Manipulating Intramolecular Charge Transfer in Terpyridine Derivatives Towards “Turn-On” Fluorescence Chemosensors for Zn2+

Abstract

A series of donor-acceptor (D−A) terpyridine derivatives with various intramolecular charge transfer interactions have been successfully synthesized bearing phenyl, methoxyphenyl, N-butyldiphenylamine (DPA), and triphenylamine (TPA) as electron-donors and terpyridine (TPY), 2,6-di(pyrazin-2-yl)pyridine (PYDPZ), and N,N-dimethylated PYDPZ (PYDPZ-2CH₃) as electron acceptors. Upon the introduction of pyrazine rings instead of pyridine ones and further selective N,N’-dimethylation, the intramolecular D−A interactions are significantly enhanced, resulting in the remarkable reduced intramolecular charge transfer (ICT) transitions and quenched PL emissions in CH₂Cl₂ solution. However, their ICT emissions are clearly recovered upon adding Zn²⁺. Especially, for double positively charged compound 12, the Zn²⁺-induced “turn-on” green emission (λ_{max, em}=518 nm, Φ=0.24) with excellent sensitivity (I/I₀=77) and selectivity (I_Zn/I_Cd=23) is detected. Under the optimized experimental condition (EtOH-H₂O (9 : 1, v/v), containing 0.05 M HEPES buffer (pH=7.4)), this probe is used to quantitative determination of Zn²⁺ in a water sample with an average recovery of 97.5 %. Furthermore, the probe is applied to imaging of Zn²⁺ ions in HeLa cells and its mechanism is confirmed by the molecular dynamics simulation, in which lower coulomb potential for membrane surface adsorption and energy barrier for membrane translocation can be found.