Highly Ordered Poly(cyclopentabithiophenes) Functionalized with Crown-Ether Moieties for Lithium- and Sodium-Sensing Electrodes

Functionalized cyclopentadithiophenes 2 and 3 were synthesized and polymerized by anodic coupling, in acetonitrile solution, in the presence of 0.1 M tetraethylammonium perchlorate as supporting electrolyte. The former carries a 16-crown-5-ether ring coplanar to the bithiophene moiety, while a perpendicular 15-crown-5-ether ring is present in the latter. The cyclic voltammogram of poly- 2 shows two redox processes at E ° = −0.3 and +0.3 V, whereas that of poly- 3 consists of a single response at E ° = 0.0 V. The difference in electrochemical behavior is attributed to the occurrence of strong polaron π-dimerization in poly- 2 . The redox potential ( E °) of this polymer moves toward more positive values passing from lithium to sodium salts as supporting electrolytes in acetonitrile solution. The redox cycle of poly- 3 is instead completely insensitive to the change of the cationic species in solution. EQCM analysis shows that (i) both neutral polymers incorporate one alkaline ion per crown ring, (ii) the p-doping process of poly- 2 produces the ejection of 50% of the cations, and (iii) complete cation retention is observed in p-doping of poly- 3 . These results are explained by a sandwich-coordination of the cation to the crown-ether moieties in poly- 2 following oxidation. Poly- 2 -coated Pt-wire electrodes give a linear potentiometric response (slope 60 mV) versus the logarithm of sodium concentration, in acetonitrile solution, in the 10 - 3 −10 - 1 M range.