The Synthesis and Characterization of Photoluminescent Conjugated Frameworks Containing Silicon and Germanium
Siloles and 9-heterofluorenes (Si, Ge) are conjugated π-electron systems containing Group 14 atoms which demonstrate unique properties such as high electron affinity, mobility, and bathochromically shifted optical spectra relative to their all-carbon analogs. This is due to their low-lying lowest-unoccupied molecular orbital (LUMO) which is resultant of the σ*-π* conjugation between the exocyclic groups at the Group 14 center and the butadiene unit of the ring. As a consequence, they have received considerable attention for their use as components for a wide variety of optical and electronic applications such as organic light emitting diodes (OLEDs), photovoltaic devices, field-effect transistors (FETs), and as chemical and biological sensors. Siloles with conjugated organic moieties incorporated in the 2,5-substituents are of interest since modification of the substituents at these positions of the silole ring have the most dramatic effect on the optoelectronic properties of the silole. Platinum complexes containing polypyridine-based ligands have been reported to show unusually rich photophysical and polymorphism properties resulting from their ability to form Pt-Pt and/or π-π interactions in solution and especially in the solid state resulting in a bathochromic shift in the emission with enhanced quantum yields. A series of platinum(II) complexes containing chelating polypyridine and phosphine ligands were coordinated to the 2,5 positions of a siloles by organic linker moieties in an attempt to form an array of photoluminescent platinum(II)-silole macrocycles. Heterofluorenes demonstrate similar properties to siloles, only they are enhanced due to their fused-ring structure. Although their photoluminescent substituent effects remain unexplored relative to that of siloles. The synthesis and characterization of a series of new highly fluorescent 2,7-alkynyl(aryl)-3,6-dimethoxy-9,9-diphenylsilafluorenes was investigated utilizing a modified multi-step synthetic pathway in which the final step utilizes palladium-catalyzed cross-coupling conditions to incorpoarate a variety of conjugated – alkynyl(aryl) groups at the 2,7-positions. Crystal structures were obtained for all successfully synthesized heterofluorenes herein.