Coordination-driven [2+2] metallo-macrocycles isomers: conformational control and photophysical properties
Abstract
In recent years, fluorescent supramolecular materials have received significant attention due to their wide application prospects. However, the relationship between the conformation of supramolecules and their photophysical properties remains an open question. In this study, two rhomboidal metallacycle isomers, SA and SB, self-assembled with trans- and cis-isomers of tetraphenylethylene-based ditopic pyridyl ligands (LA and LB), and a 120° di-platinum (II) acceptor were prepared. Compared with metallacycle SB constructed by cis- tetraphenylethylene (TPE)-based ligand LB, the curved rhomboidal metallacycle SA constructed with trans-TPE-based ligand LA can restrict molecular motions of the aromatic groups on TPE and exhibits better light-emitting properties. Moreover, curved SA also exhibited better fluorescence stability than isomer SB towards molecules with strongly electron-withdrawing groups. This work provides a new platform to explore the relationship between conformation and the corresponding photophysical properties.
Keywords
INTRODUCTION
In the past decades, fluorescence materials have attracted considerable attention for their widespread applications in biological imaging[1], environmental sensing[2], fluorescent probes[3], and other fields[4] due to their high luminous efficiency and stimulus responsiveness. However, conventional organic fluorophores often suffer from aggregation-caused quenching (ACQ) in a concentrated solution or the solid state, which limits their practical application[5]. To overcome the undesirable ACQ, in 2001, Tang and co-workers developed fluorescent species exhibiting the opposite effect, known as aggregation-induced emission (AIE)[6]: tetraphenylethylene (TPE), 1,1,2,3,4,5-hexaphenylsilole, triphenylamine, distyrylanthracene, etc[7]. The characteristic of molecules with AIE activity is that their initial free rotation is restricted in an aggregation state, resulting in reduced non-radiative decay and strong fluorescence emission. However, AIE active molecules have relatively low luminous efficiency in dilute solution because the free rotation of benzene rings leads to high non-radiative decay.
Among the diverse field of supramolecular chemistry, coordination-driven self-assembly offers a straightforward synthetic approach for constructing diverse metal-organic complexes (MOCs)[8], ranging from two-dimensional (2D) metallacycles to three-dimensional (3D) metallacages. The metal coordination bonds not only provide definite directionality and high predictability but also have moderate bond energies (15-25 kcal/mol) to ensure self-modulation and thermodynamic stability during self-assembly. These unique advantages allow MOCs to be well constructed with specific stoichiometry and precise geometry and, thus, provide the impetus for applications in catalysis, sensing, and optics[9]. Recently, MOCs combined with AIEgens have also drawn particular attention as multifunctional platforms for exploring AIE properties. However, how to provide the bright emission of AIEgens in dilute solution is still a big challenge. Rational synthesis strategies should be developed to restrict the rotation of phenyl rings by slightly adjusting the conformation of the AIE group via metal coordination. In this way, the non-radiative decay in the excited state can be further reduced, thus giving a bright emission in dilute solution, which is also known as coordination-induced emission (CIE).
Herein, we provide such a CIE strategy via constructing two rhomboidal metallacycle isomers SA and SB by the self-assembly of trans- and cis-isomers of TPE-based ditopic pyridyl ligands (LA and LB) with a di-platinum (II) acceptor. It is worth mentioning that these two ligands can be easily obtained through one-pot synthesis and separation by column chromatography. In contrast to SB constructed by cis-TPE-based ligand LB, the curved rhomboidal metallacycle SA based on trans-TPE-based ligand LA could restrict molecular motions of the TPE aromatic groups, inducing the integration of AIE and CIE and thus showing double the quantum yield as SB. Moreover, curved SA also exhibited better fluorescence stability than isomer SB towards strongly electron-withdrawing molecules.
RESULTS AND DISCUSSION
The synthesis and characterization of the 120° di-Pt(II) acceptor 1 and the two ditopic ligands LA and LB are shown in the supplementary materials [Supplementary Figures 1-11]. The two isomeric ligands LA and LB can be obtained in a 1:1 product ratio by one-pot Suzuki coupling reactions and column chromatography (SiO2). As shown in Scheme 1, the obtained 180° trans-TPE-based ligand LA and 60° cis-TPE-based ligand LB were assembled with the same 120° di-Pt(II) acceptor 1 in a 1:1 stoichiometric ratio in DMSO at 80 °C, affording the metallacycles SA and SB, respectively. After 10 h, the final discrete metallacycles were obtained in quantitative yields (94% for SA and 92% for SB) by sedimentation and centrifugation.
Scheme 1. Self-assembly of metallacycles SA and SB (energy-minimized structures). Carbon atoms of ligands LA and LB are blue, carbon atoms of acceptor 1 are green, nitrogen atoms are dark blue, oxygen atoms are red, phosphorus atoms are pink, and platinum atoms are grey.
The formation of discrete metallacycles SA and SB with highly symmetric architecture was first investigated by multinuclear NMR analysis [1H, 31P{1H}, 13C, 2D correlation spectroscopy, and 2D diffusion-ordered NMR spectroscopy (DOSY)] [Figure 1A-H and Supplementary Figures 12-23]. It is evident from the 1H NMR spectra [Figure 1A-E] that no linear structures were formed. If a linear structure were formed, the proton signal of the ligands in these complexes would show more peaks due to the asymmetric chemical environment. In the 1H NMR spectra [Figure 1A-E], the signals of protons corresponding to the pyridyl units and phenyl rings in metallacycles SA and SB exhibited downfield shifts relative to those of the free TPE ligands due to the loss of electron density after the formation of Pt-N coordination bonds. For metallacycle SB, the signals of the pyridyl and phenyl protons split into two sets upon metal coordination, being consistent with the previously reported MOC structures[10]. 2D diffusion-ordered spectroscopy (DOSY) in DMSO-d6 showed a single band at log D = -10.19 for SA and -10.26 for
Figure 1. 1H NMR spectra (500 MHz, 300 K) in DMSO-d6of: 1 (A); LA (B); SA (C), LB (D); and SB (E). 31P{1H} NMR spectra (500 MHz, 300 K) in DMSO-d6of: 1 (F); SA (G); and SB (H). ESI-MS spectra of SA (I) and SB (J). DFT-optimized ground-state structure of SA (K) and single-crystal X-ray structure of SB (L). Carbon atoms of ligands LA and LB are gray, carbon atoms of acceptor 1 are green, nitrogen atoms are dark blue, oxygen atoms are red, phosphorus atoms are pink, and platinum atoms are orange. Hydrogens and counterions OTf- (trifluoromethanesulfonate) anions are omitted for clarity.
After the successful construction of metallacycles SA and SB, their photophysical properties were studied. The absorption spectra of ligands and metallacycles are shown in Figure 2A. Ligands LA and LB displayed two similar broad absorption bands at 270 and 330 nm, originating mainly from pyridine and TPE. Due to the metal to ligand charge transfer (MLCT) after coordination and forming SA and SB, the two absorption bands were, respectively, red-shifted to 315 and 355 nm in SA and 315 and 365 nm in SB. Moreover, these assemblies exhibited significantly enhanced molar absorption coefficients because of the inclusion of two ligands in one metallacycle structure. Next, fluorescence spectra of ligands and metallacycles were also recorded [Figure 2B]. Ligands LA and LB showed weak emission at 485 nm in DMSO, which can be ascribed to the non-radiative relaxation pathway via intramolecular rotations of the phenyl and pyridyl rings. After assembling ligands into rhomboidal metallacycles, the emission intensity was enhanced and red-shifted by ~20 nm (505 nm). Compared to the normal rhombus metallacycle SB, the molecular motions of the aromatic groups on TPE are more restricted by coordination bonds in the curved metallacycle SA, decreasing the non-radiative decay and thus giving a brighter emission. Consequently, the obtained ΦF value of SA in DMSO was 6.47%, significantly higher than the ΦF of SB in DMSO (3.21%). The absorption and emission spectra of metallacycles SA and SB in the solid state were further explored
Figure 2. (A) UV-Vis spectra of LA, LB, SA, and SB in DMSO (λex = 320 nm, c = 10.0 μM). (B) PL spectra of LA, LB, SA, and SB in DMSO (λex = 320 nm, c = 10.0 μM). PL spectra of SA (C) and SB (D) recorded in DMSO/H2O mixtures containing different H2O fractions. The respective inserts show SA and SB dissolved in DMSO/H2O with 0% (left) and 90% H2O (right). Quantum yields of SA (E) and SB (F) versus increasing H2O fractions in DMSO/H2O mixtures, determined using rhodamine B (ΦF = 69.0%) at 365 nm (λex = 320 nm, c = 1.00 μM).
To investigate the AIE properties of metallacycles SA and SB, their fluorescence emission spectra in DMSO and DMSO/H2O mixture solutions were recorded [Figure 2]. Adding water to the DMSO solution reduced the solubility of SA and SB, thereby promoting aggregate formation. Upon successive increments in the water content, the fluorescence intensities of SA chronologically increased and reached a maximum at 90% H2O content [Figure 2C]. Its fluorescence quantum yield continued to increase with the rising water content and matched well with the change in fluorescence emission intensity [Figure 2D]. As anticipated, the fluorescence results demonstrate the AIE behavior of SA. Metallacycle SB displayed a similar AIE phenomenon [Figure 2E and F]. Interestingly, in the aggregated states (H2O content = 90%), the fluorescence intensity of SA increased up to 27 times (ΦF = 60.28%), whereas SB displayed only a 14-fold fluorescence enhancement (ΦF = 26.08%). The more curved structure of metallacycle SA may promote the tight packing of TPE units in the aggregated state, enhancing its fluorescence remarkably. Moreover, the restricted movement of phenyl rings of TPE in SA further enriched its fluorescent performance compared to SB.
AIE-active molecules often show the phenomenon of fluorescence quenching due to the charge transfer between AIE moieties and molecules with strongly electron-withdrawing groups. Thus, we expected that the twisted and high luminous efficient structure of SA could effectively reduce the charge transfer process and quenching phenomenon. Inspired by the excellent AIE luminescence properties observed for rhombus metallacycles SA and SB in this study, we investigated the fluorescence stability of these two isomers in the DMSO/H2O mixture with 90% H2O content towards 2,4,6-trinitrotoluene (TNT). The quenching processes could be monitored by the change in emission intensity in response to TNT addition [Figure 3A and B]. Upon adding TNT, the emission of the aggregates was gradually quenched. The fluorescence quenching could be clearly observed even at a TNT concentration as low as 0.1 μg/mL, consistent with the previous report[12]. Impressively, although metallacycles SA and SB had the same components, their quenching efficiency and quenching rate by TNT were completely different. For metallacycle SB, quenching efficiency was 21.41% when 0.1 μg/mL of TNT was added to its solution, while it was only 8.86% for SA under the same condition. For 10.0 μg/mL TNT concentration, the initial fluorescence intensity of SB reduced by 96.76%, but only 76.79% for SA. The fluorescence quenching of SA could reach 97.68% only when the TNT concentration was increased to 100 μg/mL [Figure 3C]. To determine the quenching constants of metallacycles SA and SB with TNT, the respective relative fluorescence intensity (I0/I) was plotted against the TNT concentration. Upon applying a linear Stern-Volmer equation I0/I = K[TNT] + 1[2], from the mean of linear fitting, the quenching constants of SB were calculated to be 7.289 × 105 M-1. However, the best fit was obtained with the linear fitting for metallacycle SA when the TNT concentration was lower than 176 μg/mL, and the quenching constant was calculated to be 5.774 × 104 M-1. With the further increase of TNT concentration, the quenching process of SA showed a curvy enhancement [Figure 3D]. The structural differences of metallacycles SA and SB led to the large difference in their quenching constants. The key mechanism in binding TNT with chromophoric receptors was generally considered to be through the π-π stacking interactions. Since SA is a rhombic metallacycle with a curved structure, its π-π stacking interactions with TNT was hindered, which further affected the fluorescence quenching effect.
Figure 3. Fluorescence emission spectra of metallacycles SA (A) and SB (B) in a DMSO/H2O (1/9) mixture containing different amounts of TNT (λex = 320 nm, c = 1.00 μM). (C) The fluorescence quenching efficiencies of metallacycles SA and SB at different TNT concentrations in a DMSO/H2O (1:9) mixture. (D) Plot of relative fluorescence intensities (I0/I, I = peak intensity and I0 = peak intensity at [TNT] = 0 μM) versus TNT concentrations in a DMSO/H2O (1:9) mixture (λex = 320 nm, c = 1.00 μM).
CONCLUSION
Two TPE-based metallacycles (named SA and SB) were constructed using cis-trans isomers of a TPE-based dipyridyl ligand and 120° di-Pt(II) acceptor. The rhombus metallacycle SA constructed with the trans-TPE-based dipyridyl ligand had a more curved structure than the rhombus metallacycle SB constructed with the cis-TPE-based dipyridyl ligand. The curved SA structure restricted the molecular motions of the aryl groups on TPE, resulting in the CIE phenomenon and remarkable light-emitting properties. In addition, SA with twisted configuration and high quantum yield showed better fluorescence stability than SB in the presence of TNT. This work provides a new platform to explore the relationship between the conformation of the AIE group and the corresponding photophysical properties.
DECLARATIONS
Acknowledgments
This study was supported by the National Natural Science Foundation of China (22071079 for Wang M). The authors thank the staff from the BL17B beamline of the National Facility for Protein Science in Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility for assistance during data collection.
Authors’ contributions
Completed the synthesis, conducted NMR characterization, optical tests, and prepared the draft manuscript: Zeng Y
Performed MS characterization: Li K, Fang F
Performed DFT calculations: Li J, Zhang H
Performed part of the X-ray crystal structure data collection: Yu H
Designed the experiments and wrote the manuscript: Shi J, Hao XQ, Wang M
Availability of data and materials
The data supporting this article have been included as part of the supplementary materials.
Financial support and sponsorship
This study was supported by the National Natural Science Foundation of China (22071079 for Wang M).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2022.
Supplementary Materials
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Cite This Article
How to Cite
Zeng, Y.; Shi J.; Li K.; Li J.; Yu H.; Fang F.; Hao X. Q.; Zhang H.; Wang M. Coordination-driven [2+2] metallo-macrocycles isomers: conformational control and photophysical properties. Chem. Synth. 2022, 2, 12. http://dx.doi.org/10.20517/cs.2022.11
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