 Solution

DBU removes the proton from 1, restoring its zinc-binding ability. zinc returns from 2 to 1; the regenerated free 2 captures lithium from 3, restoring the preferred initial sorting. Thus the reporter returns predominantly to 554 nm, while DBU becomes its conjugate acid and trifluoroacetate remains as counterion. This assumes the added base is strong enough under the solvent conditions and does not itself compete effectively for either metal. Reversing a fluorescence shift without checking speciation would not establish all these transfers on its own.