“…Xanthene-based structures with 3,6-diamino substitution, more commonly referred to as pyronins (Figure ), have origins in the late 19th century , and became popular as dyes originally in textiles and then later in the scientific community for imaging applications. , Pyronins, like structurally similar rhodamines, typically exhibit fluorescent properties and are commonly employed in microscopy, , cell cycle studies, electrophoresis, , flow cytometry, , paraffin section analysis, , mitochondrial studies, − detecting protein–protein interactions, and chemo/ion sensors, , among other uses. − Most applications utilize commercially available pyronin Y (PY) or pyronin B (PB) and to a lesser extent acridine red (AR) which preferentially intercalate RNA allowing for imaging/detection at an excitation wavelength (λ ex ) of ∼530 nm (AR)/∼550 nm (PY/PB) and an emission wavelength (λ em ) of ∼560 nm (AR)/∼570 nm (PY/PB); however, numerous synthetically tailored pyronins have been generated for specialized uses. ,, While PB, AR, and particularly PY remain popular dyes, the quantum yields (Φ f ) for these fluorophores in aqueous solvents are relatively low in comparison to other xanthene-based fluorophores such as rhodamines and fluoresceins, limiting the sensitivity of these fluorophore systems. As a result, pyronins are often applied in high concentrations (∼100 μM) to stain cells which can lead to cytotoxicity.…”