Emissive Platinum(II) Cages with Reverse Fluorescence Resonance Energy Transfer for Multiple Sensing

Article abstract of DOI:10.1021/jacs.9b12689

It is quite challenging to realize fluorescence resonance energy transfer (FRET) between two chromophores with specific positions and directions. Herein, through the self-assembly of two carefully selected fluorescent ligands via metal-coordination intera

Full text of DOI:10.1021/jacs.9b12689

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Article
Emissive Platinum(II) Cages with Reverse Fluorescence
Resonance Energy Transfer for Multiple Sensing
Zeyuan Zhang, Zhengqing Zhao, Lianwei Wu, Shuai Lu, Sanliang Ling, Guoping Li, Letian Xu, Lingzhi
Ma, Yali Hou, Xingchen Wang, Xiaopeng Li, Gang He, Kai Wang, Bo Zou, and Mingming Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12689 • Publication Date (Web): 15 Jan 2020
Downloaded from pubs.acs.org on January 15, 2020
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Emissive Platinum(II) Cages with Reverse Fluorescence
Resonance Energy Transfer for Multiple Sensing
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Zeyuan Zhang,†,# Zhengqing Zhao,†,# Lianwei Wu,‡ Shuai Lu,§,∇ Sanliang Ling,║ Guoping Li,⊥
⊥
⊥
,‡
Letian Xu, Lingzhi Ma,† Yali Hou,† Xingchen Wang,† Xiaopeng Li,§ Gang He, Kai Wang, Bo
,†
Zou,‡ Mingming Zhang
†State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for
Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R.
China
‡State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, P.
R. China
§Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States
∇College of Chemistry, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China
║Advanced Materials Research Group, Faculty of Engineering, University of Nottingham, Nottingham,
NG7 2RD, United Kingdom
⊥Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710049, P. R. China
Supporting Information
■ABSTRACT
It is quite challenging to realize fluorescence resonance energy transfer (FRET) between two
chromophores with specific positions and directions. Herein, through the self-assembly of two
carefully selected fluorescent ligands via metal-coordination interactions, we prepared two
tetragonal prismatic platinum(II) cages with reverse FRET process between their faces and pillars.
Bearing different responses to external stimuli, these two emissive ligands are able to tune the
FRET process, and thus making the cages sensitive to solvents, pressure and temperature. First,
these cages could distinguish structurally similar alcohols such as n-butanol, t-butanol and i-
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butanol. Furthermore, they showed decreased emission with bathochromic shifts upon high
pressure. Finally, they exhibited remarkable ratiometric response to the temperature in a wide
range (223~353 K) with high sensitivity. For example, by plotting the ratio of the maximum
emission (I600/I480) of metallacage 4b against the temperature, the slope reaches 0.072, which is
among the highest values for ratiometric fluorescent thermometers reported so far. This work not
only offers a strategy to manipulate the FRET efficiency in emissive supramolecular coordination
complexes, but also paves the way for the future design and preparation of smart emissive materials
with external stimuli responsiveness.
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■ INTRODUCTION
Energy transfer is of vital importance within many significant processes including the combustion
of matter, the generation of electricity, photosynthesis, etc. Fluorescence (or Fꢀrster) resonance
energy transfer (FRET) describes the energy transfer between different chromophores through
nonradiative dipole-dipole coupling.1 A well-known FRET system is the energy transfer from
carotenoids to chlorophyll in natural photosynthesis,2 which is considered as the most important
chemical reaction on the earth. Efficient FRET requires suitable distance between the energy
donors and acceptors as well as good spectral overlap between the emission of the donors and the
excitation of the acceptors.3 Compared with covalently linked systems, FRET systems formed via
noncovalent interactions not only affords a convenient method to avoid the time-consuming
synthesis, but also reduces the possibilities of emission changes caused by covalent
functionalization.4 Therefore, various FRET-based supramolecular systems have been designed
along with exploration in biomimicry,5 bioimaging,6 light-emitting materials,7 and photocatalysis8
during the past two decades.
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Self-assemblies formed by coordination bonds, including metal–organic frameworks (MOFs)9
and supramolecular coordination complexes (SCCs),10 have become one of the most important
themes in supramolecular materials during the past three decades. Abundant SCCs with fascinating
structures11 have been successfully constructed for guest encapsulation, catalysis, sensing and
stabilizing reactive intermediates,12 etc. Compared with other non-covalent interactions, metal-
coordination interactions are still effective even at micromolar or even nanomolar
concentrations;13 while for hydrogen bonding and host-guest interactions, the complexes
dissociate dramatically upon dilution due to their low association constants for common systems.14
Therefore, metal-coordination interactions show great advantages to prepare effective FRET
systems, especially because the study of FRET process should also be performed at the micromolar
or even nanomolar concentration to reduce the influence of molecular aggregations. Some
successful examples can be found in MOF-based systems15 in which two different chromophores
are properly manipulated to achieve efficient FRET to mimic light-harvesting process in
photosynthesis. However, the poor solubility and stability of MOFs in common solvents limit their
processability for further applications. Although the preparation of SCCs offers an alternative way
to solve this problem, metallacycles and metallacages with efficient FRET have rarely been
reported,16 due to the difficulties in the design and synthesis of two compatible chromophores with
ideal spectral overlap as well as the quenching of emission by transition metals (also known as
heavy atom effect).17
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In order to solve the heavy atom effect, Stang18 and others19 referred to the concept of
aggregation induced emission (AIE)20 raised by Tang et. al. and developed coordination induced
emission. For AIE-based systems, the fluorophores emit strongly at high concentration or in the
aggregation state while are nearly non-emissive in dilute solution. In most of the AIE-based
fluorophores, propeller-like structures offer a pathway for molecular motion to deactivate the
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molecules in the excited state in dilute solution through thermal decay. In aggregation state, the
molecular motion is restricted due to molecular aggregation, and thus reducing the non-radiative
decay to give bright emission. In coordination induced emission-based systems, the motion of the
molecules is restricted by coordination bonds, instead of molecular aggregation in AIE-based
systems, to give strong emission. A series of emissive metallacycles and metallacages was
prepared based on tetraphenylethene (TPE) for chemical sensing,21 light-emitting22 and light-
harvesting materials.23 However, the FRET process within these structures remains unexplored.
Herein, through the careful functionalization of the pillar parts of TPE-based tetragonal prismatic
platinum(II) cages,21b we are able to efficiently control the FRET either from the pillars to the faces
(cage 4a) or from the faces to the pillars (cage 4b). Moreover, because the faces (1) are an AIE
fluorophore while the pillars (2a and 2b) are aggregation caused quenching (ACQ) fluorophores20
as well as their structural differences, they show different responses to temperature, mechanical
pressure, and solvent composition and polarity. Such different responses further affect the spectral
overlap between the energy donors and acceptors, making metallacages 4a and 4b very sensitive
to environmental changes. Therefore, metallacages 4a and 4b can act as sensors for temperature,
pressure and solvents. This study not only offers a convenient method to prepare SCCs with
efficient energy transfer but also explores their application towards probing and sensing, which
will facilitate the development of stimuli-responsive light-emitting materials.
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■ RESULTS AND DISCUSSION
Design, Preparation and Characterizations of Metallacages
Based on previous study,21-23 TPE-based benzoate ligand (1) which acts as the faces of the
tetragonal prismatic metallacages generally absorbs in the UV region (250~400 nm) and shows a
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maximum emission in the range of 480~500 nm. If TPE faces act as the acceptor within the cage,
the pillar parts as the donor should emit near the UV region to achieve efficient FRET. While if
TPE faces behave as the donor within the cage, the pillar parts as the acceptor should possess an
obvious absorption band at ca. 480~500 nm. Moreover, the pillar molecules should be 180o
dipyridyl ligands with rigid configuration to meet the requirements for directional self-assembly.
Based on these principles, we designed two metallacages with distinct FRET behaviors. First,
compound 2a with a strong emission band centered at 422 nm was chosen as the donor to prepare
metallacage 4a to study the FRET from the pillars to the faces (Scheme 1). Second, Rhodamine B,
a well-known fluorophore which shows an intense absorption band from 450 nm to 600 nm and
high quantum yield, was linked to the pillar parts to prepare compound 2b as the energy acceptor
to achieve efficient FRET from the faces to the pillars within metallacage 4b. These two
metallacages possess reverse FRET process either from the pillars to the faces (4a) or from the
faces to the pillars (4b), which greatly influences their photophysical properties.
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Scheme 1. Self-assembly of the metallacages 4a and 4b with reverse FRET behaviors.
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The preparation of metallacages 4a and 4b is depicted in Scheme 1. Based on the self-assembly
of TPE-based ligand 1, dipyridyl ligands 2a or 2b and cis-Pt(PEt3)2(OTf)2 3 in a 1:2:4 molar ratio,
metallacages 4a and 4b were synthesized in good yields (Scheme 1). The structures of
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metallacages 4a and 4b were evidenced by 31P{1H} NMR and H NMR spectroscopy and
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electrospray ionization time-of-flight mass spectroscopy (ESI-TOF MS). It can be seen from
Figure 1 (spectra a-c) that the 31P{1H} NMR spectra of the two metallacages split into two doublet
peaks at 5.53 and 0.40 ppm for 4a, 5.69 and 0.63 ppm for 4b, respectively. The two peaks are of
equal intensity with concomitant 195Pt satellites because each platinum atom coordinates with one
nitrogen atom of the pyridyl group in the pillar and one oxygen atom of the carboxylic group in
the face, forming charge-separated complexes. In the 1H NMR spectra (Figure 1, spectra f-i), the
α-pyridyl protons H1 (for 4a) and H5 (for 4b) and the β-pyridyl protons H2 (for 4a) and H6 (for 4b)
shifted downfield compared with their uncoordinated precursors 2a and 2b. It is worth noting that
some protons (H5, H8, H9 and H17) on cage 4b split into two set of signals because the
functionalization of cages on the pillar parts reduced the symmetry of the cage, giving two different
types of chemical environment for the protons. ESI-TOF gives the evidence for the coordination
stoichiometry of metallacage 4a and 4b by showing peaks at m/z 1336.88 and 2304.41 (Figure 1,
spectra d and e), corresponding to [M – 5OTf]5+ species of 4a and 4b, respectively. All these
observations are consistent with previous reports,23a indicating the formation of tetragonal prisms.
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Figure 1. Partial P {1H} NMR spectra (121.4 MHz, CD3COCD3, 295 K) of 3 (a), 4a (b), and 4b (c).
Experimental (red) and calculated (blue) ESI-TOF MS spectra of 4a (d) and 4b (e) [M – 5OTf]5+. Partial
1H NMR spectra (400 MHz, CD3COCD3, 295 K) of 2a (f), 4a (g), 2b (h), and 4b (i). The labeling of
different protons are indicated in Scheme 1.
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Since all the attempts to get the crystal structures of metallacages 4a and 4b were unsuccessful,
density functional theory (DFT) calculations (see Supporting Information for computational
details) were performed in order to get insights about the structures of the metallacages 4a and 4b
(Figure 2). The tetragonal prismatic structures were formed with TPE units as the faces, dipyridyl
moieties as the pillars and 90° Pt(II) ions as the corners, respectively. The DFT optimized structure
of cage 4b shows that rhodamine groups possess two different conformations due to the steric
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hindrance, which causes some of the protons split into two set of peaks in the H NMR spectra
(Figure 1i) The DFT optimized structures of cages 4a and 4b reveal diameters of 4.18 nm and 5.50
nm, respectively. The TPE groups are partially rigidified and the molecular rotation of the aromatic
rings are limited by the metal-coordination bonds, endowing the cages with emission properties.
Figure 2. Optimized molecular structures of cages 4a (a) and 4b (b) by DFT calculations. The hydrogen
atoms were omitted for clarity. Pt atoms were represented by silver balls, and the pillars and faces were
represented by sticks.
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Photophysical Studies
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Figure 3. (a) Normalized absorption and emission spectra of the compounds 1 and 2a in DMSO. (b)
Fluorescence spectra of compound 2a (c = 40.0 μM) and 3 (c = 80.0 μM) in a mixture of DMSO and water
(v/v = 10/1) with different concentrations of compound 1. (c) Fluorescent intensity changes of (b) at 422
and 510 nm. (d) Normalized absorption and emission spectra of the compounds 1 and 2b in DMSO. (e)
Fluorescence spectra of compound 1 (c = 20.0 μM) and 3 (c = 80.0 μM) in a mixture of DMSO and water
(v/v = 10/1) with different concentrations of compound 2b. (f) Fluorescent intensity changes of (e) at 510
and 605 nm. λex = 340 nm.
The photophysical properties of the individual components were first investigated to reveal their
potential for FRET (See Supporting Information for details). As shown in the Figure 3a, the
absorption spectrum of TPE-based ligand 1 overlaps well with the emission spectrum of compound
2a. Similar overlap (Figure 3d) was also observed for the absorption of compound 2b and the
emission of 1. After the formation of the metallacages, the distance between the pillars and the
faces are significantly shortened (Figure 2), offering an electronic pathway to enable FRET process
because of the increased dipole–dipole coupling. It can be seen clearly from the fluorescence
titration experiments (Figure 3, spectra b and e) that as the gradual addition of the acceptors into
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the solution of donors and cis-Pt(PEt3)2(OTf)2 3, the emission of the donors decreased and that of
the acceptors increased. Moreover, the fluorescence intensity remained almost unchanged when
the ratios between the donors and acceptors reached their stoichiometries (Figure 3, spectra c and
f). These experiments suggested the efficient energy transfer between the pillars and faces in
metallacages 4a and 4b. Because the face molecule (TPE-based ligand 1) is an AIE-active
fluorophore and the pillar molecules (2a or 2b) are ACQ fluorophores as well as their structural
differences, their responses to the solvent compositions, mechanical pressure and temperature
differ greatly. This makes it possible to change the degree of spectral overlap between the emission
of the donors and the absorption of the acceptors under the aforementioned stimuli, thereby change
the efficiency of FRET in metallacages 4a and 4b. Therefore, the emission of metallacages 4a and
4b could be very sensitive to the solvent composition, mechanical pressure and temperature,
making them good candidates as chemical sensors.
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Solvatochromic Properties
To understand the effect of different solvents on the photophysical properties, we first measured
the UV-Vis absorption (Figure S13) and emission spectra (Figure S14) of metallacages 4a and 4b
in several commonly used solvents. Metallacage 4a shows an intense absorption peak centered at
ca. 292 nm with a shoulder at ca. 356 nm in almost all the tested solvents; while metallacage 4b
exhibits an extra intense absorption peak at ca. 565 nm, which comes from the absorption of
rhodamine moieties. However, the emission of these two cages differs a lot in different solvents
(Figure S14). For example, metallacage 4a shows only one major peak centered at 460 nm in
dichloromethane while exhibits two intense emission peaks centered at ca. 412~426 nm and
462~485 nm in acetone, acetonitrile, chloroform, DMF and DMSO. Metallacage 4b exhibits two
emission peaks at around 470 nm and 605 nm in dichloromethane, chloroform, DMF, DMSO while
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shows almost one peak at ca. 600 nm in acetone and acetonitrile. Factors which may contribute to
the vast differences on emissions of our metallacages in different solvents may include: 1) the
different emission behaviors of the faces with AIE and the pillars with ACQ; 2) the different
solubilities of the metallacages in different solvents; 3) the different FRET efficiency in different
solvents (Figure S16).
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The sensitivity of metallacages towards different solvents inspired us to explore the use of their
emission to distinguish structurally similar alcohols. The stability of the metallacages was first
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studied. The P {1H} NMR and H NMR spectra (Figures S17) of metallacage 4a in methanol
stay unchanged upon the addition of different alcohols including ethanol (EtOH), t-butanol (t-
BuOH) and ethyl glycol, indicating that the cages are chemostable in common alcohols. As shown
in Figure 4, the maximum emission and the intensity of the cages are different in the tested alcohols.
Metallacage 4a shows maximum emission centered at 439 nm, 440 nm and 414 nm in n-butanol,
i-butanol and t-butanol, respectively (Figure 4a). For metallacage 4b, the alcohols could be
distinguished by the ratio of its two emission bands (Figure 4b). Both cages show different
Commission internationale de l'éclairage (CIE) chromaticity coordinates in the three structural
similar alcohols (Figure 4c). Among three types of BuOH, t-BuOH can be easily distinguished
from n/i-BuOH by the naked eyes due to their color differences (insets of Figures 4a and 4b). With
the help of CIE chromaticity coordinates, the three types of alcohols can be readily distinguished.
We note the emission energy of 4a in t-BuOH is different from that in n/i-BuOH, perhaps due to
different solvent polarity (Table S3). On the other hand, the emission energies of 4b in different
solvents are the same, which may be associated with the different characteristics of the lowest
excited states of 4a and 4b (Figure S22). Furthermore, the different emission intensities of 4a and
4b in different solvents could be attributed to the different solvent viscosities (Table S3); more
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viscous solvent (e.g. t-BuOH) tends to show higher emission intensity, due to suppressed collisions
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between fluorescent molecules with less energy lost (as heat) through non-radiative decay.
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Figure 4. Fluorescence spectra of (a) cage 4a and (b) cage 4b and (c) their corresponding CIE chromaticity
coordinates in different alcohols (λex = 340 nm, c =10.0 μM). Insets: photographs of cages 4a and 4b upon
the excitation at 365 nm using a UV lamp at 298 K.
Mechanochromic Properties
Mechanochromic materials which change their emission in response to external forces have
been widely used in fluorescent switches and devices.24 Although the piezofluorochromic
properties of TPE-based small molecules25 and MOFs26 have been intensively studied, the
mechanochromic response of TPE-based metallacages has been rarely addressed. In order to study
the structural response under high pressure, the fluorescence upon stress was collected in situ by
diamond anvil cell technique.27 Metallacages 4a and 4b show light-green and deep-red emission
with maximum emission at ca. 500 nm and 605 nm, respectively, under ambient conditions. Upon
compression, the photoluminescence maximum of metallacages 4a and 4b experienced large
bathochromic shift to 620 nm at 14.1 GPa, and 700 nm at 12.1 GPa, respectively. Moreover, the
emission intensity decreased significantly as the pressure increased and the whole process was
fully reversible at least for 10 cycles (Figure S52). We speculate that the mechanochromic response
of our metallacages is because of more tightly molecular packing due to the mechanical
compression, and as a result, the pillars become tilted and the TPE faces become more planarized,
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which affect the dipole–dipole coupling between the pillars and the faces and therefore the
emission wavelength and intensity. We note similar mechanochromic response was also observed
in TPE-containing MOF-based systems26 and AIE-fluorophore-based cocrystals.28
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Figure 5. Emission spectra of cages 4a (a) and 4b (b) under different pressures and their optical and
fluorescence images (c).
Thermochromic Property
Temperature-responsive luminescent materials with two different emission bands can act as
ratiometric fluorescent thermometers to improve the accuracy and precision via self-calibration.29
In metallacages 4a and 4b, two fluorophores which may display different response to the
temperature were incorporated and the temperature change were expected to influence the
efficiency of FRET. This motivated us to further study their response towards temperature changes.
DMF was chosen as the solvent because it can be used in a wide range of temperature based on its
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low freezing point (-61 oC ) and high boiling point (153 oC). The NMR spectra recorded for cages
4a and 4b in DMF-d7 are similar with the spectra in acetone-d6, indicating that they are
chemostable in DMF (Figures S53-S61). As shown in Figures 6a, as the temperature increased,
the emission at 425 nm corresponding to the donor increased, while the emission at 488 nm that
ascribed to the acceptor decreased for metallacage 4a upon the excitation at 340 nm. A linear
fitting curve (slope = -0.006) of I488/I425 versus temperature (Figure 6c) was plotted, indicating that
metallacage 4a may serve as a fluorescent thermometer in the range from 223 K to 353 K. Similar
with metallacage 4a, increased donor emission at 480 nm and decreased acceptor emission at 600
nm were observed for 4b as the increase of the temperature. The ratio of the maximum emission
(I600/I480) also decreased linearly with the temperature in the range from 233 K to 333 K.
Remarkably, the absolute value of slope is 0.072, which is among the highest values for ratiometric
fluorescent thermometers reported so far.29 This indicates that the increase of temperature can
decrease the efficiency of FRET, perhaps due to the partially dissociation of the metal-coordination
bonds at high temperature.
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Figure 6. (a) Fluorescence spectra of cage 4a (c = 10.0 μM) in DMF at different temperature. (b) fluorescent
intensity changes of cage 4a at 425 and 488 nm. (c) Plots of the I488/I425 versus temperature. (d) fluorescence
spectra of cage 4b (c = 10.0 μM) in DMF at different temperature. (e) fluorescent intensity changes of cage
4b at 480 and 600 nm. (f) Plots of the I488/I425 versus temperature. Optical and fluorescent photographs of
4a (g) and 4b (h) at different temperature.
Encouraged by the fluorescence changes of the metallacages along with temperature in
solution,30 two thin films were prepared by doping the metallacages into polyvinylidene fluoride
(PVDF). Although no obvious color changes were observed from the optical photographs of the
films at different temperature, their fluorescent photographs differ dramatically at different
temperature (Figures 6g and 6h). The whole process is fully reversible after 10 cycles, indicating
that the metallacages can serve as good sensor for temperature. Different from most fluorescent
thermometers that are prepared solely based on the intensity changes, our system offers an
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approach to manipulate two fluorophores with efficient FRET in a single supramolecular system
to prepare ratiometric thermometers, which will pave the way for the further design of fluorescent
thermometers.
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■ CONCLUSION
In summary, two tetragonal prismatic platinum(II) cages with reverse energy transfer between their
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pillars and faces were prepared and characterized by P NMR and HNMR spectroscopy, ESI-
TOF-MS, UV absorption, and fluorescence spectroscopy. Because of their different fluorescent
properties of the two ligands, they show different response to external stimuli including solvent
polarity, composition, pressure and temperature, which significantly change the efficiency of
FRET, thus influencing the emission of the cages. Their use to distinguish structural similar
alcohols and as fluorescent ratiometric thermometers were explored, showing their potential
applications as highly sensitive chemical sensors. This study not only provides a type of metal-
coordinated structures in which the efficiency of FRET can be finely tuned between their building
blocks, but also takes advantage of such response to explore their practical application for chemical
and temperature sensing. As such it will open a new avenue for stimuli-responsive and light-
emitting materials, sensors and bioimaging.
■ ASSOCIATED CONTENT
Supporting Information
The supporting information is available free of charge via the Internet at http://pubs.acs.org.
Syntheses and characterization data (NMR, Fluorescence Spectra), including Figures S1−S42.
■ AUTHOR INFORMATION
Corresponding Author
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mingming.zhang@xjtu.edu.cn; kaiwang@jlu.edu.cn
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Author Contributions
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#Z. Z. and Z. Z. contributed equally to this work.
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Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (21801203 to M.
Z., 11774120 to K. W., 21725304 to B. Z.), National Institutes of Health (R01GM128037 to X.
L.), and The Key Research and Development Program of Shaanxi Province (2019KW-019 to
M.Z.). M. Z. is thankful for start-up funds from Xi’an Jiaotong University. We thank Dr. Gang
Chang and Yu Wang at Instrument Analysis Center and Dr. Aqun Zheng and Junjie Zhang at
Experimental Chemistry Center of Xi’an Jiaotong University for NMR and fluorescence
measurements.
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(30) The emission response of cages 4a and 4b as solid powders to the temperature were also
collected and shown in Figure S62.
TOC Graphic:
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