Design and synthesis of intramolecular hydrogen bonding systems. Their application in metal cation sensing based on excited-state proton transfer reaction

We reported the design and synthesis of a new type of metal-cation probes, 3-hydroxy-4-(1,4,7,10-tetraoxa-13-azacyclopentadec-13-ylmethyl)naphthalene-2-carbaldehyde (1a) and its single hydrogen-bond analogue 1-(1,4,7,10-tetraoxa-13-azacylopentadec-13-ylmethyl)-2-naphthol (2a), in which 1-aza-15-crown-5 ether in combination with the naphthol oxygen acts as a receptor, while the mechanism of excited-state intramolecular proton transfer (ESIPT) is exploited as a signal transducer. The association constant of (2.5±0.5)×10⁴, (3.8±0.4)×10⁴, (5.5±0.5)×10³ and (1.2±0.3)×10⁴ M⁻¹ for the formation of 1a/Na⁺, 1a/Ca²⁺, 2a/Na⁺ and 2a/Ca²⁺ complexes, respectively, in CH₃CN plus drastic fluorescence changes due to the fine-tuning of ESIPT reaction upon complexation, lead 1a and 2a to be highly sensitive fluorescent sensors. The results add a new class into the category of metal-cation probes, with the perspective of designing ESIPT systems capable of sensing bio-analytes.