[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. alpha-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 pi and one sigma substituent, Q(+), exhibit a directional cluster-to-Q charge transfer and increase of the net electric dipole moment to mu(e) = 17.9 D for 2a upon photo-excitation. They are weak fluorophores (Phi(f) = 2-7%). Detailed investigation of 4-heptyloxypyridinium derivative 2b revealed that it exhibits a nematic phase above 100 degrees C with Delta(epsilon) = +1.3 (130 degrees C), and a moderate negative solvatochromism. Hyper-Rayleigh scattering measurement gave beta(HRS) = 45 +/- 10 x 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 Delta G(298) = 0.82 +/- 0.06 kcal mol(-1) and Delta G(298)(double dagger) = 25.3 +/- 0.6 kcal mol(-1) established by VT NMR methods.