Bridging Color

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Science  07 Jun 2013:
Vol. 340, Issue 6137, pp. 1142
DOI: 10.1126/science.340.6137.1142-b

The ability to solution-process small light-emitting molecules makes it possible to fabricate organic light-emitting devices using large-scale methods and onto a wide range of substrates. In order to get reasonable values for the photoluminescence efficiency, larger oligomeric molecules are required; however, the increased size rapidly decreases the solubility. Solubility can be increased through the addition of alkyl side groups while also decreasing intermolecular quenching, but this often also diminishes the intermolecular charge transfer, which is the key step in converting electrical charge into light. Christensen et al. reacted a series of chromaphore molecules to form Ar-S-Ar symmetrically bridged sulfides, where the Ar ligand was either bithiophene, terthiophene, naphthalene, or pyrene, and where the sulfur bridge could be subsequently oxidized to form a sulfoxide or sulfone. The addition of the sulfur bridge enhanced the solubility of the molecules; however, it was the oxidiation of the sulfur that was the key to the enhanced photoluminescence seen in all cases. The authors speculate that the electron deficiency of the sulfoxide or sulfone groups enhances the formation of charge-transfer states. Interestingly, the photoluminescence yields of the bithiophene and terthiophene sulfone molecules were greater than the values seen for nonbridged thiophenes with an equivalent number of aromatic units.

J. Am. Chem. Soc. 10.1021/ja401383q (2013).

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