Styrylpyridinium Derivatives for Fluorescent Cell Imaging.

A set of styrylpyridinium (SP) compounds was synthesised in order to study their spectroscopic and cell labelling properties. The compounds comprised different electron donating parts (julolidine, p -dimethylaminophenyl, p -methoxyphenyl, 3,4,5-trimethoxyphenyl), conjugated linkers (vinyl, divinyl), and an electron-withdrawing N -alkylpyridinium part. Geminal or bis -compounds incorporating two styrylpyridinium ( bis -SP) moieties at the 1,3-trimethylene unit were synthesised. Compounds comprising a divinyl linker and powerful electron-donating julolidine donor parts possessed intensive fluorescence in the near-infrared region (maximum at ~760 nm). The compounds had rather high cytotoxicity towards the cancerous cell lines HT-1080 and MH-22A; at the same time, basal cytotoxicity towards the NIH3T3 fibroblast cell line ranged from toxic to harmful. SP compound 6e had IC ₅₀ values of 1.0 ± 0.03 µg/mL to the cell line HT-1080 and 0.4 µg/mL to MH-22A; however, the basal toxicity LD ₅₀ was 477 mg/kg (harmful). The compounds showed large Stokes' shifts, including 195 nm for 6a , b , 240 nm for 6e , and 325 and 352 nm for 6d and 6c , respectively. The highest photoluminescence quantum yield (PLQY) values were observed for 6a , b , which were 15.1 and 12.2%, respectively. The PLQY values for the SP derivatives 6d , e (those with a julolidinyl moiety) were 0.5 and 0.7%, respectively. Cell staining with compound 6e revealed a strong fluorescent signal localised in the cell cytoplasm, whereas the cell nuclei were not stained. SP compound 6e possessed self-assembling properties and formed liposomes with an average diameter of 118 nm. The obtained novel data on near-infrared fluorescent probes could be useful for the development of biocompatible dyes for biomedical applications.