Small water-soluble pyrimidine hexafluorophosphate derivatives with high two-photon absorption activities in the near-IR region and their biological applications

Abstract

Small, biocompatible and water-soluble molecules with moderate two-photon absorption (2PA) cross-section values (σ) are in extreme demand for specific bioimaging applications. Herein, two novel imidazole–pyrimidine hexafluorophosphate derivatives (6P and 10P) are efficiently synthesized through improved Knoevenagel condensation and Ullmann reactions with high yield. Based on systematic photophysical investigations and theoretical calculations, the structure–property relationships indicate that the different donor groups have large influences on their optical properties. The 2PA cross-section values (σ) are obtained both by Z-scan and two-photon excited fluorescence (2PEF) measurements. It is found that the two-photon absorption cross section values (σ) in the near-infrared region are significantly enhanced for 6P and 10P, which confirms that these small molecules display a suitable turn-ON fluorescence response for two-photon fluorescence microscopy (2PFM). Comprehensively considering the high solubility of the introduced hexafluorophosphate group, 6P and 10P possess high specificity for mitochondrial localization, showing a temperature-independent pathway, possibly passive infusion, which is advantageous in comparison with the commercially available mitochondrial trackers. Due to their low cytotoxicity, these small molecules offer a promising platform to directly monitor the intestinal system in live zebrafish larvae.