The First Optically Active Polycarbazoles Zhong-Biao Zhang, †,⊥ Masao Motonaga, ‡ Michiya Fujiki,* ,‡,§ and Charles E. McKenna † Department of Chemistry, University of Southern California, Los Angeles, California 90089-0744; JST-CREST; and Advanced Polymer Science Laboratory, Graduate School of Material Science, Nara Advanced Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan Received April 3, 2003 Revised Manuscript Received July 24, 2003 The highly ordered chiral structures in naturally occurring polymers play a significant role in determin- ing their precise and sophisticated biochemical func- tionalities. The design of synthetic helical polymers capable of emulating the chiral recognition properties of biopolymers is currently a focus of much interest. Enantioseparation, catalysis, and sensing are among the more promising applications of molecular recognition based on responsive three-dimensional intramolecular or intermolecular superchiral structures. 1-8 Optically active conjugated polymers 9-11 represent an attractive class of chiral macromolecules adaptable to this purpose because their chiral behavior can be augmented by nonlinear electrically conductive or optical properties 12-15 arising from conjugation along the backbone. In this Communication, we report the first example of optically active polycarbazoles. These novel chiral π-conjugated polymers are proposed to adopt a highly ordered chiral backbone conformation within a single polymer chain, but a chiral interchain π-stacking structure in their aggregates. The maintenance of the second-order chiral backbone conformation along each polymer chain during aggregation leads to a novel type of higher order chiral superstructure in the aggregates, suggesting that opti- cally active polycarbazoles will be useful tools for developing and understanding chiral polymeric materi- als. Poly[N-(R)- or (S)-3,7-dimethyloctyl-3,6-carbazole]s (R- or S-PDOC) were synthesized in 60-70% yield using our modified nickel(0) coupling method (Scheme 1). 16 The weight-average molecular weights (M w ) of R- and S-PDOC were 79 800 (M w /M n ) 1.54) and 70 600 (M w / M n ) 2.38), respectively. The structures of both the monomers and polymers were confirmed by 1 H and 13 C NMR and by elemental analyses. Differential scanning calorimetric (DSC) and thermogravimetric analyses (TGA) were carried out to determine the thermal transition and degradation behavior of the polymers. The DSC measurements on R-PDOC showed a glass transition temperature of 156 °C and 155 °C for S- PDOC. On the basis of the TGA measurements, both R- and S-PDOC display a high thermal stability. The 5% weight loss temperatures of the polymers during heating runs were 410 °C for R-PDOC and 400 °C for S-PDOC. In the circular dichroism (CD) spectra of R- and S-PDOC in tetrahydrofuran (THF) (Figure 1), mirror- image bisignated Cotton effects were clearly observed in the π-π* transition region. To elucidate the origin of the observed Cotton effects, the polymer concentration dependence of the CD and ultraviolet-visible absorption (UV-vis) signals was measured in THF (Figure 2). These results demonstrate the independence of both the CD and UV-vis signals on S-PDOC concentration over a broad range (10 -6 -10 -3 mol L -1 ), indicating that the interchain interaction is minimal. Thus, the observed † University of Southern California, Los Angeles. ‡ JST-CREST. § Nara Advanced Institute of Science and Technology. ⊥ Present address: Department of Chemistry, University of Virginia, Charlottesville, VA 22904. * To whom correspondence should be addressed. E-mail: fujikim@ms.aist-nara.ac.jp. Scheme 1. Synthesis of Optically Active Polycarbazoles Figure 1. UV-vis and CD spectra of S-PDOC (Mw ) 70 600, Mw/Mn ) 2.38, DPn (number-average degree of polymerization) ) 97) (solid lines) and R-PDOC (sample molecular weight: Mw ) 79 800, Mw/Mn ) 1.54, DPn ) 169) (dashed lines) in THF at 20 °C (sample concentration: 4 × 10 -5 M, based on carbazole repeating unit). Figure 2. Concentration (squares) (Mw ) 70 600, Mw/Mn ) 2.38, DPn ) 97) and molecular weight (circles) (sample concentration: 4 × 10 -5 M, based on carbazole repeating unit) dependence of the CD of S-PDOC in THF at 20 °C. 6956 Macromolecules 2003, 36, 6956-6958 10.1021/ma0344184 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/23/2003