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

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

    This journal is The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 12899--12902 | 12899 Published on 29 August2014. DownloadedbyEast ChinaNormal University on25/11/2014052404. ChemComm COMMUNICATION View Article Online View Journal | View Issue Cite this Chem. Commun., 2014, 50, 12899 Received 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 bridge has been efficiently synthesized by metathesis cyclopolymerization of bis1,6-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-conjugated polymer,1 has been extensively studied because of its promising properties, such as electrical conductivity,2 optical nonlinearity,3 and photoconductivity.4 Unfortunately, it suff ers from extremely low solubility and poor processability, which limited its applications in photovoltaic devices.5 Consequently, 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,6–9 producing substituted PAs with conjugated double bonds and cyclic recurring of five- or 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-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 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 p–p structures that facilitate electron-delocalization and enhance conjugation.14 Unprecedented synthesis of ladder polymers by ring-opening metathesis polymerization ROMP of bisnorbornene derivatives with various rigid linkers has been Department of Chemistry, East China Normal University, Shanghai 200241, China. E-mail ; Fax 86 21 54340058; Tel 86 21 54340058 † Electronic supplementary ination 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 semiconductor 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 expansion 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 limits 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 MCP is largely unknown. Herein, we designed bis1,6-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 poly1 Chart 1, single-stranded conjugated PA poly2 and double- stranded non-conjugated polynorbornene poly3 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. As far as the poor solubility of PBI segments and the conjugated PA backbone is concerned, monomers bearing long alkyl chains between bis1,6-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 of Chemistry 2014 View Article Online Communication ChemComm Published on 29 August2014. DownloadedbyEast ChinaNormal University on25/11/2014052404. necessary to ensure the solubility of polymers. The novel double-stranded poly1 was thus synthesized by MCP of 1 using Ru-III as catalyst under various conditions, and the results are displayed in Table S1 ESI†. It is clearly displayed that polymerization behavior is much diff erent in CHCl3 and THF, giving polymers in low or high yields. As expected, poly1 is easily soluble in common solvents such as CH2Cl2, CHCl3, and THF, while insoluble in DMF Table S2, ESI†. However, poly1 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 poly1 prepared in THF are much higher than those in CHCl3 under 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 poly1, the presence 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 poly1 should have the same cis configuration, and MCP triggered by Ru-III produced poly1 with exclusively five-membered ring units, i.e., 1,2-cyclopent-1-enylene- vinylenes.9c Similar observations were found for poly2 Fig. S14, ESI†. For poly3, two new signals of olefinic protons Hi on the backbone came at 5.79 and 5.52 ppm 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. S12, ESI†, we deemed that poly1 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 transation from monomers to polymers, and the fluorescence of poly1 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, 1 was chemically confined within one-dimensional space via covalent bonds Fig. 1b. Obviously, the ladder conation of poly1 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 polymer 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 areas have monolayer structures, where poly1 s parallel strips in the same direction, and some others jet black regions have overlapped multilayer structures, where poly1 s continuous square grids along the substrate surface. From the monolayer area Fig. S20, ESI†, we could clearly identify 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 poly1 by GPC analysis. This highly regular structure is attributed to the assembly of poly1 molecules 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*, p–p interaction between PBI segments might take place during the course of MCP, which restricted the PBI moiety 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 ation of expected ladder- like structures.15f Simultaneously, the p–p 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 SAED patterns of poly1 Fig. 1d acquired during the TEM analysis confirmed the crystallinity of the ladder polymer. TEM visualization of double-stranded poly1 unambiguously revealed that MCP of bis1,6-heptadiyne provided a new for construction of conjugated polymers with ladder architecture, which may facilitate the electron mobilities in optoelectronic devices. However, TEM analysis showed that the structure of single-stranded poly2 Fig. S21, ESI† or double- stranded poly3 Fig. S22, ESI† was amorphous, suggesting that they may have irregular stereochemistry where all PBI 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 poly2 and mixed cis/trans E1 1 double bonds on the main chain of poly3. UV-vis analysis can provide additional ination on the fully conjugated structure and optical properties. Fig. 2 exhibited the characteristic absorption 400–600 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 S0–S1 transition 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 poly1 with 4 repeat units, c HR-TEM image of poly1, and d the corresponding SAED pattern. evolves at lower wavelengths 400–460 nm, which is attributed to the electronic S0–S2 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 on25/11/2014052404. View Article Online ChemComm Communication Fig. 2 UV-vis spectra of diff 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 p–p* transition, also appears at this area. Compared to 1, poly1 with long conjugation length shows nearly 165 nm bathochromic shift from 560 nm to 725 nm in solution Fig. S23, ESI†. For poly2, an approximate 95 nm blue-shift from 725 nm to 630 nm was observed with comparison to the rigid poly1, suggesting that distortion happened for the main chain of poly2 which further caused the effective conjugation length decrease. Distinctively, besides the strong absorption between 400 and 550 nm, poly3 has another strong absorption at nearly 370 nm, which is caused by the non-conjugated main chain of polynorbornene. In addition, the lmax of polymers in a thin film state Fig. 2b is red-shifted compared to that in solution, and 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 uated from the onset absorption Table 1. The onset absorptions are 730 nm for poly1, 689 nm for poly2, and 598 nm for poly3, 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 polymer 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 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 .23 The LUMO energy was obtained indicating the much higher electron-affinity and ambient stability.9f Therefore, the wide absorption and narrow Eg of poly1 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 bis1,6-heptadiyne derivatives. 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 narrow to 1.70 eV, and the LUMO energy level lowered to 4.25 eV. Oppositely, the double- stranded non-conjugated polynorbornene with a PBI bridge by ROMP of bisnorbornene derivatives possesses an amorphous structure, having a high LUMO energy of 3.86 eV and a wide Eg of 2.07 eV. Therefore, the new 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.

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