Orientational Preference of Long,Multicenter Bonds in Radical Anion Dimers: A Case Study of π‐[TCNB]22− and π‐[TCNP]22−

The similar shape and electronic structure of the radical anions of 1,2,4,5‐tetracyanopyrazine (TCNP) and 1,2,4,5‐tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon?carbon bond in π‐[TCNP]₂^2? and in π‐[TCNB]₂^2?, in good accord with the Maximin Principle predictions. Instead, the two known structures of π‐[TCNP]₂^2? have a D_2h(θ=0°) and a C₂(θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six‐membered rings). The only known π‐[TCNB]₂^2? structure has a C₂(θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (E_int) and the variation of the E_int components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the E_int(θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of E_int(θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the E_int(θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)^.? ??? cation⁺ interactions are also included in the model aggregate are the experimental data reproduced computationally.     

Quantitative Supramolecular Heterodimerization for Efficient Energy Transfer

The challenge of quantitatively forming self‐assembled heterodimers without other equilibrium by‐products is overcome through self‐sorting favored by the introduction of designed shape‐complementary moieties. Such a supramolecular strategy based on cucurbit[8]uril‐directed dimerization is further applied to generate hetero‐chromophore dimers quantitatively, leading to efficient energy transfer (>85 %) upon photoexcitation.     

Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases

We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.     

Oxyfadichalcones A–C: three chalcone dimers fused through a cyclobutane ring from Tibetan medicine Oxytropis falcata Bunge

Oxyfadichalcones A and B, two unprecedented chalcone dimers fused through a cyclobutane ring by head-to-tail [2+2] cycloaddition of two chalcones that had never been found previously in nature, along with oxyfadichalcone C, a new head-to-head [2+2] cyclized chalcone dimer, were simultaneously obtained from Oxytropis falcata. Structural elucidation was succeeded by spectroscopic and single-crystal synchrotron radiation analysis. Additionally, the photosynthesis of the chalcone dimers was performed and the plausible biosynthesis was discussed.     

Convenient synthesis of new pregnenolone oximinyl oxalate dimers

Three new symmetrical pregnenolone oxyminyl oxalate dimers (8–10) were synthesized from the corresponding pregnenolone oximes (3, 5, and 7) at room temperature. All dimers were characterised by spectroscopic means, notably HRFABMS and comprehensive NMR spectroscopic data analyses. Published in Khimiya Prirodnykh Soedinenii, No. 3, pp. 250–252, May–June, 2008.     

Non‐symmetric liquid crystal dimers containing the 4‐nitrobenzohydrazide group: synthesis and mesomorphic behaviour

The synthesis and mesomorphic behaviour are reported of a new series of dimers containing 4‐nitrobenzohydrazide and azobenzene groups as the mesogenic units. These non‐symmetric liquid crystal dimers are found to exhibit a monolayer smectic A phase (SmA₁). Lateral hydrogen bonding and strong dipole–dipole interactions are shown to be the major driving forces for the formation of the SmA₁ phase. The present study indicates that the intermolecular interactions and thus the mesophase morphology of the liquid crystal dimers can be controlled by the appropriate selection of the molecular fragments capable of forming H‐bonds.