Pyrene-containing dyes: Reversible click/declick reaction, optical and aggregation behaviors

Article abstract of DOI:10.1016/j.dyepig.2020.108375

Pyrene-based dyes were synthesized by a clean, efficient Diels-Alder [4 + 2] click reaction where no catalyst adding or side reactions occurring. The optical behaviors, optoelectronic properties and supramolecular donor/acceptor dipolar interactions of these new dyes were investigated by UV–vis absorption, fluorescence spectroscopy, electrostatic potential and frontier molecular orbital calculations as well as electrochemical cyclic voltammetry measurements. Two pyrene-based dyes can be mutually transformed by reversible formation or cleavage of Diels-Alder covalent bonds through click/declick reaction. During the reversible reaction, switchable fluorescence ON/OFF behaviors were interestingly observed. X-ray single crystal diffraction revealed that supramolecular donor/acceptor dipolar and π-π stacking interactions work cooperatively to form stronger interaction and the shorter distance (3.38 ?) between donor and acceptor than general π-π stacking distance (3.54 ?). An amphiphilic dye is self-assembled in water into blue fluorescent, hollow vesicular aggregates with the average diameter of 115–120 nm and a narrow size distribution as revealed by dynamic light scattering, scanning electron microscopy and transmission electron microscopy.

Full text of DOI:10.1016/j.dyepig.2020.108375

Dyes and Pigments 179 (2020) 108375
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Pyrene-containing dyes: Reversible click/declick reaction, optical and
aggregation behaviors
*
Mengde Peng, Ying Wang, Xin Zhang
School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemistry Science and Engineering, Tianjin University, Tianjin, 300072, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Pyrene-based dyes were synthesized by a clean, efficient Diels-Alder [4 þ 2] click reaction where no catalyst
adding or side reactions occurring. The optical behaviors, optoelectronic properties and supramolecular donor/
acceptor dipolar interactions of these new dyes were investigated by UV–vis absorption, fluorescence spectros-
copy, electrostatic potential and frontier molecular orbital calculations as well as electrochemical cyclic vol-
tammetry measurements. Two pyrene-based dyes can be mutually transformed by reversible formation or
cleavage of Diels-Alder covalent bonds through click/declick reaction. During the reversible reaction, switchable
fluorescence ON/OFF behaviors were interestingly observed. X-ray single crystal diffraction revealed that su-
Fluorescence
Pyrene
Aggregation
Diels-Alder reaction
Supramolecular assembly
pramolecular donor/acceptor dipolar and
π
-
π
stacking interactions work cooperatively to form stronger inter-
stacking distance (3.54 Å).
action and the shorter distance (3.38 Å) between donor and acceptor than general
π-π
An amphiphilic dye is self-assembled in water into blue fluorescent, hollow vesicular aggregates with the average
diameter of 115–120 nm and a narrow size distribution as revealed by dynamic light scattering, scanning
electron microscopy and transmission electron microscopy.
1
. Introduction
Pyrene-based dyes possess excellent optical properties such as high
feature of Diels-Alder reactions is the reaction reversibility under
appropriate conditions, and the reverse reaction is known as the retro--
Diels-Alder reaction. Recently, Ji and coworkers reported mouldable
liquid-crystalline elastomer and soft actuators with switchable thermal
reprogrammability and exchangeable covalent bonds [36–38]. Zhu and
coworkers reported photoplastic transformation based on dynamic co-
extinction coefficient and absorption within blue light region, high
fluorescence quantum yield (~0.65) [1], long fluorescence lifetime
2
(
~10 ns) [2], and outstanding chemical, photo, thermal stability [3–6].
Therefore, pyrene dyes recently have attracted much interest due to
their unique optical properties and diverse applications such as indus-
trial dyes and dye precursors [7], fluorescence probes and sensors
valent
chemistry,
and
investigated
photoswitchable
self-assembly/disassembly of near-infrared fluorophores [39–41]. As
the most important dienes and dienophiles, maleimides and furans have
been widely used in Diels-Alder reaction to synthesize various structural
small molecules and macromolecules [42–44]. We have a long-term
interest in the synthesis of fluorescent dyes, preparation of dye aggre-
gates and their optical behaviors [45–51]. In this work, we synthesized
new pyrene-containing dyes, and investigated their optical behaviors,
single crystal X-ray crystal diffraction structures, self-assembly and ag-
gregation behaviors. In particular, we demonstrate the reversible
light-emitting behaviors of these dyes during Diels-Alder click/declick
reactions.
[
8–13], light-harvesting systems [14], organic electronics [15–17] and
covalent organic frameworks [18,19]. Owing to their flat polycyclic
aromatic structures, pyrene dyes have a strong -stacking tendency in
π
solution. Thus, pyrene dyes have been recently used as valuable building
blocks for various supramolecular dye aggregates in organic solvents
[
20–23], even in water [24–26].
Diels–Alder reaction is an organic [4 þ 2] cycloaddition reaction
between an electron-rich diene and an electron-poor dienophile, which
forms a substituted six-membered ring derivative [27,28]. Diels-Alder
reactions have been recently extensively studied as one class of typical
click reactions due to the nature of metal catalyst-free, functional group
tolerance and high reaction rate constant [29–35]. An interesting
*
Corresponding author.
E-mail address: xzhangchem@tju.edu.cn (X. Zhang).
https://doi.org/10.1016/j.dyepig.2020.108375
Received 21 January 2020; Received in revised form 16 March 2020; Accepted 17 March 2020
Available online 4 April 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.

M. Peng et al.
Dyes and Pigments 179 (2020) 108375
�
Fig. 1. Synthetic route towards hydrophilic furan chain, dyes 1 and 2. Reagents and conditions: (i) sodium hydride, tetrahydrofuran, 0 C, 1 h, r.t., 4 h, 71%; (ii)
�
copper(I) iodide, tetrakis(triphenylphosphine) palladium, anhydrous triethylamine, 70 C, overnight, 39%; (iii) Pd/C (10%, wt), glacial acetic acid, NaBH
4
, ethanol,
r.t., 24 h, 75%, (iv) Hydroxylamine hydrochloride, NaOH, Ethanol, reflux, 5 min, r.t., 2 h, 54%; (v) zinc, acetic acid, N
2
, r.t., 15 h, 46%; (vi) maleic anhydride,
�
�
dichloromethane, r.t., 2 h, 90%; (vii) acetic anhydride, anhydrous sodium acetate, 80 C, 12 h, 52%; (viii) chloroform, acetonitrile, 40 C, 71%.
�
2
. Experimental section
acetonitrile (5 mL) and stirred at 40 C for 60 h. After the reaction was
completed, the reaction mixture was washed with EtOAc. After filtra-
tion, the filtrate was concentrated and purified using a silica gel column
chromatography using hexane/ethyl acetate ¼ 1:1 (v/v) as eluent to
give a colorless viscous product (0.24 g, 71%). In the above Diels-Alder
reaction, two isomers, i.e., endo and exo isomers, were first formed.
Among them, the endo isomer is kinetically favored while the exo isomer
is thermodynamically stable. As the reaction proceeded, the kinetically
favored endo isomer gradually deceased, and thermodynamically stable
exo isomer increased, and became main product, and finally pure exo
2
.1. Synthesis
Dye 1. 4 mL of a solution of 1-aminomethylpyrene (0.387 g,
1
.5 mmol) in dichloromethane was slowly added dropwise to a solution
of maleic anhydride (0.22 g, 2.25 mmol) in diethyl ether (5 mL). After
stirring at room temperature for 2 h, the precipitate was collected by
filtration to give corresponding maleamic acid (0.5 g, 90%). The
resulting maleamic acid was dried overnight and then heated in acetic
�
anhydride (2.2 mL) using sodium acetate (0.21 g) as catalyst at 80 C for
isomer was obtained after purification by silica gel column chroma-
1
1
0 h. The precipitate was collected by suction filtration and washed
tography. H NMR (400 MHz, CDCl
3
, ppm): δ ¼ 8.46 (d, J ¼ 9.2 Hz, 1H,
three times with deionized water and ethanol. After the crude product
ArH in pyrene ring), 8.20–7.92 (m, 8H, ArH in pyrene ring), 5.99 (s, 1H,
–CR¼CH-), 5.36 (s, 2H, -CH -), 5.25 (m, 1H, -O-C(R)H-), 5.07 (s, 1H, -O-
C(R)H-), 3.63–3.41 (m, 10H, 5 -CH -), 3.34 (s, 3H, -CH ), 2.85 (dd,
J ¼ 15.7, 6.2 Hz, 2H, 2-CO-C(R)H-), 2.26 (s, 2H, -CH -), 1.87–1.74 (m,
was recrystallized from dichloromethane, yellow crystals were obtained
2
1
(
2.4 g, 52%). H NMR (400 MHz, CDCl
3
, ppm): δ ¼ 8.40 (d, J ¼ 9.2 Hz,
2
3
1
2
1
1
8
2
H, ArH in pyrene ring), 8.27–7.69 (m, 8H, ArH in pyrene ring), 6.52 (s,
2
–
13
13
H, -CH
–
CH-), 5.24 (s, 2H, -CH
2
-). C NMR (CDCl
3
, ppm): δ ¼ 170.58,
2H, -CH
2
-). C NMR (CDCl
3
, ppm): δ ¼ 176.48, 176.33, 151.72, 131.25,
34.19, 131.29, 130.68, 129.05, 128.81, 128.15, 127.88, 127.63,
131.12, 130.69, 128.71, 128.50, 128.40, 127.99, 127.49, 127.38,
126.94, 125.98, 125.33, 125.25, 124.83, 124.78, 124.67, 122.85, 83.31,
81.87, 77.40, 77.09, 76.77, 71.93, 70.59, 70.51, 70.31, 70.24, 59.01,
À 1
27.34, 126.02, 125.40, 125.31, 124.68, 122.83, 39.54. IR (cm ):
48.2, 909.6, 1100.9, 1197.8, 1245.4, 1349.7, 1452.3, 1728.9, 1766.6,
À 1
857.5. MS(EI): calculated for C21
H13NO
2
, 311.09; found 312.03
49.33, 47.26, 40.58, 27.20, 23.96. IR (cm ): 708.5, 864.6, 8841.2,
þ
[
MþH] .
1014.9, 1107.0, 1179.6, 1392.2, 1604.3, 1697.7, 1773.1, 2866.5. MS
þ
Dye 2. Hydrophilic furan chain (0.16 g, 0.6 mmol) and 1 (0.17 g,
(EI): calculated for C33
H
33NO
6
, 539.23; found 540.49 [MþH] .
0
.5 mmol) were dissolved in a mixture of chloroform (5 mL) and
Elemental analysis: calcd (%) for C33
H33NO
6
: C 73.45, H 6.16, N 2.60;
Fig. 2. UV–vis absorption spectra (a) and fluorescence emission spectra (b) of 1 and 2 in dichloromethane. Inset: photographs of 1 and 2 under 365 nm ultraviolet
À 5
lamp and fluorescence decay of 2 in dichloromethane, λex ¼ 277 nm, λem ¼ 376 nm [1] ¼ [2] ¼ 2 ꢀ 10 M.
2

M. Peng et al.
Dyes and Pigments 179 (2020) 108375
Table 1
UV–vis absorption and fluorescence spectroscopic data of 1 and 2 in CH
2
Cl2,
À 5
[
1] ¼ [2] ¼ 2 ꢀ 10 M.
UV/Vis absorption
Fluorescence
Dyes
4
À 1
À 1
a
λ
ab/nm
ε
/10 M cm
λ
em/nm
Φ
–
τ
/ns
1
2
277
277
3.96
4.35
–
376
–
0.21
24.54
a
Using 9, 10-diphenylanthrene (Φ ¼ 0.90 in cyclohexane) as a reference [54].
Table 2
Oxidation potentials (Eox(D)), reduction potentials (Ered(A)), free energy change
(
ΔG), highest occupied molecular orbital (EHOMO) and lowest unoccupied mo-
lecular orbital (ELUMO) energy levels of dyes 1 and 2.
a
d
E
ox(D)
E
red(A)
E
00(D*)
ΔG
/eV
E
HOMO
E
LUMO
ΔE
/eV
g
Dyes
/V
/V
/eV
/eV
/eV
b
b
b
c
1
2
1.40
1.30
À 0.71
4.48
4.48
À 2.43
À 5.68
À 5.58
À 3.57
À 1.56
2.11
4.02
/
/
Fig. 4. HOMO and LUMO energy level diagrams of pyrene-based dyes 1 and 2
a
Obtained from absorption spectroscopy.
determined by electrochemical cyclic voltammetry measurements.
b
Calculated from the oxidation potential and the reduction potentials by
using ferrocence as a reference. EHOMO ¼ À [4.78 eV þ Eox(D)- EFc/Fcþ], ELOMO
¼
absorption spectra of 1 and 2 in dichloromethane both show an ab-
À [4.78 eV þ Ered(A) - EFc/Fcþ], in which EFc/Fcþ is the oxidation potential of
1
sorption peak between 260 and 280 nm, which is assigned to the B
a
and
ferrocene.
1
B
b
bands of the pyrene group [53]. A broad absorption band was
c
Obtained by the Gaussion 09 HOMO and LUMO calculations.
observed between 310 and 350 nm with maximum absorption peak at
d
ΔE
g
¼ ELUMO-EHOMO
.
1
3
a
44 nm, which was attributed to the L band of the pyrene group [53].
No obvious spectral difference was observed in the UV–vis absorption
spectra of 1 and 2, suggesting no significant influence on the ground
state properties before and after Diels-Alder addition. Interestingly, dyes
found: C 72.66, H 6.78, N 2.47.
. Results and discussion
3
1
and 2 show distinctly different fluorescence properties. 1 is nonfluo-
rescent (Fig. 2b), and the fluorescence of pyrene chromophore of 1 is
completely quenched. However, 2 is highly fluorescent with fine
structure emission at 370–450 nm in dichloromethane. It should be
3
.1. Synthesis and optical behaviors
In this work, we first synthesized hydrophilic furan chain by a
–
–
C bond of furan ring of 2 has no any influence on its
noted that the C
fluorescence. For donor/acceptor dye 1, the C
Sonogashira reaction between 3-(2-(2-methoxy)ethoxy)ethoxy)-1-pro-
pyne and 3-bromofuran followed by reduction (Fig. 1). Pyrene-based
dye 1 was synthesized in a four-step reaction (Fig. 1). The precursor 1-
pyrenemethylamine was synthesized according to the literature [52],
and then cyclized with maleic anhydride in acetic anhydride using so-
dium acetate as catalyst to form dye 1. Pyrene-based dye 2 was syn-
thesized by Diels-Alder reaction of 1 and hydrophilic furan chain in
chloroform/acetonitrile, which is a high efficient click reaction under
mild condition without byproduct formation. Detailed synthesis pro-
cedures were provided in the Supporting Information. The chemical
–
–
C bond of maleimide
group is an electron acceptor and pyrene chromophore acts as an elec-
tron donor, thus, the fluorescence of 1 is quenched due to intramolecular
charge transfer interaction between donor and acceptor. However, for
–
dye 2, the C C bond of furan ring is electron donor, not an acceptor,
–
thus fluorescence of 2 is not quenched. The fluorescence quantum yield
of 2 was measured to be 0.21 with a long fluorescence lifetime of 24.54
ns. The detailed optical data of 1 and 2 are listed in Table 1.
1
13
structures of new dyes 1 and 2 were fully characterized by H NMR,
C
3.2. Electrochemical characterization
NMR and IR spectroscopy as well as mass spectroscopy (see details in
Supporting Information).
These dyes possess functional electron-donating and electron-
accepting groups. Thus, we investigated the electrochemical properties
of these dyes. Theoretical calculations of electrostatic potential and
Dyes 1 and 2 are well dissolved in dichloromethane, thus they exist
in molecularly dissolved monomer state in good solvent. UV–vis
Fig. 3. Molecular electrostatic potential maps of 1 and 2. The calculations were performed using the Gaussian 09 program with the DFT at B3LYP/6-31G (d) level.
3

M. Peng et al.
Dyes and Pigments 179 (2020) 108375
�
Fig. 5. (a) Retro-Diels-Alder reaction of 2 to generate 1 and furan hydrophilic chain at 80 C in toluene. (b) Fluorescence monitoring of retro-Diels-Alder reaction
from 0 h (top) to 12 h (bottom). λex ¼ 277 nm. Concentration for fluorescence monitoring in dichloromethane is 1 ꢀ 10À 5 M.
molecular frontier orbitals were executed on Gaussian 09 program using
density functional theory (DFT) at the level of B3LYP/6-31G (d) [55].
The charge transfer interaction and tendency between acceptor and
donor can be analyzed by molecular geometry minimization structures
and molecular electrostatic potential maps [4]. As shown in Fig. 3, the
red region represents the negatively charged region with high electron
density, while the blue region is the positively charged region with low
electron density. The negative potential of 1 is distributed on the ni-
trogen, oxygen atoms and benzene rings, and positive potential is
distributed on the C–
The cyclic voltammogram of 1 shows a maleimide acceptor reduc-
tion peak at À 0.71 V and an oxidation peak of a pyrene donor at 1.40 V.
The cyclic voltammogram of 2 shows the oxidation peak of the pyrene
donor at 1.30 V (Fig. S15). On the basis of the redox potentials of these
dyes measured by cyclic voltammetry, the free energy change (ΔG) of
the photoinduced charge transfer was calculated according to equation
(1) [56]:
ΔG ¼ Eox(D) - Ered(A) - E00(D*) - C
(1)
–
C group. The negative potential of 2 is distributed
in which Eox(D) is the oxidation potential of donor, Ered(A) is the
reduction potential of acceptor, E00(D*) (eV) ¼ hc/λ ¼ 1240/λex (nm) is
the excitation energy of the electron donor, and C (0.06 eV or 1.4 kcal/
mol) is stabilization energy of solvents acting with charge transfer spe-
cies. The free energy change of photo-induced charge transfer of 1
on the pyrene moiety and the hydrophilic chain, and the positive po-
tential is distributed on the nitrogen atom. For dye 1, the pyrene chro-
mophore is indicated as an electron-rich region, which acts as an
electron donor, and maleimide group is indicated as an
electron-deficient region, which serves as an electron acceptor.
The highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) calculations show that the HOMO of 1
is located in the pyrene chromophore, and the LUMO of 1 is located in
the maleimide group (Fig. 4), indicating that the orbital coupling
interaction between the donor and the acceptor is very weak, and the
oxidation active site and the reduction active site are well separated. For
dye 2, both the HOMO and LUMO are dispersed on the pyrene chro-
mophore, which reveals relatively strong orbital coupling. According to
electrochemical cyclic voltammetry measurements, the HOMO energy
levels of 1 and 2 are very close (Fig. 4), while the LUMO level of 1 is
relatively low. Therefore, the HOMO-LUMO gap of 1 (2.11 eV) is smaller
than that of 2 (4.02 eV), suggesting that dye 1 is more favorable for
charge separation.
(
ΔG ¼ À 2.43 eV) is negative (ΔG < 0), revealing that charge transfer can
occur from the pyrene donor to the maleimide acceptor when donor is
photoexcited. Thus, the pyrene chromophore fluorescence of 1 is
completely quenched due to the photoinduced charge transfer occur-
ring. While, pyrene-based dye 2 does not show a tendency for photo-
induced charge separation, thus, no fluorescence quenching occurs in 2.
3
.3. Reversible click/declick reaction
Diels–Alder reaction between furan and maleimide groups is
dynamically reversible. Pyrene-based dye 1 is non-fluorescent, while its
Diels-Alder adduct 2 is highly fluorescent. Therefore, the dynamically
reversible Diels-Alder reaction can be monitored by fluorescence spec-
�
troscopy. Upon heating at 80 C, Diels-Alder covalent bonds of 2 are
�
Fig. 6. (a) Diels-Alder covalent bond reformation by furan/maleimide Diels-Alder reaction at near room temperature 40 C in chloroform. (b) Fluorescence recovery
during Diels-Alder covalent bond reformation in Diels-Alder reaction from 0 h (bottom) to 60 h (top). λex ¼ 277 nm. Concentration for fluorescence measurements in
À 5
dichloromethane is 1 ꢀ 10 M.
4

M. Peng et al.
Dyes and Pigments 179 (2020) 108375
Fig. 7. (a) Molecular structure (ORTEP) of 1 determined by single-crystal X-ray diffraction, thermal ellipsoids are drawn at 50% probability level, all hydrogen atoms
have been omitted for clarity. (b) Crystal packing of 1 determined by single-crystal X-ray diffraction. (c) Crystal packing structure of 1 along the c-axis. (d) Schematic
illustration of head-to-tail assembly of 1 driven by cooperative donor/acceptor dipolar and
π
- stacking interactions.
π
broken (Fig. 5a), the retro-Diels-Alder reaction occurs in toluene. During
the reaction, 100 uL of the reaction mixture was taken at regular time
and dried in vacuum, then dissolved in dichloromethane for fluores-
cence monitoring at room temperature. As the reaction proceeds, the
fluorescence intensity gradually decreases until it finally disappears
X-ray diffraction structure of 1 belongs to a monoclinic system with non-
centrosymmetric space P2(1)/c, and a crystal cell contains four mole-
cules [57]. The pyrene donor and the maleimide acceptor in the mole-
cule adopt a V-type structure, and the dihedral angle between acceptor
�
and donor is 65.48 (Fig. 7a). It is well known that the pyrene chromo-
(
Fig. 5b). Thus, the change in fluorescence intensity can reflect the de-
phore is a conjugated
–
crystal data indicate that the length of C C double bond of maleimide
π
-electron aromatic system. Furthermore, the
gree of cleavage of Diels-Alder covalent bonds.
–
The broken Diels-Alder covalent bond could be repaired and self-
healed under mild conditions. Diels-Alder reaction of the above result-
acceptor is 1.300 Å and its adjacent two C–C single bonds of the mal-
eimide ring are 1.482 Å and 1.484 Å respectively, which both are shorter
than normal C–C single bonds (1.54 Å). This single crystal X-ray
�
ing maleimide 1 and furan derivative can occur at 40 C in chloroform to
reform the Diels-Alder covalent bonds (Fig. 6a). As the Diels-Alder co-
valent bonds reform, the fluorescence gradually increases, thus fluo-
rescence intensity can reflect the reformation/self-healing degree of the
Diels-Alder covalent bonds.
diffraction data suggest that maleimide ring also forms a
aromatic system.
π
-conjugated
In the crystal packing structure of 1, there is a large degree overlap
between the pyrene -donor and the maleimide -acceptor (Fig. 7b and
c). This crystal result reveals that supramolecular donor/acceptor
dipolar interaction and stacking interaction cooperatively work to
lead to a shorter pyrene-to-maleimide stacking distance (3.38 Å)
Fig. 7c) than normal stacking interactions (3.54 Å). In addition, no
π
π
π-π
3
.4. X-ray diffraction crystal structure analysis
(
π π
-
To better understand the molecular structure and assembly infor-
evidence shows the existence of hydrogen bonding interaction. There-
mation, the single crystal of 1 was prepared by slow evaporation of its n-
fore, the main driving forces for crystal packing of 1 are the cooperative
hexane/chloroform solution at ambient atmosphere. The single crystal
2
Fig. 8. (a) Fluorescence spectra of 2 in THF/H O
with water content from 0% to 95%,
À 5
[
2] ¼ 2.0 ꢀ 10
M
λex ¼ 277 nm. Inset: photo-
graph of 2 under 365 nm ultraviolet lamp. (b)
Changes in fluorescence intensity ratios (IAggr
) with increasing water content. IAggr and I
/
I
M
M
are the fluorescence intensity of aggregates and
monomers, respectively. Inset: fluorescence
colour coordinates of 2 in CIE 1931 chromaticity
diagram in the mixed solvent of THF/water (0%,
9
0%, and 95% water content). (For interpreta-
tion of the references to colour in this figure
legend, the reader is referred to the Web version
of this article.)
5

M. Peng et al.
Dyes and Pigments 179 (2020) 108375
Fig. 9. (a) Fluorescence spectra of
2
with
À 5
increasing concentration from 1.0 ꢀ 10 M to
À 4
2
.0 ꢀ 10 M in deionized water. (b) Changes in
fluorescence intensity ratio of aggregate emission
Aggr) to monomer emission (I ) with increasing
(
I
M
the concentration in water. Inset:photograph of 2
in water under 365 nm ultraviolet lamp at
À 5
À 4
À 5
À 4
2
.0 ꢀ 10 M,
4.0 ꢀ 10 M,
1.0 ꢀ 10 M,
2
.0 ꢀ 10 M, and corresponding fluorescence
colour coordinates in CIE 1931 chromaticity di-
agram. (For interpretation of the references to
colour in this figure legend, the reader is referred
to the Web version of this article.)
Fig. 10. (a) SEM image of aggregates of pyrene-
based dye 2 prepared from deionized water.
À 4
[
2] ¼ 2 ꢀ 10 M. (b) TEM image of aggregates
of
2
prepared from deionized water.
À 4
[
2] ¼ 2 ꢀ 10 M. Inset shows a magnified TEM
image of a typical vesicle aggregate. (c) Size
distribution of the vesicular aggregates of 2 ob-
tained from TEM measurements from 942 ag-
gregates. (d) Size analyses of the aggregates of 2
in water from DLS measurements. (e) Excitation-
dependent 3D fluorescence spectra of aggregates
À 4
of 2 in water. ([2] ¼ 2 ꢀ 10 M). Inset: photo-
graph of aggregates of 2 in water under 365 nm
ultraviolet lamp at 2.0 ꢀ 10À 4 M.
6

M. Peng et al.
Dyes and Pigments 179 (2020) 108375
π
-
π
stacking and donor/acceptor dipolar interactions between mal-
reversible click/declick Diels-Alder reaction occurs under mild condi-
tions without catalyst or byproduct formation. The main driving forces
eimide and pyrene rings, which form a head-to-tail supramolecular as-
sembly structure (Fig. 7c and d). The detailed crystal structure data of 1
are provided in Supporting Information.
of supramolecular assembly of these dyes are cooperative
π
-π stacking
and donor/acceptor dipolar interactions as revealed by single crystal X-
ray diffraction and electrochemical analysis and characterization. An
amphiphilic pyrene-based dye formed well-defined, hollow vesicles in
water with a narrow size distribution. These aggregates are fluorescent
and their morphologies were investigated by dynamic light scattering,
scanning electron microscopy and transmission electron microscopy.
Our results may encourage the development of functional light-emitting
dyes and the functional fluorescent aggregates in water.
3
.5. Self-assembly and aggregation behaviors in aqueous solution
Pyrene-based dye 2 is an amphiphilic structure composed of hydro-
phobic pyrene chromophore and hydrophilic furan chain connected by
Diels-Alder covalent bonds. We further investigated the aggregation
behaviours of 2 in THF/water by fluorescence spectroscopy. First of all,
the fluorescence emission spectra of 2 in a variable ratio of a mixing
solvent of THF and deionized water were measured at the excitation
wavelength of 277 nm. With increasing water contents from 0% to 95%,
a broad fluorescence emission band appears at longer wavelength of
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
4
50–600 nm, which is attributed to aggregate emission due to aggregate
formation (Fig. 8a) [58]. The fluorescence intensity of the aggregates at
85 nm reaches a maximum when the water content is 95% (Fig. 8a and
4
b). With increasing the water content, fluorescence colours of aggregate
solution change from purple to blue-green under 365 nm UV lamp
Acknowledgements
(
Fig. 8a inset). Emission colour coordinates in CIE 1931 chromaticity
We acknowledge the National Natural Science Foundation of China
(Grant No. 21674079 and Grant No. 21975177) for financial support.
diagram move from (0.172, 0.103), (0.166, 0.133) to (0.167, 0.278)
with increasing water content from 0%, 90%–95%, respectively (Fig. 8b
inset).
Appendix A. Supplementary data
To better understand the aggregation behaviours of 2, the fluores-
cence emission spectra of the variable concentration ranging from
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108375.
À 5
À 4
1
.0 ꢀ 10 M to 2.0 ꢀ 10 M were measured in pure deionized water.
Compared with the monomer emission (I ), the fluorescence emission of
the aggregates (IAggr) at 485 nm increases with increasing concentration
M
References
in water (Fig. 9a). The plot of the fluorescence intensity ratio (IAggr/I
M
)
À 5
versus concentration shows a critical turning point at 3.3 ꢀ 10 M,
revealing the critical aggregation concentration of the aggregate for-
mation in deionized water (Fig. 9b).
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