Zn(ii)-coordination and fluorescence studies of a new polyazamacrocycle incorporating 1H-pyrazole and naphthalene units

The synthesis and Zn²⁺ coordination properties of a new macrocycle (L1) obtained by dipodal (2 + 2) condensation of the polyamine 3-(naphthalen-2-ylmethyl)pentane-1,5-diamine with 1H-pyrazole-3,5-dicarbaldehyde are reported. pH-metric studies show that L1 bears five measurable protonation steps in the 2.0-11.0 pH range. Fluorescence emission studies indicate that the removal of the first proton from the H₅L1⁵⁺ species leads to a significant decrease in the emission due to a photoinduced electron transfer process. Addition of Zn²⁺ promotes a boat-like conformation that approaches both fluorophores and facilitates the formation of an excimer which reaches its highest emission for a 1:1 Zn²⁺:L1 molar ratio. Density functional theory calculations support the experimental data and suggest that the protective effect of the Zn²⁺ ion along with hydrogen bonding between the 1H-pyrazole moiety and one of the tertiary nitrogen atoms is responsible for this behaviour.