Polyfluorene as a model system for space-charge-limited conduction

Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular disorder can be described very well by one-carrier space-charge-limited conduction for a discrete set of trap levels with energy $\sim$ 0.5 eV above the valence band edge. Sweeping the bias above the trap-filling limit in the as-is polymer generates a new set of exponential traps, which is clearly seen in the density of states calculations. The trapped charges in the new set of traps have very long lifetimes and can be detrapped by photoexcitation. Thermal cycling the PF film to a crystalline phase prevents creation of additional traps at higher voltages.Comment: 13 pages, 4 figures. Physical Review B (accepted, 2007

Spectral and Photophysical Studies of Poly[2,6-(1,5-dioctylnaphthalene)]thiophenes

A complete spectroscopic and photophysical study of three alternating naphthalene-α-thiophene copolymers was undertaken in solution (room and low temperature) and in the solid state (thin films in a Zeonex matrix). The study comprises absorption, emission, and triplet−triplet spectra together with quantitative measurements of quantum yield (fluorescence, intersystem-crossing, internal conversion, and singlet oxygen formation) lifetimes and singlet and triplet energies. The overall data allow the determination of the rate constants for all the decay processes. Comparison between the behavior of analogous 1-naphthyl(oligo)thiophenes and the 2,6-naphthalene(oligo)thiophene copolymers allows several important observations. First, the polymers display higher fluorescence quantum yields and lower S1→T1 intersystem-crossing yields than the oligomers. This can be attributed to the presence of the 1,5-dioctyloxynaphthalene groups in the copolymers leading to a more rigid polymer backbone, which decreases radiationless deactivation and increases the radiative efficiency. Second, the singlet and triplet energies are significantly lower in the polymers than with the corresponding oligomers. This implies a lower HOMO−LUMO energy difference in the polymers due to an extended π-delocalization. Third, the singlet-to-triplet (S1−T1) energy splitting is higher in the oligomers than with the polymers, even though the former display higher intersystem-crossing yields. It is suggested that this may result from intersystem-crossing in the oligomers involving significant charge-transfer (CT) character (spin-orbit coupling is mediated by CT mixing involving the singlet and triplet states in matrix elements of the type 1ΨCT |H‘|3Ψ1) of the relevant excited states but that is less important with the polymers. We believe that this may be relevant to understanding the nature of CT states in conjugated copolymers

Polyfluorene as a model system for space-charge-limited conduction

Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular disorder can be described very well by one-carrier space-charge-limited conduction for a discrete set of trap levels with energy $\sim$ 0.5 eV above the valence band edge. Sweeping the bias above the trap-filling limit in the as-is polymer generates a new set of exponential traps, which is clearly seen in the density of states calculations. The trapped charges in the new set of traps have very long lifetimes and can be detrapped by photoexcitation. Thermal cycling the PF film to a crystalline phase prevents creation of additional traps at higher voltages.We gratefully acknowledge the support of this work through the National Science Foundation under grant Nos. ECS-0523656 and DMR-0413601

The Influence of Alkyl-Chain Length on Beta-Phase Formation in Polyfluorenes.

Di-n-alkyl substituted polyfluorenes with alkyl chain lengths of 6, 7, 8, 9, and 10 carbon atoms (PF6, PF7, PF8, PF9, and PF10) are studied in dilute solution in MCH using optical spectroscopy. Beta-phase is formed upon cooling in solutions (∼ 7 µg mL−1) of PF7, PF8, and PF9 only, which is observed as an equilibrium absorption peak at ∼ 437 nm and strong changes in the emission spectra. Beta-phase is formed upon thermal cycling to low temperature in solutions (∼7 µg mL−1) of PF7, PF8, and PF9, which is observed as an equilibrium absorption peak at ∼ 437 nm and strong changes in the emission spectra. Beta phase is found to occur more favorably in PF8 than in PF7 or PF9, which is attributed to a balance between two factors. The first is the dimer/aggregate formation efficiency, which is poorer for longer (more disordered) alkyl chain lengths, and the second is the Van der Waals bond energy available to overcome the steric repulsion and planarize the conjugated backbone, which is insufficient in the PF6 with a shorter alkyl chain. Beta phase formation is shown to be a result of aggregation, not a precursor to it. A tentative value of the energy required to planarize the fluorene backbone of (15.6 ± 2.5) kJ mol−1 monomer is suggested. Excitation spectra of PF6, PF7, PF8, and PF9 in extremely dilute (∼ 10 ng mL−1) solution show that beta phase can form reversibly in dilute solutions of PF7, PF8 and PF9, which is believed to be a result of chain collapse or well dispersed aggregates being present in solution from dilution of more concentrated solutions. PF7, PF8, and PF9 also form beta phase in thermally cycled solid films spin-cast from MCH. However, in the films the PF7 formed a larger fraction of beta phase than the PF9, in contrast to the case in solutions, because it is less likely that the close-packed chains in the solid state will allow the formation of planarized chains with the longer PF9 side chains

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 (τF) and of the triplet state (τT), and quantum yields of singlet oxygen production (phiΔ). 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

Synthesis and characterization of a helical step-ladder polyarylene

A helical step‐ladder polyarylene incorporating chiral (R)‐2,2′‐dioctoxy‐1,1′‐binaphthyl units was synthesized for the first time. The first step involved the preparation of a precursor poly(arylene ketone) via a palladium‐mediated Suzuki‐type cross‐coupling reaction with the aid of microwave heating. Two polymer‐analog reaction steps, the reduction of the keto groups to tertiary alcohol functionalities and subsequent intramolecular Friedel–Crafts cyclization, gave a step‐ladder polymer (6) in good yields with reasonable mean average molecular weights greater than 13,000. The regioselective cyclization pattern in the α position of the naphthalene core was confirmed by a comparison of the NMR data of the polymer with those of the corresponding model ladder oligomers, 12 and 13, and also a single‐crystal structure of 13. The optical spectra of the oligomers and polymers indicated that there was little electronic interaction across the binaphthyl units. The circular dichroism spectrum of 6 exhibited a strong bisignate Cotton effect in the π–π* absorption region of the planar chromophores, which reflected the strong exciton coupling within the helical polymer chain

Synthesis and spectroscopy of an oligophenyl based cruciform with remarkable π–π assisted folding

A facile route has been developed for the preparation of a new family of oligophenyls based on a 2,5,2′,5′-tetra-aryl substituted biphenyl structural motif. The cruciform terphenyl dimer 2,5,2′,5′-tetra(4-tert-butylphenyl)-1,1′-biphenyl (2) has been prepared in a two step protocol as a representative of this interesting class of materials. The thermal behaviour of the cruciform was analysed by DSC and shows that 2 forms an amorphous glass when cooled from the isotropic melt. Subsequent heating reveals a glass transition temperature at 130 °C. X-Ray single crystal structure analysis of 2,2′-bis(4-tert-butylphenyl)-1,1′-biphenyl (4) and 2 shows that both these molecules with a quater-phenyl substructure adopt a folded solid-state structure. Examining the 1H NMR spectra of 2 and 4 reveals that the interactions that induce this folding in the solid-state are sufficiently strong to bias foldamer formation also in solution. Consequently, it is reasonable to assume that the folded conformation within the lattice is due to intramolecular π–π interaction rather than being imposed by crystal packing. The optical properties of the cruciform terphenyl dimer 2 are discussed relative to the linear analogue 1,4-bis(4-tert-butylterphenyl)benzene (3)