An Efficient Method for the Synthesis of π-Expanded Phosphonium Salts

A new straightforward methodology for the synthesis of phosphonium salts integrated with a π-conjugated scaffold has been developed using phosphine oxides. It is now possible to obtain cyclic phosphonium salts possessing up to eight con-jugated rings and bearing e.g. pyrrole, thiophene, indole or benzofuran scaffolds from abundant and commercially avail-able materials in high yields. An enticing feature of this general strategy is that this one-step procedure typically does not require chromatographical purification. Still greater synthetic possibilities are related to the fact that even demanding scaffolds such as azulene, pyrene or fluorene can be bridged with the phospholium subunit. Starting from 1,4-dihydropyrrolo[3,2-b]pyrrole, heretofore rarely observed ladder-type bis-phosphonium salts were effectively prepared. This strategy was also extended into the preparation of cyclic arsonium salt. The ability to form phosphonium salts possessing such manifold scaffolds translated into diverse photophysical properties ranging from non-fluorescent dyes to thiophene-derivatives emitting quantitatively in the blue region. Geometry change induced by light absorption has a pre-dominant influence on the fate of the molecules’ excited state. It was shown, in analogy to previous results, that cyclic tetraarylphosphonium salts migrate through the membrane of living cells to localize in the mitochondria similarly to the well-known triarylphosphonium salts.