Access to Pyridinyl or Pyridinium Aza-BODIPYs with Tunable Near-Infrared Fluorescence through ICT from 4-Pyridinyl Pyrroles

Aza-dipyrromethene boron difluoride (Aza-BODIPY) dyes have attracted much interest in recent decades. In this manuscript, we provide a facile way to access pyridine-substituted pyrroles. A series of 1/7-pyridinyl and pyridinium near-infrared (NIR) Aza-BODIPYs with/without rigidity were constructed and their spectroscopic properties were extensively investigated. These pyridinyl Aza-BOIDIPYs displayed slight bathochromic shifts and maintained bright NIR emission. Pyridinium Aza-BODIPYs showed increased bathochromic shift, however, the molar extinction coefficients and fluorescence quantum yields were decreased owing to ICT effect. The impacts followed the order of pyridin-4-ium>pyridin-2-ium>pyridin-3-ium, which was in consistence with their ICT strength. MO calculation was performed to provide possible explanation to the red-shifted absorption/emission, and apparent charge separation in pyridin-4-ium Aza-BODIPYs. The pyridinium Aza-BODIPY localized in lysosome and potentially avoided harmful ''alkalinizing effects'' of traditional lysosome-targeting fluorescent dyes containing amine moiety. We are working on construction of pyridinyl and pyridinium Aza-BODIPY photosensitizers against microbials or tumors.