Global aromaticity at the nanoscale

Abstract

Aromaticity can be defined by the ability of a molecule to sustain a ring current when placed in a magnetic field. Hückel’s rule states that molecular rings with [4 n +2] π-electrons are aromatic, with an induced magnetisation that opposes the external field inside the ring, whereas those with 4 n π-electrons are antiaromatic, with the opposite magnetisation. This rule reliably predicts the behaviour of small molecules, typically with fewer than 22 π-electrons ( n = 5). It is not clear whether aromaticity has a size limit, or whether Hückel’s rule extends to much larger macrocycles. Here, we present evidence for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons ( n = 40); aromaticity is controlled by changing the constitution, oxidation state and conformation. Whenever a ring current is observed, its direction is correctly predicted by Hückel’s rule. The largest ring currents occur when the porphyrins units have fractional oxidation states.

Flexible C−C Bonds: Reversible Expansion, Contraction, Formation, and Scission of Extremely Elongated Single Bonds

“…Carbon-carbon (C À C) covalent bonds represent the most fundamental concept in organic chemistry.E lucidation of their nature is of great importance for further understanding of chemical phenomena;f or instance,t ou nderstand what happens at the limits of ab ond. Since deviation from the standard causes al arge loss of bonding energy,s tructural parameters such as bond length and bond angle are nearly constant among carbon atoms.H owever, strained molecules such as sterically congested polycyclica romatic hydrocarbons [1][2][3][4][5][6][7][8] as well as cyclic p-conjugated compounds [9][10][11][12][13][14][15] exhibit unusual parameters,a nd thus have attracted much attention due to their potential applications.W ith regard to the C À C single bond, which has as tandard length of 1.54 ,t hree approaches have been taken to increase the bond length in neutral organic compounds to beyond 1.7 :( i) diamondoid dimers, [16] (ii)f used or clamped hexaphenylethanes (HPEs), [17][18][19] and (iii)d iaminocarboranes (Figure 1). [20] Among these,w ef ocused on the second approach (ii)w hile adopting the core-shell strategy,w hich enables the isolation of stable dispiro[dibenzocycloheptatriene (DBCHT)]-type HPEs 1 with an extremely elongated C(sp 3 )ÀC(sp 3 )b ond [1.806(2) for 1cat 400 K].W eenvisaged that such a"hyper covalent bond" should be weak enough to exhibit reversible expansion, contraction, formation, and scission, which could be visualized by X-ray analyses.…”

“…Since deviation from the standard causes a large loss of bonding energy, structural parameters such as bond length and bond angle are nearly constant among carbon atoms. However, strained molecules such as sterically congested polycyclic aromatic hydrocarbons [1–8] as well as cyclic π‐conjugated compounds [9–15] exhibit unusual parameters, and thus have attracted much attention due to their potential applications. With regard to the C−C single bond, which has a standard length of 1.54 Å, three approaches have been taken to increase the bond length in neutral organic compounds to beyond 1.7 Å: (i) diamondoid dimers, [16] (ii) fused or clamped hexaphenylethanes (HPEs), [17–19] and (iii) diaminocarboranes (Figure 1).…”

Ein Hückelsches Riesenrad

“…Jetzt hat Andersons Team den eigenen Rekordwert mehr als verdoppelt und dabei noch neuartige Phänomene entdeckt . Die Forscher gingen dabei zunächst von dem Ring aus 6 Porphyrinen aus.…”

Flexible C−C Bonds: Reversible Expansion, Contraction, Formation, and Scission of Extremely Elongated Single Bonds

“…Carbon-carbon (C À C) covalent bonds represent the most fundamental concept in organic chemistry.E lucidation of their nature is of great importance for further understanding of chemical phenomena;f or instance,t ou nderstand what happens at the limits of ab ond. Since deviation from the standard causes al arge loss of bonding energy,s tructural parameters such as bond length and bond angle are nearly constant among carbon atoms.H owever, strained molecules such as sterically congested polycyclica romatic hydrocarbons [1][2][3][4][5][6][7][8] as well as cyclic p-conjugated compounds [9][10][11][12][13][14][15] exhibit unusual parameters,a nd thus have attracted much attention due to their potential applications.W ith regard to the C À Cs ingle bond, which has as tandard length of 1.54 , three approaches have been taken to increase the bond length in neutral organic compounds to beyond 1.7 :( i) diamondoid dimers, [16] (ii)f used or clamped hexaphenylethanes (HPEs), [17][18][19] and (iii)d iaminocarboranes (Figure 1). [20] Among these,w ef ocused on the second approach (ii)w hile adopting the core-shell strategy,w hich enables the isolation of stable dispiro[dibenzocycloheptatriene (DBCHT)]-type HPEs 1 with an extremely elongated C(sp 3 ) À C(sp 3 )b ond [1.806(2) for 1cat 400 K].W eenvisaged that such a"hyper covalent bond" should be weak enough to exhibit reversible expansion, contraction, formation, and scission, which could be visualized by X-ray analyses.…”