Small water-soluble pyrimidine hexafluorophosphate derivatives with high two-photon absorption activities in the near-IR region and their biological applicationsElectronic supplementary information (ESI) available. See DOI: 10.1039/c7ra00380c

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. Two imidazole-pyrimidine hexafluorophosphate derivatives (6P and 10P) are synthesized. The 2PA cross-section values ( σ ) were obtained by Z-scan and two-photon excited fluorescence methods. 6P and 10P can be used to quantify and image organelles in living cells and tissues.