A theoretical investigation of a series of novel two-photon zinc ion fluorescent probes based on bipyridine

In this work, a series of bipyridine core D(A′)-π-A-π-D(A′) type two–photon absorption (TPA) fluorescence probe molecules for recognition of Zn²⁺ have been studied by the density functional theory (DFT) and time dependent-DFT (TD-DFT). Furthermore, the TPA properties of these novel chromophores are explored by using DALTON program. The results of our study show that these probe molecules have the obvious TPA in the range of 600–672 nm. After combining with Zn²⁺, the fluorescence spectra red-shift and the TPA cross-section (δ) increase for most probes. These studied molecule probes, except for molecule T-8 and T-9, possess the potential to become excellent TP fluorescence imaging agents for rapid detection of Zn²⁺. More importantly, this work presents structure modification strategies for increasing two-photon response. The analysis of the relationship between structure and properties indicates that, the substitutions on the 5,5′ site are more favorable to obtain larger δ and longer emission wavelength than the modification on 4,4′ site and 6,6′ site, and the increase of electron-donating ability of end group is benefit to the red-shift of spectra and the increase of δ value. These substitutions and modifications do not affect the ability of probes to bind Zn²⁺. We hope this detailed study can provide help for the design and synthesis of new Zn²⁺ fluorescent probes.