Phenyleneethynylene-based conjugated copolymers using benzo[1,2-d:4,5-d']bis[1,3]dioxole (BDO) in the repeating unit are reported. The electronic structure of the BDO unit imparts a localized HOMO topology while the LUMO is delocalized over the polymer backbone, so that the lowest optical absorption band of the polymer has considerable intramolecular charge transfer character. This contrasts with published donor-acceptor polymers with localized LUMO and delocalized HOMO. The very large Stokes shifts of the monomers, which are due to the small oscillator strength of the lowest optical transition, are largely retained in the polymers as a result of covalently constrained dihedral angles in the substituents (not the backbone), as predicted/explained by calculations.
Supporting Information. Experimental procedures and spectroscopic data, 1 H NMR of block copolymers, IR spectra, solid state UVÀvisible absorption spectra, and wide-angle X-ray diffractograms. This material is available free of charge via the Internet at http://pubs.acs.org.
New poly(1,4-phenylene ethynylenes)s (PPEs), in which each phenylene unit is substituted with both semifluoroalkoxy (-O(CH 2 ) m (CF 2 ) n F) and dodecyloxy (-O(CH 2 ) 12 H) groups, are amphiphilic by virtue of the self-aggregating properties of the dissimilar side chains. Sonogashira polymerization of 4-iodophenylacetylenes bearing semifluoroalkoxy and alkoxy groups in the 2-and 5-positions, respectively, affords polymers with regular relative placement of the dissimilar side chains (i.e., "regioregular" materials containing only "head-to-tail" diads). This provides a Janus type structure. The assembly of these polymers was studied as a function of the length of the fluoroalkyl segment in the side chain by X-ray diffraction, differential scanning calorimetry, and UV-vis spectroscopy. The properties of these polymers were compared to analogues with random relative placement of side chains (i.e., materials containing a mixture of "head-to-head", "tail-to-tail" and "head-to-tail" diads), and to a nonfluorinated analogue. In contrast to the highly ordered and oriented solid phases formed by alkyl/semifluoroalkyl substituted poly(bithiophene)s, and despite their defined molecular structure, the amphiphilicity of the new semifluoro PPEs impedes their crystallization. While the overall structure of the regioregular polymer is amphiphilic, in which the dissimilar side chains are expected to segregate, we ascribe the lack of crystallinity to the disruption of side chain crystallization by virtue of having the alkyl and fluoroalkyl segments within a single side chain. These side chain cannot pack in an interdigitated fashion by virtue of the disparate segments, thereby leading to poorly ordered, or amorphous, solid materials.
Poly(2,5-disubstituted-1,4-phenylene ethynylene)s, PPEs, are generally synthesized by Pd-catalyzed coupling polymerizations of appropriately substituted 1,4-diiodobenzenes and 1,4-diethynylbenzenes (i.e., condensation polymerization of A-A and B-B type monomers). If the monomers are not symmetrically substituted, this results in an irregular substitution pattern of the side chains along the polymer backbone. As with other classes of conjugated polymers, the relative placement of side chains along the backbone should influence the properties of the materials. We report a new synthetic approach to prepare regioregular unsymmetrically substituted PPEs by polymerization of 4-iodophenylacetylenes (i.e., a condensation polymerization of a single A-B type monomer). We have synthesized both the regiorandom and regioregular PPEs from unsymmetrically substituted monomers. We provide a detailed discussion of various approaches to the synthesis of PPEs with different regioregularities and provide a preliminary description of the differences between regioregular and regiorandom analogues.
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