Twisted Schiff-base macrocycle showing excited-state intramolecular proton-transfer (ESIPT): Assembly and sensing properties

Article abstract of DOI:10.1039/c9cc09540c

A Schiff-base macrocyclic host (MH) showing ESIPT and AIEE is reported for the first time; the fluorescent macrocycle exhibited good pH tolerance and could serve as a probe for Cu²⁺ and Fe³⁺ ion sensing and removal. In addition, a series of size-tunable nanoparticles was fabricated by the self-assembly of free MH and its coordination complex with Fe³⁺ in THF/water solution.

Full text of DOI:10.1039/c9cc09540c

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DOI: 10.1039/C9CC09540C
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Twisted Schiff-base macrocycle showing excited-state
intramolecular proton-transfer (ESIPT): assembly and sensing
properties
Received 00th January 20xx,
Accepted 00th January 20xx
Qing Yua, Xiaodong Zhangb, Shou-Ting Wua, Huaiyu Chenb, Qi-Long Zhanga*, Hong Xua, Ya-Li
Huanga, Bi-Xue Zhub and Xin-Long Nib*
DOI: 10.1039/x0xx00000x
A Schiff-base macrocyclic host (MH) showing ESIPT and AIEE is (AIEE),13 dendrimer encapsulation,14 and restriction of
reported for the first time, the fluorescent macrocycle that intramolecular rotation (RIR),15 have been devised with the aim
exhibited good pH tolerance and could serve as a probe for Cu2+ and of improving ESIPT emission quality. Herein, we report a twisted
Fe3+ ion sensing and removal. In addition, a series of size-tunable Schiff-base macrocycle-supported ESIPT emitter for the first
nanoparticles was fabricated by the self-assembly of free MH and time, which emitted yellow fluorescence not only in dilute
its coordination complex with Fe3+ in THF/water solution.
solution, but also in the solid state. In particular, because of the
rigid three-dimensional (3D) skeleton of the twisted macrocyclic
scaffold, the ESIPT-based fluorophore exhibits AIEE.
Excited-state intramolecular proton transfer (ESIPT)1 refers to
photo-induced proton tautomerization, where by an enol
tautomer is converted into the keto form. Remarkable dual-
fluorescence emission with large Stokes shifts can be generated
by ESIPT through drastic electronic and structural changes.2
From a structural viewpoint, the prerequisite for ESIPT is the
presence of an intramolecular hydrogen bond between a
proton donor (–OH or –NH2) and a proton acceptor (=N– or –
C=O), that is to say, these groups have to be in close proximity
within a molecule.3 Reports have indicated that the spectral
properties of ESIPT fluorophores are strongly dependent on
hydrogen bonding,4 rotamerization,5 solvents,6 and the
acidity/basicity of the surrounding medium.7 Because of their
unique photophysical properties, ESIPT fluorophores have been
widely applied in various fields, including fluorescent probes
and logic gates,8 fluorescence imaging,9 and solid organic light-
emitting diodes.10
Generally, most of the reported ESIPT molecules have been
highly emissive in dilute solution in nonpolar media, but weakly
emissive at high concentration, in polar solvents, and in the
solid state,11 which has greatly limited their further application.
Therefore, various strategies, including aggregation-induced
emission (AIE),12 aggregation-induced enhanced emission
Fig. 1 Chemical (a) and X-ray (b) structure of MH.
The macrocycle host (MH; Fig. 1) was obtained by a simple
one step Schiff-base reaction in methanol solution at room
temperature. Our recent studies suggested that a series of core-
shell-based organic micro/nanospheres, including their
intermediate forms, could be directly precipitated from the
reaction solution.16 The X-ray crystal structure of solid MH
clearly revealed that the macrocycle adopted a twisted 3D
helical form, involving multiple intramolecular hydrogen-
bonding interactions. Interestingly, as shown in Fig. 1b, a rigid
coplanar arrangement (C1, C2) between the phenol and
aromatic phenyl groups, fixed by hydrogen bonds O3-N3 and
O7-N8, was observed in both side chains of the twisted MH,
a. School of Basic Medical Science/School of Public Health, Guizhou Medical
University, Guiyang 550025, China.
b. Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou
Province, Guizhou University, Guiyang, Guizhou 550025, China. E-mail:
zq810921@126.com (Q.L. Zhang); longni333@163.com (X.-L. Ni)
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: UV-vis and fluorescence
spectra, X-ray structure, SEM and TEM. CCDC 1969906. For ESI and crystallographic
data in CIF or other electronic format See DOI: 10.1039/x0xx00000x
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along with further coplanar arrangements (C3, C4) between the core-shell-based particles of MH directly precipitated from the
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phenol and aromatic phenyl moieties (hydrogen bonds O4-N4, original Schiff-base reaction.16 This phenomenon revealed that
DOI: 10.1039/C9CC09540C
and O8-N9) with dihedral angles of 34° and 38° (Fig. S1), the supramolecular assembly MH has potential applications in
respectively. The relevant hydrogen-bond distances are fabricating diverse organic nanoparticles. Moreover, pH
between 2.55 and 2.61 Å, implying that classical ESIPT emission titration experiments suggested that the macrocyclic-scaffold-
could be triggered by the coplanar moieties in MH.
supported ESIPT had high pH tolerance. As shown in Fig. S4, MH
displayed strong yellow emission (ESIPT) at pH < 12 in
water/THF solution, and a titration jump at pH >12 with a blue-
shift of its emission from 553 to 500 nm. In the reference
compound hemi-MH (L), which lacked the support of the rigid
macrocyclic scaffold (Scheme S1), the ESIPT was only observed
in the pH range 5–9 (Fig. S5). The emission changes of MH and
L at high pH could be attributed to deprotonation of the
phenolic O–H rather than decomposition the C=N bonds (Figs.
S6-S9). In addition, L exhibited a classical AIE-based ESIPT
property. As shown in Fig. S10, no fluorescence emission was
observed for L in pure THF solution and in THF/water mixtures
with fw lower than 50%, and the fluorescence emissions
enhanced remarkably when the fw were between 60% – 90%.
This result implied that the twisted macrocyclic scaffold played
a key role in the stability of the Schiff-based-derived AIEE based
ESIPT of MH.
Fig. 2 Fluorescence spectra of MH (20.0 µM) in different solvents (a), and in THF/water
mixtures (b) with different water fractions (fw). (λex = 395 nm)
A UV-vis absorption spectrum in THF solution revealed that
the absorption of the twisted MH appeared in the region 250–
400 nm (Fig.S2). The broad absorption wavelength range was
due to the formation of intramolecular hydrogen bonds in MH.
As shown in Fig. 2a, the macrocycle produced a yellow emission
with a maximum at 553 nm upon excitation at 395 nm in DMF
or DMSO solution at room temperature. The emission intensity
was increased in THF or CH2Cl2 solution, and enhanced dual-
emission was clearly observed for MH in CHCl3, CH3OH, and
CH3CH2OH solutions, which is characteristic of ESIPT. The enol
form emitted at a shorter wavelength and the keto form emits
at
a longer wavelength. The accompanying enhanced
fluorescence intensity of the dual-emission was ascribed to the
poor solubility of MH in these solvents, in which its planarity
and rigidity were enforced. This result indicated that the
macrocyclic-scaffold-supported ESIPT was AIEE-active. For
example, the process of the coplanar arrangement of C3 and C4
like the coplanar from C1, and C2 in less solvating media is the
cause of the AIEE (Fig. S1).
The AIEE properties of MH were further evaluated in
THF/water mixtures with different water fractions (fw), which
enabled fine-tuning of the solvent polarity and the extent of
solute aggregation (Fig. 1b). MH in pure THF solution exhibited
a moderate yellow fluorescence with an emission maximum at
553 nm. Upon gradual addition of water to THF from fw = 0 to
90 vol.%, the emission intensity increased steadily. From fw = 90
to 99 vol.%, the emission decreased slightly due to some
precipitation owing to the poor solubility of MH in aqueous
media. The precipitate from 99% water was collected by
filtration as a pale yellow powder. Fluorescence spectral
analysis of the solid indicated that the MH exhibited yellow
fluorescence with a maximum emission wavelength around
560 nm in the solid state (Fig. S3). The above results strongly
supported the occurrence of AIEE-based ESIPT emission in MH.
Most interestingly, SEM and TEM images demonstrated that the
precipitate collected from THF/water (1:99, v/v) was composed
of numerous discrete solid organic particles with diameters
around 500 nm (Fig. 4a, c). This was very different from the
Fig. 3 UV-vis and fluorescence spectra of MH (20.0 µM) in THF/H2O (1:4, v/v) in presence
of increased concentration of Cu2+ (a, c) and Fe3+ ions (b, d). (λex = 395 nm)
To further exploit the macrocycle-supported AIEE-based
ESIPT in a fluorescent probe, the selectivity of MH towards
metal ions was evaluated. The nitrogen-containing Schiff-base
bond and the hydroxyl moiety together formed an ideal
coordination motif for transition metal and heavy metal
cations.17 Fig. S11 shows the UV-vis spectra of MH in THF/H2O
(1:4, v/v) solution in the presence of various metal ions. No
significant changes were observed in the spectra, except in the
cases of Cu2+ and Fe3+ ions. The addition of Cu2+ and Fe3+
produced new red-shifted absorption bands at around 450 nm,
such that the initially colourless solution became yellow or pale-
yellow. These changes could be easily discerned by the naked
eye. The intensities of the new red-shifted absorption bands of
MH increased as a function of the Cu2+ and Fe3+ concentrations
(Fig. 3a, b). Essentially, these red-shifted bands were due to
2 | J. Name., 2012, 00, 1-3
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metal-coordination-induced intramolecular charge-transfer that Fe3+ ions were uniformly distributed in the nanoparticles.
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(ICT)18 transitions.
This observation revealed that MH may find broader application
DOI: 10.1039/C9CC09540C
Fig. 3c, d and Fig. S12 show the fluorescence spectral as a nanocarrier for Fe3+. On the contrary, no regularly particles
changes of the ESIPT emission for MH (in THF/H2O, 1:4, v/v) were observed in the precipitates of MH/Cu2+ complex (Fig.
under the same conditions as applied for the UV-vis spectra. As S19).
can be seen, among the metal ions tested, no significant
fluorescent spectral changes were observed upon addition of
most of the cations, but Cu2+ and Fe3+ ions induced remarkable
fluorescence quenching. This fluorescence quenching could be
partly ascribed to the coordination of Cu2+ and Fe3+ breaking the
intramolecular hydrogen bond (OH···N) with the imine nitrogen
of MH, thereby inhibiting the ESIPT emission at 560 nm, but also
partly ascribed to the paramagnetic properties of Cu2+ and Fe3+
(or the heavy atom effect). The latter involved excitation
charge- or energy-transfer from MH to a vacant d-orbital of Cu2+
or Fe3+, providing a very fast and efficient nonradiative decay
pathway for the excited state of MH.19 Interestingly, when the
hemi-MH (L) was tested as a probe for sensing the various metal
ions, it was found that the fluorescence of L was also quenched
by Cu2+ and Fe3+ ions, but a strong blue-shifted emission was
observed in the case of Al3+. As shown in Fig. S13, upon gradual
addition of Al3+ ions to a solution of L, the emission intensity at
541 nm was quenched, and a new emission maximum at
490 nm was significantly increased. We surmise that the
coordination of Al3+ removed the phenolic protons, which
disrupted and inhibited the ESIPT emission of L at 541 nm, while
the increased fluorescence emission intensity at 490 nm could
be ascribed to an Al3+-coordination-induced ICT normal
emission mechanism. Compared to Cu2+ and Fe3+, Al3+ had fully
occupied p-orbitals, such that no energy- or charge-transfer
processes took place, and hence the ICT emission was not
quenched. Notably, the different fluorescence responses of MH
and L towards Al3+ confirmed that the macrocyclic scaffold
facilitated pre-organization of the geometric structure of the
Fig. 4 SEM (a) and TEM (b) images of the precipitate of MH from THF/H2O (1:99, v/v);
ligand for the recognition of specific metal ions.
Job plots for the complexations showed a 1:2 stoichiometry
of MH with both cations (Fig. S14). Therefore, on the basis of
SEM (c) and TEM (d) images of the precipitate of MH/Fe3+ from THF/H2O (1:4, v/v); and
the corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping
images (e–h). (Scale bar: a, b is 2.0 µM, c, d is 200 nm, e-h is 500 nm.)
the UV-vis titration experiments, the two step binding constants
–1
(Ka)20 of MH were determined to be 1.10 × 103 M and 3.68 ×
106 M–1 for Cu2+, and 1.21 × 103 M–1 and 2.36 × 106 M–1 for Fe3+,
respectively (Fig. S15). This result implying that the first binding
event lead to positive cooperativity for the second association
of MH with the cations.21 No significant interference in the
detection of Cu2+ and Fe3+ was observed in the presence of other
competitive cations (Fig. S16). Furthermore, it was observed
that extensive precipitation occurred when Cu2+ or Fe3+ ions
were added to a solution of MH after 1h even in the presence
of other competitive ions. This result indicated that MH could
not only be used as a colorimetric and fluorimetric probe for
Cu2+ and Fe3+ ions, but may also serve as a separation material
through adsorption of both cations. Most importantly, SEM and
TEM images of the precipitate of MH/Fe3+ demonstrated that it
was composed of a large amount of solid nanoparticles of sizes
in the range 900–1,000 nm (Fig. 4b, d and Fig. S17), larger than
the particles of the precipitate of free MH from THF/water
solution (Fig. 4a, c and Fig. S18). Elemental mapping analysis of
energy-dispersive X-ray spectroscopy (EDS) (Fig. 4e-h) showed
Conclusions
In summary, a twisted Schiff-base macrocycle showing ESIPT
has been constructed, and its properties have been evaluated.
Experimental results have suggested the macrocyclic-scaffold-
induced AIEE-based ESIPT probe not only exhibited good pH
tolerance, but may also be utilized as a colorimetric and
fluorimetric probe for Cu2+ and Fe3+ sensing and removal.
Interestingly, SEM and TEM studies indicated that a series of
size-tunable nanoparticles could be obtained from the self-
assembly of free MH and its coordination complex with Fe3+.
This work thus provides new insights for the fabrication of
fluorescent-macrocycle-based ESIPT and AIEE probes as well as
functional organic nanoparticles.
This work was supported by the National Natural Science
Foundation of China (No. 21871063 and 21961007) and the
This journal is © The Royal Society of Chemistry 20xx
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