Thermodriven Conformational Transition of Optically Active Poly[2,7-{9,9-bis[(S)-2-methyloctyl]}fluorene] in Solution Hong-Zhi Tang, †,⊥ Michiya Fujiki,* ,†,‡,# and Takahiro Sato †,§ CREST-JST (Japan Science and Technology Corporation), 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012, Japan; NTT Basic Research Laboratories, NTT Corporation, 3-1 Wakamiya, Morinosato, Atsugi, Kanagawa 243-0198, Japan; and Department of Macromolecular Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan Received March 4, 2002; Revised Manuscript Received May 21, 2002 ABSTRACT: We report the first solution state circular dichroism (CD) spectroscopic properties of a newly designed polyfluorene (PF) featuring highly enantiopure -branched chiral alkyl side chains, poly[2,7- {9,9-bis[(S)-2-methyloctyl]}fluorene] (PFMO), in dilute THF solution. Upon cooling the solution temper- ature, the single positive-sign CD band at 380 nm at +50 °C gradually changed into an apparent bisignate CD signal with two extrema of 403 and 355 nm at -80 °C, while the broad UV-vis band centered at 380 nm changed into a major intense band at 399 nm with a shoulder around 380 nm. On the basis of these data combined with photoluminescence (PL) studies and viscometric measurement, we concluded that PFMO in THF solution underwent a certain thermodriven, order-disorder conformational transition around 0 °C. It was considered that, upon cooling, the randomly twisted local PF backbone motifs in the wormlike PF backbone at higher temperature were transformed into a highly ordered PF backbone motif, possibly a P-5 2 helix (the P-handedness is assumed here corresponding to the negative-sign CD band, and vice versa), and other unclear local backbone motifs with the opposite screw sense at lower temperature. Introduction π-Conjugated polymers are promising functional ma- terials having potential applications such as light- emitting diodes, 1 plastic lasers, 2 photovoltaic cells, 3 thin film transistors, 4 and chemical sensors. 5 Among them, especially, polyfluorene (PF) has recently attracted increasing interest due to its bluish photo- and/or electroluminescence with high quantum yield, high hole mobility, and excellent thermal, chemical, and photo- chemical stabilities. 6-12 Although much research attention has been paid to the elucidation of the PF backbone structure, the most probable conformational structure of the PF backbone is somewhat controversial. On the basis of the fiber diffraction X-ray scattering experiments, Grell et al. concluded that PF bearing n-octyl side chains, poly[2,7- (9,9-di-n-octyl)fluorene] (PFO), can assume a fully ex- tended anti-coplanar structure. 7 However, Lieser et al. demonstrated that PF having racemic 2-ethylhexyl side chains, poly{2,7-[9,9-bis(2-ethylhexyl)]fluorene} (PFEH), favorably adopts a 5 2 helical structure rather than the other possible 5 1 helix by means of transmission electron microscopy (TEM), electron and X-ray diffractions, and molecular orbital calculations. 8 Moreover, from semiem- pirical Hartree-Fock (HF) calculations, Hong et al. suggested that the 9,9-unsubstituted PF, poly(2,7- fluorene) (HPF), is able to adopt both 5 1 and 5 2 helices. 9 On the other hand, there have been several reports supporting a wormlike global structure of PF in solu- tions around room temperature. For example, based on UV-vis absorption spectra in THF at room tempera- ture, poly[2,7-(9,9-di-n-hexyl)fluorene] (PFH) was indi- cated to have an effective conjugation length of about 12 fluorene repeat units, 10 corresponding to a persis- tence length, q, of ∼10 nm. 11 Similarly, PFO was char- acterized to have a q of ca. 8.5 nm from the light scat- tering experiment in THF at +40 °C. 7b Recently, PFEH was reported to adopt a wormlike structure with a q of ca. 7 nm and a cross-section diameter, d, of ca. 1.8 nm, based on small-angle neutron scattering and transla- tional diffusion measurements in toluene at +20 °C. 11 Oda, Neher, and co-workers synthesized the first optically active PF bearing highly enantiopure chiral alkyl side chains, poly[2,7-{9,9-bis[(S)-3,7-dimethyloctyl]}- fluorene] (PFDMO). 12 They suggested that PFDMO may adopt helical backbone structures in an annealed thin film based on spectroscopic data including circular dichroism (CD), UV-vis, electroluminescence (EL), and circularly polarized electroluminescence (CPEL). How- ever, any further information on the PF backbone conformation in solution has not been reported yet, due probably to the weakness of the CD signal in solution at room temperature. To more clearly investigate PF conformational struc- ture in the solution state, an optically active PF exhibit- ing an intense CD signal in solution even at room temperature is required. For this purpose, we have designed and synthesized a new PF, poly[2,7-{9,9-bis- [(S)-2-methyloctyl]}fluorene] (PFMO, Scheme 1), 6d,e,10 on the assumption that the stereocenter at the -position in the side chains would effectively induce a more chirally ordered PF backbone structure than that at the γ-position and that a longer side chain may afford greater solubility of PF in organic solvents even at lower temperatures. † CREST-JST. ‡ NTT Basic Research Laboratories. § Osaka University. ⊥ Present address: Department of Materials Science and En- gineering, Box 352120, University of Washington, Seattle, WA 98195-2120. # Present address: Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan. * To whom correspondence should be addressed. E-mail: fujikim@ms.aist-nara.ac.jp. 6439 Macromolecules 2002, 35, 6439-6445 10.1021/ma020327f CCC: $22.00 © 2002 American Chemical Society Published on Web 06/26/2002