Synthesis and Orientation of Fluorene Containing Reactive Mesogens

Abstract

Summary: The synthesis of new fluorene containing photocrosslinkable reactive mesogens is described. Both monodisperse trimers or pentamers and oligomeric mixtures containing two photocrosslinkable acrylate end groups were obtained by Suzuki‐cross‐coupling reactions. The pentamer 12a shows an ideal phase behavior for orientation experiments with a broad nematic phase between the glass transition at −10 °C and 123 °C. In the oligomeric mixtures 14a–g the transition temperature from the nematic to the isotropic phase can be tailored from 100 to 310 °C by adjusting the molecular weight of the oligomers by end‐capping. This process can be easily characterized by MALDI‐TOF spectroscopy. The pentamer 12a and the oligomeric mixture 14c were oriented on rubbed polyimide layers and orientation ratios of 15:1 in photoluminescence were obtained. Experiments with different film thicknesses show that the orientation is not homogeneous throughout the film but decreases with increasing distance from the orientation layer. magnified image

Fluorene-Based Conjugated Oligomers for Organic Photonics and Electronics

“…Kelly et al have done considerable work with photopolymerizable groups on fluorene-containing chromophores, some of which classify as oligomers of fluorene, to form crosslinked networks [123,124]. Strohriegl et al have also developed such systems from their broad experience in crosslinkable liquid crystals [55]. Another approach to a crosslinked fluorene oligomer network is to dehydrate terminal boronic acids, with the added advantage of being reversibly recovered [82].…”

“…A dodecafluorene (compound 44) has exhibited a very high emission dichroism of 17 : 1, but even a heptamer with short enough pendants can reach a ratio of 14 : 1 [60]. Crosslinkable liquid crystalline fluorene oligomers have also yielded a high emission dichroism [55,124]. For colors other than blue, it is important that the transition dipole moment of the chromophore inserted into an oligofluorene is along the molecular axis of the entire oligomer.…”

“…7, but its short ethyl pendants render it crystalline at room temperature [119]. A number of crosslinkable nematic oligofluorenes exhibit a moderate to high degree of order [55,123,124], such as compound 48 shown in Fig. 7.…”

“…One creative approach uses boronic acid end groups to form immobilized networks of oligofluorenes [82], in contrast to the conventional functionalizations of oligofluorenes with convenient polymerizable groups to form insoluble networks [55]. Hydrophilic, or even ionic, side chains to the 9-position of fluorene have been used to realize selfcondensed nanoparticles of fluorene (with compound 30) [83] and to realize sensing of oligonucleotides [84].…”

“…This product can then be converted into a boronic acid for subsequent coupling and lengthening of the oligomer. Excess boronic acid or ester can also be used, but often leads to a side reaction where the desired remaining boronic acid/ester group is converted to a hydrogen atom [55], resulting in a low yield.…”