Liquid crystalline organic semiconductors: nematic spiro[cyclopentyl-1,9′]fluorenes

Abstract

A number of nematic spiro[cyclopentyl-1,9′]fluorene reactive mesogens with polymerisable oxetane or non-conjugated diene end-groups have been successfully synthesised. The melting and clearing points of these new spiro[cyclopentyl-1,9′]fluorenes are much higher than those of the corresponding 9,9-diethyl- or 9,9-dipropyl-substituted fluorenes, which are present in many light-emitting polymers currently used in polymer light-emitting diodes. The significant difference in transition temperature may be attributed to the lower intermolecular separation induced by the more rigid and less bulky cyclopentyl group, confirmed by molecular modelling. The presence of either these short but flexible aliphatic chains, or a rigid alicyclic group of similar size and shape, results in only small differences in the photoluminescence spectra and efficiency of these model liquid crystalline organic semiconductors, and in their improved thermal stability.

Keywords:

• spiro-fluorenes
• nematics
• organic semiconductors

Acknowledgements

We thank the EPSRC for the award of a postdoctoral fellowship to Dr S.P. Kitney, the Saudi Arabian Government for the award of a PhD studentship to Dr M.S. Al-Kalifah and the Spanish Government for a postdoctoral fellowship to Dr M. Haro. We are grateful to Dr R. Lewis and Dr K. Welham, respectively, for the ¹H NMR and MS spectroscopic measurements.

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