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Diatomic carbon (C2) is historically an elusive chemical species, considered to require high physical energy for its generation. Here, the authors describe the first room-temperature chemical synthesis of C2 and present experimental evidence for its singlet biradical (quadruple bonding) character and role as a molecular element of nanocarbons.

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [n]cycloparaphenylene dications ([n]CPP2+, n = 5-9), which have unique in-plane aromaticity. The fluorescence peak positions of the [n]CPP2+s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with n > 5. The fluorescence quantum yield of [6]CPP2+ was the highest among the [n]CPP2+s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [n]CPP2+s were smaller than those of neutral [n]CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [n]CPP2+s undergo smaller structural changes upon S0-S1 transition than [n]CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S0-S1 transition of smaller [n]CPP2+s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]CPP2+ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap. [ABSTRACT FROM AUTHOR]

Because of its hyper-leaving ability, as well as its strong oxidizing ability, diaryl(triflato)-λ³-iodane transfers one of the aryl groups to iodoarenes simply upon gentle heating (>85 °C) in nonpolar solvents. We have performed an in-depth mechanistic study of this unusual aryl transfer reaction. A combination of experimental (product analysis, kinetic study, and substituent effects) and density functional theoretical approaches revealed that the reaction proceeds through a concerted bimolecular transition state, in which ipso-carbon binds loosely to both iodine centers. We also evaluated electronic effects on the thermodynamic stability of diaryl-λ³-iodanes. [ABSTRACT FROM AUTHOR]