Thiophene-based materials have emerged as important organic semiconductors for making electronic devices such as solar cells, field-effect transistors, and light-emitting diodes. In the quest to better control the electronic properties and improve the stability of the oligomeric thiophene chains, chemists have set out to partially or completely oxidize thiophene’s sulfur. Adding oxygen to sulfur is quite the trick, however, because few reagents are powerful enough for the job.
Fluorine chemist Shlomo Rozen of Tel Aviv University, in Israel, has come up with one such approach as part of his group’s effort to develop “offspring” reagents of elemental fluorine. Fluorine is one of the strongest oxidizing reagents known, but its extreme reactivity is hard to control. To tame fluorine for oxygen-transfer reactions, one of its major uses, Rozen developed HOF•CH3CN, which is made by bubbling nitrogen-diluted F2 through aqueous acetonitrile.
Rozen described his group’s work in creating HOF•CH3CN and other easy-to-use fluorine derivatives during a symposium sponsored by the Division of Fluorine Chemistry at the American Chemical Society national meeting in New Orleans this week.
Rozen’s group used HOF•CH3CN to selectively prepare oligothiophenes containing both oxygenated and nonoxygenated thiophene units. The researchers transferred oxygen atoms to dibromothiophenes (shown) and then cross-coupled them with unoxidized thiophenes. The resulting partially oxidized oligothiophenes can be fully oxidized by further treatment with HOF•CH3CN (Chem. Eur. J., DOI: 10.1002/chem.201203936). The technique makes it possible to better regulate the electronic properties of the new materials: The prepared oligothiophenes have narrower electronic bandgaps and higher oxidation and reduction potentials compared with any other arrangement of oligothiophenes measured so far, Rozen says.
“Rozen’s discovery of a stable form of HOF by complexing it with acetonitrile has great potential for further developing this new class of organic semiconductors,” observes Columbia University’s Luis M. Campos. Campos is using Rozen’s reagent in his lab to tune the electronic properties of conjugated organic materials, such as molecular wires. Rozen’s “latest work with oligothiophenes shows how the energy levels can be modulated in a reliable fashion, opening up new possibilities to explore their applications,” Campos says.