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theses

Organic π-conjugated systems continue to attract great interest in applications ranging from organic electronics to sensing and imaging. The introduction of main group elements to π-conjugated systems provides a means to further modify and fine-tune the electronic and optical properties of π-system. In particular, the incorporation of tricoordinate boron leads to novel optical and electronic properties due to the empty p-orbital on boron that overlaps with π-orbitals of attached aryl or other unsaturated groups. The central focus of this thesis is to investigate the incorporation of organoborane moieties into conjugated macrocycles. We developed a new boron-doped cyclophane, the hexabora[16]cyclophane, B6-FMes, in which six tricoordinate borane moieties alternate with short conjugated p-phenylene linkers. Exocyclic FMes groups provide not only good stability but make the boron centers highly electron deficient. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies show that B6-FMes has a low-lying and extensively delocalized LUMO and a wide HOMO–LUMO gap which could be of interest in optoelectronic applications that require wide-gap host materials.

We also isolated an ambipolar π-conjugated macrocycle, B4N2-FMes, having triarylamine donors paired with highly electron-deficient bis(boryl)fluorene units. The pendent FMes groups play an important role to increase the electron acceptor character of the boron centers, thereby to manipulate the intramolecular charge transfer character (ICT) of B4N2-FMes. The photophysical and electronic properties of B4N2-FMes are evaluated in comparison to previously reported ambipolar macrocycles. Attempts at isolating the phosphorous analog B4P2-FMes in which N is replaced by P are also described. While this macrocycle could not be isolated, computational studies provide information on the geometry and electronic structure of B4P2-FMes. They reveal a distinctly different structure for B4P2-FMes with possible formation of syn- and anti-isomers due to the pyramidal environment at P (by comparison the N atoms in B4N2-FMes adopt a trigonal planar geometry). The HOMO and LUMO levels of B4P2-FMes are slightly lower than those of B4N2-FMes, but overall they are of similar energy. The affinity of B4N2-FMes for small anions suggests potential utility in anion sensor applications.

Finally, we extended our studies to investigate polycationic organoboron macrocycles that contain multiple bipyridylboronium moieties. Macrocycle [B4N2-4bipy]4+ (MC1) was successfully isolated and fully characterized, but efforts at isolating the corresponding hexaboron macrocycles [B6Ar-4bipy]4+ (MC2 and MC3) have not been met with success. MC1 exhibits multi-step redox properties involving the reversible transfer of up to 10 electrons. These macrocycles are also expected to display potentially interesting stimuli-responsive and self-assembly properties.

ETD_11099

Ph.D.

Includes bibliographical references

ETD doctoral

doi:10.7282/t3-7pjz-9456

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