This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016–2020, providing an update to our first review of this topic ( Chem. Rev. 2017 , 117 (4), 3479–3716).

Diradicaloid helicenes constructed formally by non‐benzenoid double π‐extension of phenanthrene were synthesized by a common strategy involving double electrophilic benzannulation. Steric effects in the second benzannulation step led to considerable structural diversity among the products, yielding a symmetrical dinor[7]helicene 1 and two isomeric unsymmetrical double helicenes 2 and 3, containing a nor[5]helicene and [4]helicene fragment, respectively, in addition to a common nor[6]helicene motif. Geometries, configurational dynamics, and electronic structure of these helicenes were analyzed using solid‐state structures, spectroscopic methods, and computational analyses. The open‐shell character of the singlet states of these helicenes increases in the order 3<1<2, with strongly varying diradicaloid indexes and singlet–triplet gaps. Compounds 1–3 displayed narrow optical gaps of 0.79–1.25 eV, resulting in significant absorption in the near infrared (NIR) region. They also exhibit reversible redox chemistry, each of them yielding stable radical cations, radical anions, and dianions, in some cases possessing intense NIR absorptions extending beyond 2500 nm.

Diradicaloid helicenes constructed formally by non‐benzenoid double π‐extension of phenanthrene were synthesized by a common strategy involving double electrophilic benzannulation. Steric effects in the second benzannulation step led to considerable structural diversity among the products, yielding a symmetrical dinor[7]helicene 1 and two isomeric unsymmetrical double helicenes 2 and 3, containing a nor[5]helicene and [4]helicene fragment, respectively, in addition to a common nor[6]helicene motif. Geometries, configurational dynamics, and electronic structure of these helicenes were analyzed using solid‐state structures, spectroscopic methods, and computational analyses. The open‐shell character of the singlet states of these helicenes increases in the order 3<1<2, with strongly varying diradicaloid indexes and singlet–triplet gaps. Compounds 1–3 displayed narrow optical gaps of 0.79–1.25 eV, resulting in significant absorption in the near infrared (NIR) region. They also exhibit reversible redox chemistry, each of them yielding stable radical cations, radical anions, and dianions, in some cases possessing intense NIR absorptions extending beyond 2500 nm.