Chiroptical Properties of Chiral Substituted Polyfluorenes

Abstract

Liquid-crystalline polyfluorene (PF) homopolymers substituted with chiral alkyl side chains were synthesized, and their chiroptical properties in the solid state were investigated by means of circular dichroism (CD), circularly polarized photoluminescence (CPPL), and circularly polarized electroluminescence (CPEL) measurements. Polarization-selective scattering of light is shown to cause artifacts in the circularly polarized absorption and emission spectra in the wavelength range near or above the absorption edge, and a measurement scheme to avoid these is presented. For all derivatives, significant chiroptical effects appeared only after the solid layers have been annealed at elevated temperatures, preferably into the liquid-crystalline state of the polymer. The largest anisotropy factors were measured for a polyfluorene substituted with chiral (R)-2-ethylhexyl side chains, yielding absolute values of up to 0.28 for CPPL and up to 0.25 for CPEL. These are among the highest ever reported for a chiral conjugated polymer. Anisotropy factors for CD, CPEL, and CPPL were consistently found to follow an "odd-even effect" concerning the position of the chiral center in the alkyl side chain. If the chiral center is placed close to the polymer backbone, the CD is dominated by one peak with its maximum close to the maximum of the π-π* absorption band. This indicates that the chiroptical properties are most probably caused by intrachain effects rather than by pure interchain exciton coupling. This interpretation is supported by the results of time-dependent Hartree-Fock calculations for the isolated fluorene dimer and trimer. In both cases, the anisotropy factor depends strongly on the torsion angle between neighboring fluorene units. For the trimer, a maximum anisotropy factor of 0.25, close to the maximum values determined experimentally, is predicted for a torsion angle of ca. 105°. Both experimental and theoretical results indicate that the chiroptical properties of these chiral substituted polyfluorenes are mainly caused by a helical conformation of the conjugated polymer backbone.

Polarized optical spectroscopy applied to investigate two poly(phenylene-vinylene) polymers with different side chain structures

“…8 and 9. The change in shape of the CD must accordingly be attributed to the formation of an interchain aggregate in accordance with what was observed by Langeveld-Voss et al 31 and Oda et al 32 In this scenario we assume that the 2B u state has been split into the two states given by the linear combination with energy eigenvalues E ± = E 2B u ± V ab / 2. As for the symmetry of the system, we note that the PPV systems can in principle be described by C 2h symmetry, 36,53 although this assumes an ideal conformation of the polymer chain.…”

“…In both systems the high energy component has a stronger absorption and this could in principle be due to an exciton splitting, presumably then from an in- trachain aggregate. The impact on the CD spectra of a polythiophene due the medium was investigated by LangeveldVoss et al, 31 and the result of their study is that a CD spectrum with a strong bisignate Cotton effect only is obtained for what is considered to be a poor solvent ͑mixture͒ for this particular polymer, and Oda et al 32 made the same observations for a polyfluorene. Both these authors therefore concluded that the medium had forced the polymers into an aggregated state.…”

“…[32][33][34] However, for the polyfluorene system it was concluded that intrachain interactions as well as interchain interactions contribute to the CD signal which in turn complicates the interpretation of the data. 32 As the CD signal is sensitive to the dipole-dipole interaction, it will also be affected by the relative ͑mutual͒ orientation of the interaction dipoles. This would, in principle, imply that the relative orientations of the interaction species are different in this case.…”

“…These studies have indicated strong interactions and exciton delocalization, mainly in the solid state ͑films͒. [31][32][33][34] The PPV polymer was the first material used that successfully produced electroluminescence in thin films. 1 The PPV system has very intriguing properties, for instance, an unusually high nonlinear response, and the system has accordingly been studied extensively within a range of experimental optical techniques, among them are site-selection luminescence and electroabsorption, etc.…”

Structure, photophysics, and the order-disorder transition to theβphase in poly(9,9-(di-n,n-octyl)fluorene)

“…Mesomorphic behavior is seen in many other conjugated polymer families containing alkyl side chain substituents 22,23,24 although the exact details are both backbone and side chain specific. It also extends to oligomers and oligofluorenes bearing chiral side chain substituents 25,26,27 and these show very rich phase behavior. The mesomorphic behavior of the PF8 polymer correlates with the existence of two distinct backbone conformations 15,28 referred to as the "alpha" or "beta" phase 29 .…”