Synthesis and Properties of Oligo[<i>n</i>]cruciforms: Nanosized Sterically Encumbered Tetraethynylphenyl-Homologated Fluorophores

A series of nanosized phenyleneethynylenes have been prepared which are sterically insulated from the surrounding environment by multiple functionalization with triisopropylsilyl (TIPS) substituents. The phenyleneethynylenes comprise oligo[<i>n</i>]cruciforms <b>1</b>−<b>4</b> (<i>n</i> = 3−5) and a dehydrotribenzo[12]annulene <b>5</b>, the former of which possess <i>para</i>-acyclic and the latter <i>ortho</i>-cyclic electronic conjugation pathways. All compounds were characterized by <sup>1</sup>H and <sup>13</sup>C NMR, IR, and mass spectroscopic techniques. The X-ray crystal structure of <b>1</b> confirmed the sterically isolating properties of the TIPS substituents. A comparison of the physical properties of these electronically differing systems revealed that they were all luminescent upon UV irradiation displayed negligible aggregation in dilute solution and that particular members of the series studied were electrochemically active, undergoing facile reversible reductions. The phenyleneethynylenes also exhibited significantly enhanced thermal stability by virtue of the presence of the TIPS substituents. The properties of <b>1</b>−<b>5</b> suggest that they are promising building blocks for the construction of materials for novel molecular electronics applications.