Synthesis and Intracellular Localization of 5,6-Disubsutituted Pyrido[1′,2′;2,3]imidazo[5,1-a]isoquinolinium Iodides

Novel pyrido[1',2';2,3]imidazo[5,1-a]isoquinolinium iodides independently substituted at their 5- and 6-position were prepared from 5-iodopyrido[1',2';2,3]imidazo[5,1-a]isoquinolinium iodides using Suzuki-Miyaura cross-coupling reaction. The desired coupling products had cationic extended pi-conjugated skeletons and fluoresced strongly in the 420-460 nm region in methanol. In cell staining assay, the 5-styryl-substituted compounds, which showed lower fluorescence quantum yields in methanol, emitted high fluorescence in the intracellular environment and were specifically distributed to the endoplasmic reticulum.