Polar (µ ≈ 16 D) and UV transparent >250 nm quinuclidinium (1) and sulfonium (2) zwitterionic derivatives of the [closo-1-CB₉H₁₀]⁻ anion were synthesized and studied as additives to nematic hosts. The molecular and crystal structures for 1 [C₁₉H₄₄B₉N triclinic, P-1, a = 9.766(2) Å, b = 10.481(3) Å, c = 12.098(3) Å, α = 93.804(9)°, β = 90.249(10)°, γ =102.587(10)°, Z = 2] were determined by X-ray crystallography and compared with the results of HF/6-31G(d) calculations. Low concentration solutions (<10 mol%) of 1 and 2 in ClEsterhost (Δε = −0.59) were investigated using thermal and dielectric methods. The results for 1 and 2 showed virtual [T_NI] values of 139 °C and 92 °C, and Δε extrapolated to infinite dilution of 70 ± 1 and 61 ± 2, respectively. Dielectric results were analyzed using the Maier-Meier relationship and calculated molecular parameters. The apparent order parameter S_app was found to be 0.63 and 0.50 for 1 and 2, respectively, which is smaller than that for the pure host (S = 0.66). Analysis of the Kirkwood factors g obtained for each concentration gave an association constant K of 63 ± 2 and 37 ± 1 (model 1) for the assumed dimerization of molecules (2M ⇌ M₂) of 1 and 2, respectively, in solutions of ClEster.