Noticeable Chiral Center Dependence of Signs and Magnitudes in Circular Dichroism (CD) and Circularly Polarized Luminescence (CPL) Spectra of alt-trans-Poly(9,9-dialkylfluorene-2,7-vinylene)s Bearing Chiral Alkyl Side Chains in Solution, Aggregates, and Thin Films

Effects of chiral alkyl side chains in poly(9,9-dialkyl-fluorene-2,7-vinylene)s [PFVs, (S)-3-methylpentyl (3mpe), (S)-4-methylhexyl (4mhex), (S)-5-methylheptyl (5mhep), (S)-6-methyloctyl (6moct), and (S)-3,7-dimethyloctyl (dmo)] toward aggregation-induced circular dichroism (AICD) and circularly polarized luminescence (AICPL) and CD/CPL spectra in solution and in thin film have been explored. The (all-trans) PFV samples with similar conjugation repeat units containing well-defined (vinyl) end groups were prepared by acyclic diene metathesis polymerization using Ru catalyst. The PFV aggregates, prepared in situ in a mixed solution of CHCl3/MeOH, showed clear CD signals ascribed to AICD, whereas these samples showed CD-silent in the CHCl3 solution. The absolute magnitude (g(CD) value) was affected by the chiral side chains (without obey the even-odd rule) and increased in the order 3mpe, 5mhep < 4mhex < dmo < 6mcot. Both the 6-moct and dmo aggregates showed clear CPL signals ascribed to AICPL, whereas signals of the others were not obvious. The lambda(max) values in the UV-vis spectra red-shifted depending upon kind of alkyl side chains due to formation of J-type aggregates. These results suggest that the optically active aggregates adopt certain helical supramolecular ordered structures induced by an interpolymer interaction through chain entanglement. The g(CD) values in the drop casted thin film (prepared from the CHCl3 solution) were lower than those in the aggregates, and the value increased in the order 5mhep, dmo < 4mhex, 6mcot < 3mpe. The lambda(max) values in their UV-vis spectra red-shifted but were not affected by the side chain. These results suggest that supramolecular structures formed by aggregate and film are different, and the formation in film could be induced by an interpolymer pi-stacking. In contrast, the basic characteristics were preserved in the thin film prepared from the PFV-6moct aggregate (CHCl3/MeOH); the film showed high both g(CD) and g(CPL) values close to those in the original aggregate.