Controlling electronic effects and intermolecular packing in contorted polyaromatic hydrocarbons (c-PAHs): towards high mobility field effect transistors

We have investigated the electronic and charge transport properties of two regioisomeric contorted polyaromatic hydrocarbons at the molecular level as well as in the crystalline state. Electron and hole transport is studied on the basis of an incoherent charge hopping model through DFT calculations. For trifluro-dibenzoperylene (CF3-DBP, 1), which crystallizes as a herringbone network, the computed drift hole and electron mobilities are 0.234 and 0.008 cm(2) V-1 S-1, respectively. The greater hole mobility in the DBP crystal (mu(h)/mu(e) = 29) can be rationalized by its lower hole reorganization energy and higher hole transfer integral simultaneously. These calculations for the pristine DBP crystal differ from recent experiments indicating its preferential electron conductivity. This might be attributed to the interaction of the molecules with the gold source/drain electrodes. Its second regioisomer, 2, having a HOMO-LUMO gap of 3.2 eV and thus expectedly inefficient, can be converted into an effective OFET material by replacing the Ph-CF3 groups by oxo groups (>C=O) in the 9 and 10 positions (9,10-dioxotribenzopyrene, 3). 3 has a suitable HOMO-LUMO gap of 2.18 eV. This bowl-shaped molecule is predicted to pack in a stacked orientation with preferential concave...concave pairs having a short intermolecular distance of 4.15 angstrom and identical inter-chromophoric electron/hole coupling (t(h) similar to t(e)). This creates an ambipolar charge transport behavior in 3. Clearly, fine tuning the structure-property relationship opens up the possibility of implanting tailored OFET properties in the existing library of molecules.