Trimethylenedipyridinium Dendrimers: Synthesis and Sequential Complexation of Anthraquinone Disulfonate in Molecular Shells

Abstract

The synthesis of a new class of polycationic dendrimers of generation 0, 1, and 2 (G0 (1.07 nm), G1 (1.63 nm), and G2 (3.03 nm)) is described. Core and 2-fold branching units consist of interconnected 1,3,5-tris(methylene)benzene and 4,4′-trimethylenedipyridinium units. 4-tert-Butylbenzyl has been used as the peripheral end group because best solubility of the dendrimers was achieved. The dendrimers act as a host for anthraquinone-2,6-disulfonate (AQDS) and can be stoichiometrically titrated and even overcharged as shown by 1H NMR, DOSY and cyclic voltammetry. Upon loading them with AQDS, the dendrimers undergo first a contraction, they reach a minimum hydrodynamic radius for complete charge compensation and they reopen when overcharging takes place. The contraction is supported by MM+ calculations. Upon stepwise loading of G2 (42 positive charges) with AQDS (2 negative charges), the first 3 mol equiv (6 negative charges) occupy the innermost dendrimer shell (consisting of 6 positive charges), the next 6 equiv (12 negative charges) occupy the middle shell (12 positive charges), and the last 12 equiv AQDS (24 negative charges) occupy the outermost shell of the dendrimer (24 positive charges), as supported by 1H NMR titrations yielding the magic equivalent numbers of 3, 9 = 3 + 6, and 21= 3 + 6 + 12.