Large area liquid crystal monodomain field-effect transistors.

Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.

Efficient intramolecular energy transfer in single endcapped conjugated polymer molecules in the absence of appreciable spectral overlap.

Intramolecular energy transfer is investigated in an endcapped conjugated polymer on the single molecule level at low temperature. While light harvesting in one dimension is on average inefficient in the ensemble, the efficiency scatters widely on the single molecule level, with some molecules exhibiting near-unity transfer probability from the polymer backbone donor to the acceptor endcap. This transfer occurs in the absence of spectral overlap between donor and acceptor, as the electronic and vibronic transitions narrow substantially at low temperatures once inhomogeneous disorder broadening is overcome. The results illustrate how far-field absorption and emission characteristics of molecular transitions are insufficient to describe resonant energy transfer processes following Förster theory in multichromophoric aggregates. Rather, exciton trapping due to efficient multiphonon emission has to be invoked with a possible contribution of strong polaronic coupling.

Exciton migration in rigid-rod conjugated polymers: an improved Förster model.

The dynamics of interchain and intrachain excitation energy transfer taking place in a polyindenofluorene endcapped with perylene derivatives is explored by means of ultrafast spectroscopy combined with correlated quantum-chemical calculations. The experimental data indicate faster exciton migration in films with respect to solution as a result of the emergence of efficient channels involving hopping between chains in close contact. These findings are supported by theoretical simulations based on an improved Forster model. Within this model, the rates are expressed according to the Fermi golden rule on the basis of (i) electronic couplings that take account of the detailed shape of the excited-state wave functions (through the use of a multicentric monopole expansion) and (ii) spectral overlap factors computed from the simulated acceptor absorption and donor emission spectra with explicit coupling to vibrations (considered within a displaced harmonic oscillator model); inhomogeneity is taken into account by assuming a distribution of chromophores with different conjugation lengths. The calculations predict faster intermolecular energy transfer as a result of larger electronic matrix elements and suggest a two-step mechanism for intrachain energy transfer with exciton hopping along the polymer backbone as the limiting step. Injecting the calculated hopping rates into a set of master equations allows the modeling of the dynamics of exciton transport along the polyindenofluorene chains and yields ensemble-averaged energy-transfer rates in good agreement with experiment.