Light‐emitting bent‐shape liquid crystals

Abstract

Symmetric and asymmetric bent‐shape LCs were synthesized, having the emissive core 1,3,4‐oxadiazole with C≡C bonds as connecting groups, and terminal nitro lateral substituents. Thermal behaviour was determined using optical microscopy and differential scanning calorimetry. Lowering of transition temperatures and/or induction of new smectic phases in the symmetric compounds was observed on the introduction of terminal polar nitro groups, as compared with unsubstituted symmetric compounds. The observed LC phases possess the classical textures of calamitic liquid crystals and no B‐phases were found, despite the fact that the bending angle of the oxadiazole ring (134°) should be large enough to give polar packing. Luminescent properties, in solution, of the final compounds were evaluated and a strong blue fluorescence was observed from 390 to 410 nm with good photoluminescence quantum yields (71–84%) for compounds without the nitro substituent. The introduction of a nitro group at the external rings of the molecules led to a very strong reduction in the emissive properties, which is attributed to n→π^* excitation of this group.

Acknowledgements

The authors thank Mrs. Marly da Silveira Soldi (DSC measurements), Prof. Ted Ray Taylor and Prof. Frank Quina for helpful discussions. This work was supported by the agencies CNPq‐Brazil and Funcitec/SC.

Notes

^aDetermined by optical microscopy and DSC measures (5°C min⁻¹).

^bDetermined by TGA, onset of decomposition in nitrogen, 10°C min⁻¹.

^cSmX = smectic mesophase not classified.

^aUnits = mol⁻¹ cm⁻¹.

^bDetermined using quinine sulphate as standard.

^cTime of decay recorded only for 1, 3 and 5.