Tunable Intermolecular Charge Transfer in Ionic Liquid Crystalline Derivatives of the [closo-B₁₀H₁₀]^2– Anion

A new paradigm of ionic liquid crystals exhibiting low-energy charge transfer bands is demonstrated. Thus, four series of ion pairs of 1,10-disubstituted derivatives of the [closo-B10H10]2– anion and substituted pyridinium cations were prepared and their thermal and photophysical properties were investigated. All pyridinium ion pairs form a smectic A (SmA) phase with clearing temperatures above 200 °C and intermolecular charge transfer (CT) band in the solid and fluid states and in CH2Cl2 solutions. The CT band maximum is in the range of 320–510 nm and its position correlates with the difference in frontier molecular orbital (FMO) energies of the ions (highest occupied molecular orbital (HOMO) of the anions and the lowest unoccupied molecular orbital (LUMO) of the cations), which, in turn, correlate well with the substituent σp parameters. Experimental results are augmented with extensive density functional theory (DFT) calculations and X-ray diffraction (XRD) analyses.