[Closo-B₁₀H₁₀]^2- as a structural element for quadrupolar liquid crystals: A new class of liquid crystalline NLO chromophores

Several electrically neutral, highly quadrupolar derivatives of the [closo-B₁₀H₁₀]^2- cluster (I) substituted in the apical positions with onium fragments derived from 4-alkoxypyridine, 4-pentylthian, 4-pentylquinuclidine, or dinitrogen were synthesized from the parent cluster in 2-5 steps, and their thermal, structural, photophysical and dielectric properties were investigated. Only derivatives 2, containing thianium and alkoxypyridinium substituents with linear alkyl chains, exhibit liquid crystalline behavior and form a nematic phase. α-Branching of the alkoxy chain leads to marked destabilization of both crystalline and nematic phases, which suggests the importance of polar interactions in phase stabilization of these compounds. Derivatives 2 and 3 with one π and one σ substituent, Q⁺, exhibit a directional cluster-to-Q charge transfer and increase of the net electric dipole moment to μ_e = 17.9 D for 2a upon photo-excitation. They are weak fluorophores (Φ_f = 2-7%). Detailed investigation of 4-heptyloxypyridinium derivative 2b revealed that it exhibits a nematic phase above 100 °C with Δε = +1.3 (130 °C), and a moderate negative solvatochromism. Hyper-Rayleigh scattering measurement gave β^HRS = 45 ± 10 × 10^-30 esu at 1064 nm. In solutions, and presumably in the melt, it exists as a mixture of interconverting trans and cis isomers at about 4 1 ratio with ΔG ₂₉₈ = 0.82 ± 0.06 kcal mol^-1 and ΔG‡298 = 25.3 ± 0.6 kcal mol^-1 established by VT NMR methods.