Red fluorescent BODIPY molecular rotor for high microviscosity environments

Microviscosity has a strong impact for diffusion-controlled processes in biological environments. BODIPY molecular rotors are viscosity-sensitive fluorophores that provide a simple and non-invasive way to visualise microviscosity. Although green fluorescent probes are already well developed for imaging, thick biological samples require longer wavelengths for investigation. This work focuses on the examination of novel β-substituted meso-phenyl-BODIPYs possessing a red emission. We report a new red fluorescent BODIPY-based probe BP-Vinyl-NO2 suitable for sensing microviscosity in rigid environments of over 100 000 cP viscosities. Furthermore, we demonstrate that changing the methyl position from ortho to meta on the β-phenyl-substituted conjugate BP-PH-m2M-NO2 redshifts absorbance and fluorescence spectra while maintaining viscosity sensitivity. Finally, we show that nitro-substitution of meso-phenyl is a versatile approach to improve the sensitivity to viscosity while suppressing sensitivity to polarity and temperature of such derivatives. In summary, we present two nitro-substituted red fluorescent probes that could be used as lifetime-based microviscosity sensors.