Shape-persistent π-conjugated Macrocycles with Aggregation

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Functional Nanostructured Materials (including low-D carbon)

Shape-persistent #-conjugated Macrocycles with Aggregation-induced Emission Property: Synthesis, Mechanofluorochromism, and Mercury(II) Detection Yi Liu, Fa Xu Lin, Yang Feng, Xiaoqing Liu, Lei Wang, Zhen-Qiang Yu, and Ben Zhong Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10702 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Applied Materials & Interfaces

Shape-persistent π-conjugated Macrocycles with Aggregation-induced Emission Property: Synthesis, Mechanofluorochromism, and Mercury(II) Detection Yi Liu,†,§,*,▽ Fa Xu Lin,‡,▽ Yang Feng,† Xiaoqing Liu,# Lei Wang,† Zhen-Qiang Yu‡,§,* and Ben Zhong Tang#,* †

Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research Center

for Interfacial Engineering of Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. E-mail: [email protected]. ‡

School of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060,

China. E-mail: [email protected]. § Centre

for AIE Research, Shenzhen University, Shenzhen 518060, China.

# Shenzhen

Grubbs Institute, Southern University of Science and Technology, Shenzhen 518005,

China #

HKUST Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hitech Park

Nanshan, Shenzhen 518057, China. E-mail: [email protected]. ▽ These

authors contributed equally to this work.

ACS Paragon Plus Environment

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KEYWORDS: Shape-persistent macrocycles, porous luminogens, aggregation-induced emission, mechanofluorochromism, mercury(II) sensor.

ABSTRACT: Shape-persistent conjugated macrocycles are fundamentally important because of their unique structure and properties. Herein, a series of π-conjugated macrocycles with shape-persistent architecture, adaptive backbone and aggregationinduced emission (AIE) properties are synthesized via oxidative coupling of acetylene terminated tetraphenylethylene precursor with half-ring topology, and following transformation from butadiynylene linkers into thienylene ones. Characterization by NMR spectroscopy and MALDI-TOF mass spectrometry provided unambiguous proofs for the macrocyclic structures. In particular, the free rotation of aromatic rings in the rigid macrocyclic backbone was validated by 2D NMR spectroscopy, variable-temperature NMR measurements, and theoretical calculations. Moreover, these shape-persistent macrocyclic chromophores all exhibited obvious AIE phenomena, and remarkable mechanofluorochromism behaviors with red-shifted luminescence upon grinding and blue-shifted emission after solvent annealing. And the introduction of S atoms into the


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macrocyclic frameworks endowed the macrocyclic luminogen the capability to selectively detect mecurry(II) ions in aqueous media among other metal ions.

1. Introduction

π-Conjugated macrocycles have recently attracted intense attentions owing to their unique structures, novel properties, and promising functions.1-5 Macrocyclic structures with shape-persistent, and non-collapsible backbones render these well-defined building blocks highly promising application toward the construction of supramolecular porous structures with diverse dimensions and applications.6-8 Recently, Nuckolls et al, assembled the capsule-shaped molecule into a cellular semiconducting material via halogen bonding interactions, and utilized these electroactive macrocycles to distinguish small guest molecules in field effect transistor devices.9

And the extended π-

conjugation skeleton with defined shape and cavity is also especially attractive for the building-up and investigation of porous carbon-rich materials as the substructure moieties.10,11 For example, the cycloparaphenylenes12-15 (CPPs) hoops and phenyleneethynylene macrocycles16,17 (PEMs) were regarded as the nano-scaled segments of


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carbon nanotube and graphdiyne, respectively. However, the exploration on the functions and applications of conjugated macrocyles are quite rare, owing to two reasons: i) the synthetic difficulty to incorporate functional units into the defined macrocyclic framework with satisfactory yields and scales; and ii) the lack of strategies to leverage the macrocyclic structure to develop materials with special functions and applications.

Contrasted to flexible macrocyclic structures like crown-ether and cycloalkanes,18 the conjugated macrocycles with a shape-persistent interior in the nanometer regime are rather rigid.19-22 However, partial building blocks of macrocycles, like phenylene rings in CPPs and PEMs, can still freely vibrate or rotate without disruption of the entire persistent conjugation skeleton.23,24 The dynamic conformational fluctuations render conjugated macrocycles the possibility to adjust their conformations within the rigid topology, and change the physical properties upon external stimulus or complexation with guest molecules.25,26


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Luminogens with aggregation-induced emission (AIE) properties are non-emissive in solution, but exhibit bright luminescence in aggregates or solid state.27-29 The photoluminescence (PL) of these AIE-active fluorophores can be boasted dramatically via restriction of the intramolecular motions, including rotation and vibration.30-33 Hence, we can utilize the changes of PL signals to explore the invisible rotations via incorporate AIE-active moieties into the macrocyclic backbones.34,35 We envision that the shapepersistent π-conjugated macrocycles containing AIE-active building blocks can probably exhibit high PL quantum yield in aggregates, and their PL signals can respond to the external stimulus (like mechanic force), or guest molecules (like small ions).36-38

In the present work, we designed and synthesized two shape-persistent conjugated macrocycles with AIE-active tetraphenylethylene (TPE) moieties incorporated in the cyclic backbone and an interior cavity of approximate 0.7 nm in diameter. Based on variable-temperature (VT) NMR spectra and theoretical simulations, the free rotations of aromatic building blocks in the rigid macrocycle were investigated. These two macrocyclic luminogens with overall rigid structure both exhibited obvious AIE and


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mechanofluorochromism properties. In addition, the introduction of thiophene rings in the macrocyclic backbones enabled the luminogen able to selectively detect mercury(II) ions in aqueous media by fluorescence quenching.

2. Experimental Sections

2.1 Materials

THF was distilled under normal pressure from sodium benzophenoneketyl under nitrogen

immediately

dimethoxybenzophenone

prior (2),

to

use.

titanium

4,4'-Dibromobenzophenone tetrachloride

bis(diphenylphosphino)ferrocene)palladium(II) bis(pinacolato)diboron,

(TiCl4),

dichloride,

1,1'-bis(diphenylphosphino)ferrocene,

(1),

4,4'-

zinc

dust,(1,1'-

anhydrous

dioxane,

potassium

acetate,

tetrakis(triphenylphosphine)palladium(0), potassium carbonate, 3-bromoiodobenzene, anhydrous dichloromethane, boron tribromide, hexyl bromide, cesium carbonate, trimethylsilylacetylene, triphenylphosphine, bis(triphenylphosphine)palladium chloride, copper(I)

iodide,

diisopropylamine,

triethylamine,

anhydrous

toluene,

tetra-n-

butylammonium fluoride, sodium sulfide, ethylene glycol methyl ether, magnesium


6

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sulfate, and acetone, were all purchased from Energy Chemical (Shanghai, China). All these materials are analytical grade and used as received.

2.2 Characterizations and Measurements

1H,13C

NMR spectra, 2D

1H-1H

correlation spectroscopy (COSY) and nuclear

overhauser effect spectroscopy (NOESY) were measured on a Bruker AVANCE Ⅲ 400MHZ or 600MHZ spectrometer using chloroform-d as solvent and tetramethylsilane (TMS, δ = 0) as internal standard. Matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) high-resolution mass spectra (HRMS) were recorded on a GCT premier CAB048 mass spectrometer. Absorption spectra were taken on a Thermo-fisher Evolution 220 spectrometer. Emission spectra were taken on a Thermo Lumina Fluorescent spectrometer. The fluorescence quantum yield (QY) data have been measured by

Hamamatsu Absolute PL quantum yield spectrometer C11347.

Fluorescence micrographs were taken on Leica Microsystems CMS GmbHErnst-LeitzStr. The sizes of the nanoaggregates were measured with dynamic light scattering (ZetasizerNano ZSP, Malvern Instruments, Malvern, UK). Powder X-ray diffraction


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(XRD) patterns were recorded on a RigakuSmartLab X-ray Diffractometer. An aliquot of a suspension of TPEMC and TPEMCS (fw=30, 50, and 90%) was spread on a silica plate and an excessive solution was removed after 5 minutes, the sample was allowed to air-dry at 40℃ and applied for scanning electron microscope (SEM) measurements using a JSM-7800F electron microscope at an accelerating voltage of 5 kV. Electron diffraction (ED) was carried out on the same samples using a JEM 2100FS operated at 40 kV.

2.3 Synthetic Procedures

The detailed synthetic procedure and structural analysis of macrocylic luminogens are given in the Supporting Information.

3. Results and discussion

3.1.Synthesis and structural characterization


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Targeted macrocyclic luminogens, named as TPEMC and TPEMCS, are composed by two TPE units with different conjugated bridging linkages. The synthesis of these two macrocycles is outlined in Scheme 1.

Scheme 1. Synthetic routes toward shape-persistent conjugated macrocycles (TPEMC and TPEMCS) with AIE property. Reagents and conditions: (i) Zn, TiCl4, THF, reflux, overnight. (ii) (a) Bis(pinacolato)diboron, Pd(dppf)Cl2, KOAc, Dioxane, reflux, overnight; (b) Pd(PPh3)4, 3-bromoiodobenzene, K2CO3, THF/EtOH/H2O, 80 °C, overnight. (iii) (a) BBr3, DCM, rt, overnight; (b) n-C6H13Br, Cs2CO3, Acetone, reflux, overnight. (iv) Pd(PPh3)4, PPh3, CuI, diisopropylamine, toluene, TMSA, reflux, overnight. (v) Bu4NF,


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THF, rt, 2h. (vi) Pd(PPh3)2Cl2, CuI, Et3N, THF, 7 days. (vii) Na2S, toluene, EGME, 120 °C, overnight. THF: tetrahydrofuran.

Firstly, TPE derivative 4 was synthesized from TPE precursor 3 via cross McMurry coupling, followed by the Miyaura borylation reaction and subsequent Suzuki coupling with 3-bromoiodobenzene. Then, BBr3 promoted deprotection of the methoxyl group in 4 and following treatment with 1-bromohexane under basic condition readily afforded the hexyl chain substituted precursor 5. Palladium catalyzed Sonagashira coupling of 5 with TMSA (trimethylsilylacetylene) and subsequent desilylation by treating 6 with Bu4NF gave the acetylene-terminated “half-ring” precursor 7. Dimeric macrocycle TPEMC were obtained in satisfactory yields of 32% via oxidative homocoupling of 7 in a diluted solution at concentration of 0.9 mM. Finally, treatment of TPEMC with Na2S in the solvent mixture of toluene and ethylene glycol methyl ether (EGME) successfully converted the butadiyne moieties into thiophene rings in a high yield of 81%.39,40

The high-resolution matrix-assisted laser ionization time-of-flight (MALDI-TOF) mass spectrum of TPEMC indicated the presence of a single species with m/z = 1461.77,


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agreeing with the desired molar mass of 1461.77 g/mol. (Figure S1) After transforming into 2,5-thienylene bridges, a single peak with m/z = 1529.74 was recorded in the MALDI-TOF mass spectrum of TPEMCS, which was also well consistent with its molar mass of 1528.74 g/mol. (Figure S2)

Figure 1. 1H NMR spectra of acyclic precursor 7 (top), macro-cyclic luminogens TPEMC (middle) and TPEMCS (bottom) in CDCl3 at 298 K.


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In addition, 1H NMR spectroscopy (Figure 1) together with 1H−1H 2D COSY and NOESY NMR spectra (Figure S3-S6) further validated the chemical identities of TPEMC and TPEMCS, where every aromatic proton signal could be unambiguously assigned. After cyclization, the proton signal located at 3.09 ppm in the 1H NMR spectrum of acyclic precursor 7, corresponding to the terminal acetylene hydrogen, disappeared in that of TPEMC and unambiguously confirmed the structure of dimeric macrocycles with the assistance of the MALDI-TOF mass spectrum. Compared with that of TPEMC, a new singlet signal appeared at 7.35 ppm in the 1H NMR spectrum of TPEMCS, arising from the hydrogen on thiophene rings. This result clearly supported the successfully conversion from butadiyne bridges to 2,5-thienylene ones.


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Figure 2. NOESY spectrum (aromatic region) of TPEMCS in CDCl3 at 298 K and proposed rotation of thiophene rings in shape-persistent macrocyclic backbone.

Interestingly, the proton Hi on thiophene ring was found to be correlated with both He and Hh in the NOESY spectrum of TPEMCS (Figure 2). This indicated that the proton Hi on thiophene ring was spatially close to both He and Hh located on the corner metaphenylene ring, which meant that the thiophene ring could rotate freely within the shape-persistent framework. Thus, variable-temperature 1H NMR (VT-NMR) spectra of TPEMCS were measured in C2D2Cl4 (Figure S7 and S8). Upon raising the temperature


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from 243 to 353 K, the signals from Hi, Hh, and Hf were remarkably upfield-shifted from 7.61, 7.48, 7.35 ppm to 7.53, 7.40, 7.26 ppm, respectively, whereas other aromatic protons showed almost tiny shifts. This was presumably because of that the 2,5thienylene bridges and meta-phenylene rings on the corner had higher freedom of motions, leading to more pronounced shielding effect from the neighboring phenylene ring at elevated temperature. And the coalescence of Hi signals with different orientation of thiophene rings suggests that the different conformers of TPEMCS are probably in fast exchange within NMR timescale.23


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Figure 3. Molecular models and energy of conformational isomers for (a) TPEMC and (b) TPEMCS as calculated at the B3LYP/6-31G(d) level of DFT. “out” means the S atom is located outside the macrocycle framework, whereas “in” means the S atom is located inside the macrocycle.


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To further elucidate the structure of the macrocyclic luminogens, their geometry was studied with density functional theory (DFT) calculations using functional B3LYP with the 6-31G(d) basis set (Figure S9). To reduce computational cost, the hexyl substitutions were replaced with methyl groups in the macrocyclic model compounds. The dimeric macrocycle TPEMC with diacetylene bridges could, in principle, exist as three isomers depending on the chirality of the TPE chromophores (M- or P- helicity).41 Structural optimization of TPEMC revealed the homochiral pair, (M,M) and (P,P)TPEMC, as the global energy minimum, whereas the heterochiral specie (M,P)-TPEMC was found to be the local energy minimum (Figure 3a). The homochiral isomers were slightly more stable than its heterochiral analogues by 0.41 kcal/mol in Gibbs free energy, which corresponded to an equilibrium constant of 2.01 at 25 °C. This suggested that the homochiral and heterochiral isomers were in equilibrium with comparable concentrations in the system due to the fast free rotation of phenylene rings in the macrocycle.


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For

macrocycle

TPEMCS

with

2,5-thienylene

bridges,

the

conformational

isomerization would be more complicated as a result of the rotation of the thiophene ring within the macrocyclic skeleton. The heterochiral isomer with S atoms pointing outward the macrocycle frameworks, namely (M,P)-(out,out)-TPEMCS, was found to be the global energy minimum for TPEMCS, and was more stable than the local energy minimum with S atoms locating inside the macrocycle backbone, (M,M)-(in,in)TPEMCS, by 2.64 kcal/mol (Figure S10). This indicated that the thiophene ring was preferred to rotate outward the macrocycle with an equilibrium constant of 86.13 at 25 °C, whereas the conformers with S atoms lying inside the macrocycle framework would exist as a minor species in the system. This was well consistent with both two correlations with Hi observed in the NOESY spectrum of TPEMCS (Figure 2). However, based on the intensity of correlation signal in NOESY spectrum, the S atom appeared to be located inward the macrocycle skeleton with higher probability. This incoherence with the theoretical investigation was probably due to the interaction between TPEMCS with solvent, which was ignored in the modeling. Meanwhile, compared with other thiophene containing conjugated macrocycles in which the S atoms are usually pointing


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inward the macrocyclic skeleton,42,43 the meta-phenylene linkages and highly congested conformations of TPE moieties in TPEMCS were perhaps the major driving forces for the outward orientation of S atoms.

3.2.Aggregation-induced emission properties

The UV−Vis absorption spectra of acyclic precursor 7, TPEMC, and TPEMCS were recorded in THF (Figure 4). A slight red-shift of ∼10 nm relative to its acyclic precursor 7 was observed for macrocyclic luminogen TPEMC, probably due to the extended conjugation after cyclization. And the absorption maximum of TPEMCS remained nearly the same after transforming the diacetylene bridges into 2,5-thienylene ones. This trend was also well consistent with the simulated UV-Vis absorption spectra based on timedependent DFT calculation using functional B3LYP with the 6-31G basis set (Figure S11). According to the calculation, their bad gap had changed from 3.71 eV for acyclic 7, to 3.51 eV for TPEMC, and 3.67 eV for TPEMCS, whereas HOMO states were delocalized on TPE moieties and LUMO states were strongly localized at macrocyclic skeleton (Figure S12). We also studied the energetics of the frontier orbitals of TPEMC


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and TPEMCS using electrochemistry (Figure S13), the onset of the first oxidation (0.88 and 0.86 eV) relative to Fc/Fc+ provided an estimate of the HOMO levels at 5.25 eV and 5.23 eV for TPEMC and TPEMCS, respectively. PL spectra suggested that all three compounds were weakly emissive in THF solution, whereas the fluorescence quantum yield of 7, TPEMC, and TPEMCS was recorded to be 1.84%, 1.11% and 1.63%, respectively.

In addition, the PL spectra of these molecules in THF/water mixtures were also recorded to investigate their AIE behaviors, in which water was used as a non-solvent (Figure S14-S16). These luminogens, no matter acyclic or cyclic, all exhibited obvious AIE phenomena. When the water fraction (fw) was below 50%, PL signals of these mixtures were quite weak due to the non-radiative delay of excited state via intramolecular motions, which was well consistent with the free rotations of aromatic rings within the macrocyclic skeletons discussed above. Upon further addition of water, the PL intensity of the solution was dramatically blasted as a result of the formation of aggregates in the mixture. Dynamic light scattering (DLS) measurements unveiled that


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nanoparticles had formed in the solvent mixture, whereas the diameter of the aggregates were determined to be 183, 228, and 115 nm, respectively, for 7, TPEMC, and TPEMC-S in THF/water mixture with fw = 90% (Figure S17). The fluorescence quantum yield of 7, TPEMC, and TPEMCS in THF/water mixture with fw = 95% was measured to be 46.3%, 63.0%, and 29.6%, respectively. Therefore, the two conjugated macrocycles, TPEMC and TPEMCS, were still evidently AIE-active fluorophores. If the intramolecular motions, such as phenyl rings rotation and cis-trans isomerization of double bond, were restricted by the macrocyclic constrains, these two macrocyclic luminogens should exhibit bright emission in good solvent as reported.44 Therefore, the obvious AIE behaviors of TPEMC and TPEMCS suggested that the intramolecular motions in these two macrocycles were overwhelming and not frozen by the shapepersistent macrocyclic skeletons, leading to the non-radiative relaxation of the excited states like normal AIE-active fluorohores.44-46


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Figure 4. (a) UV-Vis absorption spectra (in THF) and normalized PL spectra (in THF/H2O mixture with fw = 95%) of acyclic precursor 7, TPEMC, and TPEMC-S. PL spectra of (b) TPEMC and (c) TPEMCS in THF/H2O mixture with different fw. (d) Change in the PL intensity (I) of 7, TPEMC, and TPEMC-S with fw; I0 is the maximal PL intensity in THF. Concentration: 10 μM. Excitation wavelength: 330 nm.


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Interestingly, the PL spectra of TPEMCS (10 μM) in THF-water mixture were found to be time-dependent, obvious blue shifts were observed after incubation with carefully sealing for one week (Figure S18). For instance, the PL spectrum of TPEMCS at fw = 50% exhibited an obvious blue shift from 476 to 455 nm, and white flocculent precipitates appeared in 30 minutes after preparing the sample. Initially, TPEMCS samples with fw below 50% were non-emissive, however, the originally non-emissive samples with fw = 30% and 40% showed strong blue emission centered at 445 nm and 460 nm after incubating for 1 week. For the samples with fw below 30%, no changes in the PL spectra were observed. This time-dependent evolution of PL spectra was probably associated with a dynamic self-organization or

crystallization process for

TPEMCS in THF/H2O mixture, leading to the hypsochromic shift in the PL spectra.47

In order to investigate the impact of self-organized microstructure on the PL signals for TPEMCS aggregates in THF/H2O mixture, SEM was applied to study the morphological structure of precipitates or aggregates observed above (Figure S19). As seen from Figure 5a, the precipitates formed at fw = 30% were one-dimensional


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microribbons with diameters down to a few hundred nanometers and lengths up to several tens of micrometers, which emitted blue fluorescence upon excitation with UV light (Figure 5d). For the sample at fw = 50%, both microfibers and irregular precipitates with blue emission could be observed in the SEM images and fluorescent microscopy images (Figure S20), By contrast, sphere-like microstructures with diameter up to several tens of micrometers had formed at fw = 90%, and emit green emission upon excitation. Therefore, the change of PL spectra after incubation was clearly originated from the dynamic self-organization of the microstructures. And for the sample with

fw=90%, the re-dissolution and subsequent self-organization of the TPEMCS was nearly impossible in the mixture due to its extremely low solubility. Therefore, even after keeping intact for 7 days, the emission color for the sample with fw = 90% remained still unchanged before and after incubation.


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Figure 5. SEM and fluorescence images of the self-organized microstructures of TPEMCS formed in the THF/water mixtures at fw = 30% (a,d), 50% (b,e), and 90% (c,f) after standing under ambient conditions for 1 week (Concentration: 10 μM).

In order to determine the crystallinity degree of these self-organized microstructures, selected area electron diffraction patterns of these three samples were recorded through exposure to the electron beam in the transmission electron microscopy (Figure S21). Electron diffraction patterns observed provided conclusive evidence that the microstructures formed at fw = 30% and 50% was crystalized, whereas the nanoaggregates formed at fw = 90% was amorphous. Therefore, the blue-shifted emission in


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the PL spectra of TPEMCS was attributed to the packing models and crystallization degree of the aggregates. Densely packing of the molecules in the crystalized structure enforced the macrocycles into a more twisted conformation, and thus led to blue-shifted emission.28,42,48

Additionally, similar time-dependent emission color changes were also found for TPEMC (Figure S22-S24). For the TPEMC samples with fw above 60%, the emission maximum remained around 505 nm even after keeping standing at ambient conditions for 1 week. However, obvious blue emission from microfibers had been recorded for other sample with fw between 30% and 50% under fluorescent microscopy. For example, the sample at fw = 40% originally exhibited weak emission at 496 nm, and after incubation the sample showed strong blue emission at 481 nm. Similar with TPEMCS, these blue-shifted PL spectra were also probably owing to the dynamic crystallization of TPEMC in the microstructures. Efforts to obtain the X-ray crystal structures for these two macrocylic compounds had been extensively made under different conditions to investigate their detailed packing models and conformations in


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the crystalized structure. Unfortunately, crystallization experiments with these two macrocyclic luminogens had both failed to yield suitable single crystals for X-ray analysis.

3.3.Mechanofluorochromism behavior

Regarding the dependence of emission color on the self-organized microstructures of TPEMCS and TPEMC in THF/H2O mixture, the mechanochromic luminescence of these two macrocyclic luminogens was investigated with the acyclic precursor 7 as a comparison. As shown in Figure 6, when as-prepared TPEMCS sample was ground into powder in a mortar, the emission maximum moved from 458 nm in as-prepared sample to 491 nm after grinding. When the ground powder was then fumed with THF vapor for 30 minutes, the emission wavelength reverted back to 469 nm. Similarly, the TPEMC sample also exhibited subtle red-shifted emission from 501 to 510 nm after grinding the pristine samples, and the emission wavelength could recover back to 489 nm after annealing with THF vapor. By contrast, the acyclic precursors 7 exhibited no obvious mechanochromic luminescence after grinding and fuming (Figure S25).49-51


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Figure 6. Fluorescent images, PL spectra and powder XRD results of as-prepared (a,c,e) TPEMCS and (b,d,f) TPEMC samples after grinding and subsequent solvent fuming with THF for 30 minutes.

To further elucidate the underlying reasons of mechanofluorochromic process for the macrocyclic fluorophores, the powder XRD measurements were carried out on the samples to study their change on molecular arrangement in solid state. For TPEMCS,


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the powder XRD patterns of as-prepared samples showed sharp reflection peaks, which was indicative of the highly ordered crystalline structures. After grinding, the attenuated diffractions signals implied the decreased arrangement orders and loose packing in the ground sample, leading to red-shifted emission at 491 nm. After annealing the ground sample with THF vapor, the diffraction curve was reverted back to be almost identical to that of the as-prepared sample, and the luminescence blue-shifted to 469 nm again.

On the other hand, the diffraction curve of pristine TPEMC sample showed a broad peak with some faint peaks in the small angle region, which was indicative of an amorphous structure mixed with minor crystalline domains. After grinding, a similar XRD curve with some peaks disappeared in the small angle region was recorded, which suggested the more loosely organized structure of TPEMC in solid state. This decrease of packing order for TPEMC resulted in the slightly red-shifted emission from 501 to 510 nm. After fuming with THF vapor, a series of sharp diffraction peaks appeared in the XRD patterns, which hinted the formation of highly ordered crystalline structures and led to the obvious blue-shifted emission at 488 nm. Therefore, the transformation between


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the crystallized and amorphous state of TPEMCS and TPEMC was the driving force of the mechanochromic luminescence of these two macrocyclic luminogens, which also agreed well with the time dependent PL spectra described above. And the lack of appropriate crystallization capability for acyclic precursor 7 resulted in the no emission change upon mechanical stimulus.49

3.4.Mercury(II) detections

As shape-persistent macrocycles, TPEMC and TPEMCS were structurally featured with the non-collapsed voids, which endowed these macrocyclic luminogens nanoporous structures in the aggregates. Herein, we had explored the possibilities of detecting metal ions based on the nano-aggregates of these macrocyclic fluorophores in aqueous mixture.


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Figure 7. (a) PL spectra of TPEMCS with different concentration of mercury ions. (b) Curve of fluorescence intensity of TPEMC and TPEMCS with Hg(II) concentration. (c) PL spectra of TPEMCS in the presence of different metal ions (20 μM). (d) PL response of TPEMC and TPEMCS in the presence of different metal ions (20 μM). Solvent: THF/H2O = 1/9; concentration of fluorescent probes: 5 μM. Excitation wavelength: 330 nm.


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Sulfur atom was usually considered as a soft base and had strong affinity to the soft transitional metal ions, like Hg (II).52-54 Hence, we firstly investigated the fluorescent response of S-containing TPEMCS in aqueous mixture to mercury ions, which was harmful and toxic to ecosystem and human health even at extremely low concentration.55 As shown in Figure 7a, with increasing concentration of the Hg(II) ions, the fluorescent intensity of the mixture was gradually quenched and reached the plateau after addition of 28 μM Hg(II), whereas the PL intensity was decreased by approximate 90% (Figure 7a and S26). Time dependent analysis after addition of mercury ions implied that the quenched fluorescent intensity reached saturated within 30 minutes after addition of 4 equivalent of Hg(II) (Figure S27). This dynamic quenching process was attributed to the slow diffusion of the sulfurphilic Hg(II) ion into the nano-aggregates of TPEMCS, and the interaction between the Hg(II) ion and thiophene sites in the macrocyclic probes quenched the emission of the nano-aggregates probably via photoinduced electron transfer.56-58


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As a comparison, the fluorescent spectra of TPEMC in aqueous mixture after addition of mercury ions were also recorded (Figure S28, S29). The results indicated that the fluorescence intensity of TPEMC was only slightly quenched by Hg (II). These variations suggested the critical role of thiophene rings for effective detection of Hg(II). To further check the selectivity of TPEMCS toward detecting Hg(II), a series of metal ions were studied while macrocyclic analogue, TPEMC, was also investigated for comparison. As shown in Figure 7c, the fluorescence intensity of TPEMCS remained nearly unchanged except for the addition of Hg(II). Additionally, the fluorescence of TPEMC was also insensitive to the addition of any metal ions investigated in this work, which further supported the critical role of the thiophene rings for the selective detection of Hg(II) (Figure 7d).

4. Conclusions

In summary, two π-conjugated macrocyclic fluorophores were synthesized with tunable bridging units. The free rotation of aromatic rings within the shape-persistent macrocyclic framework was investigated experimentally and theoretically. These two


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macrocylic luminogens both exhibited obvious aggregation-induced emission properties. And the emission color in THF/H2O mixture was strongly dependent on self-organized microstructure due to kinetic crystallization. The mechanochromism luminescence of these two macrocylic fluorogens was thus explored in solid state as a result of the different conformations in amorphous and crystallized structure. Moreover, the emission of macrocylic chromophore TPEMCS in aqueous media was found to be selectively quenched by Hg(II) ions due to the interaction between S atoms with Hg(II). This new kind of conjugated macrocyclic compounds with AIE properties could facilitate the exploration of luminescent materials with novel structures. This work also opened up a new avenue for construction of luminescent porous materials based on rigid macrocylic luminogens.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: .


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Experimental details of synthesis and structural characterization, variabletemperature NMR spectra, theoretical calculation results, and supplementary figures.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Y. Liu).

*E-mail: [email protected] (Z. Q. Yu.).

*E-mail: [email protected] (B. Z. Tang).

ORCID

Yi Liu: 0000-0001-9510-9559

Lei Wang: 0000-0002-2313-2095

Zhen-Qiang Yu: 0000-0002-0862-9415

Ben Zhong Tang: 0000-0002-0293-964X

Author Contributions


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▽Y.

L. and F. X. L. contributed equally to this work.

Funding Sources This work is partially supported by the National Natural Science Foundation of China (21704065,

21674065),

the

Innovation

Research

Foundation

of

Shenzhen

(JCYJ20170302150014024, JCYJ20170817100547775) and Shenzhen University (No. 2018005).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is partially supported by the National Natural Science Foundation of China (21704065,

21674065),

the

Innovation

Research

Foundation

of

Shenzhen

(JCYJ20170302150014024, JCYJ20170817100547775) and Shenzhen University (No. 2018005). We thank the Instrumental Analysis Centre of Shenzhen University (Xili Campus) for NMR measurement.


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Shape-persistent π-conjugated Macrocycles with Aggregation

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