π-Extended Pyrene-Fused Double [7]Carbohelicene as a Chiral Polycyclic Aromatic Hydrocarbon

Abstract

A π-extended double [7]carbohelicene 2 with fused pyrene units was synthesized, revealing considerable intra- and intermolecular π–π interactions as confirmed with X-ray crystallography. As compared to the previous double [7]carbohelicene 1 , the π-extended homologue 2 demonstrated considerably red-shifted absorption with an onset at 645 nm ( 1 : 550 nm) corresponding to a smaller optical gap of 1.90 eV ( 1 : 2.25 eV). A broad near-infrared emission from 600 to 900 nm with a large Stokes shift of ∼100 nm (2.3 × 10 3 cm –1 ) was recorded for 2 , implying formation of an intramolecular excimer upon excitation, which was corroborated with femtosecond transient absorption spectroscopy. Moreover, 2 revealed remarkable chiral stability with a fairly high isomerization barrier of 46 kcal mol –1 , according to density functional theory calculations, which allowed optical resolution by chiral HPLC and suggests potential applications in chiroptical devices.

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).…”

Design and synthesis of chiral spirobifluorenes

“…Nevertheless, suitable chiroptical systems are scarce. Great effort has been accordingly devoted to the development of systems involving purely organic molecules, complexes, or plasmonic nanostructures with remarkable chiroptical properties in solution. In this regard, we have achieved oligomeric, cyclic, or cage‐shaped molecular geometries through axially chiral allenes.…”

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.…”