Shell-by-Shell Inside-Out Complexation of Organic Anions in Flexible and Rigid Pyridinium Dendrimers

Abstract

Polycationic flexible pyridinium and rigid bipyridinium (=viologen) den drimers were prepared. The cationic charges are persistent and equally distributed all through the dendrimers. The dendrimers are filled with monoanionic (benzenesulfonate (BS)), dianionic (anthraquinone disulfonate (AQDS), naphthalene disulfonate (NDS)), and trianionic (pyranine (Pyr)) guest molecules in a stepwise inside-out, shell-by-shell fashion. The total cationic charge per dendrimer subshell divided by the charge of a guest anion defines the maximum guest capacity of the corresponding shell. These numbers appear as ``magic numbers'' in H-1 NMR titrations. Thus, the proof for the sequential inside-out complexation scenario is based on the sequential appearance of the innermost, then the mid and finally the outermost dendrimer shell. Anions with matching size show the sharpest H-1 delta-transitions at the subshell equivalence points indicating that beside simple charge interaction molecular recognition is playing an important role. The end points of the generation 0 dendrimers were further probed by electrochemical techniques, yielding K > 10 x 10(3) M-1 for the last guest molecule. Two reasons for the inside-out filling were identified from simple MM+ based MD calculations, i.e., (i) backfolding of outer branches leading to multiple complexation of anionic sites in all except the outermost shell, and (ii) the reduced mobility of the core region as compared to the outer branches leading to an enhancement of the cationic attractor over time and space in the central dendrimer region. The sequential inside-out filling of guest counterions within the dendrimers is in agreement with a wrapping process in combination with the observed decreasing hydrodynamic radius.