期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2009
卷号：106
期号：26
页码：10444-10448
DOI：10.1073/pnas.0812660106
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：The self-assembled heterocapsule 1{middle dot}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{middle dot}2) showed that the acetoxy groups at the 1,4-positions of 3c are oriented toward the 2 aromatic cavity ends of 1{middle dot}2 and that 3c can rotate along the long axis of 1{middle dot}2. Thus, the 1{middle dot}2 (stator) with the encapsulation guest (rotator) behaves as a supramolecular gyroscope. A variable temperature (VT) 1H NMR study in CDCl3 showed that 3a, 3b, 4a, and 4b within 1{middle dot}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{middle dot}2. For 3c-3g, the enthalpic ({Delta}H{ddagger}) and entropic ({Delta}S{ddagger}) contributions to the free energy of activation ({Delta}G{ddagger}) for the guest-rotational steric barriers within 1{middle dot}2 were obtained from Eyring plots based on line-shape analysis of the VT 1H NMR spectra. The value of {Delta}G{ddagger} 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{middle dot}2.
