The compound (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,4,6‐tribromophenyl)diazomethane (1a), a precursor for persistent triplet carbene, was found to be stable enough to survive Sonogashira coupling reaction conditions to give (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,6‐dibromo‐4‐ethynylphenyl)diazomethane (2a). The diazomethane 2a was used as a starting compound for synthesis of phenylacetylene dendritic molecules having peripheral diazo groups by the convergent method. Thus, tridendrons with peripheral groups bearing 3 (3a), 6 (6a), and 12 (8a) diazo units were prepared, in which diazo groups were introduced so as to generate polycarbenes in a ferromagnetic fashion. In order to characterize the spin states of carbenes generated by the photolysis of these dendrimers, the magnetic susceptibility of the photoproducts was measured. Although the data for the photoproduct of the tris(diazo) compound were fitted with S = 2.5, those for the photoproducts of hexakis and dodecakis(diazo) compounds were fitted with S = 1.62, a value that is much smaller than the theoretically predicted value and even smaller than that of triscarbene. The results are interpreted in terms of the disjoint‐nondisjoint concept. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004

The cover picture shows a phenylacetylene dendrimer having 12 diazo units, from which a dodecakiscarbene is generated. Diazo functional groups are usually labile and hence cannot be used as building blocks to prepare more complicated poly(diazo) compounds. We have shown that a diphenyldiazomethane prepared to generate a persistent triplet diphenylcarbene is also stable and can be used as a building block to construct poly(diazo) compounds, which can generate high‐spin polycarbenes. Details are discussed in the article by H. Tomioka et al. on page 2991 ff.