Antiaromatic Dicyclopenta[b,g]/[a,f]naphthalene Isomers Showing an Open-Shell Singlet Ground State with Tunable Diradical Character

Abstract

Since the first isolation of 1,3,5,7-tetra-tert-butyl-s-indacene in 1986, core-expanded s- and as-indacenes have attracted intensive interest. However, there is no reported synthesis of such type of molecules due to their high reactivity for over 30 years. Herein, we report the successful synthesis of two relatively stable, core-expanded indacene isomers, dicyclopenta­[b,g]-naphthalene (5) and dicyclopenta­[a,f]­naphthalene (6). X-ray crystallographic analyses reveal that the backbone of 5 adopts a bond-delocalized structure, while that of 6 exhibits a bond-localized character. Variable-temperature 1H NMR/ESR measurements, electronic absorption spectra, and theoretical calculations confirm that both molecules are globally antiaromatic and have an open-shell singlet ground state. However, 6 shows stronger antiaromaticity, a larger diradical character (y 0 = 48%), and a smaller singlet–triplet energy gap (ΔE S‑T = −0.99 kcal mol–1) compared to 5 (y 0 = 30%, ΔE S‑T = −6.88 kcal mol–1), which can be explained by their different quinoidal structures.

6,6′-Biindeno[1,2-b]anthracene: An Open-Shell Biaryl with High Diradical Character

“…In contrast, the five-membered rings of BIA 1 exhibited a positive NICS(1) zz value (4.18), indicating local antiaromaticity, consistent with the observation of high-field-shifted NMR signals at ∼5.9 ppm. 52 Similar NICS(1) zz values were observed for the OSS and OST states of BIA. The UV−vis−NIR absorption spectra demonstrated remarkable changes from precursor 7 to dianion 8 and then BIA 1 (Figure 4a).…”

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

“…The existence of a weak long tail is in accordance with the previous diradical species. 5,[27][28][29][30][31] In addition, the absorption peaks of O-rubicene are slightly bathochromically shifted to those of S-rubicene, while blue shifted to those of Se-rubicene, which agree well with the B3LYP/6-31G(d,p)-calculated electronic transitions for O-, S-, and Se-rubicenes (Fig. S10).…”

Cyclopenta-fused polyaromatic hydrocarbons: synthesis and characterisation of a stable, carbon-centred helical radical

“…π-Conjugated polycyclic hydrocarbons (PHs), 1 in which six- and five-membered rings are arranged alternately, have been referred to as cyclopenta-fused polyaromatic hydrocarbons (CP-PAHs), 2 as non-alternant conjugated polycyclic hydrocarbons (CPHs), 3 or as compounds with quinoidal conjugated units. 4 Substrates with five or nine rings are occasionally only termed with their rational names as indenofluorenes (IFs) 5 and indacenodifluorenes (IDFs), 1 f ,2 a ,6 respectively.…”