EFFECTS OF SIDE ARM LENGTH AND STRUCTURE OF PARASUBSTITUTED PHENYL DERIVATIVES ON THEIR BINDING TO THE HOST CYCLOBIS(PARAQUAT-P-PHENYLENE)

The binding constants with the host cyclobis(paraquat-p-phenylene), 1( 4+), have been determined in CH3CN by UV-vis spectrophotometry for a s eries of p-phenylene guests, symmetrically substituted with side arms of varying length and functionality. Semiempirical molecular orbital t heory was employed to provide a detailed structural and energetic inte rpretation of the experimental binding data. In particular, the length of the side arms and the type and position of the heteroatoms on the side arms were systematically varied in order to understand the effect s of external interactions on the association constants of the guests with host 1(4+). A large chelate effect involving the ethyleneoxy side arm oxygen atoms and a cooperative effect between the guest aromatic core and the side arms are significant factors which determine the bin ding with this host. Sequential ethyleneoxy linkages along the side ar ms markedly increase the binding constant with respect to a compound i n which the same number of oxygens along the side arms are separated b y longer aliphatic linkages. In addition, a multiplicative rather than additive effect on the binding constant is observed which demonstrate s that the oxygen atoms exhibit a strong chelate effect. It was also d iscovered that while the side arms of these guests contribute most of the driving force fdr complexation, an aromatic core is necessary for the guest to reside in the cavity of the host. The binding of these gu ests then is dependent upon cooperation between the arms and the aroma tic core. Furthermore, elongation of the central aromatic core with al iphatic side arms containing no heteroatoms leaves the association con stant relatively unchanged and replacement of the oxygen atoms with su lfur markedly decreases the observed binding. These effects have been used to rationalize several observations regarding this system in the literature and may serve to improve the design of new supramolecular s ystems and to better understand the host/guest interaction process.