Functionalised Silicon Double Bonds

In the early 1900s the idea of compounds with silicon-silicon double bonds was conceived by F.S. Kipping and publicised with the report on what we now call a disilene. While this compound later turned out to be a saturated species, this article marked the birth of organosilicon chemistry. It took until the 1980s before a genuine stable disilene was reported thus enabling thorough investigations into such compounds. Despite this late breakthrough, the quest for silicon double bonds was marked by major milestones, e.g. semiconducting polysilanes. The most prominent spin-out from the hunt for unsaturated silicon species is, however, the area of silicones (robust, still extremely flexible coatings, lubricants, plastics, etc.), which has its origins in Kipping's misconception that he had synthesised a silicon analogue of organic ketones - a silicone .An important motivating force in disilene chemistry is the resemblance of the Si=Si bond to the so-called buckled dimer terminating the surface of elemental silicon, which is used for the majority of applications of this archetypical semiconductor. The shallow potential energy surface and the resulting conformational flexibility of the Si=Si bond renders even topologically different disilenes suitable as models for the silicon surface. Disilenes replicate the weak pi-bond inherent to the buckled dimer with the small HOMO-LUMO gap and high reactivity. The application of Si=Si moieties as functional units in molecules and materials, however, remained unexplored even though such an endeavour would literally marry the concepts of classical semiconductors with that of the newly emerging organic electronics. The reason for this is readily identified in the absence of functional disilenes in the toolbox of the preparative silicon chemist. Unlike in the case of the ubiquitous alkenes that are mainly responsible for the vast diversity of Organic Chemistry, functional disilenes capable of transferring the Si=Si moiety only became readily available in 2004 by our efforts.This synthetic project will further develop the chemistry of functional disilenes. The so far most versatile transfer reagents for Si=Si moities are nucleophilic disilenides. Alternative reagents with modified reduction potential are needed to broaden their scope towards redox sensitive substrates. Electrophilic counterparts that reverse the polarity of disilenides are going to be prepared and provide access to compound classes where only nucleophilic substrates are available and/or safe to handle. After initial successes regarding the incorporation of Si=Si moieties to the periphery of pi-conjugated organic systems using disilenides, it also became quickly apparent to us that a further development of this emerging field urgently requires di- or polyfunctional derivatives, which would open the door to various applications towards supramolecular chemistry, polymer chemistry, and surface chemistry to name only a few. Our inherently molecular approach will allow us an unprecedented level of control over atomic subunits of classical semiconductors and their incorporation into organic electronics. The synthesis of a number of di- and trifunctional derivatives based on various pi-conjugated organic scaffolds will thus be pursued. To this end, conceptually novel methodologies will be developed including the use of protecting groups to carry masked functionalities through the entire length of synthetic procedures. The reactivity of these new compounds with multiple functional Si=Si moieties will be screened in comparison with that of simple disilenides.To summarise, this project will be at the forefront of the newly emerging field of an application- and property-driven chemistry of the Si=Si double bond. It will provide the synthetic tools that are ultimately expected to enable us to utilise the unique physical and chemical properties of disilenes in various applications of nanoscalar electronics.

Since the report on the first stable disilene in 1981, the chemistry of the Si=Si double bond has dramatically developed. While the first twenty years of research along these lines were predominantly devoted to basic aspects of structure and double bond reactivity, recent years have seen increasing interest in the utilisation of the particular properties of the Si=Si bond in synthetic applications and as materials. Moreover, the Si=Si double bond in its many structural manifestations is an excellent molecular model for the silicon surface, which in turn is at the core of overwhelmingly important technologies such as integrated circuits or solar cells. The applicant's group played an active role, particularly as concerns the application of metallated disilenes in organic and inorganic syntheses. This ongoing transformation of the field will be amplified substantially by the project Functionalised Silicon Double Bonds through the synthesis of unprecedented synthetic building blocks that comprise multiple Si=Si units with residual functionalities at the periphery of the Si=Si bond. Such building blocks will allow the systematic design and preparation of extended conjugated systems incorporating Si=Si bonds, for instance electrically conducting molecular wires. The high conformative flexibility of Si=Si bond in principle should confer the possibility of twisting and coiling of the resulting wires without a significant loss of conjugation and thus conductivity. The 'marriage' of classical semiconductors with the newly emerging organic semiconductors based on conjugated systems will have various socio-economic benefits, which potentially reach far beyond synthetic chemistry by influencing neighbouring fields such as surface, polymer and nano science. Potential applications of the targeted new hybrid materials are obviously manifold and collaborations to pursue some of these will be established. Additionally, industrially and ecologically relevant sectors, e.g. information technology, electronics, and photovoltaics, will benefit from the proposed research in the long run in terms of new intellectual property and well-trained, highly skilled employees.