Fluorescent Molecular Polysilanes

Crystalline silicon is the most widely used semiconductor material today, with a market share of above 90%. Due to its indirect electronic band structure, however, the material is not able to emit light effectively and therefore cannot be used for key applications like light emitting diodes or lasers. Selected one or two-dimensional silicon compounds like linear or branched polyilsilylenes or layered structures like siloxene, however, possess a direct band gap and therefore exhibit intense visible photoluminescence. Siloxene, a solid state polymer with a sheet like layered structure and an empirical formula Si 6 H6-n (OH)n , in particular, is considered as an alternative material for Si-based luminescent devices. Detailed studies of structural and photophysical properties of the material, however, are strongly impeded by its insolubility in organic solvents. In a preceding study we were able to show, that polysiloxane polymers with two dimensional siloxene-like structures containing cyclosilanyl subunits but also molecular cyclohexasilanes bearing siloxy-groups attached to the polysilane ring are strongly photoluminescent. Novel, luminescent materials, therefore, might be accessible on the basis of cyclohexasilanyl rings and oxygen containing side groups. Thus, further investigations certainly will be helpful in order to generalize and to understand the outstanding luminescence behavior of siloxycyclopolysilanes. In the current project, therefore, we`d like to propose supplementary studies directed towards the dependence of siloxycyclopolysilane photoluminescence on substitution pattern and ring size and towards the photoluminescence of soluble well defined polysiloxane oligomers with sheet like or ladder type structures. In the first part of the project appropriate currently mostly unknown model substances shall be synthesized, structurally characterized and transformed to soluble polysiloxane oligomers with sheet like or ladder-type structures containing cyclopolysilanyl subunits. In the second part of the project the solution and solid state absorption and photoluminescence properties of the materials, thus obtained, shall be investigated in detail including quantum yields and excited states life time measurements and computational studies.

Crystalline silicon is the most widely used semiconductor material today, with a market share of above 90%. Due to certain physical properties (indirect electronic band structure), however, the material is not able to emit light effectively and therefore cannot be used for key applications like light emitting diodes or lasers. Selected silicon compounds with linear, cyclic or layered structures (polysilanes, cyclopolysilanes), however, exhibit intense visible photo- and electroluminescence and, therefore, are given considerable attention in respect to possible application as novel electronically active materials. Primary targets of the current project were the synthesis of such compounds and the detailed investigation of their light emission and absorption properties with particular emphasis on specific structure-property relations. The performed experiments successfully demonstrate, that the introduction of certain oxygen containing groups and the assembly of three dimensional cage-like structures significantly intensifies light absorption and emission of cyclopolysilanes. In the course of the project numerous novel synthetic approaches to appropriate model substances with exactly defined structure and composition had to be developed. Furthermore, a combination of experimental and computational methods allowed to gain detailed insight into the mechanism of light absorption and emission in cyclopolysilanes on a molecular level.