<italic>Carbo</italic>‐biphenyls and <italic>Carbo</italic>‐terphenyls: Oligo(phenylene ethynylene) Ring <italic>Carbo</italic>‐mers.

Abstract: Ring <italic>carbo</italic>‐mers of oligo(phenylene ethynylene)s (OPE<italic>n</italic>, <italic>n</italic>=0–2), made of C₂‐catenated C₁₈<italic>carbo</italic>‐benzene rings, have been synthesized and characterized by NMR and UV‐vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT‐optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). <italic>Carbo</italic>‐terphenyls (<italic>n</italic>=2) are reversibly reduced at ca. −0.42 V/SCE, i.e. 0.41 V more readily than the corresponding <italic>carbo</italic>‐benzene (−0.83 V/SCE), thus revealing efficient inter‐ring π‐conjugation. An accurate linear fit of <italic>E</italic>_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (−0.30 V/SCE) for a <italic>carbo</italic>‐quaterphenyl congener (<italic>n</italic>=3). Increase with <italic>n</italic> of the effective π‐conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT‐calculated excited states, one of them restricting to a HOMO→LUMO main one‐electron transition. [ABSTRACT FROM AUTHOR]