Abstract
The correlation between the molecular structures and charge carrier mobility is a key issue in the design of organic semiconductors to be used in organic electronics. The face-to-face π-orbital overlap is envisioned to be favored for charge transport because of higher electronic coupling possible compare to other relative dispositions. Molecules which have face-to-face crystal stacking have shown their superiority in organic semiconductors. Among these compounds, a promising candidate is contorted polyaromatic, because it has a high tendency to give face-to-face packing. In this thesis, two series of contorted polyaromatics were synthesized and characterized. Single-crystal field-effect transistors based on these molecules were fabricated and performance measured. A systematic comparison of the crystal packing and the calculated electronic couplings with the measured SCFET mobility was presented. The first family was the contorted tetrabenzoacenes with extended conjugation. The contorted molecular structures were clearly established except for a planar closed-ring compound which was obtained from an unexpected cyclopenta-ring fused reaction. Unsubstituted tetrabenzoacenes showed a partial or perfect face-to-face packing depending on their conjugated length. Methyl substituted tetrabenzonaphthalene derivatives gave shifted π-stacking. Planar cyclized tetramethyl tetrabenzohexacene exhibited one-dimension slipped π-stacking. Theoretical calculation was carried out based on the crystal structure. Among these, the electronic coupling of tetrabenzooctacene was the highest (131 meV). Nevertheless, tetrabenzotetracene exhibited the highest measured mobility, approaching 0.8 cm2 V-1 s-1. A comparison of the experimental measurements and the theoretical calculation is presented. Another series of contorted tetrabenzo[a,d,j,m]coronene (TBC) derivatives with substituents at 1-, 2-, 3-, 6-, 7-, 8-positions were designed, synthesized, and characterized. The unsymmetrical substitution introduced dipole moment to the molecule which also affects the crystal packing. Needle-like single crystals were used for crystal structure analyses as well as the SCFET device fabrication. Derivatives with fluoro-containing substituents exhibit antiparallel cofacial or slightly shifted - stacking, whereas those with bulky alkyl substituents show skewed and more significantly shifted - stacking. The hexa-fluorinated TBC crystal showed significant shift along long molecular axis but rather small shift in the short axis. A much larger electronic coupling and thus a highest hole-mobility was achieved, in agreement with theoretical calculation.