Dithieno[a,e]penalenes (DTPs) with various substituents were synthesized as a class of antiaromatic compounds. Annulation of thiophene rings imparts the pentalene moiety with high thermal stability even without bulky substituents, while retaining antiaromaticity. The higher magnitude of antiaromaticity in DTPs, in addition to the differences in the electronic structures of the fused aromatic rings, resulted in a narrower HOMO–LUMO gap than that of the corresponding dibenzo[a,e]pentalene analog, giving rise to red‐shifted electronic absorption that reaches the near‐infrared region. Moreover, systematic investigations on the solid‐state packing structure revealed that DTPs prefer offset face‐to‐face packing motifs rather than face‐centered π–π stacking. In particular, the thienyl‐substituted DTP bearing hydrophilic side chains exhibited thermochromic behavior in polar solvents, which was ascribed to the formation of aggregates.

A thiophene-fused 1,4-diazapentalene (TAP) was rationally designed and synthesized as a C=N-containing 4n π-electron system that exhibits restored antiaromaticity impaired by the doping with C=N bonds. X-ray crystallographic analysis and quantum chemical calculations revealed that the annulation of thiophene rings with the 1,4diazapentalene moiety resulted in a much higher antiaromaticity than the pristine 1,4-diazapentalene. These effects can be ascribed to the reduced bond alternation of the eightmembered-ring periphery caused by stabilization of the less-stable bond-shifted resonance structure upon increasing the degree of substitution of imine moieties. Consequently, TAP underwent facile hydrogenation even under mild conditions because of its pronounced antiaromaticity and the high aromaticity of the corresponding hydrogenated product H 2 -TAP. In addition, the electrophilic C=N moieties in TAP led to the formation of a dense π-stacked structure. These results highlight the effect of partial replacement of C=C bonds with C=N bonds in antiaromatic π-electron systems.

Seminaphtho-phospha-fluorescein (SNAPF), a phosphine-oxide-containing unsymmetric fluorescein dye, was synthesized based on the consecutive arylation of PhPCl, followed by Friedel-Crafts cyclization. The resulting SNAPF exhibited several attractive photophysical properties including an intense fluorescence in the NIR region and a large Stokes shift.