Chem. C ommun., 2014, 50,12899--12902.docx

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1、This journal is The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 12899-12902 | 12899 Published on 29 August2014. DownloadedbyEast ChinaNormal University on25/11/201405:24:04. ChemComm COMMUNICATION View Article Online View Journal | View Issue Cite this: Chem. Commun., 2014, 50, 12899 Re

2、ceived 17th July 2014, Accepted 29th August 2014 DOI: 10.1039/c4cc05524a www.rsc.org/chemcomm Ladder-like polyacetylene with excellent optoelectronic properties and regular architecture Wei Song, Huijing Han, Jianhua Wu and Meiran Xie* Novel double-stranded polyacetylene with a perylene bisimide bri

3、dge has been eciently synthesized by metathesis cyclopolymerization of bis(1,6-heptadiyne) derivatives, and exhibited good solubility, highly thermal and oxidative stability, low LUMO energy levels, narrow band- gaps, and regular ladder-like architecture. Polyacetylene (PA), the simplest p-conjugate

4、d polymer,1 has been extensively studied because of its promising properties, such as electrical conductivity,2 optical nonlinearity,3 and photoconductivity.4 Unfortunately, it su ers from extremely low solubility and poor processability, which limited its applications in photovoltaic devices.5 Cons

5、equently, the synthesis of soluble PA derivatives has been an enthusiastic research over a long period. One of the most powerful solutions is metathesis cyclopolymeriza- tion (MCP) of 1,6-heptadiyne derivatives,69 producing substituted PAs with conjugated double bonds and cyclic recurring of five- o

6、r six- membered rings7,10,11 along the backbone, thereby the solubility, stability, and processability have been greatly enhanced. Grubbs-type catalysts are widely used in metathesis polymerization;12 although hardly triggered using the first-generation Grubbs catalyst, MCP by the modified ruthenium

7、-based initiators has the ability to provide PAs with exclusively five-membered ring units,12a,b while generally having a broad polydispersity index (PDI).12b Very recently, a break- through in MCP was achieved by the third-generation Grubbs catalyst (Ru-III) which underwent selective a-addition to

8、produce PAs with only five-membered rings and a narrow PDI.9d,e Ladder polymers have greater resistance to irradiation as well as thermal and chemical degradation13 in comparison to their counterparts. Besides, the ladder-type arrays should have planar and rigid pp structures that facilitate electro

9、n-delocalization and enhance conjugation.14 Unprecedented synthesis of ladder polymers by ring-opening metathesis polymerization (ROMP) of bis(norbornene) derivatives with various rigid linkers has been Department of Chemistry, East China Normal University, Shanghai 200241, China. E-mail: ; Fax: +86

10、 21 54340058; Tel: +86 21 54340058 Electronic supplementary information (ESI) available: Experimental details and additional supportive data. See DOI: 10.1039/c4cc05524a demonstrated.15 Perylene bisimide (PBI) is a rigid aryl chromophore, and is capable of electron transport as the n-type semiconduc

11、tor on the basis of its optoelectronic properties, high stability, low-lying lowest unoccupied molecular orbital (LUMO) energy levels, and ease of synthesis.16,17 Very impressively, the bay-linked doubly and triply PBI oligomers were synthesized via Ullmann and Still coupling reaction,18 and the exp

12、ansion of the conjugated aromatic system enlarged the delocalization of p-electrons, which further lowered the LUMO energy and bandgap (Eg). Thus, this kind of PBI showed excellent air stability. Nevertheless, the number of linked PBI segments can hardly increase to more than four, which probably li

13、mits their flexible application in large areas. To our knowledge, only the introduction of PBI segments into the ladder architecture together has been realized through ROMP19 or polycondensation,20 while the fundamental preparation of ladder-type conjugated poly- mers that can be readily obtained by

14、 MCP is largely unknown. Herein, we designed bis(1,6-heptadiyne) derivatives as monomers for MCP, for the first time, to create new double-stranded PAs with ladder-like architecture and excellent optoelectronic properties. Except for the objective double-stranded conjugated PA poly(1) (Chart 1), sin

15、gle-stranded conjugated PA poly(2) and double- stranded non-conjugated poly(norbornene) poly(3) are selected for comparison (Scheme S1 in the ESI). It should be noted that solubility plays an important role in solution characterization of polymers and solution-processed organic electronic devices. A

16、s far as the poor solubility of PBI segments and the conjugated PA backbone is concerned, monomers bearing long alkyl chains between bis(1,6-heptadiyne) groups and the PBI core are Chart 1 The chemical structure of polymers. 2 | Chem. Commun., 2014, 50, 12899-12902 This journal is The Royal Society

17、of Chemistry 2014 View Article Online Communication ChemComm Published on 29 August2014. DownloadedbyEast ChinaNormal University on25/11/201405:24:04. necessary to ensure the solubility of polymers. The novel double-stranded poly(1) was thus synthesized by MCP of 1 using Ru-III as catalyst under var

18、ious conditions, and the results are displayed in Table S1 (ESI). It is clearly displayed that polymerization behavior is much di erent in CHCl3 and THF, giving polymers in low or high yields. As expected, poly(1) is easily soluble in common solvents such as CH2Cl2, CHCl3, and THF, while insoluble i

19、n DMF (Table S2, ESI). However, poly(1) with high molecular weight has poor solubility in THF and precipitated out from THF solvent during the polymerization process. Apparently, the values of yield and degree of polymerization (DP) for poly(1) prepared in THF are much higher than those in CHCl3 und

20、er the same conditions, suggesting that using a weakly coordinating solvent (such as THF) greatly improved the catalyst lifetime by stabilizing the propagating species through solvent coordination.9d The structure of polymers can be determined by 1H and 13C NMR spectroscopy. For poly(1), the presenc

21、e of a symmetric broad peak of polyene protons (Hh, Fig. S12, ESI) on the conjugated backbone at 6.53 ppm, and the single peak of methylene carbon (Cf) on the CH2O group at 67.1 ppm (Fig. S13, ESI) has been observed, which may imply that the double bonds of poly(1) should have the same (cis) configu

22、ration, and MCP triggered by Ru-III produced poly(1) with exclusively five-membered ring units, i.e., 1,2-(cyclopent-1-enylene)- vinylenes.9c Similar observations were found for poly(2) (Fig. S14, ESI). For poly(3), two new signals of olefinic protons (Hi) on the backbone came at 5.79 and 5.52 ppm (

23、Fig. S15, ESI), indicating that it has both trans and cis double bonds, and the trans/cis ratio is nearly 1 : 1. Consideration of the characteristic band of cis double bonds at 684 cm 1 in the IR spectrum (Fig. S17, ESI), combined with the symmetric broad peak at 6.53 ppm in the 1H NMR spectrum (Fig

24、. S12, ESI), we deemed that poly(1) is apparently based on solely cis double bonds along the backbone (Scheme S2, ESI), which induced all PBI segments aligned coherently toward the same direction. The typical fluorescence changes (Fig. S19, ESI) are also indicative of the transformation from monomer

25、s to polymers, and the fluorescence of poly(1) is quenched.21 From the chemical structure of 1 by Gaussview optimization (Fig. 1a), we can estimate the molecule dimension of 1 with a length of about 5.6 nm along the imide direction and a width of 0.7 nm along the PBI bay region. After polymerization

26、, 1 was chemically confined within one-dimensional space via covalent bonds (Fig. 1b). Obviously, the ladder conformation of poly(1) was verified from the HR-TEM image (Fig. 1c), and the black line aligned parallel to each other, suggesting that there is a strong interaction between the ladder polym

27、er molecules. The width of each strip was nearly 0.3 nm, which is narrower than that of 1, indicating that the aromatic PBI segment would align perpendicular to substrate orientation with respect to the substrate surface, which will give layered structures. In fact, we can see from Fig. 1c that some

28、 areas have monolayer structures, where poly(1) forms parallel strips in the same direction, and some others (jet black regions) have overlapped multilayer structures, where poly(1) forms continuous square grids along the substrate surface. From the monolayer area (Fig. S20, ESI), we could clearly i

29、dentify the ladder length to be 7.5 nm and the average spacing between strips to be 0.37 nm, illustrating that each ladder consists of 21 monomeric units, which is almost consistent with the DP of poly(1) by GPC analysis. This highly regular structure is attributed to the assembly of poly(1) molecul

30、es with the cis configuration, which paves the way for the access of ladder-like PAs.15e It is believed that after the insertion of a 1,6-heptadiyne group (a) into the propagation species (a*), pp interaction between PBI segments might take place during the course of MCP, which restricted the PBI mo

31、iety aligned to the same direction, and thus induced the other 1,6-heptadiyne group (b) to insert into the propagation center (b*) (Scheme S2B, ESI), which finally would be beneficial to the stereo-selectivity to guarantee the formation of expected ladder- like structures.15f Simultaneously, the pp

32、stacking interaction between conjugated double bonds along the longitudinal axis of polymers, and van der Waals interaction between the neighboring polymeric backbones in the second dimension22 may be responsible for such long ordered patterns. Lastly, the relative selected area electron diffraction

33、 (SAED) patterns of poly(1) (Fig. 1d) acquired during the TEM analysis confirmed the crystallinity of the ladder polymer. TEM visualization of double-stranded poly(1) unambiguously revealed that MCP of bis(1,6-heptadiyne) provided a new method for construction of conjugated polymers with ladder arch

34、itecture, which may facilitate the electron mobilities in optoelectronic devices. However, TEM analysis showed that the structure of single-stranded poly(2) (Fig. S21, ESI) or double- stranded poly(3) (Fig. S22, ESI) was amorphous, suggesting that they may have irregular stereochemistry where all PB

35、I segments align in a different direction thus inducing the atactic micro- structure,15b,c which is consistent with the presence of branched alkyl groups in poly(2) and mixed cis/trans (E1 : 1) double bonds on the main chain of poly(3). UV-vis analysis can provide additional information on the fully

36、 conjugated structure and optical properties. Fig. 2 exhibited the characteristic absorption (400600 nm) of the PBI core. The PBI absorption maximum (lmax) at 523 nm with a strongly pronounced vibronic fine structure is observed in solution (Fig. 2a), which belongs to the electronic S0S1 transition

37、with a transition dipole moment along the molecular axis,23 and a second absorption band Fig. 1 Optimized molecular model of (a) 1 and (b) poly(1) with 4 repeat units, (c) HR-TEM image of poly(1), and (d) the corresponding SAED pattern. evolves at lower wavelengths (400460 nm), which is attributed t

38、o the electronic S0S2 with a transition dipole moment perpendicular This journal is The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 12899-12902 | 12901 Published on 29 August2014. DownloadedbyEast ChinaNormal University on25/11/201405:24:04. View Article Online ChemComm Communication Fi

39、g. 2 UV-vis spectra of di erent polymers (a) in CHCl3 solution and (b) in film state. to the long molecular axis.24 The absorption of PAs, which is due to the pp* transition, also appears at this area. Compared to 1, poly(1) with long conjugation length shows nearly 165 nm bathochromic shift from 56

40、0 nm to 725 nm in solution (Fig. S23, ESI). For poly(2), an approximate 95 nm blue-shift (from 725 nm to 630 nm) was observed with comparison to the rigid poly(1), suggesting that distortion happened for the main chain of poly(2) which further caused the effective conjugation length decrease. Distin

41、ctively, besides the strong absorption between 400 and 550 nm, poly(3) has another strong absorption at nearly 370 nm, which is caused by the non-conjugated main chain of poly(norbornene). In addition, the lmax of polymers in a thin film state (Fig. 2b) is red-shifted compared to that in solution, a

42、nd the overall intensity is enhanced obviously, indicating that there was aggregation in the film state, which is a necessary criterion for ensuring high electron mobilities.9f From the UV-vis spectra in the film state (Fig. 2b), Eg could be evaluated from the onset absorption (Table 1). The onset a

43、bsorptions are 730 nm for poly(1), 689 nm for poly(2), and 598 nm for poly(3), correspondingly, and thus the values of Eg are estimated to be 1.70, 1.80, and 2.07 eV, respectively. Interestingly, both the single- and double-stranded polymers by MCP own rather lower Eg, which is beneficial for polyme

44、r solar cells. It is very delightful that the MCP route allowed us to obtain low Eg con- jugated polymers without complicated ring-expanded building reaction at the bay sites on PBI, which could not be readily achieved via ROMP or other approaches. To further investigate the influence of conjugated

45、structures on electronic properties, cyclic voltammetry (CV) analysis (Fig. S24, ESI) may provide the LUMO and the highest occupied molecular orbital (HOMO) energy levels of polymers, and they were calculated according to the reported method.23 The LUMO energy was obtained indicating the much higher

46、 electron-anity and ambient stability.9f Therefore, the wide absorption and narrow Eg of poly(1) suggest its prospective application in photovoltaic devices. In summary, we have demonstrated a facile strategy to obtain the PBI-contained double-stranded PAs by MCP of bis(1,6-heptadiyne) derivatives.

47、By tailoring the polymerizable diyne groups and tuning the main chain structure, novel PAs with unique properties and highly regular architecture can be readily achieved, without compli- cated ring-expanded building reaction at the bay sites on PBI. The Eg of double-stranded conjugated PAs can even

48、narrow to 1.70 eV, and the LUMO energy level lowered to 4.25 eV. Oppositely, the double- stranded non-conjugated poly(norbornene) with a PBI bridge by ROMP of bis(norbornene) derivatives possesses an amorphous structure, having a high LUMO energy of 3.86 eV and a wide Eg of 2.07 eV. Therefore, the n

49、ew type of ladder-like PA bearing a PBI bridge would be expected as the attractive alternative to fullerenes as photovoltaic materials, and also may be used as the constructing unit to further build complicated new polymers. The authors thank the National Natural Science Foundation of China (No. 21374030, No. 21074036), and the Large Instruments Open Foundation of East China Normal University (No. 2014-23) for financial support of this work. Notes and references 1 G. Natta, G. Mazzanti and P.