Microscopic details of the sensing ability of 15-crown-5-ether functionalized poly(bithiophene)

Host-guest interactions between alkali ions (Li+, Na+ and K+) and a functionalized poly(bithiophene) with a 15-crown-5-ether covalently linked to two adjacent thiophene rings have been analyzed using theoretical methods. Results indicate that the considerable conformational flexibility of the polymer backbone is reduced when the cation accommodates in the cavity of the macrocycle. The enthalphic and entropic contributions to the binding have been estimated using quantum mechanical calculations and molecular dynamics trajectories, respectively. The enthalpic term becomes more favorable when the size of the cation decreases, while the entropy calculated for the “free state → bound state” process decreases when the size of the cation increases. On the other hand, calculated atomic-centered charges reflect that the π-conjugated system of the conducting polymer undergoes a partial oxidation upon the binding process. Moreover, the accommodation of the alkali cation in the macrocycle produces an increase of both the ionization potential and the lowest π–π∗ transition energy of the polymer.