1,1-Dicyano-2-[6-(dimethylamino)naphthalen-2=yl]propene (II, DDNP) has been synthesized as a fluorescent dye whose intramolecular rotational relaxation is solvent polarity and viscosity dependent. Its fluorescence emission (lambda max : 470-610 nm in hydrophobic and viscous environments) is in a very favorable region for its use with visible fluorescence microscopy, without interference from cell or tissue autofluorescence. Its fluorescence emission quantum yield in an aqueous environment is very low, allowing for facile differentiation between lipid or protein bound state and aqueous media. II crystallizes into two solid isoforms containing one (red crystals) and two conformers (yellow crystals), exhibiting differing spectral properties. X-ray crystal structures reveal a planar arrangement between dimethylamino and naphthalenyl moieties, with out-of-plane arrangements between the malononitrile and naphthalene ring found in each of the three conformers.