Fine Tuning of Multiphoton AIE Emission Behavior, Organelle Targeting, and Fluorescence Lifetime Imaging of Terpyridine Derivatives by Alkyl Chain Engineering

In this work, a series of multiphoton terpyridine agents (ZA, ZA-Mex, and ZA-Hex) for fluorescence lifetime imaging microscopy (FLIM) are designed and synthesized. The results from photophysical property research reveal that ZA-Hex, as an N-hexylated terpyridine salt, has stronger three-photon aggregation-induced emission (AIE) properties compared to ZA-Mex due to enhanced intramolecular charge transfer (ICT) performance. All three terpyridine derivatives possess suitable fluorescence intensities and stable fluorescence lifetimes under different pH conditions (pH = 4.0–8.0), thereby performing multiphoton fluorescence lifetime imaging. For biological imaging applications, it is found that ZA shows good lipid droplet (LD) turn-on fluorescence performance, and ZA-Hex could easily accumulate in mitochondria with high specificity. This is the first report of terpyridine salts as three-photon AIE probes used for multiphoton FLIM imaging.