Conformationally Rigid Ethynylene−Cumulene Conjugated Aromatic [30] Heteroannulenes with NIR Absorption: Synthesis, Spectroscopic and Theoretical Characterization.

Two hitherto unknown conformationally rigid Hückel aromatic
ethynylene−cumulene conjugated [30] heteroannulenes have been synthesized and
characterized. A thorough solution-state spectroscopic characterization, combined
with in-depth theoretical calculations, has been performed to arrive at the proposed
geometry of the macrocycles. The most stable optimized structures for the free base
form of both the macrocycles showed absolute planar geometries without any ring
inversion with mean plane deviation (MPD) values of 0.00 and 0.00 Å, respectively,
in accordance with the NMR spectroscopic observations. The induced
correspondence of rigid ethynylene−cumulene moieties leading to near-infrared
(NIR) absorption in neutral and protonated forms of macrocycles is the important
highlight of this article. This noteworthy finding has been supported by DFT-level
theoretical calculations. There is an increasing pursuit in designing such NIRabsorbing/-
emitting systems due to their immense applications in medicine and biology for recognizing and transportation of
various substrates. The geometry of the novel 30π aromatic heteroannulenes shows promise for evolution of such novel systems
in near future.