Conformation of a Polyfluorene Derivative in Solution

Fluorescent nanomicelles for selective detection of Sudan dye in Pluronic F127 aqueous media

Novel self-assembled water-soluble nanomicelles that contain fluorescent conjugated polymers (poly(9,9-dioctylfluorene) (PFO) or poly[2,7-(9,9-dihexylfluorene)-alt-4,4'-phenylether] (PF-PE)) have been obtained and used as the highly sensitive/selective platform for Sudan dye detection. The Fluorescent nanomicelles exhibited a highly selective fluorescence quenching by the prohibited food additive Sudan I, while not for the natural pigments: Capsanthin and Beta-carotene, due to the more suitable matching of the LUMOs (lowest unoccupied molecular orbital) of the conjugated polymers with that of Sudan I molecules. The Stern-Volmer constants (K(SV)) of PF-PE/F127 and PFO/F127 for Sudan I were 1,040,480 and 665,000 M(-1), respectively, which were more than 100 times higher than those of the same conjugated polymers in the orgainc solvents. The significantly enhanced sensitivity was due to the collective effect of the F127 micelles to both chromophore and analyte, through which the fluorophone-analyte binding interaction is significantly strengthened and efficient photoinduced charge transfer occurs. The as-proposed materials and approach may be potentially applied in the real-time food safety screening.

Evidence for structural transition in hairy-rod poly[9,9-bis(2-ethylhexyl)fluorene] under high pressure conditions

We report on an x-ray scattering experiment of bulk poly[9,9-bis(2-ethylhexyl)fluorene] under quasihydrostatic pressure from 1 to 11 GPa at room temperature. The scattering pattern of high molecular weight (HMW) polyfluorene (>10 kg/mol) undergoes significant changes between 2 and 4 GPa in the bulk phase. The 110 reflection of the hexagonal unit cell disappears, indicating a change in equatorial intermolecular order. The intensity of the 00 21 reflection drops, with a sudden move toward higher scattering angles. Beyond these pressures, the diminished 00 21 reflection tends to return toward lower angles. These changes may be interpreted as a transition from crystalline hexagonal to glassy nematic phase (perceiving order only in one direction). This transition may be rationalized by density arguments and the underlying theory of phase behavior of hairy-rod polyfluorene. Also the possible alteration of the 21-helical main chain toward more planar main chain conformation is discussed. The scattering of low molecular weight polyfluorene (<10 kg/mol) , which is glassy nematic in ambient pressure, is reminiscent with that of HMW polymer above 2-4 GPa.

Control over phase behavior and solution structure of hairy-rod polyfluorene by means of side-chain length and branching

We present guidelines on how the solution structure of pi -conjugated hairy-rod polyfluorenes is controlled by the side-chain length and branching. First, the semiquantitative mean-field theory is formulated to predict the phase behavior of the system as a function of side-chain beads (N). The phase transition at N=N{ *} separates a lyotropic phase with solvent coexistence (N<N{ *}) and a metastable membrane phase (N>N{ *}). The membrane phase transforms into the isotropic phase of dissolved rodlike polymers at the temperature T_{mem}{ *}(N), which decreases both with N and with the degree of side-chain branching. This picture is complemented by polymer demixing with the transition temperature T_{IN}{ *}(N), which decreases with N . For N<N{ *}, the lyotropic phase turns isotropic with increasing T at T_{IN}{ *} . For N>N{ *}, stable membranes are predicted for T_{IN}{ *}<T<T_{mem}{ *} and metastable membranes with nematic coexistence for T<T_{IN}{ *}. Second, in experiment, samples of poly(9,9-dialkylfluorene) with N=6-10 were mixed in methylcyclohexane. For N=8 the side-chain branching was controlled by (9,9-dioctylfluorene)/(9,9-bis(2-ethylhexyl)fluorene) (F8/F2/6) random copolymers. The proportion of F8 to F2/6 repeat units was 100:0, 95:5, 90:10, 50:50, and 0:100. In accordance with the theory, lyotropic, membrane, and isotropic phases with the corresponding phase transitions were observed. For N<N{ *} approximately 6 only the lyotropic phase is present for attainable temperatures. The membrane and isotropic phases are present for N>N{ *}. T_{mem}{ *}(N) decreases from 340 K to 280 K for N > or = 8 . For copolymers, the membrane phase is found when the fraction of F8 units is at least 90%, T_{mem}{ *} decreasing with this fraction. The membrane phase contains three material types: loose sheets of two polymer layers, a better packed beta phase, and dissolved polymer. For N > or = 7 and T<T_{mem}{ *} the tendency for membrane formation becomes stronger with increasing temperature.

Direct measurement of the end-to-end distance of individual polyfluorene polymer chains

Wide-field imaging of individual multichromophoric molecules and successive photobleaching were used to determine, accurately, the relative position of the chromophores in such systems. First, a polyphenylene dendrimer with well-defined geometry was used to establish the accuracy in localization that can be obtained by this methodology. For a signal-to-noise ratio of 20, interchromophoric distances could be measured with 4 nm accuracy. Next, the method was used to determine the end-to-end distribution of an end-capped polyfluorene polymer. From comparison between the experimental and simulated distributions, information on the conformation of the polymer could be deduced. It was found that the polymer has a nonlinear conformation. A conjugation length of six monomer units gave the best fit of the experimental data to the proposed model.