Well-Defined Semiconducting Materials with Stabilized Molecular Orbitals: Thiaphospholes to Polythiophenes
The development of organic semiconductors for device applications has been intensely pursued over the past four decades. The active layer in such devices consists of both hole transporting (p– type) semiconductors and electron transporting (n–type) semiconductors. The development of p– type materials has progressed rapidly while n–type systems are relatively underdeveloped, primarily due to stability issues. Perhaps the most salient feature of electron transport materials is a low reduction potential, resulting from the stabilization of the lowest unoccupied molecular orbital (LUMO). Thus, current research is largely focused on molecular design strategies to reduce LUMO energy levels. An extension of the π–system (i.e. conjugated polymers) normally results in a lowering of the LUMO while simultaneously raising the highest occupied molecular orbital (HOMO), reducing the ambient stability. The incorporation of π-accepting functional groups is a convenient manner through which to stabilize molecular orbitals (HOMO and LUMO) and their incorporation into conjugated polymers should lead to materials with improved stability. This thesis explains our efforts to synthesize well-defined conjugated polymers that incorporate π- accepting functional groups. Moreover, heteroatom substitution also has a profound impact on the optoelectronic properties of semiconducting materials and the insertion of main group elements into conjugated organic scaffolds is another established approach to accomplish LUMO stabilization. Specifically, phosphorus is an attractive element to incorporate into organic semiconductors because of its inimitable bonding versatility. This thesis also highlights our efforts to synthesize aromatic phosphorus heterocycles for electron transport. Chapter 1 provides a general introduction into organic semiconductors as well as efforts to manipulate the molecular orbitals of π–conjugated materials. The focus is on both small molecules and polymeric materials with specific attention paid to main group substitution and functional group incorporation and how these strategies can be applied to create n-type materials. Chapter 2 describes a series of bench-stable 2-aryl-1,3-benzothiaphospholes synthesized from 1-mercapto-2-phosphinobenzene and a variety of acid chlorides. The structure of 2-phenyl-1,3- benzothiaphosphole was established using X-ray diffraction. The electrochemical and photophysical properties of each benzothiaphosphole are reported and some of these molecules exhibit reversible 1–electron reductions due to the LUMO stabilization afforded by incorporation of P=C moiety. The reduction potentials show a defined pattern: becoming incrementally more positive as the electron deficiency of the 2-aryl substituent increases and enhances LUMO stabilization. DFT calculations corroborate the electrochemical data elucidating the more pronounced effect of electron deficient groups on reduction by showing significantly more participation of those substituents in the LUMO. Chapter 3 discusses the synthesis and functionalization of the parent 1,3-benzothiaphosphole. The phosphole could not be isolated, but the compound could be manipulated in solution to produce several new phosphorus compounds. Metallation of the 2–position using lithium diisopropylamide proceeded smoothly according to 31P NMR spectroscopy, and quenching with trimethylsilyl chloride resulted in the desired 2-(trimethylsilyl)-1,3-benzothiaphosphole. However, functional substrates for cross-coupling could not be isolated using this approach. The P=C bond of the thiaphosphole was also explored as a dienophile, owing to its low lying LUMO, in Diels- Alder reactions with isoprene, 2,3-dimethylbutadiene, 2,3-dibenzylbutadiene and cyclopentadiene. The fused ring structures were fully characterized and a solid-state molecular structure of the 2,3- dimethylbutadiene cycloadduct was obtained. Chapter 4 highlights our initial efforts to expand the functional group scope of conjugated polymers. Controlled synthesis of conjugated polymers with functional side chains is of great importance, affording well–defined optoelectronic materials possessing enhanced stability and tunability as compared to their alkyl substituted counterparts. A chain–growth Suzuki polycondensation of an ester–functionalized thiophene is described using commercially available nickel precatalysts. Model compound studies were used to identify suitable catalysts, and these experiments provided guidance for the polymerization of the ester–substituted monomer. This is the first report of nickel–catalyzed Suzuki cross-coupling for catalyst–transfer polycondensation (CTP) and to further illustrate the versatility of this method, block and alternating copolymers with 3-hexylthiophene were synthesized. This Suzuki protocol should serve as an entry point into the controlled synthesis of other electron-deficient polymers and donor-acceptor copolymers. Chapter 5 describes further application of our nickel–catalyzed Suzuki CTP protocol. Ni(dppp)Cl2 was used to polymerize an amide–functionalized polythiophene – a monomer that is structurally similar to the prominent thiophene diimide electron–acceptor. Polymer molecular weights could be modulated according to catalyst loading, thus indicating a chain–growth process. Alternating and block copolymers were also prepared with reasonable polydispersities. Cyano– functionalized dimeric and trimeric monomers were explored using the Suzuki CTP protocol, however the resultant polymers were found to be highly insoluble. Chapter 6 provides a general outlook for CTP regarding state of the art conjugated polymers. The development of new catalysts for mild cross-coupling strategies should significantly enhance the monomer scope for CTP. Next generation conjugated polymers will be synthesized by CTP protocols providing control over topology, microstructure, and composition. Specifically, sequence controlled conjugated polymers should provide a major advancement to the field of organic electronics.