The torsional motions of jet-cooled 1,4-bis(phenylethynyl)benzene (BPEB), a prototype molecular wire, were studied using cavity ring-down spectroscopy in the first UV absorption band (316-321 nm). The torsional spectrum of 1,4-bis(phenylethynyl)-2,3,5,6-tetradeuteriobenzene was also recorded in the gas phase. Both spectra were successfully simulated using simple cosine potentials to describe the torsional motions. The ground-state barrier to rotation was estimated to be 220-235 cm(-1), which is similar to that of diphenylacetylene (tolane). Complementary DFT calculations were found to overestimate the torsional barrier.

International audienceThe Sonogashira cross-coupling of two equivalents of para-substituted ethynylbenzenes with 2,5-diiodothiophene provides a simple synthetic route for the preparation of 2,5-bis(para-R-phenylethynyl)thiophenes (R = H, Me, OMe, CF3, NMe2, NO2, CN and CO2Me) (1a-h). Likewise, 2,5-bis(pentafluorophenylethynyl)thiophene (2) was prepared by the coupling of 2,5-diiodothiophene with pentafluorophenylacetylene. All compounds were characterised by NMR, IR, Raman and mass spectroscopy, elemental analysis, and their absorption and emission spectra, quantum yields and lifetimes were also measured. The spectroscopic studies of 1a-h and 2 show that both electron donating and electron withdrawing para-subsituents on the phenyl rings shift the absorption and emission maxima to lower energies, but that acceptors are more efficient in this regard. The short singlet lifetimes and modest fluorescence quantum yields (ca. 0.2-0.3) observed are characteristic of rapid intersystem crossing. The single-crystal structures of 2,5-bis(phenylethynyl)thiophene, 2,5-bis(para-carbomethoxyphenylethynyl)thiophene, 2,5-bis(para-methylphenylethynyl)thiophene and 2,5-bis(pentafluorophenylethynyl)thiophene were determined by X-ray diffraction at 120 K. DFT calculations show that the all-planar form of the compounds is the lowest in energy, although rotation of the phenyl groups about the C[triple bond, length as m-dash]C bond is facile and TD-DFT calculations suggest that, similar to 1,4-bis(phenylethynyl)benzene analogues, the absorption spectra in solution arise from a variety of rotational conformations. Frequency calculations confirm the assignments of the compounds' IR and Raman spectra

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A series of 9,10-bis(phenylethynyl)anthracenes decorated with sterically demanding tert-butyl substituents have been prepared and spectroscopically characterised. We demonstrate that the introduction of two bulky substituents in the ortho position of the phenyl rings effectively locks the ground state into a conformation in which the three rings are orthogonal. Fluorescence spectroscopy reveals evidence for partial planarisation of this compound in the excited state at ambient temperature, but this is prevented in low temperature solvent glasses.

Picosecond time-resolved resonance Raman spectroscopy, TR3, reveals an intense acetylenic band in the S1 state of the prototypical molecular wire 1,4-bis(phenylethynyl)benzene.

A study of a group of compounds based on the 1,4-bis(phenylethynyl)benzene (1) architecture was undertaken to improve our understanding of their photophysics and the factors which control their geometry and hence the π-conjugation pathway in the ground and excited state of these compounds. 1 exists as a range of molecular rotamers in the ground state, resulting from the low barrier to rotation around their C(sp)-C(sp2) bonds. These compounds are highly conjugated systems with good electron conducting properties, due to delocalisation of the HOMO and LUMO over the molecule. In the electronic excited state they are capable changing their molecular conformation and will adopt a planar, or near planar, low energy conformation prior to fluorescence emission in solution. In a glassy matrix at 77 K with sterically hindering substituents on the benzene rings of 1, emission form high and low energy conformations are observed. 1 is highly emissive owing to the high oscillator strength of the S 1 →S 0 transition. All the compounds studied maintained their C≡C character in the excited singlet and triplet states. The substitution of the central benzene ring in 1 with a thiophene moiety increases the singlet oxygen generation quantum yield, which is consistent with greater intersystem crossing to the triplet excited state.

High mobility organic semiconductor formulations with excellent uniformity across large area substrates are prepared via the use of formulations containing small molecule and high permittivity semiconducting oligomers. The use of these high-k ( k > 3.3) oligomers allows control of the wetting via the manipulation of the surface energy of the substrate being coated. Organic thin fi lm transistors results with mobilities of up to 5 cm 2 V −1 s −1 , standard deviation <10 %, on/off ratios of 10 9 are presented.

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