A Chichibabin’s Hydrocarbon-Based Molecular Cage: The Impact of Structural Rigidity on Dynamics, Stability, and Electronic Properties

Abstract

A 3D π-conjugated polyradicaloid molecular cage c-Ph14, consisting of three Chichibabin's hydrocarbon motifs connected by two benzene-1,3,5-triyl bridgeheads, was synthesized. Compared with its linear model compound l-Ph4, the prism-like c-Ph14 has a more rigid structure, which shows significant impact on the molecular dynamics, stability, and electronic properties. A higher rotation energy barrier for the quinoidal biphenyl units was determined in c-Ph14 (15.64 kcal/mol) than that of l-Ph4 (11.40 kcal/mol) according to variable-temperature NMR measurements, leading to improved stability, a smaller diradical character, and an increased singlet-triplet energy gap. The pressure-dependent Raman spectroscopic studies on the rigid cage c-Ph14 revealed a quinoidal-to-aromatic transformation along the biphenyl bridges. In addition, the ellipsoidal cavity in the cage allowed selective encapsulation of fullerene C 70 over C 60 , with an associate constant of about 1.43×10 4 M -1 . Moreover, c-Ph14 and l-Ph4 exhibited similar redox behavior and their cationic species (c-Ph14 6+ and l-Ph4 2+ ) were obtained by chemical oxidation and the structures were identified by X-ray crystallographic analysis. The biphenyl unit showed a twisted conformation in l-Ph4 2+ , whereas remained coplanarity in c-Ph14 6+ . Notably, molecules of c-Ph14 6+ form one-dimensional columnar structure via close π-π stacking between the bridgeheads.zene-1,3,5-triyl bridgeheads. Our studies demonstrated how the structural rigidity in 3D π-conjugated molecular cages affected the dynamics, stability, and electronic properties of diradicaloids/polyradicaloids. ASSOCIATED CONTENTSupporting Information. The Supporting Information is available free of charge on the ACS Publications website.Synthetic procedures and characterization data of all new compounds; details for all physical characterizations and theoretical calculations; additional spectra and X-ray crystallographic data (PDF) X-ray crystallographic data (CIF)

Aggregation-Induced Radical of Donor-Acceptor Organic Semiconductors

“…33 The g value of 2.003 was in good agreement with the typical carbon-based radical rather than other defects or impurities. 26,33,34 In obvious contrast, Flu-TDPP exhibited a nearly silent ESR spectrum at the same test condition. The huge differences of the ESR signal between the two compounds are related with their different molecular geometric configuration and electronic structure according to our previous work.…”

A Chiral Molecular Cage Comprising Diethynylallenes and N‐Heterotriangulenes for Enantioselective Recognition

“…[12] In its purely organic covalent variant, Cid and co-workers have demonstrated the feasibility of realizing chiral molecular cages with outstanding chiroptical properties from the combination of shape-persistent enantiopure DEAs and simple aromatic motifs for the detection of small achiral molecules (Figure 1, cages 1 and 2). [13][14][15] On this firm basis, we advocate the use of bridged triarylamines, known as N-heterotriangulenes (N-HTAs), in the role of chromophoric aromatic lids, in order to achieve redox-active nanosized cavities [16][17][18][19][20] for larger chiral guests, aiming at setting foot on the realm of enantioselective discrimination. [21][22][23] Precedents of sensing examples involving triarylamines range from the detection of metallic cations, [24] to toxic anions [25] or anions present in chemical weapons, [26] nitroaromatic explosives, [27] biological metabolites, [28] toxic gases, [29,30] and pH determination.…”

“… [12] In its purely organic covalent variant, Cid and co‐workers have demonstrated the feasibility of realizing chiral molecular cages with outstanding chiroptical properties from the combination of shape‐persistent enantiopure DEAs and simple aromatic motifs for the detection of small achiral molecules (Figure 1 , cages 1 and 2). [ 13 , 14 , 15 ] On this firm basis, we advocate the use of bridged triarylamines, known as N ‐heterotriangulenes ( N ‐HTAs), in the role of chromophoric aromatic lids, in order to achieve redox‐active nanosized cavities[ 16 , 17 , 18 , 19 , 20 ] for larger chiral guests, aiming at setting foot on the realm of enantioselective discrimination. [ 21 , 22 , 23 ]…”

Recent advances and perspectives on supramolecular radical cages

“…Similar to the synthetic method of 17, a three-dimensional π-conjugated polyradicaloid molecular cage (18) was successfully obtained by the same group. 31 Cage 18 consisted of three Chichibabin's hydrocarbon (CH) motifs which were connected by two benzene-1,3,5-triyl bridgeheads (Fig. 8b).…”

“…For example, organic radical restricted in a confined cage usually causes the significant change of the EPR signal, which can be applied to investigate the supramolecular phenomena. Nicholas and Chechik systematically investigated the host-guest interactions between nitroxide stable radicals (28)(29)(30)(31)(32) and supramolecular coordination cages (33 and 34) in water and acetonitrile by means of EPR spectroscopy (Fig. 12a).…”