Aggregation induced emission in the rotatable molecules: The essential role of molecular interaction

Article abstract of DOI:10.1039/c2jm32405a

Aggregation-induced emission (AIE) compounds, dye2 and dye3, with a rotatable N-N single bond and aza-crown-ether, have been synthesized. The fluorescence intensities of both dyes are very weak in THF, while become extraordinarily strong in a mixture of H₂O-THF (v/v 95%). They are increased by 88.9 and 70.7 times, respectively, indicating enhanced blue-green fluorescence emissions. The λ_em of these two compounds in different states has been well studied. Besides this, dye1 and dye2 were characterized by single crystal X-ray structural determination. The results show that molecular interactions are formed in the particles, which considerably restrict the intramolecular vibration and rotation. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm32405a

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PAPER
Aggregation induced emission in the rotatable molecules: the essential role of
molecular interaction†
Zhipeng Yu, Yaya Duan, Longhuai Cheng, Zhili Han, Zheng Zheng, Hongping Zhou,* Jieying Wu
and Yupeng Tian
Received 18th April 2012, Accepted 21st June 2012
DOI: 10.1039/c2jm32405a
Aggregation-induced emission (AIE) compounds, dye2 and dye3, with a rotatable N–N single bond
and aza-crown-ether, have been synthesized. The fluorescence intensities of both dyes are very weak in
THF, while become extraordinarily strong in a mixture of H
2
O–THF (v/v 95%). They are increased by
8
8.9 and 70.7 times, respectively, indicating enhanced blue-green fluorescence emissions. The lem of
these two compounds in different states has been well studied. Besides this, dye1 and dye2 were
characterized by single crystal X-ray structural determination. The results show that molecular
interactions are formed in the particles, which considerably restrict the intramolecular vibration and
rotation.
Introduction
species, while rapidly increase in poor solvents or when made
into thin films by aggregating into nanoparticles. Research has
been devoted to the AIE systems and the structure–properties
8
relationships, trying to gain insight into their working mecha-
Recently, multifunctional materials, especially the organic
materials have been widely applied in electroluminescence
1
2
3
devices, optical sensors and biological fields. The advantages
of the organic materials are multifarious compared to polymers,
such as strong fluorescence, high purity and controllable design
strategies. The traditional studies of organic luminescent mate-
nisms. For example, Tang reported a series of AIE-active TPE
9
10
derivatives and explored their applications as sensors, blue
11
12
organic light-emitting diodes and in vivo imaging. Tian
reported a group of triaryl-amine derivatives, which exhibit
aggregation-induced emission and large two-photon absorption
4
rials are usually conducted in the dilute solutions. The solution
investigations have made great contributions to the under-
standing of the fundamental science under luminescence at
molecular levels, while the insufficiency cannot be ignored. The
conclusions drawn from dilute solution data are not applicable to
concentrated ones. It’s well known that luminescence is often
quenched at high concentrations, referred to as ‘‘aggregation-
13
cross sections and are potential non-doped red emitters for
14
organic light-emitting diodes and gas chemodosimeters.
Aprahamian developed a new kind of AIE-molecule, BF2-
hydrazone, which can be easily transformed into a solid-state
15
acid/base sensor. However, the working mechanisms of the AIE
processes are so complicated that they still remain unclear,
although various theories have been advanced to explain the AIE
16
5
caused quenching’’ (ACQ), thus limited its applications in
real world which requires materials to be solid and phragmoid
films.
phenomenon, such as planarity and rotatability, intramolecular
17
18
restrictions, intermolecular interactions, ACQ-to-AIE trans-
Since the AIE phenomenon was first noticed by Tang and
6
coworkers in 2001, aggregation-induced emission (AIE) has
19
formation etc. The intramolecular rotation, one of the most
9c,20
influential mechanisms, was researched in depth.
While most
attracted broad scientific interest by offering a new mechanism
for researchers to explore new kinds of luminogens in recent
of the studies are based on steric hindrance, Tong and
21
coworkers reported a compound with AIE attributes which are
6a,7
years.
The main characteristic of luminescent materials with
due to the intramolecular hydrogen bonds.
AIE attributes, exactly the opposite characteristic of the ACQ
effect, is that the fluorescence intensities are low as isolated
In this paper, dye2 and dye3 were synthesized (Fig. 1) with
aza-crown-ether as the substituent, in considering its good
solubility, electron donating ability and crown structure. Dye1
was fabricated without aza-crown-ether substituent for
comparison in order to further discuss the role of the aza-
crown-ether structure in the AIE systems. The advantages of
this design strategy are as follows: (1) two conjugate groups are
connected by rotatable N–N single bond rather than C–C bond;
College of Chemistry and Chemical Engineering, Anhui University and Key
Laboratory of Functional Inorganic Materials Chemistry of Anhui
Province, 230039 Hefei, PR China. E-mail: zhpzhp@263.net; Fax: +86-
551-5107304; Tel: +86-551-5108151
† Electronic supplementary information (ESI) available. CCDC
reference numbers 856816 for dye1, 848148 for dye2. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2jm32405a
(
2) the rotatable flexible aza-crown-ether is the substituent
rather than a rigid structure.
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Fig. 1 The target molecules.
Experimental section
Scheme 1 Synthetic routes to dye1, dye2 and dye3.
Materials
Tetrahydrofuran (THF) was distilled under normal pressure
from sodium benzophenone ketyl under nitrogen immediately
prior to use. Dimethylformamide (DMF) was distilled and dried
over molecular sieves type 4A. Quinine sulfate was purchased
appropriate amount of THF, water was added dropwise under
vigorous stirring to furnish 5 mL THF–H O mixtures with water
2
fractions (f ) of 0–95 vol%. The absorption and emission spectra
w
of the resultant mixtures were measured immediately.
0
from Exciton Inc. Sodium hydride and 2,2 -(phenylimino)dieth-
anol were purchased from Aldrich. Diethylamine, triethyene
glycol, tetraethylene glycol, 4-fluoro-benzaldehyde, phosphorus
oxychloride and aliquate 336 were available commercially.
Results and discussion
Synthesis
Synthetic routes of dye1, dye2, dye3 and their intermediates were
depicted in Scheme 1. The intermediate a1, a2 and a3 were
Instruments
2
2ꢀ24
IR spectra were recorded with a Nicolet FT-IR NEXUS 870
synthesized efficiently according to the literature.
The tar-
ꢀ1
spectrometer (KBr discs) in the 4000–400 cm region. A Varian
geted compounds (dye1, dye2 and dye3) were obtained by the
1
liquid-state NMR operated at 400 MHz for H-NMR and
condensation of aldehyde and 85% hydrazine.
1
3
00 MHz for C-NMR was used for NMR spectra measure-
1
ments, using DMSO-d or CDCl as solvent, respectively.
6
3
Photophysical properties
Chemical shifts were reported in parts per million (ppm) relative
to internal TMS (0 ppm) and coupling constants in Hz. Splitting
patterns were described as singlet (s), doublet (d), triplet (t),
quartet (q), or multiplet (m). The mass spectra were obtained on
a Bruker Autoflex III smartbeam mass spectrometer.
The one-photon absorption (OPA) spectra were recorded on
the UV-265 spectrophotometer. The one-photon excited fluo-
rescence (OPEF) spectra measurements were performed using
the Hitachi F-7000 fluorescence spectrophotometer. OPA and
OPEF of dye1–dye3 were measured in THF with the concen-
tration of 10 mM. The quartz cuvettes used were of 1 cm path
length.
The profiles of the absorption spectra of dye1, dye2 and dye3
resemble each other in both shapes and positions in the dilute
THF solutions, with the molar extinction coefficients (3max) in
4
4
4
the range of 3.9 ꢁ 10 , 4.9 ꢁ 10 and 4.4 ꢁ 10 , respectively
(Fig. S1, ESI†). The absorption peaks are assigned to p–p*
electronic transitions between the ground state and the excited
state of the chromophore moiety, which suggests that the
substitution of the crown do not bring great changes to the
conjugation of the molecular structure as isolated species
(Table 1).
In the emission spectra, however, the three dyes show weak
fluorescence when dissolved in organic solvents, for the fluores-
cence quantum yields (F) of the three dyes, using quinine sulfate
Synthesis
(
in 1 N H
2
SO
4
, F ¼ 0.54) as standard, are only 0.08, 0.09 and
The three compounds, dye1, dye2 and dye3, were prepared
according to the synthetic routes shown in Scheme 1. Detailed
procedures and characterization can be found in the ESI.†
0.11% in the dilute THF solutions, respectively. The results can
mainly be explained by the photoisomerization, viz, the molec-
ular planarity is reduced due to the rotating of the flexible-center
when the molecules accept energy in the dilute solutions.
Preparation of aggregates
Aggregation-induced emission
Stock THF solutions of the molecules with a concentration of
1.0 mM were prepared. An aliquot (50 mL) of this stock solution
was transferred to a 5 mL volumetric flask. After adding an
The influence of the water fractions. To investigate the AIE
attributes of these three dyes, we added different amount of
1
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Table 1 Substituent effect on the absorption and fluorescence spectra of dye1, dye2 and dye3 in the dilute THF solutions
a
ꢀ1
ab /nm (3max/M
1
l
cm
ꢀ
b
ꢀ1
)
l
em /nm
F
Stokes shift/cm
Dye1
Dye2
Dye3
388.5 (39 082)
387.5 (49 577)
387.0 (43 798)
451
451
451
0.0008
0.0009
0.0011
3567
3634
3667
a
b
Absorption maximum. Emission maximum.
water, a poor solvent for the luminogens, to the pure THF
Several findings are evident from the results in Table 2. (1) In
the pure THF solution, both dye2 and dye3 are molecularly
isolated without any interactions between the adjacent mole-
cules. (2) The molecules of dye2 and dye3 in the crystal state are
arranged so regularly that the rotatable N–N single bonds are
locked due to molecular interactions, thus strengthen the p-
conjugated systems. (3) The molecules of dye2 and dye3 in the
thin film state indicate the isolated species referring to the similar
values of lmax in both solutions, which is quite different from
other AIE-molecules. It is probably because the film was
solutions by defining the water fractions (f
w
) of 0, 0.2, 0.4, 0.6,
0
.8, 0.9 and 0.95, and then monitored the photo fluorescence
(
PL) change.
The PL intensity of dye1 increase slowly in aqueous
mixtures when f < 0.60, and it is weakened or quenched (ACQ)
w
when fw > 0.60 (Fig. S2, ESI†). However, the behaviors of dye2
and dye3 are quite different from that of dye1. The PL inten-
sities of dye2 and dye3 increase slowly in aqueous mixtures
when f
Fig. 2 and 3). From the pure THF solution to THF–H
mixture with the f
w w
< 0.60, while increase dramatically when f > 0.60
ꢀ
5
(
2
O
prepared from a dilute solution (1 ꢁ 10 M) and the solvent
evaporated quickly. Besides, the chaotic molecules caused by the
quick evaporation of solvent have no contribution to the
molecular interaction, which leads to the unlocked rotatable
center. (4) The irregular arrangement of the powder state may
cause the middle value of lem. In addition, the structure of the
particles in the solution with fw ¼ 0.95 are similar to that of
w
¼ 0.95, the PL intensities increase by 88.9-
fold for dye2, and 70.7-fold for dye3. Obviously, the PL inten-
sities of dye2 and dye3 are greatly enhanced by aggregation. In
other words, these two compounds are AIE-active materials
while dye1 is not.
The different properties with the different states. To gain
further insight into the AIE mechanism of dye2 and dye3, we
conducted a series of PL measurements in pure solutions, the
crystals due to their very similar values of l , which suggests
em
that the AIE effect is caused by the crystallization-induced
emission (CIE) effect.
mixed solutions with f
w
¼ 0.95, the film, the powder and the
single crystal (Fig. 4). As shown in Table 2, both dye2 and dye3
exhibit the minimum values of lem as the film and pure THF
solution, the middle values of lem as the powder. It is interesting
that the maximum values of lem are in the form of crystal and in
The crystal structure. In order to better understand the
mechanism, we obtained single crystals of dye1 and dye2.
Planarity. There are two different molecules in a unit cell of
dye1 taking the unique features of the ethyl into account. One is
the mixed solution with f
w
¼ 0.95.
Fig. 2 (A) Photographs of dye2 in THF–H
water fractions (f ) taken under UV illumination. (B) Emission spectra of
dye2 in THF–H O mixtures. (C) Plot of (I/I
ꢀ 1) values versus the
compositions of the aqueous mixtures. I
¼ emission intensity in pure
2
O mixtures with different
Fig. 3 (A) Photographs of dye3 in THF–H
water fractions (f ) taken under UV illumination. (B) Emission spectra of
dye3 in THF–H O mixtures. (C) Plot of (I/I
ꢀ 1) values versus the
compositions of the aqueous mixtures. I
¼ emission intensity in pure
THF solution. Solution concentration: 10 mM.
2
O mixtures with different
w
w
2
0
2
0
0
0
THF solution. Solution concentration: 10 mM.
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Fig. 4 The fluorescence spectroscopy of (a) dye2 and (b) dye3 in the pure
solvents, the mixed solution with f
w
¼ 0.95, the film, the powder and the
crystal.
Table 2 The lem of dye1, dye2 and dye3 in the pure solvent, the mixed
solution with f
¼ 0.95, the film, the powder and the crystal
w
lem/nm
Pure solvent
Mixed solution
Film
Powder
Crystal
Dye1
Dye2
Dye3
451
451
451
463 495
490
492
468
457
442
526
469
476
512
503
500
Fig. 6 (a) Molecular packing diagram of dye1 (inset: packing diagram
simplification of dye1) (b) no fixed of the rotatable N–N single bond (the
aza-crown-ether in (a) and hydrogen atoms except H22B in (b) are
omitted for clarity).
order (molecule II), and the other is disorder (molecule I). The
ꢂ
ꢂ
ꢂ
dihedral angles are 0.04 (P –P ), 9.94 (PIIa–PIIb), 2.62 (P –
Ia Ib
Ia
ꢂ
ꢂ
P ), 6.31 (P –P ), 2.96 (P –P ), respectively (Fig. 5a, P , P , P
z
x
Ib
y
IIa
z
x
y
are the planes which consist of the C12–N3–N2, N3–N2–C11,
0
C23–N6–N6 in dye1, respectively.) In dye2, the dihedral angles
ꢂ
ꢂ
are 1.69 (P2a–P2b) and 9.86 (P2a–P
i i
), respectively (Fig. 5b, P is
0
bond cannot be locked substantially by the interaction and it’s
free to twist. While in the molecule I, there is no interaction
around the flexible center. Thus, the photoisomerization process
can hardly be prevented (Fig. 6b).
the plane which consists of C1–N1–N1 in dye2.). The data of the
crystals show that the planarity of both two molecules is good.
That is to say, dye1 and dye2 are similarly planar.
However, the case is completely different in dye2 indicated
from the packing diagram shown in Fig. 7a. The distance
ꢀ
between molecule a and d is 3.06(6) A, and the dihedral angle
Rotatability. Fig. 6a shows the packing diagram of dye1. The
distance from H22B in molecule I to N6 in molecule II is 2.75(0)
ꢂ
ꢀ
A, and the angle of C–H/N is 150.16 . The flexible N–N single
ꢂ
between molecule a and c is 60.56 . This phenomenon can be
explained by the fact that the molecular interactions (H17B–C15,
H17B–O4, H17B–C16, H17B–H16A) caused by the crown
between molecule a and b, b and d shorten the distance between
molecule a and d. Likewise, molecule c also contributes to
shortening the distance owing to the molecular interactions
caused by the crown between molecule a and c, c and d (Fig. 7b).
That is the main reason for the decreasing distance between
molecule a and d. Most of all, the new molecular interactions,
from H3 in molecule c to C1, N1 in molecule a, are formed
because of the shortened distance between molecule a and b
(
ꢀ
Fig. 7c). The distances of the molecular interactions are 2.87(8)
ꢀ
A and 2.58(8) A, and the angle of C–H/C and C–H/N are
ꢂ ꢂ
1
28.45 and 168.58 , respectively, suggesting strong interac-
25
tions. The interactions inhibit free twisting motions around the
N–N single bond by fixing rotatable N–N single bond. There-
fore, the molecule becomes rigid. In other words, the photo-
isomerization process around the N–N bond is not important,
which ensures light radiation from the torsional molecules.
Besides, the face-to-face p–p interactions are restrained due to
the interactions by the aza-crown-ether substituents. Thus, the
strong interactions lead to a special packing manner and rigid
Fig. 5 The restricted twisting motions in crystals of (a) dye1 and (b)
dye2.
1
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Fig. 8 Optimized molecular structures and molecular orbital amplitude
plots of HOMO and LUMO levels of the dye1, dye2 and dye3 calculated
using the B3LYP/6-31G basis set.
highest-occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) plots are shown in Fig. 8. The
HOMO and LUMO of dye1, dye2 and dye3 are dominated by the
orbitals from the rotatable core and the phenyl units, which
reveals that their absorption and emission stem from the p–p*
transitions. Besides, the calculated bandgaps of dye1, dye2 and
dye3 are similar (3.20 eV, 3.19 eV and 3.17 eV, respectively),
which indicates the weak influence of the substituent for the
whole molecule. The theoretical study nicely explains the similar
optical properties of the three dyes in the dilute solutions.
Conclusions
In summary, a series of flexibly cored chromospheres, dye1, dye2
and dye3 with or without aza-crown-ether as the substituents,
have been designed and synthesized. The three compounds show
weak fluorescence in the pure THF solution. With the changing
water fractions, dye1 exhibits the ACQ effect, while dye2 and
dye3 show the AIE attributes. The optical properties of three
different states for the three dyes have been well studied. A new
process with the influence of the molecular interactions, in
combination with planarity and rotatability mechanisms, has
been discussed, which is identified by the X-ray analysis.
Acknowledgements
This work was supported by Program for New Century Excellent
Talents in University (China), Doctoral Program Foundation of
Ministry of Education of China (20113401110004), Science and
Technological Fund of Anhui Province for Outstanding Youth
(10040606Y22), National Natural Science Foundation of China
Fig. 7 (a) Molecular packing diagram of dye2 (inset: packing diagram
simplification of dye2) (b) the repeating unit in packing (c) the fixed of the
rotatable N–N single bond (the aza-crown-ether in (a) and (b), and
hydrogen atoms except H3, H6, H16A, H17A and H17B in (c) are
omitted for clarity).
(
21071001, 51142011, 21101001), the 211 Project of Anhui
University, Education of Committee of Anhui Province
KJ2012A024, KJ2010A030), the Natural Science Foundation of
molecular structure, making the fluorescence intensities multiply
when packing.
(
Anhui Province (1208085MB22), Ministry of Education Funded
Projects Focus on Returned Overseas Scholar.
The X-ray analyses of dye1 and dye2 show that the aza-crown-
ether has a significant effect on the molecular packing modes and
the molecular interactions, which leads to AIE-active molecules.
Notes and references
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This journal is ª The Royal Society of Chemistry 2012