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Abstract: Fluorescent molecular rotors (FMRs) can act as viscosity sensors in various media including subcellular organelles and microfluidic channels. In FMRs, the rotation of rotators connected to a fluorescent π‐conjugated bridge is suppressed by increasing environmental viscosity, resulting in increasing fluorescence (FL) intensity. In this minireview, we describe recently developed FMRs including push–pull type π‐conjugated chromophores, meso‐phenyl (borondipyrromethene) (BODIPY) derivatives, dioxaborine derivatives, cyanine derivatives, and porphyrin derivatives whose FL mechanism is viscosity‐responsive. In addition, FMR design strategies for addressing various issues (e.g., obtaining high FL contrast, internal FL references, and FL intensity‐contrast trade‐off) and their biological and microfluidic applications are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: A series of fluorescent molecular rotors obtained by introducing two rotational groups (“rotators”), which exhibit different rotational and electron‐donating abilities, are discussed. Whereas the control molecular rotor, PH, includes a single rotator (the widely used phenyl group), the PO molecular rotors consist of two rotators (a phenyl group and an alkoxy group), which exhibit simultaneous strongly electron‐donating and easy rotational abilities. Compared with the control rotor PH, PO molecular rotors exhibited one order of magnitude higher quantum yield (fluorescence intensity) and simultaneously exhibited significantly higher fluorescence contrast. These properties are directly related to the strong electron‐donating ability and low energy barrier of rotation of the alkoxy group, as confirmed by dynamic fluorescence experiments and quantum chemical calculations. The PO molecular rotors exhibited two fluorescence relaxation pathways, whereas the PH molecular rotor exhibited a single fluorescence relaxation pathway. Cellular fluorescence imaging with PO molecular rotors for mapping cellular viscosity was successfully demonstrated. [ABSTRACT FROM AUTHOR]

New pyrrolic viscosity sensors exhibit one order of magnitude higher fluorescence contrast compared to that of the conventional phenolic analogues due to the viscosity-sensitive rotation of the asymmetric pyrrole group and successfully demonstrate mapping of intracellular viscosity by fluorescence lifetime imaging microscopy. [ABSTRACT FROM AUTHOR]