Network structure of polyfluorene sheets as a function of alkyl side chain length.

The formation of self-organized structures in poly(9,9-di-n-alkylfluorene)s ∼1 vol % methylcyclohexane (MCH) and deuterated MCH (MCH-d(14)) solutions was studied at room temperature using neutron and x-ray scattering (with the overall q range of 0.00058-4.29 Å(-1)) and optical spectroscopy. The number of side chain carbons (N) ranged from 6 to 10. The phase behavior was rationalized in terms of polymer overlap, cross-link density, and blending rules. For N=6-9, the system contains isotropic areas and lyotropic areas where sheetlike assemblies (lateral size of >400 Å) and free polymer chains form ribbonlike agglomerates (characteristic dimension of >1500 Å) leading to a gel-like appearance of the solutions. The ribbons are largely packed together with surface fractal characteristics for N=6-7 but become open networklike structures with mass fractal characteristics for N=8-9, until the system goes through a transition to an isotropic phase of overlapping rodlike polymers for N=10. The polymer order within sheets varies allowing classification for loose membranes and ordered sheets, including the so-called ß phase. The polymers within the ordered sheets have restricted motion for N=6-7 but more freedom to vibrate for N=8-9. The nodes in the ribbon network are suggested to contain ordered sheets cross-linking the ribbons together, while the nodes in the isotropic phase appear as weak density fluctuations cross-linking individual chains together. The tendencies for macrophase separation and the formation of non beta sheets decrease while the proportion of free chains increases with increasing N. The fraction of ß phase varies nonlinearly, reaching its maximum at N = 8.

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 (NN{ *}). 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 NN{ *}, stable membranes are predicted for T_{IN}{ *}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

Stark spectroscopy of excited-state transitions in a conjugated polymer.

Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states.

Excited state properties of oligophenyl and oligothienyl swivel cruciforms.

A comprehensive study has been undertaken of the electronic spectral and photophysical properties of two oligophenyl (BPH and BPHF) and one oligothienyl (BTF) swivel cruciforms involving measurements of absorption, fluorescence, and phosphorescence spectra, quantum yields of fluorescence (phiF), phosphorescence (phiPh) and triplet formation (phiT), lifetimes of fluorescence (tauF) and of the triplet state (tauT), and quantum yields of singlet oxygen production (phiDelta). From these, all radiative kF and radiationless rate constants, kIC and kISC, have been obtained in solution. The energies of the lowest lying singlet and triplet excited states were also determined at 293 K. Several of the above properties have also been obtained at low temperature and in the solid state (thin films). In general, for the phenyl oligophenyl (BPH) and for the oligothienyl (BTF) compounds, the radiationless decay channels (phiIC+phiISC) are the dominant pathway for the excited-state deactivation, whereas with the fluorene based oligophenyl BPHF the radiative route prevails. In contrast to the general rule found for related oligomers (and polymers) where radiative emission from T1 is absent, with the compounds studied, phosphorescence has been observed for all of the compounds, indicating that this type of functionalization can lead to emissive triplets. Time-resolved fluorescence decays with picosecond resolution revealed multiexponential (bi- and triexponential) decay laws compatible with the existence of more than one species or conformation in the excited state. These results are discussed on the basis of conformational flexibility in the excited state.