Frank Galbrecht

Swivel-cruciform oligothiophene dimers

Synthesis and electronic properties of three swivel-cruciform oligothiophene dimers—bis(terthiophene) (BT3), bis(pentathiophene) (BT5) and bis(heptathiophene) (BT7)—with increased solubility in organic solvents are reported. We obtained a field-effect mobility of 3.7 × 10−5 cm2 V−1 s−1 and a current on/off ratio of >103 for a solution-processed OFET device with dimer BT5 as p-type semiconductor.

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).

Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores

The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported.

Decoupling 2D inter- and intrachain energy transfer in conjugated polymers.

The dimensionality of conjugated polymer systems plays an important role in energy-transfer processes, and 1D and 2D energy transfer of excitations are typically much slower than that between pi-stacked chains within a 3D polymeric solid. However, whether 2D energy transfer in conjugated polymers occurs mainly along polymer chains (intrachain), or between in-plain-adjacent polymer chains (interchain), has yet to be determined due to the difficulty of experimentally decoupling inter- and intrachain interaction in a 2D polymer system. This can be achieved by incorporating conjugated polymer chains into the planar galleries of layered matrices which sterically hinder polymer aggregation and pi-pi interchain interactions. Here, pristine blue-emitting polyfluorene chains and polyfluorene chains with known concentrations of green-emitting on-chain fluorenone defects are either separately or collectively incorporated into layered SnS(2). X-ray powder diffraction of the composite films confirms incorporation of the polymer chains into the layered galleries. Monitoring the fluorene-to-fluorenone energy transfer as a function of fluorenone concentration and distribution in the layered galleries allows differentiation between inter- and intrachain fluorene-fluorenone energy transfer. It is found that, 2D energy transfer in conjugated polymers follows mainly an interchain process, despite the absence of pi-pi interchain interactions.

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.