Chromogenic Properties of p-Pyridinium- and p-Viologen-Calixarenes and Their Cation-Sensing Abilities

The synthesis of calix[4]- and -[6]arene derivatives P6 (H) 2 2+ ·(Cl - ) 2 , V4 (H) 2 4+ ·(Cl - ) 2 ·(I - ) 2 , and V6 (H) 2 4+ ·(Cl - ) 2 ·(I - ) 2 bearing N -linked pyridinium ( P ) and viologen ( V ) units at the upper rim is described here. A rare example of an anionic conformational template is reported for p -pyridiniumcalix[6]arene P6 (H) 2 2+ , which adopts a 1,3,5-alternate conformation in the presence of chloride anions. Derivatives P6 (H) 2 2+ ·(Cl - ) 2 , V6 (H) 2 4+ ·(Cl - ) 2 ·(I - ) 2 , and V4 (H) 2 4+ ·(Cl - ) 2 ·(I - ) 2 show a negative solvatochromism, while their UV-vis acid-base titration evidenced that upon addition of a base, new bands appear at 487, 583, and 686 nm, respectively, due to the formation of betainic monodeprotonated species P6 (H) 1 + , V6 (H) 1 3+ , and V4 (H) 1 3+ . These new bands were attributable to the intramolecular charge-transfer (CT) transition from the phenoxide to the pyridinium or viologen moiety and were responsive to the presence of cations. In fact, the band at 487 nm of P6 (H) 1 + was quenched in the presence of a hard Li + cation, and the color of its acetonitrile solution was changed from pink to colorless upon addition of LiI. Consequently, this derivative can be considered as a useful host for the recognition and sensing of lithium cations.