Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes

Abstract

Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the effect of polymer chain interactions on excited state relaxation. Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also affect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.

Kinetics and Thermodynamics of Poly(9,9-dioctylfluorene) β-Phase Formation in Dilute Solution

“…On the basis of fiber X-ray diffraction studies, Bradley et al 8 concluded that this lower energy state is associated with particularly extended conformations of PFO chains, 8,9 resulting from the polymer response to some physical stress, driven by solvent quality or temperature. 3,4,8 In another study, Teetsov et al 7,10 reported that the 437 nm absorption band is also formed with dihexyl-and didodecyl-substituted polyfluorenes and attributed the phenomenon to polymer aggregation.…”

“…[8][9][10] In fact, the 437 nm absorption peak also appears when PFO is not completely dissolved, being associated with special aggregates (visible under UV light illumination and which can be removed upon filtration) present in the solid sample, which are formed during the PFO purification by addition of a poor solvent or during the solvent removal. In addition, PFO is a member of the group of so-called "hairyrod" polymers, which have a tendency to exist as supramolecular aggregates, even at very low concentrations.…”

“…4,16 Furthermore, it has been suggested that octyl side chains in PFO are particularly effective at inducing and stabilizing such intramolecular ordering in solution, when compared to hexyl and dodecyl side groups. 7,10 The concept of side chain-driven planarization upon polymer agglomeration in solvent mixtures due to an increase of the fraction of a poor-solvent was also introduced. 16 More recently, the conformations of (isolated) PFO chains were addressed by means of theoretical calculations.…”

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

“…A way to achieve this is through the introduction and modification of the side chains/ functional group structure, which in turn results in an impact on the optical and transport properties. This is best exemplified with the various LEPs within the poly͑phenylene-vinylene͒ ͑PPV͒ 1-3 and the polyfluorenes [4][5][6] that have been synthesized to date. With structural variations within these two classes of polymers, markedly different transport properties have been observed.…”

Blue Emission of a Soluble Poly(p‐phenylene) with a Cross‐Conjugated Bisimidazole‐Based Chromophore

“…The emission color of conjugated polymers is often bathochromicly shifted and highly broadened in the solid state due to the formation of aggregates or excimers. [28] This is not the case for polymer 6, which can be attributed to a suppressed aggregation in the solid state as a result of the bulkiness of the 2,2 0 -( p-phenylene)-bis(4,5-diphenylimidazole) units.…”