Reports: ND749719-ND7: Novel Approaches to Photovoltaic Devices and Materials

Craig Hawker, PhD , University of California (Santa Barbara)

The development of solution-processable small molecules and polymeric materials with good electron transporting properties is highly desired for various applications in organic-based electronics. The vast majority of materials known today exhibit p-channel (hole-transporting) behavior. In direct contrast, air-stable n-channel (electron-transporting) semiconductors and polymeric materials that show solid-state transport of both hole and electron carriers are significantly less developed. Consequently, a major research challenge in this field is to synthesize electron-poor building blocks having the energy of the lowest unoccupied molecular orbital (LUMO) decreased to such an extent that the compound is easily reduced and capable of acting as an n-channel device. It was shown that incorporation of electron withdrawing groups such as fluorine and/or carbonyl, into oligo and polythiophene skeletons can increase the n-type character of the corresponding device. Building on this strategy, thiophene-S,S-dioxides have been examined as electron-poor units with Barbarella and co-workers reporting the preparation of partially oxygenated oligothiophenes where transformation of one or several aromatic thiophene rings into the corresponding S,S-dioxides inserts a strong electron-withdrawing group into the π-conjugated main chain. This leads to an increase in electron-affinity, electron delocalization and a reduction in the optical band-gap of the oligothiophene, allowing the transport properties of oligo and polythiophenes to be tuned from p-type to n-type. These findings triggered investigations into the synthesis of various chemical architectures containing partially oxygenated oligothiophene-S,S-dioxides, their characterization and applications in electronic devices, with one noticeable example being the synthesis of oligothiophene-[all]-S,S-dioxides. In comparison with their partially oxygenated counterparts, complete dearomatization of the oligothiophenes resulted in a significant red shift of the absorption maxima, indicating a considerable narrowing of the energy gap. Theoretical calculations in combination with electrochemical measurements indicated that the energies of the frontier orbitals of oligothiophene-[all]-S,S-dioxides were reduced, with the values of the lowest unoccupied molecular orbital (LUMO) dropping almost twice as much as the highest occupied molecular orbital (HOMO). Despite their promising characteristics, the use of oligothiophene-[all]-S,S-dioxides in applications was limited due to their poor solubility in common organic solvents.

During the period of this PRF grant, we have examined the synthesis of bi-, tert-, and quaterthiophenes incorporating solubilizing alkyl groups in the oligomer skeleton and the subsequent use of these systems as modular building blocks for the preparation of conjugated systems based on thiophene-S,S-dioxides. Oxygenation of these oligothiophenes using the acetonitrile complex of hypofluorous acid, HOF•CH3CN, resulted in the formation of a new family of oligothiophene-[all]-S,S-dioxides which are completely soluble in a variety of organic solvents and can be easily processed into thin films. Oxygenated oligothiophenes were characterized utilizing UV-vis spectroscopy, thermal gravimetric analysis (TGA) and X-ray diffraction. The high solubility also allows them to be used as building blocks for the preparation of thiophene-based conjugated polymers containing bithiophene-S,S-dioxide units in the backbone. Incorporation of electron-withdrawing sulfones along the polymer backbone leads to the narrowing of the energy gap with the energies of both frontier orbitals in bithiophene-[all]-S,S-dioxide containing polymers being lower with respect to the aromatic polythiophene. This tendency is considerably stronger for the LUMO than for the HOMO, leading to materials with greater electron accepting ability. The modularity of this approach allows the energy levels of polythiophenes to be controlled based on the number and position of the thiophene-S,S-dioxide units. Small molecule and polymer based devices, such as thin film transistors, based on oligothiophene-[all]-S,S-dioxides are currently under investigation.

Continuing the examination of novel conjugated building blocks and the synthetic potential of thiophene-S,S-dioxides, the development of azulene-derivatives as viable monomers in conjugated polymers was examined. From medieval times, azure-blue colored natural products based on the azulene (C10H8) skeleton have attracted particular attention, initially for their physical properties, but more recently for their novel electronic structures. Significantly, the parent molecule, azulene is a 10 p-electron isomer of naphthalene, yet it exhibits a dipole moment of 1.08 D and a deep blue color. While unusual for small unsaturated aromatic hydrocarbons, these properties result from the fusion of an electron-rich five-membered ring and electron-poor seven-membered ring. This remarkable electronic structure of azulene allows for cation stabilization through aromatization of the seven-membered ring, which may be exploited in advanced materials for electronic, optoelectronic and electrochromic devices.

The traditional synthetic approaches to these electronically unique building blocks are characterized by long and elaborate synthetic procedures that are low yielding and in many cases do not afford the desired substitution patterns. In fact, many azulene substitution patterns are difficult to access, if they can be accessed at all. This limitation is particularly relevant for azulene-based polymeric materials with all reported systems having the azulene unit incorporated into the polymeric backbone through the five-membered ring. In the PRF grant, we have developed a versatile and modular strategy for the synthesis of azulene derivatives having a single isomeric arrangement of reactive functional groups in the seven-membered ring and the use of these functional azulenes as building blocks in order to construct stimuli-responsive oligomers.

It was demonstrated that exposure of azulene-containing materials to an acidic environment would lead to the protonation of the electron rich cyclopentadiene ring of azulene, forming a stable aromatic 6p-electron tropylium cation. The ability to prepare novel azulene-containing materials where the seven-membered ring is directly incorporated into the backbone rather than being tangential, as in the case of connectivity via five-membered ring, represents a significant opportunity to bring new properties to the field of conjugated polymers. This directly affects the optical and electrical conductivity properties, opening up new areas of research in both azulene-based materials as well as conjugated polymers. Significantly, amongst the family of conducting polymers and organic semiconductors, the closest analogy to the unique properties of these azulene systems may be polyaniline due to its unique environmental stability and simple doping/dedoping chemistry. This polyaniline-like behavior further increases the potential interest in, and application of, these novel materials.

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Dr. Ridgway
Polyene Synthesis
Dr. O'Neil
Dr. Bali
Faults and Fluid Flow
Dr. Huntington