Articles Alternating Aromatic and Transannular Chromophores with and without Linker: Effect of Transannular π-π Interaction on the Optical Property of Dithiaparacyclophane-based Copolymers Weiling Wang, † Jianwei Xu, †,‡ Yee-Hing Lai,* ,† and Fuke Wang †,‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 and Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 Received December 15, 2003; Revised Manuscript Received March 11, 2004 ABSTRACT: A series of bithiophene-cyclophane (PPP-type), acetylene-fluorene-acetylene-cyclophane (PPE-type), and ethylene-fluorene-ethylene-cyclophane (PPV-type) copolymers, namely, 4-6, was synthesized via a nickel-catalyzed reaction, a palladium-catalyzed Sonagashira reaction, and a Heck coupling, respectively. Unlike in the fluorene-cyclophane copolymer 3, the transannular π-π interaction of the cylophane unit in copolymer 4 resulted in a significant blue shift in its emission spectrum compared to a reference polymer, giving it an essentially blue emitting light. By introducing an acetylene or ethylene linker in going from 3 to 5 or 6, the effective conjugation in the polymer backbone due to higher rigidity in coplanarity decreased the effect of the transannular π-π interaction. While the PPV-type copolymer 6 still exhibited an appreciable red shift in both absorption and emission with respect to a reference polymer, negligible shifts were observed for copolymer 5. Introduction The broad applicability of conjugated polymers in light-emitting diodes, 1 field-effect transistors, 2 light- emitting electrochemical cells, 3 solar cells, 4 and sensors 5 has attracted continuing interest in the design and synthesis of structurally diverse conjugated polymers that exhibit the various desired properties. Among the organic conjugated polymers, the properties of poly(p- phenylenevinylenes) (PPVs) have been extensively stud- ied and documented, 1a,6 while poly(p-phenylenes) (PPPs) 7 and poly(p-phenyleneethynylenes) (PPEs) 8 have also received considerable attention because of their favor- able emission characteristics. Fluorene is one of the most universally employed building blocks in construct- ing conjugated copolymers that show blue-light-emitting characteristics. A large number of fluorene-based co- polymers derived from comonomers such as benzenoids, thiophene, tertiary aromatic amine, benzothiadiazole, and benzooxadiazole were prepared by various synthetic coupling reactions. 9 The PPV-type and PPE-type fluo- rene-based copolymers have also been explored 10 in attempts to tune their spectral and electrical properties by incorporating various conjugated units into the polymer backbone. 11 The unique transannular π-π interaction in [2.2]paracyclophane was employed in copolymers 1 that exhibited novel optical properties in comparison to the corresponding linear conjugated reference compounds. 12 Our preliminary study of an example of copolymers 2, namely, the fluorene-dithia- [3.3]paracyclopane polymers 3a-c, 13 showed that the transannular effect in the [3.3]cyclophane unit is suf- ficiently strong to allow tuning of the optical properties of copolymers 3a-c. [2.2]Paracyclophane-based poly- mers 1 are synthetically less accessible if functional- ization on the cyclophane is desired. Dithia[3.3]para- cyclophanes could, however, be readily prepared by coupling reactions under high dilution conditions. 14 We report here a successful modulation in the optical prop- erties of the bithiophene-dithia[3.3]paracyclophane copolymer 4 by the transannular effect of the [3.3]cyclo- phane unit. Additional effects derived from the intro- duction of a conjugated linker such as those in copoly- mers 5 and 6 were discussed in comparison to the properties of the parent copolymers 3. 13 Experimental Section General. All reactions were carried out under nitrogen. Commercially available reagents and solvents were used * To whom correspondence should be addressed. Fax: 65- 67791691. E-mail: chmlaiyh@nus.edu.sg. † National University of Singapore. ‡ Institute of Materials Research and Engineering. 3546 Macromolecules 2004, 37, 3546-3553 10.1021/ma035902+ CCC: $27.50 © 2004 American Chemical Society Published on Web 04/20/2004