Fluorodynamers Displaying Tunable Fluorescence on Constitutional Exchanges in Solution and at Solid Film–Solution Interface

Article abstract of DOI:10.1002/chem.202000981

Dynamic covalent polymers—dynamers—are adaptive materials that offer timely variant adaptive macroscopic organization across extended scales. In the current study, imine exchange reactions and fluorescence transfer can occur at the interfaces between vari

Full text of DOI:10.1002/chem.202000981

A Journal of
Accepted Article
Title: Fluorodynamers displaying tunable fluorescence on constitutional
exchanges in solution and at solid film-solution interface
Authors: Mingran Si, Weijia Zhu, Yan Zhang, Mihail Barboiu, and
Jinghua Chen
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Fluorodynamers displaying tunable fluorescence on
constitutional exchanges in solution and at solid film-solution
interface
Mingran Si,[a] Weijia Zhu,[a] Yan Zhang,*[a] Mihail Barboiu*[b] and Jinghua Chen*[a]
[a]
M. Si, W. Zhu, Prof. Y. Zhang, Prof. J. Chen
Key Laboratory of Carbohydrate Chemistry and Biotechnology, Ministry of Education, School of Pharmaceutical Sciences
Jiangnan University
1800 Lihu Avenue, Wuxi, 214122, P.R. China
E-mail: zhangyanyz@jiangnan.edu.cn; chenjinghua@jiangnan.edu.cn
Dr. M. Barboiu
[b]
Institut Europeen des Membranes, Adaptive Supramolecular Nanosystems Group
University of Montpellier, ENSCM-CNRS
Place E. Bataillon CC047, Montpellier, F-34095, France
E-mail: mihail-dumitru.barboiu@umontpellier.fr
Supporting information for this article is given via a link at the end of the document.
Abstract: Dynamic covalent polymers -the dynamers- are adaptive
materials that offer timely variant adaptive macroscopic organization
across extended scales. In the current study, imine exchange
reactions and fluorescence transfer can occur at the interfaces
between various solutions and solid state dynameric films. The
fluorescence quenching upon imine formations for designed fluorogen
was successfully demonstrated, and this tunable fluorescence was
further used to study the re-composition of a solid film. Moreover, the
dynamic covalent films also exhibited responsiveness to competing
amines and acid/base conditions, both in solutions and solid film-
solution interface. This work can provide more insights into interface
dynamic chemistry and hold great potentials for further applications in
optical and biomedical materials.
etc.[15] There are also studies using fluorescent probes in dynamic
covalent systems to indicate the switch of molecular bonds or
change of assembly states in solution between various target
entities.[16] For example, the formation of imine bonds can usually
result in the switch off the emission of amine functionalized
fluorogens, leading to in situ observation of dynamic reactions in
solution via ligand displacement assays[17] or in solid films of
optodynamers.[13,18]
Dynamic constitutional chemistry and its subfield dynamic
covalent chemistry have been continuously developed during the
last two decades.[1] The accumulated achievements have led to
enriched diversities in reaction types,[2] intricate architectures,[3]
advanced properties[4] and the increasingly expanded
applications, varying from chemical/enzymatic catalysis[5] to
biomimetic signal transfer[6] to adaptive smart materials.[7] Among
all these advances, diverse reversible covalent reactions have
been explored[8] and the arrangement of dynamic networks has
been diversified,[9] providing adaptive systems with higher
complexity and multiple responsiveness.
More recently, dynamic chemistry on interfaces has
emerged as a new research topic. Studies about dynamic
covalent reactions between liquid-solid,[10] liquid-liquid,[11] liquid-
air[12] and solid-solid[13] interfaces have been illustrated. One direct
proof of component exchanges on solid-solid interface is the self-
healing[7a, 14] or optical[13a,b] properties of the dynamers, indicating
the breaking down and reconnection of the reversible covalent
bonds through the interface. Furthermore, there are a few direct
observations of the constitutional exchanges between solid-liquid
interfaces, when sophisticated 2D structures are detected by
scanning tunneling microscopy (STM).[10b]
Figure 1. Schematic illustration of the imine exchange concept with
fluorescence tuning.
In the current study, to set up the dynamic covalent
exchanging fluorodynamer, fluorogenic amine 4-(1H-Phenanthro
[9,10-d]imidazol-2-yl)benzenamine, L0 was chosen as the core
structure. The synthesis of L0 was through two steps:[19] first, a
condensation reaction between phenanthrene-9,10-dione, 4-
nitrobenzaldehyde and ammonium acetate, followed by reduction
of the nitro group, providing L0 as a yellow powder (yield 67 %).
Using dimethyl sulphoxide (DMSO) as the solvent and 350 nm as
the excitation wavelength, L0 exhibited a fluorescence emission
band with a maximum at λem 452 nm, associated with the π-π*
transition from the phenanthrene moiety. Subsequently, the
reaction between the fluorogenic amine L0 and 3-
pyridinecarboxaldehyde 1 was performed in methanol for 24 h at
60 °C, yielding 88 % formation of imine product L1 (Figure 2a).
Under the same excitation conditions, the emission intensity of L1
in methanol was decreased ~ 80 % compared to L0 (Figure 2b),
indicating the quenching of fluorescence by transforming amine
L0 to imine L1. This can be explained by photoisomerization of
the imine bond, which accelerated non-radiative transition of the
activated state of the imine molecules.[20] Moreover, there is a
small shift of the emission peak from 452 nm (green line in Figure
Fluorescent probes with high sensitivity and selectivity are
frequently used in dynamic systems, mostly related to imaging of
protein interactions, drug delivery processes, ion-metal sensing,
1
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2b) of L0 to 449 nm of L1, owing to the change of electron density
of the phenanthrene fluorogen after the addition of imine double
bonds.[16a]The addition of HCl to a methanolic solution of L1
results in the imine-bond hydrolysis and an immediate recovery of
the fluorescence intensity (λex 350 nm, λem 452 nm) to over 85 %
of the original value of L0 (Figure 2c). Subsequently, the addition
of NaOH allow to decrease the fluorescence intensity (λex 350 nm,
λem 452 nm) to a substantial extent after 24 h, reflecting the
gradual reformation of imine bond. The slightly higher
fluorescence intensity of the final mixture than L1 can be
contributed to the decreased available aldehyde 1 from the
promoted aldol reaction in such basic condition, making it difficult
to be completely converted to imine L1 again.
With the optimized imine exchange condition in hand, other
amines were examined, including substituted anilines, such as p-
toluidine (3), 4-methoxyaniline (4), 4-nitroaniline (5) and 4-
chloroaniline (6) or aliphatic linear 2,2-(ethylenedioxy)
bis(ethylamine) (7) and cyclic cyclohexylamine (8) (Figure 3).
Among the various substituted anilines, it was apparent that the
presence of electron-donating groups accelerated the exchange
reaction, resulting in the higher formation of imines P3 (81 %) and
P4 (89 %) after 24 h, comparing to the P2 (60 %). Oppositely, the
amines substituted with electron-withdrawing groups showed
much lower conversions after 24 h, to P5 (2 %) and to P6 (34 %).
Besides, a high reactivity was observed for the double amine
substituted compound 7, leading to the imine P7 (65 %), and a
low conversion to P8 (11 %) was observed for the sterically
hindered cyclic structure of amine 8.
After establishing the fluorescent switch system tuned by
dynamic imine chemistry in solution, the fluorescence on/off
mechanism was applied to fluorodynameric species in solution
and at the solid film-liquid interfaces. First, a fluorodynamer was
prepared by connecting 1,3,5-benzenetricarbaldehyde (9),
diamine 7 and fluorogenic amine L0 (molar ratio 1:1.4:0.2)
through imine formation reaction (Figure 4a), occurring in MeOH
for 10 min at room temperature. The solution was poured then
into a Teflon mode (2 cm × 2 cm) and the solvent was evaporated
at room temperature, yielding a yellow thin film. Infrared
spectroscopy was subsequently used to analyse the film. After the
reaction, the aldehyde peak at 1694 cm-1 (marked as a green line)
from 9 diminished, meanwhile, a new peak appeared at 1643 cm-
Figure 2. a) Imine formation reaction from fluorescent amine L0 and aldehyde
1; b) fluorescent spectroscopy of L0 and L1 (under λex 350 nm); c) the
fluorescent intensity changes of L1 subjected to acid and base treatments.
1
(Figure 4b), indicating the consumption of all the aldehyde
groups and the formation of imine bonds. Furthermore, the film
surface morphology was obtained from scanning electron
microscopy (SEM), while porous structures containing micro-
sized particles with diameter around 2 μm were observed (Figure
4c), which may come as previously observed,[21] from
hydrophobic/hydrophilic phase segregation during the film
formation. Derivative thermogravimetric analysis (DTG) and
differential scanning calorimetry (DSC) showed good
thermostability up to ~ 300 °C of the fluorodynamer (Figure S12-
S13).
Another approach to tune of the fluorescence of L1 through
dynamic imine bonds can be realized by the addition of a second
amine. Thereafter, the imine exchange reaction was optimized by
using different amounts of aniline (2) in MeOH at room
temperature. The results showed that the increased amounts of 2
obviously raised the conversions and a L1 : amine = 1 : 4 molar
ratio has been selected (Table S1, Figure S10a). Then, MeOH,
H2O, EtOH and iPrOH solvents were evaluated, among which,
MeOH provided the highest conversion (60 %) after 24 h, followed
by EtOH (11 %) and iPrOH (8 %), while H2O only gave trace
amount of product under the same conditions due to the poor
solubility of L0 (Figure S10b). On the other hand, elevated
temperatures (up to 60 °C) and increased concentration of L0
(from 0.8 mM to 4.8 mM) led to only a bit higher (< 10 %) reaction
conversions (Figure S10c-d), so room temperature and 1.6 mM
was chosen for operational convenience.
Figure 4. a) Preparation of the fluorodynameric film from trialdehyde 9, diamine
linker 7 and fluorescent amine L0; b) Infrared (IR) spectroscopy of the film
compared to individual starting components; c) Scanning electron microscopy
(SEM) image of the film surface.
The fluorodynameric film was next subjected for imine exchange
reactions between the solid film-amine solution interfaces. The
imine bonds inside the solid film with quenched fluorescence can
Figure 3. Imine exchange reactions between L1 and various amines with the
indicated conversions, measured after 24 h with fluorescence spectroscopy.
2
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be replaced by another amine in solution and release the free
amine L0, turning on the fluorescence. As the competing amines,
aniline or its substituted derivatives were included in the MeOH
solution, in which the film was also immersed. After certain time
intervals, 5 μl of each solution was retrieved for analysis of
fluorescent intensity, which can be subsequently transformed into
the concentration of free L0 and reaction conversions. From the
time-course analysis, typical exponential curves were obtained for
most of the exchange reactions (Figure 5), indicating first order
kinetics. Compared to aniline, substitution of electron-donating
groups, such as methyl- aniline (3) and methoxyl- aniline (4), can
not only accelerate the initial reaction rates of imine exchange,
but also contribute to the formation of new imine bonds, leading
to higher percentages of released L0 in solution: 33 % for amine
4 and 25 % for amine 3, comparing to 18 % for aniline 2 after 3
days. On the other hand, the electron-withdrawing group in p-
chloro-aniline (6) decreased the final concentration of free L0 to
15 %, while p-nitro-substituted aniline (5) showed very slow
reaction rates, without reaching the equilibrium even after 20 days.
This trend was in consistence with their individual reactivity of the
aromatic amines, where the nucleophilic attack ability of amines
was promoted or weakened by the electron distribution of the
neighbouring aromatic ring.
variously substituted electron-donating/withdrawing groups was
observed as in solution.
The fluorescence properties can be examined by the confocal
microscopy in solid state. The L0 powder alone gave blue
emission light, induced by λex 405 nm, which was in consistence
with previous reports.20 After forming the film through covalent
imine bonds, the blue light was mostly disappeared and replaced
by green emission under λex 488 nm (Figure 6a). Moreover, a film
that was reacted with aniline 2 in MeOH for 4 h was also
measured by confocal microscopy. As a result, with the mixed
excitation of λex 405 nm and λex 488 nm, there were more blue
spots emerged on the edge of the film while the green emission
was clearly weakened (Figure 6b), indicating the release of the
free amine L0 during the solid film-solution imine exchange
processes.
Figure 6. The images of a) the original film and b) the film edge after reacting
with aniline for 4 h, obtained by confocal microscopy, under the excitations of
both 405 nm and 488 nm. The green emission light represents the imine film
(under λex 488 nm), while the blue emission light corresponds to the free amine
L0 (under λex 405 nm).
In summary, we have designed a fluorodynameric system based
on dynamic covalent bonds, and examined its uses in direct
observation of the imine exchange processes in solution and
between solid film-solution interfaces. The fluorogen L0 used
here can be quenched after forming imine bonds with aldehyde 1
and present acid/base fluorescence on/off responsiveness. The
fluorogen L0 was further embedded through imine formation in
dynameric networks, which were able to perform imine
exchanges in response to external amines effectors in solution
and at the interface solid film-solution. These results showed that
the exchange rates were highly influenced by the substituents on
the competing exchanging amines. In all cases, L0 was
successfully released from the solid film, with slower rates than
the exchanges in solution, as monitored by both fluorescence
spectroscopy and confocal microscopy results. These features
have promising potential for its implementation in optical materials
for adaptive sensing or photoactive devices responding to
external stimuli. The dynamic exchanges described here,
emphasized the important opportunities offered by the application
of the principles of constitutional dynamic chemistry to materials
science.
Figure 5. The time-course analyses of the progress of the imine exchange
reaction between the solid-solution interfaces, monitored by fluorescent
spectroscopy, fitted by non-linear regression analysis. The reactions were
performed with the solid film in MeOH with 4 eq. of competing amines 2-6.
As a reference, the competing amine/imine fluorodynamer
exchanges were examined in solution. For all tested amines, both
the reactivity and the final reached equilibrium were higher than
the case in solid film-solution interfaces (Figure S16). For instance,
the highest conversion was achieved for 4-methoxyaniline (4)
(89 %), comparing to 33 % on solid-solution interfaces. This can
probably be explained by the physical barrier presented from the
solid film, demanding extra film-diffusion for all the amines.
Furthermore, not surprisingly, the same reactivity trend among
Acknowledgements
The study was in part supported by the Natural Science
Foundation of Jiangsu Province (BK20180625), the National
First-class Discipline Program of Light Industry Technology and
3
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Entry for the Table of Contents
Sold films constituted of dynamic covalent fluorodynamers have been designed and used to analyse the interface imine exchange
reactions between solid-solution interface, leading to re-composition and tunable fluorescence in response to competing amines and
acid/base conditions.
5
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