Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 106, Issue 26, Pages 10444-10448Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0812660106
Keywords
encapsulation; self-assembly
Categories
Funding
- PRESTO
- Japan Science and Technology Agency
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The self-assembled heterocapsule 1(.)2, which is formed by the hydrogen bonds of tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2, encapsulates 1 molecule of guests such as 1,4-diacetoxybenzene 3a, 1,4-diacetoxy-2,5-dimethylbenzene 3b, 1,4-diacetoxy-2,5-dialkoxybenzenes (3c, OCH3; 3d, OC2H5; 3e, OC3H7; 3f, OC4H9; 3g, OC5H11; 3h, OC6H13; 3i, OC8H17), 1,4-diacetoxy-2,5-difluorobenzene 4a, and 1,4-diacetoxy-2,3-difluorobenzene 4b. The X-ray crystallographic analysis of 3c@(1(.)2) showed that the acetoxy groups at the 1,4- positions of 3c are oriented toward the 2 aromatic cavity ends of 1(.)2 and that 3c can rotate along the long axis of 1(.)2. Thus, the 1(.)2 (stator) with the encapsulation guest ( rotator) behaves as a supramolecular gyroscope. A variable temperature (VT) H-1 NMR study in CDCl3 showed that 3a, 3b, 4a, and 4b within 1(.)2 rotate rapidly even at 218 K, whereas guest rotation is almost inhibited for 3h and 3i even at 323 K. In this respect, 4b with a large dipole moment is a good candidate for the rotator of 1(.)2. For 3c-3g, the enthalpic (Delta H-double dagger) and entropic (Delta S-double dagger) contributions to the free energy of activation (Delta G(double dagger)) for the guest-rotational steric barriers within 1(.)2 were obtained from Eyring plots based on line-shape analysis of the VT H-1 NMR spectra. The value of Delta G(double dagger) increased in the order 3c < 3d < 3e < 3f < 3g. Thus, the elongation of the alkoxy chains at the 2,5-positions of 3 puts the brakes on guest rotation within 1(.)2.
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