Structural relevance of N2O2-donor naphthodiaza-crown macrocyclic ligands to selective fluorescence signaling behavior towards aliphatic tertiary amines

• The effect of amines on fluorescence emission of naphthodiaza-crown macrocyclic ligands was investigated.
• The dynamic quenching of the macrocylces followed from the order aromatic amine << 1° amine < 2° amine < 3° amine.
• The observed quenching correlated with the basicity of the amine as well as the macroring size.
• A proton transfer mechanism in excited state was suggested.

A series of N2O2-donor naphthodiaza-crown macrocyclic ligands, L1–L5, were synthesized and characterized by IR, 1H and 13C NMR spectroscopy, mass spectrometry as well as microanalysis. Preliminary observations employing UV–vis spectrophotometry of L3 in ethanol revealed that triethylamine (TEA) has a significant blue-shift effect on the entire of the spectra. Then, the detection of aliphatic and aromatic amines by L3 were followed by monitoring of fluorescence quenching of the resulting adducts in terms of their binding with L1-L5. Both steady-state and time-resolved spectroscopic measurements in the polar ethanol (EtOH) solvent show that the fluorescence quenching phenomenon of L1–L5 in presence of TEA is primarily due to the involvement of dynamic process. The fluorescence quenching of L1–L5 was described according to the hydrogen bonding of NH groups on the aza-crowns with TEA. A mechanism of proton transfer in excited state in terms of hydrogen bonding of the macrocyclic ligands with amines was also proposed for selective fluorescence quenching of tertiary aliphatic amines by L1–L5.

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