Cyanines represent a class of charged chromophores with an odd number of pi-conjugated carbons. Today they are most widely employed as bio labels for fluorescent imaging. Originally cyanine dyes were developed as sensitizers for the photographic technology, but only very few research groups have assessed this class of materials as organic semiconductors.
These dyes however provide unique spectral features in the near infrared to be exploited in transparent devices, for example as NIR sensitive photodetectors or in up-conversion devices. An interesting peculiarity that arises in the solid state is linked to the cationic nature of the chromophore. As we have shown for certain pentamethine cyanine dyes, charges can be generated and extracted in pristine cyanine films with high quantum yield owing to the very slow carrier recombination rate which we at-tribute to a charge screening effect. Under the influence of a strong external electric field, ions can be displaced in the organic layer and thereby tune charge injection in electrochemical light-emitting devices.
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