Quadrupolar, Highly Polarized Dyes: Emission Dependence on Viscosity and Selective Mitochondria Staining.

Quadrupolar A-D-A-type 1,4-dihydropyrrolo[3,2- b ]pyrroles (DHPPs) bearing pyridinium and quinolinium substituents emit in the 500-600 nm region. The enhancement of electronic communication between the electron-rich heterocyclic core and electron-deficient peripheral substituents turned out to be crucial for achieving emission enhancement in viscous media. DHPP bearing two 4-pyridinium substituents has optical brightness 34,000 in glycerol and only 700 in MeOH, as evidenced by measurements of the emission intensity and fluorescence lifetimes in a series of polar solvents. Such behavior makes it an excellent candidate for viscosity probes in fluorescence microscopy, as demonstrated by the fluorescence imaging of H9C2 cardiomyocytes.