It is shown for the first time that liquid crystal behavior can be induced through halogen bonding. Thus, mixing the nonmesomorphic components 4-alkoxystilbazole with pentafluoroiodobenzene leads to a 1:1 halogen-bonded complex whose integrity is shown by X-ray single-crystal analysis and which shows thermotropic smectic A and nematic phases.

Low-melting point salts, the basis of industrially relevant ionic liquids, exhibit smectic A mesophases over extended temperature ranges.It is well known that organic molecules composed of a rigid and polarisable elongated core, attached to one or two aliphatic chains, often give liquid-crystal mesophases when heated.' In particular, when the alkyl chains are long, smectic phases tend to result due to the microphase separation of the polarisable core and the aliphatic tail portions. Smectic phases are also found in mesomorphic salts, the classic examples of which are Group 1 metal salts of carboxylic acids2Room-temperature chloroaluminate(rI1) ionic liquids are attracting much academic3-5 and industrial611 interest as, in addition to an extended liquid range, they exhibit superacidity,'2 remarkable solubility properties13 and unique catalytic behaviour.7,10~11 The most widely studied systems are AlC13-[emim]Cl, AlC12Et-[bmim]Cl and AIC13-[N-Bupy]C1, where [emim]+, [bmim]+ and [N-Bupy]+ are 1 -ethyl-3-methylimidazolium, 1 -butyl-3-methylimidazolium and N-butylpyridinium cations respectively. We have further investigated the physical properties of series of imidazolium and pyridinium salts, where the alkyl group, R, has been varied from C2H5 to C18H37. We report here that the salts of [C,-mim]+ and [C,-py]+ ( n = 12-18), when associated with chloride, tetrachlorocobaltate(I1) or tetrachloronickelate(I1) counter ions, all exhibit liquid crystalline behaviour and that the thermal stability of the resultant smectic A phase is related to the length, n, of the chain. The N-alkylpyridinium and l-alkyl-3-methylimidazo-

A series of neutral pyridyl adducts involving the strong Lewis acids BF 3 and B(C 6 F 5 ) 3 has been prepared, and their second-order nonlinear optical coefficients (EFISH method) have been examined. The formation of the pyridyl-boron bond leads to enhancement of the dipole moments and second-order NLO coefficients (β and β(0)). Reactions of DMAP (4-Me 2 N-C 5 H 4 N) with BH 3 , BF 3 , and B(C 6 F 5 ) 3 give the corresponding neutral pyridine adducts DMAP‚-BH 3 (orthorhombic, Pbca, a ) 9.6914(7) Å; b ) 11.7173(9) Å; c ) 22.718(2) Å; V ) 1622.4(5) Å 3 ; Z ) 8; p ) 1.114 g cm -3 ), DMAP‚BF 3 (orthorhombic, Pbca, a ) 10.430(3) Å; b ) 8.111(2) Å; c ) 20.295(7) Å; V ) 1717.0(9) Å 3 ; Z ) 8; p ) 1.470 g cm -3 ), and DMAP‚B(C 6 F 5 ) 3 (orthorhombic, Pbca, a ) 12.322(1) Å; b ) 20.556(2) Å; c ) 28.455(3) Å; V ) 7207.4(14) Å 3 ; Z ) 8; p ) 1.508 g cm -3 ). Reactions of Me 2 N-4-C 6 H 4 -CHdCH-C 5 H 4 N, MeO-4-C 6 H 4 -CHdCH-C 5 H 4 N, and Me 2 N-4-C 6 H 4 -CtC-C 5 H 4 N with BF 3 and B(C 6 F 5 ) 3 result in materials that are highly luminescent and incorporate structural features that are known to give rise to large second-order NLO effects. X-ray diffraction analysis for the compound Me 2 N-4-C 6 H 4 -CtC-C 5 H 4 N‚B(C 6 F 5 ) 3 (monclinic, P2 1 /c, a ) 9.883(2) Å; b ) 10.717(1) Å; c ) 28.421-(4) Å; β ) 91.96(1)°; V ) 2993.4(8) Å 3 ; Z ) 4; p ) 1.629 g cm -3 ) is also described.