Substitution position effect on photoluminescence emission and chain conformation of poly(diphenylacetylene) derivatives

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(1)View Online / Journal Homepage / Table of Contents for this issue. COMMUNICATION. www.rsc.org/chemcomm | ChemComm. Substitution position effect on photoluminescence emission and chain conformation of poly(diphenylacetylene) derivativesw Wang-Eun Lee,a Chang-Jin Oh,a Guen-Tae Park,a Jin-Woo Kim,a Heung-Jin Choi,*b Toshikazu Sakaguchi,c Michiya Fujiki,d Ayako Nakao,d Ken-ichi Shinoharae and Giseop Kwak*a. Downloaded by Nara Institute of Science & Technology on 03 October 2012 Published on 09 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CC01481H. Received 19th May 2010, Accepted 7th July 2010 DOI: 10.1039/c0cc01481h The significant variation in photoluminescence emission of poly(diphenylacetylene) derivatives according to the substitution position is due to the differences in the intramolecular p-stack structure and chain conformation. Many types of conjugated polymers have been extensively studied for various applications such as polymer light-emitting devices (PLED)1 and organic thin film transistors (OTFT).2 These polymers have been precisely designed in order to satisfy the requirements of specific functions and applications. From the viewpoint of the molecular design, the electronic action,3 bulkyness,4 and symmetry5 of substituents attached to the main chain significantly influence their photophysical and electro-optical properties. That is, whether to attach an electron-donating or an electron-withdrawing group to the main chain, whether to attach a smaller or a bulkier group, and whether to attach the substituent symmetrically or unsymmetrically have been largely considered in order to optimize their HOMO/LUMO energy levels and the band gap energy in the desired direction. Thus, such molecular designs are very important in relation to fluorescence-color tuning, efficient exciton confinement, and charge-carrier transport performance. Recently, some polymers having stable and regulated p-stacked structures in the side chains have been reported by many research groups.6 Such chain conformation significantly influenced their properties and functions. Tsuchihara et al. have synthesized poly[1-phenyl-2-(p-trimethylsilyl)phenylacetylene] [p-PTMSDPA in Fig. 1a] in 1991.7,8 The corresponding meta-substituted polymer [m-PTMSDPA in Fig. 1a], having a trimethylsilyl group on the meta-position of the side a. Department of Polymer Science, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702-701, Korea. E-mail: [email protected]; Fax: +82-53-950-6623; Tel: +82-53-950-7558 b Department of Applied Chemistry, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702-701, Korea c Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, Bunkyo, Fukui 910-8507, Japan d Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan e School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan w Electronic supplementary information (ESI) available: Detailed experimental methods, absorption/emission spectra, time-resolved emission decay data, X-ray diffraction patterns, AFM movies, histogram of the circularity and schematic explanation for energyminimized structures of 10-mer model compounds. See DOI: 10.1039/ c0cc01481h. This journal is. c. The Royal Society of Chemistry 2010. Fig. 1 (a) Chemical structures of p- and m-PTMSDPA (paraposition: p-PTMSDPA; meta-position: m-PTMSDPA). Photographs of p- (left) and m-PTMSDPA (right) in (b) solution (c = 1.0 10 5 mol L 1 in toluene) and (c) film (spincoating, thickness E 30 nm) under UV irradiation at >365 nm.. phenyl ring, was also synthesized in the same year. Surprisingly, these two polymers show completely different photoluminescence (PL) emission properties merely due to the subtle difference in substitution position, despite their structural similarity. As shown in Fig. 1b and c, the m-PTMSDPAshows much weaker emission at a longer wavelength relative to the p-PTMSDPA (see Fig. S1 and the description in ESIw for the detailed absorption/emission spectrum analysis). It is a remarkable event that only the slight difference in the chemical structure, i.e., whether the trimethylsilyl group is attached to the para- or meta-position on the side phenyl rings, results in a significant difference in the PL emission property. However, the reason has not been clarified yet and has remained unknown.8 The PL emission of diphenylacetylene polymer derivatives originates from the intramolecular excimer emission due to the cofacial phenyl–phenyl stack in the side chains.9 Thus, the emission origin of the present polymers is completely different from the intermolecular excimer emission of the coplanar aromatic p-conjugated polymers with the cofacial chain packing structure in a solid state.10 Such an intramolecular excimer emission should be shown in dilute solution as well as in bulk solids. In comparison of both the polymers, the emission of m-PTMSDPA in solution is very weak (quantum emission yield, FPL: 2.9 in toluene), while the p-PTMSDPA in solution shows about 11 times more intense emission (FPL: 32.0). The dominant fluorescence lifetime (tPL: 0.611 ns in toluene) of p-PTMSDPA is much longer than that (tPL: 0.114 ns in toluene) of m-PTMSDPA (Table S1, ESIw). In general, the greater the degree of aromatic stack in a fluorophore molecule, the less intense the emission, and shifts into the longer wavelength of the emission band. This suggests that the electronic clouds of m-PTMSDPA are highly dense in the phenyl–phenyl stack structure and that the molecular torsional and scissoring motions of the side phenyl rings are highly restricted; thus, the cross-sectional phenyl–phenyl stack area becomes greater, resulting in a chain conformation Chem. Commun., 2010, 46, 6491–6493 | 6491.

(2) Downloaded by Nara Institute of Science & Technology on 03 October 2012 Published on 09 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CC01481H. View Online. mainly with a nonradiative electronic structure even at the isolated chain state in a dilute solution. On the other hand, p-PTMSDPA should have a less dense phenyl–phenyl stack structure and thus the side phenyl rings have a smaller crosssectional phenyl–phenyl stack area to undergo a radiative electronic transition process. The intramolecular excimer, due to the cofacial aromatic stack structure, will be more easily generated in a solid film rather than in an ideal solution, because the molecular torsional and scissoring motions of the side phenyl rings should be further restricted in the denser space of the solid film than in the solution-state isolated chains. The FPL of p-PTMSDPA in film is 1.2 (tPL: 0.167 ns in film). Especially, the FPL value of m-PTMSDPA in film is as small as o0.2 (tPL: 0.094 ns in film). It should be also noted that the unsubstituted poly(diphenylacetylene) with no trimethylsilyl groups showed the weaker PL emission (FPL: 0.09 in film) at a little longer wavelength (emission maximum wavelength, lmax,em: 547 nm in film) relative to both p- and m-PTMSDPA (Fig. S2, ESIw).11 In a previous study, it was found that the PL emission of diphenylacetylene polymer derivatives in bulk films significantly depended on the lamellar layer distance (LLD).9f,12 The longer the LLD, the smaller the crosssectional area of the cofacial phenyl–phenyl stack, and the more intense the emission. Thus, when a polymer derivative film with a shorter LLD is swollen by an appropriate nonsolvent, the PL emission is significantly enhanced to be able to stand a comparison with that of a longer LLD polymer derivative.9f,13 However, both p- and m-PTMSDPA showed almost the same LLD of 13.0 A˚ in the X-ray diffraction (XRD) patterns (Fig. S3, ESIw). When both the polymer films were swollen by a hydrocarbon liquid such as hexane, the PL intensity dramatically increased, probably due to the enhanced chain mobility accompanied with the stack structure relaxation. Actually, the FPL values of p- and m-PTMSDPA, after fully swelling with hexane, increased about 10 times relative to those before swelling, to reach up to 11.0 and 2.0, respectively. However, the difference in emission intensity between the p- and m-PTMSDPA is still remarkable. These results suggest that the difference in emission between both polymers should be ascribed to the intramolecular chain conformation, but not to LLD. Theoretical calculations were carried out on the 10-mer model compounds of both polymers to see their energyminimized structures (Spartan 04, V1.0.1, AM1 semiempirical). Fig. 2 shows the side and top views of 10-mer model compounds of p- and m-PTMSDPA polymers. Both model compounds show repeating conformational patterns, especially for spatial arrangements of trimethylsilyl groups. The difference in chain conformation between the para- and meta-polymers is clearly seen: The trimethylsilyl groups of p-PTMSDPA are discontinuously arranged in a zigzag pattern, while the m-PTMSDPA is continuously coiled in a helical manner. As for p-PTMSDPA, the dihedral angles between two planes of molecular backbone with neighboring ipso-carbon of the p-trimethylsilylphenyl group are sequentially twisted with angles of +63 11, 109 21 and +54 11 in a repetitively zigzag pattern (Fig. S4, ESIw). However, in the case of m-PTMSDPA, the corresponding dihedral angles are fixed at 6492 | Chem. Commun., 2010, 46, 6491–6493. Fig. 2 Views of energy-minimized structures of 10-mer model compounds of both p- and m-PTMSDPA polymers as obtained with AM1 geometry. Side views are shown as space-filling model, in which carbon atoms appear in gray, silicons in red, and hydrogens in white. For viewing clarity, hydrogen atoms are omitted for the top views in tube model.. 55 0.51 in a helical manner (Fig. S5w). Thus, the molecular backbone of m-PTMSDPA is also helically coiled, and consequently, one turn of the helix exactly involves six monomer units. In m-PTMSDPA, both the stack of phenyl rings and the stack of phenyl rings with a trimethylsilyl group on the meta-position are coiled in a helical manner in a direction parallel to each other. The side phenyl groups are stacked one on top of the other, with an average distance of ca. 2.4 A˚ between two phenyl rings. Because six phenyl rings are involved in one complete turn of the helix, the pitch of the helix is calculated to approximately 12.0 A˚. Accordingly, there is a considerable difference in the stack degree of the aromatic pendant groups between the para- and meta-polymers. The difference in chain conformation is also reflected in the main chain rigidity. The m-PTMSDPA shows the stiff nature of the main chain, as determined by the high viscosity index of a = 1.07 (in THF at 40 1C). On the other hand, p-PTMSDPA has a lower value of 0.80. The higher main chain rigidity of the meta-polymer should be ascribed to the helix-like, continuous phenyl–phenyl stack structure, while the lower rigidity of the para-polymer is due to the discontinuous stack structure in a zigzag pattern. On direct observation of single chains in solution using a high-speed atomic force microscope (AFM), the meta-polymer chain behaves as a rigid rod while the para-polymer does exist as a shrunk coil (Fig. S6, Movies S1 and S2, ESIw). In summary, we clearly verified the substitution position effect on PL emission of poly(diphenylacetylene) derivatives. The steady-state/dynamic PL spectroscopy and XRD studies revealed that the PL emission properties were significantly dependent on the substitution position of a trimethylsilyl group on the side phenyl ring. The theoretical calculations on both model compounds revealed that the remarkable difference in the PL emission property is because the intramolecular cofacial phenyl–phenyl stack structure significantly varies according to the substitution position of the trimethylsilyl group. These results help us to comprehensively understand the origin of the PL emission of poly(diphenylacetylene) derivatives. It would be also a new guideline for synthetic This journal is. c. The Royal Society of Chemistry 2010.

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