Resorcin[4]arene cavitands with four quinoxaline bridges are a family of macrocycles that adopt, at elevated temperature, a contracted, vase‐type conformation, capable of guest inclusion, whereas at low temperature they switch to an expanded, kite‐type conformation with a large flat surface. The present investigations lay the foundation for the use of such dynamic cavitands as miniaturized mechanical grippers for supramolecular construction at the single‐molecule level. New vase–kite switching modes, stimulated by pH changes or stoichiometric metal‐ion complexation, have been discovered and monitored by 1H NMR and optical absorption spectroscopy. The solid‐state geometries of the two states have been revealed by X‐ray crystallography, and the kinetics and thermodynamics of the switching processes in solution as well as their solvent dependency has been investigated in great detail. Monolayers of the cavitand in the vase form have been studied by scanning tunneling microscopy at molecular resolution; conformational switching is also observed in Langmuir monolayers at the air/water interface. Synthetic protocols have been developed for preparation of partially and asymmetrically bridged resorcin[4]arene cavitands, which are also shown to undergo conformational switching. These synthetic advances pave the way to new, dynamic molecular receptors for steroids, tetrathiofulvalene‐bridged grippers with the potential to undergo electrochemically induced conformational switching, and systems with greatly extended, rigid cavity walls functionalized at the termini by dipyrrometheneboron difluoride dyes. The latter cavitands are shown by fluorescence resonance energy transfer to undergo geometrically precisely defined motions between a contracted (≈ 7 Å linear extension) and a strongly expanded (≈ 7 nm linear extension) state.
The reversible switching from the C(4v)-symmetric vase to the C(2v)-symmetric kite conformation of an amphiphilic resorcin[4]arene cavitand was induced by Zn(II) ion coordination. Langmuir monolayers were obtained of both conformers with the area per molecule increasing dramatically from 120 A(2) for the vase to 270 A(2) for the kite form. (1)H NMR spectroscopy in chloroform solution supports the formation of a stoichiometric kite-cavitand[radical dot](Zn(II))(2) complex, with the metal ions coordinating to pairs of neighbouring quinoxaline N-atoms.
A new linear switchable diporphyrin array has been synthesized in good yield by a Williamson coupling of 5‐(4′‐hydroxyphenyl)‐10,15,20‐triphenylporphyrin with tetraethylene glycol ditosylate. 1H‐NMR binding studies reveal that the title receptor binds to Na+ and K+ salts to give a supramolecular complex with stability constants (K) of 30 and 160 M–1, respectively (CD3CN/CDCl3, 1:1). The formation of the host‐guest complexes promotes a change of the geometry of the system toward a topology in which the two tetrapyrrolic macrocycles tend to face together. UV/Vis and fluorescence spectroscopy give indications of the actual geometry of the assembly. The system reported represents an important step toward the construction of a supramolecular model of relevant biological systems.
The weak interactions responsible for the aggregation and organization of micellar aggregates have been shown to control the regioselectivity of the epoxidation of limonene catalyzed by manganese porphyrins toward the endo double bond. The same reaction carried out in the presence of aggregates formed by sodium N-dodecanoyl-L-prolinate has shown the same regioselectivity, a selectivity toward the R enantiomer of limonene and a slight but significative stereoselectivity due to the organization of the molecular aggregates.
The synthesis of a series (1–5) of fullerene and Zn(II)‐porphyrin amphiphiles with polar dibenzo[24]crown‐8 headgroups is described. Their ability to form Langmuir monolayers at the air‐water interface was investigated in a systematic study. The Langmuir films were characterized by their surface pressure versus molecular area isotherms, compression and expansion cycles, and Brewsterangle microscopy. Complexation of larger alkali metal cations (K+ and Cs+) by the polar headgroups leads to higher molecular area requirements and a better anchoring to the aqueous subphase. The monolayers of the porphyrin‐(dibenzo[24]crown‐8) conjugates were transferred as Langmuir—Blodgett films onto glass slides and the films characterized by UV‐vis spectroscopy and grazing‐incidence X‐ray diffraction. Good evidence was obtained that the porphyrin‐fullerene‐crown ether triad 5 adopts a sandwich geometry in the LB films. In such an arrangement, the electron‐attracting carbon sphere benefits from attractive interactions with the sandwiching electron‐rich porphyrin and crown ether chromophores. The described films could have potential applications as functional materials in optical and electronic technology.
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