Lipidic carbo-benzenes: Molecular probes of magnetic anisotropy and stacking properties of a-graphyne
Résumé
Solubilization of the C-18 fundamental circuit of alpha-graphyne has been envisaged by decoration with aliphatic chains R =n-CnH2n+1. The synthesis and characterization of p-dialkyl-tetraphenyl-carbo-bentenes (n = 2, 8, 14, 20) are thus presented and compared to the monoalkyl series produced concomitantly. In both series, a dramatic enhancement of solubility in organic solvents (CH2Cl2, CHCl3) is observed for n >= 8, and in the dialkyl series, the melting-decomposition temperature of the solid products is shown to decrease linearly from 208 degrees C for n = 2 to 149 degrees C for n = 20. Fluoroalkyl analogues with R = n-C8H4F13 are also described. The products display classical UV-vis electronic spectra of carbo-benzenes in solution (lambda(max) = 445.5 +/- 1 nm, epsilon approximate to 200 000 L.mol(-1).cm(-1)). They are also characterized by UV-vis absorption in the solid state, which is found to be correlated with the color and crystal packing. The methylene groups of R provide an experimental probe of the magnetic anisotropy and aromaticity of the C-18 ring through the NMR shielding of the H-1 nuclei from ca. 4.70 to 1.25 ppm going away from the border of the ring (as far as 8 angstrom away). All alkyl-carbobenzenes were also found to be highly crystalline. Seven of them have been characterized by X-ray diffraction analysis and the C-18 columnar packing compared in a systematic manner. Crystals of the diethyl and bistetradecyl derivatives, containing no solvent molecule, provided the first examples of direct pi-stacking of carbo-benzene rings, with inter-ring distances very close to calculated interlayer distances in AB and ABC alpha-graphityne (3.255 and 3.206 angstrom vs 3.266 and 3.201 angstrom, respectively).