Sensing Abilities of Crown Ether Functionalized Polythiophenes

Abstract

Tuning sensing abilities! The affinity of three different [15]crown-5 ether functionalized polythiophenes for alkali ions has been explored (see figure). Ab initio and DFT quantum mechanical calculations show that the binding energy between neutral conducting polymers and metallic ions, which interact attractively, decreases as the size of the ion increases. Oxidation of these polythiophene derivatives significantly reduces their affinity towards alkali ions, becoming low or even nonexistent.This work reports a theoretical study of the affinity of three different [15]crown-5 ether functionalized polythiophenes for alkali ions. Ab initio and DFT quantum mechanical calculations have evidenced that the binding energy between neutral conducting polymers and metallic ions, which interact attractively, decreases as the size of the ion increases. However, the preferences of these polythiophene derivatives have been found to be influenced by the entropic contribution, which favors binding with Na(+) in preference to Li(+). Molecular dynamics simulations have shown that this behavior can be accounted for on the basis of the large fluctuations that Li(+) undergoes when it is contained in the cavity of the [15]crown-5 ether macrocycle. In contrast, the fluctuations detected for complexes with Na(+) and K(+) were particularly low, even though only the former ion fits into the center of the macrocyclic cavity. Oxidation of these polythiophene derivatives produces a drastic change in their sensing abilities, their affinities towards alkali ions becoming low or even nonexistent. Moreover, calculations indicate that the presence of unsubstituted thiophene rings directly attached to a functionalized thiophene modulates the strength of the binding in the oxidized state by reducing the repulsive interaction between the metallic cation and the pi-conjugated system. On the other hand, the influence of alkali ions on the electronic properties, in particular, on the ionization potentials and the lowest pi-pi* transitions, of both the neutral and oxidized polythiophenes is discussed.