Drum-like Metallacages with Size-Dependent Fluorescence: Exploring the Photophysics of Tetraphenylethylene under Locked Conformations

Zhewen Guo, Guangfeng Li, Heng Wang, Jun Zhao, Yuhang Liu, Hongwei Tan, Xiaopeng Li,

Article

Drum-like Metallacages with Size-Dependent Fluorescence:

Exploring the Photophysics of Tetraphenylethylene under Locked

Conformations

Peter J. Stang,* and Xuzhou Yan*

Cite This: J. Am. Chem. Soc. 2021, 143, 9215 −9221

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sı Supporting Information

ABSTRACT: Materials with aggregation-induced emission (AIE)

properties are of growing interest due to their widespread

applications. AIEgens, such as tetraphenylethylene units, display

varying emission behaviors during their conformational changes.

However, the structure−property relationships of the intermediate

conformations have rarely been explored. Herein, we show that the

conformational restriction on TPE units can aﬀect the structural

relaxation in the excited state and the resulting photophysical

behaviors. Speciﬁcally, three metallacages of diﬀerent sizes were

prepared via the coordination-driven self-assembly of a TPE-based

tetrapyridyl donor with length-increasing Pt(II) acceptors. While

the metallacages share similar scaﬀolds, they exhibit a trend of red-

shifted ﬂuorescence and attenuated quantum yield with the increase of their sizes. Furthermore, spectroscopic and computational

studies together with a control experiment were conducted, revealing that the degree of cage tension imposed on the excited-state

conformational relaxation of TPE moieties resulted in their distinct photophysical properties. The precise control of conformation

holds promise as a strategy for understanding the AIE mechanism as well as optimizing the photophysical behaviors of materials on

the platform of supramolecular coordination complexes.

INTRODUCTION

exhibiting piezoﬂuorochromic behavior due to conformational

changes of the TPE linkers during the compression process.

■

Fluorescent materials have gained extensive research interest in

recent years due to their widespread applications in biological

However, in all these cases, only the structures of the

beginning and ﬁnal states are known, and the details of the

intermediate conformation of the TPE unit constitute a black

box wherein the relationship between the distorted con-

formation and the accompanying ﬂuorescence is rarely studied.

Supramolecular coordination complexes (SCCs), prepared

via coordination-driven self-assembly, have been an emerging

3,4

5,6

sensing, lighting devices, stimuli-responsive materials,

−11

and so on.

However, traditional ﬂuorophores oftentimes

suﬀer from aggregation-caused quenching (ACQ) eﬀects in

condensed states wherein the ﬂuorescence would be quenched

precipitously because of the exciton interactions and non-

2,13

radiative decays.

This issue was subtly addressed when

Tang et al. reported a completely opposite phenomenon

area of interest and oﬀer a promising way to deal with the

14−16

23−27

known as aggregation-induced emission (AIE).

In such

scientiﬁc question above.

Discrete metallacages, a typical

cases, certain ﬂuorogens that emit weakly in dilute solution

type of SCC, can be eﬃciently synthesized based on the

spontaneous formation of labile metal−ligand bonds between

become highly emissive in the aggregated state on account of

8−33

the restriction of intramolecular rotation (RIR).

ligands and metal acceptors.

Through adjusting the

Tetraphenylethylene (TPE) is an emblematic AIE ﬂuo-

rophore in which the initial free rotation of phenyl rings and

distortion of CC double bond would be restricted in the

angularity, directionality, and stoichiometry of the precursors,

metallacages could possess well-deﬁned sizes and shapes and

aggregated state. It is noteworthy that the TPE molecule is

Received: April 24, 2021

Published: June 9, 2021

highly twisted, and diﬀerent degrees of conformational

19,20

distortion could lead to variable emissions.

Typically,

TPE ﬂuorogens may continuously alter the resulting

ﬂuorescence, as long as the external stimulus causes the

corresponding conformation change. For example, Zhou et

al. reported a TPE-based metal−organic framework (MOF)

2021 American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. 2021, 143, 9215−9221

215

coordination-driven self-assembly.

Article

Figure 1. Schematic representation of the formation of drum-like metallacages 1, 2, and 3 with diﬀerent tensions and photophysics via

Figure 2. Simpliﬁed chemical structures of metallacages 1 (a), 2 (b), and 3 (c). Partial H NMR spectra (400 MHz, DMSO-d , 298 K) of ligand 4

(

d, g, j), acceptors 5 (f), 6 (i), and 7 (l), as well as metallacages 1 (e), 2 (h), and 3 (k). ESI-TOF-MS spectra of metallacages 1 (m), 2 (n), and 3

o).

serve as versatile platforms to explore AIE properties. ⁴⁻³⁸

regulate the conformation of the TPE core when incorporated

into such systems. It is reasonable that even subtle modulation

Therefore, we expect that metallacages can be applied to

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Figure 3. Ball−stick views of the optimized structures of metallacages 1 (a and d), 2 (b and e), and 3 (c and f). Carbon atoms are green, nitrogen

atoms are blue, phosphorus atoms are brown, and platinum atoms are gray.

may have a profound inﬂuence on the photophysical

properties, and thereby the structure−property relationship

could be investigated in detail at the level of the molecular

structure.

Herein, we designed a suite of TPE-based drum-like

metallacages, 1, 2, and 3, with increasing sizes via

coordination-driven self-assembly to explore the emission

property of TPE in multiple locked conformations (Figure

and S30). The results supported the formation of discrete

multi-TPE metallacages. In the H NMR spectrum of

metallacage 1 (Figure 2e), distinct signals H_a₋_f_,₁, corresponding

to coordinated structures, were assigned with downﬁeld shifts

relative to those of ligand 4 (Figure 2d,g,j) and acceptor 5

(Figure 2f). The H protons on the pyridine rings were split

into two sets upon metal coordination, which is consistent with

39,40

the previously reported SCC structures.

Metallacages 2

). In the ﬁne structure of these metallacages, the TPE-based

and 3 also displayed a similar trend wherein all the proton

signals corresponding to pyridine and phenyl rings exhibited

downﬁeld shifts and broadening (Figure 2h,i,k,l). The typical

downﬁeld shifts were due to the formation of Pt−N

coordination bonds that reduced the electron density of the

tetrapyridyl ligand was combined with suitable mono-, di-, and

terphenyl-bridged organoplatinum(II) acceptors. The cage

matrix would restrict the ligands from further distortion, thus

restraining the ﬂexibility of the TPE core and leading to the

enhanced emission. The feature of our design is that, with the

TPE ligands and counterpart anions being the same, diﬀerent

tensions of the metallacages would induce the TPE core to

take diﬀerent conformations, which presumably would impact

the resulting ﬂuorescent properties. In order to get rid of

interference, we used the metallacycle as the comparison to

exclude the conjugation eﬀect. We expect that the

conformation-locking strategy using metallacages may further

contribute to a better understanding of structure−property

relationships as well as oﬀering a facile way of tuning the

emission.

original moieties. 2D H diﬀusion-ordered spectroscopy

(

DOSY) also suggested the formation of single products

−9

−1

with di ﬀ usion coeﬃcients (D) of 5.56 × 10 m s for 1

(

−9

2 −1

Figure S12), 5.05 × 10 m s for 2 (Figure S19), and 4.24

−9

−1

10 m s for 3 (Figure S26). The P{ H} NMR spectra

of metallacages 1, 2, and 3 exhibited doublets (ca. 13.88 and

4.10 ppm for 1, 14.46 and 14.65 ppm for 2, as well as 14.43

195

and 14.62 ppm for 3) with concomitant Pt satellites, which

could be attributed to the diﬀerent chemical environments of

phosphorus inside and outside the metallacages. The NMR

signals of metallacages 1, 2, and 3 were shifted upﬁeld from

those of Pt(II) acceptors by approximately 6.09, 6.26, and 6.23

ppm, respectively. The coupling of the ﬂanking Pt satellites

decreased (ca. ΔJ = −140.3 Hz for 1, ΔJ = −143.9 Hz for 2,

ΔJ = −136.2 Hz for 3) due to the electron back-donation

eﬀect of metal coordination. The well-deﬁned signals in both

the H and P{ H} spectra of these species supported the

formation of such discrete structures as simplex assemblies.

Electrospray ionization time-of-ﬂight mass spectrometry

(ESI-TOF-MS) is a highly reliable approach to characterize

the stoichiometry of the multicharged supramolecular

structures. In the ESI-TOF-MS spectra (Figure 2m−o),

RESULTS AND DISCUSSION

■

According to the principle of coordination-driven self-

assembly, the directionality and angularity of each component

will determine the ﬁnal structure. In this study, we combined

TPE-based tetrapyridyl ligand 4 with diﬀerent lengths of di-

Pt(II) acceptors, 5, 6, and 7, to construct metallacages 1

(

Figure 2a), 2 (Figure 2b), and 3 (Figure 2c), respectively.

The mixture of ligand 4 and each di-Pt(II) acceptor in a 1:2

ratio was heated to 70 °C in deuterated DMSO and then

stirred for 8 h. The three self-assembled metallacages were

obtained respectively in almost quantitative yield.

multiple prominent peaks of [M − x(OTf)] (x = 4−11)

can be found that correspond to the [3+6] assembly with

charge states resulting from the loss of the OTf counterions

The reaction mixture and ligand 4 were characterized

−

through multinuclear H and P{ H} NMR (Figures 2e,h,k

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Figure 4. (a) Absorption and (b) ﬂuorescence spectra of ligand 4 and metallacages 1−3 in CH Cl (λ = 350 nm, c = 10.0 μM). (c) Quantum

yield and Stokes shift of metallacages 1−3 in CH Cl (λ = 350 nm, c = 10.0 μM). (d) Absorption and (e) ﬂuorescence spectra of ligand 4 and

metallacycles 9−11 in CH Cl (λ = 350 nm, c = 10.0 μM). (f) Fluorescence decay proﬁles of metallacages 1−3 (λ = 365 nm, c = 10.0 μM).

Insets in (b) and (e): Photographs of the metallacages/metallacycles obtained under irradiation by a 365 nm UV lamp (c = 10.0 μM).

for example, peaks at m/z = 1185.6371 corresponding to [M −

shown in Figure 4a. Ligand 4 displayed two broad absorption

bands with peaks at ca. 310 and 345 nm. After metal

coordination, all the metallacages only displayed single

absorption bands at around 315 nm. These observations

indicated that the metallacages possessed similar ground-state

photophysical properties on account of the similar cage

scaﬀolds while diﬀering from ligand 4 due to the formation

of N−Pt coordination bonds. Furthermore, ﬂuorescence

spectra of ligand 4 and metallacages 1−3 were recorded

(Figure 4b). Ligand 4 was weakly emissive at ca. 519 nm in

CH Cl , and the lack of emission originated from the

OTf] for metallacage 1 (Figure 2m), m/z = 939.9957

corresponding to [M − 9OTf] for metallacage 2 (Figure 2n),

and m/z = 990.6281 corresponding to [M − 9OTf] for

metallacage 3 (Figure 2o). All the dominating sets of peaks

were isotopically resolved and in good agreement with their

calculated theoretical distributions. Moreover, diﬀerent charge

states revealed by traveling wave ion mobility−mass spectrom-

etry (TWIM-MS) of these three metallacages had narrow drift

time distributions, further indicating the formation of intact

structures (Figures S15, S22, and S29 ). On the basis of the

above combined results, it was conﬁrmed that no other

assemblies were constructed at a detectable level during the

self-assembly process leading to these metallacages.

nonradiative decay through the intramolecular rotation of

pyridine and benzene rings. After the TPE-based ligands were

anchored into metallacages, they exhibited salient ﬂuorescence

enhancements with variable emission maxima. In the emission

proﬁle of metallacage 1, which was the smallest size and owned

the highest tension, the maximum emission wavelength blue-

shifted to 466 nm. A trend of red-shifted emission for 2 (491

nm) and 3 (507 nm) was observed, which can be attributed to

the decrease in the conformational restraint. The red shift of

the emission is clearly visualized as depicted in the

ﬂuorescence images of 1−3 (inset in Figure 4b). Also, the

energy diﬀerences between absorption and emission were

further translated into Stokes shifts (Figure 4c). The changes

in Stokes shifts are presented in the order of 1 (1.32 eV) < 2

(1.44 eV) < 3 (1.52 eV). These results implied that the degree

of conformational relaxation for the metallacages was in the

order of 1 < 2 < 3 induced by cage tension. Given that these

metallacages emitted eﬀectively even in dilute solvent, the

quantum yields (Φ ) of 1−3 in CH Cl solution were also

Given the diﬃculty of growing single crystals suitable for X-

ray diﬀraction, molecular simulations were performed to obtain

further insight into the architectural features of these multi-

TPE metallacages 1−3 (Figure 3a−c). The geometry

optimizations were performed using Gaussian09 software

by a semiempirical PM6 method in C symmetry, and the PEt

ligands on the platinum atoms were simpliﬁed as PH . The

simulated structure of metallacage 1 possessed a well-deﬁned

drum-shaped structure with a ca. 1.2 × 2.3 × 1.2 nm cavity

(Figure 3d). The molecular simulations showed very similar

drum-like structures for metallacages 2 and 3, with increased

cavity sizes (1.2 × 2.5 × 1.2 nm for 2 and 1.2 × 2.7 × 1.2 nm

for 3) (Figure 3e,f). The simulated structures suggested that

metal coordination induced the TPE ligands to adopt the

twisted conformations and further restrained their intra-

molecular rotation in the metallacages, which is responsible

for the enhanced emission (vide infra). It is notable that, as the

metallacages increase in size, they become more capacious and

allow more intramolecular movement of the TPE-based

ligands.

recorded. The values were measured as 18.4% for 1, 12.5% for

2, and 10.2% for 3, displaying a decreasing trend. Because

metallacage 1 has a smaller skeleton which may induce more

restriction on the intramolecular movements of TPE units, it

exhibited better ﬂuorescent performance than those of 2 and 3.

To exclude the inﬂuence of conjugation of the phenyl

groups and achieve a thorough understanding of our system, a

After the successful synthesis of these metallacages, their

photophysical properties were studied. The normalized

absorption spectra of ligand 4 and metallacages 1−3 are

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control experiment was carried out in which discrete

metallacycles with similar scaﬀolds were designed and

synthesized. In this case, all the di-Pt(II) acceptors 5−7

were kept unchanged except for the replacement of tetrapyridyl

ligand 4 by bipyridyl ligand 8 , leading to the formation of 2D

metallacycles 9, 10, and 11 (Figure S31). Our design ensures

that the cages and cycles have the same conjugated structure

but diﬀerent frameworks to constrain the TPE units.

Multinuclear NMR ( H and P{ H}) together with ESI-

TOF-MS conﬁrmed the formation of metallacycles 9−11 with

undetectable impurities (Figures S32 −S40 ). The absorption

and emission proﬁles were depicted in the same way as for the

metallacages. As expected, the absorption spectra of metalla-

cycles 9−11 (Figure 4d) were almost identical to those of the

corresponding metallacages 1 −3 on account of the same

absorption groups (Figures S41 −S43). However, due to the

presence of freely rotating phenyl rings and CC double

bonds that led to the nonradiative decay, the ﬂuorescence

intensity of the metallacycles decreased signiﬁcantly relative to

the metallacages (Figure 4e). Meanwhile, all the metallacycles

displayed identical emission maximum at ca. 480 nm,

indicating no change of the Stokes shift with diﬀerent ring

sizes. Therefore, the conjugation of the phenyl groups in the

acceptors had little inﬂuence on the emission behaviors of the

assemblies, while the restriction of conformation played a key

role.

Figure 5. (a) Structures of ligand 4 in the ground state and optimized

excited state with the recorded values for the bending angle Θ . (b)

Relative energy calculated at diﬀerent Θ angles in metallacages 1−3.

(

c) Schematic illustration of the Franck−Condon eﬀect and Stokes

shift.

Furthermore, the time-resolved ﬂuorescence decay of the

multi-TPE metallacages 1−3 was recorded in CH Cl solutions

1,44

(

Figure 4f). Each dynamic decay was ﬁtted well with a double-

C2−C3−C4, Θ ) was dominant,

while other distortions

exponential curve of τ (around 10 ps) and τ (around 1 ns)

were negligible. Furthermore, metallacages 1, 2, and 3 were

components (Figures S54 −S59). The coeﬃcients of the

optimized while Θ was ﬁxed at 100−130° (gradually closer to

component τ displayed negative amplitudes, which were

the excited state). The change of energy was recorded as

associated with the geometry relaxation from the Franck−

shown in Figure 5b. Speciﬁcally, the increased energy (ΔE)

Condon conﬁguration, while the second component τ

exhibited a uniform order of 1 > 2 > 3 with variable Θ values,

represented the nonradiative decay of the excited state.

indicating that metallacage 1 required the highest energy for

deformation in the excited state, followed by metallacages 2

and 3. According to the principle of vertical transition (Figure

5c), lower cage tension prolonged the relaxation time of

conformational adjustment at the excited state and ultimately

resulted in higher Stokes shift. Therefore, metallacage 3 allows

the TPE unit to have the maximum Stokes shift and

ﬂuorescent lifetime, while metallacage 1 has the minimum.

The time constants τ for the metallacages followed the order

of 1 (10 ps) < 2 (13 ps) < 3 (14 ps), and this could be related

to the cage tension at the excited state. As discussed above, the

trend of the Stokes shifts, quantum yields, and the ﬂuorescence

lifetimes corresponding to the degree of conformational

relaxation was uniﬁed and self-consistent, which aﬃrmed that

even subtle conformational diﬀerences could have a signiﬁcant

inﬂuence on the light-emitting properties of multi-TPE

materials.

CONCLUSION

■

To obtain better insight into the structure−property

relationship, a computational analysis was also performed. By

comparing the structure of ligand 4 moieties in the three

metallacages, we found that the size and shape of the rectangle

framework formed between four nitrogen atoms were slightly

diﬀerent, with 12.6 × 11.1 Å for metallacage 1, 12.3 × 11.4 Å

for metallacage 2, and 12.1 × 11.6 Å for metallacage 3 (Figure

S64 ). It is plausible that these small conformational changes

reﬂected diﬀerent degrees of constraint on TPE units in these

metallacages. Furthermore, such a diﬀerence in cage tension

could aﬀect the structural relaxation in the excited state. In

order to study the conformational change from the ground

state to the excited state, the geometry of ligand 4 in the

metallacage 1 was selected and optimized via the time-

dependent density functional theory (TDDFT) method with

In summary, we have synthesized and characterized three

multi-TPE metallacages with increasing sizes and further used

these cages as the platform to investigate the conformational

inﬂuence of TPE moieties on the resulting photophysical

characteristics. A TPE-based tetrapyridyl ligand and three

Pt(II) acceptors of incremental length were designed and

synthesized by means of metal-coordination-driven self-

assembly. This strategy allows for precise control on the

restriction of the intramolecular rotation of anchored TPE

cores. All the metallacages were characterized by multinuclear

NMR and ESI-TOF-MS to denote the molecularity of these

species. Although the three metallacages shared similar

absorption peaks, they exhibited varying emission peaks and

quantum yields due to diﬀerent cage tension. In order to

exclude the conjugation eﬀect in the acceptors, a control

experiment was conducted, wherein the metallacycles corre-

sponding to the cages were designed and synthesized for

further demonstration. The computational results conﬁrmed

that a metallacage with higher tension led to a smaller Stokes

42,43

the CAM-B3LYP/6-31G* basis set

(Figure 5a). The four

N atoms were ﬁxed to mimic the coordination environment. In

the excited state, we found that the change of the dihedral

angle between the phenyl ring and the ethylene plane (∠C1−

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Feng, L.; Dou, C.; Yin, D.; Xu, H.; Cheng, Y.; Zhang, F. J -Aggregates

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shift. These metallacages exhibited variable yet orderly

emission behaviors depending on the size, which provided a

whole new understanding of the intermediate conformations of

TPE moieties in the deformation processes.

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AUTHOR INFORMATION

Corresponding Authors

Xuzhou Yan − School of Chemistry and Chemical Engineering,

Frontiers Science Center for Transformative Molecules,

Shanghai Jiao Tong University, Shanghai 200240, P. R.

China; orcid.org/0000-0002-6114-5743;

Email: xzyan@sjtu.edu.cn

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Zhewen Guo − School of Chemistry and Chemical

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P. R. China

Heng Wang − College of Chemistry and Environmental

Engineering, Shenzhen University, Shenzhen 518055, P. R.

China

Jun Zhao − School of Chemistry and Chemical Engineering,

Frontiers Science Center for Transformative Molecules,

Shanghai Jiao Tong University, Shanghai 200240, P. R.

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(15) Mei, J.; Leung, N. L.; Kwok, R. T.; Lam, J. W.; Tang, B. Z.

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Z.G. and G.L. contributed equally to this work.

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The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS

X.Y. acknowledges the ﬁnancial support of the NSFC/China

21901161 and 22071152) and Interdisciplinary Program of

Shanghai Jiao Tong University (YG2019QNA16). G.L.

acknowledges the China Postdoctoral Science Foundation

■

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