Tony Farrell

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.

Low threshold blue conjugated polymer lasers with first- and second-order distributed feedback

We report on the fabrication of low threshold distributed feedback (DFB) polymer lasers based on a polyfluorene derivative containing statistical binaphthyl units (BN-PFO). First- and second-order feedback lasers have been realized. The emission was tuned in the wavelength range from 438to459nm by varying the grating period and the film thickness. A threshold energy of 280pJ/pulse was observed in second-order DFB structures, which could be further reduced to 160pJ/pulse by employing first-order feedback in electron beam lithographically patterned structures with a period of 140nm. In these first-order structures, laser oscillation at both edges of the photonic stop band was observed. These very low threshold values render BN-PFO a very promising material for future electrically pumped organic semiconductor laser diodes.

Alternating quinoxaline/oligothiophene copolymers—synthesis and unexpected absorption properties

The optical and electronic properties of novel, alternating quinoxaline/oligothiophene donor–acceptor copolymers show an unexpected independence of absorption and photoluminescence, as well as of the HOMO/LUMO energy levels, on the length of the oligothiophene segments in the copolymer main 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).

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.

Novel rod-like fluorene-trimers exhibiting smectic LC mesophases

Two novel fluorene-trimers were synthesised following a Suzuki-type cross-coupling reaction. Methyl pendant chains in 9-position of the fluorene building blocks ensured a rod-like backbone of a small molecular diameter while the introduction of terminal, flexible n-octyl and n-hexadecyl alkyl chains in the 2- and 7″-positions causes the occurrence of two different liquid-crystalline (LC) mesophases for both trimers. The terminal chain length was found to strongly affect the phase transition temperatures but not the number and type of LC phases formed. By using several analytical techniques (differential scanning calorimetry DSC, polarizing microscopy, and X-ray diffractometry) the exact nature of the mesophases was determined. The n-hexadecyl- and n-octyl-substituted fluorene-trimers characteristically form smectic LC phases at transition temperatures of 108 and 171 °C, respectively, which when heated further are transformed into lower ordered nematic LC mesophases at 140 and 187 °C, respectively.

Low threshold blue conjugated polymer DFB lasers

We report low threshold blue polyfluorene distributed-feedback (DFB) lasers. Second-order lasers exhibit a laser threshold of 0.28 nJ/pulse while the threshold is reduced to 0.16 nJ/pulse for first-order gratings. Laser diode pumping is demonstrated.

On the angular dependence of the optical polarization anisotropy in ladder-type polymers

Two ladder-type polymers have been examined using polarized optical absorption and fluorescence spectroscopy in solution and solid state. The fluorescence anisotropy excitation spectra of ladder-type methyl-substituted poly(para-phenylene) and poly(naphthylene-phenylene) follow their absorption profiles, contrary to the typical behavior in other conjugated polymers, which show excitation wavelength dependent anisotropy until the onset of the typical red edge effect. Using Raman microscopy and linear dichroism of aligned stretched polyethylene films containing the ladder polymer, it has been found that the orientation of the absorption transition dipole moment is dependent on the conjugation length. The transition dipole of the shorter conjugation lengths of the distribution is localized approximately 7 degrees off axis, and that of the longer conjugation lengths is delocalized along the chain.