The halochromism of the crown-ether-substituted betaine dyes 1 is very sensitive to the cation size of the added electrolytes. The strongest hypsochromic shift of the longest- wavelength UVIVIS absorption results for n = 0 with Na⊕, n = 1 with K⊕, and n = 2 with Cs⊕. Thus, the dyes 1 may be useful as indicators.
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Reichardt et al. (1991) studied this question.
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