The Charge-Transfer Motif in Crystal Engineering. Self-Assembly of Acentric (Diamondoid) Networks from Halide Salts and Carbon Tetrabromide as Electron-Donor/Acceptor Synthons

Unusual strength and directionality for the charge-transfer motif (established in solution) are
shown to carry over into the solid state by the facile synthesis of a series of robust crystals of the [1:1]
donor/acceptor complexes of carbon tetrabromide with the electron-rich halide anions (chloride, bromide,
and iodide). X-ray crystallographic analyses identify the consistent formation of diamondoid networks, the
dimensionality of which is dictated by the size of the tetraalkylammonium counterion. For the tetraethylammonium bromide/carbon tetrabromide dyad, the three-dimensional (diamondoid) network consists of
donor (bromide) and acceptor (CBr4) nodes alternately populated to result in the effective annihilation of
centers of symmetry in agreement with the sphaleroid structural subclass. Such inherently acentric networks
exhibit intensive nonlinear optical properties in which the second harmonics generation in the extended
charge-transfer system is augmented by the effective electronic (HOMO−LUMO) coupling between
contiguous CBr4/halide centers.