Color changes anion substitution

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Fabbrizzi and coworkers have synthesized a naphthalimide-substituted urea capable of double depro-tonation. The addition of TBA-F to 122 in DMSO leads to a yellow to red color change after the addition of a few equivalents of anion, and on further addition, a second deprotonation step occurs leading to a blue coloration. This process can be monitored by UV-vis spectroscopy with the emergence of a new band at 540 nm for the single deprotonated species and a decrease in the free host band at 400 nm. With further addition of F , a new band at 600 nm forms corresponding to the doubly deprotonated species, with a decrease in the band intensity at 540 nm (Figure 32). Isosbestic points are observed for all new bands showing a clear transition between species. Carboxylates such as acetate also lead to a similar effect. 

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