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Cite this: Chem. Commun., 2015, 51, 4643 Received 15th January 2015, Accepted 5th February 2015 DOI: 10.1039/c5cc00398a

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FRET-capable supramolecular polymers based on a BODIPY-bridged pillar[5]arene dimer with BODIPY guests for mimicking the light-harvesting system of natural photosynthesis† Lu-Bo Meng,a Dongqi Li,a Shuhan Xiong,a Xiao-Yu Hu,a Leyong Wang*a and Guigen Libc

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AA/BB-type and A2/B3-type FRET-capable supramolecular polymers based on a BODIPY-bridged pillar[5]arene dimer and two BODIPY derivative guests have been successfully constructed and their application in mimicking the light-harvesting system of natural photosynthesis was studied.

Energy issues are a global concern for human beings. Because of the excessive consumption of fossil fuels, severe environmental pollution, and increasing pressure for energy supply, the search for economical and clean energy sources is a highly challenging and urgent task. Fortunately, solar energy is a very promising energy source, which is nearly inexhaustible and distributes all over the earth. However, one difficult problem is how to take full advantage of sunlight and convert it into other forms of energy we can conveniently use. In this regard, nature provides us with a fantastic lesson: photosynthesis, which is a natural process, takes place in plants, algaes and cyanobacteria and plays a significant role in life on this planet. Based on photosynthesis, sunlight can be converted into chemical energy that is stored in carbohydrate molecules, such as sugars.1 Generally, the absorption and transfer of solar energy is the first and a vital step in the process of photosynthesis.1a,2 Therefore, mimicking the photosynthetic light-harvesting system with synthetic models is significantly meaningful not only for our understanding of the function and mechanism of photosynthesis but also in the development of potential applications in terms of organic light-emitting diodes (OLED),3 dye-sensitized solar cells (DSSC),3a,4 and other optoelectronic devices.5 This area is currently

a

Key Laboratory of Mesoscopic Chemistry of MOE, Center for Multimolecular Organic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. E-mail: [email protected]; Fax: +86-25-83597090; Tel: +86-25-83592529 b Institute of Chemistry and BioMedical Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China c Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, USA † Electronic supplementary information (ESI) available: Experimental details, NMR spectra, 2D NOESY, SEM images, fluorescence excitation spectra, Job plots and specific viscosities. See DOI: 10.1039/c5cc00398a

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attracting great interest from biologists and chemists. So far, lots of ¨rster) artificial light-harvesting systems based on fluorescence (or Fo resonance energy transfer (FRET) have been developed, most of which were constructed by covalent donor–acceptor models.2b,d,6 Although impressive achievements have been made in this field, non-covalent artificial light-harvesting systems7 have been much less frequently explored. Recently, with the rapid development of supramolecular chemistry, more and more efforts were contributed towards developing such non-covalent artificial light-harvesting systems, and among them crown ethers (CEs),8 cyclodextrins (CDs)9 and host–guest complexes containing multiple hydrogen bonds8,10 were frequently employed as building scaffolds. As a new class of macrocyclic hosts, pillararenes, which are cyclized by several hydroquinone monomers linked with methylene bridges at para-positions, possess many advantages, including facile preparation, selective functionalization, unique host–guest chemistry, and so on.11 These properties make pillararenes excellent host molecules for constructing rotaxanes, supramolecular polymers, and vesicles, which are widely applied as fluorescent sensors,12 drug delivery systems,13 and other functional materials.14 However, until now, pillararene-based artificial light-harvesting systems have never been reported except for the only example of a pillar[5]arene-based FRET system reported by Ogoshi in 2013,15 in which a mechanically interlocked [2]rotaxane was fabricated based on a di-pyrene appended pillar[5]arene and an axle with a perylene stopper, and an effective FRET process occurred from pyrene to perylene due to the smart interlocked structure, suggesting a potential application for mimicry of the light-harvesting system. In order to develop ideal light-harvesting systems, which generally possess three key factors, namely intense and broad absorption in the range of the solar spectrum, fast and efficient energy transfer from ‘‘antenna’’ complexes to the ‘‘reaction center’’, and good photostability,16 herein we prepared a boron-dipyrromethene (BODIPY)-bridged pillar[5]arene dimer (H, as donor, for detailed synthesis, see Scheme S1, ESI†) and two BODIPY derivatives with mono-styryl/di-styryl group substituents (G1 or G2, as acceptors, for detailed synthesis, see Scheme S2, ESI†) to construct AA/BB-type or A2/B3-type FRET-capable supramolecular polymers and further

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Fig. 1 1H NMR (300 MHz, CDCl3, 298 K) spectra of (a) 4 mM H, (b) G1CH ([G1] = [H] = 4 mM, [G1]/[H] = 1 : 1), and (c) 4 mM G1 (blue italics represent complexed host and guest and the solvent peaks are marked with asterisks).

Scheme 1 Chemical structures of H, G1, G2 and G0 and the cartoon representation of the construction of two kinds of FRET-capable functional supramolecular polymers.

investigated their application for mimicking a photosynthetic light-harvesting system (Scheme 1). The neutral guests bearing two or three short alkyl chains with a triazole site and a cyano site at either end (unit G0) were designed due to their strong binding ability towards pillar[5]arene (Ka = (1.2  0.2)  104 M 1 in chloroform).17 BODIPY dyes, which are perfectly competent for the requirements of ideal light-harvesting systems, were chosen as the chromophores, since they have a high absorption coefficient, high fluorescence yields and remarkable photostability.18 Particularly, they are conveniently derivatized to cover the entire visible spectrum and energy transfer can efficiently take place between well-designed analogues.19 In this work, two kinds of assemblies exhibited very strong absorption in a broad region from 300 to 700 nm and showed slightly different FRET effects. To the best of our knowledge, this is the first example of pillar[5]arene-based supramolecular polymers for mimicking the light-harvesting system of natural photosynthesis. Although the transfer efficiency was not as high as those of covalent models, the results we achieved (51% for G1CH and 63% for G2CH) were comparable with previous artificial architectures. Therefore, the present study developed a novel supramolecular model for mimicking the light-harvesting system. The host–guest complexations between compounds H and G1 or G2 were first investigated by 1H NMR. Fig. 1 shows the proton NMR spectra of H, G1, and G1CH (1 : 1 molar ratio), in which a new species is observed, along with the corresponding

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signals for uncomplexed host and guest, indicating a slowlyexchanging system on the NMR timescale. As shown in Fig. 1b, after complexation, all proton signals became broad, and the signals of the phenyl protons (H1, H2, and H3), methylene protons (H4), and methoxyl protons (H5 and H6) from the pillar[5]arene dimer H shifted downfield slightly. Meanwhile, the signals derived from the triazole protons (Ha and Hb) of G1 shifted upfield slightly and the signals of the methylene protons (Hc, Hf, Hd, and He) from G1 shifted upfield remarkably due to the shielding effect of the electron-rich cavities of pillar[5]arene. These results clearly demonstrated that both of the pillar[5]arene motifs in H were fully threaded by guest G1 with the protons Ha, Hb, Hc, Hf, Hd, and He in the pillar[5]arene cavities. Similar complexation-induced chemical shift changes could be observed from the 1H NMR spectrum of G2CH (2 : 3 molar ratio) (Fig. S17, ESI†). Moreover, 2D NOESY experiments were also performed (Fig. S18 and S19, ESI†) and the NOE correlation signals marked with circles confirmed the above binding mode.12a 2D diffusion-ordered NMR spectroscopy (DOSY) experiments were further performed to investigate the self-assembly behavior of H and G1 or G2 during linear or cross-linked supramolecular polymerization. A plot of diffusion coefficient against concentration (Fig. 2a) shows that as the concentration of H increased from 5 to 40 mM, the measured weight-average diffusion coefficient of G1CH decreased remarkably from 2.37  10 10 to 0.51  10 10 m2 s 1, and a similar observation could also be seen for G2CH with the diffusion coefficient decreasing from 1.93  10 10 to 0.31  10 10 m2 s 1 (Fig. 2b). These results clearly

Fig. 2 DOSY (400 MHz, 298 K) plots of solutions in CDCl3 of: (a) G1CH (1 : 1 molar ratio) and (b) G2CH (2 : 3 molar ratio) at multiple H concentrations.

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reveal that larger polymeric structures gradually formed from small oligomers to linear supramolecular polymers (G1CH) or large-sized cross-linked supramolecular aggregates (G2CH), which was intrinsically concentration dependent.17a Moreover, SEM micrographs (Fig. S20, ESI†) of a rodlike fiber drawn from a very concentrated solution of G1CH or G2CH in chloroform provided further direct physical evidence for the formation of such supramolecular polymers, which generally resulted from the entanglement of linear or cross-linked macro-sized aggregates. Specific viscosities of two polymers were further measured and the critical polymerization concentration (CPC) values were 8.8 mM for G1CH and 9.6 mM for G2CH (Fig. S24, ESI†). In order to develop the application of these two assemblies for mimicking the light-harvesting system, photophysical properties of these compounds were initially examined via their ground-state UV-vis absorption and steady-state fluorescence spectroscopy in chloroform. Fig. 3a shows the normalized UV-vis absorption spectra of compounds H, G1, G2 and complexes G1CH and G2CH. For single BODIPY derivatives H, G1 or G2, only one intense and narrow absorption band is exhibited in the range of 400–700 nm (peaks at 503, 570 and 642 nm for H, G1, and G2, respectively), and the maximum absorption wavelength redshifted gradually with the continuously extended p-system by introducing one or two styryl groups. As expected, the spectra of G1CH and G2CH were essentially equal to the superposed patterns of the two individual components, suggesting that there was no interaction between donor H and acceptors G1 or G2 in the ground-state.6d Moreover, with the addition of an absorption band in the range of 300– 400 nm, the whole spectra of G1CH and G2CH spanned a very broad region from 300 to 700 nm, which overlapped with the most intense radiation domain of sunlight. As shown in Fig. 3b, upon excitation at different wavelengths, the fluorescence emission of compounds H, G1, and G2 showed signals at 522, 595 and 669 nm, respectively, which were also as sharp as those in their absorption spectra. In particular, the absorption spectrum of G1 overlapped with the fluorescence emission spectrum of H in the range of 500– 600 nm just as in our design goals, which perfectly satisfied one of the basic principles for an excellent FRET donor–acceptor system. In contrast, the overlap between the spectra of G2 and H was overwhelmingly smaller because the absorption band of

Fig. 3 (a) Normalized UV-vis absorption spectra of H, G1, G2 and the G1CH (1 : 1 molar ratio) and G2CH (2 : 3 molar ratio) complexes in CHCl3. (b) Normalized UV-vis absorption spectra of G1, G2 and fluorescence emission spectra of H (lex = 490 nm), G1 (lex = 560 nm), and G2 (lex = 630 nm) in CHCl3.

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Fig. 4 Changes in the fluorescence emission spectra of H (10 mM) upon gradual addition of (a) G1 (lex = 470 nm) and (b) G2 (lex = 490 nm) in CHCl3. The attached pictures are the corresponding changes in fluorescence emission color.

G2 shifted towards the red region much more than that of G1 as discussed above. After studying the individual properties of H, G1, and G2, the excited-state interactions of complexes G1CH and G2CH were further investigated by fluorescence titration experiments. The excitation wavelengths of H were chosen as 470 nm for G1CH and 490 nm for G2CH to avoid the excitation of G1 or G2 at the same time. With gradual addition of G1, the emission peak of H at 522 nm decreased gradually, while the emission peak of G1 at 595 nm appeared and then increased remarkably (Fig. 4a). When 1.0 equiv. of G1 was added to the H solution, the intensities of both H and G1 showed almost no change, indicating a 1 : 1 stoichiometry of G1 : H which was in good accordance with their chemical structures.9a A similar phenomenon could also be observed for complex G2CH, except that the stoichiometry of G2 : H was about 2 : 3, corresponding to the designed structures as well. The above stoichiometry was also supported by Job plot experiments (Fig. S22 and S23, ESI†). In addition, compared with the strong fluorescence intensity (2862 counts) of G1, the intensity (1009 counts) of G2 turned out to be much weaker, probably due to the smaller overlap of its absorption spectrum with the fluorescence emission spectrum of H (Fig. 3b). Moreover, changes in fluorescence emission color before and after the addition of the guests could be easily distinguished by the naked eye. The above results clearly demonstrated that an efficient FRET effect from donor H to acceptors G1 or G2 existed in both supramolecular complexes. To further confirm the energy transfer process, the fluorescence excitation spectra of G1CH and G2CH were recorded by monitoring the emission at 595 nm corresponding to G1 and at 669 nm for G2 (Fig. S21, ESI†). In the fluorescence excitation spectra of both G1CH and G2CH, all absorption bands of the parent compounds could be observed. Moreover, the similarity between the excitation and the absorption spectra provided further evidence for energy transfer.2b

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The FRET efficiencies were further calculated to be 51% for G1CH and 63% for G2CH (for details see ESI†). Although the fluorescence intensity of G2 increased less than that of G1, the energy transfer efficiency of G2CH was actually a little higher than that of G1CH, which was out of our expectation. We speculated that the greater number of binding sites (G2 vs. G1 = 3 vs. 2) towards the pillar[5]arene dimer H might result in the stronger complexation between G2 and H, which may explain the difference in energy transfer efficiency. In summary, we have successfully prepared a BODIPY-bridged pillar[5]arene dimer H and two BODIPY derivative guests G1 and G2 to construct AA/BB-type and A2/B3-type FRET-capable supramolecular polymers for mimicking the photosynthetic light-harvesting system. Both assemblies exhibited very strong absorption in a broad region from 300 to 700 nm. Due to the high complexation stability of the host–guest pair, they showed efficient FRET effects and the transfer efficiencies were 51% for G1CH and 63% for G2CH, which are comparable with previous artificial models. To the best of our knowledge, this is the first example of pillar[5]arenebased supramolecular assemblies for mimicking the lightharvesting system. Therefore, this work not only provided a novel model for fabricating artificial light-harvesting systems but also extended the potential applications of pillararenes in the field of optoelectronic materials. We are grateful for the financial support from the National Basic Research Program of China (2014CB846004 and 2013CB922100), the National Natural Science Foundation of China (No. 21472089 and 21202083), and the National Science Foundation of Jiangsu (No. BK20140595).

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FRET-capable supramolecular polymers based on a BODIPY-bridged pillar[5]arene dimer with BODIPY guests for mimicking the light-harvesting system of natural photosynthesis.

AA/BB-type and A2/B3-type FRET-capable supramolecular polymers based on a BODIPY-bridged pillar[5]arene dimer and two BODIPY derivative guests have be...
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