The effect of molecular shape and chemical structure on the photo-physical properties of liquid crystals

Fluorescence in liquid crystals (LCs) is valuable for applications such as optoelectronic devices, photovoltaics and light emitting diodes. The molecular shape and chemical structure of LCs greatly influence their fluorescent properties, although these relationships are not well understood. We here provide a systematic comparative study of a selection of cyanobiphenyl-based LCs with both calamitic and dimeric molecular shapes. The influence of both these molecular shapes and of lateral fluorination on the observed fluorescence is determined. Our results show that cyanobiphenyl-based calamitic nematic LCs exhibit a constant increase in the excimer-to-monomer emission ratio as the temperature is reduced from the isotropic phase. For a cyanobiphenyl-based dimeric LC with an average bent (or banana) molecular shape, that exbibits both nematic and twist-bend nematic phases, the excimer-to-monomer ratio is found to be constant within both the isotropic and the nematic phases. However, interestingly, a significant increase in the excimer-to-monomer ratio is observed in the lower temperature twist-bend nematic phase of this material, indicating an increased presence of anti-parallel (AP) pairing modes. When the same dimeric LC is fluorinated laterally to the cyano group, the material shows significantly less excimer emission compared to the non-fluorinated dimeric LCs due to an interruption in AP pairing. Furthermore, the results on cyanobiphenyl-based LCs are compared with results for an oxadiazole-based nematic bent-core LC that does not support the same AP pairing modes. For the oxadiazole-based LC, we observe a constant excimer-to-monomer ratio that is significantly smaller compared with that observed for the other LCs throughout the entire temperature-range of the nematic phase. Our results clearly demonstrate that the fluorescence of cyanobiphenyl-based LCs, directly linked to excimer formation, is almost exclusively a result of AP pair formation. Importantly, we also demonstrate that this pair formation, and thus the fluorescence, can be tuned both through modification of the molecular shape and chemical structure.