Helix Induction to Polyfluorenes Using Circularly Polarized Light: Chirality Amplification, Phase-Selective Induction, and Anisotropic Emission

Chirality induction (preferred-handed helix induction) to poly(9,9-dioctylfluorene-2,7-diyl) (poly(DOF)), poly(9,9-didodecylfluorene-2,7-diyl) (poly(DDF)), and poly(9,9-dihexylfluorene-2,7-diyl) (poly(DHF)) using circularly polarized light (CPL) was thoroughly investigated mainly using film samples. We found that the presence of the beta-phase is indispensable in chirality induction to poly(DOF) which suggests the importance of interchain interactions and that chirality induction first occurs preferentially in the beta-phase and then extended to the alpha-phase. Chirality induction only to the beta-phase was attained by irradiating at the beta-phase absorbance band (435 nm) or by irradiating the full wavelength range for a short time while pure alpha-phase chirality is induced for poly(DHF) having no beta-phase using an achiral aid molecule. Chirality induction is feasible for polymers having octyl and dodecyl chains, poly(DOF) and poly(DDF), whereas it is not possible for the polymer having a hexyl chain, poly(DHF), unless an aid molecule is used. CPL-driven chirality induction occurs through enantiomer-selective photoexcitation of either a right- and left-handed twisted diad composing the polymer chain into achiral, coplanar conformation, and deactivation of the coplanar conformation leading to right- and left-handed twisted diads. Chirality amplification takes place through biased deactivation of coplanar conformation preferentially to the hand which is the same as that of preformed single-handed diad or longer chain in the vicinity (deactivation chirality amplification). In addition, ground-state chirality amplification is also proposed on the basis of significant temperature effects in chirality induction. Chiral structure induced to the film samples is stable if they are stored at room temperature without irradiation with intense light. Optically active poly(DOF) films showed efficient circularly polarized luminescence.