A pyrene-based e
xcited
-
state intramolecular proton transfer (ESIPT)
active probe PMHMP was synthesized, characterized, and
employed for the ppb-level, dual-mode, and high-fidelity detection
of Cu2+ (LOD: 7.8 ppb) and Zn2+ ions (LOD: 4.2
ppb) in acetonitrile medium. The colorless solution of PMHMP turned yellow upon the addition of Cu2+, suggesting its
ratiometric, naked-eye sensing. On the contrary, Zn2+ ions
displayed concentration-dependent fluorescence rise till a 0.5 mole
fraction and subsequent quenching. Mechanistic investigations indicated
the formation of a 1:2 exciplex (Zn2+:PMHMP) at a lower concentration of Zn2+, which eventually turned
into a more stable 1:1 (Zn2+:PMHMP) complex
with an additional amount of Zn2+ ions. However, in both
cases, it was observed that the hydroxyl group and the nitrogen atom
of the azomethine unit were involved in the metal ion coordination,
which eventually altered the ESIPT emission. Furthermore, a green-fluorescent
2:1 PMHMP–Zn2+ complex was developed
and additionally employed for the fluorimetric analysis of both Cu2+ and H2PO4
– ions.
The Cu2+ ion, owing to its higher binding affinity for PMHMP, could replace the Zn2+ ion from the preformed
complex. On the other hand, H2PO4
– formed a tertiary adduct with the Zn2+–complex,
leading to a distinguishable optical signal. Furthermore, extensive
and organized density functional theory calculations were performed
to explore the ESIPT behavior of PMHMP and the geometrical
and electronic properties of the metal complexes.