Tweezering the core of dendrimers: Medium effect on the kinetic and thermodynamic properties

(Chemical Equation Presented) We have investigated the complex formation between dendritic guests and a molecular tweezer host by NMR, absorption, and emission spectroscopy as well as electrochemical techniques. The dendrimers are constituted by an electron-acceptor 4,4′-bipyridinium core appended with one (DnB2+) or two (Dn2B2+) polyaryl-ether dendrons. Tweezer T comprises a naphthalene and four benzene components bridged by four methylene groups.Medium effects on molecular recognition phenomena are discussed and provide insight into the conformation of dendrimers: change in solvent polarity from pure CH2Cl2 to CH2Cl 2/ CH3CN mixtures and addition of tetrabutylammonium hexafluorophosphate (NBu4PF6, up to 0.15M), the supporting electrolyte used in the electrochemical measurements, have been investigated. The association constants measured in different media show the following trend: (i) they decrease upon increasing polarity of the solvent, as expected for host-guest complexes stabilized by electron donor-acceptor interactions; (ii) no effect of generation and number of dendrons (one for theDnB2+ family and two for the Dn2B2+ family) appended to the core is observed in higher polarity media; and (iii) in a low-polarity solvent, like CH2Cl2, the stability of the inclusion complexes is higher for DnB2+ dendrimers than forDn2B2+ ones, while within each dendrimer family it increases by decreasing dendron generation, and upon addition of NBu4PF6. The last result has been ascribed to a partial dendron unfolding. Kinetic investigations performed in lower polarity media evidence that the rate constants of complex formation are slower for symmetricDn2B2+ dendrimers than for the nonsymmetricDnB2+ ones, andthatwithintheDn2B 2+ family, they decrease by increasing dendron generation.The dependence of the rate constants for the formation and dissociation of the complexes upon addition of NBu4PF6 has also been investigated and discussed. ©2009 American Chemical Society.