Π-conjugated Small Molecules Research Articles

New low bandgap molecules based on ethylene-separated benzothiadiazoles: synthesis and bandgap comparison

New ethylene-separated benzothiadiazoles were synthesized for the first time by using a facile procedure, and they showed lower bandgaps than the reported benzothiadiazole-containing compounds. This new benzothiadiazole-containing unit could be introduced into the backbone of the π-conjugated small molecules or polymers to develop new materials with a low bandgap that may have potential applications in optoelectronic fields.

Electronic structure of delocalized singlet biradical Ph2-IDPL solid film

The film structure and electronic structure of a biradical hydrocarbon, diphenyl derivative of s-indacenodiphenalene (Ph(2)-IDPL) solid film has been investigated. A small energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) compared with that of typical π-conjugated small molecules was observed even for the amorphous film of Ph(2)-IDPL. This result indicates that the small HOMO-LUMO gap is an important characteristic of the singlet biradical electronic structure and well explains the previously reported ambipolar field effects of amorphous Ph(2)-IDPL film by Chikamatsu et al., Appl. Phys. Lett. 2007, 91, 043506. It was found that the gas-deposition method substantially improved the crystallinity of the film where Ph(2)-IDPL molecules form quasi one-dimensional (1D) molecular chains normal to the substrate surface. An extremely small HOMO-LUMO gap was observed in the polycrystalline Ph(2)-IDPL film, which is possibly caused by strong intermolecular coupling. The photon energy dependence of ultraviolet photoemission spectra shows that the stacked Ph(2)-IDPL molecular chain in the polycrystalline film develops an energy band structure in the direction of the surface normal of the film. The intermolecular covalency therefore evolves into the quasi 1D energy band along the molecular stacking direction.

Comparing small molecules and polymer for future organic spin-valves

We report spin polarized injection and transport in organic spin-valves made from both organic small molecules and polymers. The devices with the structure La 0.67Sr 0.33MnO 3 (LSMO)/organic spacer/Co showed inverse magnetoresistance and low temperature operation with the vacuum evaporated π-conjugated small molecule 8-hydroxyquinoline aluminium (Alq 3) while normal MR and high temperature operation was observed in devices with the π-conjugated polymer poly(3-hexylthiophene). Due to the proximate values of the work functions of LSMO and Co with the highest occupied molecular orbital (HOMO) energy of the polymers, spin polarized carrier injection is much more efficient in the polymeric spin-valves compared to the organic spin-valves with Alq 3 where carrier injection is hindered due to greater barrier height. Efficient spin transport is also observed in polymeric spin-valves and can be attributed to longer conjugation in the polymeric chains compared to the small molecules.

Light-emitting conjugated molecule containing 1,3,4-oxadiazole, carbazole and naphthalene units

A novel π-conjugated small molecule VNCO, 2,5-bis{3′-[3″-vinyl-9″-(α-naphthyl)carbazolyl]phenyl}-1,3,4-oxadiazole, containing hole-transporting carbazole moieties, electron-injecting 1,3,4-oxadiazole moieties and chromophore naphthalene was designed and synthesized by Wittig reaction of 2,5-bis(3-tolylene-triphenylphosphonium bromide)-1,3,4-oxadiazole and 3-formyl-9-(α-naphthyl)carbazole. The UV–vis absorption, fluorescence excitation and emission spectra have been obtained in solution for VNCO. The photoluminescence (PL) of VNCO were examined in different solvents and the luminescence quantum yield was 0.746 in chloroform. It emitted blue and blue–green light, with the band gap of 3.30eV estimated from the onset absorption. In addition, the light-emitting can be quenched by both electron donor (N,N-dimethylaniline) and electron acceptor (dimethylterephalate). Furthermore, the molecular interactions of VNCO with fullerene (C60) or carbon nanotubes (CNTs) were also carefully investigated.

Near-ultraviolet electroluminescence from polysilanes

We report the electroluminescent (EL) characteristics of a new class of polymeric material, polysilanes, which were employed in light-emitting diodes (LEDs) as an emissive material. In contrast to the LEDs utilizing π-conjugated polymers and small molecules that have been reported to date, LEDs made from polysilanes exhibit EL in the near-ultraviolet (NUV) or ultraviolet (UV) region due to their σ-conjugation. Three types of polysilanes, dialkyl, monoalkyl-aryl and diaryl polysilanes, have been used as the emissive material, together with an indium-tin-oxide (ITO) and metal electrode for the injection of holes and electrons, respectively. The LED characteristics were observed to depend strongly on the chemical, optical and electronic properties of the emissive polysilanes. The development of emissive polysilanes has led to the successful fabrication of single-layer LEDs which emit NUV light at 407 nm (3.05 eV) with a quantum efficiency of 0.1% photons/electron at room temperature.