Abstract
The synthesis of calix[4]- and -[6]arene derivatives P6(H)22+·(Cl–)2, V4(H)24+·(Cl–)2·(I–)2, and V6(H)24+·(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)22+, which adopts a 1,3,5-alternate conformation in the presence of chloride anions. Derivatives P6(H)22+·(Cl–)2, V6(H)24+·(Cl–)2·(I–)2, and V4(H)24+·(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)13+, and V4(H)13+. 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.
Highlights
Chromogenic molecules1 can respond to external stimuli by varying their optical properties
The synthesis of p-pyridiniumcalix[6]arene P6(H)22+·(Cl−)2 is outlined in Scheme 1
Even though a cationic conformational template19,20 is widely described for calixarene macrocycles,19,20 this is a rare example of an anionic conformational template for this class of hosts.21. With these results in hand, we focused our attention on the synthesis of calixarenes bearing 4,4′-bipyridinium units at the upper rim
Summary
Chromogenic molecules can respond to external stimuli by varying their optical properties. The CD3CN solution of P6(H)22+·(Cl−) was titrated with (nBu)4NOH, and the formation of P6(H)1+ was confirmed by the upfield shift of the signals of the pyridinium units of about 0.5−1.0 ppm (Figures S45−S47) This result confirmed the charge transfer from the phenoxide to the pyridinium moiety already discussed by UV−vis experiments and DFT calculations (Figure 6a). PViologencalix[6]arene betainic trication V6(H)3+ shows a band at λ = 680 nm in DMSO, which experienced a hypsochromic shift at 580, 550, and 510 nm in isopropanol, methanol, and water, respectively (Figure 7c) In this case, a negative solvatochromism is observed, with a corresponding color change from green to pink (Figure 7c). In this case, the stronger electron-withdrawing effect of the doubly charged viologen unit on the phenoxide group weakens the O−···M+ interactions
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