New Class of n-Type Discotic Liquid Crystals with N3B3O3 Core: Probable Materials for Organic Electronics
C. V. Yelamaggad
Centre for Nano and Soft Matter Sciences(CeNS)
P.B.No.1329 , Professor UR Rao Road, Jalahalli
Bengaluru, 560 013.
An entirely new class of n-type discotic liquid crystals (DLCs) namely, tris(boranil)s, the pseudo-triphenylene boron(III) complexes with NBO sequence, have been realized and characterized. The novelty of these DLC materials stems from the fact that the association of π-electron rich chelate ligands with electron-deficient boron bearing vacant p-orbitals enables an intramolecular electron delocalization resulting in the formation of rigid π-conjugated (shape-persistent) skeletons. This structural stiffness with NBO sequence not only lowers the electronic state of the lowest unoccupied molecular orbital (LUMO) level to enhance the electron affinity but also restricts π-conjugated framework to intensify the light emission. NMR spectroscopy studies evidence the formation of tris(boranil)s through the isomerization of keto-enamine tautomeric forms of ligands to enol-imine form so as to form B(III) complexes. These new discotics display technologically significant columnar (Col) phase over wide thermal range; notably, some of them are room temperature Col LCs. The powder X-ray diffraction studies suggest the self-organization of these mesogens into rectangular or oblique 2D Col assemblies, which seems to be dictated by the length of terminal tails. The primary investigations demonstrate the organic field effect transistor (OFET) device characteristics of the materials synthesized. The solid state, solution and fluid / frozen Col structure of these discotics emit light. Notably, the room temperature Col structure intensifies the light emission. The LUMO energy value viz, ~ - 4.27 eV, as revealed by cyclic voltammograms of these discotics is remarkable in view of the fact that organic n-type semiconductors with such energies are suitable materials for electronic device applications.