Compounds with an ultralong CC single bond have been successfully constructed in three steps from commercially available dihalo aromatics. The intramolecular ''core-shell strategy'' is a key tactic for stabilizing compounds with an ultralong CC bond. Using this concept could lead to an even longer CC bond (''hyper covalent bonds'' with a bond length of 1.8-2.0 Å) because the covalently bonded state and non-bonded state are seamlessly connected in terms of the interatomic distance.

Since carbon-carbon (C À C) covalent bonds are rigid and robust, the bond length is,ingeneral, nearly constant and depends only on the bond order and hybrid orbitals.W e report herein direct visualization of the reversible expansion and contraction of aC (sp 3)ÀC(sp 3)s ingle bond by light and heat. This flexibility of aC À Cb ond was demonstrated by X-ray analysis and Raman spectroscopyo fh exaphenylethane (HPE)-type hydrocarbons with two spiro-dibenzocycloheptatriene units,t he intramolecular [2+ +2] photocyclization of which and thermal cleavage of the resulting cyclobutane ring both occur in as ingle-crystalline phase.T he force constant of the contracted CÀCb ond is 1.6 times greater than that of the expanded bond. Since formation of the cyclobutane ring and contraction of the C À Cb ond lower the HOMO level by approximately 1eV, the oxidative properties of these HPEs with aflexible CÀCbond can be deactivated/activated by light/ heat.

In the crystals of 4,7-dihalobenzo[c][1,2,5]chalcogenadiazoles, the molecules are connected by two competing secondary bonding interactions: chalcogen bond (ChB) and halogen bond (HaB). Because the strengths of ChB and HaB change according to the kind of chalcogen (E: S, Se, Te) and halogen (X: Cl, Br, I), their relative contributions in determining the crystal packing can also change. Six newly determined crystal structures as well as 3 previously reported structures can be categorized into two groups: ChB-dominant and HaB-dominant structures. HaB in which X = I is strong enough to dictate the crystal packing of compounds with E = S and Se, but not where E = Te, whereas HaB in which X = Cl or Br has little effect on ChB-dominant molecular networks in crystals. The observed changing contributions of ChB and HaB may be useful for designing new supramolecular synthons for crystal engineering.

The title nonplanar electron acceptor (1) fused with a selenadiazole ring selectively forms a crystalline charge-transfer complex (CT crystal) with 2,6-dimethylnaphthalene (2,6-DMN). On the other hand, the sulfur analogue (2) has less recognition ability and forms CT crystals with both 2,6-and 2,7-DMN. X-ray analyses of 1, 2, and their CT crystals revealed that the Se ••• N chalcogen bond (ChB) in 1 is strong enough to determine the crystal packing with the formation of a cavity suitable for 2,6-DMN. On the contrary, ChB through S ••• N contact in 2 competes with other weak interactions such as a C-H ••• N hydrogen bond. The stronger ChB involving Se is the key for 1 to separate 2,6-DMN (>97 wt%) from a complex isomer mixture containing ca. 10 wt% each of 2,6and 2,7-DMN by a simple, efficient and straightforward mixingfiltration-heating process.

The title dispiro hydrocarbon 1 was designed as a new electrochromic material. This multiply clamped hexaphenylethane-type electron donor was prepared from 2,2′-diiodobiphenyl via biphenyl-2,2′-diylbis(dibenzotropylium) 22+ salt. X-ray analysis of 1 revealed a highly strained structure as reflected by an elongated “ethane” bond [bond length: 1.6665(17) Å] and nearly eclipsed conformation. The weakened bond was cleaved upon two-electron oxidation to regenerate the deeply colored dication 22+. The reversible interconversion between 1 and 22+ is accompanied not only by a drastic color change but also by C–C bond formation/cleavage. Thus, the voltammogram showed a pair of well-separated redox waves, which is characteristic of “dynamic redox (dyrex)” behavior. The tetrahydro derivative of 1 with two units of spiro(dibenzocycloheptadiene), which suffers from more severe steric congestion, was also prepared. The crystallographically determined bond length for the central C–C bond [1.705(4) Å] is greatest among the values reported for 9,9,10,10-tetraaryl-9,10-dihydrophenanthrene derivatives.