Molecular simulation of the high temperature phase behaviour of α-unsubstituted sexithiophene

We examine the high-temperature phase behaviour of α-unsubstituted sexithiophene (α-6T) by means of Molecular Dynamics (MD) and Monte Carlo (MC) simulations using a recently developed state-of-the-art algorithm based on internal bridging moves. In the MD simulations, a realistic fully flexible united-atom model is used. In the MC simulations, a stiffer version of this united-atom model is implemented by restricting atoms on thiophene rings to remain strictly co-planar by employing holonomic constraints for all bond lengths and intra-ring bond bending angles; on the other hand, inter-ring torsion and bond bending angles are considered to be fully flexible subject to suitable potential energy functions. The MD simulations, which are started from the isotropic (Iso) phase at a relatively high temperature (above 700 K) and continued to lower temperatures under isobaric conditions using a very large simulation cell containing 8960 α-6T molecules, show four phase transitions: an isotropic-to-nematic (Iso-to-Nem) at 640 K, a nematic-to-smectic A (Nem-to-SmA) at 630 K, a smectic A-to-smectic C (SmA-to-SmC) at 620 K demonstrating smectic polymorphism, and a SmC-to-crystal-like (SmC-to-Cry) at 600 K. In the corresponding MC simulations, no Nem phase is observed; the system, as it is isobarically cooled down to lower temperatures from its Iso phase, undergoes directly a transition to a SmC phase at 690 K. This is attributed to the stiffer nature of the forcefield employed in these simulations. Both methods (MD and MC) shed light on the type and degree of molecular self-assembly, orientational and positional ordering as a function of temperature, and manifestation of liquid crystalline behaviour of α-6T. We provide a thorough characterization of structural ordering in all mesophases observed, in terms of several measures (radial correlation functions, orientational order parameters and X-ray diffraction patterns). According to the results, at the phase transition temperatures, drastic configurational changes take place driving α-6T molecules to positionally-ordered phases accompanied by self-assembly into characteristic layers which, in turn, are self-organized into macroscopic smectic phases. Our methodology opens up the way to exploring the rich phase behaviour and anisotropic ordering of the condensed phases of several longer (and perhaps more complex) thiophene-based polymers.

Tuning of the photophysical and electrochemical properties of symmetric and asymmetric conjugated thiophenoazomethines

A synthetic route towards symmetric and asymmetric thiophenoazomethines was accomplished by reaction of the readily available amine with various aldehydes. Investigation of a series of thiophenoazomethines obtained by this method indicates that the terminal groups and the degree of conjugation have a great effect on the electronic absorption and energy levels of the conjugated compounds, particularly the effect of terminal groups. The terminal withdrawing and donating groups of thiophenoazomethines led to the formation of an electronic push–pull, push–push and pull–pull system, which can perturb the electronic transitions between the ground and excited states. The flexible chain substituents on the thiophene units, which improve its solubility, also result in bathochromic absorption, but have limited effect on the energy level.

A synthetic route towards symmetric and asymmetric thiophenoazomethines was accomplished by reaction of the readily available amine with various aldehydes.

Hydrogen-bonding-facilitated layer-by-layer growth of ultrathin organic semiconducting films

We demonstrated that the layer-by-layer growth of thin film crystals of conjugated organic molecules is facilitated by their hydrogen-bonding capabilities. We synthesized bis(3-hydroxypropyl)-sexithiophene (bHP6T), which includes two hydroxyalkyl groups that promote interlayer and intermolecular molecular interactions during the crystal growth process. Under the optimal deposition conditions, the crystals grew in a nearly perfect layer-by-layer mode on the solid substrate surfaces, enabling the formation of uniform charge transporting films as thin as a few monolayers. A thin film transistor device prepared from a bHP6T film only 9 nm thick exhibited a charge carrier mobility well above 1 × 10(-2) cm(2)/(V s) and an on/off ratio exceeding 1 × 10(4). These properties are better than the properties of other sexithiophene-based devices yet reported. The devices exhibited enhanced stability under atmospheric conditions, and they functioned properly, even after storage for more than 2 months.

Synthesis, photophysics, and electrochemistry of thiophene–pyridine and thiophene–pyrimidine dyad comonomers

A series of new π-conjugated donor (D) and acceptor (A) dyad comonomers were prepared using Suzuki coupling protocols. The D–A comonomers consisting of thiophene/bithiophene as donors and pyridine/pyrimidine as acceptors were prepared to investigate their photophysical and electrochemical properties. The dyads were spectroscopically confirmed to be highly conjugated. This was further supported by the X-ray crystal structure of the bithophene–pyridine dyad that showed all the heterocycles to be coplanar. It was further found that the fluorescence yields (Φfl) of the dyads were highly dependent on the number of thiophenes. The bithiophene derivatives exhibited Φfl values ≥ 0.3 while the thiophene derivatives did not fluoresce. The suppressed fluorescence observed for the thiophene derivatives was due to their higher triplet energy resulting in efficient intersystem crossing (ISC) to the triplet state with ΦISC ≥ 0.8. This was confirmed both by time-resolved and steady-state measurements. The singlet excited state of both thiophene and bithiophene dyads was deactivated solely by either fluorescence and (or) ISC. Owing to their donor and acceptor character, the dyads could be oxidized and reduced both electrochemically and photochemically to afford the radical cation and anion, respectively.