An organic semiconductor, expressed as a quadrupolar linear conjugated system (DPP Linear-c), consisting of a 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) central unit and two quaterfluorenylphenyl arms at the 3- and 6-positions of the DPP fragment, is studied in organic field-effect transistors (OFETs). The strong propensity of DPP Linear-c to aggregate in the solid state favours the hole mobility of the semiconductor due to the alignment of HOMOs in the aggregates. The fabrication of OFETs from this novel p-type semiconductor is reported, using different dielectrics to evaluate their effect on the overall device performance. Despite its aggregation, DPP Linear-c remains luminescent in the solid state. Electroluminescence of a non-optimised OLED fabricated from this material is demonstrated.

In this Research News article we discuss the significance of dimensionality on the charge-transport properties of organic semiconductors. Dimensionality is defined in two ways: as a function of (i) the π-conjugated framework within the molecular structure, and (ii) the degree and direction of intermolecular close-contacts between molecules in the bulk. In terms of dimensionality, silicon is a good role model for organic semiconductors, since it demonstrates 3D architecture in the bulk through covalent bonding. Achieving this for organics is challenging and requires not only a 3D molecular structure, but also a network of intermolecular short contacts in three dimensions. This review identifies the limitations of low dimensional materials and summarizes the challenges faced in progressing towards fully 3D organic semiconductors.

The fused, bicyclic molecule, 2,1,3-Benzothiadiazole (BT), has become a key ingredient in the design of new organic semiconductors for light emission and energy harvesting applications. Here, the synthesis is reported of a series of trigonal, star-shaped compounds comprising a truxene core and three quater-dialkylfluorene arms into each of which a BT unit is inserted sequentially at each possible position (T4BT-A to T4BT-E). Analysis of the resulting electronic properties shows that as a consequence of conjugative coupling to the core and the resulting symmetry there are three distinct locations for the BT unit and the influence that these locations have on light emission and other spectroscopic characteristics is discussed. The systematic variation in photophysical properties for the different structural isomers helps to clarify the influence of BT unit addition to 9,9-dialkylfluorene chains. It also helps to establish a design template for the construction of donor-acceptor conjugated materials with targeted properties. For T4BT-E with a BT unit at the terminal position of each arm, the photoluminescence quantum efficiency is significantly reduced and no amplified spontaneous emission is observed under typical pumping conditions. Theoretical calculations assist in understanding the variation in behaviors among the T4BT-X family of compounds, especially in relation to their photoluminescence decay times and the Raman scattering intensities of their dominant BT-unit-centred molecular vibrations.

The synthesis of two [4]-dendralene compounds incorporating thiophene-(p-nitrophenyl) donor–acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron-donating components of the dendralenes are represented by bromothienyl (in 13) and ethylenedioxythiophene(EDOT)-thienyl (in 15) end-groups. The most facile transformation involves the isomerisation of donor–acceptor conjugated systems (a conformers) into structures in which only the thiophenes are conjugated (b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single-crystal X-ray diffraction studies and the structural changes in both compounds have been monitored by ¹H NMR spectroscopy and absorption studies. The transformations were found to be first-order processes with rate constants of k=0.0027 s⁻¹ and k=0.00022 s⁻¹ for 13 and 15, respectively. Density functional theory calculations at the B3LYP/6-31G* level give credence to the proposed mechanism for the ab conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative (15) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.