= Solution
<protonation> of the basic <amine> host weakens its <zinc> binding. <zinc> transfers to 2, displacing <lithium>, which can then bind the crown-like site of 3. The predicted switched distribution is \b[protonated 1, $[\mathrm{Zn}(2)]^{2+}$ and $[\mathrm{Li}(3)]^+$], giving \b[512 nm emission]. The strain penalty for <zinc> binding 2 is accepted when the better competing host is protonated.
Without <fluorescence>, compare $^1$H <NMR> spectra of the mixture with independently prepared free and metal-bound hosts, looking for changes near the <amine>, pyridyl and crown-ether donors. $^7$Li <NMR> can follow <lithium> environments; chemical-shift changes and exchange-dependent line shapes should be interpreted using titrations rather than treating one peak as definitive proof of one complex. <zinc>(II) is diamagnetic, so ordinary <NMR> observation is possible. UV–visible <absorption> can independently track changes in 2 and 3. Acid-only controls for each component test whether direct <protonation> shifts the reporter independently of <lithium>; metal-only and acid titrations test the proposed redistribution. The stipulated one-equivalent response is an idealized acid–base sorting cycle, not a pH measurement in aqueous buffer.
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