Aqueous highly emissive host-guest systems by host enhanced intramolecular charge transfer

Article abstract of DOI:10.1016/j.dyepig.2019.107919

Highly emissive host-guest systems have been an emerging topic which combines dynamic supramolecular system with fluorescent dyes. In this work, we report the host-guest interactions between (E)-1-benzyl-4-(2-(pyren-1-yl)vinyl)pyridin-1-ium bromide (PVP)

Full text of DOI:10.1016/j.dyepig.2019.107919

Dyes and Pigments 173 (2020) 107919
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Aqueous highly emissive host-guest systems by host enhanced
intramolecular charge transfer
a,
b
b
b
b
a,
**
b,*
Yaxin Si , Qi Zhang , Weihang Jin , Shun Yang , Zhenwei Wang
, Dahui Qu
a
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai, 201418, China
b
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Highly emissive host-guest systems have been an emerging topic which combines dynamic supramolecular
Supramolecular assembly
Cucurbit[n]uril
system with fluorescent dyes. In this work, we report the host-guest interactions between (E)-1-benzyl-4-(2-
(
pyren-1-yl)vinyl)pyridin-1-ium bromide (PVP) and cucurbit[n]uril (CB[n], n ¼ 7, 8), which were investigated in
Fluorescent dye
1
aqueous solution by H NMR, UV/Vis, fluorescence spectroscopic techniques and isothermal titration calorimetry
ITC). The results indicate the simultaneous formation of 1:1 inclusion complexes between CB [7] and PVP with
Host-guest chemistry
(
6
À 1
the binding constants as 4.97 � 10 M . Meanwhile a 2:2 homoternary inclusion complex CB [8]-PVP with a
5
À 1
binding constant of 2.49 � 10 M was detected. The two macrocycles are found to effectively increases the
fluorescence efficiency via the host-enhanced intramolecular charge transfer (ICT) effect. This system provides a
distinct strategy to construct high-quantum-yield fluorescent systems.
1
. Introduction
to avoid aggregation and to suppress the quenching effect of the
interactions.
π-π
The supramolecular host-guest complexes with macrocycles and
Herein, we explore the host-guest interactions between the CB[n]
(n ¼ 7, 8) and the chromophoric monomer (E)-1-benzyl-4-(2-(pyren-1-
yl)vinyl)pyridin-1-ium bromide (PVP). Because of the tendency to form
fluorescent dyes plays a vital role in supramolecular materials. The
classical water-soluble macrocyclic hosts included cyclodextrins [1]、
calix[n]arenes [2]、 Pillar[n]arenes [3]、Cucurbit[n]urils [4] and
other macrocycles [5]. Due to the advantages of the binding interactions
of high affinity [6]、 high selectivity of binding [7] and low toxicity in
vivo or vitro, cucurbit[n]urils as the common supramolecular macro-
cyclic molecules facilitate biologically relevant applications [8] and also
contribute to nanotechnology [9] and dynamic self-assembly system
the fluorescent
π
- stacking, the fluorescence emission of this kind of
π
fluorophores has remained low in aqueous solution. Therefore, the ag-
gregation behavior of ICT compounds can be controlled by the move-
ment of macrocyclic components [14]. To efficiently suppress
aggregation, we establish a supramolecular approach to prevent PVP
dyes from aggregating by forming a strong host-guest inclusion complex
with the macrocyclic host; cucurbit[n]urils (n ¼ 7, 8). CB [7] can form
1:1 complexes with a range of PVP which efficiently suppress aggrega-
tion of PVP. In this case, the PVP dye encapsulation resulted in profound
changes in the emission wavelength and quantum yield, as well as
increased photostability. Compared with CB [7], CB [8] has the
increased volume of its cavity, the uniqueness of CB [8] as molecular
receptor is expanded by the very nature of its hollow structure [15].
Based on the research of Scherman and Biedermann [16], CB [8] is a
unique hydrophobic molecular receptor, its polar cavity can stabilize
charge-transfer interactions between two guests of complementary na-
ture, such as aromatic electron acceptors and donors [17]. In this case,
[
10]. Water-soluble fluorescent dyes as molecular containers (nano-
cavities) have been found particularly useful to monitor the formation of
discrete host-guest complexes with macrocyclic structures [11]. Pyrene
is a typical water-soluble aromatic fluorescent dye and intramolecular
charge-transfer (ICT) fluorophore with an electron acceptor (A) and an
electron donor (D) exhibit a broad and structure-less fluorescent emis-
sion [12]. To improve the emissive properties and ease charge transfer
between the donor and acceptor parts in the excited state, several
conjugated systems including benzene, thiophene and furan rings
generally have been incorporated into pyrene dyes as -linkers [13]. The
great efforts have been devoted to the structural modification of pyrene
π-
π
π
*
Corresponding author.
** Corresponding author.
E-mail address: dahui_qu@ecust.edu.cn (D. Qu).
https://doi.org/10.1016/j.dyepig.2019.107919
Received 25 July 2019; Received in revised form 22 September 2019; Accepted 22 September 2019
Available online 25 September 2019
0
143-7208/© 2019 Elsevier Ltd. All rights reserved.

Y. Si et al.
Dyes and Pigments 173 (2020) 107919
1
Fig. 1. Partial H NMR spectra (400 MHz, 293K) of PVP (0.05 mM) with addition of molar equivalents of CB [7]: a) 0; b) 0.3; c) 0.5; d) 0.7; e) 0.8; f) 1.0 in DMSO‑d
6
/
ꢀ
DCl (50:1, v/v) at 25 C.
while PVP dyes encapsulated by CB [8], the charge-transfer interaction
is enhanced between pyrene and bipyridinium moieties of PVP.
Designing PVP dyes with structural and functional diversity and tai-
lorability is a promising means of fabricating well-formed supramolec-
ular architectures with functions as well as understanding their
self-assembly processes [18].
2.3. Synthesis of the guest
The intermediate compound 1 was prepared by the routes in Scheme
2, the detailed characterization data are deposited in supporting infor-
mation (Scheme S1, Fig. S1).
Compound PVP: Piperidine (36.65 mg, 0.43 mmol) was added
dropwise into a methanol (10 mL) solution of pyrene-1-carbaldehyde
(100 mg, 0.43 mmol) and Compound 1 (136.52 mg, 0.51 mmol), the
2
. Experimental section
ꢀ
reaction solution was stirred for 48 h at 60 C and the reaction mixture
2
.1. Materials and reagents
was allowed to cool to room temperature. Then, methanol (3 � 20 mL)
was added to the mixture. The resulting precipitate was separate by
filtration and the filtrate was concentrated and washed successively with
methanol (3 � 20 mL) to give crude PVP as an orange solid (154 mg,
Parts of chemicals were used as received from Adamas-beta, Acros,
Aldrich, or Merck. CB [7] and CB [8] were synthesized and separated
according to the reported literature [19]. All solvents were reagent
75%). 1H NMR (400 MHz, DMSO‑d
, 298K) δ 9.14 (d, J ¼ 6.6 Hz, 2H),
6
grade, which were dried and distilled prior to use according to standard
9.09 (d, J ¼ 16.0 Hz, 1H), 8.93 (d, J ¼ 9.4 Hz, 1H), 8.63 (d, J ¼ 8.3 Hz,
1H), 8.53 (d, J ¼ 6.6 Hz, 2H), 8.38 (td, J ¼ 7.7, 3.2 Hz, 4H), 8.22–8.29
(m, 2H), 8.12 (t, J ¼ 7.6 Hz, 1H), 7.83 (d, J ¼ 16.0 Hz, 1H), 7.57 (d,
O was used as solvent in 1H NMR and
13
C
procedures. D
2
O or DCl/D
2
NMR experiments.
1
3
J ¼ 6.9 Hz, 2H), 7.51–7.43 (m, 3H), 5.80 (s, 2H). C NMR (600 MHz,
2
.2. Instrumentation
All absorption and fluorescence measurements were performed in
DMSO‑d
6
, 298K) δ 153.72, 144.70, 137.77, 135.19, 132.84, 131.35,
1
30.75, 130.11, 130.01, 129.74, 129.43, 129.29, 129.08, 127.87,
127.22, 126.78, 126.52, 126.09, 125.94, 125.10, 124.75, 124.63,
þ
quartz cuvettes (1 cm � 1 cm � 4.5 cm). The UV/Vis absorption spectra
were recorded on an Agilent Technologies Cary 60 UV–Vis spectro-
photometer. The fluorescence spectra were obtained on a Varian Cray
Eclipse fluorescence spectrophotometer. The excitation and emission
124.21, 123.59, 62.74. HRMS (ESI) (m/z): [M À Br] calcd for C30
H22N,
396.1747; found, 396.1733. (For details, see Support Information,
Figs. S2–S5).
1
13
bandwidths were both set at 10 nm. H NMR and C NMR spectra were
carried out on a Bruker AM400 and AM600 spectrometer. The MS
spectra were recorded on a 4800 Plus M-TOF/TOF analyzer. The elec-
tronic spray ionization (ESI) mass spectra were tested on a LCT Premier
XE mass spectrometer. The binding constant were tested by NANO ITC
instrument. All measurements were performed at room temperature.
3
. Results and discussion
.1. The formation of inclusion complexs
To get an insight into the structure of host-guest complexes, H NMR
3
1
titration experiment of PVP dyes were performed by adding of varied
amounts of CB [7] and CB [8] and the 1H NMR assignments were
ascertained by 2D NMR technique ROESY(Figs. S8–S9).
2

Y. Si et al.
Dyes and Pigments 173 (2020) 107919
1
Fig. 2. Partial H NMR spectra (400 MHz, 293K) of PVP (0.05 mM) with addition of molar equivalents of CB [8]: a) 0; b) 0.6; c) 1.0; d) 1.4; e) 1.5; f) 1.9 in DMSO‑d
6
/
ꢀ
DCl (50:1, v/v) at 25 C.
2 2
Fig. 3. UV/Vis absorption (a, b) spectra and PL (c, d) spectra of PVP (0.2 mM) in H O upon addition of CB [7] (a, c) and CB [8] (b, d) in H O. The insets show the
corresponding absorbance and PL intensities with increasing CB [7] and CB [8] concentration.
1
Compared with the H NMR spectrum of the free PVP (Fig. 1a), most
protons of PVP with the increasing addition of CB [7] were found
distinctly upshifted. Upon addition of 1.0 equiv of CB [7], the signals for
affected [20]. That suggests that these protons were encapsulated into
CB [7] cavity and the methylene-linked benzene rings can rotate into CB
[7] cavity spatially.
proton
Δδ ¼ À 0.09 ppm and Δδ ¼ À 0.13 ppm, respectively). The proton sig-
nals for H , H and H of benzene units moved upfield slightly. In gen-
H
e
and
H
f
on pyridine group underwent upfield shift
As a comparison, while adding 1.0 equiv of CB [8] (Fig. 2c), the
(
e d
pyridine proton H and the methylene proton H experienced a
a
b
c
downfield-shift (Δδ ¼ þ 0.01 ppm; Δδ ¼ þ 0.03 ppm). Continue to add
CB [8], the pyridine proton and other protons almost show a trend of
upfield shift. It obviously means that the dynamic process of PVP dyes
eral, the hydrophobic cucurbit[n]uril cavity inner protons of guest
undergo shielding effect, and those near the carbonyl rim are hardly
3

Y. Si et al.
Dyes and Pigments 173 (2020) 107919
Scheme 1. Schematic representation of the complex formation of CB [7]-PVP and CB [8]-PVP.
Scheme 2. Synthetic routes of PVP.
encapsulation into CB [8] cavity can be clearly observed by the change
of CB [7]. and the maximum absorption peak in 438 nm was reduced
from 0.38 to 0.34 when the chromophore was encapsulated into the
cavity of the CB [7] (Fig. 3a).
1
of H NMR and the aromatic protons encapsulated by CB [8] were
placed near the inter-pyridinium moiety owing to the stacked intermo-
lecular interactions in CB [8] cavities.
The fluorescence intensity of CB [7]-PVP was significantly increased
and the color of the solution changed from orange to bright yellow
(Scheme 1). When 1.0 equiv of CB [7] was added to the system, the
fluorescence intensity of PVP in 600 nm increased by about 800%.
Moreover, the fluorescence emission spectra of it were blue shifted from
607 nm to 569 nm (Fig. 3c). In most cases, the fluorescence emission has
Matrix-assisted laser desorption/ionization time-of-flight mass
spectrometry (MALDI-TOF MS) results, gave a response of 1:1 complex
formation for CB [7]-PVP and CB [8]-PVP (Figs. S6–S7). The coexis-
tence of pyrene and bipyridinium moieties in the cavity of CB [8] was
further indicated by the red-shifted absorption peak observed in UV/Vis
spectra, indicating the formation of charge-transfer interaction in CB
remained low, likely on account of
π
- stacking. However, some de-
π
[
8]-PVP complex [21].
rivatives can become highly fluorescent under the deaggregating con-
ditions of high dilution [22]. Apparently, the pyrene units tend to form
π
- stacking in the polar solution and the CB [8] efficiently suppress
π
3
.2. Spectral properties
aggregation of PVP dyes in water.
In this context, our host-guest combination strategy offers a much
more straightforward approach to protect the PVP dye from self-
aggregation than synthetic modifications of the molecule. Different
from the CB [7]-PVP system, the absorption maximum of CB [8]-PVP
showed shorter red-shifts from 437 nm to 464 nm and the maximum
absorption peak in 437 nm is reduced from 0.38 to 0.31 (Fig. 3b). The
ICT fluorophore in solution exhibits significant emission wavelength
changes by altering the excited state [23] and the ICT fluorophores
suffer from a very low fluorescence efficiency [24] due to the charac-
teristic of forbidden electronic transition. In our case, considering the
pyrene moiety in PVP as strong electron-rich group, the broad
The photophysical properties of PVP were investigated by UV/Vis
absorption and photoluminescence (PL) intensity in water. UV/Vis ab-
sorption titration experiments were carried out to better understand the
interaction between Cucurbit[n]uril (n ¼ 7, 8) and guests. The results
demonstrated the existence of binding modes between the host and
1
guest, which was consistent with H NMR data. Fig. 3 shows changes in
the optical properties upon complexation of PVP with CB [7] and CB
[
8].
The absorption intensity of CB [7]-PVP in H
2
O undergoes a slight
hypsochromic shift (from 606 to 568 nm) which provide the spectral
signature for the deaggregation of the PVP chromophore upon addition
4

Y. Si et al.
Dyes and Pigments 173 (2020) 107919
Table 1
pyridinium unit when the two binding sites are encapsulated by CB [8].
Remarkably, the bathochromic shift also explained that the stacking
occurred after PVP dyes enter CB [8] cavity.
Excitation maxima (λex), emission maxima (λem), stokes shift (Δλ) and fluores-
cence quantum yield (Φ
f
) values for PVP, CB [7]-PVP system and CB [8]-PVP
system (0.2 mM PVP with addition of 1 equivalents of CB [7] and CB [8]
Interestingly, after mixing with CB [8] or CB [7], the absorbance of
PVP both decreased due to the formation of inclusion complex. The
different self-assembly forms result in an opposite spectral shifts. During
the gradual increase of CB [8], the fluorescence intensity first decreases
slightly (Fig. 3d). However, compared with CB [7]-PVP, the color
brightness of the CB [8]-PVP system enhanced but there was no obvious
color change. The structure of CB [8]-PVP system has not been further
calculated. Therefore, we assume a stacking form of CB [8]-PVP to
roughly represent the difference from the CB [7]-PVP complex (Scheme
ꢀ
determined in H
2
O, 25 C).
System
λex/nm
λem/nm
Δλ/nm
Φf (%)
PVP
437
430
464
600
570
600
163
140
136
9.22
CB [7]-PVP
CB [8]-PVP
35.68
16.78
1
).
Combining the UV/Vis absorption and fluorescence emission spec-
trum of PVP (0.2 mM in water with 1.0 equiv CB [7] or CB [8]), the
fluorescence quantum yields of CB [7]-PVP and CB [8]-PVP complexes
were calculated as 35.68% and 16.78%, respectively (Table 1).
Stoichiometric binding ratio of CB [7]-PVP were verified to be 1:1 by
Job’s plot (Fig. 4) and ITC studies (Fig. S10). Applying a modified
Benesi-Hildebrand equation (Table S1) fit the experimental data pre-
a
sented at 430 nm, the binding constants K value was estimated as
6
À 1
4
.97 � 10 M . Otherwise, CB [8] has a larger cavity than CB [7], it can
synchronously bind two planar aromatic molecules [25]. Due to the
poor solubility of CB [8], the Job’s plot titration was not successful. The
CB [8]-PVP complex appears to be the 1:1 as measured by ITC and the
5
À 1
binding constants K
a
value was estimated as 2.49 � 10 M (Fig. S11).
Fig. 4. Job’s plot for the complexation of CB [7]-PVP (the total concentration
of CB [7] and PVP is 0.04 mM).
To further confirm the co-conformations of CB [7]-PVP and CB [8]-
PVP and approve our host-guest encapsulation strategy offered a much
more straightforward approach to protect the PVP dyes from self-
aggregation than arduous synthetic modifications of the dyes, the
stronger binding ammonium hydrochloride (AD) was used [26].
Whether it is CB [7]-PVP or CB [8]-PVP systems, when 1.0 equiv of AD
is gradually added, the UV/Vis showed the opposite trend. With addition
absorption band centered at 463 nm could be contributed to the
charge-transfer interaction with the electron-deficient pyridinium unit
in water. The phenomenon is probably the result of the enhancement of
intramolecular charge-transfer caused by the deshielding effect to the
Fig. 5. UV/Vis absorption (a, b) spectra and PL (c, d) spectra for CB [7]-PVP and CB [8]-PVP (2 � 10À 4 M) upon addition of ammonium hydrochloride (AD) (0–1.0
equiv) in water.
5

Y. Si et al.
Dyes and Pigments 173 (2020) 107919
binding modes. CB [7]-PVP system is demonstrated a facile supramo-
lecular approach to eliminate the self-aggregation of fluorophores in
aqueous and while the CB [8]-PVP can be the example as enhancement
of intramolecular charge-transfer fluorophore complex system.
Furthermore, the PVP dye molecules encapsulated in the cucurbit[n]
uril(n ¼ 7, 8) have good optical stability as fluorescent printing mate-
rials. This approach provides an attainable method to prepare lumi-
nescent materials, which could be a simple encryption printing
technology widely applied in identification, information storage,
markers, writing, and printing.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
We appreciate the Research Center of Analysis and Test of East China
University of Science and Technology for help on the compound char-
acterization. This work was supported by NSFC/China (21790361,
21871084, 21421004, 21672060), the Fundamental Research Funds for
the Central Universities (WJ1616011, WJ1213007, 222201717003),
the Programme of Introducing Talents of Discipline to Universities
Fig. 6. a) The digital photographs of using free PVP dyes as the printing ma-
terial; b) The digital photographs of using CB [7]-PVP as the printing material;
c) The Digital printing picture of CB [7]-PVP placed for one week. (The power
of UV lamp is 8 W, and the wavelength is 420 nm).
(
(
B16017), Program of Shanghai Academic/Technology Research Leader
19XD1421100), and the Shanghai Municipal Science and Technology
of AD to CB [7]-PVP and CB [8]-PVP in water, the decrease in the CB
Major Project (Grant No.2018SHZDZX03).
[
7]-PVP solution absorption was observed as well as a red shift on
4
4
39 nm and the CB [8]-PVP solution undergoes a slight blue shift from
65 nm to 447 nm. The CB [8]-PVP solution also shows the contrary
Appendix A. Supplementary data
phenomena compared with the black CB [8]-PVP system(Fig. 5). This
phenomenon suggests that adding AD can make self-assembling system
return to the state without cucurbit[n]uril (CB[n], n ¼ 7, 8).
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2019.107919.
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