Ring-Expanding Rearrangement of Benzo-Fused Tris-Cycloheptenylenes towards Nonplanar Polycyclic Aromatic Hydrocarbons.

Invited for the cover of this issue are Klaus Müllen and co-workers at the Max-Planck-Institute of Polymer Research and the University of Heidelberg. The image depicts the ring-expanding rearrangement of hexabrominated benzo-fused tris-cycloheptenylenes towards nonplanar polycyclic aromatic hydrocarbons. Read the full text of the article at 10.1002/chem.202203735.

Ring-Expanding Rearrangement of Benzo-Fused Tris-Cycloheptenylenes towards Nonplanar Polycyclic Aromatic Hydrocarbons.

A strongly twisted benzo-fused tris-cycloheptenylene, containing three dibenzosuberenone units fused to a common benzene ring, was subjected to Ramirez olefination and subsequent palladium-catalyzed Suzuki-Miyaura cross-coupling with 4-substituted phenylboronic acids. The high steric demand within the overcrowded, benzene-rich benzo-fused tris-cycloheptenylenes enforced an unprecedented 1,2-rearrangement upon π-extension during the Suzuki coupling reaction. According to crystal structure analysis, the resulting negatively curved polycyclic aromatic hydrocarbons consist of two heptagons and one octagon surrounding a central benzene ring as a result of strain release. In the solid state, the materials exhibit a blue to blue-green fluorescence with increased quantum yields and a hypsochromic shift of the emission maxima compared to their respective solutions.

On-Surface Debromination of 2,3-Bis(dibromomethyl)- and 2,3-Bis(bromomethyl)naphthalene: Dimerization or Polymerization?

We describe the on-surface dehalogenative homocoupling of benzylic bromides, namely bis-bromomethyl- and bis-gem-(dibromomethyl) naphthalene as a potential route to either hydrocarbon dimers or conjugated polymers on Au(111). While bis-gem-(dibromomethyl) naphthalene affords different dimers with naphthocyclobutadiene as the key intermediate, bis-bromomethyl naphthalene furnishes a poly(o-naphthylene vinylidene) as a non-conjugated polymer which undergoes dehydrogenation toward its conjugated derivative poly(o-naphthylene vinylene) upon mild annealing. A combination of scanning tunneling microscopy, non-contact atomic force microscopy and density functional theory calculations provides deep insights into the prevailing mechanisms.

Benzo-fused Tri[8]annulenes as Molecular Models of Cubic Graphite.

Cyclotrimerization of 9,10-dibromo-9,10-dihydrodibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrene with potassium tert-butoxide in the presence of a transition-metal catalyst afforded two polycyclic aromatic hydrocarbon stereoisomers consisting of three cyclooctatetraene (COT) moieties connected via a central benzene ring. Both isomeric tri[8]annulenes were obtained selectively through the choice of the catalyst: The α,α,α-form (Ru catalyst) displayed a threefold symmetrywith the COT subunits forming the side walls of a (chiral) molecular cup. In the thermodynamically more stable α,α,ß-isomer (Pd catalyst), one of the three boat-shaped COTs was flipped over and faced the opposite molecular hemisphere with respect to the central benzene ring as evidenced by crystal structure analysis. Both title compounds are small segments of "cubic graphite", an elusive carbon allotrope.

Chrysene-Based Blue Emitters.

Chrysene and its bisbenzannulated homologue, naphtho[2,3-c]tetraphene, were synthesized through a PtCl2 -catalyzed cyclization of alkynes, which also furnished corresponding biaryls subsequent to a Glaser coupling reaction of the starting alkynes. The optoelectronic properties of 5,5'-bichrysenyl and 6,6'-binaphtho[2,3-c]tetraphene were compared to their chrysene-based "monomers". Oxidative cyclodehydrogenations of bichrysenyl and its higher homologue towards large nanographenes were also investigated.