2015 Theses Doctoral

Explorations of controted polycyclic aromatics as electronic materials

Wu, Ying

This thesis describes the design, synthesis, characterization, and application of organic molecular materials made from strained polycyclic aromatic compounds. The strain in the molecular subunits induces non-planar structures. We coined the term “contorted aromatics” to describe these molecules. Contorted polycyclic aromatic hydrocarbons (PAHs) are not only structurally intriguing, but also promising small active molecules for organic electronic devices. This thesis explores the contorted PAHs in two directions, (1) making c-PAHs with high enantiomeric purity and (2) studying c-PAHs with expanded size, for desirable physical, chemical, and electronic properties. The synthesis and chiral resolution of trichloro c-hexabenzocorones (c-HBCs) stereoisomers in Chapter 2 enables the study of conversion barrier in expanded aromatic systems with adjacent cove-units. We also rationalized the formation of thermodynamic stereoisomers and kinetically trapped stereoisomers with density functional theory (DFT) calculations. The organic film effect transistors (OFETs) fabricating from racemic trichloro c-HBC are photo-responsive, suggesting enantiopure OFETs may have potential application in detecting polarized light. Chapter 3 describes the expeditious synthesis of a supersized contorted aromatic molecule, c-octabenzocircumbiphenyl (c-OBCB). Spectroscopic and cyclic voltammetry characterizations show that c-OBCB has a smaller band-gap relative to its analog c-HBCs as designed. The expanded contorted shape results in strong association of c-OBCB with shape-complementary PC70BM fullerenes, as demonstrated by NMR and fluorescence spectroscopies. Chapter 4 studies c-OBCB as an active component in OFETs and solar cells. The tetradodecyloxy-substituted c-OBCB self-assembles to form the active layer in OFETs. Transistor characteristics show that c-OBCBs transport holes readily in thin films. Bulk hetero-junction (BHJ) solar cells of c-OBCB: PC70BM fullerene improved solar power conversion efficiency to 2.88%. External quantum efficiency (EQE) spectra reveal that the red-shift in absorbance is responsible for the higher PCE for c-OBCB compared to the smaller c-HBC series, which charts a clear path to improving the properties of these materials in OPVs by further red-shifting the absorbance. Chapter 5 describes the selective dispersion of single-walled carbon nanotubes (SWCNTs) using c-OBCB. Our dispersion of SWCNTs with c-OBCB selectively disperses semiconducting SWCNTs in toluene with high purity, enabling direct film process with a simple filtration method. We fabricate SWCNTs network transistors using the sorted semiconducting SWCNTs. Our method demonstrates the use of small molecules for facile sorting of semiconducting SWCNTs in high purity (97%) and subsequent direct transistor fabrication from the dispersion.