPP are shown in
Figure 2. TPVPP exhibits two main absorption bands at 339 and
407 nm in THF which are attributed to the characteristic vibronic
pattern of the pyrene moiety and the intramolecular
p–
p⁄ transi-
tions, respectively. The longest absorption band is red-shifted by
53 nm in comparison with the monosubstituted PVPP. This should
be ascribed to the enlarged
p-conjugated system. With the
increase of water content, the absorption and fluorescence spectra
reveal the same change trend. In contrast to the fluorescent behav-
ior of compound PVPP, the emission intensity of TPVPP shows a
for such
p–p interactions. In the pyrene crystals, the molecules
exhibit an interplanar separation of ca. 3.5 Å with strong
p–p

Z.-Q. Liang et al. / Tetrahedron Letters 52 (2011) 1329–1333
1331
Figure 2. (a) UV–visible absorption spectra of TPVPP (5 ꢁ 10ꢂ6 mol Lꢂ1) in pure
THF and in a 90% water/THF mixture. (b) PL spectra of TPVPP (5 ꢁ 10ꢂ6 mol Lꢂ1) in
pure THF and in a 90% water/THF mixture.
Figure 1. (a) UV–visible absorption spectra of PVPP (5 ꢁ 10ꢂ6 mol Lꢂ1) in THF/
water mixtures with different volume fractions of water. (b) PL spectra of PVPP
(5 ꢁ 10ꢂ6 mol Lꢂ1) in THF/water mixtures with different volume fractions of water.
Inset shows the relationship between the PL peak intensity of PVPP and the water
fractions in THF.
structure, the key stacking forces are still the intermolecular
C–Hꢀ ꢀ ꢀ
p interaction, without intermolecular p–p interactions, as
shown in Figure 4b. The molecules are packed into molecular col-
stacking through the involvement of 14 carbons in each pyrene
molecule.12,5c Therefore, 4-(2,2-diphenylvinyl)phenyl plays an
umns along the b-axis (Figure S2) and the molecular columns are
important role in suppressing the
planar pyrene moieties. The key driving forces of the molecular
packing for PVPP are intermolecular C–Hꢀ ꢀ ꢀ interactions. There
are three different kinds of C–Hꢀ ꢀ ꢀ hydrogen bonds between adja-
interactions I
and II are the key stacking forces for the second stacking mode of
pyrene moieties. Furthermore, these interactions may help to hin-
der the rotation of the terminal phenyl rings.
As for TPVPP, single crystals were prepared by vaporizing a mix-
ture of chloroform and hexane slowly at room temperature. Its
structure diagram is shown in Figure 4a. The molecule lies on a
twofold axis, thus half is crystallographically unique. Similarly,
TPVPP also adopts torsion conformation. The four phenylene rings
are not coplanar with the central pyrene ring and have twist angles
of 57.7° for the phenyl ring (C15–C20) and 54° for the phenyl ring
(C29–C34), relative to the central pyrene ring. The twist angle be-
tween the phenyl rings (C37–C42 and C43–C48) is 88.1°, and that
between the phenyl rings (C1–C6 and C7–C12) is 73.2°. The former
p
aggregates formation between
connected together by C–Hꢀ ꢀ ꢀ
p interactions (interaction IV, V,
and VI). Moreover, the three kinds of interactions may hinder the
rotation of the terminal phenyl rings. For each molecular column,
it is arranged due to the interaction VII.
p
p
cent molecules as shown in Figure 3c. The CAHꢀꢀꢀ
p
A direct comparison of the two crystal structures clearly indi-
cates that molecular packing modes and intermolecular interac-
tions are influenced by the number of substituents. The distinct
fluorescent behavior between PVPP and TPVPP is mainly caused
by the different intermolecular interactions. In the crystal of PVPP,
the free twisting motion of 1,1-diphenylvinyl is restricted by the
intra- and intermolecular C–Hꢀ ꢀ ꢀ
p hydrogen bonds. Thus, the non-
radioactive decay channel is closed. More importantly, the molec-
ular structure of PVPP is significantly distorted and the twisted
geometry can prevent the strong
tense fluorescence in the solid state. For TPVPP, its substituents
are also fixed and there is no interaction in the crystal. How-
ever, TPVPP is still AIEE-inactive. The weaker fluorescence in the
inter-
p–p interaction to produce in-
p–p
solid state should be ascribed to the intermolecular CAHꢀꢀꢀ
p
is larger than the latter, while the intramolecular C–Hꢀ ꢀ ꢀ
p
interac-
actions. The molecule lies on a twofold axis in the crystal and has
tion between C(31)–H(31) and phenyl ring (C37–C42) is smaller
than that between C(16)–H(16) and phenyl ring (C7–C12). It sug-
gests that the twist angle of terminal phenyl rings may also be con-
four identical substituents. These structural features indicate the
presence of considerable C–Hꢀ ꢀ ꢀ
(12 interactions for each molecule). Although the C–Hꢀ ꢀ ꢀ
gen bond is a relatively weak intermolecular force, the sum of
p
packing forces in the crystal
p
hydro-
nected with the intermolecular C–Hꢀ ꢀ ꢀ
p interaction. In the packing

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Z.-Q. Liang et al. / Tetrahedron Letters 52 (2011) 1329–1333
Figure 3. (a) Molecular structure of PVPP. (b) Representation of two different arrangements observed in the case of pyrene moieties. (c) Weak C–Hꢀꢀꢀ
p
interactions between
adjacent molecules of PVPP. The interaction distance and the angle of C–Hꢀꢀꢀ
interaction III are 2.89 Å and 131°, respectively.
p for interaction I are 2.64 Å and 96°, those for interaction II are 2.77 Å and 91°, and those for
multiple C–Hꢀ ꢀ ꢀ
influence the optical properties of materials, especially for organic
molecules bearing
groups.13 In order to prove this interpretation,
p
interactions can become significant, and may
In conclusion, we have synthesized two novel light emitting
materials PVPP and TPVPP, and determined their crystal structures
by X-ray diffraction. PVPP exhibits an interesting aggregation-in-
duced enhanced emission behavior. Despite the same substituent,
TPVPP shows a different fluorescent behavior. The distinct fluores-
cent behavior was discussed by analyzing their crystal structures.
The results manifest that the weaker fluorescence of TPVPP in
p
the PL properties of PVPP and TPVPP in crystalline state were
investigated, as shown in Figure 5. The crystalline powder of PVPP
emits at 454 nm, with only a 9 nm red-shift in its emission maxi-
mum as compared to that in THF. TPVPP shows an emission max-
imum at 469 nm in THF (Figure 2b) and at 489 nm in the
crystalline state. The longer red-shift is observed in the PL spectra
of TPVPP, which indicates that there are larger intermolecular
interactions in the crystal state.14 The observation is in good agree-
ment with our analysis in their packing structures.
the solid state is ascribed to multiple intermolecular C–Hꢀ ꢀ ꢀ
p inter-
actions. For PVPP, it has successfully suppressed aggregation-in-
duced fluorescence quenching of pyrene units, and shows bright
blue emission in the solid state. Therefore, it is a promising candi-
date for fabricating efficient blue OLEDs. In addition, these mole-

Z.-Q. Liang et al. / Tetrahedron Letters 52 (2011) 1329–1333
1333
Figure 5. Normalized PL spectra of PVPP and TPVPP in crystalline state.
Acknowledgments
This work was supported by the State National Natural Science
Foundation of China (Grant Nos. 50721002, 50802054, and
50873001) and the 973 Program of the People’s Republic of China
(Grant No. 2010CB630702).
Supplementary data
Supplementary data (synthetic procedures, characterization,
and crystal packing diagram, crystallographic data) associated with
this Letter can be found, in the online version, at doi:10.1016/
j.tetlet.2011.01.060.
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Figure 4. (a) Molecular structure of TPVPP. (b) Weak C–Hꢀ ꢀ ꢀ
p
interactions between
for
adjacent molecules of TPVPP. The interaction distance and the angle of C–Hꢀ ꢀ ꢀ
p
interaction IV are 2.87 Å and 128°, those for interaction V are 2.90 Å and 116°, those
for interaction VI are 2.83 Å and 126°, and those for interaction VII are 2.86 Å and
132°, respectively.
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cules provide a possible design strategy to prevent the aggrega-
tion-induced fluorescence quenching of pyrene units.